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Cooling food with transcritical CO₂

The majority of scientists agree that the world’s climate is changing, and that the cause is human activity.

SWEP Ultra-pressure BPHEs in action

While the biggest impact comes from the consumption of fossil fuels, the use of refrigerants in applications such as vehicles, air conditioning and supermarket chillers is increasing. Leakage from these applications is injecting into the atmosphere substances that have a particularly high impact on global warming. One example is the widely used R404A, whose global warming effect is 3900 times greater than that of carbon dioxide. Replacing these synthetic refrigerants with carbon dioxide therefore reduces the impact on global warming. 

However, carbon dioxide presents a major challenge as a refrigerant: the high operating pressures it demands. SWEP’s rugged new Ultrapressure range overcomes this challenge and allows global warming from refrigeration to be tackled safely and economically. Customer website: www.scmfrigo.com

The problem

The use of brines in medium-temperature food display cabinets combined with a single-stage carbon dioxide refrigerant cycle imposes extra design requirements due to the high pressure of carbon dioxide (approx. 100 bar) at ambient temperatures. Such systems also have high power demands, so the challenge is twofold: producing components approved for very high pressures that also enable systems to work with greater efficiency.

The solution

The key to the solution is SWEP’s new Ultra-pressure BPHEs. These are reinforced with steel frames, enabling design pressures to be increased to cope with the very high operating pressures required by carbon dioxide. A heat recovery stage further reduces costs by supplying hot water for space heating in retail premises.
A suction gas heat exchanger (SGHX) increases the unit’s capacity while protecting the compressors from liquid droplets.
The result is a well-designed and efficient unit for cooling food. The refrigerant is carbon dioxide, which inherently less damaging to the environment than older HFC blends such as R404A. Furthermore, the refrigerant charge is reduced by the use of a brine.

System description

The purpose of the unit is to supply cooled glycol at - 8°C to retail display cases containing provisions such as dairy products. The brine is cooled by evaporating CO in a SWEP BPHE approved for operation at 45 bar [1]. The glycol is pumped into the retail display cases where it cools the products. It is then returned to the evaporator at -4°C to be recooled. The evaporated CO is superheated in the SGHX [2] while the liquid CO entering the evaporator is subcooled. The SGHX is a SWEP B12H-U, reinforced with a steel frame and approved for operation at 65 bar. The three parallel compressors [3] increase the pressure of the superheated CO to approx. 90 bar before it enters the heat recovery exchanger.

In the heat recovery stage [4], which is a SWEP B16DW-U approved for operation at 140 bar, the gaseous CO is cooled while water is heated to approx. 60°C. This warm water can then be used for space heating in the store or adjacent premises. The CO is further cooled to approx. 20°C in an outdoor air-cooled gas cooler [5]. The pressure is then reduced to approx. 50 bar in the receiver [6]. Liquid CO is supplied to the SGHX, completing the cycle.