ASHRAE Journal - February 2022 - 44

TECHNICAL FEATURE
heating during cold weather when no
mechanical cooling is needed as discussed
later in Figure 7.
Many advantages result from this
configuration, but all depend on how
the coils are selected. If the sizes of
coils and water loops are kept the
same as in a traditional system, this
configuration can reduce pump
energy uses as chilled or hot water
will split between the two coils during
full cooling or full heating periods. It
can enhance heat transfer between
water and air as more surface areas
(both coils instead of just one coil) are
used. However, to reduce the size of
AHU and water loop pipes, combined
coils and associated water loops can
be designed to meet the peak cooling
loads and the peak heating loads, cutting
the size of those coils and piping
diameters compared to a traditional
system.
FIGURE 5 Typical Air-Handling Unit AHU vs. Option 3 or Option 4. Two coils.
Exhaust Air (EA)
Return Air (RA)
CI1 CI2
Outdoor
Air (OA)
Mixed
Air (MA)
VAV Boxes
Water Loop 1
Water Loop 2
A. Option 3 or Option 4
FIGURE 6 Option 3 Configuration. Two coils.
A.
57°F
Air
DB = 63°F
DP = 63°F
CI2
Cooling
Air
DB = 55°F
DP = 54°F
21.6 Tons
400 gpm
42°F
C.
115°F
Another advantage is related to
the opportunity to run the chiller
in Loop 1 serving Cl1 at an elevated
chilled-water supply temperature
(e.g., 52°F [11°C]). Looking at the
example in Figure 6b, the first coil
(Cl1), which is typically designed for
heating with two rows, cools the air
to an elevated value-for instance
to 63°F (17°C)-and the chillers in Loop 1 can produce
a higher supply water temperature of 52°F (11°C) and
operate more efficiently than the chillers in traditional
configurations.
Air
DB = 44°F
10,000 cfm CI2
140°F
400 gpm
Heating
Air
DB = 70°F
286 kBtu/h
Loop 2
B1
On
Ch1
Off
Ch2
Off
B2
On
Loop 2
B1
Off
Ch1
On
Ch2
On
B2
Off
Supply Fan
Supply
Air (SA)
Exhaust Air (EA)
Outdoor
Air (OA)
Return Air (RA)
Supply Air
(SA)
Chilled Water Loop
Mixed
Air (MA)
Supply Fan
VAV Boxes
Hot Water Loop
B. Typical AHU
Design Cooling Conditions
B.
67°F
Cooling
Air
DB = 85°F
DP = 65°F
10,000 cfm
Loop 1
400 gpm
52°F
Design Heating Conditions
85°F
Heating
Air
DB = 38°F
10,000 cfm
Loop 1
800 gpm
110°F
VAV Boxes
A similar scenario for heating can be obtained as
shown in Figure 6d: Cl1 heats the air to 44°F (6.7°C) for
example, the boilers in Loop 1 can operate at lower supply
water temperature (e.g., 110°F [43.3°C]), but this
is possible only if the VAV box is designed for this low
temperature.
Looking at the control, both loops and associated
coils (Cl1 and Cl2) will provide cooling when the OAT
is greater than 65°F (18°C) (adjustable). The boilers in
both loops should be locked off. When the OAT drops
below 65°F (18°C), the chillers in this loop should
44
ASHRAE JOURNAL ashrae.o rg
F E B R U A RY 2022
be locked off, and Loop 1 will be available in heating
so the VAV box reheat coil can provide heating if
needed. Loop 2 continues providing cooling through
Cl2 if needed. When the OAT drops below 40°F (4.4°C)
(adjustable) so no mechanical cooling is needed, Loop
2 and Cl2 become available for heating. The chillers
in Loop 2 should be also locked off. When the
OAT increases above 45°F(7.2°C) (adjustable), Loop 2
and Cl2 become available for cooling. When the OAT
increases above 70°F(21°C) (adjustable), Loop 1 and
Cl1 become available for cooling. Five degrees is a dead
band to avoid cycling between cooling and heating.
The two coils could simply be controlled in sequence
as shown in Figure 7. In cooling, the SAT is controlled by
Cl2, and when the valve position becomes fully open,
CI1
Air
DB = 44°F
66 kBtu/h
CI1
Air
DB = 63°F
DP = 63°F
21.6 Tons
VAV Boxes
D.
22.8 gpm
35 gpm
400 gpm
400 gpm
400 gpm
400 gpm
CC
HC
5.2 gpm
35 gpm
http://ashrae.org
ASHRAE Journal - February 2022

Table of Contents for the Digital Edition of ASHRAE Journal - February 2022
