![[logo] US EPA](http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Module 1: Basic Concepts - Pressure
Gas PressurePressure Scales
Atmospheric Pressure
Common Units of Measurement for Pressure
Calculator:
Pressure Converter
Relative Pressure
Practice Problems
Objective
- Convert from one pressure scale to another.
The pressures of the gas streams throughout the particulate control system are very important because gas pressure data is often used to evaluate operating conditions. The total pressure of a gas stream is the sum of the static pressure and velocity pressure of the gas stream.
-
Where:
The term static pressure is used to describe the pressure exerted by all gases. This pressure is basically related to the number of gas molecules in a given volume and at a given temperature. If the number of molecules in the space increases, the pressure increases. An increase in the gas temperature increases the kinetic energy of the molecules, and the static pressure increases.
Pressure Scales
Like temperature, gas pressure can be expressed in both relative and absolute terms. The absolute pressure scale starts at zero gas pressure (no molecules 
a vacuum) and has no practical maximum limit. The absolute temperature and pressure scales are most useful for the scientific and engineering calculations necessary to evaluate the following:
- Hoods and ventilation systems
- Source emission rates
- Air pollution control equipment performance
Absolute pressure is used whenever it is necessary to use the Ideal Gas Law to calculate gas flow rates. However, in the air pollution control and emission measurement fields, gas pressures are often monitored in terms of relative pressures and it is necessary to convert the data to absolute pressure prior to performing the calculations.
The static pressure exerted by ambient air is termed either atmospheric or barometric pressure. The atmospheric pressure is an absolute pressure because it is directly related to the number of molecules and their kinetic energy. A barometer measures atmospheric pressure by comparing the pressure of the air against a chamber that approximates a vacuum (0 pressure).
Figure 1 shows a simple mercury barometer.
Common Units of Measurement for Pressure
The absolute and relative air pressures can be expressed in a number of units, which are listed in the Table below. The values of atmospheric pressure under standard conditions are listed in the last column.
Standard conditions are arbitrarily selected valves for absolute temperature and pressure. They are used as a convenience when expressing physical values that are dependent on temperature and pressure. For example, standard conditions are used for expressing gas volumes, gas flow rates, gas densities, liquid densities, and liquid velocities. By using standard conditions, a consistent basis is available to evaluate changes in these physical characteristics. EPA's definition of standard conditions are 14.7 psia and 528 °R (68°F).
Standard atmospheric pressure (14.7 psi) is the average pressure that exists at sea level at 45° latitude. On a mountain, the atmospheric pressure is lower than 14.7 psi because the column of air above the measuring point is smaller. When weather systems pass over, there are slight variations in the atmospheric pressure that are slightly above or slightly below 14.7 psi. Nevertheless, the value of 14.7 psi will be used as a "standard."
Pressure expressed as inches W.C. is generally the most convenient format due to the magnitudes of the gas pressures commonly found in industrial sources. For example, the normal positive pressures (see next section) in gas streams and emission testing equipment are generally from +1 to +20 in. W.C., which are equivalent to the awkward values of +0.036 to +0.722 psi or +0.0735 to +1.47 in. Hg. The normal negative pressures range from -0.20 to -60 in. W.C., which are equivalent to -0.0072 to -2.17 psi. Furthermore, gauges often used in air pollution control and industrial ventilation systems indicate relative pressure directly in inches of water.
Pressure Converter (Calculator)
|
Purpose: This calculator converts a given pressure to five different units of measurement: atmospheres, pounds per square inch, inches of water column, millimeters of mercury, and kilopascals. Values for both relative and absolute pressures may be entered.
Restrictions on Use: Output is not valid if you input values below absolute zero pressure. |
Notes:
- This Java applet may take a few minutes to load.
- 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 - Inputting non-numeric characters such as commas and percent signs will result in erroneous output. (Decimal points are fine to use.)
In the field of air pollution the magnitude of the gas pressures commonly found in industrial sources can be slightly above or below atmospheric pressure. Therefore it is often convenient to measure pressure relative to atmospheric pressure. When fans used in industrial systems create gas static pressures above the prevailing atmospheric pressure, the condition is termed positive pressure. When the fans create a gas static pressure below the prevailing atmospheric pressure, negative pressure exists. Both positive and negative pressures are considered relative terms because the static pressure is being described in a form that is compared to the atmospheric pressure.
-
#1
-
What is the relative pressure for Filters A and B in Figure 2?
Filter A has negative relative pressure (or negative pressure).
Filter B has positive relative pressure (or positive pressure).
Figure 3 graphically depicts the concept of negative and positive relative pressure for Filters A and B.
Figure 3. Positive and Negative
Relative Pressures
-
#2
-
In Figure 4 (Cylinder pressure gauge), the pressure gauge on the cylinder filled with compressed gas reads 300 psig. Is this the absolute pressure or relative pressure in the cylinder?
The absolute pressure is calculated simply by adding the gauge pressure (relative pressure) to the atmospheric pressure.
.gif){kind=small}
-
#3
- In Figure 4, what is the absolute pressure (i.e. total pressure) of the gas in the cylinder?
Throughout Basic Concepts in Environmental Science and U.S. EPA APTI courses, the pressure indicated by an instrument will be called the "gauge pressure," and therefore will be a relative pressure. It is the pressure in the device being evaluated compared with the atmospheric pressure at the time of the measurement.
Practice Problems
Pressure
-
Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.