Type of heat exchangers used

There are two types of heat exchangers to chose from: plate-and-frame, and tube-and-shell. Plateand-frame heat exchangers are most economical for commercial systems as they take up little space and do not need insulation. They provide the highest approach temperature (the leaving temperature of the secondary fluid compared to the entering temperature of the hotter primary fluid). Plate-and-frame exchangers need very clean surfaces to obtain this performance and thus require a nightly reverse backflush to break free any accumulated deposit on the water side.

Tube-and-shell heat exchangers are a good option for light commercial applications since they are also practical in this setting (in terms of size), and because they have having good resistance to fouling since the water passages are large. The leaving temperature of the secondary fluid will be lower with this type of heat exchanger.

Conversely, water quality also affects system components; for areas with hard water (hardness > 100ppm), a closed loop or water softener should be used.

Generally stainless steel or copper is selected as a material for heat exchanger construction because of their good heat transfer properties and corrosion resistance.

If the heat transfer fluid is not safe for human consumption or is toxic (like ethylene glycol) then a double wall heat exchanger is required. Figure 5.11 shows an example of this type of heat exchanger where a volumetric space is filled with a nontoxic heat transfer fluid between the two circulating fluids.

A leak in one side would become visible and the other fluid could not become contaminated. Leak detection would involve noting a change in the fluid level in the interim space or a change in color of the interim fluid. When plate-and-frame heat exchangers are used leak detection can be provided by an additional heat exchanger circulating loop filled with a colored fluid for detection fluid between the glycol used in the collector circulating loop and the domestic water system. A check of the current building code should be made to determine the acceptable method of isolation.

To help minimize pumping energy, the pressure loss of the heat transfer fluids passing through the heat exchanger should be limited to 1 to 2 psi (6.9 to 14 kPa). Heat exchangers used to heat domestic water are exposed to the potable water pressure and thus should be rated for that pressure, typically above 75 psi (517 kPa).