Home » Charting the Proper Psychrometrics of Humidification and Dehumidification
Buildings require various levels of temperature and humidity controls, but when it comes to preserving historical artifacts or performing an operation, the temperature and humidity levels in a space become crucial to keep a building operational. For instance, a typical office building will require a space temperature of 75°F ± 2°F and a humidity level of 50% ± 5%; however, a typical museum gallery may require a space temperature of 70°F ± 2° and a relative humidity (RH) of 45% ± 5%. Museums, hospitals, and lab buildings all generally need stricter temperature and humidity set points, making psychrometric analysis an important step in the design of the HVAC systems that serve these types of buildings. A psychrometric analysis involves using a psychrometric chart, as seen in Figure 1, to determine the dry bulb (°F), wet bulb (°F), dew point (°F), RH (%), humidity ratio (gr/lb), and enthalpy (btu/lb) of air at a given condition. Moving the plotted point in different directions on the chart represents various ways of conditioning the air, as seen in Figure 2. With strict operating conditions, humidifying and dehumidifying strategies will need to be set in place in order to condition the air to a precise location on the psychrometric chart.
For example, take an interior 10,000-square-foot interactive museum gallery located in Washington, D.C., that has 20% outside air, a lighting power density of 1 W/ft2, a plug load of 5 W/ft2, and an occupancy of 400 people. From this, the sensible heat ratio, which is the sensible heat load divided by the total heat load, can be determined to be 0.79. The sensible heat ratio is used in tandem with the psychrometric chart by plotting the ratio, or slope, on the chart running through the space temperature and RH set points. Supply air must fall on this line in order to properly condition the space due to the specific sensible and latent load present.
As seen in Figure 2, the museum gallery is required to operate at 70° dry bulb and 45% RH. The outside air condition can be taken from ASHRAE climatic data, which, in this summer for Washington, D.C., is 92° DB and 74.7° WB. The mixed air condition can be determined by drawing a line between the return and outside air points, with the mixed air condition being based off the proportion of return air versus outside air. During summer, the mixed air is sent through a cooling coil to dehumidify and cool the air as seen in Figure 2. Typical cooling coils cool air down to 50°-52°; however, the room condition of 70° DB and 45% RH has a dew point of 47.7°. Exhibit spaces, in particular the immersive type, tend to experience high occupancy rates almost instantaneously and for periods of 30 minutes to two hours. This means that sizing a cooling coil to cool air below 47.7° to avoid condensation in the space may not suffice; the cooling coil will need to have enough additional capacity to address these high ‘instantaneous’ space sensible and latent loads. These authors recommend using no more than 3° chilled water approach with 40° chilled water entering temperature into the cooling coil in order to achieve a cooling coil leaving air temperature of 43°. Once the air is cooled to 43°, the air is require to be reheated to be on the sensible heat ratio line. The reheat can be done indirectly by having the chilled water coil upstream of the supply fans and by using the reheat coils at the terminal units.
The resulting supply air condition is 57.7° DB and 50.7° WB, which will provide proper sensible and latent cooling the museum gallery requires.
FIGURE 1: A psychrometric analysis involves using a psychrometric chart to determine the dry bulb (°F), wet bulb (°F), dew point (°F), relative humidity (%), humidity ratio (gr/lb.), and enthalpy (Btu/lb) of air at a given condition. Images courtesy of SmithGroup