Comparative Test Data Assessment and Simplified Math Modelling for the Vapor Compression Distillation Subsystem 932194

Space Station Freedom (SSF) has an extended mission duration of 30 years. Trade studies for extended missions of manned spacecraft almost invariably show that large resupply weight and consequent cost savings can be achieved by recovering potable water from wastewater sources. This rationale has led to the present baseline Water Recovery and Management (WRM) system for the Permanently Manned Capability (PMC) phase of SSF. The baseline WRM includes the Vapor Compression Distillation (VCD) subsystem for recovering water from urine. This process serves as a preliminary processing step in achieving potable water from wastewater sources.

The basic principle of the VCD is that water is evaporated from urine and then condensed in a zero-gravity device containing an evaporator and a condenser in a rotating drum.

The VCD was selected for the baseline WRM following the assessment of test results from competitive urine processing subsystems obtained from the Comparative Test (CT) program. This test program was performed by the Boeing Missiles & Space Division at Huntsville, Alabama. The testing of the VCD consisted of processing 29 batches of pretreated urine followed by processing 2 batches of 50/50 mixtures of pretreated urine and reverse osmosis (RO) process brine. The specific goals of the testing program were to determine production rate, water recovery efficiency, required power, product water quality and expected flight supply needs. The intended testing program was completed and the measured performance data for the VCD were successfully used in assessing the competitive standing of the VCD.

A mathematical model for the VCD has been prepared. This simplified model is a first step in the VCD modeling activity. It includes thermodynamic relationships concerning pressures, temperatures and flow rates in the evaporator, condenser and compressor. Empirical relationships which can be used to predict operational performance parameters were obtained from analyses of considerable test data from extensive testing of VCD subsystems, assemblies, and components. The independent variable for many of these empirical relationships is the percentage of solids in the recycle loop. This quantity along with the ambient temperature determines the operating pressure and temperature levels in the VCD's evaporator and condenser. These empirical relationships along with others for predicting water production rate, required power, heat rejection, and water recovery efficiency are included in the mathematical model.

The CT program's operations test data for the VCD have been compared with predicted data from the mathematical model. The results of these comparisons have shown some good agreements between the test data and the analytical predictions. Some other comparisons have indicated areas where the math modelling needs to be improved. The math model will be needed to support flight VCD operations including normal processing operations and anomalies.