Optimal Flow Rate Heat Exchanger

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whisper

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Aug 6, 2008
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Hi,
I'm a new pool owner with a fairly advanced question. I got into a discussion today about flow rates and heat transfer coefficients. I was discussing my disappointment with the fact that when running my natural gas heater, the water coming out the returns isn't very hot(perhaps about 10-15* hotter at full heat). I know that this will heat up the pool over an extended period of time. However, I blamed it on the fact that I currently have a 3.5 HP spa/pool pump running my recirculation system. I claimed that the water was running so quick through the heater, it didn't have time to heat up much. Someone else claimed that as long as I was having turbulent flow (vs. laminar), the transfer of heat was about the same, regardless of flow rate. Does anyone have any input on this? If it matters, my pump is plumbed for 2" lines, but everythingon my pool is 1.5", so everything is reduced to that at the pump. Thanks in advance for any input.


Chris
 
Generally speaking, the faster the flow rate the more efficient the heat transfer, but the efficiency has to do with the temperature difference between the heat source and the fluid being heated so there are diminishing returns at faster and faster flow rates. The rate of heat transfer between two points is a function of the thermal conductivity of the material between the two points and the temperature difference between the two points (and the amount of area of contact). A faster flow rate means the temperature of the water doesn't rise as much so that makes the rate of heat transfer somewhat higher.

The fact that the amount of temperature rise is lower at the faster flow rate doesn't mean the pool water rises in temperature more slowly. On the contrary, it rises a little faster since more heat is transferred as noted above. Ignoring the efficiency improvement noted above, the same heat is transferred at all flow rates and temperatures. The product of the flow rate and the temperature rise is proportional to the amount of heat transferred so a doubled flow rate with half the temperature rise is the same amount of heat.

If you lowered the flow rate in your system, the temperature of the water flowing through the heater would rise more but the amount of heat transferred would be somewhat less due to the lower efficiency of transfer in the gas heater due to the smaller temperature difference between the gas heat and the water (since the water is now hotter). If you want more efficiency of heat transfer, an even faster flow rate would be a little better. The limit of heat transfer is the BTU output rating of your heater. By the way, 10-15F is not low or unusual. If your turnover rate were 5 hours, that would mean a temperature rise of 2-3F per hour which is pretty fast for a pool. Generally if you want to heat a pool that is too cool, you need to run the gas heater overnight. It's not like a spa where the temperature comes up much faster (but the volume of water is much smaller).

I have a gas heater that has an output rating of 200,000 BTU/hr. which would theoretically heat my 16,000 gallon pool at 1.5F per hour. The turnover rate is around 6.7 hours at 40 GPM so the pool rises in temperature by 10F in one turnover so that is the temperature difference coming out of my returns.

I'm not sure where your plumbing is 2" and where it's 1.5" from your description. Generally, the 2" would be out of your pump and when it splits into individual lines for each return it would be 1.5".

Richard
 
A 3.5 H.P pump is way oversized for your system. You need to match the flow rate of the pump to match that given by the heater and the filter. Too much flow will damage the heater. Although the heater should have an internal bypass, it is not necessarily reliable. Too much flow through the heater will cause mechanical erosion of the heat exchanger. Your pump should be a 1.0 Hp, not a 3.5. You should have a separate pump for the hot tub jets. 3.5 HP will also consume way too much electricity. As long as your flow rate to the heater is within the specified rates given by the manual, your heater will do fine.
 
Thank you all for your replies. I know the pump is way oversized, I have a 1HP with a bad seal plate waiting to go in. It is definitely a bad seal plate, the PB tried replacing the seals. The liner was just replaced and when we hooked up the pump, we realized there was a problem. I happened to have a bad hot tub that I could "borrow" a pump from temporarily. I bought the house from the bank, so I didn't know the status of any of the equipment. The fitting on the filter basket inlet and pump outlet are 2" reduced to 1.5".

Richard, thank you for your response, it's been about 8 years since I took fluid dynamics and it was more than a little rusty. Your post is exactly what I was looking for.


Chris
 
To put it simply, if your heater is rated for 200K BTU's per hour, then that is how many BTU's the pool will get. Whether it's warm water flowing fast or hot water flowing slow, the net effect is that 200K BTU's of energy will be added to the pool each hour the heater is in operation. :)
 
HarryH3 said:
To put it simply, if your heater is rated for 200K BTU's per hour, then that is how many BTU's the pool will get. Whether it's warm water flowing fast or hot water flowing slow, the net effect is that 200K BTU's of energy will be added to the pool each hour the heater is in operation. :)
Click to expand...

Heater are only between 60% and 80% efficient, and the efficiency is lower at lower flow rates, so this isn't really true. Just about the best you can hope for getting into the water is 160,000 BTU, and most heaters are worse than that.
 
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The heater rating that was referred to could have been its output rating. My gas heater, for example, is rated at 250,000 BTU/hr. input with 200,000 BTU/hr. output so is 80% efficient -- typical for standard not very expensive gas heaters. There are more expensive gas heaters that have 95% efficiency. From measurements of the temperature rise in my pool, I would say that the output rating is a best-case rating at high flow rates with no other losses (in the pipe on the way to the pool, for example)
 
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