Compressed Air Moisture Calculator

• Ambient temperature, relative humidity and altitude. The intake air state. Relative humidity times the saturation vapour pressure at the ambient temperature (Buck equation) gives the water-vapour partial pressure; altitude sets the barometric pressure P_atm by the standard-atmosphere formula (101.325 kPa at sea level), and a lower P_atm raises the humidity ratio for the same RH.
• Free air delivery (FAD). The compressor's intake volume flow, taken on the normal basis (0 °C, 1.013 bar; Nm³ and the cfm options are converted to it). The mass flow is the normal volume × 1.293 kg/m³ — the dry-air normal density, used as the standard proxy for the intake air — and the water vapour at the intake humidity is then split off it to give the dry-air mass flow that is conserved through the system. Water is a mass balance, so the mass flow is what matters.
• Discharge pressure. The line pressure the air is compressed to. A gauge value is added to the local P_atm — 7 bar(g) is 8.013 bar(a) at sea level and less at altitude. Compression does not change the water mass, but the higher pressure makes the saturation humidity ratio smaller — that is the whole mechanism of condensation.
• Aftercooler outlet temperature. The air temperature leaving the aftercooler, typically 8–15 K above the cooling air or 5–8 K above the cooling water. The air is taken as saturated at this temperature and the line pressure with everything above that saturation ratio condensed and drained — the design ideal; in the field the outlet air is 80–95 % RH with a little liquid carry-over past the separator.
• Dryer and pressure dew point. A refrigerated dryer cools the air to about 3 °C PDP and drains the condensate; a desiccant dryer adsorbs water to −40 °C PDP or lower. The dryer stage removes whatever is above the saturation ratio at its pressure dew point. A heatless desiccant dryer's purge, typically 10–20 % of the flow, is not subtracted here — it does not change the water balance, but it does reduce the air delivered to the plant.
• Humidity ratio W. Grams of water vapour per kilogram of dry air: W = 0.622 · p_w / (P − p_w). It is the quantity that is conserved through the compressor and reduced at each stage; every condensate figure is a difference in W times the dry-air mass flow.
• Pressure dew point and atmospheric dew point. The PDP is the temperature at which the compressed air is saturated at line pressure — the figure ISO 8573-1 classes and dryer ratings use (Class 4 is ≤ +3 °C PDP, Class 2 ≤ −40 °C). The atmospheric dew point is the same air expanded to P_atm; it is always lower (about −23 °C for a 3 °C PDP at 7 bar(g)), which is why a 3 °C dryer protects an outdoor line well below freezing point.
• Condensate. Litres per hour and per day of liquid water removed at each stage (1 kg ≈ 1 L). It sizes the separator drains and the oil–water separator that treats the drained condensate (not the air); the inlet moisture rate is the total water entering with the air.

• Saturation vapour pressure by the Buck (1981) equation over liquid water, accurate to about 0.1 % between −40 and 50 °C (sub-zero dew points are therefore over water, not ice — the convention dryer ratings use); humidity ratio from the ideal-gas relation W = 0.622 p_w/(P − p_w) without the pressure enhancement factor, which would raise W by roughly 0.4–0.6 % at 8 bar.
• Water is conserved through compression: the compressor adds no moisture and removes none (oil-injected machines add a little oil aerosol, which is not water and is not modelled).
• Each stage leaves the air exactly saturated at its outlet temperature and line pressure with 100 % separation of the liquid formed. Real separators pass a few percent of the condensate as carry-over, and an aftercooler that runs hotter than entered leaves proportionally more water in the air.
• No pressure drop across the aftercooler, separator or dryer — the line pressure is used for every stage. A dryer with a 0.3 bar drop changes the saturation ratio by well under 1 %.
• Free air delivery is taken on the normal basis (0 °C, 1.013 bar) at the intake humidity. Compressor ratings per ISO 1217 are quoted at the actual intake conditions; convert with the intake density if your rating is on that basis.
• Desiccant dryer purge air (typically 10–20 % of the flow) is not subtracted from the delivered flow, and dryer performance is taken as its nominal pressure dew point regardless of load, inlet temperature or ageing.
• Steady state at one operating point. Condensate over a day is the hourly rate × 24; a compressor that cycles or a climate with diurnal swings needs the load profile and the ambient extremes — the design case is the hottest, most humid hour.

What happens after the dryer — cooling and condensation in the distribution pipe — is the pipe condensation calculator's question. The SimuPipe solver's own accuracy for the compressed-air network itself is documented on the validation page.

Correlations, standards and handbooks behind this page:

• Buck, A. L. (1981). "New Equations for Computing Vapor Pressure and Enhancement Factor." Journal of Applied Meteorology, 20(12), 1527–1532. doi:10.1175/1520-0450(1981)020<1527:NEFCVP>2.0.CO;2 — the saturation vapour pressure equation and the enhancement factor discussed above.
• ASHRAE (2021). Handbook — Fundamentals, Chapter 1: Psychrometrics — humidity ratio, dew point and the psychrometric relations.
• ISO 8573-1:2010. Compressed air — Part 1: Contaminants and purity classes — purity classes defined on pressure dew point.
• ISO 1217:2009. Displacement compressors — Acceptance tests — the free-air-delivery definition compressor ratings use.
• Atlas Copco. Compressed Air Manual, 8th ed. — aftercooler, separator and dryer practice with typical condensate figures.
• Compressed Air and Gas Institute. Compressed Air and Gas Handbook, 7th ed. — compressed-air moisture and treatment design guidance.