130 000 BTU Heat Exchanger for Salt Water

Swimming pools that utilise Salt for water purification cannot be connected directly to solar panels - this is because Salt can attack copper. The solution is to fit a stainless steel heat exchanger. These are available in various sizes up to 460 000 btu. For solar use, heat exchangers should be rated at least one size larger than for a conventional heat source. Pipe fittings are 1 bsp on solarboiler input and output and 1.5 bsp on pool connections. Both connections are male fittings.
Navitron Pool Heat Exchangers have 1 BSP connectors for boiler and 1½ BSP for pool connections as well as a sensor pocket at the top. They should be fitted downstream of the pool pumpfilter unit.

The procedure adopted for rating pool heat exchangers requires high flow high temperature input. This is unrealistic of solar installations, which would not run efficiently under these conditions. Therefore it is customary to over-size pool heat exchangers when used with solar. Assuming you maintain the flow rates quoted in the table below, you can assume that the 170 000 BTU heat exchanger (50kW) will be suitable for up to 6 x 20tube 47mm Navitron solar panels. However, if flow rates are reduced, then output should be derated accordingly.

Rated Output: BTU 130,000
KW: 38.1
Filter Secondary Flow: gpm 31 M3hr 9
Boiler Primary Flow: gpm 11 m3hr 3
Primary Water Design Flow Temperature: degC 82
Primary Water Design Return Temperature: degC 71
Primary Head Loss: ft 2.5 mb 95

DIMENSIONS
Width: mm 118
Length: mm 417
Depth: mm 140
P Primary Connections BSP Male Thread: In 1?
S Secondary Connections BSP Male Thread: Ins 1.5?
Weight: Kg 4.5

If you wish to explore this further, the following equations may be of help in calculating the heat exchanger performance:
The rate of heat transfer Q = U x A x dTmean = kW
U (kWm2 K) = 1Rt  and Rt  = total thermal resistance (m2 KkW)
A = surface area (m2)
dTmean = True mean temperature difference (K)

The temperature of both the primary (Th) and the secondary (Tc) fluids will generally vary as they pass through the heat exchanger and you need to know the true mean dT which is not the same as the dT between the boilersolar flow and the pool water.  The heat transfer will also vary depending on whether it is a parallel or counter-flow heat exchanger and you may need to calculate the log mean temperature difference (LMTD) depending on the resulting dTmax and dTmin.

For a parallel flow heat exchanger dTmax= Th1-Tc1 and dTmin= Th2-Tc2

For a counter flow heat exchanger dTmax= Th1-Tc2 and dTmin= Th2-Tc1

Th1 Th2 = Primary (boilersolar side) inletoutlet temp.
Tc1 and Tc2 = Secondary (pool side) inletoutlet temp.

LMTD dTmean= (dTmax - dTmin) In(dTmax dTmin)

If dTmax=dTmin, then LMTD dTmean = O, so therefore the arithmetic mean temperature difference is used
(many thanks to Lightfoot for providing these)