With the advent of increased outside-air requirements to combat indoor- air quality problems — in some cases requiring 15 to 20 ft3/min per occupant — the volume of outside air introduced to many buildings will have to increase. This is particularly true for schools and other facilities with high occupancies, such as restaurants, theaters, and retail stores. These are the very places that can ill afford to pay the extra cost of conditioning a copious volume of fresh outside air.
A heat reclaim system is an apparatus to recover a portion of the thermal energy of exhaust air, transferring it to the incoming outside air. There are several different mechanisms to accomplish this:
1. Air-to-air heat exchange by way of a flat-plate heat exchanger.
2. Heat pipes that extend between the air streams, transferring heat by evaporation and condensation of the fluid within. These devices can vary the heat-transfer rate by tilting the pipes, affecting the thermal efficiency of the change in state of the fluid.
3. Run-around coils, which place a multi-row finned coil in each air stream, circulating water between them. The latter system can be used with any duct configuration, whereas the other two must have the exhaust and outside-air ducts adjacent and parallel.
These systems transfer only sensible heat between the air streams. An enthalpy wheel, with a desiccant fixed into the wheel surface, will also transfer latent energy between the air streams. The wheel rotates slowly between the two (parallel) air streams, absorbs thermal energy from outgoing air stream, and then discharges it into the incoming air flow.
A hybrid thermal recovery unit will use a flat-plate heat exchanger or a heat-pipe configuration, but with a humidifier type of sprayer in the exhaust duct ahead of the unit. The spray causes extra evaporative cooling of the air stream, decreasing its temperature before reaching the heat exchanger, thus making the system more effcient. The process is further enhanced because the spray wets the heat-transfer surface, which boosts the effciency of the coils.
One drawback to this system is that dissolved solids and minerals can precipitate out of solution and solidify on the heat-exchange surface, reducing its effectiveness and restricting the airflow. One manufacturer avoids this problem by wetting a filter-type medium upstream of the heat exchanger. The air is cooled as it passes through the medium, but very little of the fluid passes downstream to the heat exchanger itself.
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Excerpt from Energy Conservation Projects.