Why does the International Space Station require ammonia coolant?

Answer 1

Anhydrous Ammonia is used in passive cooling systems aboard the ISS, primarily in Ammonia Heat Pipes.

Liquid AA, when heated, turns to a gaseous state and draws heat along with it. It is put into a sealed aluminum pipe that has internal ridge veins running its length; as one end of the pipe is heated (e.g., by a power unit or computer), using a process of capillary action, it turns to a gas and moves to the other (cool) end of the pipe, drawing the heat with it. Once it gets to the cool end, it changes back to a liquid state and returns along the ridges to the hot end, completing the cycle.

Liquid AA is also used for air-conditioning, and its system is similar to the Ammonia Boiler System aboard the Shuttles. It's the same process, just a different type of system; ammonia is heated, turned to a gas, drawing the heat from the area and thus cooling the air.

The ISS uses ammonia heat pipe cooling systems for electronic systems, power units, internal cooling, etc. Systems are installed both inside and outside the station as well. The Hubble Telescope also uses ammonia heat pipes for electronics cooling, using a different method called a Capillary Pumped Loop. The system was designed to compensate for extra heat from new electronic systems that were installed on HST Servicing Mission 3A, used to save the NICMOS Camera experiment.

If you go to my bio page and scroll down toward the middle, you can see one of the large cooling panels (the largest in the world at the time) built for the ISS that I was responsible for, as well as the CPL panel for HST. The picture of the ISS Radiator panel has many yellow lines going across it - those are the heat pipes. The yellow tint is due to the adhesive glue sheets that were added prior to the top panel being installed. After that, the whole panel is covered in heavy mats and sealed, then put under extreme vacuum for pressure to keep everything together while being baked for many hours in a large oven. The ISS Radiator required an extremely large oven; the one that was eventually used was previously used for baking enamel finishes on cars.

Much smaller versions (essentially mounting plates with several pipes underneath) are used for electronic systems aboard the station.

The pictures of the smaller white panel being worked on, installed in the Shuttle Bay, and finally attached to the Hubble is HST's CPL Radiator Panel. It was installed during Servicing Mission 3A to help dissipate heat from new electronic systems that were installed at the time.

The ISS Radiator was eventually coated on one side with a diffused silver tape (literally silver), that covered one side, or 100 square feet. A diffusion pattern, etched into the tape, helps to diffuse reflected sunlight off of the panel to prevent blinding any astronauts working in space.

Each 25' roll of tape cost just over $4,000 in 1998; I'll let you do the math on how much it was per foot then, and contemplate how much it would be today with the increase in silver prices.