Free Cooling Heat Exchanger Variable flow vs Variable temp

[BronYrAur]

I think this is a straight forward heat transfer-101 question, but that was a long time ago for me. I have a free cooling heat exchanger on a chilled water system and was thinking of adding a VFD to the chilled water flow entering the heat exchanger. It acts as a primary "chiller" when ambient permits and is connected through closely-spaced Tees to a bridge with a variable flow secondary system on the other side (not shown). Please see my attached sketches. I understand that if the secondary flow exceeds the primary, I will lose temperature, so I don't want this thread to go down that road. My concern here is what happens when the system is primary-strong.

The top sketch is the design condition. A constant volume of 60 deg water enters and is cooled to 46 deg. The other side of the plate goes to a cooling tower which delivers 43 deg water. The cooling tower is constant volume. I know that return water temp back to the tower will vary with each scenario, but let's assume a constant 43 degree and constant tower flow for this discussion.

When I go primary-strong, some of the water leaving the Hot Side will recirculate backwards through the bridge and blend with the water entering the hot side. So in the Option A sketch, I show a hot side inlet of 53 deg. What temperature will I now get leaving the hot side? I'm not looking for an exact number, I just want to know if it would be higher or lower than the 46 degs.

The Option B sketch would throttle Hot Side flow to avoid blending. So in that case, I still have the design 60 degree entering water, but my flow is less. I used half of the flow as an example. Again, what can i expect to happen with my temperature leaving the hot side?

In all cases, the temperature going back to the tower will be different, so I am having trouble with the heat balance. The tower side will remain constant flow and constant 43 degree entering water.

I tried making a heat exchanger selection. But, the software won't allow me to hold a selected unit constant and see how it performs under various conditions. I am only able to make new selections each time and try to compare them.

Again, I'm not looking for calculations of those exact hot side dischange temps, I just want to know how they change relative to the design condition so I can decide whether or not to vary the hot side flow. Which Option will give the lower discharge temperature on the hot side?

Thanks

 

[MatthewL]

In both cases, I would expect the outlet of the hot side to be less than 46 but more than 43 degrees. However, the B option outlet temp may be higher than the A option because heat transfer is related to fluid velocity due to mixing effects. To maintain fluid velocity (and therefore mixing) use the first case with a recycle stream. Don't worry about the tower heat balance, since you are looking at reduced loads, and you know the tower can already meet the higher load.

Matt

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