ASHRAE Journal - May 2024 - 44

TECHNICAL FEATURE
FIGURE 3 Heat transfer with four-row/six-row/eight-row coils and varying
air velocity (based on outdoor airflow of 5,000 cfm (2360 L/s) at 0.4%
dehumidification design condition for Atlanta).
600
fpm
400
fpm
200
fpm
600
fpm
400
fpm
200
fpm
600
fpm
400
fpm
200
fpm
Four-Row Coils
Six-Row Coils
Eight-Row Coils
depression of the dew-point. With beefi er SER coils,
the dew-point drops by 2.5°F (1.4°C) and 3.0°F (1.7°C)
for six-row and eight-row coils, respectively. In this
particular case, using SER reduces the absolute humidity
of the conditioned air by about 7% for four-row coils, 9%
for six-row coils and 11% for eight-row coils.
Looking at the water side, without the benefi t of
SER, the chilled-water fl ow through the cooling coil
must increase by about 20% (in this case, 58 gpm
[3.7 L/s] versus 48 gpm [3.0 L/s]) to achieve the design
50°F (10°C) leaving air temperature. Of course, it is
recommended to modulate the cooling-coil setpoint
according to real-time dehumidifi cation measurements
in the spaces served by the DOAS. As a matter of fact, for
humidity control systems that use mechanical cooling,
ASHRAE/IES Standard 90.1-2022 Section 6.4.3.66 sets
the lower limit for indoor humidity at 55°F (13°C)
dew-point, with some exceptions. (Please note that all
ASHRAE Standard 90.1 citations in this article are to the
2022 edition, unless noted otherwise.)
ENERGY SAVINGS
WITH SER ANNUAL
(Btu)
5B
4A
3A
2A
1B
44
Denver
New York City
(LGA)
Atlanta
Tampa, Fla.
Phoenix
47,312,948
ANNUAL
COOLING
SAVINGS
$434
ANNUAL
REHEAT
SAVINGS
$473
158,972,316 $1,457 $1,590
250,250,212 $2,294 $2,503
440,011,515 $4,033 $4,400
265,115,927 $2,430 $2,651
ASHRAE JOURNAL ashrae.org MAY 2 0 2 4
ANNUAL
$241
$241
$241
$241
$241
ADDITIONAL
FAN LOSS
ANNUAL
PUMP
LOSS
$28
$45
$68
$107
$70
Figure 3 compares the energy-recovery performance
of four-row, six-row, and eight-row coils at varying air
velocities. Increasing the number of rows from four to
six increases the heat transfer by 16%, and increasing
rows from six to eight boosts the heat transfer by an
additional 9%. Similarly, slowing the air from 600 fpm
(3 m/s) to 400 fpm (2 m/s) increases heat transfer by
about 12%, and further slowing the air from 400 fpm
(2 m/s) to 200 fpm (1 m/s) boosts the heat transfer by
another 8%. Obviously, the additional energy recovery
must be balanced against the added cost of providing
more heat transfer area, as well as the additional fan
power required to overcome the coil pressure drops.
The deeper the required drying of the outdoor air,
the more energy SER can save. With 50°F (10°C) air
leaving the cooling coil, the estimated annual cooling
energy savings is between 19% and 33% for the fi ve sites
listed in Table 1. Field measurements in Climate Zone
5A showed a 20% reduction in peak cooling load when
using SER. Simulation of fi ve Climate Zones (Table 1)
show a simple economic payback from about one year
up to about 10 years.
During unoccupied periods, a DOAS may be called
upon to dehumidify the building without introducing
outdoor air. When the supply air is recirculated, a
SER system can continue to save considerable energy,
whereas an EAER system, by itself, may not be helpful.
Energy Considerations-Standard 90.1
Energy Recovery
Series energy recovery (SER) was fi rst mentioned in
the 1999 edition of Standard 90.1, where it was allowed
in lieu of exhaust air energy recovery (EAER). That
concession has continued through the 2022 edition.
However, beginning with the 2019 edition, the following
constraints were introduced:
1. For nontransient dwelling units, such as apartments
TABLE 1 SER annual energy savings and estimated payback based on 8,760 h/yr operation.
CLIMATE ZONE & SITE
HOURS PER
YEAR PUMP
OPERATION
1,836
2,984
4,545
7,108
4,657
ANNUAL
ADDITIONAL
MAINTENANCE COST
$150
$150
$150
$150
$150
ANNUAL NET
UTILITY
SAVINGS
$489
$2,612
$4,338
$7,936
$4,621
INCREMENTAL
INSTALLATION
COST
$5,100
$5,100
$5,100
$13,400
$13,400
SIMPLE
ECONOMIC
PAYBACK (YEARS)
10.4
2.0
1.2
1.7
2.9
Series Heat Transfer Rate
87,485 Btu/h
94,707 Btu/h
102,390 Btu/h
103,830 Btu/h
109,947 Btu/h
113,860 Btu/h
115,695 Btu/h
119,776 Btu/h
124,951 Btu/h
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ASHRAE Journal - May 2024

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