Energy Recovery is used throughout the various thermodynamic cycles, in many types of equipment like feedwater heaters, heat exchangers and reheat coils, etc. Each piece of equipment is discussed in this section, but the concept of energy recovery is best explained by looking at energy recovery devices. The important thing to understand is that the heat transfer in energy recovery is not perfect and that there is efficiency loss. On the exam, this concept is tested with the device effectiveness or efficiency term.
Air to Air Energy Recovery
In the Thermal & Fluids 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.
The amount of heat transferred by the device is determined by the effectiveness of the device. 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 air-streams 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.
The above equations can also be adjusted from air to air energy balances to air to liquid or liquid to liquid energy balances.
The book also includes brief discussions on the different types of devices like Rotary Sensible Wheel, Rotary Enthalpy Wheel, Wrap Around Heat Pipe and Run-Around Loop. See the technical study guide to get more information.