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Module 2: Characteristics of Gases - Flow Rate
Actual Gas Flow Rate-
Animation #1:
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Gas Flow Rate
Conversion Between Actual and Standard Gas Flow Rates
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Calculator:
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Gas Flow Rate Converter (Actual
Standard Conditions)
Objectives
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Convert gas flow rates between actual and standard conditions.
- Convert gas flow rates from the American Engineering units (ACFM and SCFM) to Cgs (m3/min and Nm3/min) and vice versa.
Gas flow rate is a measure of the volume of gas that passes a point in an industrial system during a given period of time. The ideal gas law tells us that this gas flow rate varies depending on the temperature and pressure of the gas stream and the number of moles of gas moving per unit of time.
When gas flow rates are expressed at actual conditions of temperature and pressure, the actual gas flow rate is being used. As you will learn later, gas flow rates can also be expressed at standard conditions of temperature and pressure; this is referred to as the standard gas flow rate.
Common units of measure for gas flow rate are Actual Cubic Feet per Minute (ACFM) in the American Engineering system of units and Actual Cubic Meters per Minute (m3/min) in the Cgs system of units.
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The gas flow rate generated by an industrial process can vary substantially depending on the operating rate of the process equipment and on the temperature and pressure of the gas stream generated. The actual gas flow rate (as opposed to the standard gas flow rate, discussed later) must be used whenever gas velocity is calculated in process equipment, air pollution control equipment, ductwork, stacks, and other locations in an industrial process. Therefore, changes in the actual gas flow rate affect gas velocities in the system. Changes in gas velocity can have an impact on the performance of the air pollution control system.
The gas velocity is calculated simply by dividing the actual gas flow rate by the cross-sectional area of the equipment or duct through which the gas passes. The gas velocity calculation is illustrated below and discussed further in the Lesson on Velocity (Module 2).
Example Problem 1.
Using Actual Gas Flow Rate to Calculate Gas Velocity
What is the velocity of a gas stream moving at a flow rate of 960,000 ft3/min (27,187.8 m3/min) through the electrostatic precipitator shown in Figure 1 (Gas Velocity Calculation Using ACFM)? The height and width of the precipitator are 40 ft (12.2 m) and 80 ft (24.4 m) respectively. Solve in both American Engineering units and Cgs units.
Solution:
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Calculate the cross-sectional area through which the gas flows.
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Calculate the gas velocity through the electrostatic precipitator.
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#1
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Do collectors A and B process the same amount (pound moles) of combustion gas per minute? See Figure 2.
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Yes
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No
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Cannot be determined
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Yes
As you can see, knowing the gas volume or gas flow rate does not definitively quantify the amount of gas. Question 1 above shows that 120,000 ACFM of gas at Point A is not always equal to 120,000 ACFM of gas at Point B. Expressing gas volumes at standardized conditions of temperature and pressure eliminates this problem. This is discussed in the next section.
TIP: The term n/minute [or more generally (n/unit of time)] is an important concept. This is the molar flow rate, which is the number of moles per unit time that are moving through the air pollution control system. The molar flow rate is also related to the mass flow rate (or mass emission rate), which is the amount of material passing through the system (or going to the atmosphere) in a given unit of time. The relationship between mass flow rate and molar flow rate is indicated in Equation 2.
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Where:
For example, a mass flow rate of 128 lbm/min would equal a molar flow rate of 2 lb moles/min of SO2 since the molecular weight of SO2 is 64 lbm/lb mole.
The standard gas flow rate is the gas flow rate expressed at standard conditions of temperature (20°C or 68°F) and pressure (14.7 psia or 407 in. W.C.). Standard gas flow rates can be expressed in units of Standard Cubic Feet per Minute (SCFM) and Standard Cubic Meters per Minute (Nm3/min). The SCFM and Nm3/min values are measures of the total quantity of gas being handled per minute and therefore are directly related to the number of gas molecules handled during that time period. They are also the volumes that these amounts of gas occupy at standard temperature and pressure.
Unlike actual gas flow rates, standard gas flow rates can be added and subtracted as necessary to describe the industrial source system. For example, if 1,000 SCFM enter an air pollution control device, and none is reacted or lost, then 1,000 SCFM exit the control device (Figure 3). If an air pollution control system operating at negative pressure is suffering air inleakage of 125 SCFM, and the inlet gas flow rate is 2,000 SCFM, then the outlet gas flow rate is 2,125 SCFM. The SCFM quantity can be calculated from the ideal gas law when standard conditions of absolute temperature and pressure are used.
Standard gas flow rates are useful in the following situations:
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When material balances are being prepared
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For calculations in which one or more of the gas stream constituents participate in chemical reactions
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For comparing the flow rate in a system with the flow rate capacity of a fan
However, as mentioned earlier, this form is not very useful when evaluating the gas flow characteristics inside the ductwork, air pollution control devices, and process equipment. When it is necessary to evaluate gas velocities and other flow conditions, use the actual gas flow rate expressed in terms of ACFM or m3/min.
TIP: It's easy to convert from flow rates expressed as moles/min to SCFM or Nm3/min (at EPA-defined standard conditions) by using these conversion factors:
Conversion between Actual
and Standard Gas Flow Rates
It is possible to convert between the actual and standard gas flow rate values using the ideal gas law relationship discussed earlier in this Module. This has been expressed in the form of simple ratios shown in Equations 4 through 7.
From Standard to Actual Gas Flow Rate:
From Actual to Standard Gas Flow Rate:
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Where:
Note: Conversions between ACFM and SCFM and between m3/min and Nm3/min use absolute temperature and absolute pressure data.
Gas Flow Rate Converter (Actual Standard Conditions).
Purpose: This calculator converts gas flow rates between actual and standard conditions of temperature and pressure. Standard conditions are defined as 20°C (68°F) and 1 atm (14.7 psia) pressure.
Restrictions on Use: Do not use commas when inputting values.
Notes:
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This Java applet may take a few minutes to load.
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Sometimes calculator output is provided in an exponential format in base ten as shown below.
5.2e003 = 5.2
103
5.2e-003 = 5.2 10-3
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Inputting non-numeric characters such as commas and percent signs will result in erroneous output. (Decimal points are fine to use.)
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When this calculator converts temperatures between the relative and absolute scales, it uses the more exact values for absolute zero expressed in the two relative scales:
273.15°C and 459.67°F, instead of using the rounded values, 273°C 460°F.
- Reminder - Converting gas flow rates between actual and standard conditions requires the use of absolute temperatures and pressures. (This calculator converts relative temperatures and pressures to their absolute equivalents.)
Example Problem 2.
Conversion from ACFM to SCFM
A gas stream has a flow rate of 10,000 ACFM, a temperature of 300°F, and a pressure of -6 in. W.C. What is the gas flow rate in SCFM? Assume a barometric pressure of 407 in. W.C.
Solution:
Example Problem 3.
Conversion from m3/min to Nm3/min
A gas stream has a flow rate of 1,000 m3/min, a temperature of 373°K, and a pressure of -2 kilopascals (abbreviated kPa). What is the gas flow rate in Nm3/min?
Solution:
Example Problem 4.
Conversion from SCFM to Nm3/min
A gas stream has a flow rate of 12,000 SCFM. What is the gas flow rate in Nm3/min?
Solution:
Practice Problems
Gas Flow Rate
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Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.