CHAPTER 19 SYSTEMS: ENVIRONMENTAL CONTROL AND LIFE SUPPORT SYSTEM—SUPPORTING THE HUMAN ELEMENT OF THE ISS 340 Temperature and Humidity Control Not only does the crew need to be comfortable, some equipment requires the air in the ISS to remove excess heat (i.e., air cooled). CCAAs in the Laboratory, Node 2, Node 3, and airlock, and similar air conditioners in the International Partner modules, circulate, cool, and dehumidify the air. The CCAAs are connected directly to the IMV ducting (Figure 4). The ducting passes air between all the modules to ensure uniform mixing. Air is pulled into ducts that feed the CCAAs and passes through High Efficiency Particulate Air (HEPA) filters, in the same way as household systems do on Earth. Although the crew and spacecraft are kept as clean as possible on the ground, a fair amount of particulates are present on the ISS. These filters have to be vacuumed fairly regularly since, in microgravity, objects collect on the inlet of vents and not on the floor. This process also acts as a handy way to find missing items—e.g., lost screws, misplaced washers, small tools, or even a pack of gum or a fork. A fan pulls the air in through the HEPA filter and past the Condensing Heat Exchanger (HX). The air passes over metal layers that are cooled by water from the Low Temperature Loop (see Chapter 11). When air hits the cool plates, the water condenses on a plate where it is drawn into small holes and separated from the air. The cooler and dehumidified air is then vented directly into the cabin. If the crew members want the air temperature to be warmer, they (or the ETHOS flight controllers) can command the system to slightly close doors within the ducting of the fan, which reduces the amount of air passing over the cooler plates. For a cooler cabin, the doors are opened more fully, thus more air is passed over the plates. Since air is used to push the water into the collection holes, a mixture of water (~90%) and air (~10%) comes out the other side. The water enters a small centrifuge. As the centrifuge spins, the heavier water is pulled out from the air. The water is then routed to the condensate tubing where it can go either to a big condensate tank for storage or into the Water Processor Assembly (WPA). Why do we process the urine on the ISS instead of taking up fresh water? The basic answer is cost. Six crew members produce about 9 kg (~20 lbs) of urine a day. About 70% of that, or 7.7 kg (17 lbs), is processed into water. In 2015, it cost about $25,000 to launch 0.5 kg (1 lb) of water to the ISS. This translates into approximately $425,000 of cost savings per day, or more than $155,125,000 a year in water that doesn’t have to be launched. Table 1. Types of Water in use on the ISS Type Use Potable Water Drinking water via the Russian systems for the crew containing a silver-biocide to retard microbial growth and minerals for taste. Technical Water Contains silver-biocide to retard microbial growth and is used for crew hygiene, Russian toilet flush water, and in the Russian O2 generator to produce O2. Condensate Water recovered from the atmosphere, which is processed back into potable water. Waste Urine or water that was used in the EMU (see Chapter 17). Special Fluid Water used for the internal cooling system or other uses, and which may contain chemicals to retard biological growth or chemical reactions and cannot be used for drinking. Iodinated Similar to technical water but with iodine biocide this type of water is used in the USOS water systems for toilet flush, the OGA, and, when stripped of iodine, the crew drinks it. Water Recovery and Management As on Earth, water is a precious commodity in space and, therefore, is managed carefully by the flight control team. Water is transported to the ISS by one of the various cargo vehicles. The six types of water on the ISS are listed in Table 1. One of the key functions of ETHOS is to track the various quantities of water to ensure an adequate amount of each type. This section will discuss the life cycle of water on the ISS, which is depicted in Figure 5. As mentioned in the previous section, condensate water from the USOS