Environmental Design Considerations for Hospital Operating Rooms (Part-3)

Part-3: Outdoor Air Loads

Once the operating room (OR) space temperature and humidity requirements have been established, it’s time to begin identifying the individual air conditioning (A/C) load components and the impact each contributes to sizing the HVAC system. According to the ASHRAE HVAC Design Manual for Hospitals and Clinics, second edition, the purpose of the HVAC system in an OR is to minimize infection and maintain staff and patient comfort. Because of heightened infection concerns, ORs have prescribed air change and pressurization requirements. The current recommendation in 2014 FGI Guidelines for Design and Construction of Hospitals and Outpatient Facilities and ANSI/ASHRAE/ASHE Standard 170-2013 (per Table 7-1) is 20 air changes per hour (ach) supply air including 4 ach of outdoor air (20% outdoor air). This outdoor air requirement can contribute to over 40% of the peak air conditioning load, so choosing the outdoor ambient conditions (to be used in the analysis) is a critical step in determining its total impact. 

The ASHRAE Handbook of Fundamentals (2013) Chapter 14 provides climatic design information for 6443 locations in the United States, Canada and around the world. This includes summaries of values for dry bulb, wet bulb, and dew point temperature. Warm season temperature and humidity conditions are based on annual percentiles of 0.4, 1.0 and 2.0 (% of 8760 hours). The use of annual percentages ensures that they represent the same probability of occurrence in any climate; so for example, using the 0.4 percentile would represent a “to exceed” occurrence at this condition for no more than about 36 hours per year. The user must determine from which percentile category to select the weather data based on how many hours per year the A/C system can “miss the mark,” possibly falling short of meeting the load if actual outdoor ambient conditions exceed the data set for a short period. This is rarely an issue in practice because most HVAC designers add a safety factor to their analysis to help compensate for unexpected weather or internal load occurrences. In addition, A/C equipment is typically chosen for the “next size larger” when selected.

When referring to this climatic information, ASHRAE provides five different data sets from which to choose. We will review only two. The first, Cooling db/wb is considered “Cooling Design Day” data and is traditionally chosen when sizing “less-critical” applications such as commercial office buildings. The second data set, Dehumidification DP (dew point)/HR (humidity ratio)/MCDB (mean coincident dry bulb), is considered “Dehumidification Design Day” data and is traditionally chosen when sizing buildings where there is a “more critical concern” in maintaining the required indoor relative humidity condition at all times, particularly on days of the year that are “wetter” than others, such as a warm mid-summer day when it has just rained. Someone outdoors on a day like this may describe it as feeling like “wearing a wet blanket.”

To show the impact in choosing one set of conditions over the other, let’s examine weather data compiled for Houston Intercontinental Airport at the 0.4 percentile:

1. Cooling Design Day: 97.2°F db / 76.6°F wb = 39.95 Btu/lb.
2. Dehumidification Design Day: 78.2°F db / 147.1 HR / 82.9 MCDB = 42.95 Btu/lb.

This small difference may seem insignificant, but it actually has a major impact on the HVAC system’s ability to meet higher latent load requirements when necessary. If attempting to cool 1000-cfm of outdoor air from Dehumidification Design Day conditions to a supply air temperature of 50°F db / 49°F wb, an A/C unit sized using the Cooling Design Day weather data would be undersized by 2.42 tons of latent cooling capacity and a little over 1 ton in total cooling capacity. This error (particularly in critical care environments) can result in disastrous effects by limiting the ability of the HVAC system to produce proper environmental space conditions. This can contribute to an OR that will rise above acceptable indoor relative humidity levels and possibly result in the formation of condensed moisture (from the air) on interior surfaces.

Part 4 in this series will continue to explore the environmental design of hospital operating rooms by discussing some of the additional impacts outdoor ambient conditions have on sizing the A/C system.