DOAS, Indoor Air Quality

The Economiser In a DOAS

As part of the Heating, Ventilation and Air Conditioning (HVAC) control system, such as a dedicated outdoor air system (DOAS), the function of the economiser is, as its name suggests,to economise or save on cooling costs by pulling cooler outside air into buildings, reducing the load on the mechanical cooling system. In this post, we will look at the two different types of economisers, and their benefits on indoor air quality.

Air-side Economisers

 

An air-side economiser is a collection of dampers (introduced, recirculated or exhausted), sensors (for example, temperature, humidity, flow, pressure, smoke, CO2), actuators, and controls that work together to determine how much outside air to bring into the building to reduce, or eliminate, the need for mechanical cooling during mild and cold weather.

  • Dampers regulate the amount of air introduced, recirculated, or exhausted from the building.
  • Outdoor temperature sensors and logic controllers determine whether conditions are right for the economiser to operate.
  • Actuators open or close dampers based on signals from the logic controllers.

When the logic controller decides the outside-air temperature is low enough to take some or all of the cooling load, the outside-air damper opens and the air conditioning compressors are turned off. An exhaust fan always runs when the economiser operates. For the economiser to work effectively, the same amount of air should be exhausted from the building as is taken in. When the outdoor air temperature rises too high to provide useful cooling, the outside-air damper moves to a minimum position, maintaining minimum ventilation. Then the return opens and exhaust closes, and the compressors take over the cooling process.

economiser in a doas

Water side Economisers

 

This is a system by which the supply air of a cooling system is cooled indirectly with water that is itself cooled by heat or mass transfer (evaporative cooling) to the environment without the use of mechanical cooling. It is generally used in instances where space for the very large ductwork is scarce, or where floor-by-floor air handlers are used. Its application is largely reserved for systems that employ water cooled chillers. As such, they use a cooling tower, and the tower leaving water temperature available is a strong function of the ambient wet bulb temperature. Generally, in the winter time, the OA dry bulb and dew point temperatures are low enough that dehumidification is no longer a mechanical refrigeration requirement. And in fact, if ceiling radiant cooling is used with a DOAS system, the desired fluid temperature is around 60ºF. There are many possible arrangements, types of evaporative cooling equipment, and controls – too many to explore in this post, perhaps in a future piece.

 

Economisers and Humidity Control

 

In an effort to reduce mechanical refrigeration, it is fairly common to allow the supply air temperature (SAT) to be reset upward, to 60ºF or higher. That is highly unrecommended – a consequence of SAT reset is an increase in the fan energy, commonly the largest energy user in the mechanical system, even without SAT reset. In addition, elevating the SAT often results in flooding the building with very humid air that can lead to unwelcome biological growth and the associated odor and indoor air quality (IAQ) problems.

humidity control doas

If a DOAS system is to be used to control humidity, it is always best to build some excess air handling capacity into the unit to assure that unforeseen latent loads can be accommodated. Since the DOAS is generally required to have total energy recovery (see blog post 1), increasing the airflow rate has only a limited impact on the OA load seen by the mechanical cooling equipment. It is generally recommended to design a DOAS for at least 15 cfm/person, or 0.2 cfm/ft2, even though for many high occupancy density spaces ASHRAE Std. 62.1-2004 does not require that much OA flow. As a rule of thumb, the focus of good design must be to deliver at least 15 cfm/person of OA and maintain space relative humidity below 60%.

In conclusion, economisers in DOAS, especially the water side type, can save significant mechanical cooling energy. Coupled with ceiling radiant cooling or water cooled chillers, DOAS can provide the same quality of cooling at a much lower load, at a much lower operating cost. And with effective humidity control, it can comfortably operate several degrees above normal, reducing the envelope conduction load by about 22% – a further energy and demand savings.

 

doas

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