ASHRAE Journal - June 2021 - 44

COLUMN ENGINEER'S NOTEBOOK
project? You do not want to unnecessarily add
cost to your project if extra dehumidification
measures are not required, but you also
do not want to cut corners and leave them
out when they really are needed to maintain
proper space conditions.
ASHRAE Standard 62.1 has for many years
limited the maximum indoor dew point
to 60°F (16°C). The most recent edition of
Standard 62.1 also adds a dew-point limit.
Generally, 30% to 60% indoor relative humidity
is considered ideal for occupant comfort.
Some specialty spaces will require more strict
relative humidity control.
Knowing when it is appropriate to use an
extra dehumidification strategy is a tricky
judgment call. You need to assess how critical
it is to maintain certain space conditions.
An office building owner may have a higher
tolerance for having high space humidity for
a few hours on a rare sticky summer day than
would the owner of a museum storing rare
paintings. Both building owners will have a
very low tolerance if those sticky summer days
occur all summer long every year.
Since we care about how often we would
FIGURE 4 Weather bin data for Saint Louis (Top) and Ontario, Calif. (Bottom).
St. Louis
Weather Data Location:
ST_LOUIS_INT'L_ARPT, MISSOURI, USA
Weather Hours
14 to 1
28 to 15
42 to 29
56 to 43
70 to 57
84 to 71
98 to 85
112 to 99
126 to 113
0.4% OA Design
Mixed Air-57% MOA
Cooling Coil Discharge
Space
Space SHR = 0.8
Ontario, Calif.
Weather Data Location:
expect the space conditions to be out of tolerance,
it is valuable to review the bin data
for the project location. Doing so allows you
to determine how many hours per year you
expect the system to maintain design space
conditions. Viewing the bin data on a psychrometric
chart can help you visualize your
climate conditions.
Looking at Figure 4 you can see that there are
far more hours above a 60°F (16°C) dew point
in St. Louis (top) than there are in Ontario, Calif. (bottom).
To determine if your equipment can meet your
dehumidification needs, you can calculate the coil sensible
heat ratio (SHR) and compare it to the capacity of
your proposed cooling coil. If your proposed coil cannot
satisfy the calculated latent load on more days than you
are willing to accept, consider an additional dehumidification
method.
Let's look at a 30 ft × 30 ft (9 m × 9 m) high school
classroom served by a standard single-zone packaged
rooftop unit (RTU) located in St. Louis and one located
44
ASHRAE JOURNAL ashrae.o rg
J U N E 2021
RIVERSIDE_MUNI, CALIFORNIA, USA
Weather Hours
25 to 1
50 to 26
75 to 51
100 to 76
125 to 101
150 to 126
175 to 151
200 to 176
225 to 201
Space SHR = 0.8
0.4% OA Design
Cooling Coil Discharge
Space
Mixed Air-57% MOA
in Ontario, Calif., and assess how the dehumidification
needs differ. In both cases we use the 0.4% design
outside air conditions. Keeping all aspects of the load
calculations the same (envelope properties, ventilation
rate, internal setpoints, etc.) except for the location and
associated outside air conditions, we get these summer
mixed-air conditions:
* Saint Louis, Mo.: 86.3°F DB/70.1°F WB/62.4°F DP
(30.2°C DB/21.2°C WB/16.9°C DP)
* Ontario, Calif.: 86.9°F DB/66.7°F WB/55.5°F DP
(30.5°C DB/19.3°C WB/13.1°C DP)
COURTESY DAVID SELLERS, FACILITY DYNAMICS ENGINEERING
https://www.ashrae.org/
ASHRAE Journal - June 2021

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