Compressed Air Dew Point and Moisture Calculator
Convert pressure dew point to atmospheric dew point and back with the ISO 8573-1 water class, and calculate the condensate produced at each stage of a compressed air system: aftercooler and dryer.
P_atm = 101.3 kPa
Typically ambient + 8-15°C (air-cooled) or + 5-8°C (water-cooled)
Aftercooler
Compressed air dew point converter: pressure dew point to atmospheric dew point
Convert a pressure dew point (PDP, measured at line pressure) to the atmospheric dew point (ADP, the same air expanded to ambient pressure) or back, and read the water content it corresponds to. Compressing air raises its dew point; expanding it lowers it. Dryer ratings and ISO 8573-1 classes are stated as PDP.
Atmospheric pressure 101.33 kPa, from the site altitude entered above.
The water-vapour mole fraction is what compression conserves, so the dew point at the other pressure is where the saturation pressure equals that fraction times the pressure. Frost point: below 0 °C the saturation pressure is taken over ice — what dew-point transmitters report and what forms in the pipe. Over water: supercooled-water basis throughout, used by some published conversion charts; it gives lower sub-zero dew points and about 50 % more water content at −40 °C.
Pressure dew point to atmospheric dew point table
Atmospheric dew point for common pressure dew points at the usual line pressures, with the ISO 8573-1 water vapour class and the water content at 7 bar(g). Sea level, 101.325 kPa.
Frost-point basis below 0 °C. Water content in g/m³ is at ISO 8573-1 reference conditions (20 °C, 1 bar(a)). A refrigerated dryer's 3 °C PDP at 7 bar(g) is an atmospheric frost point near −21 °C; a −40 °C desiccant PDP is near −57 °C. Values computed by the converter above.
- 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. Relative humidity is taken relative to liquid water at all temperatures, which is how humidity sensors report it, including below 0 °C.
- 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 −21 °C for a 3 °C PDP at 7 bar(g)), but it only applies once the air has been released to atmosphere. Inside the pressurised pipe, water condenses — and freezes — below the PDP itself, so a 3 °C dryer does not protect an outdoor line in frost; that needs a desiccant dryer.
- 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. With a desiccant dryer the dryer's share is adsorbed and leaves in the purge air rather than through a drain, so the total is then labelled water removed and is an equivalent liquid volume.
The calculator's defaults: intake air at 25 °C and 60 % RH at sea level, a 60 Nm³/min compressor discharging at 7 bar(g) through an aftercooler leaving the air at 35 °C — then the same plant with a refrigerated dryer set to 3 °C PDP.
- Intake moisture: p_ws(25 °C) = 3.169 kPa (Buck), so at 60 % RH p_w = 1.901 kPa and W = 0.622 · p_w / (P_atm − p_w) = 11.89 g/kg; the intake air's dew point is 16.7 °C.
- Mass flows: 60 Nm³/min × 1.293 kg/m³ (the dry-air normal density, the standard proxy) = 1.293 kg/s of intake air, of which 1.278 kg/s is dry air — carrying 54.7 L/h of water vapour into the compressor.
- Aftercooler: 7 bar(g) + 1.013 = 8.013 bar(a) at sea level; at 35 °C and that pressure the air can hold only W_sat = 4.40 g/kg, so the excess condenses: 34.5 L/h, or 827 L/day — 63 % of the intake water, drained by the separator.
- The air leaves saturated: pressure dew point 35.0 °C, which expanded to atmosphere is a 2.1 °C dew point — still wet enough to condense in any line cooler than the aftercooler outlet.
- Add a refrigerated dryer at 3 °C PDP: W falls to 0.59 g/kg, another 17.5 L/h is drained (1248 L/day in total) and the atmospheric dew point drops to -20.8 °C — dry enough for an unheated indoor system, not for a −20 °C yard line, which needs a desiccant dryer.
These figures are computed by the same function the calculator runs; the defaults reproduce steps 1–4, and selecting the refrigerated dryer reproduces step 5.
- Saturation vapour pressure by the Buck equations — over liquid water above 0 °C and over ice below 0 °C, so sub-zero values are frost points, which is what dew-point transmitters report and what actually forms in the pipe (the converter can be switched to the over-water basis used by some published charts); accurate to about 0.1 % between −40 and 50 °C. 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 2–3 % at 8 bar(a).
- 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).
- Where a stage cools the air below its dew point, the air leaves saturated at that stage's outlet temperature and line pressure with 100 % separation of the liquid formed; air that arrives drier than that passes through unchanged and stays unsaturated. A dryer is represented by its rated pressure dew point, which for a desiccant dryer is not its outlet air temperature. 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 0.3 bar drop at 8 bar(a) raises the saturation humidity ratio at the dryer by about 4 %, but that ratio is small next to the water being removed, so in this worked example the condensate changes by well under 1 %. Close to the condensation threshold, where little water is being removed, the relative change can be much larger.
- 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.
About Compressed Air Moisture
Ambient air always contains water vapor. When this air is compressed to typical industrial pressures (7-13 bar), the same mass of water vapor occupies a much smaller volume. Since the air can only hold a limited amount of moisture at any given temperature and pressure, the excess condenses as liquid water. A typical 60 Nm³/min compressor operating at 7 bar in warm, humid conditions can produce over 500 litres of condensate per day.
Aftercoolers
Most compressors include an aftercooler that cools the compressed air from the high discharge temperature (80-200°C depending on compressor type) down to a practical temperature, typically 8-15°C above the cooling medium temperature. This is where the majority of condensate is removed. A moisture separator and automatic drain downstream of the aftercooler collect the liquid water before it enters the distribution system.
Dryers
For applications requiring dry air (painting, pneumatic instruments, food processing, pharmaceuticals), a dryer is installed downstream of the aftercooler. Refrigerated dryers cool the air to approximately 3°C pressure dew point, removing most remaining moisture at modest energy cost. Desiccant dryers use adsorption to achieve pressure dew points of -40°C or lower, required for critical applications and outdoor piping in freezing climates.
Pressure Dew Point vs Atmospheric Dew Point
The pressure dew point (PDP) is the temperature at which compressed air becomes saturated at line pressure. The atmospheric dew point (ADP) is the equivalent at ambient pressure — it is always lower than the PDP because at lower pressure the air can hold more moisture. ISO 8573-1 air quality classes reference pressure dew point for specifying compressed air dryness.
Measuring dew point in compressed air
Dew point is measured on a sample of air at line pressure, so the instrument reads the pressure dew point directly. Inline transmitters and portable dew-point meters use a capacitive polymer or ceramic sensor whose capacitance tracks the water-vapour partial pressure; a chilled-mirror hygrometer, which cools a mirror until condensation forms and reads the mirror temperature, is the reference method of ISO 8573-3. The sample must reach the sensor without being expanded to atmosphere first: if it is expanded, the instrument reads the atmospheric dew point and the value must be converted back to line pressure with the converter above. Two practical points catch people out. A sensor moved from room air into a desiccant-dried line can take hours to dry down to a stable reading at -40 °C, so a spot check that is stopped early reads too wet. And a sampling line whose wall is colder than the sample's dew point condenses water on it and reads too dry. Typical accuracy of a well-maintained transmitter is about ±2 °C of dew point.
For moisture condensation in pipes carrying warm air through cold environments, see our pipe condensation calculator. For pipe insulation heat loss analysis, use the insulation thickness calculator. For full pipe network simulation, try SimuPipe.
Frequently Asked Questions
How much condensate does a compressor produce?
What is pressure dew point and why does it matter?
What is the difference between a refrigerated and desiccant dryer?
How does altitude affect compressed air moisture?
What happens if I don't remove moisture from compressed air?
How do I convert pressure dew point to atmospheric dew point?
What is the dew point of compressed air at 7 bar?
How is dew point measured in compressed air?
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