Effect of changing cooling fluid in heat exchanger

[adrian1906]

I have a chilled water heat exchanger used to cool oil in a gearbox,
From the energy balance equation:

mdot_water * cp_water *deltaT_water = mdot_oil * cp_oil * deltaT_oil.

Re-written, this becomes:

(mdot_water/mdot_oil) * (cp_water/cp_oil) = deltaT_oil/deltaT_water = (Th_in-Th_out)/(Tc_out-Tc_in)

What happens if I only change from water to another cooling fluid with a lower specific heat, ie cp_new = 1/2*cp_water? Would this effect only _deltaT_water and leave Th_in and Th_out unchanged? Or is the change spread across Tc_in, Tc_out, Th_in, and Th_out? If it is the latter, how could one predict the 4 different temperatures?

In my case, I can measure Tc_in and Tc_out. I also know the sump temperature, Thi but am unable to measure Tho.

I ran a similar experiment but mdot_water and cp_water was changed. The corresponding deltaT_water did not cancel the effects of the modified mdot_water and cp_water, somewhat answering my original question. But since I am unable to fully calculate the right side of original equation, I cannot verify this.

Thank you.

 

[georgeverghese]

What you've written is only one part of how the HX will react to this change in coolant, which is the overall heat balance. The other part is the effect on overall heat transfer coeff U.
1/U = 1/hi + 1/ho + 1/fi + 1/fo
Pls go through your Uni heat transfer text to get the new value for either hi or ho (where the coolant flows through). And get the other values from the original HX datasheet. Then recompute U for new operating case.

 

[Rputvin]

Yes, you will have different temperature deltas on both sides of the heat exchanger if the only change is the cooling fluid.

If Q = mcΔT and you're changing c to a lower value, as well as most likely decreasing your mass flow, you get less energy transfer for the same temperature difference. So if your cold side temperatures were to not change (they would), your hot side would not see as much cooling and Th_out would increase. You need to increase your volume flowrate to provide balance back to the terms and compensate for the lower specific heat, as well as the change in specific gravity which changes your mass flowrate term. If your intent is to provide the same amount of energy to the oil with a different coolant.

And as george pointed out- theoretical energy balance is great, but different fluids respond differently inside the heat exchanger and your efficiency of transfer also decreases as you're doing more than theoretically removing some amount of energy. Not only that, but whatever you're using to produce that flow of water (a pump?) will not produce the same flow and pressure with another liquid. How much all of that matters will depend on how far you're deviating from water. If you just want to use water with 25% glycol the practical change might only be a slight increase in required flow. If you want to switch to dielectric fluid or something else you'll need to re-engineer the entire circuit as all of your pumps, piping, heat exchanger surface area, etc will likely be ill-suited to the fluid.