ASHRAE Journal - October 2019 - 20

TECHNICAL FEATURE

OR HVAC Load Analysis
In dealing with OR design where the mandate is for
an environment that is both cool and not too humid, it
is imperative to perform a comprehensive load analysis
which includes all loads associated with the outdoor
ambient conditions, desired indoor space conditions
(both temperature and humidity), ventilation air
requirements, building heat gain, infiltration, internal
heat sources, and any additional factors that will influence HVAC system sizing. Achieving the desired environmental conditions require simultaneously satisfying
both the sensible and latent load components. OR load
analysis is a relatively straightforward process with the
exception of a few key considerations.
One important aspect of the HVAC load analysis that
should not be overlooked is properly defining the latent
component contributed by the vapor pressure differential between the OR and all surrounding environments.
Vapor pressure differentials result from the differences
in absolute humidity between areas. Absolute humidity
can be defined as the amount of water vapor present in a
unit volume (cubic foot of air) and is measured in inches
of mercury (in. Hg vp). Note that absolute humidity does
not fluctuate with the temperature of the air. Air pressure is typically measured in inches of water (in. H2O).
Realizing that mercury weighs over 13 times more than
water, it becomes apparent that the partial pressure of
water vapor exerts a considerable force in comparison.
Figure 1 shows that this can create a substantial driving force, greatly contributing to the transmission of
moisture from the ambient surroundings into the OR.
Consider that the infiltration of air and its accompanying load must pass through cracks and penetrations
in the construction, whereas water vapor can diffuse
through the entire surface of a building component
(permeation), impeded only by its permeance rating
and any added resistance due to vapor barriers. Many
interior and some exterior walls and ceilings within a
hospital do not have true vapor barriers installed within,
and if there may be poorly installed or severely damaged. An OR that is positively pressurized for an airflow
differential of +0.01 in. H2O/0.0025 kPa (+0.0007 in.
Hg vp) may stand little chance of opposing the migration of moisture into the space due to the higher pressures driving the water vapor, along with the way moisture transfers through building materials.
It is important to understand that underestimating
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O C T O B E R 2 0 19

FIGURE 1 Potential vapor pressure differentials across building components of the OR.

Annual Design
Conditions

97.2°F DB
76.6°F WB
0.704 in. Hg

82.9°F DB
79.4°F WB
0.974 in. Hg

9.57 in.
H 2O

13.24 in.
H 2O

Dehumidification
Design Conditions

Operating Room
5.38 in. H2O
72°F DB
50% RH
0.396 in. Hg

5.38 in. H2O
60°F DB
40% RH
0.209 in. Hg
(2.84 in. H2O)

72°F DB
50% RH
0.396 in. Hg

the amount of vapor migration the space will experience
can have a negative effect when trying to maintain a hospital OR designed for deeply cooled and dehumidified
conditions such as 60°F (15.5°C) dry bulb and 40% to
50% RH. In addition, latent loads associated with people
and other internal and external moisture sources must
be addressed. An analysis of this magnitude may be outside the scope of traditional commercial building HVAC
load calculation software, requiring additional psychrometric evaluation to confirm the humidity component
of the load is properly addressed.
Another critical component of the load analysis is
the contribution made by the outdoor ventilation air.
Outdoor air ventilation requirements can contribute
to over 40% of the peak air-conditioning load in an OR,
depending on climate zone, so choosing the outdoor
ambient conditions to be used in the load analysis, and
properly assessing the HVAC system capacity requirement are important steps in determining its total
impact.
When referring to climatic information, ASHRAE
provides five different data sets from which to choose.
We will discuss only two. The first, cooling dry bulb
(DB)/mean coincident wet bulb (MCWB) 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 dew point (DP)/humidity ratio (HR) /mean
coincident dry bulb (MCDB), 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

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ASHRAE Journal - October 2019

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