Woolworths Kyalami takes CO2 to the next level

By Commercial Refrigeration Services (CRS)

The custom-built, locally manufactured CO2 refrigeration pack for the Woolworths store in the upmarket Kyalami shopping centre, takes care of the store’s refrigeration and heating needs.

WW00 12Woolworths Kyalami chose to go with CO2 for their refrigeration needs.

For this project, CRS manufactured a compound (two-stage) trans-critical booster pack with parallel compression. It consists of medium-stage, low-stage, and intermediate-stage racks, which are controlled by individual rack controllers with their own separate suction, operating, and safety set-points.

Each medium-temperature cycle consists of three compressors and each low-temperature cycle consists of two compressors. The intermediate temperature cycle also consists of two compressors and share a common discharge line with the medium-temperature rack.

The medium- and low-temperature racks share a common liquid line and separate suction lines to accommodate two separate evaporating temperatures; the low-temperature refrigeration discharges into the medium-temperature suction and in this regard, the low-temperature rack is reliant on the medium-temperature rack to maintain the low-temperature compressor’s discharge pressure within the design envelope.

The pack is designed to have a plant room evaporator coil to ensure the plant room area is maintained within acceptable ambient temperatures of a maximum of 25°C.

Plate heat exchangers are used for heat reclaim; these heat exchangers are connected to the discharge of the medium-temperature rack and heat produced from the refrigeration cycle is used to heat up a secondary water cycle for use within the store area.

The adiabatic gas coolers are situated in a properly ventilated area. In high-ambient conditions, the gas cooler may use water to maintain acceptable discharge pressures and temperatures. The water is evaporated, thereby lowering the dew point temperature below ambient temperatures, allowing the system to continue to run within acceptable limits.

The pack is designed to have a plant room evaporator coil to ensure that the plant room area is maintained within acceptable ambient temperatures of a maximum of 25°C.

Woolworths is now working with CRS to use dry gas coolers instead of adiabatic to help preserve the use of water.

All high-pressure piping is done with stainless-steel tubing, fittings, and components, where necessary copper piping is used for all the low-pressure and suction line piping. Where copper piping is used, it is ensured that high-pressure copper piping is fitted.

It is important to note that all lines are fitted with relief valves:

  • Medium-temperature suction: 48 bar
  • Low-temperature suction: 30 bar
  • Medium-temperature discharge: 130 bar
  • Low-temperature discharge: 48 bar
  • Liquid line: 48 bar
  • Receiver: 48 bar.

The receiver (or flash tank) is also fitted with a purge valve; this is operated through a pulse timer and controlled by a pressure switch to relieve pressure from the receiver should the pressure rise uncontrollably for whatever reason. This pre-emptive safety feature is to help prevent the total loss of the refrigerant charge.

All electrical components are linked with an array of cables, and the racks are operated by switch gear and protected by various electrical and mechanical safety switches.

A Danfoss system manager, case, and pack controllers have been used to ensure that the refrigeration cycle is continuously run at optimum performance.

Underfloor heating

There are two heat exchangers on the CO2 discharge line, which heat up the water to the required 55°C. This is then circulated through two 250ℓ storage vessels. It is controlled by a hot water pump and variable speed drive (VSD) to maintain the required temperature. This was part of CRS’s scope.

WSP’s scope involved pumping water from two storage vessels through the heat pump if further heating is required, or bypassing directly to a series of manifold stations positioned in the store. This split up the hot water supply into various underfloor water heating pipes that were cast into the surface bed at the stage of construction.

Plant operation

The refrigerant used within the refrigeration cycle is CO2 (R744) and is a natural refrigerant that is odourless, non-flammable, food safe, and heavier than air.

Leak detectors are therefore essential in particular areas of low ventilation; that is, cold rooms, and are required to be situated close to the ground.

Each rack consists of more than one compressor, where the first compressor is controlled by a VSD — this is used to control the suction temperature as close as possible to set-point. The rest of the compressors will start up direct on line as needed when the required capacity of the cycle increases. The compressors that are not driven will switch on in a cyclic manner to ensure that running time is spread between each non-driven compressor.

The Danfoss pack controllers will always attempt to operate to the best coefficient of performance (COP) possible, unless set-up is altered for heat reclaim. The medium-temperature cycle will operate in a trans-critical manner in high-ambient temperatures where the condensing pressure is above 72 bar or 31°C. When operating in this state, the refrigerant is unable to condense into a liquid. Below a condensing temperature of 31°C, the medium-temperature cycle will operate in a subcritical manner; this means that the refrigerant will condense to a liquid and the discharge pressure will vary between 55 and 70 bar. The head pressure varies according to ambient temperature and is controlled via the Danfoss controllers installed.

When the system is unable to condense below 72 bar and to condense CO2 to a liquid, a high-pressure expansion valve is used. In the case of Danfoss, the device is known as the ICMT valve. The pressure in the gas cooler leaving as well as the outlet temperature are calculated to vary the opening of the ICMT. The pressure in the receiver is maintained at 38 bar, or 3.3°C where CO2 exists as a liquid.

The liquid is then fed to the evaporator coils where direct expansion occurs.

The medium-, low-, and intermediate-temperature cycles are equipped with a common oil management system, which ensures that the refrigeration compressors stay lubricated. The system is of a closed loop configuration type and only requires maintenance should contamination occur. The oil management system has built-in safeties that automatically switch the compressors off when it senses low or no oil levels and automatically resets once minimum oil level has returned.

All switch gear for the packs are situated in the main board within the plant room and within the pack boards that are situated on the packs themselves.

Woolworths is now working with CRS to use dry gas coolers instead of adiabatic to help preserve the use of water.

All controls that operate the evaporator coils in the store are situated within the store at each individual coil respectively. Each compressor is fitted with a manual overload and thermistor, which will protect the compressor electrically; should this fail, it usually indicates a faulty compressor and a technician will be required before resetting.

The heat reclaim to supply hot water can be connected through the discharge of the medium-temperature cycle and is piped in such a manner that the heat exchanger could be bypassed with the help of a three-way ball valve. A signal is received when heating is required and in turn, this opens the three-way ball valve so that hot water is generated and supplied. The hot water can be controlled to a temperature close to boiling point.

The medium-, low-, and intermediate-temperature racks are fitted with low- and high-pressure safety switches, which will stop the cycle if the operating pressures are out of the desired set-points. Should the physical safeties trip, a technician will be required to manually reset and check that the system is safe to operate.

Due to the high operating pressures of CO2, the pack controller is set up so that it will stop certain compressors should the discharge pressure and/or temperature of the cycle exceed the safety limits and eventually, it will stop the whole cycle should the problem not resolve itself. The controllers are fitted with pressure transducers, which the controller reads to calculate the operating parameters.

When the pressures are within the desired range, the cycle will start up automatically.

Every pack and its specific controllers are connected to a main safety circuit that will stop all refrigeration completely if none of the compressors are safe to run. The safety circuit will be de-energised, preventing migration of the refrigerant. With CO2, this can occur in a very short period and can cause excessive pressures, relief valve blow-offs, and very high start-up loads in the system.

Pressure relief valves are fitted on critical main pipelines and receiver to relieve pressure when necessary; this is to ensure the safety of the valves and the components fitted in the refrigeration cycle. CO2 sensors are installed in the plant room and on the floor area, which will detect a leak and sound an alarm if the CO2 concentration gets too high. The sensor in the plant room will activate an extraction fan when the concentration of CO2 within the plant room gets too high.

List of professionals

Owner   - Woolworths
Developer - Iguana
Architect / Designer  - R&L
Project manager   - Woolworths
     
Consulting engineers    
Electrical - RWP
Mechanical - CRS | WSP
Wet services and civil - Iguana
     
Contractors    
Main building - Iguana
HVAC&R - Iguana
Wet services - Iguana
Electrical - RWP
Refrigeration - CRS
     
Product suppliers     
Refrigeration plant - CRS
Condensers - HC Heat
Compressors - Bitzer
Electronics - Danfoss 
Underfloor heating - Pioneer Floor Heating

 


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