Gasketed plate and frame type heat exchangers can be used for swimming pool heating.
Used with a hot water boiler, heat pump or any other hot water source.
Advantages of using a gasketed plate:
- The gasketed PHE can be opened for cleaning and inspection purposes whilst other types of heat exchangers cannot.
- The gasketed plate can be manufactured with wetted metal parts of 316 stainless steel for standard pool use, or titanium for use with saltwater pools. Other types of heat exchangers may contain copper which will eventually corrode away due to the acids used in the pool water to adjust / stabilise the PH level.
- The heat exchanger does not have to be sized to accept the full pool water circulation rate – it can be used on a divert loop to good effect. This feature can offer great cost savings.
- High heat transfer rates that only require a small physical size and low water hold up volume.
How do they work?
The heat exchanger has two circuits. One for the boiler water and the other for the pool water. This means that heat from the boiler water will be transferred inside the plate heat exchanger across to the pool water, without the two liquids ever mixing.
A gasketed plate heat exchanger can be used to provide heat from a boiler, heat pump, or any other hot water source to all sizes of swimming pools ranging from large municipal swimming pools down to small garden sized pools.
The heat exchanger is external to the pool and can be located anywhere that it convenient. The hot water supply and the swimming pool water are then pumped through the heat exchanger.
Sizing the amount of heat required:
A common method used to determine the heat input is to base the calculation upon a temperature rise of 0.5°C per hour. This figure is used because it does not allow the pool to be heated too fast. Rapid pool heating can cause damage to the pool structure if it is of concrete with a tiled lining. If the temperature is raised too quickly then the concrete and the tiles expand at a different rate, and this can cause the tiles to crack and/or fall off.
Equation to use:
If the pool is located un-covered outside, then change the 1.1 in the above equation to 1.25.
Heat Exchanger Sizing:
Especially in larger pools, the swimming pool recirculation flow rate is relatively high as the entire pool volume is turned over one, or more, per hour. It is not necessary to use a heat exchanger that can accept the full recirculation rate of the swimming pool. To cut down the size and cost of the heat exchanger, it is recommended that the exchanger is used on a divert loop off the main circulation pipe work. The pipe work on the divert loop can be down sized and valved to suit the heat exchanger.
As the portion of the diverted flow goes through the heat exchanger, it is effectively “over heated” to a temperature of 50ºC but when mixed back into the main circulation loop, the temperature is brought down to the safe / required limits.
Mixed end temperature example calculation:
A swimming pool requires 244 Kw of heat input and has a pool circulation rate of 100 m³/hour.
Temperature of pool required is 29 ºC.
If a heat exchanger was put directly in line to this flow rate, then the connection sizes would have to be large enough to be fitted with DN100 or larger sizes of connections. Therefore, to reduce size and cost of the exchanger in this example, 10% of the circulation flow is diverted through the exchanger.
Main pool circulation rate: 100 m³/hour
Diverted flow to heat exchanger: 10 m³/hour
Amount of circulated flow not heated: 90 m³/hour
Inlet to heat exchanger from loop: 29 ºC.
Outlet from heat exchanger: 50 ºC
Circulation loop temperature: 29 ºC
Equation to use:
Using our example figures from above:
In this example, by using a diverted flow rate of 10 m³/hour, it is possible to use a much smaller heat exchanger than what would be required if the full circulation loop flow rate had to pass through the heat exchanger. The other advantage is that the divert loop can be fitted with isolation valves to enable the heat exchanger to be serviced if necessary without the need to drain down large volumes of the swimming pool water (gasketed heat exchangers tend to have a low hold up volume).
The heat exchanger selection inlet temperature to use:
Assumption: the desired swimming pool temperature is 29 ºC, and has to be maintained at this temperature.
To correctly size the heat exchanger, the inlet temperature of the pool circuit into the heat exchanger must be the same as the temperature that the pool has to be maintained at (in our example this is 29 ºC.
If this is not the case, then the heat exchanger has not been sized correctly and is potentially undersized. Using an inlet temperature for the selection of the heat exchanger that is lower than the desired temperature that the pool has to be maintained at, means that the pool is to operate at this lower temperature.
Hence for correctly selected equipment, it is important to use the correct inlet temperature to guarantee best results.