Treated Fresh Air Units and Energy Recovery: When Does a Project Need Them?

Purpose of Treated Outdoor Air

Outdoor air introduced into a building for ventilation carries a thermal and moisture load that must be handled by the air conditioning system. In hot and humid climates, this outdoor air load can represent a significant proportion of the total cooling load. A treated fresh air unit (TFAU) — also called a dedicated outdoor air system (DOAS) — conditions the outdoor air to a defined supply condition before it enters the building, reducing the load on the main air handling units and improving humidity control.

Ventilation and Indoor Air Quality

Ventilation standards require a minimum quantity of outdoor air to be supplied to occupied spaces to dilute internally generated pollutants and maintain acceptable indoor air quality. The ventilation rate is determined by the occupancy, the activity and the applicable standard. In some applications — laboratories, pharmaceutical manufacturing, food processing — the ventilation rate is determined by process requirements rather than occupancy.

Sensible and Latent Loads

The outdoor air load has two components: the sensible load (the heat content associated with the temperature difference between outdoor and indoor conditions) and the latent load (the heat content associated with the moisture content of the outdoor air). In humid climates, the latent load can be larger than the sensible load. The TFAU must be sized to handle both components at the design outdoor conditions.

Temperature and Humidity Control

The TFAU typically cools and dehumidifies the outdoor air to a defined supply condition. The cooling coil removes both sensible and latent heat. The supply condition is selected to provide the required ventilation air at a temperature and humidity that does not impose a cooling or heating load on the served space. In some applications, the supply air is reheated after dehumidification to prevent overcooling.

Heat-Recovery-Wheel Principle

A heat recovery wheel — also called a rotary heat exchanger — transfers heat and moisture between the exhaust air stream and the incoming outdoor air stream. In cooling mode, the wheel pre-cools and pre-dehumidifies the outdoor air using the cooler, drier exhaust air. In heating mode, the wheel pre-heats the outdoor air using the warmer exhaust air. The wheel rotates slowly between the two air streams, transferring both sensible and latent energy. The actual recovery achieved depends on the wheel type, rotational speed, face velocity and the conditions of both air streams.

Plate Heat-Exchanger Principle

A plate heat exchanger transfers sensible heat between the exhaust and supply air streams through a fixed matrix of plates. Unlike the rotary wheel, there is no physical contact between the two air streams, which eliminates the risk of cross-contamination. Plate heat exchangers recover sensible heat only — they do not transfer moisture. They are preferred in applications where cross-contamination between exhaust and supply air is not acceptable.

Run-Around-Coil Concept

A run-around coil system uses two coils — one in the exhaust air stream and one in the supply air stream — connected by a pumped glycol circuit. Heat is transferred from the exhaust air to the glycol in the exhaust coil and then from the glycol to the supply air in the supply coil. This arrangement allows heat recovery between air streams that are not adjacent, which is useful in retrofit applications or where the supply and exhaust ducts are in different locations.

Cross-Contamination Considerations

In applications where the exhaust air contains contaminants — laboratory fume, pharmaceutical process exhaust, kitchen exhaust — the risk of cross-contamination between the exhaust and supply air streams must be assessed. Rotary wheels carry a small risk of cross-contamination due to the rotation of the wheel between the two streams. Plate heat exchangers and run-around coils do not have this risk. The cross-contamination risk assessment should be documented and reviewed as part of the system design.

Filtration

The outdoor air entering the TFAU must be filtered to protect the heat recovery device and the downstream equipment from fouling. The filter efficiency class must be appropriate for the outdoor air quality at the project location. The heat recovery device itself must be accessible for cleaning, as fouling reduces its effectiveness.

Climate and Application Suitability

The benefit of heat recovery depends on the difference between the outdoor and indoor conditions and the number of hours per year that the system operates. In climates with a large temperature or humidity difference between outdoor and indoor conditions for a significant proportion of the year, heat recovery can reduce the cooling or heating load on the main system. Actual recovery and savings depend on operating conditions, equipment selection and system design — no guaranteed saving percentage can be stated without analysing the specific project conditions.

Maintenance Requirements

Heat recovery devices require periodic cleaning to maintain their effectiveness. Rotary wheels must be cleaned to remove dust and biological growth from the matrix. Plate heat exchangers must be cleaned to remove fouling from the plate surfaces. Filters must be replaced at intervals defined in the maintenance schedule. The condensate drain from the cooling coil must be kept clear.