GB2300934A

GB2300934A - Controlling heater for glass door of refrigeration equipment

Info

Publication number: GB2300934A
Application number: GB9604851A
Authority: GB
Inventors: Dennis Cotter; Richard Stevenson
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-05-13
Filing date: 1996-03-07
Publication date: 1996-11-20
Anticipated expiration: 2016-03-07
Also published as: GB9509734D0; GB9604851D0; GB2300934B

Abstract

A system for controlling the heat produced by a heater provided on a glass pane 3 of a refrigerator door 1. The system is arranged to cause heat to be generated at a high level when the door 1 is opened so as to remove or prevent condensation forming on the inner surface of the inner glass pane. The system is further arranged to maintain the high level of heat for a predetermined period of time following subsequent closure of the door whereafter the level of heat is gradually reduced to a relatively low level.

Description

REFRIGERATION EOUIPMENT The present invention relates to refrigeration equipment and in particular to fridges or freezers having heaters for de-misting glass doors.

Conventional glass door fridges and freezers suffer from the common problem of condensation forming on the inside surface of the glass door when the door is opened, causing the glass to mist up. When the door is subsequently closed, the condensation formed prevents customers from viewing goods contained in the fridge or freezer and also results in ice or frost forming on the interior surface of the glass. This can be a major problem in a busy store where customers are regularly opening and closing fridge and freezer doors.

In order to overcome this problem, it is known to provide heat to the glass such that the temperature of the glass is raised above the temperature at which condensation will form. Glass door heaters take the form of heaters which heat the door frame and heaters which heat the glass itself. The latter are provided by thinfilm metal coatings deposited on the glass surface, with low voltage electric power being supplied to the coating.

Generally, glass door heaters are operated continuously, 24 hours a day and seven days week, although in stores it is known to provide timers which disconnect the heaters when the stores are closed. It is also often the case that heater elements are provided around the casing of the refrigeration cabinet in order to prevent condensation forming on cabinet surfaces.

However, known heaters for glass door refrigerators and freezers remain inefficient.

It is an object of the present invention to provide an apparatus and method for preventing the glass door of an item of refrigeration equipment from misting up in a simple and economical manner.

According to a first aspect of the present invention there is provided a system for controlling a heater or heaters used to heat a glass surface of a glass door of an item of refrigeration equipment, the system comprising control-means operable when the door is opened to cause said door heater(s) to provide average heat output power at a HIGH level and operable after subsequent closure of the door to cause said average output power to be gradually reduced.

Preferably, the control means is arranged to maintain the average output power of the door heater(s) at said HIGH level for a predetermined period after closure of the door and to commence said gradual reduction at the end of that period. The predetermined period is chosen to prevent moisture forming on the inner pane following closure of the door due to residual moisture allowed to enter into the refrigerator and may be, for example, between 2 and 10 minutes.

Preferably, the gradual reduction in average output power of the door heater(s) following closure of the door is substantially linear.

Preferably, the control means comprises a switch coupled between the door heater(s) and a power supply, the control means being arranged to provide a substantially square wave switching signal to the switch.

In a preferred embodiment of the invention, the control means is arranged to gradually decrease the duty cycle of the switching signal supplied to the switch after the door is closed. After a predetermined time following the closure of the door, the duty cycle is preferably maintained at a constant value. Preferably, said constant value is less than 0.2 and more preferably approximately 0.1.

In an embodiment of the present invention, the control means comprises a periodic waveform generator arranged in use to generate a triangular waveform of constant period and amplitude. The control means also comprises a ramp generator which is initialised when the door is closed to generate a decreasing or increasing ramp and comparator means for comparing the output of the periodic waveform generator with the output of the ramp generator. The output of the comparator provides the switching signal to the switch. The ramp generator may comprise an integrator in which case the control means can be implemented using no more than four operational amplifiers. Preferably said periodic waveform is a triangular waveform and the ramp is linear such that the duty cycle of the switching signal decreases linearly following closure of the door.

In an alternative embodiment of the invention, the control means is implemented using substantially only digital electronic components.

According to a second aspect of the present invention there is provided a system for controlling a heater or heaters used to heat a glass surface of a glass door of an item of refrigeration equipment, the system comprising control means operable when the door is opened to cause said door heater(s) to provide average heat output power at a HIGH level and operable a predetermined time after subsequent closure of the door to cause said average output power to be reduced.

According to a third aspect of the present invention there is provided a method of controlling a heater or heaters used to heat a glass surface of a glass door of an item of refrigeration equipment, the method comprising causing the door heater(s) to provide average heat output power at a HIGH level when the door is opened and after subsequent closure of the door causing said average output power to be gradually reduced.

Preferably, the method comprises maintaining said average heat output power at said HIGH level for a predetermined period of time and, at the end of that period, gradually reducing said average heat output power.

According to a fourth aspect of the present invention there is provided a method of controlling a heater or heaters used to heat a glass surface of a glass door of an item of refrigeration equipment, the method comprising causing the door heater(s) to provide average heat output power at a HIGH level when the door is opened and to reduce the average heat output power a predetermined time after subsequent closure of the door.

For a better understanding of the present invention and in order to show how the same may be carried into effect reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a system for regulating the provision of heat to a glass door of a refrigerator; Figure 2 shows a cross-sectional view of the door of Figure 1 taken on the line A-A; Figure 3 shows schematically a heater control unit of the system of Figure 1; Figure 4 shows waveforms generated by the system of Figure 3; and Figure 5 shows a detailed circuit diagram for implementing the control unit of Figure 3.

There is shown in Figure 1 a refrigeration cabinet 1 having mounted therein a refrigerator door lb.

Distributed over the surfaces of the cabinet 1 is a heater (not shown) which prevents condensation forming on the cabinets outer casing.

Electric power is supplied to the casing heater at 120V ac directly from the secondary winding 4a of a transformer 4 having its primary winding 4b coupled to a 240 volts mains ac supply so that the casing heater is continuously on, so long as mains power is supplied to the transformer 4.

The refrigerator door lb comprises a metal door frame 2 which supports a three-pane glass window 3.

Mounted within the door frame is a resistive heater element (not shown) whilst the glass window 3 is provided with a glass heater. Typically, the glass heater comprises a thin film metal coating 3a, covering substantially the entire internal surface of the outer pane 3b of the glass window 3 which forms a resistive heating element. Alternatively the glass heater may be in the form of an array of thin wire elements extending across the glass surface. A proportion of the heat generated by the glass heater is transmitted across the central pane 3d and the two intermediate air gaps to cause heating of the inner pane 3b, sufficient to lift moisture from the inner surface of the inner pane 3b.

Both the door frame heater and the glass heater receive power, typically 130W, via a common 'Door Supply' from the secondary winding of the transformer 4, the supply of power being regulated by a heater control unit 6 which is shown in more detail in Figure 2.

The heater control unit 6 comprises at its input a fullwave rectifier 7 which receives 120 volts ac power from the secondary winding 3 of the transformer 4 and converts this into dc power. A stabilised low voltage power supply 8 is coupled across the dc outputs of the fullwave rectifier 7 and generates low voltage power, for example 5V, for powering components of the control unit.

These components include a ramp generator 9, a triangular waveform generator 10, and a comparator 11 which is arranged to compare the outputs of the ramp generator and the triangular waveform generator and to provide a switching signal to a solid state power switch 12. The power switch is coupled between the door heaters 13 and the dc outputs of the fullwave rectifier 7.

When the refrigerator is initially switched on and the door closed, the ramp generator 9 is arranged to provide at its output a OV signal. Simultaneously, the triangular waveform generator 10 begins generating a steady state triangular waveform having a constant period of 10 seconds and a constant amplitude, the level of the signal always being greater than OV. The outputs provided by the triangular waveform generator 10 and the ramp generator 9 are supplied to the non-inverting and inverting inputs respectively of the comparator 11. In the initial power-up state, the output from the comparator is therefore high turning on the power switch and connecting the dc supply to the heaters. The heaters then begin to heat up the glass and the frame.

Following power-up, the ramp generator 9 begins to increase the level of its output at a constant rate.

After a short time, typically 5 minutes, the ramp generator output climbs above the triangular waveform at its lowest level. Thus, for a small fraction of each period of the triangular waveform the output from the comparator goes low and the solid state power switch is turned off. As the ramp signal increases further, the fraction of the period for which the solid state power switch is off increases at a constant rate. Thus, the duty cycle of the solid state power switch, i.e the ratio of the on to the off time, decreases from its initial value of 1.0 and the average heat output of the heaters also decreases accordingly.

After approximately 30 minutes, the output from the ramp generator levels off at a constant value which is selected such that the duty cycle of the solid state power switch 12 is in the range 0.1 to 0.5.

When the refrigerator door is opened following power-up, a door switch 14 is moved from its normally closed state to an open state. This causes the output 15 of the ramp generator 9 to be reset to OV, as illustrated in Figure 3, resulting in the output 16 of the comparator 11 going high and the power switch being turned on. The ramp generator output 15 is maintained at OV until the door is closed, closing the door switch 14, whereupon the output 15 begins to ramp upwardly until it impinges upon the triangular waveform 17 in exactly the same manner as occurs on initial power-up.This sequence of events has the result that the heaters are maintained in the on state whilst the door is open, and following closure are maintained initially in a continuously on state and then in the on state for a continuously decreasing fraction of the time, until the steady state situation is reached.

Of course, if the door is reopened before the steady state situation is reached, the unit resets and runs through the cycle again.

It is an important feature of this embodiment of the invention that the door heaters are maintained in the fully on state for a period after closure of the door.

This ensures that moisture does not form on the inner pane 3c after closure due to any moist air which has been allowed to enter the refrigerator but which has not yet been removed by the cooling action within the refrigerator.

The ramped, gradual, reduction in the heat generated ensures that, sufficient heat is supplied to the inner pane to remove the relatively small amounts of moisture which may otherwise form on the inner pane after prolonged closure of the door whilst still giving a significant energy saving compared to controllers which maintain the door heaters in a continuously on state.

Figure 4 shows in detail a circuit for implementing the heater control unit of Figure 2. The solid state power switch is provided by a MOSFET switch having a 0.1F capacitor coupled between its gate and source terminals. The purpose of the capacitor is to increase the switching time of the MOSFET in order to reduce the amount of electromagnetic interference (EMI) generated by the circuit during switching (a relatively slow switching time is not significant in this application given the relatively long period for which the switch is maintained in the on or off state).

The triangular waveform generator 10 is provided by a standard circuit which comprises a first operational amplifier ICla forming part of an integrator and a second operational amplifier IClb forming part of a comparator.

A bipolar junction transistor is provided at the output of the comparator and inverts the output thereof so that it is of the correct polarity for feedback to the input of the integrator.

The ramp generator 9 is provided by a long time constant integrator which comprises a third operational amplifier IClc. When the door is opened, the switch 14, which may be a magnetically operated reed switch, is also opened coupling the 120V DC line of the fullwave rectifier circuit through a set of five 1OMn resistors and a 68Kn 'door switch' resistor to the inverting input of operational amplifier IClc. The output of the integrator falls rapidly to zero (over a period of about 1 second) and remains there until the door and the switch 14 are re-closed whereupon the inverting input IClc is coupled through the five 1OMn resistors to the oV line.

As the non-inverting input of IClc is held at approximately 50mV by associated bias circuitry, the output of the integrator begins to rise.

The comparator 11 is provided by a fourth operational amplifier ICld having its inverting input coupled to the output of the integrator and having its non-inverting input coupled to the output of ICla.

It is noted that, if the inputs to the comparator are reversed, a ramp generator generating a decreasing ramp could be used.

The circuit arrangement of Figure 4 is particularly advantageous in that it uses only four operational amplifiers which can be provided by a single quad IC package and is therefore extremely economical.

It will be appreciated that various modifications may be made to the above described embodiment without departing from the scope of the present invention. For example, the triangular waveform generator could be replaced by a sine wave generator or a generator producing any suitable periodic waveform. Similarly, instead of generating a linear ramp, the ramp generator could be designed to produce an exponentially varying ramp.

In order to prevent the control unit from restoring full power to the heaters in the event that the door is opened only for a very brief period, the door switch 14 may incorporate a delay means so that the switch is only closed if the door remains open for a predetermined period of time, e.g. 3 seconds.

The system of Figure 2 may be implemented using digital electronic components. Typically a microprocessor would be arranged to continuously reduce the duty cycle of the switching signal supplied to the solid state switch following closure of the refrigerator door.

Whilst the embodiment described above comprises a glass door having three panes of glass, it will be appreciated that the invention is applicable to doors having any number of panes. It will also be appreciated that when the door comprises two or more panes, the glass heater may be provided on any one of the glass panes.

With conventional refrigeration cabinets, the door frame and glass heaters are normally fed by a common power supply. Thus, the embodiment described above may be easily retrofitted to such cabinets. However, where the door frame and glass heaters may be powered separately, it may sometimes be advantageous to maintain a permanent supply of power to the door frame heater whilst varying the level of power supplied to the glass heater in the manner described above.

Claims

1. A system for controlling a heater or heaters used to heat a glass surface of a glass door of an item of refrigeration equipment, the system comprising control means operable when the door is opened to cause said door heater(s) to provide average heat output power at a HIGH level and operable after subsequent closure of the door to cause said average output power to be gradually reduced.

2. A system according to claim 1, wherein the control means is arranged to maintain the average output power of the door heater(s) at said HIGH level for a predetermined period after closure of the door and to commence said gradual reduction at the end of that period.

3. A system according to claim 1 or 2, wherein the control means comprises a gated switch coupled between the door heater(s) and a power supply, the control means being arranged to provide a substantially square wave switching signal to the switch to selectably connect and disconnect the power supply to the door heater(s).

4. A system according to claim 3, wherein the control means is arranged to gradually decrease the duty cycle of the switching signal supplied to the switch subsequent to the door being closed.

5. A system according to claim 4, wherein the control means is arranged to maintain the duty cycle at a constant value after a predetermined time period has elapsed following closure of the door.

6. A system according to claim 5, wherein said constant value is less than 0.2 and more preferably approximately 0.1.

7. A system according to any one of the claims 3 to 6, wherein the control means comprises a periodic waveform generator arranged in use to generate a triangular waveform of constant period and amplitude, a ramp generator which is initialised when the door is closed to generate a decreasing or increasing ramp, and comparator means for comparing the output of the periodic waveform generator with the output of the ramp generator, the output of the comparator being arranged to provide said switching signal to the switch.

8. A system according to claim 7, wherein the ramp generator comprises an integrator.

9. A system for controlling a heater or heaters used to heat a glass surface of a glass door of an item of refrigeration equipment, the system comprising control means operable when the door is opened to cause said door heater(s) to provide average heat output power at a HIGH level and operable a predetermined time after subsequent closure of the door to cause said average output power to be reduced.

10. A method of controlling a heater or heaters used to heat a glass surface of a glass door of an item of refrigeration equipment, the method comprising causing the door(s) heater to provide average heat output power at a HIGH level when the door is opened and after subsequent closure of the door causing said average output power to be gradually reduced.

11. A method of controlling a heater or heaters used to heat a glass surface of a glass door of an item of refrigeration equipment, the method comprising causing the door heater(s) to provide average heat output power at a HIGH level when the door is opened and to reduce the average heat output power a predetermined time after subsequent closure of the door.

12. A system for controlling a heater substantially as hereinbefore described with reference to the accompanying drawings.

13. A method of controlling a heater substantially as hereinbefore described with reference to the accompanying drawings.