ASHRAE Journal - December 2013 - 24

technical feature

ASHRAE JouRnAl

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Cooling rate (Btu/h· Ft 2 )

systems are different from air systems.
Radiant System
Air System
a
B
The tests were carried out in a climatic
3
3
Convective Heat Gain
Cooling Load From
chamber (14 ft [4.27 m] × 14 ft [4.27 m]
Convective Heat Gain
2
2
× 9.8 ft [3.0 m]) that has been used for
standard radiant cooling panel testing.
1
1
The high insulation level of walls, floor
0
0
and ceiling makes the room almost
6:00
18:00
6:00
6:00
18:00
6:00
adiabatic. The test chamber (Figure
3
3
4) was configured to include a 1.5 in.
Radiative Heat Gain
Cooling Load From
(0.04 m) layer of concrete pavers covRadiative Heat Gain
2
2
ering most of the floor to provide thermal mass (3,000 lbs [1350 kg]). Heat
1
1
gain was simulated with a thin electric
0
0
resistance heating mat, laid on top of
6:00
18:00
6:00
6:00
18:00
6:00
the concrete blocks. The loose mesh
design of the heating mats allowed
5
5
Total Cooling Rate
13%
the radiant cooling ceiling panels to
4
4
3
3
interact directly with both the heater
2
2
and the concrete blocks. The radiant
1
1
ceiling panels were well insulated on
0 6:00
0 6:00
the top surface.
18:00
6:00
18:00
6:00
For each set of test conditions, two
Occupied
Unoccupied
Occupied
Unoccupied
separate experiments were conFigure 3 Conversion of convective and radiative heat gain into cooling load for air system (Figure 3A, left) and
ducted. First, radiant chilled ceiling
radiant cooling panel system (Figure 3B, right).
panels were used to condition the
chamber with controlled heat gain.
No air system was operated during the radiant system
systems during the 18.8 Btu/h·ft2 (59.3 W/m2) test.
test. Second, an overhead mixing air distribution sysThe shaded area around each measurement line inditem was used to remove the same heat gain profile. The
cates the experimental uncertainty. The cooling rate
12-hour test procedure was as follows. Prior to beginning
of the radiant panel system reached an average of 14
the test, the chamber and concrete thermal mass were
Btu/h·ft2(44.3 W/m2) between hours 1 and 2, and at
that time it was about 48% higher than the air system
allowed to reach a uniform, steady state initial temperature of 75.2°F (24°C). The test was started when the heater case. The radiant cooling rate slowly ramped up over
the next four hours until it reached an average of 16.9
was turned on and maintained at a constant value for six
Btu/h·ft2 (53.2 W/m2) in the last hour before the heater
hours. After six hours, the heater was turned off and the
was turned off. This value was about 18% higher than
experiment continued for another six hours. The heater
the air system case. Similar trends were observed for
schedule is shown in Figure 5. For the entire duration
the 26.1 Btu/h·ft2 (82.4 W/m2) test.
of each test, the radiant or air system was controlled to
Key findings from the experiment include:
maintain a 75.2±0.9°F (24 ±0.5°C) operative temperature
The radiant system has a higher cooling rate than the
in the chamber. Cooling rates were continuously moniair system, meaning that it is faster to remove heat gains
tored by measuring supply and return temperatures
while maintaining equivalent comfort conditions.
and flow rates for the hydronic radiant panel system and
For the tested cases, 75% to 82% of the total heat gain
the overhead air distribution system. Full details of the
was removed by the radiant system during the periexperiment are described by Feng, et al.13
Two series of tests were conducted, one at a heat
od when the heater was on, while for the air system,
input level of 18.8 Btu/h·ft2 (59.3 W/m2), and one at
61% to 63% was removed.
2 (82.4 W/m2). Figure 5a compares the
26.1 Btu/h·ft
For the radiant system, because of the direct radiant
instantaneous cooling rates for the radiant and air
exchange between radiant cooling panels and the
D ecem ber 2013

ASHRAE Journal - December 2013

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