Determine the volumetric flow to use in sizing a compressor to meet the following suction requirements:

Weight flow = 425 lb/min dry air

Inlet pressure = 14.7 psia ambient air inlet temperature = 90°F

Inlet relative humidity = 95%

Step 1. Determine the total moist air flow to provide the dry air needed. Because the air is at atmospheric pressure, psychrometric charts may be used to determine the amount of water vapor contained in the dry air (see Figures B-2 and B-3 in Appendix B).

From the psychrometric chart, for a dry bulb temperature of 90°F with a relative humidity of 95%,

Specific humidity = .0294 lbs of water vapor/lb of dry air For the 425 lb/min of dry air, the water vapor content is w;425 x .0294

w2 = 12.495 lb/min water vapor

wm - 437.5 lb/min total weight flow

Step 2. Determine the molecular weight of the moist air mixture using Equation 2.3.

M 425 lb/min 28.95 lb/lb-mol

Mm = 15.373 total mols/min mixture

Step 3. Using Equation 2.15, calculate the mol fraction of each component,


1 15.373 Xj = .955 mol fraction dry air



x? = .045 mol fraction dry air

Step 4. Calculate the molecular weight using Equation 2.22.

Step 5. Calculate the compressor inlet volume using Equation 2.10. First use Equation 2.5 to calculate the specific gas constant.

Substitute into Equation 2.10 and, using 144 in2/ft2, For comparison, assume the moisture had been ignored.

The calculation would indicate that the volume would have been short by approximately 5% if the moisture in the air was ignored.

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