Dew Point Analysis Developments for Space Station 941511
This paper reviews the recent G189A computer program developments in the area of humidity control for the U.S. Lab Module in the Space Station. The humidity control function is provided as an indirect or passive function by the Common Cabin Air Assemblies (CCAA) in pressurized elements or modules in the Space Station. The CCAAs provide active cabin temperature control through implementation of a digital/electromechanical control system (i.e., a proportional/integral (PI) control system). A selected cabin temperature can be achieved by this control system as long as the sensible and latent heat loads are within specified limits.
In this paper three pertinent analytical cases directed to determining minimum or maximum dew point temperatures are discussed. In these cases the basic sensible heat loads are set at constant values. The sources of water vapor in the module include: 1) the crew activity timeline with variations in metabolic heat load, 2) the cyclic operation of the Carbon Dioxide Removal Assembly (CDRA) which alternately removes water vapor from the cabin (along with CO2) at a steady rate and subsequently returns water vapor to the cabin as a pulse, 3) operation of Crew Systems equipment (such as the handwash facility or the shower) on its timelines. Since the sources of water vapor are all time varying, it is evident that the issue of extreme cabin dew point temperatures requires a transient analysis.
Prior to discussing the three analytical cases considered here, discussions of the operations and of the thermodynamic principles involved in the performance of the CCAA and the CDRA are included. Also, the interactions of the CDRA and CCAA are discussed. The results of the three analytical cases are presented. It is shown that for the present design performance modeling and data for the CCAA and the CDRA and the sensible and latent loads imposed on the system considered here the specified minimum and maximum limits on cabin humidity can be accommodated.
The Space Station requirement that the Environmental Control and Life Support System (ECLSS) equipment and other equipment be capable of surviving a laboratory module depressurization/repressurization event must be dealt with analytically. The depressurization process includes condensation of water vapor followed by freezing. These change-of-phase processes have been included in the G189A program's simulation logic. An example analysis of a Lab Module depressurization event has been run and the results are included here. The condensation and freezing effects are noted.