PROJECT
INCORPORATING COLD CHAIN
heaters. The average temperature
difference may not vary by more than
0.5K during the test period.
Air speed in the chamber is
maintained at the specified level by
the five axial fans mounted in the false
plenum. Once the difference in the
chamber temperature and the interior of
the vehicle stabilises, it is maintained for
the specified period. During the specified
period, the power usage (watts) of the
fans inside the vehicle is measured.
With the temperature difference and
measured heater power usage, the K
value in watts (W) per square meter of
mean exterior surface of the vehicle
per degree of temperature difference
(kelvin) is calculated as K = W/m 2 K.
According to the ATP regulation, the
K value for the transportation of frozen
loads (-20°C and lower), as tested in a
chamber, should be 0.4W/m 2 K.
AMMONIA REFRIGERATION SYSTEM
The refrigeration system was designed
to have a low charge of ammonia. The
compressor, a Grasso V300, four-cylinder
reciprocating machine, is directly coupled
to a 37kW motor at a speed of 1 040rpm.
Cooling capacity is 100kWr at -12/35°C.
The discharge of the compressor is
piped to a Vahterus shell and plate heat
exchanger (SPH) fed with pumped water
from a BAC cooling tower. For standby
capacity and to prevent a stoppage
during a test period, the system has two
Grundfos water circulating pumps.
The chamber has access doors with a clear opening of 4.9m × 4.0m on both ends. The steel frame supporting
the false ceiling, coil, heater bank, and fans is visible on the inside.
The flow of liquid ammonia from the SPH
is regulated by a Witt HR 2H high-pressure
float to feed the surge drum (separator).
Liquid ammonia at -11.5°C is gravity fed
to a second Vahterus SPH, which cools a
glycol solution for the coil mounted above
the false ceiling in the chamber.
The temperature of the glycol is
maintained within a narrow band (0.5K)
by a mixing valve and buffer tank,
separating glycol at -8°C and -4°C. Two
primary and two secondary pumps
circulate the glycol through the
cooling coil.
The width of the glycol coil being
close to the width of the inside of the
chamber, required special handling to
place in position. “Although the coil is
stainless steel and less in mass than a
steel coil, it was difficult to position,” says
Bob Vuletic of Multistage Cooling, who
did the refrigeration installation.
The refrigeration plant was built as a
package unit on a skid in Multistage
Cooling’s workshop in Randburg and
transported to site on a standard road
vehicle.
The chamber has been operational
since May 2017 and all tests done have
been on refrigerated semi-trailers, both
new and others that have been in service
for two years and more. The results of
the tests have been encouraging with K
factors much lower than expected.
The coil, heater bank, and fans
are supported by a galvanised steel
structure of uprights with horizontal
members in-between. “We started at
one end, placing the uprights in position,
lifted the coil — keeping it perfectly
horizontal so as not to jam against the
chamber walls — and then continued
building the steel frame to support the
heater bank and then the fan unit, until it
was positioned 1.8m from the chamber’s
end,” says Vuletic.
The dimensions of
the chamber can
accommodate insulated/
refrigerated trailers, rigids,
marine containers, and
tankers of maximum legal
length, height, and width.
The insulation contractor then
fixed the insulated false ceiling to the
underside of the galvanised frame.
“We also asked that three hatches
be fitted into the roof of the chamber
The Grasso V30 compressor, directly coupled to a 37kW motor, has a cooling capacity
of 100kWr at -12/35°C. The internally partitioned buffer tank on the far right serves as a
reservoir of glycol at -4°C and -8°C.
30
to allow access to the coil, heaters,
and fans if needed for maintenance or
repairs,” Vuletic explains.
www.coldlinkafrica.co.za
At a recent seminar, it was asked if hi-cube
reefer containers could be tested in the
chamber. Yes, reefer containers can be
tested; the door height has been designed
to allow hi-cube reefers on skeletal trailers
to be placed in the chamber for K-value
testing. CLA
COLD LINK AFRICA • November | December 2017