Compressed Air Aftercooler Condensate Calculator — Pressure Dew Point & Outlet Humidity
Estimate condensate generation from inlet volumetric flow or dry-air mass flow, then evaluate outlet relative humidity and pressure dew point at the compressed-air operating pressure.
Enter the known values and review the calculated result
Input parameters
Use consistent values and select the intended engineering units.
Calculation mode
Compressor inlet conditions
Compression & aftercooler
Inlet air flow
Dry-air mass flow
Results
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Method, application and limitations
Review the calculation method, intended application and engineering assumptions before using the result in a design decision.
Formula and calculation method
Compressed air aftercooler condensate calculation
The calculator models the moisture balance of compressed air as it is cooled in an aftercooler. It first determines the inlet humidity ratio, reconstructs the water-vapour state at the compressed-air pressure, and then compares that moisture content with the saturation limit at the aftercooler outlet temperature.
1. Inlet water-vapour partial pressure
Relative humidity is converted to water-vapour partial pressure using:
pw,in = RHin · pws(Tin)
where:
- pw,in — inlet water-vapour partial pressure [Pa]
- RHin — inlet relative humidity [-], entered from 0 to 1
- pws(T) — saturation vapour pressure at temperature T [Pa]
- Tin — compressor inlet temperature [K]
2. Humidity ratio
The calculator uses the ideal-gas moist-air relationship:
w = ε · pw / (p − pw)
with:
ε = 0.621945
Therefore the inlet humidity ratio is:
win = 0.621945 · pw,in / (pin − pw,in)
At the aftercooler outlet, the saturated humidity ratio is:
wsat,out = 0.621945 · pws(Tcool) / (pout − pws(Tcool))
3. Condensation criterion and condensed water per kg of dry air
The available moisture reduction is:
Δw = win − wsat,out
Condensation is predicted when:
Δw > 0
The condensed water per unit mass of dry air is:
Δwcond = max(0, Δw)
and the outlet humidity ratio becomes:
wout = min(win, wsat,out)
4. Condensate generation rate
When a dry-air mass flow is supplied directly:
ṁcond = ṁda · Δwcond
where ṁda is the dry-air mass flow.
When inlet volumetric flow is entered, the calculator first determines the dry-air partial pressure:
pda,in = pin − pw,in
and converts the actual inlet volumetric flow to dry-air mass flow:
ṁda = pda,in · Qin / (Rda · Tin)
with:
Rda = 287.042 J/(kg·K)
5. Water-vapour pressure after compression
Compression is modeled with unchanged humidity ratio before condensate removal. The corresponding water-vapour partial pressure at the aftercooler operating pressure is:
pw,compressed = pout · win / (0.621945 + win)
The outlet water-vapour partial pressure is:
pw,out = pout · wout / (0.621945 + wout)
6. Outlet relative humidity
Outlet relative humidity is calculated from vapour partial pressure, not from the ratio w/wsat:
RHout = pw,out / pws(Tcool)
When condensation occurs under the modeled equilibrium condition, the outlet vapour is saturated and RHout = 1.
7. Compressed-air pressure dew point
The pressure dew point before aftercooling is the temperature satisfying:
pws(TPDP,in) = pw,compressed
The calculator solves this relationship numerically.
If condensation occurs and the outlet vapour is saturated:
TPDP,out = Tcool
If no condensation occurs:
TPDP,out = TPDP,in
The reported dew-point margin is:
ΔTPDP = Tcool − TPDP,in
A negative value means the aftercooler outlet temperature is below the incoming compressed-air pressure dew point, so condensation is expected in the model.
8. Moisture removal fraction
For a nonzero inlet humidity ratio:
fremoval = Δwcond / win
Saturation vapour pressure correlation
Saturation vapour pressure is evaluated with the Hyland–Wexler form implemented in the calculator. Temperature is in kelvin and the resulting pressure is in pascals.
For T ≤ 273.15 K:
ln(pws) = C1/T + C2 + C3T + C4T2 + C5T3 + C6T4 + C7ln(T)
For T > 273.15 K:
ln(pws) = C8/T + C9 + C10T + C11T2 + C12T3 + C13ln(T)
The implemented psychrometric correlation is evaluated over the calculator's supported temperature range of −100 to 200 °C.
Calculation modes
- Condensate load — inlet volumetric flow: calculates dry-air mass flow from the actual inlet volumetric flow and then calculates condensate generation rate.
- Condensate load — dry-air mass flow: uses the entered dry-air mass flow directly.
- Outlet humidity / condensation state: evaluates the psychrometric state without requiring a flow rate; condensate mass flow is therefore not calculated in this mode.
When to use this calculator
When to use this calculator
Use this compressed air aftercooler calculator when you need to determine whether moisture will condense as compressed air is cooled and, when a flow rate is known, estimate the resulting condensate load.
The calculation is intended for compressed-air systems in which ambient or inlet moist air is compressed to a higher absolute pressure and subsequently cooled in an aftercooler.
Typical engineering uses
- Estimate the condensate generation rate downstream of an air compressor aftercooler.
- Compare condensate load for different inlet temperatures, inlet relative humidities, operating pressures, and aftercooler outlet temperatures.
- Calculate the compressed-air pressure dew point before aftercooling.
- Determine the pressure dew point and relative humidity of the air leaving the aftercooler.
- Check whether a specified aftercooler outlet temperature is below the compressed-air pressure dew point and therefore causes condensation.
- Estimate condensed water per kilogram of dry air and the fraction of inlet moisture removed.
- Evaluate the moisture state without specifying flow by using the outlet humidity / condensation state mode.
- Calculate condensate load from either actual inlet volumetric flow or dry-air mass flow.
Choosing the calculation mode
Use Condensate load — inlet volumetric flow when the known flow is the actual volumetric flow at the entered compressor inlet pressure and temperature. The calculator converts that flow to dry-air mass flow before calculating condensate generation.
Use Condensate load — dry-air mass flow when dry-air mass flow is already known.
Use Outlet humidity / condensation state when only the thermodynamic moisture state, outlet relative humidity, and pressure dew point are required. No condensate mass-flow result is generated because no flow rate is supplied.
When not to use this calculator
This is a moisture-balance and pressure-dew-point model. It does not size the aftercooler heat exchanger or determine its thermal duty, surface area, coolant flow, approach temperature, fan power, or pressure drop.
It also does not size a refrigerated or desiccant compressed-air dryer, calculate atmospheric dew point from pressure dew point, evaluate separator efficiency, or predict liquid carryover, oil aerosols, drainage performance, or downstream water accumulation.
Applications requiring subzero aftercooler outlet temperatures, ice or frost formation, detailed real-gas behavior, or equipment-specific performance should be evaluated with a more appropriate model and manufacturer data.
How to interpret the result
How to interpret the results
Condensate generation rate — ṁcond
This is the calculated mass of water leaving the vapour phase per unit time as compressed air is cooled to the specified aftercooler outlet temperature.
A value greater than zero means the inlet humidity ratio exceeds the maximum vapour-phase humidity ratio that can remain at the entered aftercooler pressure and temperature. Condensation in an aftercooler is an expected physical process and is not treated by the calculator as a failure condition.
Condensate generation generally increases when the inlet air contains more moisture, the aftercooler outlet temperature is reduced, the compressed-air pressure is increased, or the dry-air flow rate is increased, provided the other inputs remain unchanged.
In Outlet humidity / condensation state mode no flow rate is supplied, so the condensate mass-flow result is intentionally not calculated.
Outlet relative humidity — RHout
Outlet relative humidity is the ratio of the outlet water-vapour partial pressure to the saturation vapour pressure at the aftercooler outlet temperature.
If condensation occurs, the model sets the remaining outlet vapour to saturation, giving RHout = 1. If the specified cooling does not reach saturation, RHout remains below 1 and the condensate load is zero.
Pressure dew point before aftercooling — TPDP,in
This result is the dew-point temperature corresponding to the incoming moisture content after it has been referenced to the entered compressed-air operating pressure.
It represents the temperature at which that compressed-air moisture state would first reach saturation if cooled at the same pressure.
Pressure dew point after aftercooling — TPDP,out
When condensation occurs, the outlet vapour is saturated at the aftercooler outlet temperature, so the calculated outlet pressure dew point equals Tcool.
When no condensation occurs, the humidity ratio is unchanged and the pressure dew point remains equal to the pre-aftercooler pressure dew point at the same operating pressure.
Dew-point margin
The calculator reports:
ΔTPDP = Tcool − TPDP,in
- Negative margin: the aftercooler cools the air below its incoming pressure dew point and condensation is expected.
- Zero margin: the outlet temperature is at the saturation threshold.
- Positive margin: the outlet temperature remains above the incoming pressure dew point and no condensation is predicted.
Inlet and outlet humidity ratio
win is the mass of water vapour per kilogram of dry air entering the compression model. wout is the water-vapour humidity ratio remaining after the modeled aftercooling and condensation process.
Their positive difference is the condensed water per kilogram of dry air.
Moisture removal fraction
The moisture removal fraction is the fraction of the inlet humidity ratio that is converted from vapour to condensate in the modeled aftercooler process. It is not the efficiency of a physical water separator or drain.
Condensation state
Condensation expected in the aftercooler means the entered outlet temperature and pressure cannot retain the complete inlet moisture content as water vapour under the model.
No condensation expected at the specified aftercooler outlet state means the complete inlet humidity ratio can remain in the vapour phase at the specified outlet condition.
Neither result verifies downstream compressed-air quality. A system may still require a moisture separator, automatic drain, dryer, filtration, or a lower specified pressure dew point depending on the downstream temperature and application requirements.
Calculation example
Worked example — condensate load from inlet volumetric flow
A compressor takes in humid air at approximately atmospheric pressure and supplies an aftercooler operating at 8 bar absolute. The objective is to calculate the condensate generation rate, outlet relative humidity, and compressed-air pressure dew point.
Input data
- Calculation mode: Condensate load — inlet volumetric flow
- Inlet absolute pressure, pin = 101.325 kPa
- Inlet temperature, Tin = 25 °C = 298.15 K
- Inlet relative humidity, RHin = 0.60
- Aftercooler absolute pressure, pout = 800 kPa
- Aftercooler outlet temperature, Tcool = 35 °C = 308.15 K
- Actual inlet volumetric flow, Qin = 10 m³/min = 0.166667 m³/s
Step 1 — inlet moisture state
The implemented saturation-pressure correlation gives:
pws,in = 3169.22 Pa
Therefore:
pw,in = 0.60 · 3169.22 = 1901.53 Pa
and:
win = 0.621945 · 1901.53 / (101325 − 1901.53) = 0.0118950 kg water/kg dry air
Step 2 — saturation condition at the aftercooler outlet
At 35 °C:
pws,out = 5627.82 Pa
The saturated humidity ratio at 800 kPa absolute is:
wsat,out = 0.621945 · 5627.82 / (800000 − 5627.82) = 0.00440624 kg/kg
Step 3 — moisture condensed per kg of dry air
Δw = 0.0118950 − 0.00440624 = 0.00748881 kg/kg
Because Δw is positive, condensation is expected.
Step 4 — convert inlet volumetric flow to dry-air mass flow
Dry-air partial pressure at the inlet is:
pda,in = 101325 − 1901.53 = 99423.47 Pa
Using Rda = 287.042 J/(kg·K):
ṁda = 99423.47 · 0.166667 / (287.042 · 298.15) = 0.193623 kg/s
Step 5 — condensate generation rate
ṁcond = 0.193623 · 0.00748881 = 0.00145001 kg/s
This is equivalent to approximately:
5.22 kg/h of condensate
Step 6 — pressure dew point and outlet humidity
At 800 kPa absolute, the unchanged inlet humidity ratio corresponds to a compressed-air pressure dew point of:
TPDP,in = 53.99 °C
The aftercooler outlet temperature is only 35 °C, giving:
ΔTPDP = 35.00 − 53.99 = −18.99 K
Because the air is cooled below its incoming pressure dew point, water condenses. Under the calculator's equilibrium model the outlet vapour is saturated, therefore:
- RHout = 1.000
- TPDP,out = 35.00 °C
- Moisture removal fraction = 0.630, or approximately 63% of the inlet water-vapour mass per kilogram of dry air
Interpretation
For the specified conditions, the compressed air reaches its saturation limit before it reaches the 35 °C aftercooler outlet temperature. The model therefore predicts condensate formation at a rate of approximately 0.00145 kg/s.
This result is the thermodynamic amount of water that must leave the vapour phase. It does not verify the efficiency of a downstream moisture separator or condensate drain and does not represent aftercooler heat-exchanger sizing.
Assumptions and limitations
Assumptions and limitations
- The calculation models a steady moist-air state and a thermodynamic moisture balance through compression and aftercooling. Transient startup, cycling, and storage effects are not included.
- Compressor inlet air is treated as a valid single-phase moist-air mixture defined by absolute pressure, temperature, and relative humidity.
- Relative humidity must be between 0 and 1. The inlet state is rejected if calculated water-vapour partial pressure reaches the entered total inlet pressure.
- Compression is assumed not to change humidity ratio before the aftercooler. No upstream condensation, liquid injection, moisture removal, leakage, or additional water source is modeled.
- Aftercooler pressure must be greater than compressor inlet pressure. Both pressure inputs are absolute pressures.
- The aftercooler model is limited to outlet temperatures from 0 to 200 °C. Temperatures below 0 °C are excluded because frost and ice formation require a different phase model.
- Inlet temperature and the internal saturation-pressure/dew-point calculations use the supported psychrometric range of −100 to 200 °C.
- The calculation uses ideal-gas moist-air relationships with ε = 0.621945 and Rda = 287.042 J/(kg·K).
- Saturation vapour pressure is evaluated using the Hyland–Wexler form implemented in the calculator. The model does not apply a separate moist-air saturation-pressure enhancement factor or a detailed real-gas equation of state at elevated pressure.
- When win exceeds the saturated outlet humidity ratio, the excess water is treated as condensate and the remaining outlet vapour is assumed to be saturated at the aftercooler outlet temperature.
- Calculated condensate is the thermodynamic amount of water leaving the vapour phase. Moisture-separator efficiency, droplet entrainment, drain performance, liquid holdup, and downstream carryover are not modeled.
- In volumetric-flow mode, Qin is the actual inlet volumetric flow at the entered pin and Tin. It is not automatically interpreted as standard cubic metres, SCFM, FAD, or another reference-condition flow unless the entered inlet state matches that reference basis.
- In mass-flow mode, the entered ṁair is specifically dry-air mass flow, not the combined mass flow of dry air and water vapour.
- The calculator does not determine compressor discharge temperature, compression work, compressor efficiency, or compressor power.
- The calculator does not size an air-cooled or water-cooled aftercooler and does not calculate heat-transfer duty, heat-exchanger area, coolant flow, approach temperature, fan requirements, or pressure drop.
- Refrigerated dryers, desiccant dryers, membrane dryers, their pressure dew point ratings, and atmospheric-dew-point conversion are outside the implemented calculation.
- Oil vapour, compressor lubricant, aerosols, contaminants, dissolved gases, corrosion, condensate treatment, and disposal requirements are not included.
- A predicted condensation state does not indicate equipment failure. Conversely, a zero condensate result does not prove that a complete compressed-air system will remain free of condensation if the air is subsequently cooled to a lower temperature.
- Final equipment selection should consider actual site conditions, the full operating envelope, downstream minimum temperature, required compressed-air quality, separator and dryer performance, and applicable manufacturer requirements.