Temperature in a refrigerated storage space is controlled within a temperature range adjacent to a setpoint temperature. The refrigerated storage space may for example comprise a transport volume of a refrigerated transport container. The setpoint temperature is chosen to keep the perishable produce such as meat, vegetables and fruit, at correct temperatures to avoid quality degradation. A lot of produce (e.g. chilled meat, grape, apple, pear, kiwi, dairy, etc.) is transported/stored at a temperature just above the freezing point of the produce, i.e. at setpoints in the range between about −5° C. and 0° C.
One typical cooling unit or refrigeration unit used in refrigerated transport containers is based on the so-called vapour compression refrigeration circuit and comprises a cooling space situated inside an insulated enclosure of the refrigerated transport container. This circuit comprises at least a compressor, a condenser, an expansion device, and an evaporator. The compressor sucks refrigerant vapour from the evaporator and compresses the refrigerant vapour which subsequently flows to the condenser at high pressure. The condenser ejects its heat to a medium outside the refrigerated transport container while condensing the refrigerant vapour. The liquefied refrigerant then flows to the expansion device in which a refrigerant pressure drops. The low pressure refrigerant then flows to the evaporator, situated in the cooling space, where the refrigerant evaporates while extracting the required heat from the refrigerated storage space.
Throughout this specification, the word ice indicates frozen water, a brittle transparent crystalline solid. The word frost means small white crystals formed when water vapour deposits from saturated air. Frost is formed when solid surfaces are cooled to below the so-called dew point of the adjacent air as well as below the freezing point of water.
Operating an evaporator at an external surface temperature below 0° C. may result in frost formation on the evaporator. Frost formation decreases the efficiency of the cooling unit and eventually may completely block the flow of circulating air. Avoiding this requires periodic defrost cycles. Many types of defrost cycles exist. All or many of them typically require an interruption of the cooling process. Most of them rely on a way of supplying heat to the evaporator, for example using a heater mounted underneath the evaporator (see for example patent specification U.S. Pat. No. 6,609,388). Others quit cooling while continuing to circulate return air from the refrigerated transport container, when return air temperature is above 0° C. (see for example Australian patent AU200136250). The return air from the refrigerated transport container will then lose heat to the frost on the evaporator, simultaneously cooling the air and defrosting the evaporator. It is generally understood that defrost cycles should only be terminated when substantially all frost has melted and the melt water has received enough time to drain off to the outside of the cooling space through installed condensate collection guide(s).
When the setpoint temperature is close to 0° C., e.g. between +2° C. and −5° C., and the compressor is intermittently operated, for example to avoid energy-inefficient part-load operation, the external surface temperature of the evaporator may oscillate between melting and frosting conditions. An intermittently operated compressor is a compressor that operates between a first active state (e.g. ON, MAX, near MAX) and a second less active state (e.g. LOW, near OFF, OFF) in such a way that it completely or almost completely inactivates for a certain period of time after a certain (e.g. other) period of time. Typically, an intermittently operated compressor inactivates for temperature control purposes and inactivates more than 20% of time with more than 2 stops per hour. In this situation, a new issue may occur: if no further measures are taken it is inherent to the intermittent operation that cooling may resume before all frost is melted whereby ice starts to form on the evaporator or in the condensate collection guide(s). Ice is far more difficult to remove than frost. In addition, at least a part of the condensate collection guide(s) may be outside the reach of the installed heating device(s). Hence the ice formation is a serious problem. Ice formation in this way has the potential to first block the condensate collection guide(s), and after that gradually fill the lower part of the cooling space with ice, potentially completely blocking the air flow.