Cabin Humidity Control: Condensing Heat Exchanger

Habitat & Crew

Crew size 4

Cabin air temperature 22 °C

Condensing Heat Exchanger

Coolant / core temperature 10 °C

Airflow through core 28 L/s

Live Readouts

Cabin relative humidity

--%

Cabin dew point

-- °C

Condensate collection rate

-- mL/hr

Tank level

0.00 L / 5.00 L

How it works

A spacecraft cabin's air is cooled below its dew point by a condensing heat exchanger (CHX) — the same principle as the ISS Common Cabin Air Assembly. Saturation vapor pressure follows the Magnus formula:

e_s(T) = 6.1094 * exp(17.625*T / (T + 243.04))   [hPa, T in °C]
w_s(T) = 0.622 * e_s(T) / (P - e_s(T))            [kg water / kg dry air]

Crew respiration and perspiration add water vapor to the cabin at a steady mass rate. Air ducted through the CHX core is cooled to the coolant temperature; any vapor above the core's saturation humidity ratio w_s(T_chx) condenses onto the fins and drips into the collection tank:

source   = crew * per-person vapor rate
condense = airflow * efficiency * max(0, w_cabin - w_s(T_chx))
dw_cabin/dt = (source - condense) / (mass of cabin air)

Crew size — more people, more metabolic water vapor entering the cabin.
Cabin temperature — sets the cabin's own saturation curve, which relative humidity and dew point are measured against.
Core temperature — colder core = lower w_s(T_chx) = more moisture pulled out per pass, but real hardware must stay above 0°C or the condensate freezes and blocks the fins.
Airflow — how much cabin air is cycled through the core per second; more flow moves more moisture even at fixed core temperature.
Fresh vs fouled core — a fouled (mineral-scaled or biofilm-coated) core has poor thermal contact, cutting condensation efficiency roughly in half, exactly as ISS crews see between hydrophilic-coating servicing cycles.

The recovered condensate is exactly the feedstock the water-recovery system distills back into drinking water — this CHX is the first stage of that closed loop.

Humid cabin air Condensate droplets Collection tank

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