Challenges of refrigeration system for thermal test chamber

By John Ackermann

The construction of the chamber to test the thermal efficiency (K factor) of road vehicles at the SABS in Pretoria, presented some unique engineering challenges to the refrigeration contractor, Multistage Cooling.

GIZ001The package unit, built in the Randburg workshop of Multistage Cooling, was delivered to site on a skid.

The insulated chamber, with 150mm of polyurethane, measures 20.3m (l) × 4.4m (w) × 7.7m (h) and has doors with a clear opening of 4.9m (h) × 4.0m (w) at each end. A false ceiling provides a space of 2.0m (h) × 17.2m (l) and the full width of the chambers for the glycol cooling coil, bank of induction heaters, and five axial fans of 1 000mm diameter with 2.2kW three-phase motors.

The dimensions of the chamber can accommodate insulated/refrigerated trailers, rigids, marine containers, and tankers of maximum legal length, height, and width. With doors at both ends, the placing of the vehicle in the chamber is done with ease.

Air temperature in the chamber is maintained within a close tolerance by a glycol cooler with mixing valve and bank of induction heaters for reheating if required.

The test to validate the K value of a vehicle is done in accordance with standard SABS 1744. The standard details temperatures, air speeds, time intervals, and so on. Briefly, the test entails maintaining the air temperature in the chamber in the region of 7°C and the interior air temperature of the sealed vehicle heated to 35°C with electric 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 ceiling. 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/m2 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/m2 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 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 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 temperature of the glycol is maintained within a narrow range (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 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 insulation contractor then fixed the insulated false ceiling to the underside of the galvanised frame.

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. Access to the coil, heater bank, and fans is gained from three hatches in the roof via a ladder and catwalk. Bob Vuletic of Multistage Cooling personally supervised the installation of the equipment and refrigeration plant. The galvanised steel support structure was erected after the blower coil had been placed in position. The package unit, built in the Randburg workshop of Multistage Cooling, was delivered to site on a skid. 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.

“We also asked that three hatches be fitted into the roof of the chamber to allow access to the coil, heaters, and fans if needed for maintenance or repairs,” Vuletic explains.

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.

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.


Click below to read the November/December 2017 issue of Cold Link Africa

CLA Nov Dec 2017

 

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John Ackermann
Ilana Koegelenberg
Benjamin Brits
Paul Engelbrecht
Zeldalee Du Toit
Maxlee Marange
Timothy Sibuyi

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