Heat or enthalpy wheels are rotary air-to-air heat exchangers. Adjacent supply and exhaust air counterflow streams each flow through half of the wheel. Heat wheels have a fill that transfers only sensible heat while an enthalpy wheel's fill transfers total heat.
- These wheels are quite compact and can achieve high heat transfer effectiveness,
- Heat wheels have a relatively low air pressure drop, typically 0.4 to 0.7 in. of water,
- Freeze protection is not an issue,
- The cooling or heating equipment size can be reduced in some cases.
The use of the heat wheel
- adds to the first cost and to the fan power to overcome its resistance,
- requires that the two air streams be adjacent to each other,
- requires that the air streams must be relatively clean and may require filtration,
- requires a rotating mechanism that requires it be periodically inspected and maintained, as does the cleaning of the fill medium and any filtering of air streams,
- in cold climates, there may an increase in service needs,
- results in some cross-contamination (mixing) of the two air streams, which occurs by carryover and leakage.
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Heat wheel heat exchanger enhancement can improve system latent capacity. For example, a 1°F dry bulb drop in air entering a cooling coil can increase the latent capacity by about 3%. Both cooling and reheating energy is saved by the heat wheel's transfer of heat directly from the entering air to the low-temperature air leaving the cooling coil. It can also be used to precool or preheat incoming outdoor air with exhaust air from the conditioned spaces.
- Where lower relative humidity is an advantage for comfort or process reasons, the use of an enthalpy wheel pipe can help. An enthalpy wheel used between the warm air entering the cooling coil and the cool air leaving the coil transfers sensible heat to the cold exiting air, thereby reducing or even eliminating the reheat needs. Also the enthalpy wheel heat precools the air before it reaches the cooling coil, increasing the latent capacity and possibly lowering the system cooling energy use.
- Projects that require a large percentage of outdoor and has the exhaust air duct in close proximity to the intake, can increase system efficiency by using a heat wheel to transfer heat in the exhaust to either precool or preheat the incoming air.
- Existing buildings where codes require it or they have "sick building" syndrome and the amount of outdoor air intake must be increased,
- New buildings where the required amount of ventilation air causes excess loads or where the desired equipment does not have sufficient latent capacity.
Technology types (resource)
A small motor and belt drive rotates the wheel and its fill medium. Sensible heat is transferred as the hot air stream passes through the fill which picks up and stores heat and then releases it as the fill rotates into the cold air stream.
Latent heat is transferred as the wheel fill
- condenses moisture from the air stream with the higher humidity (either due to a fill temperature below the air dewpoint or because the fill includes a desiccant) and heat is released, and
- releases the moisture through evaporation ( and picks up heat) as the fill rotates into the air stream with the lower humidity ratio. Both latent and sensible heat is transferred simultaneously as the moist air is dried and the dry air is humidified.
Fill for the heat wheel is typically made of aluminum for HVAC applications. Fill for total heat recovery is made from a number of different materials and treated with a hygroscopic material such as lithium chloride, alumina, or aluminum oxide, each of which has specific moisture pickup properties.
Seals divide the fill from the two air streams, but there is some carry-over as the air entrained within the fill is carried into the other air stream. Leakage occurs due to the static air pressure difference between streams which drives some air from the higher pressure stream into the lower pressure one. Cross-contamination problems can be reduced by placing the fans so they promote leakage of the ventilation air into the exhaust air stream.
Carry-over can be further reduced by adding a purge section to the wheel where some supply air is bypassed through a section of the fill before the main supply passes through the wheel. This is typically done in critical applications such as hospital operating rooms, clean rooms, and labs.
Wheel heat recovery is controlled using two methods. One is to bypass supply air around the wheel and mixed with the remaining air to regulate temperature. The other is to control the rotating speed of the wheel with a variable speed drive.
Heat wheels typically have a sensible effectiveness of 50% to 80% and a total effectiveness of 55% to 85%.