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Liquid-to-liquid heat exchangers are used in a solar water heating system to transfer the heat from the solar fluid into the domestic water.

Most heat exchangers are single walled. A single-walled heat exchanger has a single membrane between the two fluids. This membrane could be made of copper, stainless steel, or (in very specific circumstances) Pex. Double-walled exchangers afford an extra layer of protection to the potable water in case the heat exchanger ever develops a leak. Many have a gap between the layers, commonly referred to as “positive leak detection,” so that a leak in a wall will clearly be visible, and the heat exchanger can be repaired or replaced. Single-walled heat exchangers are typically more efficient because the heat needs to be conducted through less material. However, some municipalities may require a double-walled heat exchanger whenever domestic water is involved in the exchange. Rules requiring double-walled heat exchangers are often a carryover from the early years of our industry, when toxic liquids were used as heat transfer fluids. There is no need for a double-walled exchanger if a nontoxic solar fluid is used.

Figure 4.4: Plate heat exchanger

Heat exchangers can be constructed using either pipes or plates. Two fluids are passed next to each other, separated by a membrane. Heat transfers across the membrane from one fluid to the other. Always plumb heat exchangers in a counter-flow arrangement, as this is the most efficient way to transfer heat. This means that the two fluids will need to flow in opposite directions when passing through the heat exchanger. This allows for the greatest difference in temperature between the two fluids, which increases the rate of heat transfer.

A plate heat exchanger consists of a number of plates spaced apart and capped around the sides. Separate waterways are designed into them to allow the different fluids to pass through adjoining spaces in a zig-zagging counter-flow pattern. This back-and-forth arrangement means that plate heat exchangers always require a pump on both waterways, as they will not convect. This type of exchanger can pack a large amount of surface area into a small package. Because heat transfer is directly related to the amount of surface areas that are available, you are unlikely to have too much heat transfer surface area. Because they have so much surface area in such a small package, plate heat exchangers also have very small waterways. Consequently, in hard-water conditions, scale can easily build up, causing reduced heat transfer or clogging. When installing one of these, you may want to place boiler drains and isolation valves on the domestic water side pipes to provide the means to periodically flush out the heat exchanger. Remember to always use a nontoxic solution when doing so. We prefer a product that is enzyme-based, making it biodegradable, non-corrosive and safe for septic systems.

A tube-in-shell heat exchanger is essentially a smaller pipe (or pipes) inside a larger pipe or tube. One fluid is circulated through the inner pipe(s) and the other fluid circulates through the outer pipe. The material of the inner tube separates the fluids. Tube-in-shell heat exchangers come in either single-wall or double-wall versions. These heat exchangers can be configured in straight lengths or coiled. The coiled configuration always requires a pump like the plate exchangers, but the straight ones can thermosiphon on the water side if properly designed for that purpose.