I saw this today and it made me very curious.

See page 8 here: https://www.nasa.gov/pdf/167129main_Systems.pdf

Ammonia has a boiling point of -30 C. The ammonia is liquid in these loops, so if ever exposed to sunlight I'd imagine intense expansion as it goes into gas phase. I noted that Ammonia volumetric expansion coef > water expansion coef.

Is the ammonia loop highly pressurized? Would that make the make the risk of leaks higher? (I see there have been several spacewalks due to ammonia coolant loop leaks)

Next point: The specific heat capacity of Ammonia is: 80.8 kJ/kg K Which is much higher than water 4.2 kJ/ kg K

====EDITED APRIL 14TH ============

***I made a mistake the specific heat capacity of ammonia is pretty close to water, at 4.7 kJ/ kg K


So I guess this means that ammonia can exchange substantially more heat energy without changing its temperature. Yet, I dont clearly see the advantage of that since its normal boiling point is at lower temperature than water.

So whats the advantages/ reasoning for using Ammonia for the coolant loop?

enter image description here


1 Answer 1


Ammonia is used because it has excellent heat transfer properties (as you mention) and a low freezing point. Because of its toxicity, the ISS has internal coolant loops which use water as a heat transfer fluid. Only the heat exchangers where the two systems interface allow for the possibility of ammonia leaks into the cabin.

After detailed engineering analysis, NASA chose ammonia as the coolant for the ATCSs on the exterior of the space station for several reasons: ammonia has a lower density than many other commercially available coolants and can therefore be launched in great quantity at dramatically reduced launch costs; it has a low viscosity so it requires little power for pumps to circulate the ammonia through cooling loops; and ammonia remains liquid down to -78°C (-108°F), which is important in the extreme cold of the ISS external environment. On the downside, ammonia is toxic to humans; therefore, the possibility of this dangerous chemical leaking into the pressurized cabin is one of the three major emergency responses discussed in Chapter 19.

ATCS = Active Thermal Control System

Ammonia is incompressible, but the ISS ammonia loops operate at a reasonably high pressure (~ 7 MPa, 1000 psi)

Reference: The International Space Station - Operating an Outpost in the New Frontier (Chapter 11 recommended for further reading on the cooling systems).

  • $\begingroup$ Yes, that ammonia is not in cabin made sense, due to toxicity. So this high specific heat capacity means that the ammonia gains and loses heat more easily than other compounds? $\endgroup$
    – dlight
    Apr 13, 2021 at 12:26
  • $\begingroup$ It means that the substance can absorb a lot of heat without changing its temperature very much. $\endgroup$ Apr 13, 2021 at 12:30
  • 1
    $\begingroup$ Ha it may be. Let me read up this evening and write back after. $\endgroup$
    – dlight
    Apr 13, 2021 at 12:39
  • 3
    $\begingroup$ High specific heat means that more heat can be carried by a fixed volume of ammonia. If C2 has 2x specific of C1, then you only need to pump C2 half as fast as C1 to remove the same amount of heat. The other factor is thermal conductivity, which is the speed at which the coolant can accept heat. Ammonia is comparable to water here. $\endgroup$ Apr 13, 2021 at 20:52
  • 1
    $\begingroup$ To add to what @LawnmowerMan said, high specific heat capacity is useful because temperature differential is crucial to heat transfer. If your part is 50°, and your coolant starts at 10°, it won't be able to cool the second half of the part if it's already heated to 45° by the half way point. High heat capacity keeps the temperature low for the same amount of energy absorbed. $\endgroup$
    – Tim
    Apr 14, 2021 at 2:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.