ASHRAE Journal - November 2023 - 35

Of great significance in this example is that the need for costly
" reheat " has been eliminated, which in conventional operating room
design may range anywhere from 15°F to 30°F (-10°C to 4.0°C).
In a time when every dollar must be accounted for, this design
allows health-care facilities to maintain the surgical procedures
that yield the most profit, without the fear of losing surgeons to
competing facilities where operating conditions are more comfortable
and productive.9
Desiccant dehumidifiers may use refrigerant-based pre- and postcooling
devices to enhance efficiency and effectiveness. But without
being coupled with desiccant dehumidification, traditional cooling
technologies (alone) may be incapable of meeting the more stringent
environmental space requirements of this example.
Improving HVAC System Outcome
Transmission by aerosols is considered the main route for the
spread of COVID-19 (and other) infections indoors. Therefore,
limiting air transfer between supply and exhaust air in HVAC
ventilation systems is critical.10 Unlike energy recovery or passive
desiccant dehumidification systems that use rotary wheel heat
exchangers that work on the premise of creating high exhaust air
transfer ratios, active desiccant dehumidifiers (as described above) do
not exchange energy from a building's return or exhaust airstreams,
avoiding this potential pitfall.
Any HVAC system surface that is not regularly cleaned or possibly
disinfected can harbor pathogenic microorganisms and biofilms.11
Areas of a critical-care AHU system that could potentially harbor such
contamination include the fan, coil, drain pan, filters and ductwork,
particularly if there has been a buildup of dust or residue on these
inside surfaces, which (when chilled and wetted) can proliferate
microbial growth. In this example, using desiccant dehumidification
to remove all associated ventilation and building-related latent
loads will keep downstream AHU componentry dry, significantly
reducing the chance of any moisture occurrence. Benefits to consider
when applying desiccant dehumidification as a method to enhance
environmental control include:
1. AHU cooling coils can be selected to handle sensible-only
cooling loads. This may lower system first costs by reducing the
number of coil-rows and fins while lowering fan horsepower
requirements to conserve energy, reduce carbon emissions and save
operational expense.
2. Downstream AHU and ductwork (internal) surfaces and
componentry can remain dry, lessening concerns with microbial
growth and its aerosolization and distribution into occupied
environments.4 While this eliminates wet final filters and extends
their useful life, it may also reduce or eliminate the need for
manually cleaning bio growth from cooling coils and drain pans or
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ASHRAE Journal - November 2023

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