When designing various types of DOAS systems, safety factors are often used by engineers. With safety factors, many DOAS systems are purposely built larger than needed for normal usage to allow for emergency situations, unexpected loads, misuse, or degradation. However, excessive use can result in much larger-than-necessary equipment, inflated installed costs, and excessive energy use.
In this post, we will try to determine the influence of safety factors on the design airflow of a dedicated outdoor air system. In the next post we will turn our attention to the design dehumidification capacity of a DOAS.
Determining Design Airflow
The design airflow of a dedicated outdoor air system is dictated by the amount of ventilation air required by industry standard or local code. In some cases, it may be wise to deliver more than code-minimum ventilation airflow to improve indoor air quality, or to earn a green building certification. Another case for example, with low ventilation requirements or high indoor latent loads, a design engineer may choose to increase the airflow delivered by the DOAS so that the conditioned outdoor air can be delivered at a higher dew point (i.e. not as dry).
ASHRAE prescribes two ventilation rates for each occupancy category:
1: for people-related sources of contaminants (Rp)
2: for building-related sources (Ra).
The following equation is used to determine the minimum outdoor airflow (Vbz) that must be delivered to each breathing zone:
Next, ASHRAE advises to account for zone air distribution effectiveness (Ez) and to calculate the design outdoor airflow for the zone (Voz). This is the outdoor airflow that must be provided to the zone by the air distribution system (i.e. through the supply air diffusers):
Finally, for a 100% outdoor air system in which one air handler supplies only outdoor air to one or more zones, a slightly revised equation is used to calculate the system-level outdoor air intake flow (Vot), by summing the zone outdoor airflows of all zones served by the dedicated outdoor air unit:
Note: In some system configurations, the dedicated outdoor air unit provides conditioned OA to the intakes of local or centralized HVAC equipment, rather than directly to each zone. In these configurations, the dedicated OA unit must be sized to deliver the sum of the outdoor air intake flows (Vot) required by each of the systems being served.
Safety factor for zone population
The first common use of a safety factor occurs when determining Pz, the number of people expected to occupy the zone. ASHRAE defines it as “the largest number of people expected to occupy the zone during typical usage”. If actual zone population is not known, default values for occupant density may be used.
Using an overly conservative estimate for zone population can result in significant overventilation and excessive energy use. It also impacts the calculation of space latent cooling load, and the resulting dehumidification capacity of the dedicated OA unit. It is therefore recommended to avoid applying safety factors to zone population (Pz), instead of using the best estimate for expected occupancy during typical usage.
Safety factor for future expansion or change of use
The other common use of a safety factor when determining the design airflow of the dedicated OA unit occurs when accounting for future expansion or a change in use of the facility. If the facility is expanded in the future, and the dedicated OA unit will be expected to serve the expansion, it would be prudent to select a unit with some amount of reserve airflow capacity. Or, if the use of the facility is likely to change in the future (from an office space to a group of meeting rooms or a retail area, for example) the ventilation requirements for the zones served by the dedicated OA unit may change.
It is recommended to calculate the design airflow as accurately as possible, without using safety factors. Then select a dedicated outdoor air unit that has reserve airflow capacity, rather than a unit operating near its maximum allowable airflow.
In conclusion, it is considered safer to avoid applying safety factors while designing a DOAS, and instead select the dedicated outdoor air equipment with reserve capacity. This way, the system is future proofed for possible expansions or increased use without over-spending or over-engineering.