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DEFROST
Air cooling evaporators whose surface temperature is lower than the dew point of the air they are cooling will accumulate moisture from the air. When the surface
temperature is below freezing, this moisture will be deposited in the form of frost and eventually, if left, will restrict heat transfer and air flow.
Various methods are used for the removal of frost. When the air inlet temperature is at 38°F or higher it is possible to use natural defrost by just switching off the
refrigeration flow for a period. At lower temperatures heat needs to be added and the fans switched off to prevent the heat being blown into the store. The heat
can be from electrical elements passing through or adjacent to the fin, hot/warm refrigerant gas passing through the tubes or, less commonly, water cascaded
over the fins or a warm glycol solution in a secondary system of tubes.
DEFROST OPTIONS
Hot-gas defrost can be the quickest and most efficient common method of defrost. The fact that heat is applied to all the same surfaces that provided the
cooling means that frost is melted quickly and evenly. The low temperature of the heat, compared to electric defrost, results in less heat, in the form of steaming
condensate, escaping from the cooler into the cold store.
HG(E)A, HG(E)B – REVERSE CYCLE
Generally used on systems with one or two evaporators. During defrost the condenser is used as an evaporator and the refrigeration system acts as a heat
pump. During defrost the flow is in the reverse direction as it is usually most convenient to redirect flow close to the compressor. As the evaporator acts as a
condenser in this regime the hot gas is at high pressure/high temperature giving a superior performance in comparison to a hot gas system.
HG(E)C, HG(E)D
Where there are two or more evaporators on a refrigeration system it is possible to defrost one using the hot-gas generated in the operating of the other. This
arrangement allows defrost flow to be in the same direction as cooling flow. Although this arrangement can operate on a two evaporator system the quantity
of low pressure, super heated gas is minimal, therefore it is recommended that this arrangement is used on systems with three or more evaporators, with one
evaporator on defrost whilst the other evaporators are operational, generating the defrost heat.
DEFROST HEADER - HG(E)B, HG(E)C
Without a defrost header the distribution system causes a restriction reducing the flow rate of hot-gas. This reduces the rate of defrost and can result in
condensed refrigerant logging in the coil and preventing even defrosting. This problem is more likely the less superheat the hot-gas has.
DRAIN PAN DEFROSTING – HGE
Hot gas can be used for drain pan defrost but the lower heating temperature makes it difficult for heat to reach the extremes of the drain-pan. Also, the
drain-pan is more exposed to the room temperature and will quickly drop to room temperature when the defrost is terminated, with the possibility of condensate
still running of the coil to freeze in the pan. Using electric defrost in the pan overcomes these disadvantages, particularly as it can be controlled to operate for a
short time before and after the main defrost.
The best configuration of hot gas coil and tray defrost is to ensure that the hot gas enters the tray matrix first then enters the coil, this is arranged via the valve
station supplied and fitted by the refrigeration contractor.
INITIAL DEFROST SETTINGS
INITIAL DEFROST SETTINGS
DEFROST TYPE
DURATION
TIME TERMINATION
TEMPERATURE TERMINATION
Off cycle defrost
20 to 30 min
45 min
4.5 to 7.2 °C
Hot gas defrost
10 to 15 min
20 min
Refrigerant pressure at 25 °C
Electric defrost
30 to 45 min
45 min
10 to 12.7 °C
