ASHRAE Journal - February 2013 - 18

HTC W/m2·K

Unfortunately, when applying the low-pressure reCombined
ceiver to ammonia systems,
Purger
Evaporator
the situation is far from ideal.
Condenser
& Level
Control
3
Properties of ammonia result
P
4 Port Valve
in low mass flow, low vapor
6
fraction by mass and very
4
3
2
low density of the vapor frac1
2
tion. The result is a tendency
Compressor
to produce, in the tubes of
Low Pressure
an air-cooling evaporator,
Receiver
stratified flow that persists
5
5
6
1
throughout the evaporator
Heat Exchanger Sight
Expansion 4
Strainer
H1 – H6 = H3 – H4
because there is not suffiGlass
Valve
H
cient evaporation to get the
vapor velocity high enough Figure 1: Low-pressure receiver circuit and Mollier diagram.
to break up the stratified flow.
This tendency was exacerbated when tube material of low
conductivity, such as stainless steel was used. The challenge
was overcome by substituting aluminum for stainless steel in
the cooler. Figure 2 shows the relative proportions of stainless
steel tubing and of aluminum that were used in comparative
tests under practical conditions in an operating cold store. The
figure also illustrates the type of stratified flow that can be
experienced.
Performance of the aluminum cooler was also improved by
treating the internal surface of the tube to promote “wicking”
5/8 in. OD Stainless Steel Tube
7/8 in. OD Aluminum Tube
as is done in heat pipes.
Site testing of the coolers indicated that overall performance Figure 2: Relative proportions of aluminum and stainless steel
of the treated aluminum cooler was at least twice as good as tubing used in comparative tests in an operating cold store.
that of a comparable stainless steel cooler, and that perfor25
mance of the aluminum cooler was less dependent on internal
velocity than was the performance of the stainless steel cooler.
Figure 3 shows the performance of the coolers at varying
20
heat flux. It can be seen that, under low flux conditions, the
y = 21.891x –0.055
SS Cooler
overall heat transfer coefficient related to the outside surface
15
AI Cooler
of the cooler is about 1.3 Btu/h·ft2·°F (7.4 W/m2·K) whereas
the coefficient for the treated stainless steel cooler is about
10
3.7 Btu/h·ft2·°F (21 W/m2·K).3 The apparent decrease in heat
y = 6.7374x –0.1905
transfer coefficient with increasing heat flux in the aluminum
5
cooler may be a pressure effect, but it is more likely that heat
transfer coefficient is more or less independent of heat flux in
this design of cooler.
0
0
2
4
6
8
10
12
These figures mask a remarkable difference. If the cooler
Circuit Loading kW
dimensions and calculated outside coefficient of heat transfer
are used to infer the internal coefficient of heat transfer, it is Figure 3: Finned air cooler test (2009); heat transfer coeffifound that the calculated internal coefficient for the stainless cient vs. circuit loading.
steel cooler is only 31.8 Btu/h·ft2·°F (181 W/m2·K) whereas
the internal coefficient in the aluminium cooler is 220.1 (1250 Not only was the system better in applications for which amW/m2·K). It was concluded from these figures that the internal monia LPR systems were already competitive, but the change
surface of the stainless steel tube was only wetted to an extent allowed the application of ammonia LPR systems to be extended
of about 15%. Photo 1 shows an aluminum cooler with cross- to other applications where they had not previously been comover connections for use in a cold store.
petitive. Such systems included economized and two-stage sysChanging to the use of treated aluminum coolers in ammonia tems where mass flow of flash gas after the expansion device had
low-pressure receiver systems was a significant breakthrough. been insufficient to allow proper functioning of the evaporator.
18

ASHRAE Journal

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February 2013

ASHRAE Journal - February 2013

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