By Steve Hamoen
Energy efficient heating systems are here to stay. Condensing boiler units as well as geothermal and solar thermal heating systems can produce considerable savings in their operation. However, they may not operate at maximum efficiency if the systems they serve are not designed to use less energy. As a designer, one must consider any limiting steps that could impede a system from operating at peak efficiency.
Heat exchangers typically used in pool and spa heating are instrumental in transferring heat from the source (heating system) to the load (pool/spa). If they are not sized, designed or built to the proper specifications they will not make efficient use of the heating source, or worse yet, fail to satisfy the load demand.
There are several types of heat exchangers on the market, including:
- Shell and tube (Type 1):
- Spiral (Type 2); and
- Braised plate and plate and frame gasket (Type 3).
When determining what design should be used on a particular project, especially if energy efficiency is crucial, it is important to look at the heat exchanger’s performance chart first to ensure the type selected is designed for efficient pool heating.
‘Type 1’ and ‘Type 2’ heat exchangers (listed above) historically have rapidly declining scales of efficiency as the entry water temperature drops. These heat exchangers are commonly applied to higher temperature applications and do not work well for high-efficiency, low-temperature applications such as geothermal, solar or condensing boiler systems.
Type 3’ heat exchangers, however, are better suited for low system EWT due to their design and larger surface area for heat transfer. Increasing water flow (litres/gallons per minute [lpm/gpm]) or the number of plates in the system will cause the load leaving water temperature to be closer to the source EWT from geothermal, solar or boiler systems. This greatly increases efficiency by permitting the use of low EWT.
However, consideration should be given to the implications of the increased pressure that is required to pump larger volumes of water through more plates on a small heat exchanger. Therefore, careful pump selection is critical to success in this area.
Ensure all wetted areas and connections are made of a material that is resistant to corrosion, which may result from the liquid (e.g. water, saltwater, chlorinated/brominated water or antifreeze) passing through the system. Other component materials in the system may contribute to corrosion effects by their reaction with the liquid stream.
Make sure the overall heat transfer is sufficient in providing heat quickly enough to satisfy the customer. For example, the system may have enough energy to maintain the heat in a spa/hot tub, however, if it takes several hours to heat the water, the customer may be unsatisfied.