Compressed Air Moisture and Condensate
How much water a compressor makes and where it comes out.
Example
You enter
- Air delivered (scfm) 100
- Intake air temperature (F) 75
- Intake relative humidity (%) 75
- Atmospheric pressure (psia) 14.696
- Line pressure (psig) 100
- Compressed air temperature after cooling (F) 100
- Dryer pressure dew point (F; refrigerated about 38) 38
You get
- Intake humidity ratio 0.01396
- Intake water (lb/hr) 6.282
- Condensed (lb/hr) 3.943
- Dryer removed (lb/hr) 2.0636
- Remaining vapor (lb/hr) 0.275
- Total liquid (gal/day) 17.286
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeFrom the air delivered, the intake temperature and humidity, and the line pressure and temperature, it gives the water vapor taken in, the liquid that drops out at the aftercooler, receiver and drip legs, and what a dryer removes to its pressure dew point. A 100 scfm compressor breathing 75 F air at 75% humidity takes in 6.3 lb of water an hour and sends about 17 gallons a day to its drains. Size drains and dryers for the most humid intake air. The equipment manufacturer's data governs.
Formula or tableHumidity ratio W = 0.621945 Pw / (P - Pw), lb of water per lb of dry air. Intake: Pw = relative humidity x Psat(intake temperature), P = atmosphere. Dry air = scfm x 0.075 lb/cu ft x 60. Water in = dry air x W. After cooling at line pressure the air holds at most Ws = 0.621945 Psat(T) / (P line absolute - Psat(T)); condensate = dry air x (W in - Ws at the line temperature), and the dryer removes dry air x (W leaving the line - Ws at the pressure dew point). Gallons per day = lb/hr x 24 / 8.34.
Source and editionThe psychrometric humidity-ratio relation, ASHRAE Handbook -- Fundamentals, Psychrometrics, cited by name; saturation pressure of water from IAPWS R7-97(2012) Eq. 30 (the shared steam-tables.js). Standard air at 0.075 lb per standard cubic foot.
Free accessA mass balance on water; the saturation equation is free at iapws.org and is held to the release's verification values by a unit test. The example, 100 scfm at 75 F and 75% relative humidity, takes in 18.1 gal/day of vapor and drains 17.3, in line with the roughly 18 to 20 gal/day compressor makers quote for a 25 hp machine in humid weather.
What governsEstimate. AHJ and licensed professional govern.
API field namesflow_scfm, inlet_temp_f, inlet_rh_pct, atmosphere_psia, line_pressure_psig, line_temp_f, dryer_dew_point_f, intake_humidity_ratio, intake_water_lb_hr, condensed_lb_hr, dryer_removed_lb_hr, remaining_vapor_lb_hr, total_liquid_gal_day
- Standard air 0.075 lb of dry air per standard cubic footcommon definition of scfm
- Saturated after cooling air leaving each stage holds no more than the saturated ratio at that temperature and line pressurepsychrometrics
- Water 8.34 lb per gallonphysical data
- Dew point range 32 F and abovescope of this tile