While designing a DOAS system, it is advisable to use energy wheels to recover energy. Systems range from one to two wheels, with an optional coating to dehumidify the airstream. In this post, we will have a closer look at the energy recovery wheel and its use in humid environments.
Choosing DOAS Energy recovery equipment
Generally speaking, there are four choices for an energy recovery ventilator: rotary heat exchanger (wheel), plate heat exchanger (fixed core), heat-pipe heat exchanger (refrigerant), and runaround coils (water).
A wheel is a plastic or metal device that rotates between the exhaust and outdoor air streams, picking up heat from one air stream and transferring it to the other.
Metal wheels can only transfer heat (sensible energy), while some plastic wheels, when impregnated with a desiccant, can absorb and release moisture (latent energy) as well.
Wheels are the most popular ERV because of their relatively low initial cost, reasonable pressure drop, ease of maintenance, and smaller physical footprint.
Plate heat exchangers generally are larger and more expensive than wheels. The air streams pass by each other through a series of channels, heating up or cooling down the material between the channels and transferring energy. Their main advantage over wheels is that they do not permit contamination between the different airstreams. In certain applications, such as hospitals and labs, fixed-core plates are recommended to avoid contamination.
Heat pipes are somewhat limited because they cannot recover latent energy. Heat pipes are copper tubes with refrigerant inside of them, running between the two air streams (exhaust and outside air).
Similar to heat pipes, runaround coils are often preferred when the exhaust and outdoor airflows are separated by large distances. This type of system requires the installation of a water coil in the exhaust air stream and a second one in the incoming outdoor/ventilation air stream.
The Energy Recovery Wheel
It is important to distinguish between the sensible and enthalpy (or latent) wheels.
The enthalpy wheel (also called a passive desiccant or total energy wheel) recovers both sensible (temperature) and latent (moisture) energy. The wheel’s desiccant-loaded honeycomb rotor design provides for high heat transfer while simultaneously reducing pressure loss parameters.
In a typical installation, the wheel is positioned so that it is divided into two half-moon sections. Stale exhaust air is drawn through one half and outdoor air through the other in a counter flow pattern. As the wheel rotates, sensible heat is transferred as the metallic substrate picks up and stores heat from the warmer air stream and transfers it to the cooler one.
Latent heat is transferred as the desiccant coating on the metallic substrate adsorbs (condenses) moisture from the air stream that has the higher humidity ratio and releases (evaporates) the moisture into the air stream that has the lower humidity ratio.
The benefits of an enthalpy wheel are more pronounced in the summer in terms of cooling coil load and energy consumption reduction. It is equally beneficial for minimizing heating and humidification energy use during the cold winter months. A word of caution concerning the winter operating conditions.
As condensation may occur in the wheel, temperatures below 0ºC may actually freeze the condensate in the wheel. In order to avoid this, preheating is required under certain conditions to prevent frosting of the enthalpy wheel.
On the other hand, the sensible wheel performs another task entirely. In many applications, the internal sensible cooling loads are not sufficiently high to prevent overcooling with the low-temperature ventilation air. Therefore, it is desirable to elevate the supply air temperature. This can be accomplished by the sensible wheel, although as mentioned above, other forms of sensible heat transfer equipment could be utilised.
For example, the air leaving a deep cooling coil may be reheated sensibly with energy extracted from the return airstream. Simultaneously, the return air is sensibly cooled in this process, thus lowering the energy content of the return airstream, further reducing the enthalpy of the outdoor air leaving the enthalpy wheel and entering the deep cooling coil. As an added advantage, when two equal flow rate airstreams exchange energy in the sensible wheel, virtually no moisture is exchanged.