Why does ISS use ammonia and not liquid nitrogen for cooling? I guess that they could store liquid nitrogen cause vacuum is essentially cold (if properly shielded from sun), and ammonia is pretty toxic so, what is really the advantage of having ammonia rather than nitrogen on the international space station?

Answer :

- Liquid nitrogen absorbs heat as it evaporates, and would have to be continuously replenished with supplies shipped from the surface of the Earth at great expense.

The ammonia is used as a refrigerant in a closed cooling system, just like your household refrigerator or air conditioner (which probably uses R-134, a different chemical). The ammonia cycles between liquid and gaseous state in a closed loop. All it needs is energy, which is supplied in orbit from the solar panels.

- Ammonia conducts heat much more than nitrogen. It is like comparing steel to styrofoam. This is important because a refrigeration system is a heat exchange engine. As the gas is compressed it releases heat and as it expands it absorbs heat. Ammonia is also easier to liquify than nitrogen, so there is less work involved for the compressor. This makes an ammonia refrigeration system more efficient in terms of energy. In fact, the first refrigerators used ammonia instead of the chlorofluorocarbons they do today. Carbon dioxide absorbs and releases heat even better, but liquid CO2 has the tendency to solidify easily and this jams the compressor. Of course water is the best, but the vapor exists at a far higher temperature than people can tolerate, so this gas is best used in steam turbines.

- If you knew anything about the thermodynamics of cooling, then I don't think you'd be asking this question.I do NOT know why they use NH3 compared to the other refrigerant gasses they could have selected, possibly its weight, but N2 would have been a very very poor choice. How does refrigerant work? Do you even know? No, you don't. Here is a link... -=- Its not that N2 can't be used. I assume that its use was considered and dismissed after a cost/risk/benefit analysis. -=- I am amused by your claim that vacuum is cold. Why don't you look up the temperature? -=- Of course, vacuum doesn't have a "real" temperature. You should know that. The temperature of the gas and plasma actually present in outer space is quite high. Of course, if cooling is predominantly by radiation, it may swamp any heating in an extremely low pressure atmosphere. I guess that is what you meant. But of course if itsshielded from the sun, then how efficient will radiative cooling be?

Source(s):

http://en.wikipedia.org/wiki/Refrigerant...