Airtoair heat exchanger or heat recovery systems

Such systems are used to transfer heat from the exhaust air of a ventilation system to the supply air. Various approaches to air-to-air heat recovery are common. Some are able to transfer latent heat, while others can work in reverse mode and provide cooling. Air-to-air heat recovery systems are used in conjunction with mechanical balanced ventilation, incorporating separate supply and exhaust networks. There are four major technical types of heat exchangers on the market:

1 plate heat exchangers;

2 run-around coils;

3 heat wheel systems; and

4 heat regenerators.

Plate heat exchangers (HX) are static devices (that is, they contain no moving parts). They consist of layers of separated, interleaved flow channels through which the supply and exhaust air flows. The channel walls or plates are constructed from very high conducting material (usually metal, but also various types of plastic may be used) across which heat rapidly transfers.

The efficiency of a plate heat exchanger is primarily associated with the flow configuration of exhaust and supply air, the spacing between plates, the surface area and the type of surface (for example, roughness can promote turbulence and enhance heat transfer coefficients). Parallel counter-flow can produce a maximum theoretical heat recovery of 100 per cent (less fan energy), while performance is reduced to a maximum of 50 per cent if exhaust and supply air flow run in the same direction. For optimum HR, combined with ease of manufacture and installation, a cross-flow system is commonly used. These systems can have thermal efficiencies in excess of 70 per cent. Fans are usually located on the supply and extract side so that air is pulled through the heat exchanger. This minimizes the pressure difference between the two air streams and, therefore, reduces the risk of cross-contamination. Heat generated by the extract fan, however, is lost to the outgoing air stream. Plate heat exchangers are used in dwellings and in other environments in which the supply and exhaust ducts can be brought closer together. They are very popular in countries with severely cold climates, as in Scandinavia and Canada.

Run-around coils are comprised of two fin-type heat exchangers, one of which is installed in the supply air and the other in the exhaust air duct. A liquid (normally a water/glycol solution) is used as the heat transfer medium and is continuously pumped between the heat exchanger by using a circulation pump. Heat in the exhaust air stream is thus transferred to the supply air via the heat exchanger. Performance is primarily related to the number of coil rows although, eventually, there is a trade-off between the benefit of additional coil rows and the extra fan energy needed to overcome increasing pressure drop.

This fan-coil approach is useful when fresh air and exhaust ducts are not adjacent to each other and, hence, often has important retrofit applications. Multiple supply and exhaust systems can be combined by a single loop. The true benefit of this technique, however, refers to larger buildings and industrial applications.

The advantage of run-around coils is that:

• the supply and exhaust air streams are totally separated; therefore, the risk of cross-contamination is eliminated.

The disadvantages of run-around coils are that:

• this type of system can only generally transfer sensible heat and has a limited thermal of 40 per cent to 60 per cent;

• the additional energy needed to operate the circulation pump has to be offset against energy savings; and

• the circulation pump presents additional maintenance requirements.

Heat wheel systems are comprised of a revolving cylinder divided into a number of segments packed with coarsely knitted metal mesh or some other inert material. The cylinder rotates between 10 and 20 times per minute, picking up heat in the warmer exhaust stream and discharging it into the cooler supply air stream. Some heat wheels contain desiccant materials that enable latent heat transfer to take place. This is especially useful in an air-conditioned environment where the system can be operated in reverse mode to dry and cool incoming air. Since it is not possible for the wheel to provide a perfect barrier between the exhaust and supply air, some cross-contamination is inevitable.

Heat wheel performance is strongly affected by the packing material. Different packing materials are applied according to need (for example, latent heat recovery). This is a unique benefit of thermal wheels combined with a low air-side pressure drop. Heat wheels tend to be preferred in large commercial or public buildings where they form an integral part of the heating, ventilating and air conditioning (HVAC) system, and high thermal efficiencies can be achieved. Smaller units have recently become available for residential applications.

Heat regenerators use two chambers with a significant thermal capacity and a switch to cycle the supply and exhaust flows between these two chambers. In the first part of the cycle, the exhaust air is flowing through the first chamber and heats up its thermal mass. After some time the switch is then moved so that the supply air now flows through the second part of the chamber, absorbing the heat from the structure and reducing its temperature for the beginning of the next cycle. Recovery of latent heat is possible. Thermal efficiencies can be quite high for these systems (up to 90 per cent).

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