Sustainability Office

Sustainable Queen's
Sustainable Queen's

Enthalpy Wheel

An enthalpy wheel exchanges heat and humidity from one air-stream into another. Rather than discard used building air, an enthalpy wheel salvages useful energy and transfers it to incoming, fresh air. This saves energy by reducing the need for cooling in the summer and heating in the winter.

Enthalpy wheels are becoming common in HVAC systems. There are currently two other wheels in use on the Queen's campus (in the Cancer Research Centre and in the new residences).

The IL Centre's enthalpy wheel is a large, turning disc made out of an aluminum honeycomb material that is coated in desiccant. The aluminum expands when it is heated, and carries that energy around with it as the wheel rotates. When it hits a cooler air-stream, the aluminum contracts and the heat energy is released into the air.

The exchange of humidity depends upon vapour pressure. When warm air flows through the wheel it heats up the desiccant material. The vapour pressure of the desiccant is raised until it is higher than the vapour pressure of the air in the opposite air-stream. When the wheel turns into that air-stream, the water evaporates into it. The result is that cooler air is humidified and warmer air is dehumidified.

The wheel is three meters (ten feet) in diameter, and sits between the main incoming and outgoing air-streams of the IL Centre's air-handling system. It rotates through the incoming and outgoing airstreams at a rate of one to six revolutions a minute, depending upon outside air temperature and humidity.

When a section of the wheel passes through the warmer air stream, the heat is transferred to the aluminum honeycomb on the wheel, and moisture is transferred to the desiccant. When that portion of the wheel rotates into the cooler air stream, the honeycomb cools and releases its heat into it, and the desiccant releases its moisture. The supply air is warmed and humidified in the winter and cooled and dried in the summer. The device is effective all year around at pre-conditioning the outdoor air.

When the temperature outside is near room temperature, the wheel shuts down, and air travels through a bypass duct. When the wheel can't save energy for the heating and cooling systems -- the fans that blow air into and out of the unit consume a lot of power -- energy is conserved by powering it down.

Purge Sector

While the wheel exchanges heat and humidity, it shouldn't be able to exchange anything else, like toxins or contaminants, from the exhaust air into the incoming air. Seals maintain a boundary between the ducts where they meet the enthalpy wheel. To increase effectiveness, the incoming air is at a higher pressure than the outgoing air, so if air does flow between the ducts, the fresh air will want to flow into the exhaust air-stream rather than the other way around.

Enthalpy wheels also remove contaminants from the air. A purge sector, a small area that the wheel rotates into between the incoming and outgoing ducts, blows any exhaust air from the wheel media before it rotates into the supply air duct. Purge sectors have helped lower cross-contamination from 4 or 6 percent to 0.04 percent, and have made enthalpy wheels suitable for sensitive environments such as chemical laboratories and hospitals.

Molecular sieves prevent molecules larger than a specific size from being absorbed into the desiccant of the wheel. The common sieve size is three angstroms, as this allows the 2.8-angstrom water molecule to be absorbed while screening larger airborne contaminants.