CLIAMS:WE CLAIM:
1. A method of operating a tandem refrigeration system having a compressor unit comprising two or more compressors for compressing a refrigerant, and delivering the refrigerant throughout the refrigerant system to at least one condenser, an expansion device and at least one evaporator, said method comprising the steps of:
operating the compressors for providing a desired capacity; sensing operating conditions of the system in real time;
starting cycle of unloading and loading of the compressors when one of the operation conditions crosses a safety parameter defined for the said operating condition wherein the cycle includes reducing capacity of the system by unloading the compressors step by step and loading the compressor to the desired capacity of the system after a predetermined period; and
providing best possible cooling by repeating the cycle of unloading and loading the compressors until the system returns normal operation wherein cycle of loading and unloading of the compressor is repeated for a predetermined number of times; or locking the compressors to safe capacity if the system fails to return normal operation in a predetermined number of cycles thereby providing best possible cooling.

2. The method as claimed in claim 1, wherein the step locking the compressors to safe capacity includes steps of determining the safe capacity of the system at which the compressor runs safely by unloading and loading the compressors step by step, comparing the capacity with a capacity provided by stopping the faulty compressor and running other compressors for providing best possible cooling and deciding to run the system as safe capacity or stopping the faulty compressor for providing best possible cooling.

3. The method as claimed in claim 1, wherein the operation conditions includes discharge pressure, suction pressure, current, oil level, discharge temperature, and the like.

4. The method as claimed in claim 1, wherein a user can define the number of cycle of unloading and loading based on the type of chiller system and nature of complexity of the operating condition.

5. The method as claimed in claim 1 or 3, wherein the predetermined number of cycles is preferably between three and five.

6. The method as claimed in claim 1, wherein the method comprises comprising step of one or more cycles of unloading and loading compressors by allowing manual resetting of locked compressors with a user password protection when the system.

7. The method as claimed in claim 1, wherein the sensing one of the parameter also includes sensing differential discharge pressure across NRVs or discharge valves provided between the discharge pipelines of the compressors and common discharge pipeline and indicating fault with NRV or discharge valve if the differential discharge pressure across NRVs or discharge valves is increased.

8. A controlling system for a chiller system having tandem compressors, said controlling system comprising:
at least one sensor for sensing a refrigerant system operating condition;
a safety parameter for each operating condition to operate the system; and
a controller for operating said refrigerant system at desired capacity based on inputs from the sensors, said controller having an unloading and loading module for operating compressors wherein said module configured to unload the compressors on detection of fault in the compressor to a best possible capacity such that the system reaches within the safety parameter and load the compressor after a predetermined time to provide desired capacity and repeat said unloading and loading cycles to until the system reaches normal operating condition or locking the compressors to provide best possible cooling if system fails to return normal operation after predetermined number of cycles.

9. A controlling system as claimed in claim 8, wherein the sensors are adapted to sense operation conditions including discharge pressure, suction pressure, current, discharge temperature, and the like.

10. A controlling system as claimed in claim 8, wherein the sensor includes a sensor adapted for sensing differential discharge pressure across NRVs or discharge valves provided between the discharge pipelines of the compressors and common discharge pipeline and indicating fault with NRV or discharge valve if the differential discharge pressure across NRVs or discharge valves is increased.
[00042]
Dated this 1st day of August 2013
FOR BLUE STAR LIMITED
By their Agent

(Girish Vijayanand Sheth)
Patent Agent No.: IN/PA 1022
KRISHNA & SAURASTRI ASSOCIATES
,TagSPECI:FORM 2

THE PATENTS ACT, 1970
(39 of 1970)
[0001] &
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[See section 10, Rule 13]
[0002]
[0003]
A METHOD OF OPERATING A TANDEM REFRIGERATION SYSTEM AND CONTROLLING SYSTEM THEREOF;
[0004]
[0005]
BLUE STAR LIMITED A COMPANY INCORPORATED UNDER THE COMPANIES ACT, 1956, WHOSE ADDRESS IS KASTURI BUILDINGS, MOHAN T. ADVANI CHOWK, JAMSHETJI TATA ROAD, MUMBAI – 400 020, MAHARASHTRA, INDIA
[0006]
[0007]
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

FIELD OF THE INVENTION:
[0008] This application relates to a refrigerant cycle. Particularly, the invention claimed in this application is related to a chiller refrigerant system having tandem compressors.

BACKGROUND OF THE INVENTION
[0009] Chiller system uses refrigerant cycle to chill the brine water or the like. In a standard refrigerant cycle, a compressor delivers a compressed refrigerant to a heat exchanger, known as a condenser, which is typically located outside. From the condenser, the refrigerant passes through an expansion device, and then to an heat exchanger, known as an evaporator. At the evaporator, in the Chiller system, temperature of fluid such as brine water is lowered with the refrigerant. From the evaporator, the refrigerant returns to the compressor. Of course, basic refrigerant cycles are utilized in combination with many configuration variations and optional features. However, the above provides a brief understanding of the fundamental concept.
[00010] In more advanced refrigerant systems, a capacity of the air conditioning system can be controlled by the implementation of two or more compressors so-called tandem compressors. The tandem compressors are normally connected together via common suction and common discharge manifolds. From a single common evaporator, the refrigerant is returned through a suction manifold, and then distributed to each of the tandem compressors. From the individual compressors the refrigerant is delivered into a common discharge manifold and then into a common single condenser. The tandem compressors are also separately controlled and can be started and shut off independently of each other such that one or both compressors may be operated at a time. By controlling which compressor is running, control over the capacity of the combined system is achieved. Presently, controller provided for controlling tandem compressors of the chiller system continue unloading & loading of the all running tandem compressor to a predetermined set point when it notices fault in the chiller system until the operating conditions become normal. This may result in damaging one or more compressors arranged in tandem. Alternatively, the controller directly stops the faulty compressor if it is found by the controller or shut down the chiller system completely.
[00011] Hence there is a need of a method for operating chiller system having tandem compressor and a controlling system thereof that provides minimum possible cooling or solve one or more above discussed problem.

SUMMARY OF THE INVENTION
[00012] Accordingly, the present invention provides a method of operating a tandem refrigeration system having two or more compressors for compressing a refrigerant, and delivering the refrigerant throughout the refrigerant system to at least one condenser, an expansion device and at least one evaporator and a controlling system thereof.
[00013] According an aspect of the invention, the method comprises the steps of operating the compressors for providing a desired capacity, sensing operating conditions of the system in real time, starting cycle of unloading and loading of the compressors when one of the operation conditions crosses a safety parameter defined for the said operating condition wherein the cycle includes reducing capacity of the system by unloading the compressors step by step and loading the compressor to the desired capacity of the system after a predetermined period, and providing best possible cooling. According to the present invention the step of providing best possible cooling comprises steps of repeating the cycle of unloading and loading the compressors until the system returns normal operation wherein cycle of loading and unloading of the compressor is repeated for a predetermined number of times or locking the compressors to safe capacity if the system fails to return normal operation in a predetermined number of cycles thereby providing best possible cooling
[00014] According to the present invention, the step locking the compressors to safe capacity includes steps of determining the safe capacity of the system at which the compressor runs safely by unloading and loading the compressors step by step and comparing the capacity with a capacity provided by stopping the faulty compressor and running other compressors for providing best possible cooling.
[00015] According to the present invention the operation conditions includes discharge pressure, suction pressure, current, discharge temperature, and the like.
[00016] According to an embodiment of the present invention, a user can define the number of cycle of unloading and loading based on the type of chiller system and nature of complexity of the operating condition.
[00017] According to the preferable embodiment of the invention, the predetermined number of cycles is preferably between three and five.
[00018] According to the further embodiment of the invention, the method comprises further one or more cycles with the user password protection thereby allowing manual resetting of locked compressor by inputting a password.
[00019] According to the present invention, the sensing one of the parameter also includes sensing differential discharge pressure across NRVs or discharge valves provided between the discharge pipelines of the compressors and common discharge pipeline and indicating fault with NRV or discharge valve if the differential discharge pressure across NRVs or discharge valves is increased.
[00020] In another aspect, the present invention provides a controlling system for a chiller system having tandem compressors. The controlling system comprising at least at least one sensor for sensing a refrigerant system operating condition, a safety parameter for each operating condition to operate the system and a controller for operating said refrigerant system at desired capacity based on inputs from the sensors, said controller having an unloading and loading module for operating compressors wherein said module configured to unload the compressors on detection of fault in the compressor to a best possible capacity such that the system reaches within the safety parameter and load the compressor after a predetermined time to provide desired capacity and repeat said unloading and loading cycles to until the system reaches normal operating condition or locking the compressors to provide best possible cooling if system fails to return normal operation after predetermined number of cycles.

DEFINITIONS:
[00021] Best possible cooling is a maximum possible cooling provided by running the system at safe capacity when there is a fault in the system;
[00022] Safe capacity is a capacity of the system wherein the operating conditions are within the safety limit/parameter defined for each operating parameter;
[00023] Safety limits/parameters are very well known which are defined for the safety of the compressors of the system for example high discharge pressure, low oil level, low suction pressure, etc of which a value is defined and depends on the type and complexity of the system; and
[00024] Normal operation is an operation wherein the system provides a desired capacity and the operating conditions are within their safety limits or parameters.

BRIEF DESCRIPTION OF DRAWINGS
[00025] Figure 1 shows a tandem refrigeration system of a chiller according to preferred embodiment of the invention;
[00026] Figure 2 shows a tandem refrigeration system of a chiller according to another preferred embodiment of the invention; and
[00027] Figure 3 shows a flow chart of a method of operating a tandem refrigeration system of a chiller according to the preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION
[00028] In general, the present invention provides a method of operating a tandem refrigeration system that ensures maximum possible cooling from the tandem refrigeration system in case of fault in the compressor.
[00029] Referring FIG. 1 and 2 illustrate refrigerant systems (100, 200) incorporating tandem compressor (120A, 120B) according to the preferred embodiments of the present invention. The tandem compressors (120A, 120B) compress a refrigerant and deliver it to a common discharge manifold (124) then through an condenser (130), to an expansion device (140), to an evaporator (110), and to a common suction manifold (122) leading back to the tandem compressors (120A,120B).
[00030] A suction pressure sensor (SP) and a discharge pressure sensor (DP) are illustrated and typically are already incorporated into the refrigerant system (100) for other control purposes. Alternatively, temperature sensors (T1, T2) and (T3, T4) may be utilized to measure water & refrigerant inlet / outlet temperature to the suction and discharge pressures (SP, DP) respectively. The system also comprises bypass line and various valves including NRVs after each discharge line connected in a known way are not shown.
[00031] Referring Figure 2 shows another embodiment of the present invention comprising all the embodiments of the system shown in the Figure 1. The system (200) shown in Figure 2 further comprises sensors to measure differential pressure across NRV (Non Return Valve) (126A, 126B) of discharge lines.
[00032] A controller (150) receives input signals from the various pressure and /or temperature sensors for operating compressors of the system (100, 200) according to the present invention. The controller (150) provides a desired capacity based on inputs from the sensors. The controller (150) monitors operating conditions of the refrigeration system to operate the tandem compressors (120A, 120B) to provide desired capacity. The controller (150) is configured to identify a refrigerant system fault based upon expected changes in said operating condition.
[00033] According to the present invention, the controller (150) comprises an unloading and loading module (not shown) for operating compressors. The said module configured for unloading the compressors step by step for a predetermined time period on detection of fault in the system and loading the compressor after the predetermined time and repeating said unloading and loading cycles until the system reaches normal operating condition. According to the present invention, if the system fails to reach normal operating condition after predetermined number of cycles, the controller either locks the compressors to the safe capacity or stops the faulty compressor and runs other compressor to provide best possible cooling.
[00034] Referring Figure 3 depicts a flow chart of the method of operating a refrigeration system in relation to the tandem compressors. The compressors in the tandem compressor configuration are run in a serial manner at periodic intervals, such as, for instance, during system startup or shutdown, and actual changes in corresponding operational parameters are compared to the expected changes. Alternatively, when extra capacity is demanded in the conditioned space and the next compressor is to be brought online, the corresponding operational parameters are monitored and the relevant changes are observed. The operating parameters such as electric current, power draw, pressures, temperatures in the compressors etc. are monitored within the refrigerant system with the help of sensors, and diagnostic procedures are performed in real time with the help of a predefined safety parameter for each operation parameter. If any one of operating parameters crosses its predefined safety parameter, then the controller is updated and a warning signal is issued. It also should be noted that the tandem compressor system shown in FIG. 1 is selected for illustration purpose only, as these system can be modified to be made more complex such as, for example, by incorporating additional condensers and/or evaporators, having more than two compressors, etc.
[00035] The unloading and loading module provided in the controller starts cycle of unloading and loading of the compressors when a warning signal is issued. The cycle of unloading and loading includes reducing capacity of the system by unloading the compressors step by step and loading the compressor to the desired capacity of the system after a predetermined period for providing best possible cooling. The best possible cooling is provided by repeating the cycle of unloading and loading the compressors until the system returns normal operation or locking the compressors to safe capacity if the system fails to return normal operation.
[00036] According to the present invention, the cycle of loading and unloading of the compressor is repeated for a predetermined number of times. Preferably, the predetermined number of cycles is preferably between three and five. Alternatively, a user can define the number of cycle of unloading and loading based on the type of chiller system and nature of complexity of the operating condition.
[00037] As shown in the flow chart in Figure 3, if the system fails to return normal operation after the predetermined number of the cycles, the controller module locks the compressors to best possible cooling. The locking of the compressor to best possible cooling includes the steps of determining the safe capacity of the system at which the compressor runs safely by using cycle of unloading and loading the compressors, comparing the capacity with a capacity provided by stopping the faulty compressor and running other compressors and deciding to run the system as safe capacity or stopping the faulty compressor for providing best possible cooling. For example, 200 Ton chiller with 2 compressors of 100 ton each arranged in tandem. The safety parameter is generally defined as 10% of total capacity as per the existing unloading and loading method. According to the present invention when a fault in operation condition arises, the controller will start reducing the capacity of the system by unloading the compressor step by step until the system reaches to safe parameter. If one of the compressors has some fault at 100% capacity when required cooling capacity is 200 ton, but running safe at 75% capacity, then after number of predetermined cycles of loading and unloading, the comptroller will lock the compressors 75% capacity to clear the safety instead of locking compressor as 10% of total capacity thereby provide maximum possible cooling. Whereas in another case, if required cooling load is 100 ton, then both compressors are generally run at 50 ton as an economical solution. When a fault arises, the controller starts cycle of unloading and loading compressor as per the present invention. If a compressor having fault fails run even if 25% capacity, then controller logic will stop the faulty compressor after predetermined cycling and run other compressor at full load i.e. at 100% capacity to provide cooling.
[00038] According to the present invention, the system comprises further one or more cycles of the unloading and loading with the user password protection if system is locked to a safe capacity after carrying out the predefined number of cycles and fails to return to the normal operation at the desired capacity. The password protection is provided to allow the user to study the system as well as stop the operator from resetting the system time to time thereby avoiding damage of the compressors due to the resetting of the compressors of the system number of times.
[00039] In reference to Figure 2, the method also comprises sensing differential discharge pressure (?P) across NRVs or discharge valves provided between the discharge pipelines of the compressors and common discharge pipeline and indicating fault with NRV or discharge valve if the differential discharge pressure across NRVs or discharge valves is increased. Accordingly, the controller stops the compressor as a precautionary measure.
[00040] The present invention provides a best possible cooling in case of the fault in the system having compressors arranged in tandem. Further, it also protects the compressors from damaging by allowing running at safe capacity. Moreover, it provides double protection for resetting the system. One more reset with user level password make aware to the concern about the existing problem to take some action. This will also ensure the nuisance caused sometime once or twice on site as after cycling if everything is normal, chiller start to works normal. Further, the present invention provides/indicates malfunctioning of the NRV’s thereby avoids various problems like reverse rotation of screw compressor, restriction to refrigerant flow may lead the centrifugal compressor to surge etc.
[00041] Although, the preferred embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. Further certain terms are not explained or values are not mentioned as the said terms or values are very well known in the field of the invention such as predetermined time is very well known which is generally defined based on the type of the chiller and capacity of the chiller and compressors are used and a user can set at his end also. For that reason, the following claims should be studied to determine the true scope and content of this invention: