Temperature control aboard spacecraft is, in general, an exercise in cooling. Separate systems are used for external thermal control and for internal control.
External systems utilize in-line heaters and recirculating thermal fluids such as liquid ammonia or chlorofluorocarbons (Freons). Since vacuum conditions exist outside spacecraft, heat flow by conduction and convection is minimal. Radiation is the sole means for heat transfer out of the system. This is accomplished using high surface area radiators. Heat is dissipated via emission of electromagnetic energy in the infrared region of the spectrum.
Internal thermal control systems maintain temperature in the cabin and usually combine other functions including control of: humidity, airborne particulates, and airborne microorganisms. These composite Temperature and Humidity Control (THC) systems utilize condensing heat exchangers (CHX) for the primary cooling effect. Water vapor and heat (latent heat of fusion for water) are removed from the cabin air by the CHX. The water recovered in this way, known as humidity condensate, contains trace levels of water soluble contaminants which are present in the air. Contaminant levels are relatively low, and for this reason, the Russians utilize a separate water reclamation system to purify humidity condensate for the production of drinking water aboard the Mir space station. Control of airborne solids including microorganisms is achieved using High Efficiency Particulate Air (HEPA) filters.
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