Cooling-coil condensate rate

Calculate moisture removal from the drop in humidity ratio across a coil.

How this calculation works

Condensate production follows a water mass balance across the air stream. Enter dry-air mass flow and the humidity ratios before and after the coil, expressed as grams of water per kilogram of dry air. Subtract the leaving humidity ratio from the entering value and convert the difference to kilograms per kilogram before multiplying by the dry-air flow. The resulting water-removal rate is shown both per second and per hour. Equal humidity ratios give no net moisture removal; a higher leaving value describes humidification and is outside this removal calculation. The balance assumes steady operation and that removed vapor becomes collected condensate. Water retained on surfaces, droplet carryover, defrost and transient storage can separate the calculated vapor removal from actual drain discharge. Drain and trap sizing require separate checks.

Inputs and units

Dry-air mass flow (kg/s)
Entering humidity ratio (g/kg dry air)
Leaving humidity ratio (g/kg dry air)

Method and formula

Water removal = dry-air mass flow × (Wenter−Wleave). Divide g/kg humidity ratios by 1000; multiply kg/s by 3600 for kg/h.

Worked example

Example inputs

Dry-air mass flow: 1.5 kg/s
Entering humidity ratio: 12 g/kg dry air
Leaving humidity ratio: 8 g/kg dry air

Calculation steps

Humidity-ratio decrease = (12 − 8) / 1000 = 0.004 kg water/kg dry air.
Water removal = 1.5 × 0.004 = 0.006 kg/s. Hourly condensate = 0.006 × 3600 = 21.6 kg/h.

Example results

Condensate mass flow: 21.6 kg/h
Condensate mass flow: 0.006 kg/s

Assumptions

Steady operation; removed vapor is collected as condensate.

Limitations

Does not size drain piping or traps. Carryover, retained water, defrost and transient storage are omitted.
Preliminary educational check; verify inputs and equipment data before design or operation.

Sources

ASHRAE Handbook: Psychrometrics: Sections 5–7 and 10: humidity ratio, ideal-gas properties, moist-air balances