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Module 5: Flowcharts and Ventilation Systems - Monitoring Performance
IntroductionHood Static Pressure
Hood Entry Loss
Gas Flow Rate Calculations Using Hood Static Pressure Data
Practice Problems
Objectives
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Explain the relationship between changes in hood static pressure and gas flow rate.
- Using duct dimensions and hood static pressure data, calculate the gas flow rate through an industrial ventilation and air pollution control system.
There are several effective ways to confirm that the hood capture effectiveness has not decreased since it was installed or tested. Visible emission observations for fugitive emissions should be conducted in the case of particulate sources. In general, you should confirm that the hood has not been moved away from the point of pollutant generation and that side baffles and other equipment necessary to maintain good operation have not been damaged or removed. Monitoring changes in the hood static pressure is a very good way to detect variations in airflow rate and hood capture effectiveness.
The material in this lesson builds on some of the basic concepts on gas pressure covered in Module 1 (see lesson on Pressure). Please refer to this lesson as necessary.
It is important to ensure that the gas flow rate is maintained at the proper level to maintain the necessary hood capture velocity. The hood static pressure, which is simply the static pressure in the duct immediately downstream from the hood, is entirely dependent on the hood geometry and the gas flow rate. As long as the hood has not been damaged or altered, the hood static pressure provides an indirect, but relatively accurate measurement of the gas flow rate. The hood static pressure should be monitored on a routine basis for this purpose.
Hood static pressures are relative pressures and are always negative (less than atmospheric pressure). Similar to pressure drops, hood static pressures are said to increase as they become more negative and to decrease as they become less negative (i.e. closer to zero).
As the gas flow rate into the hood increases, the hood static pressure increases (see Figure 1). In this Figure, a hood with an entry loss coefficient (Fd) of 0.49 has been assumed in making these calculations. Hood entry loss coefficients are determined by the shape of the hood and are discussed in the next section.
A decrease in hood static pressure, corrected for gas density changes, usually indicates that the gas flow rate entering the hood has decreased from previous levels. This may reduce the effectiveness of the hood by reducing the capture velocities at the hood entrance.
Relatively simple gauges such as water-filled manometers and Magnehelic® gauges can measure the hood static pressure. The normal range of hood static pressures is -0.2 to -2.0 in. W.C.
Equation 1 below is useful during the initial design of the ventilation system to determine the hood static pressure drop component of the overall system pressure drop. Fan selection depends on the overall system pressure drop and the required volumetric flow rate.
As indicated in Equation 1, the hood static pressure is negative and is determined by these terms: (1) the velocity pressure in the duct from the hood and (2) the hood entry loss. The velocity pressure term is due to the energy necessary to accelerate the air from zero velocity to the velocity in the duct from the hood. The hood entry loss represents the loss of pressure caused by airflow moving into the system.
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Where:
The overall hood entry loss, he, is comprised of the slot or opening loss and the duct entry loss.
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Where:
The slot or opening loss will not be discussed further in these modules. For more information on this topic see Industrial Ventilation - A Manual of Recommended Practice. For the remainder of these modules, you may equate the overall hood entry loss with the duct entry loss.
The duct entry loss is related to the velocity pressure as follows.
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Where:
Fd is tabulated in standard texts concerning hoods and ventilation systems.
Making the simplifications and substitutions mentioned above to Equation 1 yields the following equation.
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Where:
The following discussion on hood inlets will help you understand how hood designs affect the overall pressure drop and fan requirements of a system.
When air enters a duct under suction, the airflow converges as shown in Figure 2. 
The area of air convergence upon entering a duct is referred to as vena contracta. The area around the mouth inlet is void of airflow due to the formation of the vena contracta.
As air passes through the vena contracta its velocity increases. After passing through the vena contracta, the airflow expands to fill the duct. As the air expands, some of the velocity pressure converts to static pressure. The hood static pressure and entry loss are related to the size of the vena contracta.
The hood geometry determines the size of the vena contracta by influencing how smoothly the airflow will enter the duct [see Figure 3, 
Hood Entry Loss Coefficient (Fd) for Various Duct Designs]. Figure 3 shows three different duct designs and their corresponding hood entry loss coefficients. The duct inlet design illustrated in Figure 3(c) is superior to those in parts (a) and (b) because air enters the duct more smoothly (less vena contracta effect) and air is drawn into the duct primarily from the front where the contaminated air is located.
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#1
- Which of the duct designs in Figure 3 will have the highest (most negative) static pressure? On this basis alone, which will require the largest airflow rate?
Gas Flow Rate Calculations Using Hood Static Pressure Data
The velocity pressure is related to (1) the square of the velocity of the gas stream in the duct and (2) the gas density. The gas velocity can be calculated using the following Equation when the velocity pressure is measured using a standard-type pitot tube.
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Where:
Note: For characteristics of standard-type pitot tubes refer to Industrial Ventilation - A Manual of Recommended Practice. For information concerning S-type pitot tubes, refer to 40 CFR Part 60, Appendix A, Method 2. A variation of Equation 5 for use with S-type pitot tubes is provided in Method 2.
Substituting the density of air at U.S. EPA standard conditions (0.075 lbm/ft3) into the above Equation yields the following:
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Where:
Rearranging the terms in the above Equation to solve for gas velocity yields the following equation.
Example Problem 1.
Determine the Gas Flow Rate from Hood Static Pressure Data
A hood serving a paint dipping operation has a hood static pressure of -1.10 in. W.C. The baseline hood static pressure was -1.70 in. W.C. Use the data provided below to answer the questions.
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What is the estimated gas flow rate at present operating conditions?
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What is the estimated gas flow rate at baseline levels?
Solution:
Part i (Gas Flow Rate at Present Conditions)
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Calculate the velocity pressure (VP) using the following equation.
a.gif)
Calculate the value for the hood entry loss, he, as follows.
b.gif)
- Given: SPh = -1.10 in. W.C.
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Calculate the gas velocity using Equation 7 at standard conditions,
Actual = 0.075 lbm/ft3.
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Calculate the gas flow rate as follows:
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c.gif)
Solution:
Part ii (Gas Flow Rate at Baseline Conditions)
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Calculate the velocity pressure using the following.
a.gif)
- Given: SPh = -1.7 in. W.C.
b.gif)
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Calculate the gas velocity using Equation 7 at standard conditions, Actual = 0.075 lbm/ft3.
.gif)
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Calculate the gas flow rate. The duct area was calculated in Part i.
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The change in hood static pressure from -1.7 in. W.C. to -1.1 in. W.C. indicates a drop in the gas flow rate from 11,796 ACFM to 9,495 ACFM. This is a 20% decrease in the gas flow rate.
Practice Problems
Hoods - Monitoring Performance
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Instructions:
- Complete the Practice Problems before proceeding to the next section. Click on the button below.