ASHRAE Journal - December 2013 - 139

Column HVAC AppliCAtions

especially those at the perimeter of the air outlet, in
some cases can have a higher velocity than the average
over the face.
According to basic physics, we know that if a jet of air
has lower static pressure than the surrounding air mass,
the air jet will induce air from its surrounding air mass.
In other words, a perforated face air outlet will induce
some air from around the air column. The supply air is
almost certainly a bit colder than the average temperature in the room, which means it has negative buoyancy
and will gain speed and contract as it drops into the
room. The greater the temperature difference between
the jet and the room, the greater the acceleration, and
the higher the mean velocity across the jet.
An air column measuring 25 fpm (0.13 m/s) just below
the panel will likely read 50 fpm (0.25 m/s) a short distance above the operating table or patient, with a few
degrees change in temperature. Should multiple panels
be located close to one another, there is a mass effect,
which further increases the air speed at the patient.
Regardless of the design chosen, the HVAC unit that
supplies air to the operating room needs to have HEPA
filtration to remove contaminants from the airstream.
These filters may be located in the ductwork that feeds
the operating room, at the air handler, or in the operating room integrated in the diffuser. Because room cleanliness is critical, one has to be cautious when accessing
filters for removal/replacement, being careful to not
contaminate the air supply system or the operating room
during the process.
For older designs without HEPA filtration, we have seen
them retrofitted with fan filter units, which are essentially HEPA filter perforated "laminar flow" devices with
a built-in fan to provide the additional pressure required
by the filters. These units often have ECM motors, which
are able to keep a constant flow until the rpm limit is
reached, then display an alarm for a filter change.
A lot of effort is put into the design of an operating
room air distribution system. So, it is important that we
evaluate all the parameters to ensure the choices made
will generate intended and realistic results. Both computational models and physical measurements have been
used for years to accomplish this.
In fact, today's modeling programs have been
improved to provide quick solutions for very complex,
turbulent flow environments. Evaluations should not
rely on the computational models alone. Additionally,

Figure 1 Laminar array.

physical measurements that include the use of radiant
shielded temperature probes and omnidirectional low
speed air movement sensors will aid in the validation.
For further verification, tests conducted in the surgical space should include both wet and dry particles, as
it appears that moist particles seem to exhibit different dispersion patterns than dry ones. ANSI/ASHRAE
Standard 113-2013, Method of Testing for Room Air Diffusion,
provides a good reference for the minimum requirements for any air distribution measurements.
Taking physical measurements during an actual
operation will of course provide the best validation,
but in today's litigious environment, it is unlikely any
facility's legal staff would allow such procedures. A
number of tests were, however, conducted in the past.
A discussion of them can be found in a 2006 ASHRAE
Transactions paper1 by Gerry Cook and me, as well as in a
2009 ASHRAE Journal article.2 More recently, a series of
CFD and validation tests were conducted in Egypt and
reported by Khalil.3
In these tests, it was validated that using an air curtain
minimized the transport of viable particulates from the
space outside the immediate operating area by blocking
the return air pathway along the ceiling to the laminar
panel arrays required by Standard 170.
Hospital operating rooms and cleanroom environments that are in constant use require vast amounts of
air. For this reason, it is crucial to manage energy consumption. At the same time, however, a clean supply of
air (typically 100% outside air) must be maintained.
Some would suggest that to keep costs down, the
least amount of air supplied the better. Building codes,
though, mandate minimum air quantities for different
applications, ranging from 15 to 30 air changes per hour
D ecem ber 2013

ashrae.org

ASHRAE JouRnAl

139

ASHRAE Journal - December 2013

Table of Contents for the Digital Edition of ASHRAE Journal - December 2013