In this post, we will pay particular attention to designing dehumidification capacity of a dedicated outdoor air unit and which safety factors to consider and which to avoid.
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.
Calculation of the dehumidification capacity
The required dehumidification capacity of a dedicated outdoor air (OA) unit is dictated by the design airflow (Vot), humidity ratio of the entering outdoor air (Woa), and the humidity ratio of the conditioned air leaving the unit (Wca):
Humidity ratio of the outdoor air entering the unit (Woa)
The ASHRAE Handbook–Fundamentals is a good source for climatic data that represents the outdoor design conditions for various locations worldwide. It is important to use the design dew point condition when determining the required dehumidification capacity of a dedicated outdoor air unit.
The first common use of a safety factor occurs when selecting the design humidity ratio of the entering outdoor air (Woa). The ASHRAE Handbook includes three sets of design conditions: 0.4%, 1%, and 2%. These indicate the percentage of hours during a year when outdoor conditions are expected to exceed the tabulated design value. Many engineers tend to use the most conservative (0.4%) design condition, which often results in the selection of larger dedicated OA equipment. The impact of this decision on equipment capacity will be discussed below.
Humidity ratio of the conditioned air leaving the unit (Wca)
The process for determining the required humidity ratio of the conditioned air delivered by a dedicated OA unit involves the following steps:
- Define the target humidity level for the occupied space (Wsp). This is the maximum indoor humidity that is considered acceptable, usually around 50-60% relative humidity.
- Determine the design latent load for each zone (qLspace). It typically consists of moisture generated by people or other sources within the zone, as well as infiltration or diffusion of humid air from outdoors. (use load calculation software)
- Calculate the required humidity ratio of the conditioned air (Wca). If the dedicated OA system is being designed to offset the entire indoor latent load, in addition to the ventilation latent load, then the conditioned outdoor air must be dry enough to offset the latent load in each zone, such that the humidity ratio in every zone is maintained at or below the desired upper limit.
The first influence of safety factors in determining the required humidity ratio of the conditioned air (Wca) is in the selection of the target humidity level for the occupied space (Wsp). For most comfort cooling applications, engineers will typically design HVAC systems using a target relative humidity (RH) of 50%. ASHRAE recommends limiting indoor humidity to no greater than 55°F (13°C) dew point, which equates to about 51% RH at 74°F (23°C) dry bulb. This decision has a huge impact on the sizing of the dedicated outdoor air unit. The humidity ratio of the conditioned air (Wca) and the resulting dehumidification capacity of the dedicated OA unit are so determined for various RH limits. To maintain a lower RH, Wca must be lower, resulting in greater dehumidification capacity required. For example, designing the system to limit indoor humidity to 50% RH may increase the required dehumidification capacity of the dedicated OA unit by say 15% compared to allowing indoor humidity to rise to 60% RH at worst-case conditions.
Design indoor latent loads (qLspace)
The second influence of safety factors in determining the required conditioned-air humidity ratio (Wca) is in the calculation of the design latent loads for each zone. The estimated zone population (Pz) not only impacts ventilation calculations, it also affects indoor latent load calculations. Another source of the latent load in a zone is infiltration of humid air from outside. Calculating loads due to infiltration is a rather imprecise exercise, however, as it is highly dependent on the quality of the building envelope construction. Since precise calculations are often difficult, many engineers choose to select the dedicated OA unit with reserve dehumidification capacity, inflating the DOAS again.
Designing a DOAS with too many safety factors can thus result in excessively large and complicated systems. Particularly for the dehumidification load, it is best not to interfere with the design calculations. Instead, best practice suggests selecting the dedicated outdoor air equipment with reserve capacity.