GB2456744A

GB2456744A - Auto-defrost refrigeration apparatus

Info

Publication number: GB2456744A
Application number: GB0716852A
Authority: GB
Inventors: John Malcolm Elliott
Original assignee: Ebac Ltd
Current assignee: Ebac Ltd
Priority date: 2007-08-30
Filing date: 2007-08-30
Publication date: 2009-07-29
Also published as: GB0716852D0

Abstract

A refrigeration apparatus, such as a freezer or cold room, comprises a refrigeration compartment 1, which can be opened to introduce ambient air, and a heat exchanger 3 with a cooling surface exposed to air within the refrigeration compartment for cooling the contents of the refrigeration compartment. Preferably, the heat exchanger comprises a single-plate evaporator forming part of a vapour compression circuit including a compressor 4, a condenser 5, and an expansion device 6. A reservoir of liquid antifreeze 8 supplies a drip bar 10 to bathe the cooling surface of the heat exchanger and to intermix with any atmospheric condensate which forms on the cooling surface. A trough 12 is arranged to receive the mixture of liquid antifreeze and condensate, which may be discarded or recirculated after concentration in an evaporation tray 14. The heat exchanger may be continuously bathed in antifreeze to prevent icing. In a further aspect, a defrost controller 20 monitors the operating temperature of the heat exchanger and triggers a defrosting operation when predetermined temperature conditions are detected which are indicative of ice formation on the heat exchanger.

Description

AUTO-DEFROST REFRIGERATION APPARATUS

TECHNICAL FIELD OF THE INVENTION

This invention relates to auto-defrost refrigeration apparatus of the kind having a refrigeration compartment which can be opened to introduce ambient air, and a heat exchanger for cooling the contents of said refrigeration compartment and having a cooling surface exposed to air within the refrigeration compartment.

BACKGROUND

There is an increasing consumer demand for domestic appliances such as freezers and refrigerators which operate with reduced energy consumption, and there is intense competition between manufacturers who are keen to demonstrate their "green credentials" by producing devices which operate as efficiently as possible.

A domestic freezer typically includes a vapour compression refrigeration system which achieves a refrigeration temperature below -10 °C. Over a period of time, when ambient air enters the refrigeration compartment, atmospheric moisture freezes on the cooling surface of the evaporator of the refrigeration system causing a reduction in the efficiency of the refrigeration circuit. Existing freezers may include an auto-defrost system for de-icing the evaporator without manual intervention. Many schemes for controlling the onset and duration of such a defrosting operation have been proposed, but most current systems employ a defrost timer which is arranged to periodically cause warming of the evaporator until the ice has melted. Whilst the amount of energy required to defrost the evaporator only represents a small proportion of the overall running costs it is nevertheless significant. Thermal efficiency is defined in terms of heat gain, which includes the heat input required to defrost the evaporator, and as manufactures make greater efforts to improve thermal insulation the energy used in defrosting has become more important.

Auto-defrost freezers normally use a relatively small evaporator with cooling fins. A compact evaporator increases the defrosting speed by improving heat conduction during the defrosting period, but on the other hand the cooling efficiency is reduced so that a circulation fan is normally required to create an air flow over the evaporator within the refrigeration compartment. This arrangement contributes to the low efficiency of auto-defrost refrigerators compared with manual-defrost systems.

It is acknowledged that in known refrigeration apparatus liquid coolants may be used as a heat exchange medium to assist heat exchange between a product and an evaporator coil, as in US 3 888 092 for

example.

The present invention seeks to provide a new and inventive form of auto-defrost refrigeration apparatus of the kind set forth in the opening paragraph above which is capable of significantly improved thermal efficiency.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention proposes auto-defrost refrigeration apparatus of the kind set forth in the opening paragraph which is distinguished by the provision of: -a reservoir of liquid antifreeze, -washing means arranged to release a controlled quantity of liquid antifreeze from said reservoir to bathe the cooling surface of the heat exchanger which is exposed to air within the refrigeration compartment and intermix with any atmospheric condensate which forms on the cooling surface, and -collection means arranged to receive the mixture of liquid antifreeze and atmospheric condensate from the cooling surface of the heat exchanger.

One form of the apparatus may be arranged to continuously trickle antifreeze over the cooling surface of the evaporator to remove condensate before icing can occur.

In another form of the apparatus the antifreeze may be arranged to wash over the cooling surface after a buildup of ice has taken place. The defrosting will normally take place under adiabatic conditions so that the melting ice will absorb latent heat from the refrigeration compartment reducing the overall energy consumption of the equipment.

According to a second aspect, the invention proposes auto-defrost refrigeration apparatus of the kind set forth in the opening paragraph which is distinguished by the provision of a defrost controller which is arranged to monitor the operating temperature of the heat exchanger and initiate a defrosting operation when predetermined temperature conditions are detected indicative of ice formation on the heat exchanger.

In a vapour compression refrigeration system the heat exchanger will be provided by an evaporator, although it is also possible to use heat exchangers which operate by thermoelectric cooling for example.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: FiQure 1 is a diagrammatic view of auto-defrost refrigeration apparatus in accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to Fig. 1, the drawing shows a domestic upright freezer having a thermally insulated refrigeration compartment I provided with a door 2 through which food or other items can be placed into the refrigeration compartment to be maintained in a frozen condition. Although a domestic freezer is illustrated by way of example the invention can be applied to similar kinds of refrigeration apparatus such as chest freezers.

refrigerators, large commercial freezers, cold rooms etc. Door seals are provided to prevent atmospheric air from leaking in and out of the compartment, but air can still be admitted whenever the door is opened.

Within the refrigeration compartment there is a vertically mounted plate-type evaporator 3 having its external cooling surface exposed to the interior of the refrigeration compartment I. The evaporator forms part of a vapour compression circuit in which a compressor 4 circulates refrigerant vapour under pressure through a condenser 5 wherein the refrigerant condenses and gives out heat. Condensed refrigerant then passes through an expansion device 6 such as a valve or small bore tube providing a restriction which reduces the temperature and pressure of the refrigerant entering evaporator 3 causing the evaporator to absorb heat from within the refrigeration compartment 1. Refrigerant vapour from the evaporator is re-circulated by the compressor 2 in a continuous cycle of condensation and evaporation. Since the operating temperature of the evaporator will normally be considerably lower than the current atmospheric dewpoint, atmospheric water vapour within the refrigeration compartment will condense out on the surface of the evaporator 3.

With the present auto-defrost system a conventional single-plate evaporator can be used without forced air circulation rather than a finned block-type evaporator and recirculation fan.

The freezer incorporates an auto-defrost system including a reservoir of liquid antifreeze 8 which can be replenished by the user as required.

Antifreeze can flow from the reservoir 8 via a flow control device 9 such as a valve or restriction, to a drip bar 10 which is mounted vertically above the evaporator 3. Liquid antifreeze is released from the drip bar at a controlled rate to bathe the cooling surfaces of the evaporator. Vertically below the evaporator there is a collection trough 12 arranged to collect the liquid antifreeze which has trickled over the surface of the evaporator.

Significant buildup of ice is generally undesirable because it rends to reduce the heat-absorbing ability of the cooling surface and increases running costs. Icing up also lowers the running temperature of the evaporator. In one form of the apparatus the antifreeze can be released continuously at a steady rate so that the antifreeze mixes with supercooled condensate before it has a chance to freeze onto the evaporator. However, a disadvantage of continuous de-icing is that the antifreeze must be capable of reducing the freezing temperature of the antifreeze-water solution below the normal working temperature of the evaporator, generally requiring a highly concentrated antifreeze solution.

In a preferred form of the apparatus which is described below the condensate may be allowed to freeze and an auto-defrost operation is carried out periodically (usually approaching 24 hour intervals) to de- ice the cooling surface. This has the advantage that less antifreeze is required.

The basic requirements of the liquid antifreeze is that it should mix with water and have a freezing point significantly lower than the running temperature of the evaporator 3. By way of example, a food quality glycol such as propylene glycol may be suitable, although other liquids can be used. Contact between the antifreeze and the contents of the refrigeration compartment I can be prevented in any convenient manner, e.g. by providing a wire screen in front of the evaporator.

In a simple form of the apparatus the antifreeze can be discarded after use or recirculated and replaced at regular service intervals. However, a low maintenance system can be used in which the mixture of used antifreeze and condensate is diverted to an evaporating dish 14 which uses heat from the compressor 4 to drive off water from the liquid. After a period of time the concentrated antifreeze can be recirculated to the reservoir 8 using a pump 15. A sensor may also be arranged to monitor the concentration of the liquid in the evaporation tray 14 and activate the pump 15 when a suitable concentration is achieved.

The vapour compression circuit will normally operate intermittently when required in order to maintain the refrigeration compartment at a target operating temperature, typically around -18 °C. This is normally achieved by a refrigeration controller 18 which monitors the temperature of the compartment 1 using a temperature probe 19. Since the probe 19 is also used for defrosting, as explained below, it is mounted in contact with the cooling surface of the evaporator 3, at the bottom of the evaporator.

However, the refrigeration controller could be provided with a separate temperature probe mounted in any suitable position.

The apparatus includes a defrost controller 20 which monitors the evaporator temperature Tevap, e.g. using the temperature sensor 19, and the air temperature within the refrigeration compartment Tint by means of a further sensor 22. When the temperature difference between the evaporator temperature and the internal air temperture within the refrigeration compartment increases to a predetermined figure indicative of significant ice buildup, e.g. 6 °C below the internal air temperature, a defrosting operation is commenced.

An alternative method of triggering a defrost operation is to monitor the rate of fall of the evaporator temperature when the vapour compression circuit operates. Since icing of the evaporator will reduce heat exchange with the refrigeration compartment this will be reflected in an increased rate of temperature fall which can be used to trigger auto-defrost.

The defrost controller commences a defrost operation by inhibiting the refrigeration controller 18, thereby turning off the compressor so that no evaporator cooling can occur. Although it is possible to commence defrosting the evaporator immediately it is also possible to wait until the evaporator temperature approaches the internal air temperature. In either

V

case the defrost controller generates a defrost signal D which is used to open the flow control device 9 to wash the ice from the evaporator. If the evaporator is allowed to reach the internal air temperature before defrosting it is possible that the ice will be at the same temperature as the air, but if defrosting commences soon after the evaporator has been running the ice temperature may be lower than the internal air temperature. Whichever condition applies, when the antifreeze is released onto the evaporator the evaporator temperature will initially fall again by a few degrees, say 4 °C, as the ice melts. The defrost controller is preferably arranged to detect that the ice has stopped melting so that this condition may be used to terminate the defrosting operation. The controller can recognise the melting ice irrespective of the temperature conditions which prevail at the start of a defrost operation because of the temperature fall which always occurs as the ice melts. Melting starts at the top of the evaporator and progresses downwards towards the bottom of the evaporator where the temperature sensor 19 is located. The controller can keep adding antifreeze until the ice at the bottom starts to melt, i.e. drops below the internal air temperature, and then rises towards the internal air temperature when the ice is all melted. It is possible to estimate how much antifreeze is normally required to melt an iced-up evaporator and this quantity can be released at a slow rate, allowing say an hour to melt. Defrosting can be terminated sooner if the temperature at the bottom of the evaporator falls and then rises again. If all the ice has not been removed at the end of the designated defrosting period this does not pose a problem since the next defrost operation would simply commence a little sooner.

Normal operation of the refrigeration circuit can be allowed to recommence when defrosting has ended.

When condensate freezes on the cooling surface of the evaporator 3 about 80 calories per gram of water is used in simply changing state from water to ice. This represents wasted heat input since it is not used to reduce the temperature of the freezer contents. In a conventional freezer additional heat input in excess of 80 calories per gram is required to melt the ice, which further adds to the running costs. With the present invention, if the condensate is prevented from freezing there is a clear energy saving of 80 calories per gram of condensate, but even if icing is allowed to take place there is still an energy saving because, assuming adiabatic conditions, the melting ice removes 80 calories per gram of ice directly from freezer compartment, cooling the freezer contents. The energy saving is a significant proportion of the running costs when the freezer is simply maintaining the sub-zero temperature of the frozen product. With the present refrigeration apparatus the only significant energy input requirement is to remove heat conducted through the walls of the freezer compartment or entering through an opened door.

It is also possible to use a similar defrosting system with other means of de-icing the evaporator, e.g. an electrical heating element, and still achieve a significant improvement in running costs.

Whilst the above description places emphasis on the areas which are believed to be new and addresses specific 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. -10-

Claims

1. Auto-defrost refrigeration apparatus of the kind having a refrigeration compartment which can be opened to introduce ambient air, and a heat exchanger for cooling the contents of said refrigeration compartment and having a cooling surface exposed to air within the refrigeration compartment, the apparatus being distinguished by the provision of: -a reservoir of liquid antifreeze, -washing means arranged to release a controlled quantity of liquid antifreeze from said reservoir to bathe the cooling surface of the heat exchanger which is exposed to air within the refrigeration compartment and intermix with any atmospheric condensate which forms on the cooling surface, and -collection means arranged to receive the mixture of liquid antifreeze and atmospheric condensate from the cooling surface of the heat exchanger.

2. Auto-defrost refrigeration apparatus according to Claim 1 in which the washing means is arranged to continuously trickle antifreeze over the cooling surface of the heat exchanger to remove condensate before icing can occur.

3. Auto-defrost refrigeration apparatus according to Claim 1 in which the washing means is arranged such that the antifreeze washes over the cooling surface after a buildup of ice has taken place.

4. Auto-defrost refrigeration apparatus according to any preceding claim in which the collection means is arranged to recycle liquid antifreeze to the reservoir.

-11 -

5. Auto-defrost refrigeration apparatus according to Claim 4 in which the liquid antifreeze is recycled via water extraction means for concentrating the liquid antifreeze.

6. Auto-defrost refrigeration apparatus according to Claim 3, or any preceding claim which is appended thereto, including a defrost controller which is arranged to monitor the operating temperature of the heat exchanger and initiate a defrosting operation when predetermined temperature conditions are detected indicative of ice formation on the heat exchanger.

7. Auto-defrost refrigeration apparatus according to Claim 6 in which the defrosting operation is commenced when the operating temperature of the heat exchanger reaches a predetermined amount below the air temperature within the refrigeration compartment as monitored by an air temperature sensor.

8. Auto-defrost refrigeration apparatus according to Claim 6 in which the defrosting operation is commenced when the rate of temperature fall of the heat exchanger exceeds a predetermined figure.

9. Auto-defrost refrigeration apparatus according to Claim 6, 7 or 8 in which the defrost controller terminates the defrosting operation when the temperature of the heat exchanger rises to the air temperature within the refrigeration compartment as monitored by an air temperature sensor.

10. Auto-defrost refrigeration apparatus according to any preceding claim in which the heat exchanger is provided by the evaporator of a vapour compression refrigeration system.

11. Auto-defrost refrigeration apparatus according to Claim 10 in which the evaporator comprises a single heat exchange plate. -13-

12. Auto-defrost refrigeration apparatus of the kind having a refrigeration compartment which can be opened to introduce ambient air, and a heat exchanger for cooling the contents of said refrigeration compartment and having a cooling surface exposed to air within the refrigeration compartment, the apparatus being distinguished by the provision of a defrost controller which is arranged to monitor the operating temperature of the heat exchanger and initiate a defrosting operation when predetermined temperature conditions are detected indicative of ice formation on the heat exchanger.

13. Auto-defrost refrigeration apparatus according to Claim 12 in which the defrosting operation is commenced when the operating temperature of the heat exchanger reaches a predetermined amount below the air temperature within the refrigeration compartment as monitored by an air temperature sensor.

14. Auto-defrost refrigeration apparatus according to Claim 12 in which the defrost condition is commenced when the rate of temperature fall of the heat exchanger exceeds a predetermined figure.

15. Auto-defrost refrigeration apparatus according to Claim 9 or 10 in which the heat exchanger is provided by the evaporator of a vapour compression refrigeration system.

16. Auto-defrost refrigeration apparatus which is substantially as described with reference to the drawing.