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Heat Exchangers For Solar Heating Systems

Heat exchangers are devices that transfer heat from one fluid (liquid or gas) to another while keeping them separate. A hot fluid flows on one side of a heat exchanger's metal partition, transferring its heat to a lower temperature fluid flowing on the other side. The size of a heat exchanger's surface area affects it speed and efficiency: a large surface area transfers heat faster and more efficiently.

Many machines and household appliances contain heat exchangers. A car radiator is a heat exchanger: it transfers heat from the engine cooling system to the outside air, preventing the engine from overheating. Refrigerators and air-conditioners have heat exchangers to dissipate heat; furnaces have heat exchangers to heat the cool air that enters the furnace. Solar systems also use heat exchangers to transfer solar energy absorbed in solar collectors to the liquid or air that is used to heat the water or space.

Heat exchangers can be made of steel, copper, bronze, stainless steel, aluminum, or cast iron. Solar heating systems usually use copper, because it is a good thermal conductor and has greater resistance to corrosion.

Types of Heat Exchangers: Solar heating systems use three types of heat exchangers: liquid-to-liquid, liquid-to-air, and air-to-liquid. Many liquid solar energy systems use a liquid-to liquid heat exchanger, which uses a heat transfer fluid that circulates through the solar collector, absorbs heat, and then flows through a heat exchanger to transfer its heat to water in a storage tank. Heat transfer fluids, such as antifreeze, protect the solar collector from freezing in cold weather. Liquid-to-liquid heat exchangers have either one or two barriers (single wall or double wall) between the heat transfer fluid and the domestic water supply.

A single wall heat exchanger is a pipe or tube surrounded by a fluid. Either the fluid passing through the tubing or the fluid surrounding the tubing can be the heat transfer fluid, while the other fluid is the potable water. Double-wall heat exchangers have two walls between the two fluids. They are often used when the heat transfer fluid is toxic, such as ethylene glycol (antifreeze). They are often required as a safety measure in case of leaks, helping ensure that the antifreeze does not mix with the potable water supply. An example of a double-wall liquid-to-liquid heat exchanger is the "wrap-around heat exchanger," in which a tube is wrapped around and bonded to the outside of a hot water tank. The tube must be adequately insulated to reduce heat losses.

While double-wall heat exchangers increase safety, they are less efficient, because heat must transfer through two surfaces rather than one. In order to transfer the same amount of heat, a double-wall heat exchanger must be larger than a single-wall exchanger.

Liquid-to-air heat exchangers transfer heat from a liquid solar heating system to a home's forced air duct system. The most common type is the finned-tube heat exchanger, which is a finned water coil inserted in the air duct. Supply air from the fan blows over the coil and absorbs heat from it. The heated air is then distributed throughout the house.

Solar heating systems with air heater collectors usually do not need a heat exchanger between the solar collector and the air distribution system. Those systems with air heater collectors that heat water use air-to-liquid heat exchangers. These are similar to liquid-to-air heat exchangers.

Heat Exchanger Designs: There are many heat exchanger designs. One commonly used design in solar domestic water heater systems is the coil-in-tank design. The heat exchanger is a coil of tubing in the storage tank. It can be a single tube (single-wall heat exchanger) or two thickness of tube (double-wall heat exchanger). A less efficient alternative is to place the coil on the outside of the collector tank with a cover of insulation.

Another popular design is the shell-and-tube heat exchanger. The heat exchanger is separate from the storage tank. It has two separate fluid loops inside a case or shell. The fluids flow in opposite directions to each other through the heat exchanger, maximizing heat transfer. In one loop, the to-be-heated fluid (such as potable water) circulates through the inner tubes. In the second loop, the heat transfer fluid flows between the shell and the tubes of water. The tubes and shell should be made of the same material. When the collector or heat transfer fluid is toxic, double-wall tubes are used, and a non-toxic intermediary transfer fluid is placed between the outer and inner walls of the tubes.

A very efficient type of heat exchanger is the tube-in-tube design. In this design, the tubes of water and the heat transfer fluid are in direct thermal contact with each other. The water and the heat transfer fluid flow in opposite directions to each other. This type of heat exchanger has two loops similar to those described in the shell-and-tube heat exchanger.

Areas of Concern: A heat exchanger must be sized correctly to be effective. There are many factors to consider for proper sizing. These include: the type of the heat exchanger; the characteristics of the heat transfer fluids (specific heat, viscosity, and density); the flow rate; and the inlet and outlet temperatures for each fluid. Usually, manufacturers will supply heat transfer ratings for their heat exchangers (in Btu/hour) for various fluid temperatures and flow rates.

For the best performance, always follow the manufacturer's installation recommendations for the heat exchanger. Be sure to choose a heat transfer fluid that is compatible with the type of heat exchanger you are using. If you are building your own heat exchanger, be aware that using different metals in heat exchanger construction may cause corrosion. Also, because dissimilar metals have different thermal expansion and contraction characteristics, leaks or cracks may develop. Either of these conditions may reduce the life span of your heat exchanger.

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