A typical, constant-volume, mixed-air system uses a single cooling coil to cool and dehumidify a mixture of recirculated air and outdoor air. A thermostat modulates the cooling capacity of the coil to directly control zone temperature in response to changes in the sensible load. Coil capacity indirectly controls space humidity: less cooling capacity means less dehumidification and vice versa. Various options are used to improve the indirect dehumidification of a typical CV system (Figure 1). Let's examine the effectiveness of three of them:

• Total energy recovery
• Mixed-air (MA) bypass
• Return-air (RA) bypass

Note: One of the best enhancements for indirect dehumidification controls zone temperature by varying the flow of supply air rather than its temperature (see “What About VAV Systems?”). This article, however, purposely limits the discussion to constant-volume systems.

Total Energy Recovery
Some designers find that passive energy-recovery systems provide adequate dehumidification. A passive, total—energy-recovery wheel (ERW), for instance, preconditions the outdoor air and reduces the cooling capacity needed to maintain the zone temperature. It removes both latent and sensible heat from the outdoor-air stream entering the building, which indirectly reduces zone relative humidity while saving significant operating energy.

Adding an ERW also helps the designer select a packaged system based on less airflow. That is, the designer can use an airflow that more closely matches the zone requirement (within the constraints of the flow-to-capacity ratio) by raising the unit airflow per ton.

Figure 3 illustrates the psychrometric effect of adding an energy-recovery wheel to the packaged air conditioner represented in Figure 2. When the sensible design condition exists, preconditioning the outdoor air reduces the coil load from 4.66 tons to 3.5 tons and permits an equipment selection based on 1,500 cfm rather than 1,750 cfm.

This reduction of supply airflow permits colder supply air (55.7°F rather than 58.3°F), increasing the latent capacity of the coil at all loads. As a result, the relative humidity in the classroom drops from 56.2%RH (Figure 2) to 50.4%RH.

Notice, however, that the latent-design, part-load condition still requires a supply-air temperature of 63°F. Although it reduces the coil load from 3.62 tons to 2.47 tons, the energy-recovery wheel does little to improve indirect dehumidification. The resulting relative humidity (now 65%RH rather than 68.7%RH) still exceeds the 60%RH maximum recommended by ASHRAE.

Further dehumidification cannot occur without making the mixed-air humidity ratio less than that of the return air. A total-energy-recovery device such as the ERW cannot perform this task without the help of a cooling coil.

Mixed-Air Bypass
Face-and-bypass dampers arranged to bypass mixed air are often used to extend the “indirect” dehumidification range of a constant-volume air handler. Simple and inexpensive, this option blends cold, dry air leaving the cooling coil with warm, moist, mixed air (return air and outdoor air) to achieve the proper supply-air temperature. The zone thermostat controls capacity by adjusting the face-and-bypass dampers, regulating airflow through and around the coil. Chilled water flow through the coil is constant, not modulated.

All mixed air passes through the cooling coil when a sensible design load exists, making dehumidification performance identical to that shown in Figure 1.

Figure 4 illustrates the classroom condition that results at the part-load latent-design condition when the blended supply-air temperature is 63°F. Using a coil performance program, we determined that the leaving-coil temperature falls to 52.7°F. Moisture in the bypassed air prevents more than a slight decrease in relative humidity (from 66.9%RH to 64.5%RH) and increases the total coil load from 3.68 to 3.74 tons.

Return-Air Bypass
In many climates, face-and-bypass dampers arranged to bypass return air provide a cost-effective way to extend the indirect dehumidification range of a CV air handler. Although ducting may increase its cost slightly compared with mixed-air bypass, return-air bypass limits relative humidity better than any other indirect dehumidification enhancement at both sensible and latent (part-load) design conditions.

Like the mixed-air version, the return-air bypass modulates coil capacity by adjusting airflow rather than water flow. This means that the coil surface can be very cold, enhancing the ability of the system to dehumidify the zone without directly controlling humidity.

What makes the return-air bypass more effective, however, is that it directs all of the moist outdoor air through the cooling coil. Relatively dry return air (rather than moist mixed air) reheats the cold air stream leaving the coil. When a sensible design load exists, the entire mixed-air stream passes through the cooling coil. Psychrometric performance matches Figure 1.

Figure 5 summarizes the effect of adding less moisture at latent design. Again, we used a coil performance program to determine the leaving-coil temperature of 52.9°F. Relative humidity in the classroom falls below the ASHRAE-recommended high limit, dropping from 66.9%RH to 55.2%RH. Maintaining this level of dehumidification requires a total cooling capacity of 3.92 tons.

Continue on to Direct Dehumidification