- BACKGROUND ART
This invention relates to a method and apparatus for thermostat control of a refrigeration system. In particular, the invention is directed to an improved thermostat control method and apparatus in which the refrigeration mechanism is disabled in the event of abnormal temperatures in components of the refrigeration mechanism.
Refrigeration mechanisms, such as those used to refrigerate display cabinets, usually have a thermostat which maintains the temperature of the cabinet chamber at or near a desired temperature. The thermostat normally operates with a fixed hysteresis to avoid overly frequent switching of the refrigeration mechanism. That is, for a desired set temperature (Tset), the thermostat will activate the refrigeration mechanism when the temperature rises above a temperature (Tmax) which is slightly above Tset. The refrigeration mechanism will continue to operate until the temperature of the refrigerated space falls below a temperature (Tmin) which is slightly below Tset. The thermostat will not activate the refrigeration mechanism again until the temperature of the refrigerated space rises above Tmax.
In most refrigerated cabinets and similar refrigeration systems, the thermostat controls the refrigeration mechanism in response to one only of the following: the temperature of the refrigerated space, the temperature of the refrigerated product or the evaporator temperature.
- SUMMARY OF THE INVENTION
It is an object of this invention to provide an improved thermostat controller which is responsive to one or more conditions in addition to the temperature of the space or product being cooled.
In one broad form, the invention provides a refrigeration system having:
a refrigeration mechanism for cooling an object, the refrigeration mechanism including a compressor, condensor and evaporator in a refrigeration circuit;
thermostat means for regulating the operation of the refrigeration mechanism in response to the temperature of the object;
wherein the refrigeration system further includes control means for controlling the refrigeration mechanism in response to the sensed temperature of at least one component of the refrigeration mechanism, the control means overriding the normal operation of the thermostat means.
The term “object” is used in a broad sense, and includes a space such as a refrigerator chamber, or one or more items or products within that chamber, or a container, or a liquid within the container.
Preferably, the control means controls the operation of the refrigeration mechanism in response to the temperature of the evaporator. In one embodiment, the control means stops or otherwise disables the operation of the refrigeration mechanism if the temperature of the evaporator drops below a predetermined temperature (Tdisable). Further, the control means controls the refrigeration mechanism to ensure that it does not commence unless the temperature of the evaporator is above a (higher) pre-determined temperature (Tstart).
The control means will therefore override the thermostat means and disable the refrigeration mechanism if the evaporator temperature falls below Tdisable. This may be caused by the evaporator icing up or, in the case of a forced draught system, failure of the evaporator fan(s). Further, the control means overrides the normal operation of the thermostat means to ensure that the refrigeration mechanism will only start if the evaporator is completely defrosted, i.e. the evaporator temperature is above Tstart. The control means therefore ensures that the refrigeration mechanism operates only when the evaporator is able to operate effectively.
The refrigeration system suitably includes a temperature probe connected to the control means and adapted to sense the temperature of the evaporator.
Further, or in the alternative, the control means controls the refrigeration mechanism in response to the temperature of the condensor. Typically, if the condenser temperature rises above a predetermined value (Talarm), an alarm is triggered. The alarm may suitably be an audio and/or visual alarm. This alarm may be reset by switching the power off for a predetermined period, then switching it back on. However, the alarm will reactivate if the condenser temperature remains above Talarm.
If the condensor temperature rises above a (higher) predetermined valued (Tshutdown), the control means will stop or otherwise disable the refrigeration mechanism, until re-set in the same manner as the alarm.
By controlling the refrigeration mechanism in response to the condenser temperature, the control means ensures safe operation by alerting the operator to high condenser temperature and/or shutting down the refrigeration mechanism in the event of sustained high condensor temperature.
The refrigeration system may suitably include a temperature probe connected to the control means for sensing the temperature of the condensor, as well as an alarm circuit.
The control means may suitably be in the form of an electronic circuit which also incorporates the thermostat means. The electronic circuit may include a programmed micro-processor or any other suitable electrical control circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings.
FIG. 1 is a block diagram of the refrigeration system of the preferred embodiment;
DESCRIPTION OF PREFERRED EMBODIMENT
FIG. 2 contains temperature and operational charts illustrating the operation of the refrigeration system in various conditions.
As shown in FIG. 1, a refrigeration system 10 includes a conventional refrigeration mechanism 11 which comprises a compressor 12, condenser 13 and evaporator 14 connected in a refrigeration circuit. The evaporator 14 is typically used to cool a space 15 such as the product cabinet or chamber of a display refrigerator. However, the evaporator 14 may also be used to cool a product directly or indirectly.
The refrigeration system 10 also includes a thermostat controller 16 which incorporates the functions of a conventional thermostat and the control means of this invention. A temperature sensor or probe 17 located in the refrigerated space 15 is connected to an input of the thermostat controller 16. In its normal thermostat function, the thermostat controller switches the compressor 12 on and off so that the evaporator 14 maintains the refrigerated space 15 close to a pre-determined temperature (Tset) set by a temperature adjustment potentiometer 18, with typical hysteresis control. That is, when the temperature of space 15 as sensed by probe 17 rises above Tmax (slightly above Tset), the compressor 12 is switched on so that the evaporator 12 cools the space 15. When the temperature of the space 15 falls below Tmin (slightly below Tset), the compressor 12 is switched off. The abovedescribed temperature control procedure is well known, and need not be described in detail.
According to the preferred embodiment of this invention, the refrigeration system 10 further includes a temperature sensor or probe 19 connected to an input of the thermostat controller 16. The sensor 19 senses the evaporator temperature, and the thermostat controller controls the operation of the compressor 12 in response to that temperature overriding the conventional hysteresis thermostat operation described above.
More specifically, the temperature controller 16 will only activate the compressor 12 if the temperature of the evaporator is above a predetermined temperature, Tstart. At this temperature, the evaporator is completely defrosted. Hence, the thermostat controller will not start the compressor unless the evaporator is defrosted. If during operation, the temperature of the evaporator 14 falls below a (lower) pre-determined temperature Tdisable, the thermostat controller overrides the normal thermostat hysteresis operation, and stops or disables the compressor 12.
The temperature Tdisable is selected as being a temperature which would be reached if, for example, the evaporator begins to ice up or, in the case of a forced draught system, there is a failure of the evaporator fan(s) thereby reducing the evaporators capacity to cool. Hence, the thermostat controller ensures that the refrigeration mechanism is shut off if the evaporator falls to a temperature which is too low to function effectively. Moreover, the thermostat controller 16 will not allow the compressor 12 to be restarted by the normal hysteresis thermostat unless the temperature of the evaporator has risen above Tstart.
The refrigeration system also includes a second temperature sensor or probe 20 connected to an input of the thermostat controller 16. The temperature sensor 20 senses the temperature of the condenser 13, and the controller 16 actuates an alarm 21 and/or controls the operation of the compressor 12 in response to that temperature (overriding the normal hysteresis thermostat operation).
If the condenser temperature, as sensed by sensor 20, rises above a pre-determined value Talarm, the controller 16 will activate an alarm 21. This is typically an audible alarm, such as a buzzer, but may be a visual alarm, such as a warning light. The alarm may be re-set by switching off the power to the refrigeration system for a pre-determined period of time, say 30 seconds, and then switching the power back on. However, if the condensor temperature is still above Talarm the alarm 21 will remain activated. The temperature Talarm is selected to indicate some overheating of the compressor.
If the condensor temperature 13 rises above a higher temperature Tshutdown, the thermostat controller will override the normal thermostat hysteresis control of the compressor 12 and shut down or otherwise disable the compressor. The temperature Tshutdown is selected to be the maximum allowable operating temperature of the compressor. The refrigeration mechanism will remain shut down until re-set by switching the power on and off as for the alarm re-set. However, if the temperature is still above Tshutdown, the refrigeration mechanism will remain disabled. This safety mechanism ensures that the refrigeration system is not operated if the condenser temperature is unduly high.
An example of the operation of the refrigeration system is illustrated in FIG. 2. At start up, if the temperature in the chamber 15 is above Tmax, and providing that the evaporator temperature is above Tstart, the compressor will be switched on by the controller 16. Once the chamber temperature has dropped below Tmin (at T1), the compressor will be switched off in accordance with the normal hysteresis control procedure.
At T2, when the chamber temperature has risen above Tmax, and again providing that the evaporator temperature is above Tstart, the compressor will be switched on. However, if the evaporator temperature drops below Tdisable (at T3), the compressor will be switched off even though the chamber temperature has not yet reached Tmin. The compressor will not start again until the evaporator temperature rises above Tstart (at T4).
If the condenser temperature rises above Talarm (at T5), the alarm 21 will switch on, and remain on until the condensor temperature drops below Talarm (at T7) and is reset (at T7) as described above. Further, if the condenser temperature rises above Tshutdown (T6), the thermostat controller will override the normal hysteresis control, and shut down the compressor leaving the alarm on, until reset (T7) as described above, typically after the temperature has dropped below Talarm. (In the illustrated example, although the condenser temperature no longer overrides the normal hysteresis control at T7, the compressor does not switch on until the evaporator temperature rises above Tstart (at T8)). If the system is reset after the condensor temperature has dropped below Tshutdown but above Talarm, the compressor will normally start again but the alarm will sound.
The foregoing describes only one embodiment of the invention, and modifications which are obvious to those skilled in the art may be made thereto without departing from the scope of the invention.
For example, the refrigeration system may include a display panel to display the temperatures of the evaporator and/or condenser, or indicate when the temperatures of the evaporator and/or condenser are outside normal operating parameters.