FORM 2
THE PATENTS ACT, 1970 (39 of 1970)
& THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See section 10, Rule 13]
SYSTEM AND METHOD FOR
MAINTAINING OPTIMUM CONDENSING TEMPERATURE AT LOW LOAD IN HEATING MODE IN VRF SYSTEMS;
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 a variable refrigerant flow (VRF) system operating on a single mode at a time providing either heating or cooling in all zones and more particularly to heating mode of variable refrigerant flow (VRF) systems. BACKGROUND OF INVENTION
A variable refrigerant flow (VRF) or a variable refrigerant volume (VRV) is a system that comprises a plurality of indoor units (IDU) connected to a outdoor unit (ODU). The term variable refrigerant flow refers to the ability of the system to control the amount of refrigerant flowing to the multiple indoor units, enabling the use of many evaporators of differing capacities and configurations connected to a single outdoor unit. The VRF system adjust the system capacity as per the demand or load. For example, if demand is more, it increases the system capacity and if demand is low, it decreases the system capacity. The system capacity means the capacity of the compressors and adjusting of the system capacity means switching on/off fixed capacity compressors and varying capacity of the compressor of variable capacity type. The VRF system provides an individualized comfort control, and provides either heating or cooling. There are VRF systems that provides heating and cooling simultaneously in different zones. But those VRF systems are very expensive and in a country like India, either cooling or heating is required and hence the VRF systems generally operate on single mode only i.e. either cooling mode or heating mode at a time providing cooling or heating in all zones. In a typical VRF system, the refrigerant is used as cooling or heating medium and its flow

direction is varied in response to changes in* the cooling or heating requirement within
the air conditioned area.
A plurality of thermostats are used for sensing the temperature of a system so that the
system's temperature is maintained near a desired set point. The thermostat does this
by switching heating or cooling modes on or off, or regulating the flow of a refrigerant
fluid as needed, to maintain the required/set temperature.
However, in heating mode, as the number c>f thermostat-ON IDU quantity goes down,
condensing temperature of the VRF system also decreases. The minimum acceptable
condensing temperature is 38°C. If the condensing temperature fail below 38°C, the
heating capacity of the VRF system reduces and thus the required temperature or
customer set temperature is not achieved. This can be explained as follows:
As the quantity of OFF IDUs goes on increasing beyond a certain percentage, the
condensing temperature starts reducing because the system capacity falls down due to
less demand. At very few running IDUs and most of the OFF IDUs, the condensing
pressure is very less and hence the heating output is not sufficient and also sometimes
running IDU provides a cold air draft. As a result desired heating effect is not achieved
in the very few IDUs which are ON.
A solution to the above mentioned drawbacks is provided by a novel system and
method for maintaining the desired condensing temperature and the same is disclosed
herein.
SUMMARY OF THE INVENTION
An object of the present invention is to achieve desired heating effect when very few numbers of IDUs are ON during the heating mode operation of.variable refrigerant flow

(VRF) system having a plurality of indoor units (IDU) connected to a outdoor unit (ODU).
In accordance with this, a method and a system is disclosed for maintaining the optimum condensing temperature of a VRF system at low load in heating mode. Accordingly the method for maintaining optimum condensing temperature of refrigerant at low load in heating mode in VRF systems having a plurality of indoor units (IDU) connected to a outdoor unit (ODU) and said outdoor unit, the method comprises the steps of: adjusting system capacity in proportion to heating load and simultaneously sensing condensing temperature of the refrigerant, and increasing the system capacity upon sensing the condensing temperature falling below a threshold limit in order to reduce further falling in the condensing temperature thereby maintaining the condensing temperature within the acceptable range.
The present invention also provides a system for maintaining optimum condensing temperature of refrigerant at low load in heating mode, said system comprising a plurality of indoor units (IDU) connected to a outdoor unit (ODU) and a controller for sensing condensing temperature of the refrigerant, and increasing system capacity if the condensation temperature of the refrigerant falls below the threshold limit. According to the present invention, the threshold limit of the condensing temperature is 38 °C.
According to the present invention, the system capacity is increased inversely proportional to the condensing temperature of the refrigerant. Advantageously, the system capacity of the compressor is increased between 0-100% for the condensing temperature falling from 38 -25°C.

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 simple schematic of a heat pump VRF system connected to a plurality
of indoor units according to the present invention;
Fig. 2 shows a compressor of variable capacity;
Fig. 3 shows the loaded and unloaded state of the compressor of variable capacity;
Fig. 4 illustrates the cycle time consisting of loaded and unloaded state time segments
as per the present invention;
Fig. 5 is a graph comparing variations in condensing temperature as per the present
invention with the prior art; and
Fig. 6 shows the graph for compressor capacity variation as per the present invention.
DESCRIPTION OF THE INVENTION
Disclosed herein is a system and method for maintaining the optimum condensing temperature at low load in heating mode in the VRF systems which operates on one mode at a time i.e. either heating mode or cooling mode. According to the present invention, the system comprises a plurality of indoor units (IDU) connected to a outdoor unit (ODU) and a controller for sensing condensing temperature of the refrigerant. According to the system and method of the present invention, the controller is configured to vary the system capacity if and when the condensation temperature of the

refrigerant falls below the threshold limit. Advantageously, the controller increases the system capacity in reverse proportion to the condensing temperature. This reduces further falling in the condensing temperature and uniformly results in optimum condensing temperature within the acceptable range.
Fig. 1 shows the simple schematic of a VRF system (100) having an outdoor unit (110) connected to a plurality of indoor units (120) and a controller (130) according to the present invention according to the present invention. The outdoor unit (110) comprises compressor/s, heat exchanger coil, fan and motor assembly for sucking air over the heat exchanger, electronic expansion valve/s and various other known refrigeration system controls. The outdoor unit (110) according to the present invention as shown in Figure 1 comprises a compressor of variable capacity (104) and compressors (105) of fixed capacity.
Each indoor unit 102, comprises a heat exchanger, blower/fan and motor assembly for sucking air over the heat exchanger, electronic expansion valve which controls the amount of refrigerant to feed to heat exchanger, and electronic control system to control the fan speed, electronic expansion valve opening and calculate the cooling demand. Referring now to Fig. 2 shows a compressor of variable capacity type, and Fig. 3 shows the loaded and unloaded state of the digital compressor of variable capacity type. As mentioned earlier, the system capacity is varied by varying capacity of the compressor. The capacity of the compressors is varied to achieve the desired condensing temperature at low load in the heating mode in VRF systems. The variation of the compressor capacity of variable capacity is discussed herein below:

As per the present invention, the compressor is a digital scroll compressor 200. Said digital scroll compressor comprises atleast two scrolls (not shown in the figure) including a top scroll and a bottom scroll.
The top scroll of the digital scroll compressor (200) can be lifted axially upward as and when required. In other words, the top scroll of digital scroll compressor (200) has got two degree of freedom, which facilitates the scroll to move vertically up and down. Where as in case of a normal scroll compressor top scrollhas zero degree of freedom and it is at fixed position always.
The digital scroll compressor (200) further comprises a lift piston assembly 203 that is fixed to the top scroll to ensure that when the piston moves up, the top scroll also moves up. There is also a modulation chamber 201 at the top of the piston assembly 203 that is connected to the discharge pressure 305 through a bleed hole 202. In one embodiment herein the bleed hole 202 is preferably of 0.6 mm diameter. An external solenoid valve 204 connects the modulation chamber 201 with the suction side pressure 304.
When the solenoid valve 204 is in the normally closed position 204a, the pressure on either side of the piston 203 is the 'discharge pressure' 305 and a spring force 205 ensures that the two scrolls are loaded together. When the solenoid valve 204 is energized, the discharge gas 305 in the modulation chamber 201 is relieved to the low pressure. This causes the piston 203 to move up and consequently the top scroll also moves up. This action separates the scrolls and results in no mass flow through the scrolls. De-energizing the external solenoid valve 204 again loads the compressor 200 fully and the compression is resumed.

Since the movement of the top scroll is very small (of say about 1.0 mm), consequently the amount of high-pressure gas that is bled through the bleed hole 202 from the high side to the low side is very little.
At this point and as mentioned above, a cycle time of two time segments is defined as per the present invention for the purpose of capacity variation of the compressor. The cycle time comprises a loaded state time segment and an unloaded state time segment. Fig. 4 illustrates the cycle time consisting of loaded and unloaded state time segments as per the present invention.
The durations of each time segments may be varied to change the compressor capacity. For example, the cycle time is of 20 seconds. During the unloaded state, the valve is ON. During the loaded state, the valve is OFF. This means that out of 20 second cycle time, if valve is ON for 10 Seconds, then it is unloaded for 10 second, and there is no pumping. Thus there is Zero Capacity of the compressor. For rest of the 10 second the valve is OFF. This means that it's a loaded state and there is 100% capacity for 10 seconds. Thus capacity delivered = (Loaded time/Cycle Time)%
= 10/20%
= 50% Therefore, in a 20-second cycle time, if the loaded state time is 10 seconds and the unloaded state time is 10 seconds, then as per the present invention, the compressor modulation would be (10 seconds x 100% + 10 seconds x 0%)/ 20 = 50% . If for the same cycle time, the loaded state time is 15 seconds and the unloaded state time is 5 seconds, the compressor capacity is 75%.

Thus the system capacity is varied by using a variable capacity compressor according to the present invention. If the variable capacity compressor is running at full capacity and fixed compressors are off and if the condensing temperature falls below the threshold limit, then the controller starts atleast one fixed compressor and adjust the capacity of the variable capacity compressor if required to increase the system capacity in order to prevents falling of the condensing temperature from falling below acceptance range of the condensing temperature. The controller varies system capacity ranging from 0% to 100% for condensing temperature falling from 38 -25°C linearly according to the present invention.
Fig. 5 is a graph illustrating a comparison of variations in the condensing temperature as per the present invention with prior art. It is clear from the graph that as per the method of the present invention prevents falling of the condensing temperature from falling below acceptance range of the condensing temperature which is 38~46°C and thereby maintain the optimum condensing temperature.
Fig. 6 shows the graph for system capacity verses condensing temperature as per the present invention.
According to the present invention, as shown in the graph as the condensing temperature falls below certain value in heating mode, the controller (130) increases the capacity of the compressor in reverse proportion. Increase in capacity of the compressor reduce the falling trend in the condensing temperature and maintains it within the acceptable range. According to the present invention, the threshold limit of the condensing temperature is set to 38° C.
The increased in the system capacity according to the present invention is explain with an Example below. The following example is merely illustrative of preferred

embodiments of the invention. Many variations thereon may be made without departing from the spirit of the disclosed invention, as will be evident to those skilled in the art, and such variations are intended to come within the scope of what is claimed:
Example 1
A VRF system having plurality of IDUs and one ODU comprising one variable capacity compressor and two fixed capacity compressors. It is known that in heating mode, expansion valves of the thermostat-OFF IDUs are kept open to certain degree to bleed refrigerant and fans also run at low speed. This is done in order to avoid accumulation of liquid refrigerant in the OFF IDUs. Hence system capacity is maintained double the demand capacity in normal operation. When the condensing temperature of the said system in heat mode falls below 38°C, the system capacity is increased according to the present invention is shown in the Table 1 below:
Table 1

Condensing Temperature Capacity
Demand
(kW) System Capacity
variations according to
the Present invention System Capacity = Demand +
(Demand * Extra Capacity %) Increased
system
capacity (%)
43 100% 2.00 0
42 100% 2.00 0
41 100% 2.00 0
40 100% 2.00 0
39 100% 2.00 0
38 100% 2.00 0
37 108% 2.08 8
36 115% 2.15 15
35 123% 2.23 23
34 131% 2.31 31
33 138% 2.38 38

32
146% 2.46 46
31 154% 2.54 54
30 162% 2.62 62
29 169% 2.69 69
28 177% 2.77 77
27 185% 2.85 85
26 192% 2.92 92
25 200% 3.00 100
As per the present invention, and as illustrated clearly in the above table, if condensing temperature falls below 38 Deg C, then the actual compressor capacity is more than twice of the demand capacity and the ratio goes on further increasing as the condensing temperature goes further below 38 Deg C linearly. This slows down the rate of fall in condensing temperature and improves heating capacity. The system and method of present invention provides a continues hot air supply even if less number of IDU of VRF system are running in heat mode or irrespective of number of IDUs running in heat mode. It also avoids/reduces pressure fluctuations, when demand fluctuates, thereby increase in the life of the compressor. As the system and method of present invention provides continues hot air, it eliminates cold air draft from the IDU which happens in the prior art system when less IDUs are operating. 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 optimum condensing temperature of refrigerant at low
load in heating mode in VRF systems having a plurality of indoor units (IDU) connected
to a outdoor unit (ODU) and said outdoor unit, the method comprising the steps of:
adjusting system capacity in proportion to heating load and simultaneously sensing
condensing temperature of the refrigerant; and
increasing the system capacity upon sensing the condensing temperature falling below a threshold limit in order to reduce further falling in the condensing temperature thereby maintaining the condensing temperature within the acceptable range.
2. The method as claimed in claim 1, wherein the system capacity is increased inversely proportional to the condensing temperature of the refrigerant.
3. The method as in claim 1, wherein the threshold limit of the condensing temperature is 38 ° C.
4. The method as in any one of the preceding claims 1-3, wherein the system capacity of the compressor is increased between 0-100% for the condensing temperature falling from 38 -25°C linearly.
5. A system for maintaining optimum condensing temperature of refrigerant at low load in heating mode, said system comprising:
a plurality of indoor units (IDU) connected to a outdoor unit (ODU); and

a controller for sensing condensing temperature of the refrigerant and increasing system capacity if the condensation temperature of the refrigerant falls below the threshold limit.
6. The system as claimed in claim 5, wherein the controller increases system capacity inversely proportional to the condensing temperature of the refrigerant.
7. The system as claimed in claim 5, wherein the threshold limit of the condensing temperature is 38 ° C.
8. The system as in any one of the preceding claims 5-7, wherein the system capacity of the compressor is increased between 0-100% for the condensing temperature falling from 38 -25°C.