The result is that calculating either volumetric or mass water flow rates are straightforward, while the volume of a given mass of air may change significantly based on the temperature and pressure changes that take place in the measurement process. Calculating standardized or mass-related volumetric air flow in a pipe may require correction for "before and after" pressure and temperature conditions around the flow-sensing element. These corrections take place under the square root radical.
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Define the air flow application. In this case, 135-pound-per-square-inch (psi) compressed air is being conveyed through a three-inch Schedule 40 steel pipe to a treatment facility 350 feet away. The pressure gauge at the end of the pipe reads 112 psi. With this information you can calculate the air flow through the three-inch pipe.
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Calculate 5 percent of 135-psi, which would be a maximum recommended pressure loss per 100 feet of pipe at 135 psi to still be energy-efficient. Substituting values, 0.05 X 135 = 6.75 psi.
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Calculate the pressure loss per 100 feet of pipe, since this is how published pipe flow data is presented. 135 psi minus 112-psi = 23-psi/350/100 = 6.57-psi drop per 100-feet. As 6.57 psi is less than 6.75 psi, this example lies in the "efficient" realm.
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Look up the flow data for 135 psi compressed air in three-inch pipe. Since there is only data for 100 psi and 150 psi, you can correct from either point for both drop and static pressure and compare.
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Correct from the 100 psi point by interpolating drop and correcting for static pressure. The flow data would be 2600 SCFM (Standard Cubic Feet per Minute) with a pressure drop of 5 psi (5 percent of 100 psi). The air flow at 135 psi = 6.57 psi drop/5 psi drop X 135 psi/100 psi = 1.7739. Extracting the square root of 1.7739 yields 1.3318. Multiplying the 1.3318 by 2600/SCFM yields 3462 SCFM.
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Correct from the 150 psi point. The flow data is 3900 SCFM with a pressure drop of 7.5 psi. The air flow at 135 psi = 6.57 psi drop/7.5psi drop X 135 psi/150 psi = 0.7884. Extracting the square root yields 0.8879. Multiplying the 0.8879 X 3900 SCFM yields 3462.88 SCFM.
The Drip Cap
- Air flow differs from water flow in that air is compressible and water is not.
- These corrections take place under the square root radical.
- Calculate 5 percent of 135-psi, which would be a maximum recommended pressure loss per 100 feet of pipe at 135 psi to still be energy-efficient.
- The air flow at 135 psi = 6.57 psi drop/5 psi drop X 135 psi/100 psi = 1.7739.
- Multiplying the 1.3318 by 2600/SCFM yields 3462 SCFM.
- Correct from the 150 psi point.