ASHRAE Journal - June 2021 - 46

COLUMN ENGINEER'S NOTEBOOK
Yes, Missouri has different ventilation
requirements than California,
but we are keeping them the same,
so we are only analyzing climate
impact on the results.
The calculated coil SHR is 0.60
and 0.79 for St. Louis and Ontario,
respectively. In reviewing the coil
performance for a packaged RTU at a
standard 400 cfm/ton (54.7 L/s·kW),
the coil SHR is about 0.78 for the St.
Louis conditions and 0.94 for the
Ontario conditions. Those capacities
are not very close to meeting
the required coil SHR based on the
load calculation. But if you lower the
airflow to 300 cfm/ton (40.3 L/s·kW),
your latent capacity increases, and
the coil SHRs become about 0.71
for St. Louis and 0.83 for Ontario,
respectively.
The coil capacities are different
for the two locations because they
depend on the entering air conditions
and ambient temperature.
With the St. Louis wet bulb entering
air conditions being about 3.5°F
(1.9°C) higher than Ontario, the St.
Louis coil has more latent capacity.
So, at 300 cfm/ton (40.3 L/s·kW), the
Ontario coil is pretty close to meeting
the calculated load, while the St.
Louis coil still is not.
When we look, the number of
hours the Ontario coil would not
provide the calculated latent capacity
are so few that the space does
not have time to get out of control
in terms of humidity. But there are
sustained hours in St. Louis in which
the coil does not have adequate
latent capacity, which will result
in an unacceptably high relative
humidity classroom environment.
The example above only describes
one specific situation. It is not
intended to conclude that humidity
46
ASHRAE JOURNAL ashrae.o rg
control is never warranted in arid
climate classrooms. For example,
shortly after arriving in Ontario, I
was asked to assess some science
classrooms that were experiencing
high humidity. Teachers in the
science classrooms were reporting
moisture on countertops and
papers curling from the high relative
humidity. When I went to the
site and took space readings, I found
that some of the spaces were as high
as 70°F (21°C) DB and 78% RH. These
higher humidity conditions only
occur for a few days per year, but
they do occur.
Looking at the classroom from our
previous example and analyzing the
SHR at the 0.4% design dehumidification
conditions, we get a much
different story than in St. Louis.
In Ontario, the coil SHR would be
0.50, which is far below the previously
determined coil SHR of 0.83.
The science classrooms I had been
asked to assess were served by standard
packaged rooftop units with
single-stage compressors. Since they
were science classrooms, they were
bringing in a higher amount of outdoor
air than a standard classroom,
which exacerbated the humidity
issues on those few days a year they
do have high outside dew point.
A science classroom located in
Ontario could have a coil SHR as low
as 0.44 on the design dehumidification
day. The higher outdoor air percentages
of the science classrooms
increased the number of hours the
coil could not meet the required
latent load, which resulted in high
humidity levels in the classrooms.
If you analyze the system performance
throughout the entire year,
you can make an educated decision
about the best path forward for your
J U N E 2021
FIGURE 5 Window with condensation and frost.
project based on how many hours per
year you expect those more extreme
conditions and whether your design
will comply with the applicable editions
of Standards 55 and 62.1.
What About Humidification Needs?
Like the impact of the climate on
dehumidification needs, the climate
is also important to consider
when you humidify a building.
Certain buildings such as hospitals
and museums require minimum
humidity levels inside. When you
humidify the building, the dew point
increases. This can be especially
problematic in cold climates, as it
can lead to condensation on glass if
not properly mitigated (Figure 5).
In a climate like St. Louis, the summers
are hot and humid, while the
winters are cold and have low dewpoint
temperatures. A future column
will provide a more in-depth analysis
of HVAC design in cold vs. hot climates.
Below is an example contrasting
design in different climates.
A few years ago, we were designing
St. Louis's Museum of Westward
Expansion. It is the museum located
beneath the Gateway Arch. We had to
humidify the museum in the winter
to maintain high enough humidity
levels to prevent the museum displays
from drying out and degrading.
IMAGE COURTESY OF IMEG CORP.
https://www.ashrae.org/
ASHRAE Journal - June 2021

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