Maintenance Engineering -- Fans, Blowers, and Fluidizers

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Technically, fans and blowers are two separate types of devices that have a similar function. However, the terms are often used interchangeably to mean any device that delivers a quantity of air or gas at a desired pressure. Differences between these two devices are their rotating elements and their discharge-pressure capabilities.

Fluidizers are identical to single-stage, screw-type compressors or blowers.

CENTRIFUGAL FANS

The centrifugal fan is one of the most common machines used in industry. It utilizes a rotating element with blades, vanes, or propellers to extract or deliver a specific volume of air or gas. The rotating element is mounted on a rotating shaft that must provide the energy required to overcome inertia, friction, and other factors that restrict or resist air or gas flow in the application. They are generally low-pressure machines designed to overcome friction and either suction or discharge-system pressure.

configuration

The type of rotating element or wheel that is used to move the air or gas can classify the centrifugal fan. The major classifications are propeller and axial.

Axial fans also can be further differentiated by the blade configurations.

Propeller

This type of fan consists of a propeller, or paddle wheel, mounted on a rotating shaft within a ring, panel, or cage. The most widely used propeller fans are found in light- or medium-duty functions such as ventilation units in which air can be moved in any direction. These fans are commonly used in wall mountings to inject air into, or exhaust air from, a space. A belt-driven propeller fan appropriate for medium-duty applications.

This type of fan has a limited ability to boost pressure. Its use should be limited to applications in which the total resistance to flow is less than 1 inch of water. In addition, it should not be used in corrosive environments or where explosive gases are present.

Axial

Axial fans are essentially propeller fans that are enclosed within a cylindrical housing or shroud. They can be mounted inside ductwork or a vessel housing to inject or exhaust air or gas. These fans have an internal motor mounted on spokes or struts to centralize the unit within the housing. Electrical connections and grease fittings are mounted externally on the housing. Arrow indicators on the housing show the direction of air flow and rotation of the shaft, which enables the unit to be correctly installed in the ductwork. An inlet end of a direct-connected, tube-axial fan.

This type of fan should not be used in corrosive or explosive environments, because the motor and bearings cannot be protected. Applications in which concentrations of airborne abrasives are present should also be avoided.

---1 Belt-driven propeller fan for medium-duty applications.

---2 Inlet end of a direct-connected tube-axial fan.

Axial fans use three primary types of blades or vanes: backward-curved, for ward-curved, and radial. Each type has specific advantages and disadvantages.

Backward-Curved Blades

The backward-curved blade provides the highest efficiency and lowest sound level of all axial-type centrifugal fan blades. Advantages include the following:

_ Moderate to high volumes

_ Static pressure range up to approximately 30 inches of water (gauge)

_ Highest efficiency of any type of fan

_ Lowest noise level of any fan for the same pressure and volumetric requirements

_ Self-limiting brake horsepower (BHP) characteristics (Motors can be selected to prevent overload at any volume, and the BHP curve rises to a peak and then declines as volume increases) The limitations of backward-curved blades are as follows:

_ Weighs more and occupies considerably more space than other designs of equal volume and pressure

_ Large wheel width

_ Not to be used in dusty environments or where sticky or stringy materials are used, because residues adhering to the blade surface cause imbalance and eventual bearing failure

Forward-Curved Blades

This design is commonly referred to as a squirrel-cage fan. The unit has a wheel with a large number of wide, shallow blades; a very large intake area relative to the wheel diameter; and a relatively slow operational speed. The advantages of forward-curved blades include the following:

  • _ Excellent for any volume at low to moderate static pressure when using clean air
  • _ Occupies approximately the same space as backward-curved blade fan
  • _ More efficient and much quieter during operation than propeller fans for static pressures above approximately 1 inch of water (gauge) The limitations of forward-curved blades include the following:
  • _ Not as efficient as backward-curved blade fans
  • _ Should not be used in dusty environments or handle sticky or stringy materials that could adhere to the blade surface
  • _ BHP increases as this fan approaches maximum volume, as opposed to backward-curved blade centrifugal fans, which experience a decrease in BHP as they approach maximum volume

Radial Blades

Industrial exhaust fans fall into this category. The design is rugged and may be belt-driven or directly driven by a motor. The blade shape varies considerably from flat surfaces to various bent configurations to increase efficiency slightly or to suit particular applications. The advantages of radial blade fans include the following:

  • _ Best suited for severe duty, especially when fitted with flat radial blades
  • _ Simple construction that lends itself to easy field maintenance
  • _ Highly versatile industrial fan that can be used in extremely dusty environments as well as with clean air
  • _ Appropriate for high-temperature service
  • _ Handles corrosive or abrasive materials The limitations of radial-blade fans include the following:
  • _ Lowest efficiency in centrifugal-fan group
  • _ Highest sound level in centrifugal-fan group
  • _ BHP increases as fan approaches maximum volume

Performance

A fan is inherently a constant-volume machine. It operates at the same volumetric flow rate (i.e., cubic feet per minute) when operating in a fixed system at a constant speed, regardless of changes in air density. However, the pressure developed and the horsepower required vary directly with the air density.

The following factors affect centrifugal-fan performance: brake horsepower, fan capacity, fan rating, outlet velocity, static efficiency, static pressure, tip speed, mechanical efficiency, total pressure, velocity pressure, natural frequency, and suction conditions. Some of these factors are used in the mathematical relation ships that are referred to as Fan Laws.

Brake Horsepower

Brake horsepower (BHP) is the power input required by the fan shaft to produce the required volumetric flow rate (cfm) and pressure.

Fan Capacity

The fan capacity (FC) is the volume of air moved per minute by the fan (cfm).

Note: the density of air is 0.075 pounds per cubic foot at atmospheric pressure and 688F.

Fan Rating

The fan rating predicts the fan's performance at one operating condition, which includes the fan size, speed, capacity, pressure, and horsepower.

Outlet Velocity

The outlet velocity (OV, feet per minute) is the number of cubic feet of gas moved by the fan per minute divided by the inside area of the fan outlet, or discharge area, in square feet.

Static Efficiency

Static efficiency (SE) is not the true mechanical efficiency but is convenient to use in comparing fans. This is calculated by the following equation:

Static Efficiency (SE) = 0:000157 _ FC _ SP / BHP

Static Pressure

Static pressure (SP) generated by the fan can exist whether the air is in motion or is trapped in a con fined space. SP is always expressed in inches of water (gauge).

Tip Speed

The tip speed (TS) is the peripheral speed of the fan wheel in feet/minute (fpm).

Tip Speed = Rotor Diameter _ x _ RPM

Mechanical Efficiency

True mechanical efficiency (ME) is equal to the total input power divided by the total output power.

Total Pressure

Total pressure (TP), inches of water (gauge), is the sum of the velocity pressure and static pressure.

Velocity Pressure

Velocity pressure (VP) is produced by the fan when the air is moving. Air having a velocity of 4,000 fpm exerts a pressure of 1 inch of water (gauge) on a stationary object in its flow path.

Natural Frequency

General-purpose fans are designed to operate below their first natural frequency. In most cases, the fan vendor will design the rotor-support system so that the rotating element's first critical is between 10% and 15% above the rated running speed.

While this practice is questionable, it’s acceptable if the design speed and rotating element mass are maintained. However, if either of these two factors changes, there is a high probability that serious damage or premature failure will result.

---3 Fan-performance Curve #1.

---4 Fan-performance Curve #2.

Inlet-Air Conditions

As with centrifugal pumps, fans require stable inlet conditions. Ductwork should be configured to ensure an adequate volume of clean air or gas, stable inlet pressure, and laminar flow. If the supply air is extracted from the environment, it’s subject to variations in moisture, dirt content, barometric pressure, and density. However, these variables should be controlled as much as possible. As a minimum, inlet filters should be installed to minimize the amount of dirt and moisture that enters the fan.

Excessive moisture and particulates have an extremely negative impact on fan performance and cause two major problems: abrasion or tip wear and plate-out.

High concentrations of particulate matter in the inlet air act as abrasives that accelerate fan-rotor wear. In most cases, however, this wear is restricted to the high-velocity areas of the rotor, such as the vane or blade tips, but can affect the entire assembly.

Plate-out is a much more serious problem. The combination of particulates and moisture can form ''glue'' that binds to the rotor assembly. As this contamination builds up on the rotor, the assembly's mass increases, which reduces its natural frequency. If enough plate-out occurs, the fan's rotational speed may coincide with the rotor's reduced natural frequency. With a strong energy source like the running speed, the excitation of the rotor's natural frequency can result in catastrophic fan failure. Even if catastrophic failure does not occur, the vibration energy generated by the fan may cause bearing damage.

Fan Laws

The mathematical relationships referred to as Fan Laws can be useful when applied to fans operating in a fixed system or to geometrically similar fans.

However, caution should be exercised when using these relationships. They apply only to identical fans and applications. The basic laws are as follows:

  • _ Volume in cubic feet per minute (cfm) varies directly with the rotating speed (rpm)
  • _ Static pressure varies with the rotating speed squared (rpm^2)
  • _ BHP varies with the speed cubed (rpm3) The fan-performance curves show the performance of the same fan type, but designed for different volumetric- flow rates, operating in the same duct system handling air at the same density.

---3 Fan-performance Curve #1.

Curve #1 is for a fan designed to handle 10,000 cfm in a duct system whose calculated system resistance is determined to be 1 inch of water (gauge). This fan will operate at the point where the fan pressure (SP) curve intersects the system resistance curve (TSH). This intersection point is called the Point of Rating. The fan will operate at this point provided the fan's speed remains constant and the system's resistance does not change. The system-resistance curve illustrates that the resistance varies as the square of the volumetric flow rate (cfm). The BHP of the fan required for this application is 2Hp.

Curve #2 illustrates the situation if the fan's design capacity is increased by 20%, increasing output from 10,000 to 12,000 cfm. Applying the Fan Laws, the calculations are:

New rpm = 1:2 _ 440

= 528 rpm (20% increase) New SP = 1:2 _ 1:2 _ 1 inch water (gauge)

= 1:44 inches (44% increase) New TSH = New SP = 1:44 inches New BHP = 1:2 _ 1:2 _ 1:2 _ 2

= 1:73 _ 2

= 3:46Hp (73% increase)

The curve representing the system resistance is the same in both cases, since the system has not changed. The fan will operate at the same relative point of rating and will move the increased volume through the system. The mechanical and static efficiencies are unchanged.

The increased BHP required to drive the fan is a very important point to note. If a 2-Hp motor had driven the Curve #1 fan, the Curve #2 fan needs a 3.5-hp motor to meet its volumetric requirement.

Centrifugal fan selection is based on rating values such as air flow, rpm, air density, and cost. ----1 is a typical rating table for a centrifugal fan. Air-density ratios.

Installation

Proper fan installation is critical to reliable operation. Suitable foundations, adequate bearing-support structures, properly sized ductwork, and flow-control devices are the primary considerations.

Foundations

As with any other rotating machine, fans require a rigid, stable foundation. With the exception of inline fans, they must have a concrete footing or pad that is properly sized to provide a stable footprint and prevent flexing of the rotor support system.

Typical Rating Table for a Centrifugal Fan

----Bearing-Support Structures

-----Air-Density Ratios

In most cases, with the exception of inline configurations, fans are supplied with a vendor-fabricated base. Bases normally consist of fabricated metal stands that support the motor and fan housing. The problem with most of the fabricated bases is that they lack the rigidity and stiffness to prevent flexing or distortion of the fan's rotating element. The typical support structure is composed of relatively light gauge material (3/16 in.) and does not have the cross-bracing or structural stiffeners needed to prevent distortion of the rotor assembly. Because of this limitation, many plants fill the support structure with concrete or other solid material.

However, this approach does little to correct the problem. When the concrete solidifies, it pulls away from the sides of the support structure. Without direct bonding and full contact with the walls of the support structure, stiffness is not significantly improved.

The best solution to this problem is to add cross-braces and structural stiffeners.

If they are properly sized and affixed to the support structure, the stiffness can be improved and rotor distortion reduced.

--- Outlet damper with streamlined blades and linkage arranged to move adjacent blades in opposite directions for even throttling.

Ductwork

Ductwork should be sized to provide minimum friction loss throughout the system. Bends, junctions with other ductwork, and any change of direction should provide a clean, direct flow path. All ductwork should be airtight and leak-free to ensure proper operation.

Flow-Control Devices

Fans should always have inlet and outlet dampers or other flow-control devices such as variable-inlet vanes. Without them, it’s extremely difficult to match fan performance to actual application demand. The reason for this difficulty is that there are a number of variables (e.g., usage, humidity, and temperature) directly affecting the input-output demands for each fan application. Flow-control devices provide the means to adjust fan operation for actual conditions. An outlet damper with streamlined blades and linkage arranged to move adjacent blades in opposite directions for even throttling.

Air flow controllers must be inspected frequently to ensure that they are fully operable and operate in unison with each other. They also must close tightly.

Ensure that the control indicators show the precise position of the vanes in all operational conditions. The ''open'' and ''closed'' positions should be permanently marked and visible at all times. Periodic lubrication of linkages is required.

Turnbuckle screws on the linkages for adjusting flow rates should never be moved without first measuring the distance between the setpoint markers on each screw. This is important if the adjustments don’t produce the desired effect and you wish to return to the original settings.

Operating Methods

Because fans are designed for stable, steady-state operation, variations in speed or load may have an adverse effect on their operating dynamics. The primary operating method that should be understood is output control. Two methods can be used to control fan output: dampers and fan speed.

Dampers

Dampers can be used to control the output of centrifugal fans within the effective control limits. Centrifugal fans have a finite acceptable control range, typically about 15% below and 15% above their design points. Control variations outside of this range severely affect the reliability and useful life of these fans.

The recommended practice is to use an inlet damper rather than a discharge damper for this control function whenever possible. Restricting the inlet with suction dampers can effectively control the fan's output. When using dampers to control fan performance, however, caution should be exercised to ensure that any changes remain within the fan's effective control range.

Fan Speed

Varying fan speed is an effective means of controlling a fan's performance. As de fined by the Fan Laws, changing the rotating speed of the fan can directly control both volume and pressure. However, caution must be used when changing fan speed. All rotating elements, including fan rotors, have one or more critical speeds. When the fan's speed coincides with one of the critical speeds, the rotor assembly becomes extremely unstable and could fail catastrophically.

In most general-purpose applications, fans are designed to operate between 10% and 15% below their first critical speed. If speed is increased on these fans, there is a good potential for a critical-speed problem. Other applications have fans that are designed to operate between their first and second critical speeds. In this instance, any change up or down may cause the speed to coincide with one of the critical speeds.

BLOWERS

A blower uses mating helical lobes or screws and is used for the same purpose as a fan. Blowers are normally moderate- to high-pressure devices. Blowers are almost identical both physically and functionally to positive-displacement compressors.

FLUIDIZERS

Fluidizers are identical to single-stage, screw-type compressors or blowers. They are designed to provide moderate- to high-pressure transfer of non-abrasive, dry materials.

TROUBLESHOOTING FANS,BLOWERS, AND FLUIDIZERS

The common failure modes for fans, blowers, and fluidizers. Typical problems with these devices include (1) output below rating, (2) vibration and noise, and (3) overloaded driver bearings.

CENTRIFUGAL FANS

Centrifugal fans are extremely sensitive to variations in either suction or discharge conditions. In addition to variations in ambient conditions (i.e., temperature, humidity, etc.), control variables can have a direct effect on fan performance and reliability.

Most of the problems that limit fan performance and reliability are either directly or indirectly caused by improper application, installation, operation, or maintenance. However, the majority are caused by misapplication or poor operating practices. ----4 lists failure modes of centrifugal fans and their causes. Some of the more common failures are aerodynamic instability, plate out, speed changes, and lateral flexibility.

---- Common Failure Modes of Centrifugal Fans

----- Common Failure Modes of Blowers and Fluidizers

Aerodynamic Instability

Generally, the control range of centrifugal fans is about 15% above and 15% below their best efficiency point (BEP). When fans are operated outside of this range, they tend to become progressively unstable, which causes the fan's rotor assembly and shaft to deflect from their true centerline. This deflection increases the vibration energy of the fan and accelerates the wear rate of bearings and other drive-train components.

Plate-Out

Dirt, moisture, and other contaminants tend to adhere to the fan's rotating element. This buildup, called plate-out, increases the mass of the rotor assembly and decreases its critical speed, the point at which the phenomenon referred to as resonance occurs. This occurs because the additional mass affects the rotor's natural frequency. Even if the fan's speed does not change, the change in natural frequency may cause its critical speed (note that machines may have more than one) to coincide with the actual rotor speed. If this occurs, the fan will resonate, or experience severe vibration, and may catastrophically fail. The symptoms of plate-out are often confused with those of mechanical imbalance because both dramatically increase the vibration associated with the fan's running speed.

The problem of plate-out can be resolved by regularly cleaning the fan's rotating element and internal components. Removal of buildup lowers the rotor's mass and returns its natural frequency to the initial or design point. In extremely dirty or dusty environments, it may be advisable to install an automatic cleaning system that uses high-pressure air or water to periodically remove any buildup that occurs.

Speed Changes

In applications in which a measurable fan speed change can occur (i.e., V-belt or variable-speed drives), care must be taken to ensure that the selected speed does not coincide with any of the fan's critical speeds. For general-purpose fans, the actual running speed is designed to be between 10% and 15% below the first critical speed of the rotating element. If the sheave ratio of a V-belt drive or the actual running speed is increased above the design value, it may coincide with a critical speed.

Some fans are designed to operate between critical speeds. In these applications, the fan must transition through the first critical speed to reach its operating speed. These transitions must be made as quickly as possible to prevent damage.

If the fan's speed remains at or near the critical speed for any extended period of time, serious damage can occur.

Lateral Flexibility

By design, the structural support of most general-purpose fans lacks the mass and rigidity needed to prevent flexing of the fan's housing and rotating assembly.

This problem is more pronounced in the horizontal plane, but it also is present in the vertical direction. If support-structure flexing is found to be the root cause or a major contributing factor to the problem, it can be corrected by increasing the stiffness and/or mass of the structure. However, don’t fill the structure with concrete. As it dries, concrete pulls away from the structure and does little to improve its rigidity.

BLOWERS OR POSITIVE-DISPLACEMENT FANS

Blowers, or positive-displacement fans, have the same common failure modes as rotary pumps and compressors. ----4 lists the failure modes that most often affect blowers and fluidizers. In particular, blower failures occur because of process instability caused by start/stop operation and demand variations and because of mechanical failures caused by close tolerances.

Process Instability

Blowers are very sensitive to variations in their operating envelope. As little as a 1-psig change in downstream pressure can cause the blower to become extremely unstable. The probability of catastrophic failure or severe damage to blower components increases in direct proportion to the amount and speed of the variation in demand or downstream pressure.

Start/Stop Operation

The transients caused by frequent start/stop operation also have a negative effect on blower reliability. Conversely, blowers that operate constantly in a stable environment rarely exhibit problems. The major reason is the severe axial thrusting caused by the frequent variations in suction or discharge pressure caused by the start/stop operation.

Demand Variations

Variations in pressure and volume demands have a serious impact on blower reliability. Since blowers are positive-displacement devices, they generate a constant volume and a variable pressure that is dependent on the downstream system's back pressure. If demand decreases, the blower's discharge pressure continues to increase until (1) a downstream component fails and reduces the back pressure or (2) the brake horsepower required to drive the blower is greater than the motor's locked rotor rating. Either of these results in failure of the blower system. The former may result in a reportable release, while the latter will cause the motor to trip or burnout.

Frequent variations in demand greatly accelerate the wear rate of the thrust bearings in the blower. This can be directly attributed to the constant, instantaneous axial thrusting caused by variations in the discharge pressure required by the downstream system.

Mechanical Failures

Because of the extremely close clearances that must exist within the blower, the potential for serious mechanical damage or catastrophic failure is higher than with other rotating machinery. The primary failure points include thrust bearing, timing gears, and rotor assemblies.

In many cases, these mechanical failures are caused by the instability discussed in the preceding sections, but poor maintenance practices are another major cause.

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