The ventilation velocity requirements for crew comfort and heat transfer were assessed, as one of several initiatives to ensure that sufficient electric power will be available for Space Station Freedom during the early Man-Tended Capability (MTC) Phase of operations. A rigorous heat transfer analysis, to accommodate microgravity and reduced pressure conditions, was conducted to characterize cabin air effectiveness in transferring metabolic heat away from crewmembers. Maximum possible sweat evaporation rates were estimated based upon rigorous mass transfer correlations.
In the range of low work rates applicable to MTC, no single mechanism dominated the heat transfer. Those mechanisms not dependent upon ventilation velocities, radiation and respiration, appeared to be as important, under the nominal conditions, as forced convection and sweat evaporation. It was shown that ventilation velocities could be lower than 15 feet per minute, and still provide sufficient heat flux. These results, in conjunction with test results, led to a reinterpretation of the requirements permitting a lower average velocity than before, and to substantial reductions in cabin air fan power requirements.