337 SYSTEMS: ENVIRONMENTAL CONTROL AND LIFE SUPPORT SYSTEM—SUPPORTING THE HUMAN ELEMENT OF THE ISS CHAPTER 19 uniform. This is even more critical in the small phone-booth-sized crew quarters where crew members sleep. Therefore, redundant fans and an alarm system are present in those units to ensure the health of the sleeping astronaut. ~6”{ Figure 2. CSA-CP (left) attached to the wall of Node 1 and CMS (right). For the CSA-CP, the metal detector (gray), which is about 15 cm (6 in.) in length, is placed in the black holder that pumps air in through a tube, which is currently connected to a long rod that can be inserted into racks. A CDRA is located in both the Laboratory module and Node 3 module, though only one at a time is usually operating. Note that it is not practical to operate both CDRAs simultaneously to lower the CO 2 level further. The CDRA consumes a significant amount of power (around 1 kW) and generates a significant amount of heat. Operating a second unit can lower the level further, but at the expense of wearing out sooner. The CDRA is actually two sets of filter bed systems that alternately operate one set to purify the air while the other is being cleaned. Air is first pulled into the CDRA from the Common Cabin Air Assembly (CCAA) over a bed of desiccant to remove humidity. The desiccant bed uses silica gel and zeolite, and is similar to the small packets found in food products or shoes. Drying the air allows the next filter bed, an absorbent material, to remove the CO 2 more efficiently. The air is then pushed through a second desiccant bed that was previously used to remove water vapor and therefore contains excess water. This actually rehydrates the air a little before it reenters the cabin. After a while, the absorbing bed becomes saturated and can no longer remove the CO 2 . At this point, it is taken out of the air flow and heated to a high temperature, which causes the trapped CO 2 to be released. Released CO 2 is either vented to vacuum or piped over to the Sabatier (see below). The second set of desiccant and absorbent beds remove CO 2 while this bake-out is occurring, and the cycle keeps repeating.