Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25D—REFRIGERATORS; COLD ROOMS; ICE-BOXES; COOLING OR FREEZING APPARATUS NOT OTHERWISE PROVIDED FOR
  • F25D21/00—Defrosting; Preventing frosting; Removing condensed or defrost water
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
  • F25B47/00—Arrangements for preventing or removing deposits or corrosion, not provided for in another subclass
  • F25B47/02—Defrosting cycles
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25D—REFRIGERATORS; COLD ROOMS; ICE-BOXES; COOLING OR FREEZING APPARATUS NOT OTHERWISE PROVIDED FOR
  • F25D21/00—Defrosting; Preventing frosting; Removing condensed or defrost water
  • F25D21/06—Removing frost
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
  • F25B47/00—Arrangements for preventing or removing deposits or corrosion, not provided for in another subclass
  • F25B47/02—Defrosting cycles
  • F25B47/022—Defrosting cycles hot gas defrosting
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
  • F25B2400/00—General features or devices for refrigeration machines, plants or systems, combined heating and refrigeration systems or heat-pump systems, i.e. not limited to a particular subgroup of F25B
  • F25B2400/04—Refrigeration circuit bypassing means
  • F25B2400/0411—Refrigeration circuit bypassing means for the expansion valve or capillary tube
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
  • F25B2400/00—General features or devices for refrigeration machines, plants or systems, combined heating and refrigeration systems or heat-pump systems, i.e. not limited to a particular subgroup of F25B
  • F25B2400/16—Receivers

Definitions

• This invention relates to a flash defrost system for defrosting evaporators in vapour compression refrigeration systems. As will be explained more fully herein, the invention is applicable to direct expansion, flooded evaporator and liquid overfeed refrigeration systems.
• an evaporator In many applications of vapour compression refrigeration systems an evaporator is used to cool air, inter alia, in chiller rooms, supermarket chilled display cabinets, domestic freezers and air source heat pumps. In such applications the external surfaces of the evaporator become covered in ice during operation due to condensation and freezing of water vapour in the atmosphere. Ice formation adversely affects the heat transfer performance, and the power consumption of the compressor rises to compensate for loss of evaporator efficiency. All such systems are therefore designed to periodically defrost the evaporator in order to restore performance and minimise running costs.
• defrost include, in order of defrost speed : discontinuation of the refrigeration process whilst electrical heaters attached to the evaporator are used to melt and release the accumulated ice; discontinuation of the refrigeration effect but, with the compressor still running, diversion of the hot gas output along an extra line to the evaporator for a time sufficient to melt and release the ice; discontinuation of the refrigeration effect and the use of ambient air to melt the ice.
• the time of defrost needs to be short, so that electrical defrost is most commonly used in food applications.
• electrical defrost and hot gas defrost also incur a cost penalty in terms of extra energy used.
• WO 2009 034 300 Al discloses an ice maker which includes a vapour compression refrigeration system having multiple evaporators. Relatively hot refrigerant from a condenser flows through a defrost receiver before passing through the evaporators. Individual evaporators can be defrosted by means of a valve system which connects the evaporator to the defrost receiver to allow hot fluid to pass thermosyphonically from the defrost receiver to the evaporator and liquid refrigerant in the evaporator to return by gravity to the defrost receiver.
• the length of the defrost period is relatively unimportant since the remaining evaporators will continue to operate.
• the present invention seeks to provide a new and inventive form of defrost system which is capable of providing more rapid and energy-efficient defrosting of the evaporator than has hitherto been possible.
• the present invention proposes a vapour compression refrigeration system including a compressor arranged to recirculate refrigerant through a condenser, an expansion device and an evaporator, in which relatively hot refrigerant from the condenser flows through a defrost receiver before passing through the expansion device, and, in a defrost phase, a valve arrangement connects the evaporator to the defrost receiver to create a defrost circuit which allows hot fluid to pass from the defrost receiver to the evaporator and liquid refrigerant in the evaporator to flow to the defrost receiver,
• the refrigeration system is constructed and operated such that, in a pre-defrost phase, the valve arrangement closes the fluid input to the evaporator and the compressor operates to partially evacuate the evaporator before the evaporator is connected to the defrost receiver.
• the commencement of the defrost phase causes the hot refrigerant to boil and results in immediate flash flooding of the evaporator with hot refrigerant vapour.
• the invention therefore provides a means of defrosting the evaporator which uses a minimum amount of net energy from the system and which also enables a significant reduction in the defrost period. In food applications therefore, the invention minimises excursions from the ideal storage temperature of the product.
• Figure 1 is a diagram of a known form of vapour compression refrigeration circuit upon which the present invention is based;
• FIG. 2 is a diagram of a first such refrigeration circuit incorporating a defrost system in accordance with the invention
• FIG. 3 is a diagram of a second such refrigeration circuit incorporating a defrost system in accordance with the invention.
• Figure 4 is a modified form of the refrigeration circuit shown in Fig. 3;
• Figure 5 is a modified form of the refrigeration circuit shown in Fig. 2 which can be used with multiple evaporators;
• Figure 6 shows a further modification as applied to the refrigeration circuit of Fig.5.
• Fig. 1 shows a widely used direct expansion arrangement to which the present invention may be applied, comprising a closed refrigerant circuit in which a compressor 1 pressurises vapour phase refrigerant.
• the hot superheated gas leaving the compressor passes to a condenser 2 in which desuperheating and subcooling occurs.
• the warm high pressure liquid refrigerant then passes to a liquid receiver vessel 3 acting as a refrigerant reservoir.
• Liquid from the reservoir supplies an expansion device 4 where a rapid drop in pressure produces a two phase stream of cold vapour and liquid which then enters the bottom of evaporator 5. Evaporation of the liquid phase is completed in the evaporator so that the required cooling effect is achieved.
• Cold sub-cooled vapour from a top exit of the evaporator 5 then returns to the inlet of the compressor 1 via the suction line of the compressor and the cycle is repeated.
• a defrost receiver 6 is inserted into the liquid stream between the main liquid reservoir 3 and the expansion device 4, which may be an expansion valve.
• a shut-off valve 7 is inserted into the flow path between the receiver 3 and the defrost receiver 6, and an isolation valve 8 is inserted between the exit of the evaporator 5 and the inlet of the compressor 1.
• a drain valve 9 is connected in parallel with the expansion valve 4, and a defrost valve 10 is connected between the top of the defrost receiver 6 and the exit of the evaporator 5.
• the expansion valve 4 and valves 7 and 8 are open and valves 9 and 10 are closed resulting in a refrigerant flow circuit which is essentially the same as that shown in Fig. 1.
• normal operation of the circuit will result in ice formation on the outside of the evaporator due to condensation of atmospheric water vapour.
• the expansion valve 4 When defrosting of the evaporator is required the expansion valve 4 is firstly closed to close off the fluid inlet of the evaporator while the compressor 1 continues to run. The suction line to the compressor continues to draw refrigerant vapour from the evaporator 5, causing partial evacuation of the evaporator. After a sufficient period of time, valves 7 and 8 are closed and valve 10 is opened allowing high pressure liquid refrigerant in the defrost receiver 6 to flash over into the evaporator 5, which is at a very low pressure.
• the heat energy extracted from the hot liquid refrigerant and made available for defrost may be augmented by means of a phase-change unit 11 contained within the defrost receiver 6.
• a suitable phase-change medium is encapsulated within the phase-change unit 11 so that during normal operation the hot liquid refrigerant flows in contact with the phase-change unit melting the phase-change material and storing enthalpy from the liquid refrigerant stream as latent heat.
• the stored heat energy is released into the refrigerant stream circulating in the closed loop thereby accelerating the defrost process.
• the liquid reservoir 3 is arranged to act as a defrost receiver.
• the evaporator is at a higher level than the receiver, and the expansion device 4 is of a type which can be fully opened to remove the restriction, for example an expansion valve driven by a stepper motor.
• An isolation valve 12 in the compressor suction line is open when the compressor is running and closed at other times.
• a defrost valve 13 connects the exit of the evaporator to the top of the receiver 3 and is shut in normal operation.
• the expansion valve 4 is fully closed for a period to allow the evaporator to empty via the suction line.
• the compressor 1 is then switched off and valve 12 is shut.
• the expansion valve 4 is fully opened allowing hot liquid to drain back to the liquid receiver, and valve 13 opens allowing vapour from the top of the receiver 3 to flash over into the partially evacuated evaporator.
• valve 13 opens allowing vapour from the top of the receiver 3 to flash over into the partially evacuated evaporator.
• a flow will be established from the evaporator through the expansion valve back to the receiver 3.
• Vapour will continue to flow from the receiver 3 through the defrost valve 13 to the evaporator 5 where it will condense, and the condensed liquid will then flow back to the receiver 3 via the expansion valve 4.
• a heat exchanger 14 containing a phase change medium may be added between the receiver 3 and the expansion valve 4. This increases the energy storage capacity while minimising the refrigerant charge.
• a heat exchanger 15 of the fluid-to-fluid type can be used. The secondary of the heat exchanger is connected to a pump 16 which circulates an antifreeze fluid from a separate tank 17 in a closed circuit, thus acting to increase the thermal storage capacity of the defrost system.
• Fig. 5 In refrigeration installations with multiple evaporators fed from common liquid supply and suction manifolds, such as those used in supermarket display cabinets or cold storage facilities, the embodiment of the invention shown in Fig. 5 may be used.
• the individual evaporators 5 and associated defrost circuitry constructed and operated as previously described in relation to Fig. 2 are each connected to the common liquid manifold 18 and suction manifold 19. It will be noted that in this case each evaporator 5 is associated with its own defrost receiver 6 so that flash defrosting of the individual evaporators may again take place as described.
• the evaporator 5 should be higher than the heat store module formed by the defrost receiver 6 and the phase-change unit 11 (if provided) so that liquid refrigerant can return to the receiver 6 under the action of gravity.
• Fig. 6 shows how this requirement can be obviated by adding a pump 20 in series with the valve 9 between the liquid outlet from the evaporator 5 and the defrost receiver 6.
• the pump 20 will return cold liquid refrigerant from the evaporator 5 to the heat store 6, 11 where it can evaporate and return to the evaporator as vapour.
• the valve 9 could be replaced with a non-return valve, removing the requirement for actuation by the refrigeration control system.
• the invention can also be applied to flooded evaporator and liquid overfeed refrigeration systems.
• the evaporator is fed with liquid refrigerant and filled with boiling refrigerant so that a mixture of liquid refrigerant and refrigerant vapour exits from the evaporator.
• This requires the addition of a low pressure accumulator in the suction line so that the liquid can be separated from the vapour which is returned to the compressor.
• valve arrangement may need to be modified, but the basic principle of partial evacuation of the evaporator followed by flash flooding with hot refrigerant from the liquid supply line would still apply.
• the heat energy extracted from the hot liquid refrigerant can be augmented by means of electrical power supplied by a resistance heater located in or around the defrost receiver with the purpose of accelerating the defrost process.
• a resistance heater located in or around the defrost receiver with the purpose of accelerating the defrost process.
• the timing and sequencing of the valve operation, the sizing and positioning of the defrost receiver relative to the evaporator, and the use of thermal capacity enhancement by means of phase change materials, secondary fluid circuit or electrical power can be optimised for maximum overall system efficiency.
• valves which may be employed in the refrigeration units described above include, inter alia, check valves, solenoid valves, expansion valves and three-way valves.
• the control system employed to manage the operation of the refrigeration systems described above will initiate and terminate the defrost process based on information supplied by temperature and pressure sensors fitted at strategic points around the refrigerant circuits.

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• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Mechanical Engineering (AREA)
• Thermal Sciences (AREA)
• General Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Defrosting Systems (AREA)
• Compression-Type Refrigeration Machines With Reversible Cycles (AREA)

Applications Claiming Priority (2)

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• 2011
  • 2011-02-11 GB GB1102485.8A patent/GB2487975A/en not_active Withdrawn
• 2012
  • 2012-02-10 CA CA2827053A patent/CA2827053A1/fr not_active Abandoned
  • 2012-02-10 WO PCT/GB2012/050293 patent/WO2012107773A2/fr active Application Filing
  • 2012-02-10 GB GB1301403.0A patent/GB2495672B/en not_active Expired - Fee Related
  • 2012-02-10 US US13/983,794 patent/US20130312437A1/en not_active Abandoned
  • 2012-02-10 EP EP12709685.7A patent/EP2673578A2/fr not_active Withdrawn
  • 2012-02-10 RU RU2013141537/06A patent/RU2582729C2/ru not_active IP Right Cessation
  • 2012-02-10 JP JP2013553027A patent/JP5934257B2/ja not_active Expired - Fee Related
  • 2012-02-10 CN CN2012800085174A patent/CN103429974A/zh active Pending
  • 2012-02-10 BR BR112013020258A patent/BR112013020258A2/pt not_active IP Right Cessation
  • 2012-02-10 KR KR1020137023933A patent/KR20140007891A/ko not_active Application Discontinuation
  • 2012-02-10 MX MX2013009155A patent/MX2013009155A/es unknown
  • 2012-02-10 AU AU2012215130A patent/AU2012215130B2/en not_active Ceased

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