Heat exchangers

In the solar hot water system heat exchangers transfer the heat from one moving fluid to another. To achieve the temperature transfer the fluid from the collector will be hotter than the leaving temperature of the secondary fluid. This higher temperature from the collector has a negative effect on system performance. To keep this temperature difference as small as possible a plate-andframe heat exchanger is used. Where this temperature difference is not a large concern a shelland-tube heat exchanger can be used.

When the solar thermal system is used to heat domestic water there must be a guarantee that the heat transfer fluid is kept isolated from the potable water source. This is normally accomplished by separating the two fluids by a vented open space, or by using a third fluid that could be monitored for fluid leakage. The double wall or additional heat exchanger also protects the solar collector fluid from being diluted by water and becoming prone to freezing. (Refer to Section 5.3.5 [p 80]).

All heat exchangers have a small pressure drop of the fluid passing through them. This must be considered in selecting the pumps. Their materials of construction need to be compatible with the heat transfer fluids and with other materials in the piping system. They also need to be able to withstand the temperatures and pressures that will be experienced in the system. To achieve good heat transfer and to adhere to the other requirements, stainless steel and copper are the normal materials used in these heat exchangers.

The pipe design at the heat exchanger that transfers energy from the collector to the storage tank may need a by-pass pipe circuit around the heat exchanger for the collector heat transfer fluid if an antifreeze solution is used. The antifreeze solution on cold days during system startup could be several degrees below freezing and thus could freeze the water in the heat exchanger from the storage tank when flow begins. The pipe bypass would be opened when the collector pump is started. The valves would be closed to allow flow through the heat exchanger when there is usable heat in the heat transfer fluid (a temperature ~80 °F [27 °C]). This action will also avoid cooling the heat exchanger during the initial flow of the collector heat  transfer fluid that was downstream of the solar collectors.

A comparison between the two types of heat exchangers will show the following results. Plate-andframe heat exchangers have small passageways and thus have a higher pressure drop and are more prone to flow blockage due to contaminants in the circulating fluids. They have a quicker reaction time and typically take less space in the piping system. A shell-and-tube heat exchanger can handle fluids having more contamination. These heat exchangers are easier to clean and have a lower pressure drop for the fluids flowing through them.

Selection of the size of the heat exchanger depends on the amount of heat that must be transferred and the desired leaving temperature. An increased heat transfer surface area is needed for improved performance. Note that heat exchanger ratings are established using water as the heat transfer fluids. With the use of an anti-freeze fluid, heat exchange performance will decrease and the exchanger surface area will need to increase. A water-glycol mixture has a higher viscosity and lower heat capacity. Heat exchanger manufactures can provide useful information to aid selection.