Indirect dehumidification enhancements may work well for some indoor environments in some climates, even though latent and sensible load peaks occur independently. But when latent and sensible loads vary significantly, or when it is necessary to maintain a low relative humidity, both sensible and latent capacity must be controlled directly from both zone temperature and zone relative humidity.

Separate Paths
One way to directly control dehumidification is to individually treat the return air and outdoor air streams before mixing them. This can be accomplished with two entirely separate air handlers, or with a single, “dual-path” air handler (usually in a stacked configuration for a smaller footprint) that accommodates both airflow paths. Air treatment may include various degrees of cooling, dehumidifying, heating, and filtering.

Together, individual cooling coils in the return air and outdoor air streams maintain the target condition in the zone. That is:

• The humidistat directly controls the latent capacity of the outdoor-air coil to maintain the desired relative humidity limit. It provides sufficiently dry air to remove the latent load, both outdoor and zone.

• The thermostat directly controls the sensible capacity of the return-air coil, providing the balance of cooling needed to assure that the supply-air temperature satisfies the sensible load. In effect, heat in the return air tempers the preconditioned outdoor air.

As the latent load rises, the humidistat increases the capacity of the outdoor coil by reducing the leaving-coil temperature and maintaining the indoor humidity at the 52.4%RH limit. As the indoor sensible load drops, the zone thermostat reduces the capacity of the return coil accordingly to maintain the room temperature at set point, 74°F. Total coil load rises from 3.68 tons (Figure 1) to 4.16 tons, again split between the outdoor coil (3.72 tons) and the return coil (0.44 tons).

In this arrangement, the cooling system/chiller plant can be sized for “block” load (4.81 tons) rather than “peak” load (6.38 tons) because the sensible and latent loads do not peak simultaneously. Each coil, however, must be sized for its individual peak load—the return-air coil for 2.66 tons at sensible design and the outdoor-air coil for 3.72 tons at latent design.

Supply-Air Tempering
Dehumidification can also be directly controlled by applying a single cooling coil in series with a heating device (Figure 7). This approach assures that the supply air is always dry enough to neutralize both the outdoor and indoor latent loads and cool enough to maintain the desired zone temperature.

Note: More commonly known as supply-air “reheat,” we choose to describe this configuration as supply-air “tempering” because the heating device simply moderates the cooling effect of the dry supply air.

To understand how supply-air tempering works, let’s turn again to our classroom example. A coil load of 4.78 tons maintains the desired classroom condition—without tempering—at the sensible design load; air conditions are identical to those shown in Figure 1.

As the latent load rises, however, the zone humidistat increases the capacity of the cooling coil, which reduces the supply-air temperature and maintains the relative humidity at the 52.4% RH limit. As the sensible load in the classroom falls, the zone thermostat increases the tempering capacity of the heating device to maintain the zone temperature at set point, 74°F; see Figure 7. Total coil load rises from 3.68 tons to 5.21 tons.

When designing a CV system that includes supply-air tempering, size the cooling system and coil to handle both outdoor and zone loads at the sensible or latent design condition. (In our example, the peak cooling load of 5.21 tons occurs at latent-design conditions.) Remember, too, that on-site energy-recovery enhancements can provide the minimal heat required for tempering. In some cases, building codes or energy standards require the use of on-site recovered energy for tempering.

Because the chilled water system or the packaged-unit refrigeration system removes the latent load when the sensible load is low, always consider waterside or condenser heat-recovery options when using supply-air tempering to control relative humidity.

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