Performance of Plastic Pipe-Coil Heat Exchanger

Our solar water heating system uses a 300 ft coil of 1 inch diameter HDPE plastic pipe for the heat exchanger. The coil of pipe is immersed in a non-pressurized water tank that is heated by a solar collector. Cold water enters one end of the pipe coil, is heated as it goes through the pipe coil by the hot storage tank water, and comes out the other end preheated. One nice feature of this arrangement is that the pipe coil itself holds about 12 gallons of water, and this water will normally be heated up to the full storage tank temperature at the start of a water draw. Most hot water demands will be satisfied with this already heated water. Another nice thing about it is that a 300 ft coil of 1 inch HDPE, NSF approved for potable water can cost as little as $70.

Updates:
I've since changed from the 300 ft, 1 inch diameter coil of HDPE to a 300 ft, 1 inch diameter coil of PEX. I don't think that the performance is likely to be measurably different.
The PEX coil allows the tank to be run to a higher temperature for more heat storage, and I (perhaps imagined) that the HDPE contributed a little bit of odor to the water when showering.

March 2012: did a longer test with higher flow rates to see how well the big coil of PEX holds up for the somewhat unusual (but important) case of very long hot water draws and higher flow rates. The test results are here...
Gary

This approach for a heat exchanger came out working with Nick Pine on the development of our horizontal pond solar water heater.

Since this is our first real application of this unique heat exchanger I did the small test below to characterize its performance for our domestic solar water heating system.

This small test attempts to find out:

Does the initial 12 gallons of water already in the pipe coil come out a full storage tank temperature?
After the initial 12 gallons is used, how good a heat exchanger is the 300 foot pipe coil. That is, how much is the incoming cold water heated up in one trip through the pipe coil?
How much does the pipe coil HX performance fall off with very high flow rates?
How long after a big water draw does it take the pipe coil to heat back up to tank temperature for the next draw?

The reddish X's are the storage tank temps measured about 1/4 up from the bottom (F)

The blue + marks are the outlet temperature from the pipe coil heat exchanger (F)

The green triangles are the flow rates through the pipe coil heat exchanger (gpm)

Test Plan:

Using a typical shower flow rate of 1.3 gpm of hot water (maybe 2 gpm after mixing with some cold), go through this sequence.

Confirm that 12 gallons already residing in the pipe coil will be delivered at storage tank temperature.
See what happens to the pipe coil outlet temperature once the water that was already in the pipe coil is replaced by new cold water coming in the pipe coil inlet -- e.g. how does the pipe coil function as a heat exchanger.
Try a high flow rate to see how much temperature drops.
Shut the flow off for a few minutes, and then turn it back on to see how far the pipe coil recovers back up to storage tank temp.
Now give the dog a bath in all that warm water left in the bathtub.

The plot above shows the results of going through the first 4 steps listed just above:

Step 1: Minute 0 to 11 -- initial water resident in coil:

The pipe coil outlet temperature and the storage tank temperature are the same because the 12 gallons of water residing in the pipe coil has been sitting there a while and has warmed to the same temperature as the storage tank water. Until all the preheated water in the pipe coil is exhausted, the pipe coil will deliver water at the full tank temperature. The plot shows this happening as expected.

Step 2: Minute 12 to minute 20 -- pipe coil as heat exchanger:

The water in the pipe coil has been exhausted, and cold water coming into the inlet end of the pipe coil at 50F is being heated to about the tank temperature minus 3F in the single pass it makes through the pipe coil. This is at a flow rate of 1.3 gpm, which is typical of a shower. It would presumably go along in this mode for a long long time. During this time periods, and with 1.3 gpm flow, the pipe coil heats the water from 50F to about 127F with a tank temperature of 130F -- or, about (127-50)/(130-50) = 96% of the maximum possible -- not so bad.

Step 3: Minute 20 to 25 -- high flow rates:
The flow is turned up to the maximum available -- around 4 gpm. After a while, the pipe coil outlet temperature drops significantly. This is because the there is no hot water already stored in the pipe coil, and the flow rate is sufficiently larger that it goes through the pipe coil to quickly to be heated to tank temperature. Even here though, the storage tank is providing significant heat. The tankless heater under these circumstances will kick on when its inlet temp drops below the 108F that it is currently set at. The tankless burner will attempt to heat it so that its outlet temperature is 108F.

If there had been no flow through the pipe coil for a few minutes before this high flow rate segment, then the pipe coil outlet temp would have been right at the tank temp for the first 3 or 4 minutes, and then it would begin to drop significantly.

So, if you have sustained high flow rates this may not be the best solution, although I suspect it would still be capable of extracting all the energy in the storage tank.

Minute 25 --

Shut off water flow.

Step 4: Minute 32 to minute 35 -- pipe coil recovery time:

After 7 minutes of no flow, the flow is turned back on at 1.3 gpm. This is to see how much the pipe coil temperature which had gone down below 100 F would come back to tank temperature. The pipe coil outlet temperature appears to be within about 3F of the tank temperature, so in about 7 minutes it has made an nearly full recovery.

Additional Notes:

All the data from this test was hand recorded from these sources:

Storage tank temp is as displayed on the Steca dif cont display

Pipe Coil outlet temp is the from the Tagaki tankless water heater display (its inlet temperature)

Flow rates are also from the Tagaki tankless water heater

I've not checked any of these for accuracy, but the test does not really require very good accuracy.

The Steca differential controller (top) displays the tank temperature. The tank temperature sensor is placed about 1/4 of the way up from the bottom of the tank.

The Takagi tankless water heater remote (bottom) will display the temperature coming into the tankless heater, the temperature leaving it, and the flow rate through the heater.

Since the temperature coming into the heater is essentially the same as the temperature leaving the pipe coil HX, this was used for the pipe coil temperature. The flow rate was used for the pipe coil flow rate.

Material on the heat exchanger pipe coil and the storage tank...

Material on the full solar water heating system...

Gary September 26, 2008