Temperature and humidity within the Apollo Command Module was controlled with a system of heat exchangers, electrical heaters and water-glycol (62.5 % ethylene glycol and 37.5 % water) coolant loops. The coolant loop was used to cool the electronics mounted on cold plates and also the atmosphere of the capsule.
It was possible to rout the coolant at first to the electronics cold plates and then to the cabin heat exchanger if heating of the cabin was necessary. Waste heat of the electronics could be recycled for cabin heating this way. The coolant flow through the heat exchanger could be reduced proportional to the demand of heating or cooling of the cabin.
Different heat exchangers were used for cabin and suit oxygen circuits. The suit circuit heat exchangers removed also excessive humidity by condensation. The condensed water was removed by a pump from the exchanger and transported to the waste water tank.
Excessive heat could removed by radiation. There were two space radiation panels at the outside surface of the service module at a 130 ° arc. If one panel was exposed to the Sun, Earth or Moon, the other panel exposed to space was used instead. Each panel could remove up to 4,415 BTU per hour from the coolant loops, that is 1,294 W of heat energy.
A minimum flow of coolant through the radiators was necessary to prevent freezing of the coolant within the radiators. But if the coolant temperature after the radiators was too low there were a primary and a secondary electrical heater for the two coolant loops with 450 W each. If the temperature reaches 43°F the No. 1 heater comes on, and at 42°F the No. 2 heater comes on; at 44°F No. 2 goes off, and at 45°F No. 1 goes off. If coolant temperature was 45°F or higher, no electrical heating was required and done.
Waste water could be used in evaporators to remove heat from the coolant loops by evaporation of water into the vacuum of space. About 8,000 BTU per hour or 2,344 W of heat could be removed by the evaporators. Evaporators were used only if cooling by radiators was not sufficient. When the temperature
of the coolant entering the evaporator rises to 48° to 50.5°F, the evaporator mode of cooling was initiated. The coolant outlet temperature was regulated to a temperature between 40° to 43°F by control of the water steam pressure valve at the vacuum outlet of the evaporator. Water flow into the evaporator was regulated to keep the evaporator wick between too wet and too dry.
The water-glycol mixture of the coolant loops was precooled before launch using ground equipment to be used for cooling during launch through the atmosphere when neither the radiation panels nor the evaporators could be used
All information from a NASA paper titled, "Environmental Control Subsystem".