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A heat wheel is a rotating apparatus generally used to reduce the energy consumed by air-conditioning equipment in humid locations. It functions as a rotary heat exchanger, which means that it continually attempts to achieve thermal equilibrium. The device operates by having a warm air stream and a cold exhaust air stream flowing through its opposing halves, making use of these counter-streams to cool hot air and to heat cold air.
The physical structure of a heat wheel resembles an aluminum disk and an exhaust fan placed in the middle of two air ducts. One air duct has warm outdoor air passing through it, and the other duct releases cold air chilled by the exhaust fan. Filters also are in place to ensure that the air entering and exiting the apparatus are clean.
The heat wheel accomplishes thermal equilibrium by first being chilled by cool air directed by the exhaust fan. When its cold portion rotates to encounter the warm outdoor air stream, it then absorbs heat from the air. As the device absorbs warmth from the air, the outdoor air stream temperature consequently drops. When the wheel rotates back to its starting point, it then sends the absorbed heat into the cold exhaust air while simultaneously being once more chilled.
The concept of the heat wheel operates on the second law of thermodynamics, wherein heat will always move from areas of higher temperature to areas of lower temperature. The apparatus, by merely rotating, enables heat to come into contact with cold air and thus accordingly transfer. This balances out the temperature in two adjacent locations, cooling hot air and warming cold air.
There are clear benefits to using a heat wheel. The device lessens the load on ventilation air-conditioning, thus reducing energy consumption and subsequently the long-term air-conditioning costs. Another advantage to this is that it then lowers the size of the equipment needed to cool or heat the area. Note also that even a small heat wheel can be quite effective in transferring a high amount of heat, in keeping with the reduced size of the equipment.
There also are some drawbacks that must be considered, however. The initial cost of the heat wheel means that the starting costs are higher than purchasing equipment without it. Next, the apparatus requires two adjoining air streams to operate, and these air streams must be clean or otherwise pass through cleaning filters. Another factor is that it needs a turning mechanism to operate, which in turn requires maintenance.
The heat wheel is best used in locations where a large intake of outdoor air is needed, and where there is an exhaust air duct close by. One possible situation is a new building in which the air-conditioning equipment has been found to be insufficient for the amount of ventilated air. Another possibility is a structure where a certain amount of outdoor air intake is required.
The heat wheel is not recommended for areas with typically low to nonexistent maintenance, because it requires regular inspections and cleaning. It also is not suited for locations where the ducts for the air streams must be routed to a great extent. The fan will consume more energy, and the starting costs will be greater, thus nullifying the amount of money that would have been saved by the lower air-conditioning energy consumption.