NZ615009B2 - Flash defrost system - Google Patents

Abstract

Disclosed is a vapour compression refrigeration system with a defrost circuit that includes a compressor (1) arranged to re-circulate refrigerant through a condenser (2), an expansion device (4) and an evaporator (5). To achieve rapid thermodynamically efficient defrosting of the evaporator, hot refrigerant from the condenser is stored in a defrost receiver (6) before passing through the expansion device (4). In a defrost phase, a valve arrangement (7-10) forms a closed defrost circuit connecting the evaporator (5) to the defrost receiver (6) via defrost valve (10) to allow hot fluid to pass from the defrost receiver to the evaporator and liquid refrigerant in the evaporator flows to the defrost receiver (6) via drain valve (9). In a pre-defrost phase, the valve arrangement closes the fluid input to the evaporator (5) and the compressor operates to partially evacuate the evaporator before the evaporator is connected to the defrost receiver, so that flash flooding of the evaporator with hot vapour occurs. A phase change medium (11) may be included to store heat from the condenser output and return it to the evaporator during defrost. Additional heat may be supplied to the defrost liquid to further increase the defrost speed. rigerant from the condenser is stored in a defrost receiver (6) before passing through the expansion device (4). In a defrost phase, a valve arrangement (7-10) forms a closed defrost circuit connecting the evaporator (5) to the defrost receiver (6) via defrost valve (10) to allow hot fluid to pass from the defrost receiver to the evaporator and liquid refrigerant in the evaporator flows to the defrost receiver (6) via drain valve (9). In a pre-defrost phase, the valve arrangement closes the fluid input to the evaporator (5) and the compressor operates to partially evacuate the evaporator before the evaporator is connected to the defrost receiver, so that flash flooding of the evaporator with hot vapour occurs. A phase change medium (11) may be included to store heat from the condenser output and return it to the evaporator during defrost. Additional heat may be supplied to the defrost liquid to further increase the defrost speed.

Description

W0 2012/107773 2012/050293 FLASH DEFROST SYSTEM TECHNICAL FIELD OF THE INVENTION This invention relates to a flash defrost system for defrosting evaporators in vapour compression refrigeration systems. As will be explained more fully herein, the ion is applicable to direct expansion, flooded evaporator and liquid overfeed refrigeration systems.

BACKGROUND In many applications of vapour compression refrigeration systems an evaporator is used to cool air, inter alia, in chiller rooms, arket 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 er 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 ator in order to restore mance and minimise running costs.

W0 2012/107773 Common methods of defrost include, in order of defrost speed: discontinuation of the eration s 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.

To minimise temperature rises in the refrigerated products the time of defrost needs to be short, so that ical defrost is most commonly used in food applications. However, electrical defrost and hot gas t also incur a cost penalty in terms of extra energy used.

W0 2009 034 300 A1 discloses an ice maker which es 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 t receiver to allow hot fluid to pass thermosyphonically from the t receiver to the evaporator and liquid refrigerant in the evaporator to return by gravity to the defrost receiver. r, in such a system 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.

It is an object of the invention to at least provide the public with a useful choice.

BRIEF SUMMARY OF THE INVENTION In one aspect of the invention there is provided a vapour compression eration system including a compressor arranged to re-circulate refrigerant through a condenser, an expansion device and an evaporator, a t receiver with or without an additional liquid receiver through which hot refrigerant from the condenser flows before passing through the expansion device, and a valve arrangement which, in a defrost phase, connects the evaporator to the defrost receiver to allow hot refrigerant from the defrost receiver to pass h the evaporator, wherein - the valve arrangement is arranged to create, during the defrost phase, a defrost circuit through which hot refrigerant vapour flows from the defrost er to the evaporator and cool liquid erant condensate returns from the evaporator to the t receiver without passing through the compressor; and - the defrost receiver is associated with a heat storage medium in heat-exchange contact with the refrigerant and from which stored heat energy is released into the refrigerant g through the defrost t and transported to the evaporator during the defrost phase.

Followed by page 3a - 3a - In a further aspect of the invention there is provided a method of defrosting a vapour ssion refrigeration system including a ssor arranged to re-circulate refrigerant through a condenser, an expansion device and an evaporator, a defrost receiver with or t an additional liquid receiver through which hot erant from the condenser flows before passing h the expansion device, and a valve arrangement which, in a defrost phase, connects the evaporator to the defrost receiver to allow hot refrigerant from the defrost receiver to pass through the evaporator, wherein - the valve arrangement is arranged to create, during the defrost phase, a defrost circuit through which hot erant vapour flows from the defrost receiver to the evaporator and cool liquid refrigerant condensate returns from the ator to the defrost receiver without passing through the ssor; and - the defrost receiver is associated with a heat storage medium in heat-exchange contact with the refrigerant and from which stored heat energy is released into the refrigerant flowing through the defrost circuit and transported to the evaporator during the defrost phase by the s of refrigerant boiling in the defrost receiver followed by refrigerant condensation in the evaporator.

In a further aspect of the invention there is provided a vapour compression refrigeration system including a compressor, a condenser, a suction manifold, a liquid supply manifold, and a plurality of evaporators; Followed by page 3b - 3b - each of the evaporators having a respective expansion or other refrigerant feed device through which refrigerant flows from the condenser via the liquid supply manifold to the respective evaporator when the ssor is operated to re-circulate refrigerant through the condenser and via the n and liquid supply lds through the respective ator in a refrigeration cycle; wherein: a plurality of defrost receivers are provided, each defrost receiver comprising a respective heat exchanger in association with a respective heat storage unit, the heat exchanger of each defrost receiver being ed in association with a respective one of the evaporators so that refrigerant flows from the condenser via the liquid supply manifold and through the heat exchanger of the respective defrost receiver before passing through the respective expansion or other refrigerant feed device during the refrigeration cycle; and each heat storage unit comprises a phase change material arranged in heat exchange contact with the refrigerant flowing through the tive heat exchanger so that the phase change al is melted as it extracts heat from the refrigerant during the refrigeration cycle; and each evaporator is provided with a valve arrangement which is arranged, in a defrost cycle of the respective ator, to isolate the respective evaporator and defrost receiver from the suction and liquid supply manifolds and to t the respective evaporator to the respective defrost receiver to form a defrost circuit in which refrigerant from the respective heat exchanger flows through the Followed by page 3c - 3c - evaporator and extracts stored heat energy from the phase change material in the respective heat storage unit.

Y OF THE INVENTION The present invention generally proposes a vapour compression refrigeration system including a ssor arranged to re-circulate refrigerant through a ser, 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 t phase, a valve arrangement ts the evaporator to the t 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, characterised in that 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.

By isolating the input to the evaporator prior to commencement of the t phase and allowing the compressor to remove refrigerant from the evaporator, 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 es a means of defrosting the evaporator which uses a minimum amount of net energy Followed by page 4 W0 2012/107773 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.

BRIEF DESCRIPTION OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of miting example in order to rate how the invention may be put into practice. In the gs: Figure 1 is a m of a known form of vapour compression refrigeration circuit upon which the present invention is based; Figure 2 is a diagram of a first such refrigeration circuit incorporating a defrost system in accordance with the invention; Figure 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 le evaporators; and Figure 6 shows a further modification as applied to the refrigeration circuit of Fig.5.

DETAILED DESCRIPTION OF THE DRAWINGS Unless the context y requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Fig. 1, shows a widely used direct expansion ement to which the present invention may be applied, comprising a closed erant circuit in which a compressor 1 pressurises vapour phase refrigerant.

The hot superheated gas g the compressor passes to a condenser 2 in which rheating 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 es 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 ator 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.

Various embodiments of the invention will now be described which achieve rapid energy-efficient defrosting of the evaporator in such a refrigeration system. In the following description and drawings the reference numbers used in Fig. 1 are applied to corresponding items within the refrigeration system.

In the first embodiment which is shown in Fig. 2 a defrost receiver 6 is ed 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 ed 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 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 ted between the top of the defrost receiver 6 and the exit of the evaporator 5. During normal ion 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. As previously ned however, normal operation of the t will result in ice formation on the outside of the evaporator due to condensation of atmospheric water vapour.

When defrosting of the evaporator is required the expansion valve 4 is y 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 compressor may be turned off during this phase.) Refrigerant vapour condenses in the evaporator releasing latent heat and transferring it at high heat transfer ency until the pressures in the evaporator 5 and the defrost receiver 6 equalise, at which point drain valve 9 is opened to allow liquid erant in the evaporator to drain back into the er 6 under the action of gravity. When the temperature of the liquid in the receiver 6 falls to a predetermined level indicating that defrost is complete, valves 9 and 10 are closed and valves 4, 7 and 8 are opened and the normal operation of the refrigeration circuit resumes.

In a further improvement of the defrost system in accordance with the invention 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 t 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. During the defrost stage the stored heat energy is released into the refrigerant stream ating in the closed loop y accelerating the defrost process. The result of such extraction of heat from the hot liquid refrigerant stream is to increase the thermodynamic efficiency of the overall refrigeration circuit through a more ive expansion s, which largely compensates for the extra energy needed to re-cool the evaporator after a defrost. The energy PCT/G32012/050293 cost of the t process is thereby minimised.

In a second embodiment of the invention which is shown in Fig. 3 the liquid reservoir 3 is ed 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 ction, 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 ion. When defrost is initiated the ion 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. As the evaporator is above the receiver and the line from the receiver 3 through the ion valve 4 is full of liquid 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 h 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.

In a variation of this embodiment 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. Alternatively, as shown in Fig. 4, 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 eeze fluid from a separate tank 17 in a closed circuit, thus acting to increase the thermal storage capacity of the defrost system.

In refrigeration installations with multiple evaporators fed from common liquid supply and suction manifolds, such as those used in supermarket display ts or cold storage facilities, the embodiment of the invention shown in Fig. 5 may be used.

The individual evaporators 5 and associated defrost circuitry ucted and operated as previously described in relation to Fig. 2 are each connected to the common liquid ld 18 and suction manifold 19. It will be noted that in this case each evaporator 5 is associated with its own defrost er 6 so that flash defrosting of the individual evaporators may again take place as described.

In the embodiments described above 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 ed 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 W0 2012/107773 PCT/G32012/050293 return to the evaporator as vapour. It should also be noted that with such an arrangement the valve 9 could be replaced with a non-return valve, ng the requirement for actuation by the eration control system.

Although the specific embodiments described above are applied to refrigeration systems of the direct expansion type which maintain a constant superheat at the evaporator exit, the invention can also be d to flooded evaporator and liquid overfeed eration systems. In such systems the ator 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. Provided the return to the accumulator is above the fluid level in the evaporator all of the liquid in the evaporator should evaporate when the liquid feed to the evaporator is turned off during the frost phase. The 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.

In each embodiment of the invention 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 t process.

W0 2012/107773 The timing and sequencing of the valve operation, the sizing and positioning of the t receiver ve to the evaporator, and the use of thermal ty enhancement by means of phase change materials, secondary fluid circuit or electrical power can be optimised for maximum overall system efficiency.

The type of valves which may be employed in the refrigeration units described above include, inter alia, check valves, solenoid valves, ion 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 gic points around the refrigerant circuits.

Whilst the above description places emphasis on the areas which are believed to be new and addresses ic problems which have been identified, it is intended that the features disclosed herein may be used in any combination which is capable of providing a new and useful advance in the art.

Claims (14)

What is claimed 1. is:

1. A vapour compression refrigeration system including a compressor arranged to re-circulate refrigerant through a condenser, an expansion device and an evaporator, a defrost receiver with or without an additional liquid receiver through which hot refrigerant from the condenser flows before passing through the expansion device, and a valve arrangement which, in a t phase, connects the evaporator to the defrost receiver to allow hot refrigerant from the defrost receiver to pass through the ator, wherein - the valve arrangement is arranged to create, during the defrost phase, a t circuit through which hot refrigerant vapour flows from the defrost receiver to the evaporator and cool liquid erant condensate returns from the evaporator to the defrost receiver t passing through the compressor; and - the defrost receiver is associated with a heat storage medium in heat-exchange contact with the refrigerant and from which stored heat energy is ed into the refrigerant flowing through the defrost circuit and orted to the evaporator during the t phase.

2. A vapour compression refrigeration system according to Claim 1 in which the heat storage medium comprises a phase-change medium.

3. A vapour compression refrigeration system according to Claim 2 in which the phase-change medium is contained within the defrost receiver.

4. A vapour compression refrigeration system ing to Claim 2 in which the phase-change medium is included between the defrost receiver and the expansion device.

5. A vapour compression refrigeration system according to Claim 1 in which a fluid-to-fluid heat exchanger is included between the defrost receiver and the expansion device and a fluid heat storage medium is circulated through the secondary of the heat exchanger to a e reservoir.

6. A vapour ssion refrigeration system according to Claim 1 in which heating means is arranged to provide additional heat input to the hot refrigerant flowing from the t receiver.

7. A vapour compression eration system according to Claim 1 which includes a plurality of evaporators and in which each evaporator is associated with a respective defrost receiver.

8. A vapour compression eration system according to Claim 1 in which a pump is arranged to return liquid refrigerant from the evaporator to the defrost receiver during the defrost phase.

9. A vapour compression refrigeration system according to Claim 1, wherein the refrigeration system is arranged 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 t receiver.

10. A method of defrosting a vapour compression refrigeration system including a compressor arranged to culate refrigerant h a condenser, an expansion device and an evaporator, a defrost receiver with or without an additional liquid receiver through which hot refrigerant from the condenser flows before passing through the expansion device, and a valve arrangement which, in a defrost phase, connects the evaporator to the defrost receiver to allow hot refrigerant from the defrost er to pass through the evaporator, - the valve arrangement is arranged to create, during the defrost phase, a defrost circuit h which hot refrigerant vapour flows from the defrost receiver to the evaporator and cool liquid refrigerant condensate returns from the evaporator to the defrost receiver without passing through the compressor; and - the defrost receiver is associated with a heat storage medium in xchange contact with the erant and from which stored heat energy is released into the refrigerant flowing through the defrost circuit and transported to the evaporator during the defrost phase by the s of refrigerant boiling in the defrost receiver followed by refrigerant condensation in the evaporator.

11. A method according to Claim 10, wherein the valve ement closes the fluid input to the evaporator and the compressor operates to partially evacuate the ator before the evaporator is connected to the defrost receiver.

12. A vapour compression refrigeration system including a compressor, a condenser, a suction manifold, a liquid supply manifold, and a plurality of evaporators; each of the evaporators having a respective expansion or other erant feed device through which refrigerant flows from the condenser via the liquid supply manifold to the respective evaporator when the ssor is operated to re-circulate refrigerant through the condenser and via the suction and liquid supply manifolds h the respective evaporator in a refrigeration cycle; a plurality of defrost receivers are provided, each defrost receiver comprising a respective heat exchanger in association with a respective heat e unit, the heat ger of each defrost receiver being arranged in association with a respective one of the evaporators so that refrigerant flows from the condenser via the liquid supply manifold and through the heat exchanger of the respective defrost receiver before passing through the respective expansion or other refrigerant feed device during the refrigeration cycle; and each heat storage unit comprises a phase change material arranged in heat exchange contact with the refrigerant flowing through the respective heat exchanger so that the phase change material is melted as it extracts heat from the refrigerant during the refrigeration cycle; and each ator is provided with a valve arrangement which is arranged, in a t cycle of the respective evaporator, to isolate the respective evaporator and defrost receiver from the suction and liquid supply manifolds and to connect the respective evaporator to the respective defrost receiver to form a defrost circuit in which refrigerant from the respective heat ger flows through the evaporator and extracts stored heat energy from the phase change material in the respective heat storage unit.

13. A vapour compression refrigeration system according to claim 12, wherein the refrigeration system is arranged such that, in a frost phase of each ator, 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.

14. A vapour compression refrigeration system ntially as herein described with reference to any one of the the embodiments described in