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Alternative Refrigerants: Carbon dioxide: The born again refrigerant

Franz Kaltenbrunner of Eurammon, the European initiative for natural refrigerants, reveals how CO2 is growing in popularity throughout Europe
Alternative Refrigerants: Carbon dioxide: The born again refrigerant
INTENSE research and development since it was rediscovered in the early 1990s have led to CO2 now being practical for use in many different applications. One promising field of application is supermarket refrigeration, where HFCs still dominate at present.

For example, the Danish supermarket chain Super Best has installed a CO2 based cascade system at its Copenhagen branch. In Denmark, the use of more than 10kg of fluorinated gases in newly installed refrigeration systems has been prohibited by law since 2007. So refrigeration specialists Knudsen Køling designed a refrigeration system that uses two refrigeration circuits linked by a plate heat exchanger.

This allows three different temperature levels to be achieved, two of which are used for cooling and one for heat dissipation. The first temperature level is needed in frozen food cabinets and the deep freeze chambers in the cellar. At an evaporating temperature of -28°C, the CO2 is used to maintain an ambient temperature of -20°C.

The second temperature level is used for refrigerators and cold stores requiring an air temperature of a few degrees above 0°C. This is achieved by supplying the evaporators with CO2 at an evaporating temperature of -10°C. The third and highest level ensures reliable heat dissipation by condensation or gas cooling, depending on the ambient temperature. Overall the cascade system has a refrigeration capacity of 150kW.

Güntner components are used for the various temperature levels generated by the system and the cold stores are fitted with evaporators. The heat dissipation enables an evaporator to function as a gas cooler at elevated ambient temperatures and hypercritical processing. To guarantee the operational safety of the system even at pressures of around 120 bar.

Güntner used 0.7mm thick stainless steel for the gas cooler and the entire piping network. Owing to its low-noise design, the gas cooler can even be used in locations in residential areas. The injection to the evaporators and the cooling unit is controlled by Danfoss electrical components.
There are currently eleven of these combined deep-freezing/normal refrigeration systems installed throughout Europe, as well as 100 deep freezer systems in use in supermarkets.

Protecting servers

CO2 is also used in ac. The Dutch bank ABN Amro has since 2006 used a CO2 ac system to protect the powerful blade servers from overheating in the computer centre at its London office.
It was planned, implemented and commissioned by Trox AITCS in co-operation with Star Refrigeration.

The system consists of two compact heat exchanger and pumping units. The heat exchanger uses cold water to condense CO2 gas, which has been evaporated in a heat exchanger mounted on the backs of the blade server cabinets. A stainless steel piping system connects the CO2 circuit to 15 cooling units. These cooling units have five independently powered fans, which extract the heat from the cabinets. The chilled water circuit, at 6°C, condenses CO2, which is pumped out to the server cabinets by centrifugal pumps, evaporated and returned to the condenser to begin the process again, all at a constant saturated temperature of 14°C.

Electrical insulator

CO2 is ideal as a refrigerant for this application because it is an electrical insulator and is thus totally safe, and in comparison to a water cooling system the operator makes an energy saving of about 30%. The CO2 concentration in the cabinets and in the server room is monitored continuously.

The whole system has a refrigeration capacity of 300kW (20kW per cooling unit). This high cooling capacity makes it possible to put more servers than usual in each cabinet, thereby reducing the space required by more than half.

Trox AITCS and Star Refrigeration have installed similar systems for JP Morgan, Barclays Bank, Norwich Union and at Imperial College London.
The most recent development in this application of CO2 as a volatile secondary refrigerant is for close control cooling of dealer desks on the trading floors of investment banks.

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