REFRIGERATION PLANT AND METHODS OF CONTROL THEREFOR

Abstract

A refrigeration plant has a refrigerator circuit comprising a compressor, a condensing coil, an expansion valve and an evaporator. Heat is absorbed in the evaporator and exhausted at the condenser. Pressure is measured at the compressor inlet and a blower arrangement for blowing cool air over a heat transfer surface of the condenser is controlled in dependence upon the pressure measurement.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patent application Ser. No. 13/503,464, filed Apr. 23, 2012, which claims benefit to International Application No. PCT/GB10/51782, filed Oct. 22, 2010 which claims the benefit of Great Britain Application No. GB 0901635.4, filed Oct. 23, 2009, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a refrigeration plant and methods of control therefor.

SUMMARY OF THE INVENTION

A conventional refrigeration plant has a refrigerant circuit comprising compressor, a condensing coil, an expansion valve and an evaporator, the latter two items typically located in a chiller or freezer cabinet. Heat is absorbed in the evaporator and exhausted at the condenser.

The refrigeration capacity demand in a typical installation may vary significantly. In larger installations, with refrigeration capacities typically of tens of kilowatts up to hundreds of kilowatts, for example as found in supermarkets and warehouses, there may be multiple compressors (say 3 to 10) connected essentially in parallel and one or more of the compressors may be selectively powered depending on load to deal with fluctuations in compressor demand. However, multiple compressor installations are expensive, complicated to control and bulky and thus impractical for smaller installations, such as small retail premises which may only have a few chiller cabinets and hitherto smaller refrigeration installations have typically been set up for an average demand and generally operate inefficiently when demand is either side of expected demand, or ambient conditions alter from the maximum anticipated design point.

It is known to provide one or more fans to increase airflow over the condenser and/or evaporator. In some arrangements, it is known to control a fan associated with the condenser so that it only operates above a certain temperature, or above a certain pressure in the high-pressure side of the circuit associated with the condenser so that the fan only operates above a certain temperature or pressure to reduce fan noise and protect the system from insufficient discharge pressure to enable correct expansion valve operation.

Embodiments of the invention are particular concerned with medium size refrigeration installations, typically of capacities from about 1 kW to 20 kW, more typically of 2.5 kW to about 10 kW. Such capacities are typical of hermetically sealed compressors without variable capacity drives or mechanical capacity variation.

According to a first aspect, the invention provides a refrigeration plant comprising a compressor coupled to receive low pressure refrigerant from a low pressure circuit at a compressor inlet, to compress it to provide high pressure refrigerant, a condenser coupled to receive the high pressure refrigerant and having at least one heat transfer surface to discharge heat from the condenser to provide cooled high pressure refrigerant to an expansion valve and a blower arrangement for blowing cooling air over said at least one heat transfer surface characterised by a blower controller arranged to control the blower in dependence on the pressure in the low pressure circuit.

According to an embodiment, the invention provides a refrigeration plant comprising a compressor coupled to receive low pressure refrigerant gas from a low pressure circuit at a compressor inlet, to compress it to provide high pressure refrigerant gas, a condenser coupled to receive the high pressure refrigerant and having at least one heat transfer surface to discharge heat from the condenser to provide cooled high pressure liquid refrigerant to an expansion valve and a blower arrangement for blowing cooling air over said at least one heat transfer surface characterised by a blower controller arranged to control the blower in dependence on the pressure in the low pressure circuit.

According to the invention, it has been appreciated that by controlling the blower/fan in accordance with the low pressure circuit pressure, a surprising improvement in efficiency can be obtained in a typical installation with variable demand and beneficial ambient variance as compared to the conventional arrangement which simply switches a fan on at a given pressure or temperature. In particular, the output capacity of a typical plant can be significantly increased under normal operation which means that the compressor will typically run at a lower duty cycle, reducing energy consumption.

Preferably the blower controller is coupled to a sensor arranged to provide a variable output indicative of pressure in the low pressure circuit and the blower controller comprises a processor arranged to control the fan in dependence on an algorithm taking the pressure as an input.

Typically the blower controller is operable to determine a discharge pressure in dependence upon the fluid pressure at the compressor inlet and to control the blower based on the determined discharge pressure. Preferably the blower controller is operable to control the blower based on a difference between the determined discharge pressure and the measured discharge pressure. In one possibility the blower controller is operable to control the blower in dependence upon a measured ambient temperature. In one possibility there is provided a refrigeration plant having a fixed vapour displacement compressor.

In an embodiment a refrigeration plant has a fixed vapour displacement compressor.

In an aspect there is provided a method of controlling a refrigeration plant comprising a compressor, and a blower for cooling a condenser, the method comprising measuring a pressure at a compressor inlet and controlling the blower in dependence upon the measured pressure.

Typically the fan and/or the blower is controlled in dependence on an algorithm which takes the compressor inlet pressure as an input. Optionally methods according to the invention comprise determining a discharge pressure in dependence upon the fluid pressure at the compressor inlet and controlling the blower based on the determined discharge pressure. Preferably such methods comprise receiving a measurement of the discharge pressure at the compressor outlet and controlling the blower based on a difference between the determined discharge pressure and the measurement of the discharge pressure.

In one possibility a method according to the invention received a measurement of ambient temperature and controlling the blower in dependence upon the measurement of ambient temperature.

In an embodiment, a controller receives a pressure to voltage measurement corresponding to the pressure temperature relationship of the refrigerant in its vapour form from a transducer. The compressor switches on/off due to the measured suction pressure, the suction pressure falls due to refrigerant starvation caused by the evaporator controller closing a mechanical solenoid valve on satisfaction of a predetermined control value in relation to the desired space temperature and preventing the liquid flow to the expansion valve. The compressor removes the remaining vapour within the evaporator lowering the pressure and then on reaching a set LP point switches off. The unit is activated again on a rise of suction pressure caused by the evaporator controller opening the solenoid valve and allowing liquid refrigerant to flow into the evaporator space and consequently raising the pressure to the required value where compression is required. As the compressor switches on the blower is activated, initially at a minimum preset value and then in adjustment to meet the desired HP value read by a second pressure-voltage transducer in relation to the measured LP value of. The Blower operates in a control band either side of the desired discharge point according to the algorithm adjusting automatically to achieve the required hp value. The control parameters are pre-set within a computer program/algorithm which constantly receives and interprets the variable input voltage signals from the two transducers and provides output signals to the compressor (fixed) and blower (variable). The blower output provides for a scaled voltage output between minimum acceptable blower speed and maximum blower speed, the blower will speed up/slow down to maintain the minimum condensing temperature (HP) associated with the measures LP value.

Examples of the invention provide refrigerator units, freezer units, chest freezers, storage containers and warehouses comprising a refrigeration plant operated in accordance with examples of the invention.

The invention also provides a computer program and a computer program product for carrying out any of the methods described herein and/or for embodying any of the apparatus features described herein, and a computer readable medium having stored thereon a program for carrying out any of the methods described herein and/or for embodying any of the apparatus features described herein.

The invention also provides a signal embodying a computer program for carrying out any of the methods described herein and/or for embodying any of the apparatus features described herein, a method of transmitting such a signal, and a computer product having an operating system which supports a computer program for carrying out any of the methods described herein and/or for embodying any of the apparatus features described herein.

The invention extends to methods and/or apparatus substantially as herein described with reference to the accompanying drawings.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa.

Furthermore, features implemented in hardware may generally be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a very schematic representation of a refrigeration unit and control system according to an example of the invention; and

FIG. 2 shows a plot of temperature in the high pressure and low pressure sides of a refrigeration unit according to the invention.

The example of FIG. 1 shows a refrigeration unit and control system having a refrigeration circuit 22 comprising compressor 10 coupled to compress refrigerant fluid (typically a vapour) into condenser 14 in which the fluid is condensed to a liquid to release heat before being forced through expansion valve 20 and pumped through evaporator 18 where the fluid absorbs heat and is vapourised before being passed back through the compressor 10. Fluid flow to the expansion valve 20 is controlled by a solenoid valve 8 coupled between the condenser 14 and the expansion valve 20.

Controller 16 is coupled to control cooling fan or blower 12 which forces air over condenser 14 to cool it. Controller 16 is coupled to condenser 14 and evaporator 18 to monitor the temperature and/or the pressure of these parts of the refrigeration unit. In other words, controller 16 is coupled to pressure sensors 16a and 16b which are positioned to measure pressure at the compressor inlet (suction point) and the compressor output (discharge point) respectively. The condenser pressure (HP hereinafter) and the evaporator pressure (LP hereinafter) are compared by controller 16 and used to derive an optimum running condition based on the compressors operational envelope and the optimum condensing pressure at the measured suction point (compressor input). The fan speed is controlled dependent on these parameters to achieve the desired running condition. Evaporator sensor 6 comprises a temperature sensor such as a thermocouple, coupled to the evaporator 18 side of the system (for example in the refrigerated space or chiller cabinet) and is operable to control the solenoid valve 8 based on the measured space temperature.

Transducer 16a provides controller 16 with a voltage based on the pressure temperature relationship, for example based on the pressure and/or temperature, of the refrigerant in its vapour form. In operation when the evaporator controller 6 senses that a desired space (or cabinet) temperature has been achieved it controls solenoid valve 8 to be closed. This prevents the flow of liquid to the expansion valve. The compressor is arranged to switch on and off in response to the measured suction pressure i.e. the pressure at the compressor inlet. When the suction pressure exceeds a selected value the compressor is switched on to remove refrigerant from the evaporator, thereby lowering the pressure. When a selected low pressure value is reached, the compressor is switched off.

When the evaporator controller 6 senses that a threshold space (or cabinet) temperature has been exceeded it controls solenoid valve 8 to be opened. This permits the flow of liquid to the expansion valve and into the evaporator. When the evaporator controller 6 senses that a desired space (or cabinet) temperature has been achieved it controls solenoid valve 8 to be closed and the cycle repeats.

As the compressor switches on the blower 12 is activated, initially at a minimum preset speed. Then the blower speed is adjusted based on the difference between pressure measurements made by transducer 16a and transducer 16b. Thus the blower 16 operates to control pressure at the discharge point (compressor output) to maintain the HP value within an acceptable range. Controller 16 controls the blower speed and switches the compressor on and off based on pre-set control parameters and voltage signals from the two transducers, 16a and 16b. Controller 16 converts the input voltage signals from the two transducers to provide output signals to control the compressor (fixed speed) and the blower (variable speed). The blower is controlled using a scaled voltage output which varies the speed between a minimum acceptable blower speed and a maximum blower speed. Thus the blower is controlled to speed up or slow down to maintain the minimum condensing temperature (HP) associated with the measured LP value.

In an alternative example controller 16 receives a measurement of the compressor inlet pressure from sensor 16a. Controller 16 compares the measured compressor inlet pressure with a selected switch-on pressure. When the measured pressure exceeds this switch on pressure the fan controller 16 provides a control signal to switch on the blower 12 to operate at a first fan speed. Whilst the measured compressor inlet pressure remains above this selected threshold level the fan speed is controlled in proportion to the measured pressure. A constant of proportionality for this control is selected according to the features of the particular system such as the size and shape of the condenser and the operating power of the compressor.

Typically, in examples of the invention the suction point (compressor input) varies depending on refrigeration demand.

In general either the compressor runs or it does not, it has no capacity for variable speed operation and a pressure switch will turn the compressor on when the pressure in the low pressure section rises above a turn on threshold and the compressor will run until the pressure falls below a turn off threshold, typically slightly lower than the turn on threshold to give hysteresis so that the compressor does not repeatedly cycle on and off excessively. Controller 16 monitors the suction pressure using sensor 16a and controls the blower 12 to cool the condenser. This helps achieve a more optimum (minimum) corresponding discharge pressure. In some examples the optimum discharge pressure is calculated taking into account the ambient temperature but this is not required.

In a low noise mode of operation, influence of readings from the low pressure parts of the system (e.g. the evaporator) may be downgraded and the operation of blower 12 is reduced, controlled based on the high pressure readings, to maintain high pressure above a minimum. The HP reading is used to regulate the fan/blower to maintain the low noise high pressure condition. Generally this is selected as the minimum condensing point at the potential maximum evaporating pressure setting. For efficiency reasons the ‘average’ condition is generally the highest value of the compressor's minimum condensing temperature, for example 30° C. To further reduce fan noise at the cost of increasing the power consumption this value could be increased. In other words, for correct and efficient operation of a pre-configured packaged unit capable of operating at different LP conditions this condition is generally pre-configured to the highest value of the compressor's minimum condensing temperature, for example 30° C.

The minimum condensing temperature depends on the compressor suction. Typically, for a lower condensing temperature the compressor may operate with a lower overall pressure differential to improve coefficient of performance (COP—ratio of cooling power to input power). Therefore the required compressor input power is reduced. The standard setting which regulates the system to maintain a desired high pressure value cannot achieve this beneficial increase in COP.

Advantageously in examples of the invention the low pressure and high pressure are measured which allows operation closer to the known optimum minimum condensing temperature. In one example only the low pressure value is measured.

In the plot shown in FIG. 2 the function of an example of the invention is illustrated by a plot of high pressure values against low pressure values. These values correspond to temperatures in the condenser and evaporator respectively. Typically in ‘economic’ setting the condenser pressure (HP setting) is adjusted in relation to the suction pressure reading (i.e. the pressure measured at the suction point). This provides an adaptable refrigeration plant having improved adjustability and efficiency.

As shown in FIG. 2 the values change most markedly at two points. Data corresponding to these two points (point 1 and point 2) are set out in Table 1. The HP max and HP min settings can be related to (chosen dependent upon) a differential value above and below the required HP average value. These are stated as the target pressures for point 1 and point 2.

As shown in FIG. 2, which relates to a 2 bar pressure differential, the fan speed is regulated within the constraints of the ambient temperature, to maintain the minimum condensing temperature so that the unit can operate in its most efficient mode.

Claims

1. A refrigeration plant comprising a compressor coupled to receive low pressure refrigerant from a low pressure circuit at a compressor inlet, to compress it to provide high pressure refrigerant, a condenser coupled to receive the high pressure refrigerant and having at least one heat transfer surface to discharge heat from the condenser to provide cooled high pressure refrigerant to an expansion valve and a blower arrangement for blowing cooling air over said at least one heat transfer surface characterised by a blower controller arranged to control the blower in dependence on the pressure in the low pressure circuit.

2. A refrigeration plant according to claim 1 wherein the blower controller is coupled to a sensor arranged to provide a variable output indicative of pressure in the low pressure circuit.

3. A refrigeration plant according to claim 2 wherein the blower controller comprises a processor arranged to control the fan in dependence on an algorithm taking the pressure as an input.

4. A refrigeration plant according to claim 3 in which the controller is operable to determine a discharge pressure in dependence upon the fluid pressure at the compressor inlet and to control the blower based on the determined discharge pressure.

5. A refrigeration plant according to claim 4 wherein the blower controller is operable to control the blower based on a difference between the determined discharge pressure and the measured discharge pressure.

6. A refrigeration plant according to claim 1, wherein the blower controller is operable to control the blower in dependence upon a measured ambient temperature.

7. A refrigeration plant according to claim 1, the plant having a cooling capacity of between 1 kw and 20 kw.

8. A refrigeration plant according to claim 1, having a single compressor.

9. A refrigeration plant according to claim 1, having a fixed speed compressor.

10. A method of controlling a refrigeration plant comprising a compressor, and a blower for cooling a condenser, the method comprising measuring a pressure at a compressor inlet and controlling the blower in dependence upon the measured pressure.

11. A method according to claim 10 further comprising controlling the fan in dependence on an algorithm which takes the compressor inlet pressure as an input.

12. A method according to claim 11 further comprising determining a discharge pressure in dependence upon the fluid pressure at the compressor inlet and controlling the blower based on the determined discharge pressure.

13. A method according to claim 10 comprising receiving a measurement of the discharge pressure at the compressor outlet and controlling the blower based on a difference between the determined discharge pressure and the measurement of the discharge pressure.

14. A method according to claim 10, comprising receiving a measurement of ambient temperature and controlling the blower in dependence upon the measurement of ambient temperature.

15. A non-transistory computer program product comprising instructions operable to program a processor to measure a pressure at a compressor inlet and control the blower in dependence upon the measured pressure.

16. A refrigerator comprising the refrigeration plant of claim 1.