Practice Problems
Operating Principles
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Instructions:
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Work these problems on a sheet of paper and check your answers against those provided below.
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#1
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Determine the new fan speed if the baseline gas flow rate had increased from 1000 ACFM to 1200 ACFM and has a baseline fan rpm of 850. (Assume no changes in gas density.)
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Answer: 1,020 fan rpm
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Solution:
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Use the fan law:
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#2
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Explain the importance of gas flow rate and fan static pressure rise when selecting a fan for air pollution control systems.
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Answer: The gas flow rate must be sufficient to provide adequate pollutant capture at the hoods and to ensure proper transport of pollutant-laden air to the air pollution control systems (i.e minimum transport velocity is maintained). Treatment time requirements for the air pollution control devices and operating costs impose upper limits on gas flow rates.
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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 airflow velocities for each respective piece of equipment.
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#3
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At a fan speed of 700 rpm, the static pressure drop across the system at the fan inlet is -8 in. W.C. and the gas flow rate is 6,500 ACFM. Estimate the gas flow rate in ACFM if the fan speed changes and the new static pressure drop is -5.5 in. W.C. Assume that the rest of the system remains the same.
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Answer: 5,386 ACFM
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Solution:
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Estimate the fan speed, rpm2, for the new static pressure drop across the system. The fan laws can be used to solve this problem because the fan static pressure rise is essentially equivalent to the total system pressure drop.
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Estimate the gas flow rate, Q2, associated with the new static pressure drop across the system.
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#4
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What effect will the following conditions or situations have on a fan's baseline operating point?
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A hole in the ductwork.
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Insufficient fabric filter cleaning.
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Belts that connect the fan and motor sheaves start to slip.
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Answer: i. The operating point will shift down and to the right on the fan characteristic curve.
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The industrial ventilation system will draw air from the path of least resistance. The hole in the ductwork will have less resistance than the ductwork, bends, and hoods that are upstream of the hole. Thus a portion of the air will enter the hole and the amount of air entering the hood will be reduced. The decrease in the system static pressure (due to the fact that some air is bypassing the equipment upstream of the hole) will cause the system characteristic curve to shift down and to the right and increase the gas flow rate (Point C on Figure 1).
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Answer: ii. The operating point will shift up and to the left on the fan characteristic curve.
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Insufficient fabric filter cleaning will lead to increased buildup of particulate matter on the fabric filters. This condition will lead to a higher static pressure drop across the baghouse and increase the static pressure at the fan inlet. The airflow rate will decrease. The system characteristic curve will shift up and to the left (see Point B on Figure 1).
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Answer: iii. The operating point will shift down and to the left.
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When belts connecting the fan to the motor sheaves start to slip, all the power from the motor sheave does not get transferred to the fan sheave. Thus a loss in energy occurs. This situation causes the fan speed as measured in revolutions per minute to decrease. Thus the fan characteristic curve shifts down and to the left (Point E on Figure 2); while the system characteristic curve stays the same. The airflow rate and the static pressure are both lower at the new operating point.
Practice Problems
Performance
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Instructions:
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Work these problems on a sheet of paper and check your answers against those provided below.
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#1
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What are two methods of determining that the system characteristic curve has shifted?
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Answer:
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Measure the gas flow rate.
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Measure the fan motor current.
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The most direct, yet time consuming, method to check the fan performance is to measure the gas flow rate. The fan motor current data provides an indirect, but nevertheless very useful, indication of gas flow changes from the baseline conditions. An increase in the fan motor current is generally associated with an increase in the gas flow rate.
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#2
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What effect does a decrease in the gas flow temperature have on the fan motor requirements?
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Answer: A decrease in gas temperature increases the density and weight of the air, thereby increasing the fan motor current and the required brake horsepower (BHP).
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#3
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A centrifugal fan is operating with a motor current of 100 amps. The gas density entering the fan during normal operation is 0.065 lbm/ft3. Estimate the motor current at standard conditions when the gas density is approximately 0.075 lbm/ft3.
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Answer: 115 amps
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Solution:
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