The paper reports on the results of a simple comparison between the energy consumption of a small walk-in frozen food storeroom chilled by a conventional direct expansion refrigeration system with electric defrost, and the same store but with the refrigeration system modified to allow the use of a novel flash defrost system. The energy consumption characteristics of a commercial 8m~3 walk-in freezer were established under reasonably reproducible conditions for both the electric and flash defrost systems. The effects of door openings were modelled using steam injection and electrical heating and the effect of partial store loading was modelled using three 200 litre drums of ethylene glycol. Product temperature history was measured using an instrumented tub of ice cream. Electrical power consumption of the whole unit and of the defrost heaters was monitored using a power meter and melt water was collected and weighed for each defrost. Using a system of valves a heat storage unit was built into the liquid line from the condenser so that heat was removed from the liquid line and stored in a volume of wax with a melting point of 15C. Consequently the liquid refrigerant leaving the heat store was subcooled before entering the expansion valve. When a defrost was called for the valving allowed the heat store to be connected to the evaporator in a closed loop and the compressor to be switched off. Refrigerant trapped in the heat store then boiled and flashed over to the evaporator where the vapour condensed and released heat. With the evaporator higher than the heat store the condensate then ran by gravity back to the heat store where boiling was repeated and so on until the heat store was exhausted and the evaporator defrosted. Clearly no additional power was used to defrost the coil and the subcooling created during recharging of the heat store led to an overall increase in cooling capacity. The power consumption of the two systems is compared and it is shown that the flash defrost system reduces the power consumption of the unit by at least 20% in mid-winter conditions.
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