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Flowcharts and
Ventilation Systems - Fans - Operating Principles
Fan Speed and Gas Flow RateFan Static Pressure Rise
Static Pressure Profile of the System
Fan Operating Point
Changes in Fan Operating Conditions
Practice Problems
Objectives
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Use the fan laws to explain the relationship between the following industrial ventilation parameters: fan speed, airflow rate, and fan static pressure rise.
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Name the two most important parameters to consider when selecting an appropriate fan for an air pollution control system.
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Explain how changes in the system resistance and fan operating conditions can affect the fan's baseline operating point and the gas flow rate.
- Explain how changes in the gas flow rate and fan static pressure rise affect pollutant capture and collection efficiency.
This lesson covers operating principles for centrifugal fans. A basic understanding of fan operating principles is necessary to evaluate the performance of an industrial ventilation system. The fan speed, expressed as revolutions per minute (rpm), is one of the most important operating variables. Most centrifugal fans, such as the radial-blade fan discussed in the previous section can operate over a modest range of speeds.
The flow rate of the air moving through the fan depends on the fan wheel rotational speed. As the speed increases, the airflow rate increases as indicated in the example data in Table 1.
It is important to recognize that a 10% decrease in fan speed results in a 10% decrease in the airflow rate through the ventilation system. This relationship is expressed in the first fan law.
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Where:
Note: The rate of airflow through a fan is always expressed in terms of ACFM.
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#1
- Assume the fan speed and operating conditions remain constant. What happens to the airflow moved by the fan (in ACFM) if the density of the gas changes?
The air stream moving through the fan experiences a static pressure rise due to the mechanical energy expended by the rotating fan wheel. As indicated in Figure 1, the static pressure at the outlet is always higher than the static pressure at the inlet. The general equation for calculating the static pressure rise across a fan is provided below.
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Where:
Note: This Module follows the notation used in industrial ventilation as defined in the Industrial Ventilation - A Manual of Recommended Practice. This notation may vary from that used in general engineering.
The fan 
SP is related to the square of the fan speed as indicated in the second fan law shown below. The fan static pressure rise is usually expressed in units of inches of water column.
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Where:
The static pressure rise across the fan increases rapidly as the fan speed is increased. This is illustrated in Table 2 using example data.
Note: The fan laws presented in this Module apply when all of the following conditions hold:
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The fans have the same design and geometric shape.
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The fans have not been altered in shape or form.
- The system characteristic curve has not changed.
The specific fan for an industrial ventilation system must be selected based on the specific airflow rate and fan static pressure rise needed to properly capture, transport, and control the emissions. As indicated in the block-type flowcharts introduced earlier in this Module, each industrial ventilation system includes one or more capture hoods, ductwork, air pollution control systems, the fan, and a stack. The gas flow rate through the ventilation system must be sufficient to provide adequate pollutant capture at the hoods and to ensure proper transport of the pollutant-laden air to the air pollution control systems.
The fan static pressure rise must be sufficient to accelerate the air entering the hoods and to overcome the flow resistances of the hoods, ductwork, air pollution control systems, and stack at the prescribed hood, ductwork, and air pollution control system airflow velocities.
Example Problem 1.
Estimating Changes in the Fan Static Pressure Rise as the Fan Speed Changes
A portion of a ventilation system is shown in Figure 2. At a fan speed of 900 rpm, the fan static pressure rise is 16.5 in. W.C. and the gas flow rate is 8,000 ACFM. Suppose the fan speed changes while the rest of the system remains the same. Estimate the new fan static pressure rise if the flow rate increases to 12,000 ACFM.
Solution:
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Calculate the new fan speed, rpm2, when the flow rate is increased from 8,000 to 12,000 ACFM.
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Calculate the new fan static pressure rise, Fan SP2, due to the higher fan speed.
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Note: This solution is based on the assumption that there are no significant changes in gas density due to the increase in gas flow rate.
Example Problem 1 illustrates that an increase in the gas flow rate of 50% more than doubled the fam static pressure rise across the system.
Static Pressure Profile of the System
The changes in the air stream static pressure from the point of entry into the hood to the fan are illustrated in Figure 3. This static pressure profile of the system is useful in illustrating the necessary fan static pressure rise.
The designer of a system, such as the one shown at the bottom of Figure 3, starts by specifying the air velocities in the hoods, ductwork, air pollution control system, and stack. These velocities are selected based on established engineering design principles to ensure high efficiency hood capture, proper operation of the air pollution control systems, and proper dispersion of the effluent gas stream from the stack.
The overall static pressure drop across each component of the overall system is related to the square of the airflow rates as illustrated in Figure 4.
This general relationship between total system static pressure drop and airflow rate is termed the system characteristic curve. For example, if the designer of the system needed 12,000 ACFM to achieve the necessary velocities in the system, he or she would know that the total static pressure drop across the system would be 10 in. W.C. Therefore, the fan would need to generate an airflow of 12,000 ACFM at a fan static pressure rise of at least 10 in. W.C.
The static pressure drop for a set of hoods, ductwork, and air pollution control devices can be calculated using the "velocity pressure" methods described in the Industrial Ventilation Manual. This calculation method takes into account the energy losses throughout the ventilation system. A partial list of these energy losses include the following:
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Turbulence at the hood inlet
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Acceleration of the gas from zero velocity to the velocity of the duct
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Frictional losses on the surfaces of the duct
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Turbulence losses at duct expansions and contractions
- Static pressure drop across the air pollution control device
The fan is used to increase the air static pressure from the low level exiting the last air pollution control system, to a static pressure close to, or even slightly above ambient absolute pressure levels. This is illustrated in Figure 5, which is simply the completed version of the static pressure profile chart shown in Figure 3.
An appropriate fan is selected based on the fan manufacturer's performance data. Usually, these data are provided in terms of multi-ratings tables published for each specific fan model and size. Based on these data it is possible to select a fan model, the specific model size, and the fan speed necessary to achieve the airflow rates and static pressure rise conditions necessary for the overall air pollution control system. An excerpt from a multi-rating table for a centrifugal fan is shown in Appendix D.
The match between the fan performance data and the system characteristic curve is illustrated in Figure 6 for the specific fan rotational speed chosen. As long as both the overall system and the fan remain in good condition, the system will operate at the point shown in Figure 6. This point is termed the operating point.
When the total system static pressure drop and the fan static pressure rise are shown on the same graph, as in the case with Figure 6, it is convenient to simply delete the total system static pressure drop axis.
Air pollution control systems and other types of industrial ventilation systems, however, do not necessarily remain exactly at the conditions anticipated by the system designer and the fan manufacturer. A number of normal operating changes and operating problems can cause changes in the overall system airflow rates and the static pressure rises across the fan. The extent of the changes in airflow rate and static pressure rise depends on the fan's performance conditions and the system characteristic curve. Some of these changes are illustrated in Figures 7 through 10.
Figure 7 illustrates an example fan characteristic curve for a given fan speed. The multi-rating data used to select the fan represented a subset of the total data set that defines this fan characteristic curve. There is a specific fan characteristic curve for each fan model, model size, and speed. The intersection of the fan characteristic curve and the system characteristic curve is illustrated as Point A. This point was determined previously by the system designer who selected the fan.
Changes in Fan Operating Conditions
If the gas flow resistance increases due to the build-up of dust in an air pollution control device or because a damper is closed, the system characteristic curve will shift upwards as indicated in Figure 8. With this increased gas flow resistance there will be a new operating point (Point B). At this new operating point, the fan static pressure rise will be slightly higher while the airflow rate will be slightly lower.
If the airflow resistance decreases due to changes in an air pollution control device or the opening of a damper, the system characteristic curve will shift downwards. This results in a new operating point (Point C) that has a slightly reduced fan static pressure and increased airflow rate.
Some changes in the system characteristic curve are normal due to factors such as (1) air pollution control system cleaning cycles, (2) gradually increasing air infiltration between maintenance cycles, and (3) the opening and closing of individual dampers on individual process sources ducted into the overall ventilation system. The system must be designed to provide adequate pollutant capture even at the lowest normally occurring airflow rates.
When changes in the system characteristic curve are outside of the anticipated range, operators often have the option of modifying the fan to increase its capability. Most fans on industrial systems are selected to operate at a speed near the middle of its safe operating range. Slight increases in the fan speed can improve airflow rates and static pressure rises without exceeding the safe operating speed limits. The impact of a slight increase in the fan speed is illustrated in Figure 9.
The increased fan speed results in a new fan curve and a new operating point (Point D) having an airflow rate and fan static pressure rise that are both larger than the conditions represented by the original operating point (Point A).
Not all fans can be easily adjusted to change the fan speed. For example, direct-drive fans where the fan wheel shaft is directly driven by the fan motor operate only at the motor rotation speed and cannot be adjusted. Belt-driven fans can be adjusted but only by changing one or both of the sheaves on the fan and motor. Some large fans with hydraulic or magnetic drives have easily adjustable fan speeds.
Some inadvertent reductions in fan speed are possible for belt-driven fans. If the drive belts become slightly loose, they can slip as they move across the sheaves. All the power from the motor sheave does not get transferred to the fan sheave. This often results in a decreased fan speed of 100 to 200 rpm. The decrease in the airflow rate is directly proportional to the decrease in the fan speed.
The operating point of a system can also be changed due to the opening and closing of a fan inlet damper. The fan inlet damper is a special damper, mounted immediately ahead of the fan, which changes how air enters the fan wheel. Changes caused by the opening and closing of a fan inlet damper are illustrated in Figure 10. The operating point changes to lower airflow rates and fan static pressure rises as the inlet damper is closed.
The fan inlet damper is often used to ensure safe opening of a fan that operates with air streams at elevated gas temperatures. During start-up when the air is cold, the fan inlet damper is kept partially closed to minimize the quantity of heavy cold air moving through the system. As the air heats and becomes less dense, the fan inlet damper opens to increase the airflow rate and fan static pressure rise. This approach minimizes the electrical power demand on the fan motor. Starting with the fan inlet dampers wide open would often exceed the safe current levels for the motor and thereby result in burnout of the motor windings. It is very important to avoid excessive fan motor currents.
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#2
- What effect could a decrease in gas flow rate have on pollutant capture at the hood?
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#3
- What effect could a decrease in system resistance have on an air pollution control system?
Practice Problems
Fans - Operating Principles
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Instructions:
- Complete the Practice Problems before proceeding to the next section. Click on the button below.