SUPERMARKET SYSTEMS AND EQUIPMENT
The countdown to zero defrosts
Roland Rees, engineering manager at Adande Refrigeration’s parent company, Applied Design & Engineering, looks at how the implementation of disruptive technology is opening the door to zero defrosts on refrigerated display cabinets.
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n conventional open front display cases moisture from ambient air is entrained into the cabinet and as the system air is cooled when it passes through the evaporator, the moisture condenses and then freezes on the surfaces of the evaporator.
The consequent build-up of ice restricts the flow of air across the evaporator, reducing its efficiency and, without intervention, blocking the airflow completely. Hence the need for defrost cycles, during which it is necessary to ‘switch off’ the evaporator to melt the ice and drain the water away from the cabinet.
Hitherto, retailers have accepted defrost cycles as a necessary evil and refrigeration OEMs have done little to address the issue.
Defrost cycles warm up cabinet temperature During the defrost cycle the temperature of the air leaving the evaporator is raised, creating a warmer cabinet environment, which has a detrimental effect on the quality and shelf life of perishable merchandise. This rise in temperature could also provoke bacterial proliferation, compromising food safety. Moreover, once the defrost period is complete, the refrigeration system needs to work harder to re-establish optimum cabinet temperature, increasing duty and energy consumption. The frequency and duration of defrost cycles will vary according to cabinet design and store conditions, but with some equipment requiring 20 to 30 minute cycles every four hours, we estimate that the cooling period following a defrost may account for 10% – 12% of a cabinet’s overall energy consumption.
Reducing the entrainment of warm air The key to reducing the entrainment of warm and moist air is the integrity of the cabinet air curtain. One way of
46 April 2018
dealing with this is to divide the merchandising envelope into separate cells between shelves, unlike the single canopy to base air curtain used on conventional open front cabinets. The smaller cells have a shorter air column, resulting on less pressure on the inside of the air curtain of each cell and a substantial reduction in cold air spillage.
The multiple air curtain concept is particularly suitable for harsh store environments with cross draughts and higher ambient conditions, which have a detrimental effect on conventional open front displays.
The retention of chilled air within the cabinet means that less new chilled air needs to be generated to maintain cabinet temperature.
In laboratory tests, we have established that cabinets incorporating our own Aircell technology as an example, have a tight temperature bandwidth, typically 3°C, compared with anything up to 11.5°C in conventional open front cabinets.
Raising evaporating temperatures
Tighter temperature bandwidths allow the evaporating temperature to be raised. On conventional open front cabinets evaporating temperature is typically between -5°C and -10°C, which causes ice formation on the coil. By contrast, on cabinets with a narrow and accurate temperature bandwidth, the evaporating temperature may be set above 0°C, preventing the build-up of ice on the coil and eliminating the need for defrosts.
Raising the evaporating temperature also has benefits in terms of energy efficiency: for each 1°C rise in evaporating temperature there will be an increase in the compressor’s CoP of 2% to 4%, making an energy saving of up to 24% if raising the evaporating temperature from -5°C to 1°C. The combined energy savings from the elimination of the defrost cycle and a higher evaporating temperature is over 30%.
www.acr-news.com
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