Industrial Fans and Blowers
Industrial Fans and Blowers
Fans are normally identified as machines with relatively low pressure rises which move air, gases, or vapours by means of rotating blades or impellers and change the rotating mechanical energy into pressure or work on the gas or vapour. The result of this work on the fluid is in the form of pressure energy or velocity energy, or some combination of both. Fans are widely used in industrial and commercial applications. Fans are critical for process support and human health from shop ventilation to material handling to boiler applications.
Industrial fans and blowers are machines, whose primary function is to provide a large flow of air or gas to various processes. This is achieved by rotating a number of blades, connected to a hub and shaft, and driven by a motor or turbine. The flow rates of these fans range from around 5.7 cubic meter (cum) to 57,000 cum per minute. A blower is another name for a fan which operates where the resistance to the flow is primarily on the downstream side of the fan.
Fans, blowers, and compressors are differentiated by the method used to move the air, and by the system pressure they are to operate against. The American Society of Mechanical Engineers (ASME) uses the specific ratio, which is the ratio of the discharge pressure over the suction pressure, to define fans, blowers, and compressors as shown in Tab 1.
|Tab 1 Difference between fans, blowers, and compressors|
|Equipment||Specific ratio||Pressure increase|
|Blowers||1.11-1.20||1,136 to 2,066|
|Compressors||More than 1.20||–|
There are several uses for the continuous flow of air or gas which the industrial fans generate. These include combustion, ventilation, aeration, particulate transport, exhaust, cooling, air-cleaning, and drying, etc. A number of plants use fans and blowers for ventilation and for industrial processes which need an air flow. There are two primary types of fans namely (i) centrifugal fans, and (ii) axial fans. These types are characterized by the path of the airflow through the fan. Centrifugal fans use a rotating impeller to move the air stream. As the air moves from the impeller hub to the blade tips, it gains kinetic energy. This kinetic energy is then converted to a static pressure increase as the air slows before entering the discharge. Axial fans move the air stream along the axis of the fan. Fan and blower selection depends on the volume flow rate, pressure, type of material handled, space limitations, and efficiency. Fan efficiencies differ from design to design and also by types.
Centrifugal fans increase the speed of an air / gas stream with a rotating impeller. The speed increases as the air / gas reaches the ends of the blades and is then converted to pressure. These fans are capable of generating high pressures, which makes them suitable for harsh operating conditions, such as systems with high temperatures, dirty air / gas streams (high moisture and particulate content), and material handling. In centrifugal flow, airflow changes direction twice, once when entering and second when leaving.
The centrifugal design uses the centrifugal force generated by a rotating disk, with blades mounted at right angles to the disk, to impart movement to the air or gas and increase its pressure. The assembly of the hub, disk, and blades is known as the fan wheel, and frequently includes other components with aerodynamic or structural functions. The centrifugal fan wheel is typically contained within a scroll-shaped fan housing, resembling the shell of the nautilus sea creature with a central hole. The air or gas inside the spinning fan is thrown off the outside of the wheel, to an outlet at the housing’s largest diameter. This simultaneously draws more air or gas into the wheel through the central hole. Inlet and outlet ducting are frequently attached to the fan’s housing, to supply and / or exhaust the air or gas to the industry’s requirements. There are several varieties of centrifugal fans, which can have fan wheels that range from less than 0.3 m (meters) to over 5 m in diameter. Centrifugal fans are categorized by their blade shapes (Fig 1). The categories are (i) radial fans, with flat blades, (ii) forward curved fans, with forward curved blades, and (iii) backward inclined fan, with blades which tilt away from the direction of rotation (flat, curved, and air foil).
Fig 1 Categories of centrifugal fans
Radial fans, with flat blades are suitable for high static pressures (up to 1,400 mm water column) and high temperatures. Simple design allows custom build units for special applications. These fans can operate at low air flows without vibration problems. They have high durability and have efficiencies up to 75 %. They have large running clearances, which is useful for airborne-solids (dust, wood chips, and metal scraps) handling services. They are only suitable for low to medium air flow rates.
Forward curved fans, with forward curved blades can move high air volumes against relatively low pressure and are of relatively small size. These fans have low noise level (due to low speed) and are well suited for residential heating, ventilation, and air conditioning (HVAC) applications. They are only suitable for clean service applications but not for high pressure and harsh services. The fan output is difficult to adjust accurately. In these fans, driver is to be selected carefully to avoid motor overload since power curve increases steadily with airflow. These fans have relatively low energy efficiency (55 % to 65 %).
Backward inclined fan can operate with changing static pressure (as this does not overload the motor). These fans are suitable when system behaviour at high air flow is uncertain. They are suitable for forced-draft services. Flat bladed fans are more robust while the curved blades fans are more efficient (exceeding 85 %). Thin air foil blades fans are most efficient. These fans are not suitable for dirty air streams (as fan shape promotes accumulation of dust). Air foil blades fans are less stable because of staff as they rely on the lift created by each blade. Thin air foil blades fans are subject to erosion.
Axial fans, as the name implies, move an air stream along the axis of the fan. The air is pressurized by the aerodynamic lift generated by the fan blades. Although they can sometimes be used inter-changeably with centrifugal fans, axial fans are normally used in clean air, low-pressure, high-volume applications. Axial fans have less rotating mass and are more compact than centrifugal fans of comparable capacity. Additionally, axial fans tend to have higher rotational speeds and are somewhat noisier than in-line centrifugal fans of the same capacity. However, this noise tends to be dominated by high frequencies, which tend to be easier to decrease.
Axial fans move an air / gas stream along the axis of the fan. The way these fans work can be compared to a propeller on an aeroplane i.e., the fan blades generate an aerodynamic lift which pressurizes the air. The axial design uses axial forces to achieve the movement of the air or gas, spinning a central hub with blades extending radially from its outer diameter. The fluid is moved parallel to the fan wheel’s shaft, or axis of rotation. The axial fan wheel is frequently contained within a short section of cylindrical ductwork, to which inlet and outlet ducting can be connected. They are popular with industry since they are inexpensive, compact, and light.
Axial fan types have fan wheels with diameters which normally range from less than 0.3 m to over 9 m, although axial cooling tower fan wheels can exceed 12.5 m in diameter. In general, axial fans are used where the principal requirement is for a large volume of flow, and the centrifugal design where both flow and higher pressures are needed. The main categories of axial flow fans (propeller, tube-axial and vane-axial) are shown in Fig 2.
Fig 2 Categories of axial fans
Propeller fan generate high air flow rates at low pressures. These fans are not combined with extensive ductwork since they generate little pressure and are inexpensive because of their simple construction. They achieve maximum efficiency, near-free delivery, and are frequently used in roof top ventilation applications. They can generate flow in reverse direction, which is helpful in ventilation applications. These fans have relative low energy efficiency and are comparatively noisy.
Tube-axial fan, essentially a propeller fan placed inside a cylinder but with higher pressures and better operating efficiencies than propeller fans. These fans are suited for medium-pressure and have high airflow rate applications, e.g., ducted HVAC installations They are relatively expensive and have moderate air flow noise. These fans can quickly accelerate to rated speed because of their low rotating mass. They generate flow in reverse direction, which is useful in several ventilation applications. They create sufficient pressure to overcome duct losses and are relatively space efficient, which is useful for exhaust applications air flow noise. These fans have relatively low energy efficiency (65 %).
Vane-axial fan are suited for medium pressure to high pressure applications (up to 500 mm water column), such as induced draft service for a boiler exhaust. These fans can quickly accelerate to rated speed because of their low rotating mass. They generate flow in reverse directions, which is useful in many ventilation applications. These fans are suited for direct connection to motor shafts and are most energy efficient (up to 85 % if equipped with air foil fans and small clearances). These fans are relatively expensive compared to propeller fans.
Blower is a very important fluid machine. It has characteristics of energy transfer between continuous stream of fluid and rotating element about an axis. Blower is a head generating machine which employs the dynamic action of a rotating elements. Blowers are pressure increasing machines in which the fluid enters axially and is discharged by the rotor into a static collector system casing and then into a discharge pipe. They are also used to produce negative pressures for industrial vacuum systems. The centrifugal blower and the positive displacement blower are two main types of blowers. Blowers can achieve much higher pressures than fans, as high as 1.2 kilograms per square centimetre (kg/sq cm).
Main components of blower are impeller which is having rotary motion, where energy is transferred and followed by stationary part casing, in which energy transformation takes place. Casing decides the size and pressure rise in the system. Blowers are used where large volumes of air or gas at low pressure are needed. They normally operate at low speeds and pressure ratios. In blower there is normally a considerable density change. Blower efficiencies differ with designs of each other.
There are two types of blowers namely (i) centrifugal flow blower, and (ii) axial flow blower. The impeller is typically gear-driven and rotates as fast as 15,000 rpm (revolutions per minute). In centrifugal flow, air flow change direction two times, first when entering and second when leaving. They are also used to produce negative pressures for vacuum systems normally used in industries. Centrifugal blowers look more like centrifugal pumps than fans. Centrifugal blowers typically operate against pressures of 0.35 to 0.7 kg/sq cm, but can achieve higher pressures. One characteristic is that air flow tends to drop drastically as system pressure increases, which can be a disadvantage in material conveying systems that depend on a steady air volume. Because of this, they are more frequently used in applications which are not prone to clogging.
In multi-stage blowers, air is accelerated when it passes through each impeller. In single-stage blower, air does not take many turns, and hence it is more efficient. Positive displacement blowers have rotors, which ‘trap’ air / gas and push it through housing. These blowers provide a constant volume of air even if the system pressure varies. They are especially suitable for applications prone to clogging, since they can produce enough pressure (typically up to 1.25 kg/sq cm) to blow clogged materials free. They turn much slower than centrifugal blowers (e.g., 3,600 rpm) and are frequently belt driven to facilitate speed changes.
Important characteristics of a fan system
The term ‘system resistance’ is used when referring to the static pressure. The system resistance is the sum of static pressure losses in the system. The system resistance is a function of the configuration of ducts, pickups, elbows, and the pressure drops across equipment, such as bag filter or cyclone. The system resistance varies with the square of the volume of air flowing through the system. For a given volume of air, the fan in a system with narrow ducts and multiple short radius elbows is to work harder to overcome a higher system resistance than in a system with bigger ducts and a minimum number of long radii turns.
Long narrow ducts with several bends and twists need more energy to pull the air through them. As a result, for a given fan speed, the fan is able to pull less air through this system than through a short system with no elbows. Hence, the system resistance increases substantially as the volume of air flowing through the system increases (square of air flow). On the other hand, resistance decreases as flow decreases. To determine the volume which the fan is producing, it is necessary to know the system resistance characteristics. In existing systems, the system resistance can be measured. In systems which have been designed, but not built, the system resistance is to be calculated. Typically, a system resistance curve is generated for various flow rates on the x-axis and the associated resistance on the y-axis.
Fan characteristics can be represented in form of fan curve(s). The fan curve is a performance curve for a particular fan under a specific set of conditions. The fan curve is a graphical representation of a number of inter-related parameters. Typically, a curve is to be developed for a given set of conditions normally including (i) fan volume, (ii) system static pressure, (iii) fan speed, and (iv) brake power needed to drive the fan under the stated conditions. Some fan curves also include an efficiency curve so that a system designer knows where on that curve the fan is operating under the chosen conditions. Of the several curves, the curve of static pressure versus flow is especially important. The intersection of the system curve and the static pressure curve defines the operating point. When the system resistance changes, the operating point also changes. Once the operating point is fixed, the power needed can be determined by following a vertical line which passes through the operating point to an intersection with the power curve. A horizontal line drawn through the intersection with the power curve leads to the needed power on the right vertical axis. In the depicted curves, the fan efficiency curve can also be presented.
In any fan system, the resistance to air flow (pressure) increases when the flow of air is increased. It varies as the square of the flow. The pressure needed by a system over a range of flows can be determined and a ‘system performance curve’ can be developed. This system curve can then be plotted on the fan curve to show the fan’s actual operating point (say A) where the two curves intersect. This operating point is at air flow (say F1) delivered against pressure (say P1). A fan operates at a performance given by the manufacturer for a particular fan speed. The fan performance chart shows performance curves for a series of fan speeds. At fan speed (say R1), the fan operates along the R1 performance curve. The fan’s actual operating point on this curve depends on the system resistance i.e., fan’s operating point at A is flow (F1) against pressure (P1).
Two methods can be used to reduce air flow from F1 to F2. The first method is to restrict the air flow by partially closing a damper in the system. This action causes a new system performance curve where the needed pressure is higher for any given air flow. The fan now operates at different point (say B) to provide the reduced air flow F2 against higher pressure P2.
The second method to reduce air flow is by reducing the speed from R1 to R2, keeping the damper fully open. The fan now operates at new point (say C) to provide the same air flow F2, but at a lower pressure P3. Hence, reducing the fan speed is a much more efficient method to decrease airflow since less power is needed and less energy is consumed. The fans operate under a predictable set of laws concerning speed, power, and pressure. A change in speed (RPM) of any fan predictably changes the pressure rise and power necessary to operate it at the new RPM.
Fan systems are necessary to keep manufacturing processes working, and consist of a fan, an electric motor, a drive system, ducts or piping, flow control devices, and air conditioning equipment (filters, cooling coils, and heat exchangers, etc.). The components of a typical fan system are shown in Fig 3.
Fig 3 Components of a fan system
Assessment of fans and blowers
Here how to evaluate the performance of fans is described, but the same is also applicable to the blowers. Fan efficiency is the ratio between the power transferred to the air stream and the power delivered by the motor to the fan. The power of the air flow is the product of the pressure and the flow, corrected for unit consistency. Another term for efficiency which is frequently used with fans is static efficiency. Static efficiency uses static pressure instead of total pressure in estimating the efficiency.
When evaluating fan performance, it is important to know which efficiency term is being used. The fan efficiency depends on the type of the fan and impeller. As the flow rate increases, the efficiency increases to certain level (peak efficiency) and then decreases with further increasing the flow rate. The peak efficiency has different ranges for different types of centrifugal fans and axial fans. Fan performance is typically estimated by using a graph which shows the different pressures developed by the fan and the corresponding needed power. The manufacturers normally provide these fan performance curves. Understanding this relationship is necessary for designing, sourcing, and operating a fan system and is the key to optimum fan selection.
In the manufacturing sector, fans consume around 15 % of the electric power used by motors. Similarly, in the commercial sector, electric power needed to operate fan motors composes a high portion of the energy costs for space conditioning. In manufacturing, fan reliability is critical to plant operation. For example, where fans serve material handling applications, fan failure immediately creates a process stoppage. In industrial ventilation applications, fan failure frequently forces a process to shut down (although there is frequently enough time to bring the process to an orderly stoppage). Even in heating and cooling applications, fan operation is necessary to maintain a productive work environment.
Fan failure leads to conditions in which employee productivity and product quality declines. This is especially true for some production applications in which air cleanliness is critical for minimizing production defects. In such case, fan operation has a considerable impact on the plant production. The importance of fan reliability frequently causes system designers to design fan systems conservatively. Concerned about being responsible for under-performing systems, designers tend to compensate for uncertainties in the design process by adding capacity to fans. Unfortunately, oversizing fan systems creates problems which can increase system operating costs while decreasing fan reliability.
Fans which are oversized for their service requirements, do not operate at their best efficiency points. In severe cases, these fans can operate in an unstable manner because of the point of operation on the fan air flow-pressure curve. Oversized fans generate excess flow energy, resulting in high air flow noise and increased stress on the fan and the system. As a result, oversized fans not only cost more to purchase and to operate, they create avoidable system performance problems. The use of a ‘systems approach’ in the fan selection process typically yields a quieter, more efficient, and more reliable system.
Fan selection is a complex process which starts with a basic knowledge of system operating requirements and conditions such as air flow rates, temperatures, pressures, air stream properties, and system layout. The variability of these factors and other considerations, such as cost, efficiency, operating life, maintenance, speed, material type, space constraints, drive arrangements, temperature, and range of operating conditions, complicate the selection of fan. However, knowledge of the important factors in the fan selection process can be helpful for the purpose of reducing energy consumption during system retrofits or expansions.
A fan type is frequently chosen for non-technical reasons, such as price, delivery, availability, or designer or operator familiarity with a fan model. If noise levels, energy costs, maintenance requirements, system reliability, or fan performance are worse than expected, then the issue of whether the appropriate fan type has been initially selected is to be revisited. Fans are normally selected from a range of models and sizes, rather than designed specifically for a particular application.
Fan selection is based on calculating the air flow and pressure requirements of a system, then finding a fan of the right design and materials to meet these requirements. Unfortunately, there is a high level of uncertainty associated with predicting system air flow and pressure requirements. This uncertainty, combined with fouling effects and anticipated capacity expansion, encourages the tendency to increase the specified size of a fan / motor assembly. Designers tend to protect against being responsible for inadequate system performance by over specifying. However, an oversized fan / motor assembly creates a different set of operating issues, including inefficient fan operation, excess air flow noise, poor reliability, and pipe / duct vibrations.
Knowledge of the problems and costs associated with poor fan selection helps the designers and the operators to improve fan system performance through better fan selection and improved operating and maintenance practices. Some of these problems are described below.
Noise – In industrial ventilation applications, noise can be a considerable concern. High acoustic levels promote employees’ fatigue. The noise generated by a fan depends on fan type, air flow rate, and pressure. Inefficient fan operation is frequently indicated by a comparatively high noise level for a particular fan type. If high fan noise levels are unavoidable, then ways to decrease the acoustic energy is to be considered. Noise reduction can be accomplished by several methods like (i) insulating the duct, (ii) mounting the fan on a soft material, such as rubber or suitable spring isolator as needed to limit the quantity of transmitted vibration energy, or (iii) installing sound damping material or baffles to absorb noise energy.
Rotational speed – Fan rotational speed is typically measured in rpm. Fan rotational speed has a considerable impact on fan performance. Rotational speed is to be considered alongside with other issues, such as variation in the fan load, air stream temperature, ambient noise, and mechanical strength of the fan. Variations and uncertainties in system requirements are critical to fan type and fan rotational speed selection. Fans which generate high air flow at relatively low speeds (for example, forward-curved blade centrifugal fans) need a relatively accurate estimate of the system air flow and pressure demand.
If, for some reason, system requirements are uncertain, then an improper guess at fan rotational speed can cause under-performance or excessive air flow and pressure. Air stream temperature has an important impact on fan-speed limits because of the effect of heat on the mechanical strength of the majority of materials. At high temperatures, all materials show lower yield strengths. Since the forces on shafts, blades, and bearings are proportional to the square of the rotational speed, high-temperature applications are frequently served by fans which operate at relatively low speeds.
Air stream characteristics – Moisture and particulate content are important considerations in the selection of the fan type. Contaminant build-up on fan blades can cause severe performance degradation and fan imbalance. Build-up problems are promoted by a shallow blade angle with surfaces which allow contaminants to collect. Fans with blade shapes which promote low-velocity air across the blades, such as backward inclined fans, are susceptible to contaminant build-up. In contrast, radial tip fans and radial blade fans operate so that air flow across the blade surfaces minimizes contaminant build-up. These fans are used in ‘dirty’ air streams and in material handling applications.
Corrosive air streams present a different set of problems. The fan material, as well as the fan type, is to be selected to withstand corrosive attack. Also, leakage into ambient spaces can be a concern, needing the fan to be equipped with a shaft seal. Shaft seals prevent or limit leakage from around the region where the drive shaft penetrates the fan housing. For example, in corrosive environments fans can be constructed with expensive alloys which are strong and corrosion resistant, or they can be less expensively constructed with fiberglass reinforced plastic or coated with a corrosion resistant material.
Since coatings are frequently less expensive than superalloy metals, fan types which work well with coatings (for example, radial fan blades because of their simple shape) are widely used in corrosive applications, however, wear reduces the reliability of coatings. Alternately, materials such as reinforced fiberglass plastics have been developed for fan applications and function effectively in several corrosive environments. However, there can be size and speed limitations for composite materials and plastic materials.
Air streams with high particulate content levels can also be problematic for the fan drive train. In direct drive axial fans, the motor is exposed to the air stream. Sealed motors can be used in these applications but tend to be more expensive and, in the event of lost seal integrity, they are susceptible to expensive damage. In axial fans, belt drives offer an advantage by removing the motor from the air stream. In centrifugal fans, the particulate content is less of a factor since the motor or sheave can be located outside of the fan enclosure and connected to the impeller through a shaft seal. Gear drives are occasionally used in applications where speed reduction is needed but the use of belt drives is unfeasible because of access or maintenance requirements.
In flammable environments, fans are normally constructed of non-ferrous alloys to minimize the risk of sparks caused by metal-to-metal contact. In some applications, certain components of the fan can be fabricated out of spark-resistant materials. Fans which operate in flammable environments are to be properly grounded, including rotating components, to minimize sparking because of static discharge.
Temperature range – To a large degree, temperature range determines fan type and material selection. In high-temperature environments, several materials lose mechanical strength. The stresses on rotating components increase as the fan’s operating speed increases. As a result, for high-temperature applications, the fan type which needs the lowest operating speed for a particular service is frequently recommended.
Radial blade fans can be ruggedly constructed and are frequently used in high-temperature environments. Component materials also considerably influence a fan’s ability to serve in high-temperature applications, and different alloys can be selected to provide the necessary mechanical properties at high temperatures.
Variations in operating conditions – Applications which have wide fluctuating operating requirements are not to be served by those fans which have unstable operating regions near any of the expected operating conditions. Since axial, backward inclined air foil, and forward-curved fans tend to have unstable regions, these fans are not recommended for this type of service unless there is a means of avoiding operation in the unstable region, such as a recirculation line, a bleed feature, or some type of anti-stall device.
Space constraints – Space and structural constraints can have a considerable impact on fan selection. In addition to dimensional constraints on the space available for the fan itself, issues such as maintenance access, foundation and structural support requirements, and ductwork are to be considered. Maintenance access addresses the need to inspect, repair, or replace fan components. Since downtime is frequently costly, quick access to a fan can provide cost savings.
Foundation and structural requirements depend on the size and weight of a fan. Selecting a compact fan can free up valuable floor space. Fan weight, speed, and size normally determine the foundation requirements, which, in turn, affect the installation cost. If the available space needs a fan to be located in a difficult configuration (for example, with an elbow just up-stream or down-stream of a fan), then some version of a flow straightener is to be considered to improve the operating efficiency. Since non-uniform air flow can increase the pressure drop across a duct fitting and degrades fan performance, straightening the air flow lowers the operating costs.
An important trade off regarding space and fan systems is that the cost of floor space frequently motivates designers and architects to configure a fan system within a tight space envelope. One way to accomplish this is to use small-radius elbows, small ducts, and very compact fan assemblies. Although this design practice can free up floor space, the effect on fan system performance can be severe in terms of maintenance costs. The use of multiple elbows close to a fan inlet or outlet can create a costly system effect, and the added pressure drops caused by small duct size or a cramped duct configuration can considerably increase fan operating costs. System designers are required to include fan system operating costs as a consideration in configuring fan assemblies and ductwork.
Fan performance curves – Fan performance is typically defined by a plot of developed pressure and power needed over a range of fan-generated air flow. Understanding this relationship is necessary for designing, sourcing, and operating a fan system and is the key to optimum fan selection.
Best efficiency point – Fan efficiency is the ratio of the power imparted to the air stream to the power delivered by the motor. The power of the air flow is the product of the pressure and the flow, corrected for units’ consistency. The equation for total efficiency is an important aspect of a fan performance curve and it is the best efficiency point (BEP), where a fan operates most cost-effectively in terms of both energy efficiency and maintenance considerations. Operating a fan near its BEP improves its performance and reduces wear, allowing longer intervals between repairs. Moving a fan’s operating point away from its BEP increases bearing loads and noise.
Another term for efficiency which is frequently being used with fans is static efficiency. Static efficiency uses static pressure instead of total pressure in the above equation. When evaluating fan performance, it is important to know which efficiency term is being used.
Region of instability – Normally fan curves arc downward from the zero-flow condition, i.e., as the back-pressure on the fan decreases, the airflow increases. Majority of fans have an operating region in which their fan performance curve slopes in the same direction as the system resistance curve. A fan operating in this region can have unstable operation. Instability results from the fan’s interaction with the system as the fan attempts to generate more air flow, which causes the system pressure to increase, reducing the generated air flow. As the air flow decreases, the system pressure also decreases, and the fan responds by generating more air flow. This cyclic behaviour results in a searching action which creates a sound similar to breathing. This operating instability promotes poor fan efficiency and increases wear on the fan components.
Fan start-up – Start-up refers to two different issues in the fan industry. Initial fan start-up is the commissioning of the fan, the process of ensuring proper installation. This event is important for several reasons. Poor fan installation can cause early failure, which can be costly both in terms of the fan itself and in production losses. Like other rotating machinery, proper fan operation normally needs correct drive alignment, adequate foundation characteristics, and true fit-up to connecting ductwork.
Fan start-up is also the acceleration of a fan from rest to normal operating speed. Several fans, particularly centrifugal types, have a large rotational inertia, meaning that they need considerable torque to reach operating speed.
In addition to the inertia load, the air mass moved by the fan also adds to the start-up torque requirements on the fan motor. Although rotational inertia is not typically a problem in heating, ventilation, and air conditioning (HVAC) applications, it can be a design consideration in large industrial applications. Proper motor selection is necessary in ensuring that the fan can be brought to its operating speed and that, once there, the motor operates efficiently. Because the start-up current for majority of the motors is 2 times to 5 times, the running current, the stress on the motor can be considerably reduced by starting a fan under its minimum mechanical load and allowing the motor to achieve normal operating speed more quickly than when under full load. In several applications, system dampers can be positioned to reduce the load on the fan motor during start-up. For example, the power needed by a centrifugal fan tends to increase with increasing flow (although in ‘non-overloading’ fan types, the power drops off after reaching a peak).
In axial fans, the power tends to decrease with increasing flow. As a result, for majority of centrifugal fan types, large fan start-ups are to be performed with down-stream dampers closed, while for majority of axial fan types, start-ups are to be performed with these dampers open. However, there are exceptions to these guidelines, and the actual power curve for the fan is to be evaluated to determine how to soften the impact of a large fan start-up. The power surges which accompany the starting of big size motors can create problems. Among the effects of a high start-up current are power quality issues and increased wear on the electrical system.
In response to increasing demand for equipment which minimizes the problems associated with big size motor starts, electrical equipment manufacturers are offering several different technologies, including special devices known as soft starters, to allow gradual motor speed acceleration. A key advantage of variable frequency drives (VFDs) is that they are frequently equipped with soft starting features which decrease motor starting current to around 1.5 times to 2 times the operating current. Although VFDs are primarily used to reduce operating costs, they can considerably reduce the impact of fan starts on an electrical system.
In axial fan applications, controllable pitch fans offer a similar advantage with respect to reducing start-up current. Shifting the blades to a low angle of attack reduces the required start-up torque of the fan, which allows the motor to reach operating speed more quickly.
The system effect is the change in system performance which results from the interaction of system components. Typically, during the design process, the system curve is calculated by adding the losses of each system component (dampers, ducts, baffles, filters, tees, wyes, elbows, grills, and louvers etc.). The result of the governing equation for pressure loss across any particular component is a parabolic line. This system curve assumes all components display pressure loss characteristics according to their loss coefficients. However, in reality, non-uniform air flow profiles which are created as the air stream develops swirls and vortices cause system components to show losses which are higher than their loss coefficients. The overall effect of these added losses is to move the system curve up. The system effect can be minimized by configuring the system so that the flow profile remains as uniform as possible. However, if space constraints prevent an ideal system layout, then system effect consequences are to be incorporated into the fan selection process.
The system effect can be particularly problematic when the air flow into or out of a fan is disrupted into a highly non-uniform pattern. Poor configuration of ductwork leading to or from a fan can severely interfere with a fan’s ability to efficiently impart energy to an air stream. For example, placing an elbow close to the fan outlet can create a system effect which decreases the delivered flow by up to 30 %. This can need an increase in fan speed, which in turn results in an increase in power and a decrease in system efficiency.
Although under-estimating the system effect causes insufficient air delivery, several designers over-compensate for it and other uncertainties by selecting over-sized fans. This practice creates problems such as high energy costs, high maintenance, and reduced system reliability. A more reasonable approach is to combine proper system layout practices with an accurate estimate of the system effect to determine an appropriate fan size.
Fan system components – A typical fan system consists of a fan, an electric motor, a drive system, ducts or piping, flow control devices, and air conditioning equipment (filters, cooling coils, heat exchangers, etc.). For effective improving the performance of fan systems, designers and operators are to understand how other system components function as well. The ‘systems approach’ needs knowing the interaction between fans, the equipment which supports fan operation, and the components which are served by fans.
Prime movers – Majority of the industrial fans are driven by alternating current (AC) electric motors. Majority of these motors are induction motors supplied with three-phase, 240V (volts) or 480V power. Since power supplies are typically rated at slightly higher voltages than motors because of anticipated voltage drops in the distribution system, motors are typically rated at 230V or 460V. In recent years, because of various reasons, the efficiency of general-purpose motors has considerably improved. For improving motor efficiency, motor manufacturers have modified motor designs and incorporated better materials, resulting in slight changes in motor operating characteristics. Although initial costs of the motors have increased by 10 % to 20 %, for high run-time applications, improvements in motor efficiency create very attractive pay-backs through lower operating costs.
A characteristic of induction motors is that their torque is directly related to slip, or the difference between the speed of the magnetic field and the speed of the motor shaft. As a result, in several fans, actual operating speeds are normally around 2 % less than their nominal speeds. Fans which are driven by older motors are probably operating at much lower efficiencies and at higher levels of slip than what is available from new motors. Upgrading to a new motor can reduce operating costs, because of improved motor efficiency, while offering slightly improved fan performance. High efficiency motors operate with less slip, which means fans rotate at slightly higher speeds. For applications which can effectively use this additional output, this high efficiency can be attractive. However, if the additional output is not useful, the added power consumption increases operating costs.
Another component of the prime mover is the motor controller. The controller is the switch mechanism which receives a signal from a low power circuit, such as an on / off switch, and energizes or de-energizes the motor by connecting or disconnecting the motor windings to the power line voltage. Soft starters are electrical devices which are frequently installed with a motor controller to reduce the electrical stresses associated with the start-up of large motors.
In conventional systems, the high in-rush and starting currents associated with majority of the AC motors creates power quality issues, such as voltage sag. Soft starters gradually ramp up the voltage applied to the motor, reducing the magnitude of the start-up current. As industrial facilities increase the use of computer-based equipment and control systems, soft starters are becoming important parts of several motor control systems. In fact, a major advantage associated with the majority of the VFDs is that they frequently have built-in, soft-start capabilities.
Another common characteristic of motors used in fan applications is multiple speed capability. Since ventilation and air-moving requirements frequently vary considerably, the ability to adjust fan speed is useful. Motors can be built to operate at different speeds in two principal ways namely (i) as a single set of windings equipped with a switch which energizes or de-energizes an additional set of poles, or (ii) with the use of multiple windings, each of which energizes a different number of poles. The first type of motor is known as a result pole motor and normally allows two operating speeds, one twice that of the other. The second type of motor can have two, three, or four speeds, depending on application. Normally multiple-speed motors are more costly and less efficient than single-speed motors. However, the flow control benefit of different motor speeds makes them attractive for several fan applications.
Drive system – The drive system frequently offers substantial opportunities to improve energy efficiency and to lower overall system operating costs. There are two principal types of drive systems (i) direct drive, and (ii) belt drive. Gear drives are also used but are less common. In direct drive systems, the fan is attached to the motor shaft. This is a simple, efficient system but has less flexibility with respect to speed adjustments. Since majority of fans are operated with induction motors, the operating rotational speeds of direct drive fans are limited to within a few percent of the synchronous motor speeds (normally 1,200 rpm, 1,800 rpm, and 3,600 rpm).
The sensitivity of fan output to its operating rotational speed means that errors in estimating the performance requirements can make a direct-drive system operate inefficiently (unlike belt drives, which allow fan rotational speed adjustments by altering pulley diameters). One way to add rotational speed flexibility to a direct-drive system is to use an adjustable speed drive (ASD). ASDs allow a range of shaft speeds and are quite practical for systems which have varying demand. Although ASDs are normally not a practical option for fans which are only required to operate at one speed, ASDs can provide a highly efficient system for fans which operate over a range of conditions.
In axial fans, direct drives have some important advantages. Applications with low temperatures and clean system air are well-suited for direct drives since the motor mounts directly behind the fan and can be cooled by the air stream. This space-saving configuration allows the motor to operate at higher-than-rated loads because of added cooling. However, accessibility to the motor is somewhat restricted.
Belt drives offer a key advantage to fan systems by providing flexibility in fan speed selection. If the initial estimates are incorrect or if the system requirements change, belt drives allow flexibility in changing fan speed. In axial fans, belt drives keep the motor out of the air stream, which can be an advantage in high temperature applications, or in dirty or corrosive environments. There are several different types of belt drives, including standard belts, V-belts, cogged V-belts, and synchronous belts. There are different cost and operating advantages to each type. Normally synchronous belts are the most efficient, while V-belts are the most commonly used. Synchronous belts are highly efficient since they use a mesh type contact which limits slippage and can lower operating costs.
However, switching to synchronous belts is to be done with caution. Synchronous belts normally generate much more noise than other belts. They also transfer shock loads through the drive train without allowing slip. These sudden load changes can be problematic for both the motors and the fans.
Another issue with synchronous belts is the limited availability of pulley sizes. Since the pulleys have a mesh pattern, machining them alters the pitch diameter, which interferes with engagement. As a result, pulleys are available in discrete sizes, which precludes an important advantage of belt drives which is the ability to alter operating rotational speeds by adjusting sheave diameters. Because of these factors, synchronous belts are not as widely used as V-belts in fan applications. In contrast, V-belts are widely used because of their efficiency, flexibility, and robust operation.
V-belts have a long history in industrial applications, which means there is a lot of industry knowledge about them. An important advantage of the V-belts is their protection of the drive train during sudden load changes. Service conditions which experience sudden drive train accelerations cause accelerated wear or sudden failure. While synchronous belts tend to transfer these shock loads directly to the shafts and motors, V-belts can slip, affording some protection. Although they are less efficient than synchronous belts, V-belts offer several advantages such as low cost, reliable operation, and operating flexibility. In applications which use standard belts, upgrades to V-belts are to be considered.
Although they are not normally used, gear systems offer some advantages to belt systems. Gear systems tend to be much more expensive than belt drive alternatives. However, gears tend to require less frequent inspection and maintenance than belts and are preferable in applications with severely limited access. Gears also offer several motor / fan configurations, including in-line drives, parallel offset drives, and 90-degree drives, each of which can provide an attractive advantage in some applications. Gear-system efficiency depends largely on speed ratio. Normally gear efficiencies range from 70 % to 98 %.
In high power applications (higher than 75 kilowatts), gear systems tend to be designed for higher efficiency because of the costs, heat, and noise problems which result from efficiency losses. Since gears need lubrication, gear box lubricant is to be periodically inspected and changed. Also, since gears, like synchronous belts, do not allow slip, shock loads are transferred directly across the drive train.
Ductwork or piping – For the majority of the fan systems, air is directed through ducts or pipes. Normally ducts are made of sheet metal and used in low-pressure systems, while pipes are sturdier and used in higher-pressure applications. Same principles apply to both ducts and pipes. In ventilation applications in which a fan pulls directly from a ventilated space on one side and discharges directly to an external space (like a wall-mounted propeller fan), duct / pipe losses are not a considerable factor. However, in majority of applications, ducts / pipes are used on one or both sides of a fan and have a critical impact on fan performance.
Friction between the air stream and the duct / pipe surface is normally a considerable portion of the overall load on a fan. As a rule, larger ducts / pipes create lower air flow resistance than smaller ducts. Although bigger ducts / pipes have higher initial costs in terms of material and installation, the reduced cost of energy because of lower friction offsets some of these costs and are to be included during the initial design process and during system modification efforts.
Other considerations with ducts are their shape and leakage class. Round ducts / pipes have less surface area per unit cross section area than rectangular ducts and, as a result, have less leakage. In hot or cool air streams, this surface area also influences the quantity of heat transferred to the environment. Duct leakage class, typically identified by a factor is an indicator of duct integrity. Variables which determine duct leakage include the type of joints used in construction, the number of joints per unit length of duct, and the shape of the duct. Depending on the length of the duct system, leakage can account for a significant portion of a fan’s capacity. This is especially applicable to systems with rectangular ducts which have unsealed joints. In several cases, the system designer can improve the performance of the ventilation system by specifying ducts which have low leakage factor.
Flow control devices – They include inlet dampers on the box, inlet vanes at the inlet to the fan, and outlet dampers at the outlet of the fan. Inlet box dampers are normally parallel blade dampers. Inlet vanes adjust fan output in two principal ways (i) by creating a swirl in the airflow which affects the way in which the air hits the fan blades, or (ii) by throttling the air altogether, which restricts the quantity of air entering the fan. The inlet vanes and dampers are to be designed for proper fan rotation and are to be installed in such a way that these inlet vanes and dampers open in the same direction as the fan rotation. The pre-rotation or swirl of the air helps reduce the brake power of the fan. If the inlet dampers on the inlet box are located too far away from the inlet of the fan, the effect of pre-rotation can be lost or reduced, and the power savings can be negligible.
The outlet damper, when used for controlling air flow, is normally of opposed-blade design for better flow distribution on the discharge side of the fan. If the outlet damper is going to be used for open / close service or for isolating the fan, a parallel blade discharge damper can be used. Typically, fans with inlet vanes provide better power savings while operating the fan at part load conditions, as opposed to fans with inlet box dampers operating in a similar situation. Inlet vanes provide better controllability with optimum power savings compared to other dampers.
Outlet dampers adjust resistance to airflow and move the operating point along the fan’s performance curve. Since they do not change air entry conditions, outlet dampers do not offer energy savings other than shifting the operating point along the fan power curve. Dampers can be used to throttle the air entering or leaving a fan and to control air flow in branches of a system or at points of delivery. Dampers control air flow by changing the quantity of restriction in an air stream. Increasing the restriction creates a larger pressure drop across the damper and dissipates some flow energy, while decreasing the restriction reduces the pressure differential and allows more air flow.
From a system perspective, proper use of dampers can improve energy efficiency over traditional system designs, especially in HVAC systems. In variable-air volume (VAV) systems, dampers are effective at rerouting air flow and at controlling the quantity of air delivered to a particular work space. Since VAV systems are much more energy efficient than their precursors (constant-volume or dual-supply systems), dampers can be used to lower system operating costs. However, in several applications, dampers can decrease fan efficiency. Dampers decrease total fan output by increasing back pressure, which forces the operating point of a fan to shift to the left along its performance curve. Frequently, as the fan operating point moves to the left along its curve, it operates less efficiently and, in some cases, can perform in an unstable manner.
Unstable fan operation is the result of an aerodynamic phenomenon in which there is insufficient air moving across the fan blades. The air flow rate surges back and forth resulting in inefficient performance, annoying noise characteristics, and accelerated wear on the fan drive system. Another air flow control method which is available for axial fan applications is the use of variable pitch blades. Variable pitch blade fans control the fan output by adjusting the fan blade angle of attack with respect to the incoming air stream. This allows the fan to increase or decrease its load in response to the system demand. In effect, this method is similar to that provided by inlet vanes, which adjust the angle of attack of the entering air stream by creating a swirl in the air flow pattern. Variable pitch fans provide a highly efficient means of matching fan output to system demand.
Another method of air flow control is fan speed adjustment. As per the fan laws, speed has a linear relationship with air flow, a second-order relationship with pressure, and a third-order relationship with power. By slowing or speeding up a fan, its output can be adjusted to match system demand. In general, fan speed adjustment is the most efficient method of air flow control. There are two primary speed control options namely (i) multiple-speed motors, and (ii) ASDs.
Multiple-speed motors have discrete speeds, such as ‘high’, ‘medium’, and ‘low. Although these motors tend to be somewhat less efficient than single speed motors, they offer simplicity, operating flexibility, a relatively compact space envelope, and considerable energy savings for fan systems with highly variable loads. ASDs include several different types of mechanical and electrical equipment. The most common type of ASD is a VFD. VFDs control the frequency of the power supplied to a motor to establish its operating speed. Unlike multiple speed motors which operate at discrete speeds, VFDs allow motors to operate over a continuous range of speed. This flexibility provides accurate matching between fan output and the flow and pressure requirements of the system.
Other equipment normally found in air-moving systems includes devices used to condition the air stream to achieve certain properties. Heat exchangers are used to heat or cool an air stream to achieve a particular temperature or to remove moisture. Filters are used to remove unwanted particles or gases. Conditioning equipment influences fan performance by providing flow resistance and, in some cases, by changing the air density.
Filters, including cyclone types or mesh types, inherently create pressure drops, which are frequently significant components of the overall system pressure drop. Mesh-type filters create increasingly higher pressure drops as they accumulate particles. In several systems, poor performance is a direct result of inadequate attention to filter cleanliness. Cyclone filters remove particulates by rapidly altering the direction of the air flow so that heavy particulates, unable to change direction quickly, get trapped. Although cyclone filters are less effective than mesh filters, they tend to need less maintenance and have more stable pressure-drop characteristics.
The effects of heating and cooling coils on fan system performance depend largely on where in the system the heat exchangers are located, the extent of the temperature change, and how the heat exchangers are constructed. Where there are big changes in the air stream temperature, fan performance can change as the air density changes. Heat exchangers which have closely spaced fins can accumulate particulates and moisture which not only impact heat transfer properties, but also increase pressure losses.
Assessment of fan system needs
There are three principal opportunities in the life cycle of a system which can be used to improve fan system performance. These are (i) during initial system design and fan selection, (ii) during trouble shooting to solve a system problem, and (iii) during a system capacity modification. Fan selection starts with a basic knowledge of system operating conditions consisting of air properties (moisture content, temperature, density, and contaminant level etc.), air flow rate, pressure, and system layout. These conditions determine which type of fan (centrifugal or axial) is needed to meet service needs.
Axial fans move air along the direction of the fan’s rotating axis, much like a propeller. Axial fans tend to be light and compact. Centrifugal fans accelerate air radially, changing the direction of the air flow. They are sturdy, quiet, reliable, and capable of operating over a wide range of conditions. Several factors are used to determine whether axial or centrifugal fans are more appropriate for certain applications. After deciding which fan type is appropriate, the right size is to be determined.
Fans are normally selected on a ‘best-fit’ basis rather than designed specifically for a particular application. A fan is chosen from a wide range of models based on its ability to meet the anticipated demands of a system. Fans have two mutually dependent outputs namely air flow and pressure. The variability of these outputs and other factors, such as efficiency, operating life, and maintenance, complicate the fan selection process.
A conservative design tendency is to source a fan / motor assembly which is large enough to accommodate uncertainties in system design, fouling effects, or future capacity increases. Designers also tend to oversize fans to protect against being responsible for inadequate system performance. However, purchasing an oversized fan / motor assembly creates operating problems such as excess air flow noise and inefficient fan operation. The incremental energy costs of operating oversized fans can be considerable. Some fan system problems, such as abnormally high operating and maintenance costs and ineffective air flow control, are sufficiently troublesome to justify a system assessment. If the system problems are considerable, then a change to the fan, its drive system, or the air flow control devices becomes justifiable.
Strangely high operating costs are frequently caused by inefficient fan operation which, in turn, can be the result of improper fan selection, poor system design, or wasteful air flow control practices. Improper fan selection frequently means the fan is oversized for the application, resulting in high energy costs, high air flow noise, and high maintenance requirements. Poor system design can lead to high operating and maintenance costs by promoting poor air flow conditions. As an example, duct configurations which create large system effect factors can cause considerable efficiency and airflow losses. An effective way of minimizing maintenance and operating costs is to keep a fan operating within a reasonable range of its BEP. However, this practice is frequently difficult in systems which have changing demands.
Poor air flow control refers to a wide range of causes and problems, including inadequate delivery to a system branch, surging operation, and high air flow noise. Inadequate delivery can be the result of poor system balancing or leakage. If a branch has a damper which is stuck open or a duct develops a large leak, then this branch can provide a low resistance flow path which robs air flow from other delivery points. Fans typically react to this loss of back pressure by generating high air flow rates. In severe cases, several centrifugal fan motors get overload if operated against little or no back pressure. If not corrected, an overloaded motor typically shuts itself down with thermal or current safety switches.
Several situations can cause surging. Fans in a parallel configuration can be shifting load between each other. A single fan can be operating in a stalled condition or hunting for the right operating point along an unstable part of its performance curve. In these cases, the system resistance is very high.
Frequent start-ups of high loads can add considerable stress to an electrical system. The in-rush current and the starting current for motors can create voltage sags in the electrical system and cause the motor to run hot for several minutes. In fan applications where sensitive loads can be affected by fan start-ups, the use of soft starters is to be considered. Soft starters are electrical devices which gradually ramp up the voltage to the fan motor, limiting the in-rush and starting current. Soft starters can extend fan motor life by keeping the motor temperature low. VFDs are also normally used to soft start fans. By gradually bringing fan speed up to operating conditions, VFDs reduce stress on the electrical system.
For a system which is to be modified or upgraded, an assessment of the available fan capacity is required to be performed. Unless the existing fan is considerably oversized, added capacity needs the installation of a larger fan or an additional fan. On the contrary, a system with excess fan capacity can frequently be accommodated by operating the fan at a slower speed. In these applications, the effects of operating a motor at less than half of its rated load is to be considered. Motor efficiency and power factor fall considerably when the motor is operated below half its rating.
One option to accommodate the increased demand is to operate the fan at a higher speed. In belt driven applications, the sheave diameters can be changed to increase fan speed. The relationship between fan speed and the air flow rate is linear. However, the relationship between fan speed and power consumption is cubed. As a result, increasing the air flow rate of the fan by increasing its speed needs considerably more power and can need a bigger motor. The structural integrity of the rotating elements, bearings, shafts, and support structure is required to be evaluated for the higher speeds.
If the fan is oversized for normal operating conditions, the feasibility of operating it at lower rotational speeds need to be considered. Reducing fan speed can considerably reduce energy consumption. For example, according to the fan laws, reducing fan rotational speed by 20 % decreases fan power by 50 %. Unfortunately, this speed reduction can cause motor efficiency and power factor to drop to low levels. The costs of inefficient operation and low power factor can justify motor replacement or the installation of a VFD.
Air flow rate can also be increased by installing a separate fan next to an existing one. Multiple-fan configurations have several advantages, including flexibility in meeting widely varying system demands and redundancy in case of equipment failure. When adding a fan to an existing system, the system can be configured so that both fans operate together or either fan operates independently. The operation of two fans together creates a combined performance curve which can be more appropriate for the system requirements than that of a single fan.
Replacing an existing fan with a different model is also an option. Selecting a new, larger fan needs consideration of the same factors which are involved in any initial fan selection. A new fan can be more feasible if the existing one has degraded or needs extensive refurbishment. In high run-time applications, the installation of a new fan with an energy-efficient motor can provide an attractive payback.
Normal maintenance for the fan systems includes (i) periodic inspection of all system components, (ii) bearing lubrication and replacement, (iii) belt tightening and replacement, (iv) motor repair or replacement, and (v) fan cleaning. The costliest result of improper maintenance is unscheduled downtime. Causes of this downtime vary according to the demands of the application. Since each system places particular demands on its air-moving equipment, maintenance needs vary widely.
For minimizing the quantity of unscheduled downtime, basic system maintenance is to be performed at reasonable intervals, the length of which is to be determined by either hours of operation or calendar periods. The maintenance interval is to be based on the recommendations of the manufacturer and experience with fans in similar applications. Factors which weigh into this schedule include the cost of the downtime, the cost and risk of the catastrophic failure, and the availability of back-up equipment. In systems which do not have abnormally severe operating demands, a typical maintenance schedule include the items such as belts, bearings, system cleaning, leaks, and motor condition.
In belt-driven fans, belts are normally the highest maintenance-intensive part of the fan assembly. As belts wear, they tend to lose tension, reducing their power transmission efficiency. Even new, properly adjusted belts suffer losses of 5 % to 10 %. As belt conditions degrade, these losses increase. Since noise is one of the ways in which the energy loss of belts is established, poor belt condition can add considerably to the ambient noise level. Belt inspection is particularly important to the operation of large fans because of the size of the power losses.
Although belt inspection and tightening are normally a routine task for any maintenance person, increased awareness of the costs associated with poorly adjusted belts can improve the attention devoted to this maintenance effort. In multiple-belt arrangements, whenever one belt degrades to the point of requiring replacement, all the belts are to be replaced at the same time. As belts wear and age, they show different properties. As a result, replacing only one or two belts in a multiple-belt arrangement creates a risk of overloading one or more of the belts. Exposing all the belts to roughly the same operating time minimizes the risk of uneven loading. Establishing proper belt tightness is necessary for minimizing the energy losses associated with belt drives. However, care is to be taken to prevent overtightening the belts. This leads to high radial bearing loads, accelerated wear, and shorter bearing replacement intervals.
In several fans, performance decline is mainly because of contaminant build-up on fan blades and other system surfaces. Contaminant build-up is frequently not uniform, resulting in imbalance problems which can result in performance problems and drive train wear. Since fans are frequently used in ventilation applications to remove airborne contaminants, this problem can be particularly acute. Fans which operate in particulate-laden or high-moisture air streams are to be cleaned regularly. Certain fan types, such as backward-inclined air foil, are highly susceptible to build-up of particulates or moisture. These build-ups disturb the airflow over the blades, resulting in decreased fan efficiency and higher operating costs. In high-particulate or moisture-content applications, radial-blade, radial-tip, and forward-curved blade type fans are normally used because of their resistance to contaminant build-up. If, for some other reason, a different type of fan is used in a high-particulate or high-moisture service, then fan inspection and cleaning are to be performed more frequently than normal.
System leaks degrade system performance and increase operating costs. Leaks tend to develop in flexible connections and in areas of a system which experience high vibration levels. Leakage decreases the quantity of air delivered to the point of service. As a result, one of the first steps in trouble shooting a system which has experienced declining performance is to check the integrity of the ductwork. Sources of leaks can be identified visually by inspecting for poorly fitting joints, and tears or cracks in ductwork and flexible joints. In systems with inaccessible ductwork, the use of temporary pressurization equipment can determine if the integrity of the system is adequate.
Worn bearings can create unsatisfactory noise levels and risk seizure. Bearings are to be monitored frequently. Bearing lubrication is to be performed in accordance with the instructions of the manufacturer such as (i) for high-speed fans in severe environments, lubrication intervals can be necessary weekly or more frequently, (ii) for oil-lubricated bearings, the oil quality is to be checked and, if necessary, the oil is replaced, (iii) for grease-lubricated bearings, the grease quality is to be checked and, if necessary, the bearings are repacked. One is to be careful not to over-grease bearings as this interferes with ball or roller motion and can cause overheating. The bearings are to be adequately protected from contamination. In axial fans, anti-friction bearings (ball, roller-type) are predominantly used because of the need for a robust thrust bearing to handle the axial thrust load.
Even properly maintained motors have a finite life. Over time, winding insulation inevitably breaks down. Motors in which the winding temperatures exceed rated values for long periods tend to suffer accelerated insulation breakdown. When faced with the decision to repair or replace a motor, several factors are to be considered, including motor size, motor type, operating hours, and cost of electric power. Normally in applications needing less than 40 watts, replacement motors meeting the efficiency requirements are to be selected, while larger motors are to be rebuilt. Of course, each facility is to establish its own repair / replacement strategy.
For motor rewinds, it is to be ensured that the repair facility has a proper quality assurance programme, since poor quality motor rewinds can compromise motor efficiency. Although motor rewinds are frequently cost-effective, motors which have undergone rewinding previously can suffer additional efficiency losses during subsequent rewinds.
For motor replacements, high-efficiency motors are to be considered. High-efficiency motors are normally 3 % to 8 % more efficient than standard motors. In high-use applications, this efficiency advantage frequently provides an attractive payback period. One is to avoid the problem of installing a motor which, because of its higher operating speed, causes the fan to generate more air flow and consume more energy than the previous motor / fan combination.
Under majority conditions, fan blades are supposed to last the life of the impeller. However, in harsh operating environments, erosion and corrosion can reduce fan-blade thickness, weakening the blades and creating an impeller imbalance. In these cases, either the impeller is to be replaced or an entirely new fan is to be installed.
In several applications, fan maintenance is reactive rather than proactive. For example, bearing lubrication is performed in response to audible bearing noise. Fan cleaning is performed to correct an indication of poor fan performance or vibration because of the dust build-up. Unfortunately, several fan system problems remain unaddressed until they become a nuisance, by which time they can have resulted in considerably higher operating costs. Vibration analysis equipment is essentially a refined extension of the human ear. By ‘listening’ to the vibrations of a motor or similar piece of machinery, the instrumentation can detect the early symptoms of a bearing problem, motor winding problem, or dynamic imbalance. By identifying problems before they become worse, repairs can be effectively scheduled, reducing the risk of catastrophic failure. Fortunately, recent improvements in instrumentation and signal analysis software have increased the availability of vibration monitoring and testing equipment. These devices can be permanently installed with a fan and incorporated into an alarm or safety shutdown system. Vibration monitors offer relatively inexpensive insurance for avoiding costly failures and can improve the effectiveness with which fan maintenance is planned.
Portable vibration instruments can also be used as part of a facility’s preventive maintenance system. Vibrations measured during operation can be compared against a base line set of data, normally taken when the machinery was first commissioned. Vibration signatures taken at different points in a fan’s operating life can be evaluated to determine whether a problem is developing and, if so, how fast.
A written log or record documenting observations and inspection results is a useful supplement to a maintenance schedule. Frequently a machinery problem develops over time. A history of the repairs, adjustments, or operator observations regarding the conditions under which the problem becomes noticeable improves the ability to effectively schedule a repair.
Like most other rotating machinery, fans experience wear and need periodic maintenance and repairs. Dynamic surfaces in bearings and belt drives degrade over time. Fan blade surfaces can erode from abrasive particles in the air stream, and motors eventually need replacement or rewinding. Although some degree of wear is unavoidable, operating the system at high efficiency levels reduces the risk of sudden equipment failure and can lower the cost and frequency of maintenance.
Fan system problems can be grouped into two principal categories namely (i) problems which are related to the fan / motor assembly, and (ii) problems associated with the system. A systems approach is important to help understand the total costs and performance impacts of these problems. Problems with the fan / motor assemblies can result from improper component selection, poor installation, or poor maintenance.
Belt drives are frequently the most maintenance-intensive component of a fan / motor assembly. Common problems include belt wear, noise, and rupture. Belt wear can lead to efficiency and performance problems. As belt slippage increases, it can translate directly into lower fan output. Insufficient belt tension can also cause high noise levels through belt slap or slippage. In some cases, belts develop one or more smooth spots which lead to vibrations during the fan operation. In contrast, belt tension which is too high increases the wear rate, increases load on the bearings, and can create an increased risk of unexpected down time.
In multiple-belt drive assemblies, uneven loading of the belts causes uneven wear, which can affect the life and reliability of the whole drive unit. Poor belt drive maintenance also promotes costly system operation. Contaminant build-up on the belts frequently results in increased slippage and noisy operation. The presence of abrasive particles tends to accelerate belt wear. Belts are not the only item in a belt drive assembly which develop problems. The sheaves themselves are subject to wear and are to be periodically inspected. Since sheave diameter has a considerable effect on fan speed, the relative wear between the driven and the driving sheave can affect fan performance.
As with the majority of the rotating machinery, the bearings in a fan / motor assembly wear and, over time, can create operating problems. To prevent such problems from causing unplanned downtime, bearings are to be a principal maintenance item. There are two primary bearing types in fan / motor combinations namely (i) radial, and (ii) thrust. In general, radial bearings tend to be less expensive than thrust bearings in terms of material cost and installation requirements. Because of the nature of the air flow, axial fans typically need heavier thrust bearings. These bearings tend to be comparatively expensive, making proper fan operation and effective maintenance important.
Common bearing problems include noise, excessive clearance, and, in severe cases seizure. Since operating conditions vary widely, the history of other fans in similar applications are to be used to schedule bearing replacement. Vibration analysis tools can improve confidence in determining bearing condition and planning bearing work. In oil-lubricated bearings, oil analysis methods can help evaluate bearing condition.