Energy Recovery Devices
An energy recovery device is an air to air heat exchanging device. In the HVAC and Refrigeration field, energy recovery devices are used to exchange energy from outgoing exhaust air to incoming outside air. During the winter months the outside air is pre-heated prior to entering the air handler and during the summer the outside air is pre-cooled.
Energy recovery devices are governed by the following equations.
The effectiveness of an energy recovery device is defined as the ratio of the actual heat transferred to the maximum amount of heat that can be transferred. The effectiveness can be rated in terms of sensible heat transfer, latent heat transfer or total heat transfer.
The actual amount of energy transferred is found by multiplying each individual airstreams mass flow rates by the change in conditions, whether it is a change in temperature, change in humidity or change in total enthalpy.
The maximum amount of energy transferred is met if the entering condition of the 1st air stream exits the energy recovery device at the same conditions as the entering condition of the 2nd air stream. However, if one airstream has more air flow than the other, then the smallest airstream should be used.
There are various types of energy recovery devices listed below:
Rotary Sensible Wheel
A rotary sensible wheel is typically a metal wheel that rotates and exchanges heat from one air stream to another. The wheel is connected to a gear and motor, which rotates the wheel. As a section of the wheel picks up heat from air stream, the wheel then rotates to the other air stream to move the heat to the cooler air stream.
Rotary Enthalpy Wheel
A rotary enthalpy wheel is similar to a rotary sensible wheel, in that it has the same type of construction and parts. But in addition, a rotary enthalpy wheel has a desiccant material that is used to absorb moisture. A section of the wheel absorbs moisture from the more humid air stream, then rotates and transfers the moisture to the more dry air mixture.
Wrap-Around Heat Pipe
A wrap around heat pipe is used typically in warm humid climates in spaces with a high amount of outside air requirements. In these types of environments, warm, humid outside air is conditioned to a low temperature in order to condense the water out of the air. A wrap-around heat pipe is used to pre-cool the incoming warm humid outside air by transferring heat to the exiting cool supply air. This has the effect of providing sensible re-heat to the supply air, which also decreases the need for additional re-heat.
The heat pipe contains a pressurized refrigerant, which proceeds through the vapor compression cycle with the design temperatures. In the first phase, warm air passes over the cool liquid refrigerant. This effectively pre-cools the outside air before it enters the main cooling coil. During this first phase, the liquid refrigerants gains heat, causing it to vaporize and move to the other side of the coil. In the second phase, on the other side of the coil, the cool air passes over the warm vapor, which re-heats the air. In addition, the warm vapor is condensed to a liquid, allowing the process to start over again.
In this example, energy is transferred in the same air stream from the entering outside air to the existing supply air. The heat pipe can also be used to transfer energy between two different air streams. For example, it can be used between the outdoor/supply air and the return/exhaust airstreams.
The last energy recovery device that is explained in this section is the run-around coil. The run around loops consists of two heat exchange coils connected by piping, a fluid and a pump. A heat transfer fluid, typically water or a glycol-water mixture is pumped between the two coils. The fluid transfers heat from one air stream to the other air stream.