COLD STORES


can have single or dual evaporator, the latter being called a booster system. The fi rst option consists in adding two valves and a receiver to the traditional layout, so that simultaneous control of gas cooler pressure and evaporator superheat can be achieved, together essential for optimising the effi ciency of transcritical CO2


systems. However, this solution can be made even more


effi cient by adding parallel compressors, so that the refrigerant coming from the receiver is no longer expanded but rather fl ows directly into the compressors in the parallel line. If an ejector is added to the three-valve layout with parallel compression, system performance is improved even further, in almost all warm climates. In simple terms, an ejector is a device capable


of using the potential energy of the high-pressure refrigerant to draw in low pressure refrigerant and bring it to an intermediate pressure, thus reducing the compression ratio and the fl ow- rate handled by the compressor. Working with a three-valve system featuring a multilevel liquid receiver and zero K superheat evaporators (FTE) is another way of increasing the effi ciency in booster systems.


As concerns propane, the traditional cycle with a compressor, an expansion valve and two heat exchangers usually performs even better than with HFC refrigerants. There is another option, i.e. to make a multi-circuit system, if the refrigerant charge is higher than the maximum charge limit allowed by the corresponding safety standard. Lastly, there are diff erent types of systems that can be used with ammonia as the refrigerant and that have been successfully implemented on the market. Ammonia vapour absorption refrigeration systems are one of the oldest refrigeration systems, yet are not very popular now due to low effi ciency. Single-stage compression systems comprise the same components as a traditional refrigeration system, as well as a pump and a liquid separator, to ensure that the compressor does not receive any liquid. Two-stage compression systems are the next evolution of industrial refrigeration systems, suitable for low temperature refrigeration applications, providing high effi ciency and low compressor discharge temperatures. Additionally, low-charge ammonia systems have emerged in recent years. On one hand, optimised systems comprise a traditional industrial ammonia refrigeration system further optimised by low-charge components. On the other, a packaged ammonia system eliminates the huge quantities of ammonia inventory and piping by moving to smaller self-contained systems that are usually placed on the roof/ground outside, avoiding


any dangers due to leaks.


Ammonia is also chosen as the refrigerant in both traditional and low-charge cascade systems, normally on the high temperature side, limiting its use to the equipment room.


5. Can energy effi ciency be improved? If you have already chosen the cycle to design, you may also have thought about incorporating high-effi ciency technologies. These, combined with an effi cient control and supervisory system, can be key to obtaining a system with optimised performance.


On the one hand, variable-capacity compressors with inverters provide the best way to avoid ineffi cient on/off cycles that reduce the compressor’s seasonal effi ciency. This means that, at part load or in low load conditions, an inverter- driven compressor adapts its cooling capacity to the system requirements without stopping completely. The ideal solution is to combine


variable-capacity compressors with electronic expansion valves (EEV), which bring energy savings due to adaptive optimisation of system operating parameters.


There are other components that can increase effi ciency and are mainly used in CO2


systems.


A subcooler exploits the gas temperature after expansion in the fl ash valve to cool the liquid refrigerant before the EEV.


An economizer is a type of subcooler that extracts a small amount of CO2


from a specifi c


point in the system and expands it through an expansion valve in order to cool the main fl ow coming from the gas cooler. Lastly, evaporative cooling in CO2


refrigeration systems reduces the


temperature of the gas cooler by spraying micro particles of water, bringing signifi cant energy savings due to the reduction in power consumed by the compressor. These solutions are especially suitable to increase the effi ciency of CO2


systems in warm climates.


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