The Primary Life Support System (PLSS) contains a battery to run it, oxygen tanks, communications gear, a LiOH and charcoal air filter and scrubber cartridge, a sublimator, and water for running the cooling system and sublimator. The sublimator is a simple heat exchanger that works by transferring the heat from a cooling garment worn by the astronaut and from the suit's air supply to ice that is formed on it.
This slowly sublimates and is leaked out of the pack via an exhaust rather than discharging explosively.
...backpack, also called an Extravehicular Mobility Unit (EMU,) contains the astronaut's communications gear, two high pressure (1020 PSIA, upgraded to 1430 PSIA for later missions) oxygen tanks for replenishing the astronaut's suit pressure as he uses it up, two Oxygen Purge System tanks with about 30 minutes of oxygen in case the main supply failed, a Lithium Hydroxide and Charcoal canister that acted as a filter for the air as well as a scrubber to remove carbon dioxide, a battery to run it all and two water tanks, one to collect wastewater from the recycled air and a 3.9 litre bladder (later increased to 5.2 litres) for feedwater to be used with the cooling system.
The cooling system in the PLSS worked by use of a Liquid Cooled Garment (LCG,) worn by the astronauts under their spacesuit, a heat exchanger, and what was called a sublimator. The LCG was essentially a pair of long johns with tubing through it. Water was circulated around the tubing and would warm up as the Astronaut exerted himself. This circulation was called the Cooling Loop and once it was charged with water, there was very little loss from it, though if needed it could be topped up from the PLSS supply. By using a diverter valve, which was located on the bottom left of the PLSS, the astronaut could divert water from the cooling loop and into the PLSS's heat exchanger where chilled water from the cooling feeder tank would remove the excess heat from the Cooling Loop water before being feed into the sublimator. The more water that the Astronaut diverted out of the Cooling Loop and through the heat exchanger, the colder the water in the loop would be. The diverter valve had three distinct settings, Minimum (24-27C), Intermediate (15-18C), and Maximum (4-8C).
The feedwater, now hot from the heat exchanger, was then slowly feed through the porous nickel plates of the sublimator and out into the vacuum of space. This drop in pressure would also lead to a drop in temperature, resulting in the water freezing into ice on the outside of the sublimator before subliming into a gas and leaking from the backpack through an exhaust.
Oxygen was also recycled through this system to cool it. Removed from the suit through vents near the feet and elbows, it was first cooled by the sublimator, exchanging its heat to the ice formed on the outside, before being passed through the LiOH scrubbers. It was then mixed with new oxygen from the oxygen tank to keep the suit pressure up (the suits were kept at a constant 3.7 psi,) and passed back into the suit through vents in the helmet to keep the Astronaut's head cool, help stop the helmet fogging and to prevent any carbon dioxide build up in the helmet. These two methods of cooling were so effective that even on the Intermediate setting, several of the Astronauts commented that it made them too cold.
Due to the very high vaporisation energy of water, this constant sublimation of ice could remove a large amount of heat even though the amount of water being sublimated at any time was quite small, meaning that the larger 5.2 litre tanks would hold enough water to run for around nine hours of moderate exertion. Since there is only ever a small amount of gaseous water and as the backpack is not closed, but rather open to the environment, there could never be a large enough build up of water vapour inside of it to discharge explosively.
This system was also used in larger units on various parts of the Apollo spacecraft as cooling systems and lead to the modern day EMU that the space shuttle and ISS crews use on all of their spacewalks, none of which display an explosive discharge either. Other than the communications gear, the 279 watt-hours PLSS batteries (this was upgraded to 390 watt-hours for the later J-Missions) were only required to keep two small electronic motors running, one for a pump to circulate the water, the other for a fan to do the same with the air supply. The pump and fan could be individually turned on or off by the astronaut via the PLSS remote control unit on the front of his chest. While some of the consumables in the PLSS (Oxygen and Feedwater) would be restored from the LM supplies, the Battery and LiOH cartridges were simply swapped out for new ones. This whole procedure of recharging the PLSS would take about 30 minutes and was done prior to exiting for an EVA.
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