Transcritical CO2 for   Woolworths Distribution Centre

Transcritical CO2 for Woolworths Distribution Centre

By John Ackermann | All images by John Ackermann

One of the first Woolworths Distribution Centres and landmarks in Montague Gardens, has undergone a major conversion.

What was previously an ambient area for non-temperature perishables, has been converted to 2 763m² of floor space with an air temperature controlled at 2 to 5°C.

Before work could commence, the refrigerated banana box within the 2 763m² facility, was relocated to another building on the same site. The trans-critical CO2 plant for the new area, designed, manufactured, and installed by Mainstream Refrigeration, is not the only CO2 plant at the distribution centre.

In 2013 Mainstream Refrigeration installed a cascade R134a/CO2 plant for the low temperature store in the same building. The cascade system maintains -25°C in a store for frozen foods and ice cream and at the time was also used to ‘freeze’ eutectic plates for mini containers used in vehicles for local deliveries to Woolworth retail outlets.

The new refrigeration plant has two packs each with eight Frascold compressors, five operate on the medium temperature circuit and three are for parallel compression to improve the COP. With parallel compression the plant efficiency is improved incrementally depending on the ambient temperature.

The high cooling load of 880kW of the new area presented some unique challenges in sourcing equipment. Ten Frascold S40-26TK compressors, which tops the range of CO2 semi-hermatics, were needed, making the two racks the largest designed and manufactured by Mainstream Refrigeration to date. The receiver was not available off the shelf and is of a special design manufactured by Viola in Johannesburg.

The new outside brick and mortar refrigeration plant room at ground level, is close to the building to minimise pipe runs.

“The high cooling load of 880kW of the new area presented some unique challenges in sourcing equipment.”

Each pack has a ThermoCoil gas cooler with 10 ebmPapst 800mm diameter EC fans with variable speed control. The packs are piped to eight stainless steel blower coils mounted at ceiling level in the cooled area. Each alternate coil is piped to one of the packs to ensure continuous cooling in the event of a pack failure.

The blower coils are mounted in a plenum of 1.8m in height and discharge into a false ceiling of 600mm in height across the full length of the cooled area. Compliance with the fire regulations required sprinklers to be installed in the blower plenum. In the cooled area the required sprinklers are fixed to droppers that pass through the false ceiling.

Energy efficiency, life cycle costs and environmental impact were key considerations in the design and installation of the trans-critical plant. Features to optimise energy efficiency include variable frequency drives on each compressor, electronic stepper valves to modulate CO2 flow to the blower coils, the speed of blower coil fans are varied and all operating parameters are monitored and controlled by a tailor made Guardian plant management system developed over many years for Mainstream Refrigeration systems installed for Woolworths.

Of equal consideration in the plant design was plant safety and in many instances beyond the legal requirements. CO2 sensors are mounted at ground level in the plant room and in the return air plenums of the evaporator blower coils. The plant room has an extractor fan which automatically starts in the event of the CO2 level exceeding the first set point. If it continues to rise, the plant is shut down.

Goods are received into and dispatched from the new area through four insulated docking bays with vertical roll up doors, docking levellers and protection bollards and guards.

The old temperature-controlled area, installed in 1989, is charged with R22 and the conversion to natural refrigerant CO2 is on the horizon. The blower coils in this area are floor mounted and by raising such to ceiling level, as in the new area, will increase the 2 500m² of floor area by approximately 100m².

The conversion project was done to a tight schedule to meet the deadline ahead of the high-volume retail trade in December 2019. Mainstream Refrigeration started on site in November 2019 and the plant was operational by 1st week in December and fully commissioned by 20th December.

The plant performance during the hot summer months experienced in December and January, proved the choice for trans-critical CO2 was not only in keeping with an environmentally-friendly policy, but also an efficient option. There would also be no looming threat of having to convert the plant in years to come, because of the refrigerant being banned by international environmental agreements.

Completion of the challenging project ahead of the deadline is attributed to a close co-operation between suppliers, contractors and consultants.

List of professionals


Structural consultants


Electrical consultants

Cool Maintenance

Insulated Panels contractor


Fire Compliance consultants

Mainstream Refrigeration

Design, manufacture and installation of refrigeration plant

Guardian Controls (UK)

Plant Management System,


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