GB2167579A

GB2167579A - Temperature controls of refrigerated cabinets

Info

Publication number: GB2167579A
Application number: GB08528257A
Authority: GB
Inventors: C Wilkinson
Original assignee: Barker & Co George
Current assignee: Barker & Co George
Priority date: 1984-11-27
Filing date: 1985-11-15
Publication date: 1986-05-29
Also published as: GB8528257D0; GB8429904D0; GB2167579B

Abstract

The present invention provides a control system for a refrigerated display cabinet to keep the cabinet temperature constant or within a narrow range. To achieve this the system comprises two temperature sensors 50, 52 located at strategic positions in the flow path of the air which cools the storage space, and specifically the temperature sensors are located to either side of a refrigeration unit evaporator 30 over which the air flows. The temperature sensors are temperature sensitive resistances and the resistances thereof connected in series so as to be summed for the purposes of generating a control signal based upon the extent to which the aggregate resistance of the sensors deviate from a mean aggregate valve corresponding to the required cabinet temperature, as this provides for desirable response characteristics. <IMAGE>

Description

SPECIFICATION Temperature controls of refrigerated cabinets This invention relates to a method and apparatus for controlling the temperature of the produce receiving space of a regrigerated display cabinet.

Typically, a refrigerated display cabinet to which this invention relates comprises an open fronted box or chest having a base, a rear wall, a top wall, end walls and a short part front wall which makes the front typically knee or waist high. The cabinet space usually will be provided with shelves on which produce can be supported, and the base also has a produce supporting surface. The base may also include the refrigeration equipment comprising an evaporator. An air circulation means may cause a flow of air over the evaporator in order to cool the air, and cool air passes up through the back of the cabinet, through the top, and the air then floods as a curtain from the under side of the top at the front edge thereof, and the curtain is collected at an inlet along the top of the part front wall.Additionally, some of the cooled air issues through that surface of the back wall which faces into the interior cabinet, so that the cool air will pass over the shelves, and the produce thereon, this air eddying and gravitating towards the inlet at the top of the part front wall so that it is eventually re-cycled along the paths described above.

As these refrigerated display cabinets are used mainly for food, and mainly for food which requires to be kept at an accurately controlled low temperature if the food is to remain fresh and not to deteriate unacceptably, control of the temperature of the cabinet storage space to keep the temperature constant or within a narrow range is important.

Indeed, very demanding temperature control and temperature tolerances are demanded by users, and typically a refrigerated display cabinet for the storage of meat and dairy produce maybe required to have the storage space maintained at a temperature of 0 C with a temperature tolerence of + 2" C. The storage space may be required to be maintained at 4"C with a temperature tolerance of + 2"C when food such as vegetables is to be stored, The present invention is concerned with sensor means for the accurate control of the temperature of, typically, a refrigerated display cabinet, although it is to be mentioned that this control method and means can be used where equivalent conditions exist i.e.

where there is a air circulation through a space to be kept at a low temperature. In the following (including the claims), reference is made only to a refrigerated display cabinet because this is the main application of which we are aware and also because it is convenient to refer to such a cabinet for explanation purposes.

In the known arrangement for controlling the temperature of the airflow in a refrigerated display cabinet, a single temperature sensor is used and this is positioned at what was thought to be the most convenient location in the display cabinet, in fact under a central shelf of the cabinet and out of sight, behind front lip on the shelf. The temperature sensor is coupled electrically or pneumatically or by any other means to a regulator valve controlling the flow of the refrigerant through the refrigerator apparatus, thereby to control the heat exchange with the flowing air and thereby to control temperature within the display cabinet.This method has been found quite simply not to be sufficiently accurate for current demands, and has been found to be inaccurate for giving an overall temperature control, because the temperature of the air can be affected by a number of variables such as the volume of produce contained within the cabinet at any one time, and also the temperature of the environment surrounding the cabinet, because the surrounding evironment air is induced into the air curtain, and in addition heat exchange takes place between the air curtain and air of the surrounding environment.

The present invention provides a method and means whereby control accuracy is improved as compared with the conventional method described above, and in accordance with a first aspect of the invention there is provided a method of controlling the temperature of the air flowing through a refrigerated display cabinet comprising utilising two temperature sensors which are spaced apart in the air flow cycle, and one of which is located at or near the inlet at the top of the part front wall, whereby the temperature of inflowing air delivered to the refrigeration unit will be monitered, and wherein the temperature of the air issuing from the outlet of the refrigeration unit is controlled depending upon the temperatures sensed by the respective temperature sensors.

The other temperature sensor may be located in the air flow where it issues from the refrigeration unit, and before the air is charged into the cabinet.

In the preferred arrangement, the temperature sensors preferably comprise resistors which are sensitive to temperature change, and the respective resistors are connected in series so that the resistances thereof are in fact summed, an electrical signal derived from the summed resistances being arranged to control a solenoid valve embodied in the refrigeration unit thereby to switch on and offthe flow of refrigerant, thereby to control the heat exchange between the flowing air and the refrigeration unit.

For example, the sensors may be coupled to a printed circuit board arranged to control the solenoid valve, and for example the sensors may nominally comprise 100 ohms resistors, and the printed circuit board may be set to operate around a mean summed sensor resistance of 200 ohms, and to close the valve when the summed sensor resistance drops to 199 ohms, and open the valve when the summed sensor resitance reaches 201 ohms.

The printed circuit board may be provided with a control means to provide for the adjustment of the level of the mean summed resistance, or in an alternative arrangement there may be a two position switch, for example a toggle switch, whereby the apparatus can be arranged by throw of the switch to operate at one or other of two pre-set mean summed resistance levels. Such arrangement is appropriate for setting the display cabinet for operating selec tively at two different temperatures corresponding to the storage of two different types of produce e.g.

meat on the one hand and vegetables on the other hand. By providing such a switch arrangement, the user can set the display cabinet operation easily without requiring the presence of a refrigeration engineer.

With the above figures, typically the sensors maybe 100 ohms resistors which changed by .39 of an ohm for each C rise in temperature sensed. The sensors may furthermore be constructed as described in our co-pending patent application No.

8429903.

With the preferred arrangement of the invention, any increase in temperature sensed by the sensors is cumulative as regards control of the refrigeration unit, which means that the refrigeration unit will respond quickly if the sensors are each detecting an increase or decrease in temperature of the flowing air, as this indicates an overall increase or decrease in temperature of the cabinet. If only one sensor detects an increase of temperature however, then the response will be slower, and if one sensor shows an increase whilst the other sensor a decrease, the response will be slower still, which is what is required for accurate control.

An embodiment of the invention will now be described, by way of example with reference to the accompanying diagrammatic drawings, wherein; Figure 1 is a side elevation of a refrigerated display cabinet which operates in accordance with the method of the invention, and is in accordance with the apparatus of the present invention; and Figure2 is a block diagram illustrating the control circuit of the present invention.

Referring to the drawings, and firstly to Figure 1, the refergerated display cabinet 10 comprises a base 12, a back 14, a top 16 and part front 18. Additionally, the cabinet has ends to define a storage space 20 which is to be maintained in a refrigerated condition.

The back is provided with shelves 22,24 and 26 for the storage and display of produce such as meat, dairy produce, fruit and vegetables. The base surface 28 is also used for supporting this produce. It is necessary to maintain such products in refrigerated condition, and this is done in the example illustrated by means of a refrigeration unit 30 of conventional construction comprising an evaporator and appropriate circuit connections to permit circulation therethrough of the refrigerant. The liquid line in the refrigeration cycle is provided with a solenoid valve 32 (illustrated in Figure 2). Air is circulated over the evaporator of the regrigeration apparatus 30 by means of a fan 34.The fan 34 draws in air through an inlet 36 atthetop ofthe partfront 18, and causes the air to flow over the evaporator of the apparatus thereby to cool the air, typically to a temperature of the order of -2"C. The cold air issues from the unit 30 and then passes as shown by arrows 38 up through the hollow interior of the back 14 of the cabinet. Some of the air, as shown by arrows 40 issues through the surface of the back 14 which faces into space 20 whereby the cold air will flow over the produce on the shelves 22.The remainder of the air passes through the top 16, and eventually issues out of an outlet 42 at the front and underside of the top 16 so that an air curtain 44 travelling from outlet 42 to inlet 36 exists, sheilding the interior space 20 from the air of the surrounding atmosphere 46, which air may be at a relatively high temperature compared to the temperature of the inside of the cabinet. The air which passes through the front wall of the back 14 as indicated by arrows 40 also eddies and gravitates towards the inlet 36, so that basically all of the air is circulated.

In an arrangement such as this, the temperature of the air in the space 20 varies from place to place, as there are various temperature gradients, but generally speaking, the air is colder as it travels through the back 14, and is at warmest on entering the inlet 18. The temperature distribution throughout the space 20 can be effected by a number of factors such as the amount of produce on the shelves, and the temperature of the surrounding enviroment 46, and the previous method of controlling the temperature space 20 using a single temperature sensor at the position 48 was not found to be sufficiently accurate, as the temperature of the air entering inlet 36 could be quite high on a particularly hot day.

The present invention seeks to provide a means for the more accurate control of the temperature in the space 20 in order to keep same within more accurately controlled limits, and in the example illustrated two sensors are used, one positioned at location 50 in the relatively cold air stream, and one positioned at location 52 in the relatively warm air stream. It will be noticed that position 50 in the back wall 14 is spaced somewhat from the outlet from the unit 30, where the air is at its coldest, whilst the sensor 52 is located in the inlet 36 so as to sense the temperature of the returning air. These two sensors are used to control the solenoid valve as illustrated in Figure 2, and Figure 2 it should be mentioned also shows the sensors 50 and 52.The sensors 50 and 52 are temperature sensitive resistances of nominal value 100 ohms, and vary in resistance at .39 of an ohm for each degree of temperature variation sensed. The sensors are coupled in series across a pair of terminals 54,56 of the printed circuit board 58 which controls the solenoid valve 32 depending upon what is sensed by the sensors 50 and 52.An adjustable temperature set point potentiometer control 68 can be used to set the mean sensor input resistance at which the printed circuit board is operative, and typically this may be set, with 100 ohm sensor resisters, to be 200 ohms corresponding to 0 C in the cabinet, but the potentiometer may be adjustable over a range (10 turns) giving an operational temperature range of as much as 30"C. When the printed circuit board 68 for example by means of a comparitor senses a reading of 200 ohms from the resistors 50 and 52, the solenoid valve 32 will be opened, but when the resistance drops to 198.5 ohms for example, the solenoid valve is closed, and the refrigeration effect ceases whereby the temperature of the air flow from unit 30 will start to increase.

The dropping of the resistance reading to 198.5 ohms, for the values given, would represent a temperature drop of 2"C, but 20C spread between sensors 50 and 52. For example it might mean a drop of 2"C sensed by sensor 50, with no detection of change of sensor 52, or it might mean a drop of 1"C at sensor 50 and 1"C at sensor 52. Equally, the printed circuit board 58 may be set to open the valve 32 when the resistance sensed is 201 ohms, meaning an increase in temperature from the normal position of 2"C. This increase could be a net increase i.e. it could be represented by an increase of 3"C at sensor 52 and a decrease by 1"C at sensor 50.By setting the board 58 to operate denpending upon the aggregate change sensed by the sensors 50 and 52, a faster reponse and more aCcurate control is achieved.

Sensor 52 senses the temperature of the air when it is at its warmest in the cycle, whilst 50 senses where the air is in the region of its coldest. If there is a general drift in one direction or the other i.e. increase or decrease in temperatures, then both sensors will change in the same direction, making the response quicker than if one simply sensed on the basis of a single sensor. As the sensor 52 is located in the inlet, then it will respond quickly if the temperature of the airflow is being effected by the surrounding atmosphere 46.

In a modification potentiometer as associated with a two position switch, the respective positions corresponding to aggregate resistances which corresponding to display cabinet temperature of typically 0 C and say 4"C when the cabinet is to hold meat and dairy produce on the one hand and fruit and vegetables on the other hand. It is then a simple matter for an operator to switch the cabinet between the two settings if it is required to change the contents of the cabinet.

It is to be mentioned that the method and apparatus can be utilised in connection with other forms of refrigerated display cabinets such as the open top chest type, where an air flow is to be maintained for preserving the produce contained therein at a refrigerated temperature, and the invention can also be applied to parallel instances requiring air for the cooling of a space.

Claims

1. A method of controlling the temperature of the airflowing through a refrigerated display cabinet comprising utilising two temperature sensors which are spaced apart in the air flow cycle, and one of which is located at or near the inlet at the top of the part front wall, whereby the temperature of inflowing air delivered to the refrigeration unit will be monitered, and wherein the temperature of the air issuing from the outlet of the refrigeration unit is controlled depending upon the temperatures sensed by the respective temperature sensors.

2. A method according to claim 1, wherein the other temperature sensor is located in the air flow where it issues from the refrigeration unit, and before the air is charged into the cabinet.

3. A method according to claim 1 or 2, wherein the temperature sensors comprise resistors which are sensitive to temperature change, and the respective resistors are connected in series so that the resistances thereof are in fact summed, an electrical signal derived from the summed resistances being arranged to control a solenoid valve embodied in the refrigeration unit thereby to switch on and off the flow of refrigerant, thereby to control the heat exchange between the flowing air and the refrigeration unit.

4. A method according to claim 3, wherein there is adjustment means to adjust the effective summed resistance to adjust the temperature at which the flow of refrigerant is switched on and off.

5. A method of controlling the temperature of the air flowing through a refrigerated display cabinet substantially as hereinbefore described with reference to the accompanying drawings.

6. In a refrigerated display cabinet, a temperature control system comprising two temperature sensors which are spaced apart in the air flow cycle, and one of which is located at or near the inlet at the top of the part front wall, whereby the temperature of inflowing air delivered to the refrigeration unit will be monitered, and wherein the temperature of the air issuing from the outlet of the refrigeration unit is controlled depending upon the temperatures sensed by the respective temperature sensors.

7. A control system according to claim 6, wherein the other temperature sensor is located in the air flow where it issues from the refrigeration unit, and before the air is charged into the cabinet.

8. A control system according to claim 6 or 7, wherein the temperature sensors comprise resistors which are sensitive to temperature change, and the respective resistors are connected in series so that the resistances thereof are in fact summed, an electrical signal derived from the summed resistances being arranged to control a solenoid valve embodied in the refrigeration unit thereby to switch on and off the flow of refrigerant, thereby to control the heat exchange between the flowing air and the refrigeration unit.

9. A control system according to claim 6,7 or 8, including adjustment means to adjust the effective summed resistance to adjust the temperature at which the flow of refrigerant is switched on and off.

10. A control system according to claim 6,7,8 or 9, including a multi-position switch of which each setting corresponds to different operating temperatures of the display cabinet and by positioning of said switch in any selected position, sets the system to endeavour to maintain the cabinet at the temperature corresponding to such selected position.

11. In a refrigerated display cabinet, a temperature control system substantially as hereinbefore described with reference to the accompanying drawings.