I have two hot fluids that need to be cooled. The first is the water loop that is used to cool the motor and power electronics. The second is an oil loop used to cool a gear box. Both these will be cooled by air but two options are possible - 1) Cool both the water and oil in a traditional radiator (heat exchanger (HX)) with an airflow coming from a fan. 2) Cool the oil with the water first (the oil is at a much higher temperature and can be cooled by a liquid-liquid HX) and then cool the water as in (1).

The second option is essentially a HX within a HX. My question : Is there any obvious advantage to (2) over (1)? By advantage, I mean in terms of overall weight and power consumption of the cooling schemes. Or to rephrase my question, thermodynmically which of the two is "better"?

  • $\begingroup$ Heat transfer is driven by temperature difference so option 1. $\endgroup$
    – Solar Mike
    Commented Dec 31, 2022 at 7:33
  • $\begingroup$ It won't depend on thermodynamics, just what makes best sense mechanically for the specifics of the system. $\endgroup$
    – Phil Sweet
    Commented Dec 31, 2022 at 14:26

1 Answer 1


You need two heat exchangers to do this either way. You can't cool both water and oil in a single radiator, though I guess you could use the same fan like an auto engine. Either solution is viable, but two separate air-liquid heat exchangers would be easier to size than one water loop going through 2 heat exchangers.

I am concerned about your statements about the lube oil temp. Lube oil should never see temps over 170 degrees F to keep it from oxidizing, 140 is better.

  • $\begingroup$ I believe that is the maximum temp of the supply to the gearbox. Temps leaving the gearbox are typically higher, but that does increase the oil change frequency. ATF is good to 225 F or so. Oil cooling loops are usually considered when temps above 200 F are expected. Hard-to-maintain or remote setups might be run cooler. $\endgroup$
    – Phil Sweet
    Commented Dec 31, 2022 at 15:28
  • $\begingroup$ @phil, lube oil life drops rapidly over 160F $\endgroup$
    – Tiger Guy
    Commented Jan 1, 2023 at 7:34
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    $\begingroup$ Can you post a reference to an engine or transmission mfg's data sheet that lists an operating temp range that tops out at 160 F or so? This might be true for a big wind turbine or submarine, but most stuff isn't like that. Of course it depends where the sensor is and how that temp relates to the actual peak temps experienced by the film in metal contact. Aftermarket transmission coolers usually have a 180 F cut-in for the fan, valve, or baffle system. etrailer.com/p-D13900.html Overtemp alarms run around 250F(warning yellow)-275F(danger red). $\endgroup$
    – Phil Sweet
    Commented Jan 1, 2023 at 16:52
  • $\begingroup$ @PhilSweet, this is a well-known lube-oil standard for industrial use. I think the life is cut in half for every 10 degrees C above oxidation point which is around 145-155 F I think. I am assuming this is industrial use, not vehicle. Most rotating machinery not in a vehicle is exactly like that. You just put up with it in an IC engine. efficientplantmag.com/2015/12/consider-lubricant-temperature $\endgroup$
    – Tiger Guy
    Commented Jan 2, 2023 at 3:17

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