In related question How was temperature and humidity maintained inside the Apollo mission capsules? the basic concept of conditioning Apollo mission capsules environment is described. After depressurization of the cabin though there is, obviously, no environment to condition, and during repressurization fresh oxygen needs to be supplied at high flowrate from oxygen tank at cryogenic-temperature to fill the entire cabin. What heating mechanism was applied to heat up the supply of fresh cryogenic oxygen for CM cabin repressurisation?

P.S. In original question I asked about repressurization of both CM and LM, but it seems LM stored oxygen in compressed gas form rather than cryogenic liquid or supercritical fluid, therefore I have removed LM from the question.

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    $\begingroup$ The LM used compressed gaseous oxygen, in the SM the oxygen was stored as a supercritical fluid. $\endgroup$ – Uwe Nov 18 '19 at 10:09
  • $\begingroup$ @Uwe thanks for the comment, I wrongly assumed LM oxygen storage to be same as in CM. To be in gaseous phase the pressure has to be below 50bar, but at what temperature though? I've just tried, it doesn't seem to be easily googleable. $\endgroup$ – Sergiy Lenzion Nov 18 '19 at 10:37
  • $\begingroup$ Storing gaseous oxygen in a tank at 200 bar and room temperature is possible. $\endgroup$ – Uwe Nov 18 '19 at 10:49
  • $\begingroup$ But above 50.4bar and -118degC it becomes supercritical fluid, not gas.space.stackexchange.com/questions/39232/… i.stack.imgur.com/UpZQF.png $\endgroup$ – Sergiy Lenzion Nov 18 '19 at 11:11
  • $\begingroup$ In shuttle it ran through a hx with the Freon cooling loops to warm it up. Not sure about Apollo, will have to look. $\endgroup$ – Organic Marble Nov 18 '19 at 11:59

Unsurprisingly, it worked exactly like it did in shuttle.

To assure uniform flow, the capillary restrictors are coiled around a warm water-glycol line to increase the oxygen temperature.

Page 2.7-3

The aforementioned oxygen supply capillary restrictors are wound around the line routed to the space radiators and relief valves. The other line is routed to the mixing valve. To insure proper operation of the oxygen supply restrictors, in the line between the cryogenic O2 storage in the S/M to the surge tanks in the C/M during cabin repressurization, full water - glycol flow through the line to the space radiators is required. Sufficient heat must be available to prevent cryogenic oxygen entering the C/M oxygen system and preclude the possibility of freezing the water-glycol. To achieve this, the mixing valve must be manually placed to the full closed position 15 to 30 minutes before repressurization and remain closed until the surge tank returns to maximum pressure after repressurization of the C/M.

Page 2.7-17

These diagrams show the interface between the O2 lines and the coolant loop.

enter image description here

enter image description here

Following page 2.7-46

Source: Apollo Operations Handbook

Schematic of a shuttle Freon loop showing the O2 restrictor.

enter image description here

From ECLSS Training Manual

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    $\begingroup$ Must have been pretty decent HEX area then, or really hot glycol to heat up gas to that much deltaT. $\endgroup$ – Sergiy Lenzion Nov 18 '19 at 12:51
  • $\begingroup$ @LeoS It's a bit surprising that the hx is placed in the coldest part of the coolant loop at the radiator exit. But it was the same way on shuttle. $\endgroup$ – Organic Marble Nov 18 '19 at 12:56
  • $\begingroup$ I was thinking the same. Maybe what you've highlighted are wrong capillaries. The next page shows "O2 supply capillary restrictions" on somewhat hotter part of the glycol loop. $\endgroup$ – Sergiy Lenzion Nov 18 '19 at 13:10
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    $\begingroup$ Very interesting, it looks the manual intervention was required from the crew to operate the mixing valve in advance of repressurization to accumulate the heat in the glycol loop. I think the puzzle now is solved in my head :) Thanks for the great and prompt answer. $\endgroup$ – Sergiy Lenzion Nov 18 '19 at 13:26
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    $\begingroup$ @LeoS You're welcome! And welcome to space stack exchange! $\endgroup$ – Organic Marble Nov 18 '19 at 13:28

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