Psychrometrics

Before discussing the various types of air-handling system con­figurations, an overview of the fundamentals of psychrometrics is in order. Psychrometrics is the dynamics of moist air. To diagnose problems in an HVAC system, a basic understanding of how air and water vapor react when heated and cooled is helpful.

Publications on psychrometrics and psychrometrics software are available that may allow the hospital engineer to better understand some of the principles discussed in this section.
Air is dry and is composed mostly of nitrogen and oxygen, with small amounts of carbon dioxide and hydrogen. It also includes water vapor in varying amounts. The amount of water vapor that air is able to hold in suspension depends on the pressure and tem­perature of the air. Low-temperature air has a lower water vapor capacity than does high-temperature air.

A sling psychrometer can determine the dryf-bulb temperature of air, which is the temperature of air registered by an ordinary ther­mometer, and the wet-bulb temperature of afir, which is the tem­perature registered by a thermometer whose bulb is covered with a wetted wick. The drier the air, the more evaporation will occur on the wick and the lower the wet-bulb temperature will be. The psy­chrometer is slung in a circular pattern, simulating a wind blowing across the bulb with the wetted wick. For moisture (i.e., water) to evaporate, it must absorb heat from a source. In this case, the wick draws heat out of the thermometer. The same phenomenon occurs when a person’s body feels cooler after stepping out of a swim­ming pool when the wind is blowing. Heat is drawn out of the body as water on the skin evaporates. The difference between the wet-bulb temperature and the dry-bulb temperature can be used to determine the relative humidity of the air. If the air is 100 percent saturated, or at 100 percent relative humidity, the dry-bulb tem­perature and the wet-bulb temperature will be the same, because no net moisture will be evaporated from the wick when saturated air is blown across the wick-covered bulb.

Relative humidity refers to the ratio of the actual water vapor in the air compared to the amount of water vapor the air could hold at that particular temperature.

Specific humidity refers to the absolute amount of water vapor in the air expressed in pounds of water vapor or grains of water vapor per pound of dry air. (See paragraph below on specific humidity.) One pound equals 7,000 grains.

Enthalpy refers to the measurement of the total heat contained in the air–water vapor mixture. It has an arbitrary zero point, which is listed as 0°F at sea level atmospheric pressure. Enthalpy is used to measure the amount of heat energy contained in the air–water vapor mixture. Enthalpy is measured in Btu/LB dry air.

Sensible heat refers to the addition of heat to or deletion of heat from the air with no change in the amount of water vapor, or mois­ture, in the air. This process is represented by a horizontal line on the psychrometric chart. Latent heating or cooling, measured at Btu per hour, or Btuh, occurs when moisture is either added to or taken away from the air–water vapor mixture. A person at rest pro­duces approximately 250 (Btuh) of sensible heat, or dry heat, which leaves the body because body temperature is higher than surround­ing air temperature. People also produce 200 Btuh of latent heat, or moisture, which is added to the air in the form of evaporated per­spiration. The psychrometric chart (see Figure 5-1) shows process lines representing the condition of air and what happens to it when either heat or moisture is added or taken away.

Figure 5-1 Psychrometric Chart

Dewpoint temperature, or saturation temperature, is the tempera­ture at which the air is fully saturated, or at 100 percent relative humidity. This is also the temperature at which moisture will begin to condense out of the air. For example, if the space conditions were 75°F dry-bulb and 60 percent relative humidity, the correspond­ing dewpoint would be slightly above 59°F. This means that if any surface in the room (e.g., supply diffusers, microscope lenses, metal tables) has a temperature lower than 59°F, moisture from the air will begin to condense on this surface.

Dry-bulb temperature, wet-bulb temperature, dewpoint tempera­ture, and relative humidity are all related, so if only two designated properties are known, all the other properties may be read from the psychrometric chart. The curved line, number 1 in Figure 5-2, is a saturation line. In all conditions occurring along this line, the air is fully saturated. If saturated air is further cooled, the water vapor in it begins to condense and tiny droplets form. These droplets create fog or clouds.

The psychrometric chart shows the air temperatures normally encountered in buildings. Psychrometric charts also exist for very high-temperature and very low-temperature air, but for most cases, the chart in Figure 5-1 will cover points normally encountered in a building environment. Figure 5-2 shows a skeleton chart with the location of the various characteristics of moist air. Many software programs exist to help maintenance personnel learn the psychro­metric chart and the processes. Appendix A-3 explains how to use the psychrometric chart.

Figure 5-2 Skeleton Psychrometric Chart 

Understanding the relationships of the data conveyed on the psy­chrometric chart helps hospital personnel see how changes made to systems can have negative effects. Many times in the quest to reduce energy consumption, hospital engineers raise air-handling unit dis­charge air temperature to lighten the load on the chillers. This may be fine in dry climates or when the relative humidity outside is relatively low, but once the temperature of the discharge air rises above 55°F, very little dehumidification will occur. Consequently, humidity in occupied spaces will rise. This is particularly critical in operating rooms and other sensitive areas of the hospital. It becomes even more critical if the room must be maintained at a lower temperature than 75°F, as is the case in operating rooms.

Using chilled water air-handling units with 42°F chilled water tem­perature and supplying 50°F leaving air temperature, many operat­ing rooms are designed to maintain room temperatures between 70 and 74°F, with a relative humidity range of 30 to 60 percent, typically at 50 percent. When surgeons request lower temperatures, the relative humidity cannot be maintained below the 60 percent limit. This is easy to see when these conditions are plotted on the psychrometric chart. As the temperature in the space is lowered to 65°F, the relative humidity in the space will exceed 60 percent. If the temperature is lowered even further to 60°F, the space relative humidity will exceed 75 percent. An understanding of the proper­ties of moist air and the air-conditioning process can demonstrate to maintenance personnel and surgeons why relative humidity cannot be maintained as temperatures are lowered unless the air-handling system is designed to deliver low dew point supply air temperature.

For various reasons, hospital surgeons request very low tempera­tures in operating rooms. Common requested design conditions are 60 to 62°F and 50 percent relative humidity. This is a dew point temperature of 41°F. Normal chilled water supply temperature of 42°F cannot produce supply air at this dew point temperature. Chilled water coils must be enhanced to provide dew point tem­perature of 41°F or cooler. There are two common approaches to maintaining low dew points in ORs:

  • A low-temp, glycol chiller, either air or water cooled, to produce chilled water supply temperature of 34 to 35°F
  • A desiccant wheel to dehumidify the supply air

Excerpt from: Mechanical Systems Handbook for Health Care Facilities
J. Robbin Barrick, PE, and Ronald G. Holdaway, PE
ASHE copyright 2014. Available at the ASHE Store.

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