FORM 2
THE PATENTS ACT, 1970 (39 Of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See section 10, Rule 13]
METHOD AND SYSTEM FOR
MAINTAINING UNINTERRUPTED
COOLING OPERATION BY VRF SYSTEMS AT HIGH PRESSURE;
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
THE FOLLOWING SPECIFICATION
PARTICULARLY DESCRIBES THE
INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

FIELD OF INVENTION
The present invention relates to refrigerant systems and more particularly to cool mode operation of variable refrigerant flow (VRF) systems. The present intention more particularly related to a variable refrigerant flow systems comprising a digital scroll compressor.
BACKGROUND OF INVENTION
A variable refrigerant flow (VRF) or a variable refrigerant volume (VRV) is a system that comprises a plurality of indoor units connected to a modular outdoor condensing unit. In a typical VRF system, the refrigerant is used as the cooling or heating medium and its How is varied in response to changes in the cooling or heating requirement within the air conditioned area. In VRF or VRV systems, either digital scroll compressor or inverter compressor are used. In digital scroll compressors, the motor runs continuously at full speed, and the capacity of the compressor is adjusted by a digitally controlled mechanical unloader (slide bar) which adjusts the vertical positioning of the fixed scroll with respect to the orbital scroll thereby reducing its capacity. In inverter scroll compressors, the relative positions of the scrolls does not change, but the speed of the motor is adjusted by a variable speed drive, which adjusts the capacity. However, the inverter scroll compressor is more energy efficient at part load conditions than a comparable digital scroll compressor. The digital scroll compressor will more closely match a specified load, but is not as energy efficient as a comparable inverter driven

compressor. Both types of compressors have associated built-in mechanisms to maintain compressor lubrication at low flow.
In cooling mode at higher ambient temperature condition, the discharge pressure of the system reaches to safety trip limit and stops the system operation. This interrupts the cooling operation.
In an inverter fitted VRF system, it is very easy to control the system high pressure below certain limit. Here, when system high pressure goes above this limit, the operating frequency of the inverter is reduced to reduce the operating high pressure of the system and thereby preventing the system to trip on high pressure protection.
However, in case of a digital scroll compressor fitted VRF system, unloading the compressor does not help to reduce the high pressure. This is because while unloading the digital scroll compressor, it loads for few seconds and the system discharge pressure increases to trip limit in those few seconds of loaded condition.
Fig. 1 shows the difference between peak pressures obtained as in case of (a) digital scroll compressor and (b) inverter compressor. As can be seen in the figure, in case of a digital scroll compressor, high pressure fluctuates when the compressor is in part load condition as the compressor loads and unloads continuously. During loaded state, pressure increases and reduces during unloaded state. This keeps on happening continuously. In other words, the peak pressure at part toad is not reduced. However, in inverter compressor, high pressure reduces when the compressor is in part load condition as the motor frequency reduces and pumping rate comes down. Motor runs at lower RPM at part load.

Further, unloading of a digital scroll compressor, means reducing the PWM (Pulse Width Modulation) cycle from 100% to some less value. But, at any value of PWM, there would be certain amount of time during which the digital scroll compressor is in loaded state. During that period the system pressure is going to rise.
For example and as shown in Fig. 2, in a cycle time of say 20 seconds, and 60% PWM, the compressor would be loaded for 12 seconds and would be unloaded for 8 seconds. During the period of 12 seconds of loading time, the system high pressure increases and reaches to high pressure protection trip point. It means at any PWM the maximum pressure remains almost same. This does not happen in case of inverter compressor. Inverter compressors simply run at lower rotation per minute (RPM) in unloaded state hence high pressure is limited. However, digital scroll compressor systems are still used as the Digital Scroll compressor can change capacity output instantaneously compare to inverter compressor which has to go through the intermediate stages as the speed requirement changes. Further the loaded and unloaded cycle operation of the Digital Scroll compressor helps in better dehumidification compare to inverter compressor. Furthermore systems comprising inverter compressor are highly expensive than the systems comprising digital scroll compressor.
Hence there is a need of a system or method that solves above mentioned problem in the VRF system using a digital scroll compressor for providing uninterrupted cooling operation at high pressure which may takes place due to higher ambient temperature conditions.

SUMMARY OF THE INVENTION
Accordingly, the present invention provides a system and a method to maintain uninterrupted cooling operation preferably at high pressure which may happens due to higher ambient temperature condition during the cooling mode operation of variable refrigerant flow (VRF) systems having a digital scroll compressor.
According to the system and method of present invention, when pressure of the VRF system becomes equal to or goes beyond a set high pressure limit, the fix-speed compressor is put "OFF" and PWM of the digital scroll compressor is increased to 100% for a duration in order to provide an uninterrupted cooling operation. Accordingly the system comprises a controller for sensing pressure of the system in real time and configured with the compressor of the VRF system.
According to the present invention, a counter of duration of 'n' number of seconds is provided to initialized during this duration wherein the fix speed compressor is kept to 'OFF' condition and PWM of digital scroll compressor is retained to 100%. According to the present invention the controller keep sensing the pressure of the system in the said duration. If the pressure of the system reduces below the set high pressure, the system returns to normal operation else the counter is reinitialized by keeping fix-speed compressor is put "OFF" and PWM of the digital scroll compressor is increased to 100%.
According to the present invention, the a duration is 10 minutes.
According to present invention, the high pressure limit may be 605 PSIg.
BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
Fig. 1 shows the peak pressures as in case of (a) digital scroll compressor and (b) inverter compressor;
Fig. 2 illustrates fluctuation in system high pressure with respect to the pulse width modulation (PWM);
Fig. 3 is a flowchart illustrating the method for maintaining uninterrupted cooling operation at higher ambient temperature conditions in VRF Systems as per the present invention; and
Fig. 4 is a block diagram of a VRF system according to the present invention.
DESCRIPTION OF THE INVENTION
Disclosed herein is a VRF system and method for maintaining uninterrupted cooling operation during the cool mode operation of variable refrigerant flow (VRF) systems preferably at higher ambient temperature condition.
Figure 4 shows a VRF system (400) according to the present invention. The VRF system for maintaining uninterrupted cooling operation at higher ambient temperature conditions comprises a plurality of indoor units (not shown) connected to atleast one outdoor unit (420) having one digital scroll compressor (428) and one fix-speed compressor (424) and a controller (430) for sensing a pressure of the system in real time and configured

with the said compressor. The said controller (430) is configured to switch off the fix-speed compressor and increase PWM of digital scroll compressor to 100% for a duration in order to provide uninterrupted cooling operation at higher ambient temperature conditions.
Fig. 3 is a flowchart illustrating the method for maintaining uninterrupted cooling operation at higher ambient temperature conditions in VRF systems having a digital scroll compressor and atleast one fix speed compressor as per the present invention. According toe the present invention, the VRF system comprises a controller to sense the pressure and configured with the compressors.
As shown in Figure 3, when the pressure of the system is normal, the system runs in a normal way and the controller sense the pressure in real time. When the pressure of the system becomes equal to or crosses the set high pressure limit then the controller sense the pressure, and putt 'OFF' the fix-speed compressor and simultaneously increases PWM of the digital scroll compressor to 100% as shown in step 303 in order to provide uninterrupted cooling operation at high ambient temperature.
As shown in step 304, a counter of duration of 'n' number of seconds of the digital scroll compressor is initialized. The controller sense the pressure in real time and if the pressure of the system reduces below the set high pressure, the system returns to normal operation. However, if the pressure does not decrease below the set the high pressure, then the counter is reinitialised and continues by keeping , the fix-speed compressor on 'OFF" condition and retaining PWM of digital scroll compressor to 100% until a normal cooling operation is obtained as shown in step 306.

According to the present invention, the compressor is a Digital Scroll Compressor of variable capacity type. The counter may be set for duration of 10 minutes.
According to the present invention, the set high pressure limit is 605 PSIg. However, the set high pressure can be differ based on the capacity of the compressor, VRF and atmospheric conditions where the system is installed.
As explained above, the present invention provides a system and method that provides uninterrupted cooling operation at high pressure. The cooling may get reduce but the present invention avoids tripping of the compressor thereby increases the life of the compressor.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow:

We Claim
1. A method for maintaining uninterrupted cooling operation at higher ambient
temperature conditions in VRF Systems having a digital scroll compressor and atieast
one fix speed compressor, the method comprising the steps of:
sensing a pressure of VRF system;
putting OFF the fix speed compressor if the pressure of the system is equal to or crosses set high pressure limit and increasing digital scroll compressor PWM to 100% in order to provide uninterrupted cooling operation at higher ambient temperature conditions
initializing a counter of duration of 'n' number of seconds; and
keeping the fix speed compressor to 'OFF' and PWM of digital scroll compressor to 100% in the said duration in order to reduce pressure below the set high pressure.
2. The method as in claim 1 .comprises further step of reinitializing the counter if the
pressure does not decrease below the set high pressure limit and continuing the said
step until the pressure reduces below the set high pressure.
3. The method as in claim 1, wherein the duration of the counter is set for 10 minutes.
4. The method as in claim 1, wherein the set high pressure limit is 605 PSIg.
5. A VRF system for maintaining uninterrupted cooling operation at higher ambient temperature conditions comprising a plurality of indoor units connected to a outdoor unit

having one digital scroll compressor and atleast one fix-speed compressor and a controller unit for sensing a pressure of the system in real time and configured with the said compressor wherein said controller switches off the fix-speed compressor and increases PWM of digital scroll compressor to 100% for a duration in order to provide uninterrupted cooling operation at higher ambient temperature conditions.
6. The VRF system as claimed in claim 5 wherein the controller also initialises a counter of duration of 'n' number of seconds for maintaining PWM of digital scroll compressor to 100% in order to reduce pressure below the set high pressure.
7. The VRF system as claimed in claim 5 or 6 wherein the duration of the counter is preferably set for 10 minutes.
8. The VRF system as in claim 1, wherein the set high pressure limit is 605 PSIg.