Sizing a Heat exchanger with Low Delta T for pool heat (geothermal heat pump)
Hello, I'm having some issues sizing a heat exchanger on a project. I originally left this to my hydronic engineer and he is really good at what he does but there have been a lot of Maxi-Flo MF-200 (200,000 BTU) purchases recently and large heat exchangers are expensive. He has a good point though when I asked him how he concluded a MF-200, his response is that the load of the pool is constantly changing but only a high would be required because the lower the water temp the larger the Delta T and the smaller the heat exchanger (read on).... Regardless, I would assume that just like any load calc design conditions need to be set to some extent so would it be bad to have a range of maybe a low temp of 60f and a high 80f??? There's a point that the geothermal heat pump is out of range anyways and a boiler would be required to get the pool up and geo to maintain. He may have sized it correct but I'm not surprised that his selection is the largest in the residential product line.... I am probably going to use Water Furnace's NSW050 hydronic geothermal heat pump to heat a small swimming pool. The issue I have is that when a company like Maxi-Flo rates their pool heat exchangers (just a basic rating) they rate them for use with a boiler or something similar for quick selection and a boiler has a much larger delta T then a heat pump.
So for example; if I was theoretically heating a pool and had a 100 degree f delta T running through the heat exchanger more heat would be absorbed by the load than if I was in the same scenario with a smaller delta T (let's say 30f) due to the larger temp difference. Therefore I would require a larger Heat Exchanger to get the job done.
So in short Maxi-Flo's MF-80 that is rated for 80,000 BTU's is not going to transfer 80,000 BTU's when used with a geothermal heat pump or any heat pump for that matter under the normal conditions (not even close). I think the Water Furnace NSW050 under normal conditions will produce 62,000 BTU's. I am setting up a bypass on the pool side (load side) so that I can control the GPM as well as isolate the heat exchanger when they are shocking the pool and I am using a Grundfos UP26-99FC 3 speed circulator on the source side installed using 1 1/4" type L copper pipe. I haven't received the designed flow rates yet but I should be moving quite a bit of water with the short run, the 3 speed version of the Grunfos UP26-99 pump isn't as powerful as the single speed version but the pump curves are only slightly off. Regardless, I am looking for a formula to calculate this and leaves a spot that I can input the GPM at that time if required (I'm assuming it would have to be required for this calculation?). Water Furnace sent me a document but it was on their heat of extraction/rejection and did not relate to what I was asking. Maybe I should be on a interpersonal communications forum instead on an HVAC forum cause I am horrible at explaining myself...... Any help is appreciated. Thanks.
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