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Pressure Drop Considerations in Air
Filtration
We’ve always known
that providing good air filtration to a building had many
benefits from providing a cleaner healthier environment to
protecting equipment, fixtures and processes. In January,
the American Society of Heating Refrigerating and Air
Conditioning Engineers (ASHRAE) released the, “Report of
Presidential Ad Hoc Committee for Building Health and Safety
under Extraordinary Incidents.” HVAC air filtration moved to
the forefront as a valuable way to help protect a building
against bioterrorism.
In response to the
current situation, many facility managers (FM) are either
moving to or have upgraded their levels of air filtration.
Some FM’s are not fully cognizant of the fact that
increasing efficiency without regard to pressure drop can
result in dirtier air and increases in energy usage/cost.
Additionally, there is an increase risk of system
malfunction, compromising overall HVAC system performance
caused by reduced airflow.
Three major
components to life cycle costing formula are initial
investment and maintenance, energy consumption, and disposal
(Chart 1). Based on operating characteristics, we know the
cost breakdown is:
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 Chart
1 |
Because energy
comprises such a large portion of the cost and because
pressure drop is the precursor of this energy usage,
providing clean air depends most on efficiency and pressure
drop.
Even before a FM can
determine what level of cleanliness they want, they must
look at the capabilities of the system they have (we are
assuming this is a retrofit with no new addition of HVAC
equipment). Most large HVAC commercial grade systems are
designed to handle pressure drops of one inch, possibly
more, for the air filter resistance. Matching filter
initial, final and average resistance to the system is
critical for proper air filtration and air exchange rates.1
Also, providing pressure drop reading devices such as
manometers or electronic pressure sensors is an absolute
requirement.
Next, examine the
pressure drops of various types and efficiencies of filters
to find one that meets the requirements, or retrofit the
filter bank or filter to include more filter surface area to
lower the resistance.
The Chart 2 shows
the visual on the optimal change-out point of an air filter
– that point where the pressure drop increases electrical
consumption and overtakes the initial cost of the filter.
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 Chart
2 |
The equation used to
derive the optimum final pressure drop is:
Energy
Consumption
(kWh)= Q ΔP t η 1000
Q = Air Flow
(m3/sec)
ΔP = Avg. Pressure Loss
t = Time in Operation (hrs)
η = Fan Efficiency
Putting the numbers
in this formula, FM’s can expect to find lower overall
electrical usage/operating costs for those filters that have
lower average pressure drops. It can help determine the best
time to change-out their filters.
Summary
System pressure is critical to the correct operation of the
HVAC equipment. Facility managers can look to increase their
level of air filtration efficiency and provide added
protection for their building, but must also keep an eye on
any added resistance to the system. Air filters that provide
lower pressure drops through increased media area or newer
synthetic medias can achieve the desired efficiencies and
save energy. Working with a NAFA CAFS to determine the best
recommended filter is critical to installing the correct
filter for the system. Pressure drop reading devices are
essential to determine optimum performance results and
filter change-out frequency. Maintaining the HVAC air filter
system with the proper measuring and monitoring devices will
greatly assist the FM’s with managing the building’s air
filtration systems.
References:
1
Chart 1: Carlsson, Thomas, "Indoor
Air Filtration: Why Use Polymer Based Filter Media,"
Filtration+Separation, Volume 38 #2, March 2001, pp 30 - 32
2 Chart 2:
R.H. Avery, Optimum Final Pressure Drop, NAFA Guide to
Air Filtration, 3rd Edition, Chapter 13: Owning
and Operating Costs.
3 ASHRAE
Standard 62-2003, "Ventilation for Acceptable Indoor Air
Quality"
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