ASHRAE Journal - September 2022 - 10

LETTERS
The Perfect
Economizer
I want to congratulate David
Sellers, P.E., Member ASHRAE, on
an excellent column, " The Perfect
Economizer, " in the April 20222
ASHRAE Journal on airside economizers
and the identification of system
malfunctions that can lead to energy
waste rather than recovery during
winter variable air volume (VAV)
supply fan operation.
You are quite correct in identifying
accurate mixed air temperature
measurement as a leading contributor
to lost energy savings in economizer
systems, especially mixed air
wet bulb (WB) and dew point (DP)
temperatures.
The ASHRAE Epidemic Task
Force has published a list of best
practices that HVAC designers
may follow to mitigate the risk of
spread of airborne pathogens in
infectious aerosols indoors such as
COVID-19. Indoor relative humidity
(RH) should be maintained between
40% to 60% to reduce the spread of
airborne pathogens indoors, within
the human breathing zone, during
cold and dry ambient conditions.
Has your team installed and tested
a WB airside economizer using a
high saturation efficiency (97% to
99% RH) rigid media adiabatic evaporative
cooler/humidifier (AC/H)
to mix building return air with
outdoor air to produce a supply air
dew point that ranges between 45°F
DP to 55°F DP during cold and dry
ambient conditions (see my handdrawn
figure below)? The psychrometric
chart shows a VAV system at
50% fan turndown with an assumed
minimum 25% outdoor air to meet
code ventilation requirements. The
high saturation efficiency, at fan
turndown to 50% flow, ensures that
the delivery DB temperature off the
AC/H is within a fraction of 1°F of
both the WB and DP temperatures
at the saturation curve. A low-cost
commercial-grade DB sensor may
be used with acceptable accuracy in
determining the delivery DP condition
of the supply air. Parasitic
losses for this economizer are quite
low with a static pressure loss at
full flow of 0.2 in. w.g. and a water
recirculation pump sized under
1.5 HP. Both the chiller and boiler
are off, in the Portland Ore., climate
zone for 67.4% of the annual hours of
the year (yellow shaded area on the
psychrometric chart), while room
conditions are maintained between
40% RH and 55% RH at comfortable
room air temperatures (see room
target in orange shaded area). In
your Portland climate, only 6% of the
annual ambient conditions (below
37°F DB) would require preheating.
A 1980 Bin Weather Data reference
was used to predict the Portland
climate performance. Perhaps your
team has more current TMY2 hour
by hour ambient weather data that
might more accurately reflect the
effects of global warming in your
area. If you do, would you be willing
to share those numbers with your
readers?
Mike Scofield, P.E., Fellow ASHRAE,
Sebastopol, California
The Author Responds
I suspect folks with a health-care
background were not surprised to
see ASHRAE suggest designers consider
maintaining relative humidiA
wet-bulb economizer for VAV systems with ceiling-mounted high induction diffusers
and pinch down VAV terminals that include reheat and airflow measuring stations.
10
ASHRAE JOURNAL ashrae.o rg
S E PTEM B E R 2022
ties in the 40% to 60% range in the
" ASHRAE Position Document on
Infectious Aerosols " (https://tinyurl.
com/2nd3t7p5). In that context, I
suspect the system configuration
you suggest may merit consideration
as long as due consideration
was given to the application issues
the committee mentions in the
Journal's May 2021 IEQ Applications
column ( " ASHRAE Epidemic Task
Force Core Recommendations:
Reducing Airborne Infectious
Aerosol Exposure, " https://tinyurl.
https://tinyurl.com/2nd3t7p5 https://tinyurl.com/2nd3t7p5 https://tinyurl.com/ASHRAEEndemicCorRec https://ashrae.org/
ASHRAE Journal - September 2022

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