Maximising the efficiency of warm-air heating

Published:  15 February, 2008

heat exchangers
As heat loss through the fabric of a building has fallen, ventilation heat losses become more significant. Recuperative heat exchangers can transfer as much as 85% of heat in exhaust air from a building to the incoming fresh air. These plate heat exchangers from Reznor are listed on the Energy Technology list, allowing qualifying end users to offset costs against tax in the year of installation.

Energy-efficient warm-air heating of large commercial and industrial buildings can be effectively achieved using recuperative plate heat exchangers within air-handling units. Andrew Patch explains.

As buildings become better insulated, significant heat losses through the building structure are becoming a thing of the past. However, ventilation losses still account for a large percentage of heat loss, however well insulated the building. The challenge for building-services specifiers is to achieve the best balance of ventilation and air quality to insulation and heating efficiency.

Heat recovery

With this in mind, it makes economic and environmental sense to recover as much heat as possible from exhaust air expelled from the building. Among the most efficient ways of achieving this is installing a recuperative plate heat exchanger within the air-handling unit.

An unsung hero of energy efficiency in warm-air heating systems, the recuperative plate heat exchanger is an exchanger with very high thermal conductivity that optimises heat transfer from warm exhaust air to incoming fresh air.

Crossflow plate heat exchangers have been used for several years in air-distribution systems supplying ventilation. High airflows and heat-recovery rates can be achieved typically with efficiencies of 50 to 60% in the past. However, the new generation of counterflow plate heat exchangers increase the efficiency for standard HVAC applications up to 85%, giving designers the freedom to incorporate adequate ventilation whilst at the same time reducing energy consumption and carbon emissions.

The heat-exchange process is one of continuous recycling of heated air, with an input of fresh air to maintain air quality. Incoming air from outside crosses the heat exchanger and is distributed around the building, either directly or via ductwork, before being drawn back into the heater unit for extraction of re-useable heat.

Good ventilation

It is important that good ventilation rates are maintained; the latest regulations have increased minimum ventilation rates to 10 l/s/person and even higher in school classrooms.

The recuperative heat exchanger ensures that heat is re-used effectively while, at the same time, optimising air quality. It works on the principle of transferring energy from heated exhaust air to the incoming fresh air. The two air streams are completely separated by thin heat-transfer plates that transfer heat from exhaust air to the incoming fresh air. The two air streams never come into contact with each other and cannot mix. Consequently, there is no cross- contamination of humidity, pollution, bacteria or odours from the exhaust air to the incoming fresh air.

Advanced-technology heat-exchanger plates are manufactured from corrosion-resistant aluminium, instead of lower-cost plastic, taking advantage of the high thermal conductivity of aluminium to optimise heat transfer and extend operational life expectancy. The material can also be recycled, thereby minimising the overall environmental impact.

High-efficiency energy recovery can play a important role in reducing carbon emissions. For each 1m3/s of fresh air, up to 17 000kg per annum of carbon-dioxide emissions can be saved, based on 24/7 operation.

Financial sense

As well as making significant environmental contribution, high-efficiency heat recovery also makes sound financial sense. The additional cost can often be recovered in under a year and the size of heating plant can be reduced. Heat-recovery efficiencies of up to 85% also make it possible to eliminate low-pressure-hot-water or electric coils from air-input units, with the benefit of saving on both equipment and installation costs.

Andrew Patch is OEM sales manager with Reznor.

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