EP2321595A1

EP2321595A1 - Methods and systems for compressor operation

Info

Publication number: EP2321595A1
Application number: EP08788911A
Authority: EP
Inventors: Fabienne Peyaud; Jean-Philippe Goux; David Veillon; Frederic Brisset
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2008-07-23
Filing date: 2008-07-23
Publication date: 2011-05-18
Anticipated expiration: 2028-07-23
Also published as: HK1159234A1; CN102105759A; CN102105759B; US20110113797A1; WO2010010414A1; EP2321595B1

Abstract

There is provided a refrigeration system (10) comprising a compressor (12) having a suction (11) and a discharge (13), a heat rejecting heat exchanger. (14), an expansion valve (16), and a heat accepting heat exchanger (18). Preferably the system (10) comprises any one or more of: a pressure equalisation valve (4O3 42) for equalising the pressure differential between the compressor suction (11) and compressor discharge (13); a liquid valve (44), preferably a liquid solenoid valve or an electronic expansion valve, the liquid. valve (44) arranged in a flow line (24) between the heat rejecting heat exchanger (14) and the expansion valve (16); and a check valve (46), preferably a solenoid valve or an electronic expansion valve, arranged in a flow line (22) between the heat rejecting heat exchanger (14) and the compressor (12). The valves (40, 42, 44, 46) are operated in a variety of manners upon compressor shutdown and startup to avoid damage to the components of the compressor (12). Preferably the system further comprises means for heating at least one component of the compressor (12) and preferably also control means for activating the heating means when appropriate, such as when compressor startup is required, and starting the compressor after heating.

Description

METHODS AND SYSTEMS FOR COMPRESSOR OPERATION

The present invention relates to methods and systems for compressor operation before and during compressor startup and/or shutdown and in particular to methods and systems for reliable startup of a compressor, even at low ambient temperatures.

Conventional refrigeration or airconditioning systems typically comprise a compressor, a heat rejecting heat exchanger or condenser, an expansion valve or device and a heat accepting heat exchanger or evaporator. In operation, refrigerant is circulated through these components in a closed circuit. The pressure and temperature of the refrigerant vapour is increased by the compressor before entering the heat rejecting heat exchanger where it is cooled. The high pressure, lower temperature liquid is then expanded to a lower pressure by means of the expansion valve. In the heat accepting heat exchanger, the refrigerant boils and absorbs heat from its surroundings. The vapour at the heat accepting heat exchanger outlet is drawn into the compressor, completing the cycle.

However on compressor shutdown and restart the components of the system, particularly the compressor, can be damaged or fail particularly in low ambient temperatures.

It is therefore an object of the present invention to provide a refrigeration system and a method of operating a refrigeration system, particularly although not exclusively for a transport refrigeration unit, the refrigeration system comprising a compressor and being operable such that failures of the system due to or on compressor start up are minimised. particularly when the compressor is started at low ambient temperatures.

In accordance with the present invention, from a first broad aspect, there is provided a refrigeration system comprising a compressor, a heat rejecting heat exchanger, an expansion valve and a heat accepting heat exchanger. The system further comprises a pressure equalisation valve (PEV) for equalising the pressure differential between the compressor suction and compressor discharge. Preferably the pressure equalisation valve comprises a bypass passage connecting the compressor suction to the compressor discharge and bypassing the compressor. Preferably the system further comprises a liquid valve (LV), preferably a liquid solenoid valve (LSV), the liquid valve preferably arranged in a flow line between the heat rejecting heat exchanger and the expansion valve.

Therefore there is provided a refrigeration system comprising a main refrigerant flow path from the compressor to the heat rejecting heat exchanger, from the heat rejecting heat exchanger to the liquid valve, from the liquid valve to the expansion valve and from the expansion valve to the heat accepting heat exchanger. A bypass refrigerant path or other pressure equalisation valve is provided across the compressor, i.e. from the compressor suction to the compressor discharge that, when opened, bypasses refrigerant flow around the compressor and enables the pressure differential between the compressor suction and the compressor discharge to be minimised, or more preferably the suction and discharge pressures to be balanced, particularly on compressor shutdown and most preferably at or shortly before compressor startup. Preferably the refrigeration system is operable in at least one of a plurality of predetermined sequences, and the particular sequence (or sequences) used is preferably determined based on at least one parameter of the refrigeration system. Preferably the parameter (or parameters) comprises a system parameter measured by at least one sensor. In preferred embodiments, when it is determined that compressor startup is required, a preheating sequence is initiated in which one or more components of the compressor is heated. Preferably the compressor body or shell, and/or the oil in the compressor, and/or the compressor motor, and/or any other suitable component, is heated. Preferably the component(s) is heated for a predetermined period of time, which may be determined based on one or more system parameters. In a preferred embodiment, the period for which the component is heated is determined based on the temperature of

, the oil in the compressor, and/or the compressor shell temperature, and/or the compressor discharge temperature, and/or the temperature of the environment in which the compressor is located (i.e. the ambient temperature), etc. The period for which the component is heated is additionally or alternatively deterrnined based on the length of time for which the compressor has been shut down (for example if the compressor was only recently shut down, it may only be necessary to heat the component for a relatively short time, as the component may have retained some of the heat from its normal operating conditions and temperature).

Heating of the component(s) of the compressor is carried out in any suitable manner. In a particularly preferred embodiment, the stator windings of a motor associated with the compressor, for example the internal electric alternating current motor (synchronous or asynchronous) of the compressor are electrically connected to an electrical source, e.g. a direct current source, to thereby heat the windings and thus heat the compressor.

The pressure equalisation valve can be opened prior to compressor start up, but in preferred embodiments the pressure equalisation valve is opened when the compressor is started (preferably at substantially the same time as the compressor is started). For example in the embodimenrwhete the pressure equalisation valve is a bypass passage, the passage is opened as the compressor is started to allow pressure balancing between the compressor suction and discharge by bypassing the compressor. Preferably the pressure equalisation valve is opened after the preheating steps discussed above.

Preferably the compressor is started slowly, e.g. at a speed or frequency considerably lower than a standard operating frequency. Starting the compressor and opening the pressure equalisation valve allows oil in the compressor to be mixed. This is advantageous because in a shutdown compressor the temperature of the oil is not homogeneous in the compressor shell. When the compressor is started slowly with the pressure equalisation valve open (and therefore with very low refrigerant flow), the hot oil from the motor is mixed with the cold oil from the other parts and the oil is warmed. Furthermore the oil and other parts of the compressor are heated by the refrigerant that bypasses the compressor via the pressure equalisation valve, the vapour refrigerant from the discharge valve in the compressor being hotter than the actual suction gas refrigerant and when passing through the bypass and the compressor suction, the vapour heats the mechanical parts of the compressor and the oil. That is the bypass line generally emits heat to the compressor and heats the oil that is circulating in the compressor. Pressure in the compressor body or shell is limited by the bypass.

In particularly preferred embodiments the oil temperature of the compressor is maintained above the saturated discharge temperature of the refrigerant in the compressor shell. At temperatures below the saturated discharge temperature the vapour refrigerant condenses and if the oil temperature is below the saturated-discharge temperature, refrigerant will condense into the oil. Preferably the compressor shell temperature is also maintained above the saturated discharge temperature of the refrigerant. If the oil, and preferably also the mechanical components and the shell of the compressor, are. above the saturated discharge temperature refrigerant will not condense in the compressor.

Preferably the speed of the compressor at startup is lower than the normal running speed of the compressor as previously mentioned. For example in preferred embodiments the compressor at startup operates at a frequency of 30 Hz. Low compressor speed is desirable at startup because a low flow rate through the compressor minimises refrigerant condensation in the compressor.

Preferably the liquid valve is closed prior to and during compressor startup. In particularly preferred embodiments the liquid valve closes as the compressor stops and remains closed during compressor shutdown. Closing the liquid valve on compressor shutdown limits^' the flow of refrigerant into the compressor limiting condensation in the compressor oil.

Preferably at an appropriate time after compressor startup the liquid valve is opened. Therefore the system is operating in some states with both the pressure equalisation valve and the liquid valve open and the compressor operating at low frequency. This enables increased flow of refrigerant at the compressor suction, although the refrigerant flow is still lower than during normal system operation because the pressure equalisation valve is open (i.e. the compressor remains bypassed at this stage). Preferably the liquid valve is opened when it is determined that a system parameter is at a desired level. For-example— in- a preferred embodiment the system parameter is the oil temperature and when the oil temperature is determined (for example by measurement with a temperature sensor) to be sufficiently high (for example above a predetermined limit, and/or above the saturated discharge temperature of the refrigerant in the compressor, etc.), then the liquid valve is opened. Other suitable parameters and limits could of course be used. For example in a preferred embodiment the parameter is alternatively or additionally the pressure in the compressor shell. Furthermore it is envisaged that the liquid valve could instead or additionally be opened in response to other events, for example after a predetermined period of time (e.g. from compressor startup, and/or from opening of the pressure equalisation valve, or from any other action or event, etc.).

Preferably after the liquid valve is opened the pressure equalisation valve is closed. This occurs in response to any one or more of the following: after a predetermined period of time since opening the liquid valve; after a period of time has elapsed following any other suitable event; after a period of time has elapsed following one or more system parameters being determined to have reached a particular level; immediately after opening the liquid valve; etc. In a preferred embodiment the system parameter comprises either the compressor discharge temperature or the oil temperature and when the temperature is determined (for example by measurement with a temperature sensor) to be sufficiently high (for example above a predetermined limit, and/or above the saturated discharge temperature in the compressor, etc.);, then the pressure equalisation valve is closed. The pressure of the suction and discharge of the compressor are therefore no longer balanced and refrigerant passes through the compressor at a greater flow rate than-wheirtbre~pressure-- equalisation valve was open (e.g. refrigerant no longer bypasses the compressor).

The compressor speed is preferably then increased, either immediately or preferably in response to a measured system parameter reaching a predetermined limit and/or after a period of time has elapsed, etc. Preferably the compressor speed is slowly increased, preferably by a predetermined amount and/or at a predetermined rate, until a maximum or optimum speed is achieved and/or a predetermined time period has elapsed. Alternatively or additionally, when a measured system parameter is determined to have reached a predetermined level, the compressor speed may be set to the maximum (i.e. standard) operating speed (which is preferably after a period of slow increase in compressor speed from the initial startup speed). In a preferred embodiment, when the compressor discharge temperature and/or oil temperature of the compressor has reached a predetermined level, the compressor is controlled to operate at normal operating speeds.

The above preferred systems and operational steps provide compressor starting sequences that enable a compressor to be started with minimal risk of failure which otherwise might occur due to condensation of refrigerant in the compressor, particularly at low ambient temperatures, after compressor shutdown. Refrigerant condensation is detrimental in a compressor because condensed refrigerant can become mixed with oil in the compressor sump, and if the oil temperature in the compressor is below the saturated discharge temperature of the refrigerant then refrigerant can condense in the oil. When the compressor is started refrigerant in the oil is pumped by the oil pump and may fail. Furthermore " the^{^}rsil^{^}^rsXusity^{^} is -affected by the presence of refrigerant and therefore may be inappropriate for compressor operation causing damage to components that should be lubricated. These problems are solved by the preferred embodiments of the present invention and by the preferred methods discussed below.

In a further broad aspect of the present invention, there is provided a method of optimising compressor startup for a compressor of a refrigeration system comprising the steps of: preheating at least one component of a compressor in a refrigeration system; opening a pressure equalisation valve that connects a suction and a discharge of the compressor to thereby reduce^" the pressure differential therebetween; starting the compressor, preferably with a predetermined frequency ft, preferably at substantially the same time as opening the pressure equalisation valve. Preferably the method further comprises the steps of opening a liquid valve that is located in a refrigerant flow path of the system in response to a first event, closing the pressure equalisation valve in response to a second event, and increasing the frequency of operation of the compressor.

Preferably the starting speed of the compressor is less than the compressor standard operating speed f_s. Preferably ft is about 30 Hz. Preferably fs is about 60 Hz, preferably about 65 Hz or greater. Preferably the method comprises the further steps of: further increasing the frequency of operation of the compressor to a standard operating frequency, preferably in response to a third event.

Preferably the first event comprises at least one of a first predetermined period of time elapsing and a measured compressor oil temperature being deterrømed— to be above a predetermined threshold. Preferably the second event comprises at least one of a second predetermined period of time elapsing and a measured compressor oil temperature being determined to be above a predetermined threshold. Preferably the method further comprises the steps of providing at least one system sensor and measuring at least one parameter of the system with the sensor, and operating the system in at least one of a plurality of predetermined sequences based on the at least one parameter measured by the sensor. Preferably the step of preheating at least one component of the compressor comprises the steps of providing means for heating at least one component of the compressor, and activating the heating means to heat the component when it is determined that compressor startup is required. Preferably the means for heating at least one component of the compressor comprises means for heating at least one of the compressor body or shell, the oil in the compressor, and the compressor motor: In a particularly preferred embodiment the heating means comprises means for supplying DC electricity to the stator windings of an internal AC motor of the compressor. Preferably the method further comprises providing at least one sensor and measuring at least one parameter of the system with the sensor, and the heating means is activated for a predetermined period of time based on the at least one parameter. Preferably the predetermined period is based on at least one of the temperature of the oil in the compressor, the compressor shell temperature;, the compressor discharge temperature, the ambient temperature and the length of time for which the compressor has been inactive.

Preferably the method further comprises the steps - ef-m-easurmg the- temperature of oil in the compressor, determining the saturated discharge temperature of refrigerant in the compressor, and heating at least one component of the compressor such that die oil is maintained at a temperature above the saturated discharge temperature.

As discussed above, preheating a refrigeration system prior to compressor start up and controlling a liquid valve and a pressure equalisation valve before and during start up can advantageously reduce or eliminate refrigerant condensation problems, particularly in low ambient temperatures. However even the above system may experience failure or other problems on compressor startup for further reasons on or after compressor shutdown.

Therefore in accordance with a further broad aspect of the present invention, there is provided a refrigeration system comprising a compressor, a heat rejecting heat exchanger, an expansion valve and a heat accepting heat exchanger. The system further comprises a liquid valve (LV), preferably a liquid solenoid valve (LSV) or an electronic expansion valve (EXV), the liquid valve arranged in a flow line between the heat rejecting heat exchanger and the expansion valve, and a check valve arranged in a flow line between the heat rejecting heat exchanger and the compressor. The refrigeration system is advantageous in that when the compressor is shut down the liquid valve is actuated to close and the check valve is also closed. Preferably the check valve is closed by the pressure differential between the high pressure side of the system and the compressor. In other preferred embodiments the check valve is a solenoid valve and is actuated to close. In further embodiments the check valve comprises a combination of a solenoid valve and a pressure-actuated -valver- -In a standard circuit without this valve arrangement the pressure differential would cause refrigerant to migrate to the compressor from the high side, e.g. from the- condenser and also from the evaporator, particularly during long periods of compressor shutdown, for example 12 hours or more. Any refrigerant that migrates to the compressor during shutdown can condense in the compressor or otherwise migrate into the compressor oil, which can lead to compressor failure on start up, for example due to the low oil viscosity.

Therefore the system of the embodiments of this aspect of the present • invention advantageously reduces the amount of, or even prevents, refrigerant reaching the compressor during shutdown and therefore little or no refrigerant can mix with the compressor oil. This is advantageous because the amount of refrigerant in the oil is minimised and therefore the oil viscosity will remain sufficiently high, whereas low viscosity oil is dangerous for the compressor. Preferably the check valve is configured such that a pressure differential between the valve inlet and the valve outlet is required to open the check valve, preferably a significant pressure differential. The system of this embodiment is further advantageous in that the check valve in combination with the liquid valve will maintain the refrigerant in the condenser (and in any other component that may be present between the check valve and the liquid valve, such as in preferred embodiments an accumulator and/or a dryer, etc.). Preferably the check valve is configured such that if the pressure at the inlet and the outlet of the check valve is balanced, which may occur over time after the compressor is shut down, the valve remains closed. That is in preferred embodiments the check valve is configured such that refrigerant can pass ^■titi-rough-the valve- only when the valve inlet pressure is higher than the valve outlet pressure, for example when the compressor is started. Preferably the check valve comprises a resilient means such as a spring or the like that biases the valve into a closed position when the pressure at the inlet and the outlet of the check valve is balanced. This is advantageous because prior art valves leak refrigerant when the pressure at the inlet and the outlet is balanced, whereas in the embodiment of the present invention having the inventive check valve, a greater inlet pressure is required to open the valve and the amount of refrigerant leak under other conditions is minimised. - The above preferred refrigeration system prevents or substantially reduces refrigerant migration from the condenser and other high side components of the system, and/or from the low side of the system, to the compressor after compressor shutdown and thus enables the compressor to be started, even after a long shutdown period, with minimised risk of failure from refrigerant having mixed with the compressor oil.

From a further broad aspect of the present invention, there is provided a method of controlling a refrigeration system comprising the step of: initiating shutdown of a compressor of the system, closing a check valve provided in a flow line between the compressor and a heat rejecting heat exchanger of the system, and closing a liquid valve of the system that is located in a flow path between the heat rejecting heat exchanger and an expansion device of the system. In preferred embodiments a pressure differential between the condenser and the compressor that occurs on shutdown is such that the pressure on the condenser side of the check valve is higher than the pressure on the compressor side of the check valve after shutdown and this closes the check valve to preventHfow-of refrigerant therethrough. In other preferred embodiments, the check valve comprises a solenoid valve and preferably both valves are closed as soon as possible after compressor shutdown and preferably at substantially the same time as each other to prevent refrigerant migration.

Preferably the. method further comprises the steps of: starting the compressor thereby causing a pressure differential between the condenser and the compressor and opening the check valve provided in the flow line therebetween, and opening the liquid valve. The pressure differential between the condenser and the compressor is such that the pressure on the condenser side of the check valve (e.g. the outlet) is lower than the pressure on the compressor side of the check valve (e.g. the inlet) after the compressor is restarted. This opens the check valve to enable flow of refrigerant therethrough. Preferably the method further comprises the step of biasing the check valve into a closed position, wherein the biasing force must be overcome in order to open the valve.

As discussed above, providing a liquid valve and a check valve of the present invention enables the system to be operated to prevent or substantially reduce refrigerant migration from the condenser and other high side and/or low side components of the system to the compressor after compressor shutdown. The compressor can therefore be restarted and the risk of compressor failure due to the presence of refrigerant in the oil after refrigerant migration is minimised. However even the above system may experience failure or other problems on compressor start up due to a further effect that also arises from compressor shut down.

Therefore in- aceordanee-with a further broad aspect of the present invention, there is provided a refrigeration system comprising a compressor, a heat rejecting heat exchanger, an expansion valve and a heat accepting heat exchanger. The system further comprises a pressure equalisation valve (PEV) for equalising the pressure differential between the compressor suction and compressor discharge. The pressure equalisation valve is operable to open after or at the same time as the compressor is shutdown, preferably substantially immediately after compressor shutdown is effected. Preferably the pressure equalisation valve comprises a bypass passage connecting the compressor suction to the compressor discharge and bypassing the compressor.

By opening the pressure equalisation valve at the same time as or just after the compressor is shutdown or stopped, the pressure differential between the compressor suction and discharge is balanced as quickly as possible. This is advantageous because a high pressure differential between the compressor suction and discharge on compressor shutdown causes oil from the compressor to migrate towards the suction side of the compressor, leaving the compressor and entering the suction line, whereas in the present invention, equalisation or balancing of the discharge and suction pressures prevents such migration. It is undesirable for oil to leave the compressor as upon restarting the compressor., little or no oil is available for compressor lubrication. Furthermore upon restart the compressor has a mixture of oil and refrigerant in the suction line, which can lead to compressor failure as the mixture is sucked into the compressor. Still further the oil that leaves the compressor and enters the suction line can migrate to the evaporator which can cause -further failure and also if the expansion valve is controlled in relation to the evaporator temperature (e.g. the evaporator outlet temperature) then the presence of oil at the sensor can lead to opening of the expansion valve even though the system is shut down. However the embodiments of the present invention provide an improved refrigeration system in which migration of oil from the compressor to the low side of the system is minimised or prevented after compressor shutdown by equalising the pressure differential between the compressor discharge and suction just after compressor shutdown. A method of carrying out the invention is also envisaged and therefore from a further broad aspect of the present invention, there is provided a method of controlling a refrigeration system comprising the steps of: initiating shutdown of a compressor of the system; and opening a pressure equalisation valve (PEV) for equalising the pressure differential between a compressor suction and a compressor discharge. The pressure equalisation valve is opened preferably as or just after the compressor is shutdown, preferably substantially immediately after compressor shutdown is effected, thereby substantially equalising the high side discharge pressure and the low side suction pressure of the compressor. Preferably the pressure equalisation valve comprises a bypass passage connecting the compressor suction to the compressor discharge and bypassing the compressor.

The above-mentioned and other features of the various embodiments of the present invention will now be described, by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows a schematic representation a refrigeration system in accordance with an embodiment of the present inventions- Figure 2 shows a flow diagram illustrating the modes of operation of a refrigeration system in accordance with an embodiment of the present invention; ' Figure 3 shows a first, standard operating sequence for starting a compressor of a refrigeration system in accordance with an embodiment of the present invention;

Figure 4 shows a second, long operating sequence for starting a compressor of a refrigeration system in accordance with an embodiment of the present invention; -

Figure 5 shows a third, short operating sequence for starting a compressor of a refrigeration system in accordance with an embodiment of the^' present invention;

Figure 6 shows a schematic representation of another refrigeration system in accordance with an embodiment of the present invention;

Figure 7A shows a schematic representation of another refrigeration system in accordance with an embodiment of the present invention after compressor shutdown;

Figure 7B shows a system, that does not have the pressure equalisation valve of the embodiment of figure 7A, in two states, the first being shortly after compressor shutdown and the second being some time after compressor shutdown; and

Figure 8 shows a schematic representation of a refrigeration system in accordance with another embodiment of the present invention. The principles of the present invention can be incorporated within any suitable system. -Examples-of- such suitable systems include refrigeration and airconditioning systems and particularly, although not exclusively, transport or truck refrigeration systems. For ease of reference, the specific embodiments discussed herein are described with reference to a refrigeration system suitable for a transport refrigeration unit or the like.

Figure 1 schematically shows a refrigeration system 10 having a refrigerant cycle or circuit 20 such that refrigerant can flow around the system. The system comprises a compressor 12 connected from an outlet or discharge 13 thereof via flow path 22 to a heat rejecting heat exchanger, which in this embodiment is -a condenser 14. The condenser 14 is connected via flow path 24 to expansion device 16, which is connected via flow path 26 to a heat accepting heat exchanger, which in this embodiment is an evaporator 18. The evaporator 18 is connected to the compressor 12 at an inlet or suction 11 thereof via flow path 28. The expansion device 16 is preferably a thermostatic expansion valve and in this embodiment is controlled in response to conditions of the system 10 via control line 36. The system condition which controls opening of the expansion valve 16 could for example be the temperature of the evaporator 18, or a related temperature such as a bulb temperature at the evaporator outlet, etc. In this embodiment. additional optional components accumulator 32 and dryer 34 are also provided on flow path 24 between the condenser 14 and the valve 16.

The system 10 further comprises a pressure equalisation valve (PEV) across the compressor 12, i.e. connecting the compressor suction 11 to the discharge 13. The PEV comprises a bypass passage 40 and means 42, such as a valve, for opening and closing the passage 40.

The system 10 further comprises a liquid valve (LV), which in preferred embodiments is a liquid solenoid valve 44, in the flow path 24 between the condenser 14 and the expansion valve 16. The LV 44 can be energised to open or close as required, thereby opening or closing the flow path 24 to enable or disable refrigerant flow around the circuit 20.

In operation, high pressure and high temperature refrigerant vapour exits the compressor 12 and enters the condenser 14 where it is cooled to a lower temperature, high pressure liquid refrigerant. This liquid is then expanded to a_ lower pressure by the expansion valve 16 and passes to the evaporator 18 where the refrigerant boils and absorbs heat from its surroundings. The vapour at the evaporator 18 outlet is drawn into the compressor 12, completing the cycle.

When the compressor 12 is shut down, refrigerant may be present in the compressor 12 and, particularly if the compressor 12 is shut down for prolonged periods, additional refrigerant can migrate from the condenser 14 to the compressor 12 as discussed in more detail below.

The refrigerant in the compressor 12 may condense on the compressor shell, particularly at low ambient temperatures, and the condensed refrigerant will mix with the compressor oil which has an affinity for refrigerant. If the compressor oil temperature is below the saturated discharge temperature of the refrigerant, the refrigerant can condense in the oil. The refrigerant dilutes the oil and, when the compressor 12 is restarted, the diluted oil is less effective at lubricating the components of the compressor 12, which may lead to damage. Furthermore the compressor oil pump will draw in refrigerant which may also lead to damage.

Therefore in accordance with embodiments of the present invention, one or more compressor startup sequences are employed to minimise or eliminate refrigerant condensation in the compressor 12. Figure 2 is a flow diagram of one embodiment of the present invention in which a control means or the like determines the state of a system 10 (for example the system 10 of any of figures 1, 6, 7 A or 8) and in particular the length of time T_Sto_P that the compressor has been shut down and the discharge temperature T_ref of the compressor 12. If the compressor 12 is shut down for a reasonably long period of time, the discharge temperature T_ref is substantially equal to the ambient temperature. In other embodiments the ambient temperature may be measured. The control means determines from these parameters what steps before and during compressor startup should be taken to minimise or eliminate the problems of refrigerant condensation in the compressor 12. Of course, other parameters could alternatively or additionally be used in this determination, such as the temperature of the oil in the compressor 12, the temperature of the refrigerant in the compressor 12, and/or the pressure inside the compressor shell, etc. The parameters used may depend on the sensors that are present in the system 10 and so what can be measured for making this determination. In the figure 2 embodiment, the sequence begins at step 1.1 and the time since the compressor 12 stopped Tstop^' is determined in step 1.2. If it is less than 1 hour, the time is further determined in step 2.1 and still further in step 3.2 if Tstop is less than 1/2 hour. For shutdown periods less than 1/2 hour, it is determined unnecessary to preheat the compressor 12 and a normal starting sequence (for example as shown in figure 3) begins in step 4.3. For shutdown periods between 1/2 and 1 hour, the discharge temperature, T_ref is determined in step 3.1 and if it is low (less than 20°C), a short (3 minute) preheat of the compressor 12 is initiated in step 4.2 and as discussed below, before the normal starting sequence begins in step 5.2. However if T_ref is sufficiently high already, no preheat is required and a short starting sequence (for example as shown in figure 5) begins in step 4.1.

When the compressor 12 has been shut down for longer than 1 hour, the discharge temperature T_ref is determined in step 1.3 and dependent on the temperature, also in steps 1.4, 1.5. 1.6, 1.7 and 1.8. Furthermore, dependent on Tref, the compressor 12 is preheated for 12, 9, 6 or 3 minutes (steps 2.2, 2.3, 2.4 and 2.5 respectively) before a long starting sequence (for example as shown in figure 4) begins in step 3.3. However if T_ref is sufficiently high (between 0 and 2O⁰C) as determined in step 1.7, a 3 minute preheat is initiated in step 2.6 before the normal starting sequence begins in step 3.4. If Tref is already even higher than 20°C as determined is steps 1.8 and 1.9, then no preheat is required and either a normal starting sequence is initiated in step 2.7 or for very high temperatures (greater than 4O⁰C) a short starting sequence is initiated in step 2.8. The above sequences ensure that if the discharge temperature of the compressor 12 is low, the compressor 12 is heated, preferably prior to compressor startup, to raise the compressor temperature, including the oil temperature. This is advantageous not only because the viscosity of the oil is improved making the oil more suitable for lubricating the compressor components on startup, but also because a sufficiently high oil temperature (greater than the saturation discharge temperature of the refrigerant) reduces or eliminates refrigerant condensation in the compressor 12 that occurs when the compressor shell and oil are cool. Figures 3₃ 4 and 5 schematically illustrate preferred embodiments of the starting sequences for starting a compressor 12 after shutdown. Figure 3 shows a "normal" or default starting sequence, figure 4 shows a long starting sequence and figure 5 shows a short starting sequence. In preferred embodiments the starting sequences disclosed in figure 2 correspond with the figure 3, 4 and 5 sequences, but it is also envisaged that this could differ or be modified by the skilled person. Furthermore the preheat sequences disclosed in figures 3, 4 and 5 may correspond with the preheat. sequences of figure 2, or may differ or be modified.

Figure 3 shows a normal starting sequence for a compressor 12. In preferred embodiments, some or all of the steps of the figure 2 sequence are carried out as the first step of the normal starting sequence. Therefore the discharge temperature of the compressor 12 is preferably at least 2O⁰C or the compressor shutdown period was less than 1/2 hour. Referring to a system 10 as exemplified in figure 1, the pressure equalisation valve (PEV) 40, 42 is initially closed and so is the liquid valve (LV) 44. When the compressor 12 is to be started, the PEV is opened thereby opening a bypass of the compressor 12. The compressor 12 is started, but with a relatively low frequency of about 30 Hz (which is significantly less than the full operating speed of the compressor 1,2). Heat from the bypassed refrigerant gas that passes through the PEV is transferred to the compressor 12 and to the compressor oil when the compressor 12 is started. Thus the oil in-the -compressor -!^■£ is (further) heated and the condensation risk is further minimised, particularly as the refrigerant bypassing the compressor 12 cannot condense in the compressor 12. Furthermore the LV is closed to limit the flow of refrigerant into the compressor 12 thus further reducing the risk of condensation.

When the oil temperature in the compressor 12 is sufficiently high, e.g. when it is measured, determined or otherwise expected to be greater than the saturated discharge temperature of the refrigerant, and/or in this embodiment after the oil has been heated for a sufficient period of time which in the normal starting sequence is 20 seconds, the liquid valve is opened and the PEV remains open. The refrigerant flow at the compressor suction 11 increases slightly, but -is still relatively low as the compressor 12 is still bypassed by the open PEV. When the oil temperature in the compressor 12 is sufficiently high, e.g. when it is again measured, determined and/or expected to be greater than the saturated discharge temperature of the refrigerant, and/or in this embodiment after it has been heated for a sufficient period of time which in the normal starting sequence is a further 20 seconds, the PEV is closed whilst the liquid . valve remains open. Refrigerant therefore flows around the circuit 20 of system 10 under the influence of the compressor 12, which is no longer bypassed. When the oil temperature in the compressor 12 is sufficiently high, e.g. when it is yet again measured, determined or otherwise expected to be greater than the saturated discharge temperature of the refrigerant, and/or in this embodiment after it has been heated for a sufficient period of time which in the normal starting sequence is a still further 20 seconds, the speed of the compressor^" 12 is^~graτdτrarllynncreased,- preferably by 5 Hz per second until an optimum or normal operating frequency is reached, after which standard compressor speed control is applied as is known in the art. In other embodiments, the standard operating speed control can be started after it is again determined or otherwise expected that the oil temperature is still higher than the saturated discharge temperature.

However a normal starting sequence may not be appropriate under certain circumstances, for example when the temperature of the shutdown compressor 12 is low (e.g. less than about 5⁰C) and/or when the compressor 12 has been shutdown for a long period (more than about one hour). Instead a long starting sequence as shown in figure 4 may be more appropriate. The long starting sequence differs from the normal starting sequence in that the periods between events are generally significantly longer. For example the LV is kept closed after compressor startup for 5 minutes rather than 20 seconds, thereby allowing the oil additional time to heat up. The delay before closing the PEV is also longer and is about 2 minutes thus allowing the system 10 to operate at a reduced flow rate for longer. The period of time before the compressor frequency is increased is also longer and is about 2 1/2 minutes after which the frequency is increased more slowly than the normal starting sequence, at about 1 Hz per 5 seconds. The long starting sequence differs at this stage from the normal starting sequence in that an additional step is included before standard compressor speed control is initiated, during which the compressor is operated at a maximum frequency of 60 Hz for 1 minute. The long starting sequence is significantly slower than the normal starting sequence thereby allowing the system temperature to increase gradually before fully loading the compressor 12, which is appropriate in -eeldeϊ- conditions, particularly if the compressor 12 has been inactive for a long period of time. Furthermore it may be appropriate to heat the oil for a longer period prior to initiating the long starting sequence, as discussed in relation to figure 2.

However under other circumstances neither a normal nor a long starting sequence may be appropriate, for example when the temperature of the shutdown compressor 12 is relatively high (e.g. more than about 40⁰C) and/or when the compressor 12 has been shutdown for only a brief period (less than an hour). In such circumstances a short starting sequence as shown in figure 5 may be more appropriate. The short starting sequence differs from the normal starting sequence in that the periods between events are generally much shorter and in some embodiments, little or no delay between events is needed. For example the LV is not kept closed after compressor startup but instead is opened up quickly as the oil perhaps does not need any additional time to heat up at the lower operating speed. The delay before closing the PEV is also short or may not even be required and the PEV can be closed quickly after compressor startup. The period of time before the compressor frequency is increased is also short and is about 5 seconds, after which the frequency is increased at a slower- rate than the normal starting sequence, at about 1 Hz per second. The short starting sequence is significantly faster than the normal starting sequence as the system temperature does not need to increase gradually and the compressor 12 is capable of operating under full load relatively quickly. Furthermore it may not even be necessary to heat the oil prior to initiating the short starting sequence, as shown in figure 2.

Figure 6 schematically illustrates an -alternative embodiment of the present invention;, although this system 10 could be and preferably is combined with the system 10 shown in figure 1 simply by adding the PEV of figure 1 (for example as shown in figure 8) or indeed with figure 7A. In the figure 6 embodiment, the components are mostly the same as those of the figure 1 embodiment and have like reference numerals. However the figure 6 embodiment further comprises a check valve 46 which is a one-way valve that prevents flow or migration of fluid in one direction (from the condenser 14 towards the compressor 12) but permits flow of fluid in the other direction (towards the condenser 14 from the compressor 12).

The system 10 of figure 6 operates as normal when the compressor 12 is running. However, in prior art systems when the compressor is shut down refrigerant migrates from the condenser and/or evaporator to the compressor due to the pressure and temperature differences, and the refrigerant can condense in the compressor and mix with the oil which is undesirable as discussed above. In the figure 6 embodiment however, refrigerant is effectively trapped between the check valve 46 and the liquid valve (LV) 44 and therefore does not reach the compressor 12. This embodiment operates as follows. When the compressor 12 is stopped, the LV is closed (preferably the LV is a liquid solenoid valve and the valve is closed by energising the solenoid) and the check valve 46 is closed (preferably the check valve is also a solenoid valve and the valve is closed by energising the solenoid, or the check valve may be closed by the pressure differential between the condenser 14 and the compressor 12. Refrigerant, that would otherwise migrate to the compressor 12, is therefore retained in the refrigerant circuit 20 between the two valves 44τ-46- — Even- if -the- pressure at the inlet and outlets of the check valve 46 are balanced, the check valve 46 will not open, because the check valve 46 in this embodiment comprises a spring inside (not shown) so that no leak occurs if the pressure is balanced. The inlet pressure of the check valve 46 must be above the outlet pressure to permit circulation of the fluid. Any additional components such as an accumulator 32 and a dryer 34 in the circuit 20 help to store the refrigerant during compressor shutdown. When the compressor 12 is restarted, the LV is energised to be opened and the check valve 46 is energised or opens due to the changed pressure differential. Figure 7A illustrates another embodiment of the present invention, which although as shown is a separate embodiment, it is within the scope of the invention for this system 10 to be combined with any one or more of the systems 10 shown in the other figures. The system 10 comprises similar components as the other embodiments, including the pressure equalisation valve 40, 42 discussed with regard to figure 1 and the figure 1 embodiment can be operated in accordance with the following disclosure as well.

A conventional refrigeration system 110 is shown in figure 7B and is shown in a first state shortly after compressor shutdown and in a second state a longer time after shutdown. When the compressor 112 is shut down a large pressure differential exists between the compressor suction 111 and the compressor discharge 113, which effectively pushes the compressor oil 100 out of the compressor 112 on the suction side of the circuit 120, into line 128 and towards the evaporator 118. Therefore on compressor startup there is less oil than should be present, and in some cases little or no oil, in the compressor 112 and the compressor components are likely to be damaged. Furthermore as shown in the second diagram of figure 7B, after a peried--©f- time the oil 100 can migrate and begin to fill the evaporator 118 and also the relatively hot oil can fill the bulb of the expansion valve 116 control means 136 that is located at the exit of the evaporator 118 (i.e. in line 128). This can cause the expansion valve 116 to open even if that is not desired, further affecting the system performance on compressor startup and liquid in the compressor suction line can damage the compressor.

The refrigeration system 10 of the embodiment of figure 7 A overcomes this problem by provision of the PEV, which is opened during or preferably just after compressor shutdown. This equalises the pressure differential between the compressor suction 11 and discharge 13 and thus prevents migration of oil from the compressor to the low side of the system 10.

Figure 8 schematically illustrates another embodiment of the present invention. In this embodiment, the system 10 comprises a check valve 46, a liquid valve 44 and a pressure equalisation valve 40, 42. Therefore all of the advantages disclosed in relation to the. other embodiments and discussed above are provided by this system having the combination of all the valves.

Claims

1. A refrigeration system comprising: a compressor having a suction and a discharge; a heat rejecting heat exchanger; an expansion valve; a heat accepting heat exchanger; and a pressure equalisation valve for equalising the pressure differential between the compressor suction and compressor discharge.

2. A refrigeration system as claimed in claim 1, wherein the pressure equalisation valve comprises a bypass passage connecting the compressor, suction to the compressor discharge to enable the compressor to be bypassed and a valve to control flow of refrigerant therethrough.

3. A refrigeration system as claimed in claim 1 or 2, further comprising: a liquid valve, preferably a liquid solenoid valve, arranged in a flow line between the heat rejecting heat exchanger and the expansion valve.

4. A refrigeration system as claimed in any of claims I₃ 2 or 3, further comprising: control means for operating the system in at least one of a plurality of predetermined sequences; and at least one sensor, wherein the control means operates the system in a particular one of the plurality of predetermined sequences based on at least one parameter of the system measured by the sensor.

5. A refrigeration system as claimed in any preceding claim, further comprising: means for heating at least one component of the compressor; and control means for activating the heating means when it is determined that compressor-startup is required, the control means starting the compressor after heating the at least one component.

6. A refrigeration system as claimed in claim 5, wherein the means for heating at least one component of the compressor comprises means for heating at least one of the compressor body or shell, the oil in the compressor, and the compressor motor.

7. A refrigeration system as claimed in claim 5 or 6, further comprising: at least one sensor, wherein the control means activates the heating means for a predetermined period of time based on at least one parameter of the system measured by the sensor.

8. A refrigeration system as claimed in claim 7, wherein the predetermined period is based on at least one of the temperature of the oil in _] the compressor, the compressor shell temperature, the compressor discharge temperature, the ambient temperature and the length of time for which the compressor has been inactive.

9. A refrigeration system as claimed in any of claims 5 to 8, further comprising: a sensor for sensing the temperature of oil in the compressor; means for determining the saturated discharge temperature of refrigerant in the compressor; and control means, wherein the means for heating at least one component

-of- the compressor comprises means for heating at least the -oil- irr-the compressor, and wherein the control means controls the heating means such that the oil is maintained at a temperature above the saturated discharge temperature.

10. A refrigeration system as claimed in any of claims 5 to 9, wherein the heating means comprises: means for providing an electrical current to windings of a motor of the compressor to thereby heat the windings,

11. A refrigeration system as claimed in claim 10, further comprising: control means for controlling heating of the windings, wherein the electrical current provided to the motor windings comprises direct current (DC) and is provided for at least one predetermined periods of time.

12. A refrigeration system as claimed in any preceding claim, further comprising: control means, wherein the control means opens the pressure equalisation valve at substantially the same time as starting the compressor.

13. A method of optimizing startup of a compressor of a refrigeration system comprising the steps of: providing a refrigeration system comprising a compressor, a heat rejecting heat exchanger, an expansion valve, a heat accepting heat exchanger, and a pressure equalisation valve that connects a suction and a discharge of the compressor for equalising the- -pressure- differential between the compressor suction and compressor discharge; preheating at least one component of the compressor; opening the pressure equalisation valve to thereby reduce the pressure differential between the compressor suction and discharge; and starting the compressor, preferably at substantially the same time as opening the pressure equalisation valve.

14. A method according to claim 13₃ wherein the step of starting the compressor comprises operating the compressor at a predetermined frequency fi that is less than the operating frequency f_n of the compressor during normal operating conditions of the system.

15. A method as claimed in claim 14, further comprising the step of: opening a liquid valve, provided in a refrigerant flow path between the heat rejecting heat exchanger and the expansion valve, in response to a first event; closing the pressure equalisation valve in response to a second event; and increasing the operating frequency of the compressor.

16. A method as claimed in claim 15 further comprising the step of: further increasing the frequency of operation of the compressor to the operating frequency f_n of the compressor during normal operating conditions of the system,

17. A method as claimed in claims 15 or 16, wherein: the first event comprises at least one of a first predetermined period of time elapsing and a measured compressor oil temperature being determined to be above a predetermined threshold; and the second event comprises at least one of a second predetermined period of time elapsing and a measured compressor oil temperature being determined to be above a predetermined threshold.

18. A method as claimed in any preceding claim further comprising the steps of: providing at least one system sensor and measuring at least one parameter of the system with the sensor; and operating the system in at least one of a plurality of predetermined sequences based on the at least one parameter measured by the sensor.

19. A method as claimed in any preceding claim, wherein the step of preheating at least one component of the compressor comprises the steps of: ^■ providing means for heating at least one component of the compressor; and activating the heating means to heat the component when it I iS determined that compressor startup is required.

20. A method as claimed in claim 19, wherein the means for heating at least one component of the compressor comprises means for heating at least one of the compressor body or shell, the oil in the compressor^~and-the compressor motor.

21. A method as claimed in claim 19 or 20, further comprising the step of: providing at least one sensor and measuring at least one parameter of the system with the sensor; and wherein the heating means is activated for a predetermined period of time based on the at least one parameter.

22. A method as claimed in claim 21, wherein the predetermined period is based on at least one of the temperature of the oil in the compressor, the compressor shell temperature, the compressor discharge temperature, the ambient temperature and the length of time for which the compressor has been inactive.

23. A method as claimed in any of claims 14 to 22, further comprising the step of: measuring the temperature of oil in the compressor; determining the saturated discharge temperature of refrigerant in the compressor; and heating at least one component of the compressor such that the oil is maintained at a temperature above the saturated discharge temperature.

24. A refrigeration system comprising: a compressor; a heat rejecting heat exchanger; an expansion valve; a heat accepting heat exchanger; a liquid valve, preferably a liquid solenoid valve or an electronic expansion valve, the liquid valve arranged in a flow line between the heat rejecting heat exchanger and the expansion valve; and a check valve, preferably a solenoid valve or an electronic expansion valve, arranged in a flow line between the heat rejecting heat exchanger and the compressor.

25. A system as claimed in claim 24, wherein the check valve is configured such that a pressure differential between an inlet of the valve and an outlet of the valve opens the check valve.

26. A system as claimed in claim 24 or 25, wherein check valve is configured such that when the valve is closed, the valve remains closed if a pressure balance between an inlet of the valve and an outlet of the valve exists.

27. A system as claimed in claim 26, wherein the check valve comprises resilient means, preferably a spring or the like, that biases the valve into a closed position when the pressure at the inlet and the outlet of the check valve is balanced.

28. A system as claimed in any of claims 24 to 27, further comprising at least one of, and preferably both of, a dryer and an accumulator.

29. A method of controlling a refrigeration system comprising the steps of: providing a refrigeration system comprising a compressor, a heat rejecting heat exchanger, an expansion valve, a heat accepting heat exchanger, a liquid valve between the heat rejecting heat exchanger and the expansion valve, and a check valve between the heat rejecting heat exchanger and the compressor; initiating shutdown of the compressor; closing the check valve and the liquid valve, preferably substantially simultaneously with shutdown of the compressor.

30. A method as claimed in claim 29, wherein a pressure differential between the condenser and the compressor that occurs on shutdown is such that the pressure on the condenser side of the check valve is higher than the pressure on the compressor side of the check valve thereby closing the check valve to prevent flow of refrigerant therethrough.

31. A method as claimed in claim 29 or 30, wherein the liquid valve and preferably also the check valve, comprise solenoid valves and the step of closing the check valve and the liquid valve comprising activating the solenoid valve (s).

32. A method as claimed in claim 29, 30 or 31, further comprising the steps of: starting the compressor thereby causing a pressure differential between the condenser and the compressor and opening the check valve provided in the flow line therebetween; and opening the liquid valve.

33. A method as claimed in any of claims 29 to 32, further comprising the step of: biasing the check valve into a closed position, wherein the biasing force must be overcome in order to open the Valve.

34. A refrigeration system as claimed in claim 1 or 2> wherein the pressure equalisation valve is operable to open after or at the same time as compressor shutdown, preferably substantially immediately after compressor shutdown is effected.

35. A method of controlling a refrigeration system comprising the steps of: providing a refrigeration system comprising a compressor., a heat rejecting heat exchanger, an expansion valve, a heat accepting heat exchanger, and a pressure equalisation valve that connects a suction and a discharge of the compressor for equalising the pressure differential between the compressor suction and compressor discharge; initiating shutdown of the compressor; and opening the pressure equalisation valve for equalising the pressure differential between the compressor suction and discharge.

36. A method as claimed in claim 35, wherein the step of opening the pressure equalisation valve comprises opening the valve as or just after the compressor is shutdown, preferably substantially immediately after compressor shutdown is effected.