A Pop Quiz: Water in Compressed Air – Dew Point of Compressed Air

Is your your air treatment knowledge up to snuff?  Take this quick quiz and find out.

1. Why does compressing atmospheric air to 100 PSIG almost always results in water condensation?

A. Heat of compression causes atmospheric water vapor to condense.

B. Turbulent air flow through piping causes compressed air to cool and condense water.

C. Water condensation only occurs in air systems using reciprocating or oil lubricated screw compressors. Other compressor types do not produce water.

D. None of the above.

2. An after-cooler and high efficiency moisture separator typically remove what percentage of moisture entrained in an air system operating at 100 PSIG? (For the geeks: assume sea level, 80⁰F ambient, 60% ambient relative humidity, 10⁰F approach to ambient cooler efficiency.)

A. 70

B. 50

C. 30

D. 10

3. Why do refrigerated air dryers not protect air lines from sub-freezing ambient temperatures?

A. Current refrigerant technology limits mechanical refrigeration evaporator temperatures to 35⁰ F or greater.

B. The premise is false. Refrigerated dryers do protect air lines against freeze-ups in low ambient conditions.

C. The evaporator (heat exchanger) in a refrigerated dryer would freeze if the dryer were designed to chill compressed air below 32⁰ F.

D. None of the above.

4.  A typical dual tower regenerative desiccant dryer with activated alumina desiccant stops performing above what temperature?

A. 110⁰ F

B. 115⁰ F

C. 120⁰ F

D. Compressed air temperature does not affect a well engineered regenerative desiccant dryer.

5. Which of the following technologies cannot remove water vapor from a compressed air system?

A. Pressure swing adsorption

B. Centrifugal separation

C. Chemical absorption

D. Hollow fiber membrane separation

ANSWERS

1. D – Liquid water forms in compressed air lines because atmospheric water vapor is drawn in through the compressor intake. Compression concentrates the water vapor, elevating the dew point. Pressurized air cools below its dew point after leaving the compressor and liquid forms.

2. A – Yes, 70 percent! Under the stated conditions a cubic foot of atmospheric air contains 6.636 grains of water vapor. Multiply that amount of atmospheric moisture by the compression ratio [(100 + 14.7 / 14.7) = 7.8], and an actual cubic foot of 100 PSIG compressed air will contain 51.77 grains of water vapor (7.8 x 6.636 = 51.77). Cooling this ACF to 90⁰F leaves 14.96 grains.   In other words, 36.81 grains have been removed through cooling. 36.81/51.77 = 71.1%.   In reality, you’ll never need to do this calculation for your air system or for the air system of a customer. Just remember the end result: an efficient after-cooler and separator do most of the water removal, not the dryer.

3. C – To protect outdoor air lines an air dryer needs to lower the compressed air dew point below the lowest expected ambient temperature. If a refrigerated air dryer were to chill humid compressed air below 32⁰F ice would form within the dryer, blocking flow through the dryer.

4. C - Since water vapor pressure is exponential with temperature and thermal aging accelerates with temperate, it's not practical to design a dual tower dryer using activated alumina for inlet temperatures above 120F.   The dryer would be excessively large, have a high reactivation/regeneration cost, and the desiccant would quickly age.  Search for the terms activated alumina and isotherm for further reading.

5. B – A moisture separator can only remove liquid water. Water vapor passes through a separator. The other technologies listed remove water vapor.

How’d you do?  If you didn’t answer each question correctly, maybe you need to come see us at a Van Air Systems training school.  Email info@vanairsystems.com to learn more.