The present disclosure relates to an air conditioning apparatus with a sub-cooling unit.
BACKGROUND
An air conditioning apparatus is a device used to maintain a temperature of space lower than the temperature of the surrounding. The air conditioning apparatus works on a vapour compression cycle in which a refrigerant undergoes temperature, pressure, and phase change to achieve cooling of the space. The air conditioning apparatus includes a compressor that compresses the refrigerant, a condenser that removes heat from the compressed refrigerant, an expansion valve that lowers the pressure of the refrigerant coming from the condenser, and an evaporator that transfers the heat from the space to the refrigerant coming from the expansion valve and supplies the refrigerant back to the compressor. Generally, the performance of an air conditioning apparatus is measured as a coefficient of performance which is the ratio of the amount of heat removed from the space by the air conditioning apparatus and the amount of work done or energy is supplied to the air conditioning apparatus.
Conventional air conditioning apparatus have a lower coefficient of performance owing to their design and their operating environment. For instance, one of the factors that govern the coefficient of performance is the capability of the condenser to remove heat from the compressed refrigerant, which is further dependent on the ambient temperature of the surrounding. Accordingly, on a hot day, a high ambient temperature of the surroundings lowers the amount of heat removed by the condenser. One way to mitigate this issue is to add an additional heat exchanger to remove additional heat. While additional heat exchangers

remove the heat from the refrigerant, the additional heat exchangers increase the unit cost.
Another technique to improve the coefficient of performance is to use water to remove additional heat from the condenser. However, adding a water-based cooling system warrants changes in the design of the air conditioning unit.Usage of water in the system complicates the reliability of the product due to varied water quality available at each area of installation.
SUMMARY
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention. This summary is neither intended to identify key or essential inventive concepts of the invention and nor is it intended for determining the scope of the invention.
The present subject matter relates to a sub-cooling unit for an air conditioning apparatus. The sub-cooling unit removes a portion of heat from the refrigerant exiting from the condenser to the refrigerant exiting the evaporator without preventing superheating of refrigerant entering the compressor thereby marginal increase in load on the compressor with increased net cooling capacity.
In an embodiment, an air conditioning apparatus is disclosed. The air conditioning apparatus includes a compressor adapted to compress a refrigerant to form a compressed refrigerant and a condenser installed downstream to the compressor and adapted to release heat from the compressed refrigerant to ambient to form a condensed refrigerant. The air conditioning apparatus also includes an expansion device installed downstream to the condenser and adapted to reduce the condensed refrigerant pressure and temperature and an evaporator

installed downstream to the expansion device to transfer the heat from the surrounding area to the refrigerant and evaporate the refrigerant.
The air conditioning apparatus also includes a sub-cooling unit having a first supply line formed from the condenser to the expansion device. The sub-cooling unit also includes a second supply line formed from the evaporator exit to the compressor, wherein the second supply line is in thermal communication with the first supply line to extract heat from the condensed refrigerant in the first supply line with marginal superheating the refrigerant in the second supply line.
According to the present subject matter, the sub-cooling unit reduces liquid refrigerant temperature exiting the condenser heat exchanger thereby increasing the heat removing capacity of the refrigerant. Moreover, the sub-cooling unit removes the portion of heat from the condensed refrigerant with marginal superheating of the refrigerant with marginal burden on the compressor to compress the superheated refrigerant. Therefore, the amount of heat removal capacity is increased with marginal increased work input thereby increasing the COP of the air conditioning apparatus as compared with the COPs of the conventional air conditioning apparatus.
To further clarify advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Figure 1 illustrates a schematic of an air conditioning apparatus with a sub-cooling unit, according to an embodiment of the present disclosure;
Figure 2 illustrates a schematic of the sub-cooling unit, according to an embodiment of the present disclosure;
Figure 3 illustrates a temperature vs specific entropy diagram of the refrigeration cycle of the air conditioning apparatus, according to an embodiment of the present disclosure; and
Figures 4 illustrates a pressure vs enthalpy diagram of the refrigeration cycle of the air conditioning apparatus, according to an embodiment of the present disclosure;
Figure 5 illustrates typical pressure vs enthalpy diagram of refrigerant cycle with liquid suction sub cooling heat exchanger.
Further, skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have necessarily been drawn to scale. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.
DETAILED DESCRIPTION OF FIGURES
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the

drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.
For example, the term "some" as used herein may be understood as "none" or "one" or "more than one" or "all." Therefore, the terms "none," "one," "more than one," "more than one, but not all" or "all" would fall under the definition of "some." It should be appreciated by a person skilled in the art that the terminology and structure employed herein is for describing, teaching, and illuminating some embodiments and their specific features and elements and therefore, should not be construed to limit, restrict or reduce the spirit and scope of the present disclosure in any way.
For example, any terms used herein such as, "includes," "comprises," "has," "consists," and similar grammatical variants do not specify an exact limitation or restriction, and certainly do not exclude the possible addition of one or more features or elements, unless otherwise stated. Further, such terms must not be taken to exclude the possible removal of one or more of the listed features and elements, unless otherwise stated, for example, by using the limiting language including, but not limited to, "must comprise" or "needs to include."
Whether or not a certain feature or element was limited to being used only once, it may still be referred to as "one or more features" or "one or more

elements" or "at least one feature" or "at least one element." Furthermore, the use of the terms "one or more" or "at least one" feature or element do not preclude there being none of that feature or element, unless otherwise specified by limiting language including, but not limited to, "there needs to be one or more..." or "one or more elements is required."
Unless otherwise defined, all terms and especially any technical and/or scientific terms, used herein may be taken to have the same meaning as commonly understood by a person ordinarily skilled in the art.
Reference is made herein to some "embodiments." It should be understood that an embodiment is an example of a possible implementation of any features and/or elements of the present disclosure. Some embodiments have been described for the purpose of explaining one or more of the potential ways in which the specific features and/or elements of the proposed disclosure fulfil the requirements of uniqueness, utility, and non-obviousness.
Use of the phrases and/or terms including, but not limited to, "a first embodiment," "a further embodiment," "an alternate embodiment," "one embodiment," "an embodiment," "multiple embodiments," "some embodiments," "other embodiments," "further embodiment", "furthermore embodiment", "additional embodiment" or other variants thereof do not necessarily refer to the same embodiments. Unless otherwise specified, one or more particular features and/or elements described in connection with one or more embodiments may be found in one embodiment, or may be found in more than one embodiment, or may be found in all embodiments, or may be found in no embodiments. Although one or more features and/or elements may be described herein in the context of only a single embodiment, or in the context of more than one embodiment, or in the context of all embodiments, the features and/or elements may instead be provided separately or in any appropriate combination or not at all. Conversely, any

features and/or elements described in the context of separate embodiments may alternatively be realized as existing together in the context of a single embodiment.
Any particular and all details set forth herein are used in the context of some embodiments and therefore should not necessarily be taken as limiting factors to the proposed disclosure.
Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
For the sake of clarity, the first digit of a reference numeral of each component of the present disclosure is indicative of the Figure number, in which the corresponding component is shown. For example, reference numerals starting with digit "1" are shown at least in Figure 1. Similarly, reference numerals starting with digit "2" are shown at least in Figure 2.
Figure 1 illustrates an air conditioning apparatus 100 having a sub-cooling unit 102, according to an embodiment of the present disclosure. The air conditioning apparatus 100 is designed to provide cooling to a space, such as a room or a hall by maintaining comfort cooling in the space. The air conditioning apparatus 100 may also be referred to as a Heating, ventilation, and air conditioning (HVAC) apparatus that not only regulates the temperature of the space but also regulates other aspects of the space, such as humidity. The air conditioning apparatus 100, as the name suggests, cools the air, controls the humidity and dust particulate level in the space. Moreover, air conditioning apparatus 100 relies on the ambient air to discharge heat generated during the operation of the air conditioning apparatus 100.
The air conditioning apparatus 100 may include, but is not limited to, the sub-cooling unit 102, a compressor 104, a condenser 106, an expansion device

108, and an evaporator 110, details of which will be provided in the subsequent embodiments.
In an example, the compressor 104 is designed compress a low temperature and low-pressure refrigerant to form compressed refrigerant at high pressure and high temperature. In an example, the compressor 104 can be a reciprocating/ rotary/scroll type compressor that may have high pressure of the compressed refrigerant. Further, the refrigerant can be R410A, R32, R-290 or R-600A. The compressor 104 may be designed to supply the refrigerant at a predefined first supply rate. Alternatively, the compressor 104 may supply the refrigerant at a variable supply rate based on a cooling requirement.
As shown in Figure 1, the condenser 106 received the compressed refrigerant from the compressor 104. The condenser 106 designed to release heat from the high pressure and high temperature compressed refrigerant to form condensed liquid refrigerant. The condenser 106, in one example, can be a heat exchanger with fins adapted to remove the heat accumulated in the compressed refrigerant by discharging the heat to the ambient air. Although not shown, the condenser 106 may have a fan that flows air through the fins of the condenser 106 to increase the heat discharge from the condenser 106. The refrigerant exiting from the condenser 106 after losing the heat may be termed as the condensed refrigerant may be in liquid form.
Downstream to the condenser 106 is the expansion device 108 that may be adapted to reduce the pressure and temperature of the condensed refrigerant. In one example, the expansion device 108 converts the condensed liquid refrigerant at high temperature and high pressure into a vapor refrigerant. The reduction in the pressure causes the condensed refrigerant to cool further down. The evaporator 110 installed downstream to the expansion device 108 is adapted to receive the refrigerant exiting from the expansion device 108. The evaporator 110 transfers the heat from the surrounding to the vapor refrigerant and evaporate the

refrigerant to gas at low temperature and pressure. In one example, the evaporator 110 is a heat exchanger that transfers the heat from the air in the space to the refrigerant in the evaporator 110. Accordingly, the evaporator 110 cools the air and heats the refrigerant. The refrigerant exiting the evaporator is supplied back to the compressor 104 via the sub-cooling unit 102.
Details of the sub-cooling unit 102 is now explained with respect to detailed schematic in Figure 2 in conjunction with Figure 1. According to the present disclosure, the sub-cooling unit 102 is installed in such a way that the refrigerant exiting from the condenser 106 and the refrigerant exiting the evaporator 110 passes through the sub-cooling unit 102. Accordingly, the sub-cooling unit 102 may include a first supply line 112 and a second supply line 114 that is in thermal communication with each other. In other words, the first supply line 112 and the second supply line 114 can transfer heat therebetween. The first supply line 112 is connected to a hose from the condenser 106 and the expansion device 110. In other words, the first supply line 112 is coupled to a condenser outlet and an expansion device inlet. Similarly, the second supply line 114 is coupled to an evaporator outlet and a compressor inlet. Further, the sub-cooling unit 102 is a coaxial heat exchanger that works on a counter-flow heat transfer principle.
In one example, the sub-cooling unit 102 performs two tasks. First, the sub-cooling unit 102 provides a low temperature to the refrigerant exiting the condenser 106 which is lower than condensing temperature which enhances the heat removal from the refrigerant. Second, the sub-cooling unit 102 prevents the liquid refrigerant entering to the compressor 104 with marginal increased load on the compressor 104. Moreover, the sub-cooling unit 102 is thermally isolated from the surrounding, such as the ambient air, such that the heat transfer between the first supply line and the second supply line is not affected by the surrounding. A manner in which the sub-cooling unit 102 operates is explained with respect to Figures 3 to 5.

Figure 3 illustrates a temperature vs. specific entropy (T-S) diagram 300 of the refrigeration cycle of the air conditioning apparatus 100 while Figures 4 and 5 illustrate pressure vs enthalpy (PH) diagrams 400 and 500 respectively of the refrigeration cycle of the air conditioning apparatus 100. On the other hand, as shown in Figure 1, the flow of refrigerant at different aforementioned components are indicated with numerals 1, 2, 3, 4, 5, and 6 which corresponds to the corresponding data points 1, 2, 3, 4, 5, and 6 in Figures 3 and 4. In one example, the refrigerant is supplied to the compressor 104 at a low temperature and pressure. The refrigerant is compressed by the compressor 104 resulting in high temperature as shown by lines between points 1-2 in Figure 3 and high pressure/ high enthalpy of the compressed refrigerant as shown by lines between points 1-2 in Figures 4 and 5. The compressed refrigerant is in the gaseous phase while exiting the compressor 104.
The compressed refrigerant enters the condenser 106 in which the heat of the compressed refrigerant is released to the ambient air. The condenser 106 reduces the heat shown as a reduction in enthalpy by the line 2-3 in Figures 4 and 5. Similarly, the temperature also drops from point 2 to an arbitrary point 7 and then remains constant while the entropy further drop. The drop in heat and enthalpy causes phase change resulting in the liquefied condensed refrigerant. The liquefied condensed refrigerant then enters the first supply line 112 in which the liquified condensed refrigerant lose a portion of heat at constant pressure as shown by lines 3-4 in Figures 4 and 5. This removal of heat increases the heat removal capacity of the liquified condensed refrigerant.
The liquefied condensed refrigerant exiting the first supply line 112 enters the expansion device 108 in which the expansion device 108 reduces the pressure of the liquified condensed refrigerant. The drop in the pressure as shown by lines 4-5 in Figures 4 and 5 causes the temperature drop as shown by lines 4-5 in Figure 3. The cooled refrigerant then enters the evaporator 110 in which the air in the space releases its heat to the cooled refrigerant in the evaporator 110 thereby

becoming cooled air. At the same time, the cooled refrigerant starts boiling and exits the evaporator 110 shown by point 6.
Thereafter, the refrigerant enters the second supply line 114 in which the portion of the heat discharged by the liquified condensed refrigerant in the first supply line 112 is absorbed by the refrigerant in the second supply lines 114. Since the temperature of the second supply line 114 is lower than the temperature of the ambient air, the liquified condensed refrigerant is able to release additional heat to the refrigerant in the second supply line 114 as shown by arrow line A. The receipt of heat by the second supply line is indicated by the lines 6-1 which cause an increase in temperature shown in Figure 3 and an increase in enthalpy shown in Figures 4 and 5.
In one example, the lower temperature of the second supply line 114 can cause a greater amount of heat transfer to the refrigerant which can superheat the refrigerant. However, the second supply line 114 is designed in such a way that the refrigerant is prevented from superheating above 3 deg C of evaporating temperature. In one example, the second supply line 114 cool the refrigerant in the first supply line 112 by 4-6 deg C below condensing temperature. Restricting superheating of the refrigerant temperature up to 3 deg C above evaporating temperature helps in marginal increase in load on the compressor 104 to compress the refrigerant thereby enhancing the COP of the air conditioning apparatus 100. In one example, the first supply line 112 and the second supply line 114 is sized, such that the flow rate of the liquified condensed refrigerant in the first supply line 112 is greater than the flow rate of the refrigerated in the second supply line 114. The lower flow rate in the second supply line 114 ensures the refrigerant in the second supply line 114 does not receive excess heat and thus restricting superheating of the refrigerant up to 3 deg C above evaporating temperature. In one example, the temperature of the refrigerant exiting the second supply line 114 is maintained at the gaseous phase with a temperature less than the optimum temperature range of compressor 104. Moreover, the temperature of the

refrigerant exiting the second supply line 114 is maintained based on the optimal efficiency of the compressor 104.
In order to achieve the aforementioned objective, the sub-cooling unit 102 is designed based on the type of refrigerant, the COP to be achieved by the air conditioning apparatus 100. An exemplary graph indicating the calculation is shown in Figure 5. The COP is calculated by the following formula:
q + dq
COP =— —
W + dW
Assuming that there is no heat exchange between the surroundings and the sub-cooling unit 102 and negligible kinetic and potential energy changes across the sub-cooling unit 102, then, the heat transferred between the refrigerant liquid and vapour in the sub-cooling unit 102, QLSHX is given by:
QL3IK=inr(h}-h^)=mr(h1-h6)
In which the mr indicates the mass of refrigerant and hi to h6 corresponds to the enthalpy of the refrigerant at the data points 1 to 6. Similarly, in order to design the coaxial heat exchanger, the following formulas are used. Co axial heat exchanger design:
Total heat transfer between liquid refrigerant to suction line is

At

Rtot
*w I
h1A] + 2iriU A 1 2*2
LMTD = AV ;.%
InCA^/A^)
where A/^ = temperature difference between two fluids at position A, K AtB = temperature difference between two fluids at position B, K
Nusselt number calculation:

•-©"(#*
Nu = C Re" Pr"1 where n and m are exponents. The constant Cand exponents in the equation are
hD
T
where /i = convection coefficient, W/m2 * K D = IDof tube.m
A = thermal conductivity of fluid, W/m * K K = mean velocity of fluid, m/s p = density of fluid, kg/m3 IJ - viscosity of fluid, Pa * s c - specific heat of fluid, J/kg ■ K
Pressure drop calculation details:
L V2 D 2
where Ap = pressure drop, Pa
/= friction factor, dimensionless L ■ length of tube, m

Using above co -axial heat exchanger formulas, first supply line 112 and second supply line 114 areas are arrived to achieve temperature drop of 4-6 deg C in first supply line-112 and 3 deg C super heating above evaporating temperature in the second supply line 114. Based on practical data, optimum COP is achieved with the above temperature range only. Higher temperature drops in first supply line 112 and second supply line 114 will lead to no improvement on COP.
While specific language has been used to describe the present disclosure, any limitations arising on account thereto, are not intended. As would be apparent to a person in the art, various working modifications may be made to the method in order to implement the inventive concept as taught herein. The drawings and the foregoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment.

Claim:

An air conditioning apparatus (100) working on vapor compression refrigeration principle, the air conditioning apparatus (100) comprising:
a compressor (104) adapted to compress a refrigerant to form compressed refrigerant; and
a condenser (106) installed downstream to the compressor (104) and adapted to release heat from the compressed refrigerant to form condensed refrigerant;
an expansion device (108) installed downstream to the condenser (106) and adapted to convert the condensed refrigerant into a cooled refrigerant;
an evaporator (110) installed downstream to the expansion device (108) to transfer the heat from surrounding to the cooled refrigerant and evaporate the refrigerant; and
a sub-cooling unit (102) comprising:
a first supply line formed from the condenser (106) to the
expansion device (108); and
a second supply line formed from the evaporator (110) to the
compressor (104), wherein the second supply lines is in thermal
communication with the first supply line to extracts heat from the
condensed refrigerant in the first supply line with marginal
superheating the liquid refrigerant in the second supply line.
The air conditioning apparatus (100) as claimed in claim 1, wherein phase of the condensed refrigerant exiting the first supply line is maintained at liquid phase.
The air conditioning apparatus (100) as claimed in claim 1, wherein a flow rate of the refrigerant in the first supply line is greater than a flow rate of the refrigerant in the second supply line.

4. The air conditioning apparatus (100) as claimed in claim 1, wherein the sub-cooling unit (102) is thermally isolated from the surrounding.
5. The air conditioning apparatus (100) as claimed in claim 1, wherein the sub-cooling unit (102) is a co-axial heat exchanger.
6. The air conditioning apparatus (100) as claimed in claim 1, wherein a temperature of the refrigerant exiting the second supply line is maintained based on optimal efficiency of the compressor (104).
7. A sub-cooling unit (102) for an air conditioning apparatus (100) working on vapor compression refrigeration principle, the sub-cooling unit (102):
a first supply line downstream to a condenser (106) and upstream to an expansion device (108) and adapted to receive a refrigerant of a first flow rate and a first temperature; and
a second supply line downstream to the evaoprator (110) and adapted to discharge refrigerant at a second flow rate and at a second temperature, wherein the second supply lines extracts heat from the condensed refrigerant in the first supply line and preventing the second temperature exceed from a superheating temperature of 3 deg C above evaporating temperature of the refrigerant in the second supply line.
8. The sub-cooling unit (102) as claimed in claim 7, wherein the first flow rate is greater than the second flow rate.
9. The sub-cooling unit (102) as claimed in claim 7, wherein phase of the refrigerant exiting the first supply line is maintained at liquid phase.
10. The sub-cooling unit (102) as claimed in claim 7, wherein the second temperature is maintained at gaseous phase with temperature less than optimum temperature range of compressor (104).