CA2628065A1

CA2628065A1 - Refrigeration apparatus and evaporation duty cycle

Info

Publication number: CA2628065A1
Application number: CA002628065A
Authority: CA
Inventors: Sikander Jaffer
Original assignee: QBD Cooling Systems Inc
Current assignee: QBD Cooling Systems Inc
Priority date: 2007-04-04
Filing date: 2008-04-02
Publication date: 2008-10-04
Also published as: US20080245085A1

Abstract

A cooling apparatus for objects such as convenience foods and beverages has an enclosure and a vapour cycle refrigeration system. The refrigeration system includes a compressor, a condenser, a metering device and an evaporator, all connected in a circuit.
Fans force air across the evaporator coils during operation of the compressor. When the compressor is 'off', or dormant, the evaporator fans are operated in a partial or reduced duty cycle in which the overall power draw and average power consumption is less than it would be if the evaporator fans were run continuously.

Description

COOLING APPARATUS WITH EVAPORATOR
Field of the Invention This invention relates to the field of cooling apparatus for foods and beverages.
Background of the Invention Commercial coolers for foodstuffs and beverages are well known. Typically, such coolers include an enclosure that may be insulated to some extent. The enclosure may typically include a door or access panel, and may include internal shelving on which objects to be refrigerated may be placed. Most typically these units are cooled using a vapour cycle system that includes a compressor, a condenser, a metering or throttling device, and an evaporator. The condenser is located outside the zone to be cooled, and rejects heat to ambient. The evaporator is most typically located inside the zone to be cooled. Heat transfer between the evaporator and the air of the zone to be cooled may be enhanced by a fan.

Summarv of the Invention In an aspect of the invention, there is a cooling apparatus The cooling apparatus includes an enclosure in which to place objects to be cooled and a compressor, a condensor, an expansion member, and an evaporator connected together in a vapour cycle cooling circuit. The evaporator is located within the enclosure. An air moving apparatus is mounted within the enclosure.
Control circuitry is connected to operate the compressor and the air mover.
The compressor has a duty cycle. The compressor duty cycle includes a first portion and a second portion, the compressor being in operation in the first portion and dormant in the second portion. The air moving apparatus has a duty cycle, the air moving apparatus duty cycle including a first portion and a second portion. In the first portion of the air moving apparatus duty cycle the air moving apparatus being in operation while the compressor is in operation. In the second portion of the air moving apparatus duty cycle the compressor is dormant and the air moving apparatus alternates between 'on' and 'off conditions.

In another feature of that aspect of the invention, during steady state operation the second potion of the air moving apparatus duty cycle includes at least three alternations between 'on' and 'off' while the compressor remains in a single dormant portion of its duty cycle. In a further feature, the enclosure has a volume V, and a resistance to heat loss R, and a thermal time constant T. In steady state operation when the compressor is dormant the second portion of the air moving apparatus duty cycle includes one of (a) an 'on' portion that is less than 5 % as long as the time constant; and (b) the combination of an 'on' portion and a following 'off portion that sum to a length of time that is less than 15 % of the time constant. In still another feature, the compressor is in operation between 25 and 40 % of the time (i.e., the run time), and dormant the balance of the time. In another feature, and during the balance of the time the air moving apparatus is 'on' less than 80 % of the time when the compressor is 'off. . In yet another feature the second portion of the duty cycle of the compressor is longer than the first portion, and the air moving apparatus cycles 'on' and 'offl during the second portion, each 'on' cycle of the air moving apparatus during the second portion of the duty cycle of the compressor being less than 10 % as long as the second portion. Is a still further feature, during the second portion of the duty cycle of the compressor the air moving apparatus is more often 'off than 'on'. In still yet another feature the air moving apparatus remains 'off no longer than 5 minutes at a time. In a further feature the air moving apparatus is 'off between 25 and 80 % of the time. In yet another feature, the cycle of 'on' and 'offportions of the air moving apparatus has a total cycle duration in the range of 1 minute to 10 minutes. In another feature, the total cycle time is in the range of 2 minutes to 5 minutes.

In another feature, the enclosure has first and second temperature sensors operable to sense first and second temperatures at respective locations in the enclosure.
The control circuitry is operable to observe a temperature difference between the first and second temperatures, and, when the compressor is dormant, the air moving apparatus comes 'on' when the temperature difference exceeds a first threshold value, and turns 'off when the temperature difference falls below a second threshold value. In another feature the cooling apparatus has a temperature sensor mounted to observe a temperature within the enclosure. The compressor comes 'on' when the sensor senses a high temperature TH, and turns off when the sensor senses a low temperature, TL. The first threshold value is less than the difference between TH and TL. In a further feature, the cooling apparatus monitors an internal temperature therein, the compressor comes 'on' when the internal temperature exceeds a high temperature threshold TH, and turns off when the internal temperature falls below a low temperature threshold, TL. The air moving apparatus is operable in response to readings from another monitored temperature within the enclosure. The apparatus operates to maintain ripple in the other monitored temperature as compared to the internal temperature exceeds 40 % of the difference between TH
and T,_,. In another feature the air moving apparatus operates in an 'on' condition when the sum of the internal temperature and the ripple exceeds TH.

In another aspect of the invention, there is a method of operating cooling apparatus. The cooling apparatus includes an enclosure in which to place objects to be cooled; a compressor, a condensor, an expansion member, and an evaporator connected together in a vapour cycle cooling circuit; the evaporator being located within the enclosure. Air moving apparatus is mounted within the enclosure. Control circuitry is connected to operate the compressor and the air mover. The method includes operating the compressor on a duty cycle, the duty cycle including a first portion and a second portion, the compressor being in operation in the first portion and dormant in the second portion; operating the air moving apparatus for at least a portion of the time in which the compressor is in operation; and operating the air moving apparatus intermittently when the compressor is dormant.

These and other aspects and features of the invention may be understood with reference to the detailed description of the invention and the accompanying illustrations as set forth below.
Brief Description of the Figures The principles of the invention may better be understood with reference to the accompanying figures provided by way of illustration of an exemplary embodiment, or embodiments, incorporating principles and aspects of the present invention, and in which:
Figure 1 shows an exploded view of a refrigeration apparatus such as might include one or more aspects of the present invention;
Figure 2 is a schematic of a cooling apparatus such as that of Figure 1;
Figure 3 is a duty cycle representation for the cooling apparatus of Figure 1;
and Figure 4 shows an enlarged potion of the duty cycle of the cooling apparatus of Figure 3.
Detailed Description The description that follows, and the embodiments described therein, are provided by way of illustration of an example, or examples, of particular embodiments of the principles of the present invention. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the invention. In the description, like parts are marked throughout the specification and the drawings with the same respective reference numerals. The drawings are not necessarily to scale and in some instances proportions may have been exaggerated in order more clearly to depict certain features of the invention.
In terms of general orientation and directional nomenclature, for the cooling apparatus 20 described herein, the height, in most common use, is measured vertically, and may be measured from the base of the unit. The width of the unit is a dimension measured generally horizontally across the unit as a person facing the unit might see it. The depth of the unit, or portion thereof, may be the front-to-back distance through the unit.

By way of general overview, a cooling apparatus according to an aspect of the present invention is shown in Figures 1 and 2 as 20. At a global level, apparatus 20 includes a housing, such as may be termed a housing structure or assembly, 22. Housing 22 may include a first portion or zone 24 and a second portion or zone 26. Zone 24 may defme an insulated enclosure, 28.
Insulated enclosure 28 may have wall panels 30 which may include a top, bottom, left and right hand sides, front and back. Those wall panels may include insulation layers as at 32. Enclosure 28 may also include an access member, such as a door, or lid, or curtain, 34, which may be hinged or sliding, or loose. Enclosure 28 may have mounting fittings such as tie-downs or shelves 36 upon which objects to be refrigerated 38 may be placed. Shelves 36 may have the form of grids or gratings or spaced rods that permit the passage of air. Zone 26 may adjoin zone 24, and may, for example be located below zone 24. Zone 26 may tend not to be insulated, and may tend to have both an air intake 38 (such as may, for example include a louvered grill as at 40) and an air outlet or exhaust as at 42. Often the air intake is at the front of the unit, and the air exhaust is at the back of the unit, although the terminology of 'front' and 'back' is somewhat arbitrary.

Cooling apparatus 20 may include refrigeration equipment 50, which may be vapour cycle refrigeration equipment. Equipment 50 may include a compressor 52, a first heat exchanger designated as a condenser 54, a metering or throttling device 56 (which may be a nozzle, athrotding valve or a capillary tube, for example), and a second heat exchanger designated as an evaporator 58, with the customary high pressure lines 60 and low pressure lines 62 linking the output of the compressor to the input of the condenser, the output of the condenser to the throttling device, the throttling device to the evaporator, and the evaporator to the compressor to define a flow path, or closed circuit, through which the heat transfer working fluid (i.e., the refrigerant) may flow. The evaporator may be mounted in, or mounted in fluid communication with, the enclosed space 48 of zone 24. The compressor, the condenser and the throttling device may typically be located in the adjacent machinery space of second zone 26. The compressor may typically have a co-axially mounted fan 46 such as may tend to draw air through the intake 38, across the heat exchanger coils of condenser 54, and then urge that air over the body of compressor 52 (and such electric motor as may be driving compressor 52), and out exhaust 42, the direction of airflow being indicated generally by arrows W.

As noted, evaporator 58 is typically mounted within the zone to be cooled, and the zone to be cooled is most typically an insulated box with an access door. Evaporator 58 may often be mounted in an upper region of the enclosed space. Air moving apparatus 64, most typically in the nature of one or more fans 66, 68 may be mounted in the enclosure 28. Fans 66, 68 may tend to be mounted in a common mounting with the heat exchange coils and fmwork of evaporator 58, such as may tend to force air from the enclosure 28 over those coils, and then to circulate throughout the enclosure 28 more generally. This may tend to enhance convective heat transfer from the coils, enhance convective heat transfer to objects in the refrigerated space, and to reduce thermal stratification within the enclosure 28 generally. That is, the flow of air caused by the urging of the air moving apparatus may tend to even out the internal temperature within the enclosure 28 more generally. Fans 66, 68 need not necessarily be mounted next to, or only next to, evaporator 58, but may be mounted in various locations in the enclosures, as at 67, 69, which may be mounted to various shelves, for example.
In operation, the compressor draws heated vapour from the evaporator, compresses it to a higher temperature and pressure, and impels the heated gas to the condenser.
It may be that during initial cool-down of the refrigeration apparatus from ambient temperature generally, or during initial cooling after new objects to be refrigerated have been placed in the enclosure 28, compressor 52 may operate continuously for a relatively lengthy time. However, at some point a more or less steady state condition may be reached in which compressor 52 operates on a more or less repetitive duty cycle of 'on' and 'of~ portions. The need to be able to manage a transient temperature pull down means that the compressor will be sized to have a capacity rather greater than required to handle the typical steady state operation. Quite typically, refrigeration apparatus 50 may include control circuitry, symbolized by controller 70, and one or more temperature sensors 72 mounted within the zone to be cooled. For the purposes of this discussion, sensor 72 may be considered a datum temperature sensor. The datum may be considered to be a mean temperature within the enclosure 28, or a baseline reference temperature, and may be a calculated temperature based on the average, or weighted average, of temperatures sensed by a plurality of sensors at different locations within the space to be cooled. However it may be, the control circuitry 70 (which may include a digital processing apparatus) may be connected to cause compressor 52 to come 'on' (i.e., to be in an operating condition) when the datum temperature rises above an upper or higher threshold temperature or upper set point temperature TH, and to continue running until the datum temperature falls below a lower threshold temperature or lower set point temperature TL.
This will tend to yield an on-off hysteresis loop giving the sawtooth temperature performance shown in the upper portion of Figure 3. The control circuitry may include a temperature adjustment member 74, such as a thermostat, by which the range defined by the upper and lower threshold or set point temperatures may be raised or lowered. The length of the duty cycle, and the proportion of the duty cycle in which the compressor is operating, or 'on' or 'on-line', (and, the proportion of the duty cycle in which the compressor is non-operating, or dormant, or 'off, or 'off-line') is a function the difference between the temperature range selected and the ambient temperature to which the condenser rejects heat, as well at the overall resistance to thermal leakage of the insulated enclosure 28 more generally. Typically, where the refrigeration apparatus is rejecting heat to a room temperature ambient (i.e., about 70 - 75 F or 22 - 25 C), and is being used to maintain foodstuffs at 35 - 40 F (2 - 5 C), the compressor may operate 25 - 40 % of the time, or even up to 70 or 80 %, and may be dormant, or off-line, the balance of the time.

In apparatus made heretofore, the evaporator fans e.g., fans 66, 68, have operated continuously. For the purposes of this discussion, the electrical power draw for refrigeration apparatus 20 of this nature may be taken to include the sum of the power draw of the compressor and the power draw of the evaporator fan, or fans. During compressor operation, operation of the evaporator fans enhances heat transfer, and reduces the operating time required of the compressor.
When the compressor is off-line, or dormant, the evaporator fans serve not only to even out temperature distribution in the enclosure 28 but, since the enclosure temperature is above freezing, may tend to cause frost accumulated on the evaporator coils during compressor operation to melt, the drip condensate being collected and transported away from the enclosure 28, typically to an evaporation tray on the condenser side of the unit.

However, the airflow rate required, or desired, for encouraging forced convection heat transfer to and from the evaporator coils and finwork, such as may be, may be quite different from the airflow rate that may be suitable, optimal, or merely adequate, for agitating or mixing, or stirring the air within the enclosure 28 to discourage undue temperature stratification within the enclosure 28 when the compressor is not in operation.

As illustrated in Figures 3 and 4, fans 66, 68 (or 67, 69, as maybe) of refrigeration apparatus 20 may, during the dormant portion of the duty cycle of compressor 52, be run on their own intermittent 'on' -'off duty cycle, i.e., a partial duty cycle, rather than being run continuously.
That is, the evaporator fans 66, 68 may have different operating regimes according to whether the compressor is running in the 'on' condition, or dormant in the 'offl condition. For example, fans 66 and 68 may include, or be controlled by, switches or power relays to permit them to be turned on and off as may be desired. Operation of these switches or relays may be governed by controller 70.
As illustrated, fan 68, for example, has an on cycle period of a first duration, tl, and an off cycle period of a second duration, t2, the 'on' and 'off periods transitions being step functions, and the period of the wave, or cycle, being defined by the total to tl and t2. It may be that ti and t2 are fixed, that their duration is fixed, that their proportions of duration relative to one another are fixed, and that the combined cycle time of tl + t2 is also fixed, in the manner of a repetitive square wave, or square pulse train, where the up (or 'on') pulses may have different duration from the down (or 'off) pulses. However, in the general case this need not be so. Each 'on' period may be of a different length from its neighbours, and each off period may be of a different length from its neighbours. On occasion there may be perturbations in the cycle, as suggested by the excursion following t3, such as may represent the effect of opening the door to introduce or remove an object.
The values of ti and t2 may be set (or pre-set) arbitrarily, either by a digital input, or by a hard wired means, such as an analogue device such as a timed relay and which may employ a variable component such as a variable potentiometer to permit adjustment of the timed interval. In one embodiment in the dormant compressor regime the ratio of ti : t2 may be in the range of 1:4 to 4:1 (i.e., 'on' 20 % to 80 % of the time), or in the narrower range of 1:2 to 3:1 (i.e., on 1/3 to'/a of the time). In one embodiment it is the fan may be on about 2/3 or 70 % of the time, +/- 10%, or less.
In one embodiment, the 'on' cycle of the fans may be less than 5 minutes. In another, the 'on' cycle may be less than 3 minutes. In another embodiment, the 'on' cycle may be in the range of 20 to 120 seconds. In another embodiment the 'on' cycle may be of the order of 40 to 90 seconds. In one embodiment the 'on' cycle may be about 3 minutes and the 'off cycle may be about 2 minutes. In another the 'on' cycle may be about 1%Z minutes, and the 'off cycle' may be about 1 minute. In this mode of operation, fan 66 or 68 may simply be controlled by a timer, or clock (whether through digital or analogue devices) until the enclosure temperature rises and the compressor begins to run again. In this system, the compressor 'on' signal may be triggered when the monitored temperature in the enclosure, (i.e., the datum temperature) exceeds TH.
Optionally, where the evaporator fans have been'off, there may be frost on the evaporator coils. In some embodiments, the controller may turn on the evaporator fans prior to turning on the compressor to encourage frost to melt prior to the delivery of more cold refrigerant to the evaporator. This time interval or delay, or lag prior to starting the compressor may be 20 to 30 seconds in some embodiments.

Alternatively, the values of t, and t2 may be determined by a feedback loop.
For example, enclosure 28 may include first and second (or more) evaporator or internal circulation fan feedback temperatures sensors 76, 78 mounted at different, and relatively far apart locations in enclosure 28.
One of these sensors 76, 78 may be one of (or the) temperature sensor whose output is used to determine the on and off points for the compressor, but this need not be so.
It may be that the first sensor 76 is located in an upper region of the enclosure 28, and the second sensor 78 is located in a lower region of the enclosure 78. The operation of the evaporator or circulation fan may then be responsive to the temperature difference T76 - T78 = Delta(T), the temperature divergence between the two sensed temperatures being taken as a measure of, or a proxy for, temperature stratification within enclosure 28. It may also be that as the higher temperature, i.e., T76, approaches the compressor turn on temperature, compressor operation may be delayed by reducing the temperature divergence in the cabinet. Thus, within a certain temperature difference TH -T76 < Epsilon, circulating or evaporating fan operation of, for example, fan 66, may be continuous. In one embodiment, Epsilon may have a value in the range of 5 to 20 % of the difference in temperature between TH and TL. To the extent that the overall temperature curve at each of the high and low sensors (i.e., as T76 and T78) may tend to track the main temperature sensor controlling the compressor 'on' and 'off operation, and the main temperature may tend to follow an exponential decay sawtooth waveform, this local internal temperature divergence may be thought of as a ripple on a rectified wave, where Delta(T) is the amplitude of the ripple (or, in a similar manner, the double amplitude if the main temperature on which compressor performance is based is taken as the mean temperature calculated by averaging the local temperatures throughout the enclosure 28 at locations such as T76 and T78. In one embodiment, the ripple (i.e., the divergence from the main sensed or calculated enclosure temperature) is less than 5 F (roughly 3 C). In another embodiment the ripple is less than 3 F(2C) In another the divergence is less than 20 % of the difference Tnmb;eõt - TL. In another, embodiment the divergence is less than 10 %.

Expressed differently, the enclosure 28 may have an overall thermal resistance RTb (i.e., a resistance to heat loss or heat gain), and may have a deemed operational thermal mass from which a thermal capacitance CTh can be determined. When the compressor is 'offl, or dormant, the mean temperature in the enclosure 28 may tend to approach the ambient external temperature on an exponential decay curve. The time constant of this decay curve may be taken as the product of the thermal resistance multiplied by the thermal capacitance. Empirically, a value forthistime constant RTh CTb may be obtained by filling 2/5 of the volume of the enclosure 28 with containers of water, stabilising the enclosure at a given cold temperature Tc, placing the compressor in the 'off condition, and measuring the length of time required for the temperature difference between the mean internal temperature and ambient temperature to fall to a value that is 36 % of the initial difference between TAm;nieot and Tc. In some embodiments, when the compressor is 'off , the 'on' cycle of the evaporator or air circulating fans may be less than 1'/z % of this time constant (and in some embodiments less than 3/4 %), and the 'off cycle of the evaporator or circulating fan may be less than 1%(and in some embodiments less than '/2 %) of this time constant.
That is, qualitatively and conceptually, the 'on' and 'off periods of the circulating fans are very short relative to the length of time it takes for appreciable heating or cooling of the enclosure 28 and its contents to occur, such that the ripple effect on the internal temperatures caused by cycling of the circulation fans can be taken as being relatively insignificant. As opposed to running the circulation fans continuously at a slower speed, the intermittent running may tend to have a surge and flush effect.
In one, alternate, embodiment, the fans may be provided with either a discrete or continuously variable speed control, and may be run continuously, but at reduced speed or flowrate during the dormant period of the compressor. However, an 'on' -'of control may tend to be a relatively simple and inexpensive device.

In another mode of operation, the compressor may cut out at the low set point temperature TL. In one embodiment the evaporator fan (or fans, as may be) 66, 68 may then run until the temperature sensed at the governing temperature sensor (be it 72, 76, 78, or some other observed or calculated value based on one or more such readings) is such that it may be inferred that the evaporator coil is at a temperature above freezing (i.e., and so therefore is defrosted), at which time the evaporator (or circulating) fan (or fans) 66, 68 may also cut out i.e., be turned 'off. . The temperature in the enclosed space of enclosure 28 may then tend to rise. When the datum temperature reaches a certain level, the circulating fans may be turned on again, and then run continuously until the compressor cuts in again, and the system operates in the 'compressor 'on' regime. Alternatively, the operation of the circulating fan (or fans) may tend to cause an evening out of the temperature in the enclosure, and may tend to cause heat transfer from the objects to be cooled (assuming those objects to have been at a lower, more or less steady state temperature when the compressor reached the shut off condition) to the enclosed environment.
When the enclosure temperature falls back below a shut off set point for those fans, the circulating or evaporator fans, as may be, may then be turned off again, and the temperature permitted to rise.
If the circulating fans are unable to draw the temperature down to the shut off set point for the fans, they may tend to run continuously until the compressor comes back on, shiffting the system into the compressor 'on' regime. Under this mode of operation, the intervals in which the fan (or fans) run will tend to be shorter initially, and get longer as the underlying temperature of the objects to be cooled creeps upward. Clearly, once the objects to be cooled reach or exceed the fan cut-out temperature, those fans will run continuously, since they will no longer be able to cool the enclosure temperature sensor far enough to turn themselves off. The shut off set point for the fans may be set at some temperature difference from the compressor 'on' set point of TH. For example, in one embodiment the fans may cut in at a temperature that is I C below TH, in another embodiment 2 C below TH.
The cut-out temperature may similarly be based on a temperature difference, such as 1 C or 2 C
below the cut-in temperature, or some value referenced directly to TH that is lower then the fan cut-in temperature.
It follows from the foregoing that running the evaporator fan, or fans, on a reduced duty cycle, there is a reduced current draw to apparatus 20, more generally, and hence lower power consumption. That is, the effective mean power consumption of the evaporator fans 66, 68 is reduced during the period when the compressor is dormant. Further, although the evaporator fans may tend to be relatively low power devices, the power consumed by these devices is, ultimately, converted into heat that is released in the enclosure to be refrigerated.
While this may not seem particularly large, the fans tend to run over lengthy periods of time (indeed, continuously), and every Joule of energy consumed by the fan must, eventually, be removed as heat by the operation of the compressor and condenser. Thus there is the secondary benefit of not having later to remove that heat, which may tend to permit the 'off cycle of the compressor to be incrementally longer in proportion to the 'on' cycle, yielding an improvement, however slight, in the performance of the system overall.

Various embodiments of the invention have been described in detail. Since changes in and or additions to the above-described best mode may be made without departing from the nature, spirit or scope of the invention, the invention is not to be limited to those details.

Claims

1. A cooling apparatus including:
an enclosure in which to place objects to be cooled;
a compressor, a condensor, an expansion member, and an evaporator connected together in a vapour cycle cooling circuit;
said evaporator being located within said enclosure;
air moving apparatus mounted within said enclosure;
control circuitry, said control circuitry being connected to operate said compressor and said air mover;
said compressor having a duty cycle, said compressor duty cycle including a first portion and a second portion, said compressor being in operation in said first portion and dormant in said second portion;
said air moving apparatus having a duty cycle, said air moving apparatus duty cycle including a first portion and a second portion;
in said first portion of said air moving apparatus duty cycle said air moving apparatus being in operation while said compressor is in operation;
in said second portion of said air moving apparatus duty cycle said compressor is dormant and said air moving apparatus alternates between 'on' and 'off' conditions.

2. The cooling apparatus of claim 1 wherein, during steady state operation, said second potion of said air moving apparatus duty cycle includes at least three alternations between 'on' and 'off' while said compressor remains in a single dormant portion of its duty cycle.

3. The cooling apparatus of claim 1 wherein said enclosure has a volume V, a resistance to heat loss R, and a thermal time constant T; and, in steady state operation when said compressor is dormant said second portion of said air moving apparatus duty cycle includes one of (a) an 'on' portion that is less than 5 % as long as said time constant; and (b) the combination of an 'on' portion and a following 'off' portion that sum to a length of time that is less than 15 % of said time constant.

4. The cooling apparatus of claim 1 wherein said air moving apparatus is 'on' less than 80 % of the time when said compressor is dormant.

5. The cooling apparatus of claim 1 wherein said second portion of said duty cycle of said compressor is longer than said first portion, and said air moving apparatus cycles 'on' and 'off ' during said second portion, each 'on' cycle of said air moving apparatus during said second portion of said duty cycle of said compressor being less than 10 % as long as said second portion.

6. The cooling apparatus of claim 1 wherein, during said second portion of said duty cycle of said compressor said air moving apparatus is more often 'off' than 'on'.

7. The cooling apparatus of claim 6 wherein said air moving apparatus remains 'off' no longer than 5 minutes at a time.

8. The cooling apparatus of claim 6 wherein said air moving apparatus is 'off' between 25 and 80 % of the time.

9. The cooling apparatus of claim 6 wherein during said second portion of said duty cycle of said compressor said cycle of 'on' and 'off' portions of said air moving apparatus has a total cycle duration in the range of 1 minute to 10 minutes.

10. The cooling apparatus of claim 9 wherein said total cycle time is in the range of 2 minutes to 5 minutes.

11. The cooling apparatus of claim 1 wherein said enclosure has first and second temperature sensors operable to sense first and second temperatures at respective first and second locations in said enclosure, said control circuitry being operable to observe a temperature difference between said first and second temperatures, and, when said compressor is dormant, said air moving apparatus comes 'on' when said temperature difference exceeds a first threshold value, and turns 'off' when said temperature difference falls below a second threshold value.

12. The cooling apparatus of claim 11, wherein said cooling apparatus has a temperature sensor mounted to observe a temperature within said enclosure, and said compressor comes 'on' when said sensor senses a high temperature T H, and turns off when said sensor senses a low temperature, T L; and said first threshold value is less than the difference between T H and T L.

13. The cooling apparatus of claim 11 wherein said cooling apparatus monitors an internal temperature therein, said compressor comes 'on' when said internal temperature exceeds a high temperature threshold T H, and turns off when said internal temperature falls below a low temperature threshold, T L; and said air moving apparatus is operable in response to readings from another monitored temperature within said enclosure; and said apparatus operates to maintain ripple in said other monitored temperature as compared to said internal temperature exceeds 40 % of the difference between T H and T L.

14. The cooling apparatus of claim 13 wherein said air moving apparatus operates in an 'on' condition when the sum of said internal temperature and said ripple exceeds T
H.

15. A method of operating cooling apparatus, the cooling apparatus including an enclosure in which to place objects to be cooled; a compressor, a condensor, an expansion member, and an evaporator connected together in a vapour cycle cooling circuit; said evaporator being located within said enclosure; air moving apparatus mounted within said enclosure;
control circuitry, said control circuitry being connected to operate said compressor and said air mover, said method comprising:
operating the compressor on a duty cycle, the duty cycle including a first portion and a second portion, said compressor being in operation in said first portion and dormant in said second portion;
operating the air moving apparatus for at least a portion of the time in which said compressor is in operation; and operating the air moving apparatus intermittently when the compressor is dormant.