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Heat exchangers key to energy- efficient warm air distribution

A new generation of recuperative plate heat exchangers, designed to be incorporated into air handling units or as part of an air distribution system, is now available from Reznor UK which, the company claims, will be a critical element in the drive to achieve effective and economic ventilation, heating, cooling and air distribution for buildings. Keith Sprague, market development manager for the AmbiRad Group, explains
Heat exchangers key to energy- efficient warm air distribution
WITH global warming and carbon reduction still high on the political agenda, the government and the EU are putting the squeeze on end-users to bear the brunt of responsibility for achieving national and international green targets.

End-users cannot do this alone. Manufacturers have their part to play in developing innovative systems and creating new design options for consultants and contractors.

Innovation comes in different guises and recently a new development of an old favourite has emerged as a potential component in the drive for increased energy efficiency and carbon reduction.

The new generation of recuperative plate heat exchangers has the potential to make air heating, cooling and ventilation systems far more energy-efficient. The exchanger has very high thermal conductivity which optimises heat transfer from warm exhaust air to incoming fresh air. In other words, it sustains comfort conditions in a building by efficient re-use of heat that has already been generated.

Ventilation conundrum

As buildings become better insulated and their thermal properties are improved, heat lost through the building structure has reduced dramatically. However, ventilation losses still account for a large use of energy. Although new building regulations have reduced air leakage rates, the need to maintain air quality in office, leisure and factory environments is paramount.

In the latest regulations, minimum ventilation rates have been increased from 8l/s/person to 10l/s/person. In school classrooms an increase in ventilation rates of up to threefold is now recommended to reduce CO2 levels and improve concentration. In industrial environments, it is essential air quality is maintained and contaminant levels reduced below levels harmful to health. The trick is to ensure heat is used effectively and air quality is optimised.

The recuperative heat exchanger does both. It works on the principle of transferring energy from either heated or cooled exhaust air to the incoming fresh air ventilation. The two air streams are separated by thin heat transfer plates which transfer the heat from exhaust air to the incoming fresh air. The two air streams never come into contact with each other and cannot mix. Consequently, the transfer of humidity, pollution, bacteria and odours is eliminated, guaranteeing there is no cross contamination from the exhaust air to the incoming fresh air.

Cross plate heat exchangers have been used for several years in air distribution systems supplying ventilation, typically achieving efficiencies of 50-60%.

Increased efficiency

The new generation of counterflow plate heat exchangers increases the efficiency for standard HVAC applications up to 85% thereby allowing designers the freedom to incorporate adequate ventilation while at the same time reducing energy consumption and carbon emissions.

The heat exchanger plates in the high efficiency heat exchanger are manufactured from corrosion resistant aluminium. Using aluminium instead of lower cost plastic plates takes advantage of the high thermal conductivity of the lightweight metal - 237W/mK compared with plastic at 0.08W/mK - to optimise heat transfer and provide extended operational life.

High efficiency energy recovery can play an important role in reducing carbon emissions. For each 1m3/s of fresh air input, up to 17,000kg a year of CO2 emissions can be saved (based on 24hour/7day operation).

The Carbon Trust already recognises the contribution recuperative plate heat exchangers can make to reducing carbon emissions and units are listed on the Energy Technology list to allow qualifying end users to offset costs against tax.

As well as making a significant environmental contribution, high efficiency heat recovery also makes financial sense and the cost can often be recovered in under a year, the size of heating plant can be reduced and with efficiencies of 85% it is also possible to eliminate low pressure hot water or electric re-heat coils from air input units.

The increased efficiency of the new generation recuperative heat exchanger technology offers designers, consultants and contractors new options in air distribution systems, whether for heating or cooling, or both.

Installation is straightforward since units can be supplied in a variety of forms, from a basic heat exchanger for incorporating into an air handling unit, to a cased unit complete with drain connection, by-pass and damper for simple fitting to existing ductwork. Once fitted, it is maintenance free.

The green credentials of the high efficiency recuperative heat exchanger are impeccable. It is capable of handling large volumes of air and distributing it efficiency and economically in buildings of all sizes. Few other individual elements in heating systems could claim such versatility. The recuperative heat exchanger is a product whose time has come.
1 January 2008

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