Proper ventilation, airflow, temperature, and humidity are needed for successful surgical operations. The design requirements for these parameters in healhtcare settings are defined by ASHRAE Standard 170, and are generally straightforward. However, OR (operating room) users often try to operate OR HVAC (heating, ventilating, and air conditioning) systems at temperatures and humidity levels outside of the standard design range. A better understanding of OR HVAC parameters would help OR designers and users achieve more effective OR functionality.
ORs must be designed to provide a space relative humidity (RH) of 20 to 60 percent, and a space temperature of 68 to 75°F. However, many surgeons prefer a space temperature below 68°F, typically as low as 64 degrees. Given the ventilation requirements, OR HVAC systems typically have enough airflow to cool OR spaces below 68°F. What they often lack, however, is the dehumidification capability to achieve a 64 degree room while maintaining less than 60% RH
The relationship between temperature and humidity
Unfortunately, many users and operators of OR spaces are unaware of the fact that as space temperature decreases, space relative humidity increases. For example, a 68°F space with 60 percent RH, if cooled to 64°F, would then have a space relative humidity of 68 percent (see Figure 1). High humidity levels (above the allowable design range) can be alarming for OR users and operators. It should be known that per ASHRAE Standard 170 the design relative humidity for ORs is applicable only within the design range of space temperatures (see design range outlined in Figure 1).
Healthcare facility managers are tasked with providing OR users with the required and desired space conditions. When facility managers face complaints of high relative humidity readings at low temperatures in ORs, there is a need to educate OR users of the range of acceptable space temperature and relative humidity levels in OR. High relative humidity (RH) in conditioned spaces is most likely to occur during the humid seasons of the year, when HVAC systems typically deliver supply air at 55 °F and 95 percent RH. As cool supply air picks-up sensible heat from a space, the relative humidity decreases (space conditions move on the horizontal access, from left to right, in Figure 1).
Options for meeting ASHRAE standard 170
Although the design range of acceptable RH per ASHRAE Standard 170 is explicitly limited to the temperature range shown in Figure 1, high RH in ORs remain a concern for OR users and facility managers. Therefore, facility managers and designers need to consider options for providing < 60 percent RH for OR users that prefer space temperatures < 68°F:
- Specify air handler chilled water coils for 45°F supply air temperature.
- Provide supplemental DX (direct expansion) cooling coils for OR zones.
This first option can require significantly higher reheat energy for the air handler system, especially if other, non-OR zones are served by the same air handler system. Furthermore, this option can significantly increase cooling energy by reducing the annual hours of economizer operation (i.e., there are fewer hours in the year with lower outdoor air temperature or lower enthalpy). The second option can help to mitigate the energy impacts on the air handler system, but controllability of DX cooling can be an issue if precise and responsive compressor modulation is absent.
Problem solving together
Working together, OR designers and healthcare facility managers need to look beyond the prescriptive design requirements of ASHRAE Standard 170, and satisfy the space conditioning demands of OR users with effective and efficient strategies.