When it comes to designing an HVAC system, the moisture content of the air within a space is just as important as the supply air temperature. In a system with active chilled beams, the moisture content becomes a critical factor, so it is important to understand the concept of moisture depression.
| Price’s Linear Active Chilled Beam installed in the ceiling of HMFH Architects’ office in Cambridge, MA (photo courtesy of Ed Wonsek) |
In the world of HVAC, moisture depression refers to the difference between the dry-bulb and wet-bulb temperatures, indicating the air’s moisture content and its capacity to absorb additional moisture. A small depression indicates high humidity, while a large depression means that the air is dry and has high dehumidification potential.
This is significant to HVAC system design because it is a direct measure of how much moisture the air can hold. The moisture depression should be large enough that the system is able to satisfy the latent load of a space.
| In a decoupled system, a dedicated outdoor air system (top left) supplies moisture-depressed air, and a chiller supplies cold water (top right). The active chilled beam (bottom) works with both to deliver air directly into the space. |
Active chilled beams are part of a decoupled system. A decoupled system uses a chiller to supply cold water and a dedicated outdoor air system to supply moisture-depressed air. Active chilled beams use a hydronic coil that satisfies the majority of the sensible load and a ducted plenum delivering primary air that is responsible for the latent load and minimum ventilation requirements.
| Active chilled beams use a water coil to satisfy the sensible load and a ducted plenum delivering primary air to satisfy the latent load |
Considering moisture depression is important when designing an active chilled beam system for two reasons. First, as with any HVAC system, it is essential to satisfy the latent load. When a space’s latent load is higher than what the primary air can absorb – that is, when the moisture depression is too small for the latent load – you risk several issues, including poor indoor air quality, occupant discomfort and, if prolonged, property damage due to excessive humidity in the space.
Second, and something that’s unique to chilled beams, is the risk of condensation on the hydronic coil. Condensation on the coil occurs when the temperature of the chilled water within the coil is below the wet-bulb temperature of the space. While condensation is not instantaneous, if conditions are prolonged and the active chilled beam does not have a drain pan or drip tray, water droplets could fall onto occupants below the unit.
| Condensation can form on the beam’s coil if the temperature of the chilled water within the coil is below the wet-bulb temperature of the space |
Since the hydronic coil in an active chilled beam is designed to deal with the sensible load, systems must use the air handling unit delivering the primary air to ensure the moisture depression is high enough to satisfy the latent load and to keep the room’s wet-bulb temperature low enough that the chilled water in the coil does not cause condensation.
Increasing the moisture depression of the supply air (also known as dehumidification) can be achieved in different ways. The most common method of drying out air is evaporative cooling. This involves passing air through a coil where the coil surface is below the moist air’s dew point. This will allow water vapor to condense on the coil surface, and the condensate is then drained.
| The evaporative cooling process |
Another popular strategy for increasing the moisture depression is active desiccant dehumidification, which is essentially the opposite of evaporative cooling. With this method, water vapor is absorbed by desiccant media, and as the water vapor condenses, it releases latent heating energy into the air, which raises the dry-bulb temperature.
| A typical desiccant dehumidification wheel with reactivation/regeneration section |
Once you’ve ensured the moisture depression of the air coming from the air handling unit is sufficient to handle the latent load of the space, designing an active chilled beam system becomes much more straightforward. The goal is then to ensure that the remaining the sensible load is satisfied through the beam’s hydronic coil and that the primary air delivered to the space meets the minimum ventilation requirements.
For more information about moisture depression and how it relates to active chilled beam design, reach out to the Sustainable Systems team at Sustainable@priceindustries.com.
| Austin Campbell is an Application Engineer for Price’s Sustainable Systems team. He is based out of Price’s Crestridge facility in Suwanee, GA. |