ASHRAE Journal - February 2022 - 42
TECHNICAL FEATURE
The main advantage of this configuration
is the use of one coil instead
of two coils in each AHU, leading to
smaller AHU size, less piping and
control valves and lower fan energy
use due to reduced flow resistance.
This configuration also eliminates the
need for a preheat coil.
Option 2:
One Common Coil With Heat Pumps
Some or all air-cooled chillers and
boilers in Option 1 can be replaced
with air-source heat pumps (HPs).
Figure 4 shows two chillers replaced
by heat pumps of the same capacity.
Figure 4a and Figure 4b show the design
cooling performance, and Figure 4c
and Figure 4d show the design heating
performance. This configuration
holds the same advantages of Option 1
but with fewer boilers to be installed.
This option reduces burning fuel at
the site and provides a pathway to
the future trend in electrifying and
decarbonizing buildings. The coil
design and control are identical to
Option 1 discussed earlier and shown
in Figure 3.
A.
FIGURE 3 Supply Air Temperature Control for Option 1 and Option 2. Single coil with and without heat pumps.
Water Loop 1 in Heating
100
CI
100
Control Signal
FIGURE 4 Option 2 Configuration. One common coil with heat pumps.
Design Cooling Conditions
57°F
Cooling
Air
B1
Off
Loop 2
Off
VAV Boxes
C.
115°F
Heating
B1
On
In cold climates when the air-source
heat pump efficiency reduces significantly,
dual-fuel heat pumps can
be used. In addition, at lower OATs,
when the heating load is too high and
when the SAT is maintained by the heating coil rather
than the economizer dampers, the SAT setpoint can be
reset to a lower value (e.g., 60°F [16°C] or even lower) to
shift the heating to the local reheats served by the boiler
and consequently reduce the load on HPs when they
are not that efficient. This will also reduce the load on
the coil (Cl) that is designed and selected for the cooling
season.
Loop 2
140°F
400 gpm
VAV Boxes
Option 3: Two Coils
Now let us examine a different scenario when two coils
in each AHU and two separate water loops are installed
as in a traditional system but with the possibility of using
42
ASHRAE JOURNAL ashrae.o rg
F E B R U A RY 2022
HP1
On
HP2
(May Be
Dual-Fuel
HP)
Design Heating Conditions
85°F
Heating
Air
DB = 38°F
10,000 cfm
Loop 1
110°F
400 gpm
CI
Air
DB = 70°F
352 kBtu/h
To Other AHU Coils
any of those coils in heating or cooling modes. To achieve
that, each water loop contains both heating and cooling
sources as shown in Figure 5 and Figure 6. Loop 1 serves
the first AHU coil (called Cl1), one chiller and one boiler.
Loop 2 serves the second coil (Cl2) and contains one
chiller and one boiler. The zone VAV boxes will be connected
to Loop 1.
The Cl1 coil can be designed for heating with two
rows, as an example, and the Cl2 coil can be designed
for cooling with four to eight rows. Cl1 will provide the
second stage of cooling during warm weather when no
heating is needed. Cl2 will provide the second stage of
3,200 gpm
42°F
HP1
On
HP2
On
DB = 85°F
DP = 65°F
10,000 cfm
Loop 1
CI
Air
DB = 55°F
DP = 54°F
43.2 Tons
To Other AHU Coils
D.
200
300
Free Cooling
Economizer
Damper Control
OAT = 45°F
OAT = 50°F
Water Loop 1 in Cooling
CI
B.
Valve Position (%)
800 gpm
400 gpm
28 gpm
70 gpm
http://ashrae.org
ASHRAE Journal - February 2022
Table of Contents for the Digital Edition of ASHRAE Journal - February 2022
Contents
ASHRAE Journal - February 2022 - Intro
ASHRAE Journal - February 2022 - Cover1
ASHRAE Journal - February 2022 - Cover2
ASHRAE Journal - February 2022 - 1
ASHRAE Journal - February 2022 - Contents
ASHRAE Journal - February 2022 - 3
ASHRAE Journal - February 2022 - 4
ASHRAE Journal - February 2022 - 5
ASHRAE Journal - February 2022 - 6
ASHRAE Journal - February 2022 - 7
ASHRAE Journal - February 2022 - 8
ASHRAE Journal - February 2022 - 9
ASHRAE Journal - February 2022 - 10
ASHRAE Journal - February 2022 - 11
ASHRAE Journal - February 2022 - 12
ASHRAE Journal - February 2022 - 13
ASHRAE Journal - February 2022 - 14
ASHRAE Journal - February 2022 - 15
ASHRAE Journal - February 2022 - 16
ASHRAE Journal - February 2022 - 17
ASHRAE Journal - February 2022 - 18
ASHRAE Journal - February 2022 - 19
ASHRAE Journal - February 2022 - 20
ASHRAE Journal - February 2022 - 21
ASHRAE Journal - February 2022 - 22
ASHRAE Journal - February 2022 - 23
ASHRAE Journal - February 2022 - 24
ASHRAE Journal - February 2022 - 25
ASHRAE Journal - February 2022 - 26
ASHRAE Journal - February 2022 - 27
ASHRAE Journal - February 2022 - 28
ASHRAE Journal - February 2022 - 29
ASHRAE Journal - February 2022 - 30
ASHRAE Journal - February 2022 - 31
ASHRAE Journal - February 2022 - 32
ASHRAE Journal - February 2022 - 33
ASHRAE Journal - February 2022 - 34
ASHRAE Journal - February 2022 - 35
ASHRAE Journal - February 2022 - 36
ASHRAE Journal - February 2022 - 37
ASHRAE Journal - February 2022 - 38
ASHRAE Journal - February 2022 - 39
ASHRAE Journal - February 2022 - 40
ASHRAE Journal - February 2022 - 41
ASHRAE Journal - February 2022 - 42
ASHRAE Journal - February 2022 - 43
ASHRAE Journal - February 2022 - 44
ASHRAE Journal - February 2022 - 45
ASHRAE Journal - February 2022 - 46
ASHRAE Journal - February 2022 - 47
ASHRAE Journal - February 2022 - 48
ASHRAE Journal - February 2022 - 49
ASHRAE Journal - February 2022 - 50
ASHRAE Journal - February 2022 - 51
ASHRAE Journal - February 2022 - 52
ASHRAE Journal - February 2022 - 53
ASHRAE Journal - February 2022 - 54
ASHRAE Journal - February 2022 - 55
ASHRAE Journal - February 2022 - 56
ASHRAE Journal - February 2022 - 57
ASHRAE Journal - February 2022 - 58
ASHRAE Journal - February 2022 - 59
ASHRAE Journal - February 2022 - 60
ASHRAE Journal - February 2022 - 61
ASHRAE Journal - February 2022 - 62
ASHRAE Journal - February 2022 - 63
ASHRAE Journal - February 2022 - 64
ASHRAE Journal - February 2022 - 65
ASHRAE Journal - February 2022 - 66
ASHRAE Journal - February 2022 - 67
ASHRAE Journal - February 2022 - 68
ASHRAE Journal - February 2022 - 69
ASHRAE Journal - February 2022 - 70
ASHRAE Journal - February 2022 - 71
ASHRAE Journal - February 2022 - 72
ASHRAE Journal - February 2022 - Cover3
ASHRAE Journal - February 2022 - Cover4
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