ASHRAE Journal - June 2014 - 26

Technical FeaTure

ASHRAE JouRnAl

ashrae.org

J u n e 2014

Load (MBH)

Load (Tons)

cooling at AHU cooling coils neces10,800
900
Line indicates the amount of energy available for heat recovery
sary to meet supply air set point temCHW
from the CHW loop when exhaust and relief air are used.
Energy Available
peratures. When AHU cooling is not
9,600
800
HW
available, the third source of energy
HW + DHW
8,400
700
are exhaust heat recovery coils. The
exhaust heat recovery coils consist of
7,200
600
normal CHW coils placed in exhaust
6,000
500
airstreams. These coils cool the outgoing exhaust air to provide the load
4,800
400
being called for by the heat recovery
3,600
300
chiller. These coils are only used when
heating demand exceeds genuine
2,400
200
cooling demand in the building. When
the heating demand exceeds the
1,200
100
cooling load and all the exhaust heat
0
0
recovery coils are full open, the last
J
F
M
A
M
J
J
A
S
O
N
D
source of energy the system will search
Figure 2: Building hydronic hourly loads.
for is energy available from relief air of
AHUs operating in economizer mode.
Temperatures
If AHUs are operating in economizer mode, the DDC
The heat recovery chiller operating efficiency is a funcsystem will calculate the appropriate mixed air temtion of the lift on the compressor. The lower the heating
perature required for the AHU cooling coil to meet the
water temperature and higher the CHW temperature,
load being called for by the heating water system. Then,
the higher the heat recovery chiller operating efficiency.
economizer operation will be over-ridden, the AHU will
Design temperatures were chosen to balance operating
mix return air with outdoor air supply to achieve the calefficiency of the heat recovery chiller with size and first
culated mixed air temperature, and the AHU cooling coil
will be used to extract the amount of energy necessary for cost of CHW and heating water coils. The heating water
system was designed with a 120°F (49°C) supply and 100°F
the heat recovery chiller to meet heating demand.
(38°C) return temperatures. With a supply temperature
equipment Sizing
of 120°F (49°C), most air terminal reheat coils were two
The heat recovery chiller was not sized to meet design
row coils and only a few required three row coils. The
conditions. Instead, an energy model was used to proCHW system was designed using 42°F (6°C) supply temduce the scatter plot shown in Figure 2 and the heat
perature to meet humidity requirements. To keep the
recovery chiller was sized using a combination of averlift on the heat recovery chiller as low as possible without
age heating demand in the summer, spring, fall, and
dramatically increasing the size of equipment, the heat
available energy in the CHW system during winter.
recovery chiller was designed to supply 48°F (9°C) CHW.
The heat recovery chiller is controlled to meet heating
Fan coil units serving process cooling loads were designed
demand. It does not have the ability to reject heat anyusing 48°F (9°C) CHW and a bypass valve was included so
where except the heating water system. When cooling
the main chillers could be bypassed when no ventilation
demand exceeds heating demand in the building, three cooling is required, and all process loads could be served
variable speed water-cooled chillers serve the additional by the heat recovery chiller directly.
cooling required. Because the water-cooled chillers have
Commissioning
lower condenser temperatures, they have the ability to
A hydronic heat recovery system is highly interactive.
meet cooling demand much more efficiently. When winter heating demand exceeds the amount of energy avail- AHUs, chillers, boilers, and other components that operate independently in a traditional system may impact the
able in the CHW loop, six condensing boilers suppleoperation of each other in a hydronic heat recovery system.
ment the heat recovery chiller to meet the load.

ASHRAE Journal - June 2014

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