With much of a health-care facility's profits impacted by the type and quantity of surgeries performed, hospital administrators are prioritizing indoor environmental conditions not just for the safety of patients, but to improve the efficiency and quality of work life of surgeons.

The multiple layers of clothing, face masks, hair covers, and protective gloves surgeons wear trap body heat and exacerbate perspiration. A sweaty brow is counterproductive for a concentrating surgeon. Thus, surgeons are demanding cooler, drier operating rooms.

HOSPITAL DESIGN CONDITIONS

General comfort is significantly greater and perspiration issues are minimized when relative humidity is kept below 60 percent. Additionally, lower relative humidity decreases the likelihood that condensation and fogging will occur on magnifying lenses and polished-metal working apparatus.

Common areas of hospitals generally are maintained at 75°F and 50-percent RH, which equates to a 55°F dew point, while operating rooms often are maintained at 60°F and 50-percent RH, which equates to a 41°F dew point (Figure 1). While the relative humidity is the same, operating rooms require substantially drier air.

FIGURE 1. Hospital design conditions.

Requirements for drier air magnify the challenges of moisture-management design. Unwanted moisture is most likely to be introduced into a space via ventilation air. For example, in Greensboro, N.C., each 1,000 cfm of fresh air represents 7.1 gph of water relative to a space set point of 60°F/50-percent RH. Conventional equipment that heats and cools air can remove some of this moisture, if the cooling load is great enough. Heating and cooling equipment controlled by a thermostat provides dehumidification by chance, not by design.

Typical-meteorological-year weather-bin data indicate the ambient dew point in Greensboro is above 55°F 3,623 hr. The ambient temperature is below 75°F 2,418 of those hours, meaning 66.7 percent of the dehumidification season is cool and wet simultaneously. Those cool and wet conditions demand a means of positive dehumidification.

Following are general guidelines for outside-air preconditioning:

• Positively dehumidify all makeup air to the required dew point. Supply dew point must be depressed to at least space-dew-point set point at all times, regardless of space-temperature needs.

• Account for internal latent load. People load, open water, and infiltration represent internal sources of moisture. Dehumidification equipment must supply air with a dew point lower than the desired space dew point to support internal loads.

• Provide reheat to prevent overcooling of a space. This is especially critical with operating-room applications (considering the conventional method of achieving 40°F dew point is to overcool air to 40°F).