Process fluid thermal management with a supplementary cooling system

FIELD OFTHE INVENTION

The present invention broadly relates to a supplementary cooling system. The invention also generally relates to a method of supplementary cooling of a process fluid.

BACKGROUND TO THE INVENTION

Traditionally most refrigeration and air conditioning systems (R&AC) struggle to meet the cooling demands on days of high ambient temperature. Although the condensers are chosen for a design ambient condition for a specific location, due to global warming, the number of high ambient temperature days (degree days per year) are gradually creeping up. For example, although condensers in R&AC plants in Australia are chosen for a 42 ⁰ C condensation with a typical 5 ⁰ C temperature differential for heat transfer, there are typically 10 or more days in a year when the ambient can be above 37°C, augmenting condensation temperatures well above the design conditions. On such days either the temperatures in the freezers and medium temperature display cases tend to increase or the chilled water temperature in AC plants tend to increase.

Liquid to suction sub-cooling is a traditional method used in most plants mainly to ensure that there is no liquid entrainmβnt in the suction line. This also provides some marginal relief in reducing the liquid temperature from the condenser. However, on high ambient temperature days any increase in the suction gas temperature will also demand a higher condensing capacity which is already at a premium. Thus, this method will be counter-productive.

Basically, any reduction of liquid refrigerant temperature below ambient is possible only by refrigeration. There are several ways in which this can be obtained. One method is to use the liquid refrigerant in the main system to provide this additional high ambient day cooling. This is done by flashing a small fraction of liquid and using the refrigeration capacity created from there. But this again will be counter-productive since, indeed, more refrigerant is required on such days. This is further compounded by a loss in volumetric efficiency associated with higher condensing pressures. It may be recalled that the liquid refrigerant needs to be cooled from above 45°C to about 37 ⁰ C, which is typical liquid inlet temperature for most cooling systems.

US Patent No. 3,852,974 to Brown describes a method of cooling the liquid refrigerant line to achieve the subcooling. This method requires a large and/or a long liquid refrigerant line to accommodate the necessary cooling surface area. Most modern refrigeration systems are built to be compact with the shortest possible line runs mainly to reduce the total amount of refrigerant charge held in a cooling system and such a direct liquid line cooling to achieve subcooling may not be feasible.

Maintaining oil temperatures below some set value in the case of cooling of transformers and hydraulic motors on high ambient days poses a serious industrial problem when only heat rejection to ambient is resorted to.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a supplementary cooling system comprising: a fluid bypass line adapted to connect to and bypass at least a portion of a process fluid line; one or more heat exchangers connected to the fluid bypass line; and a coolant fluid recirculation line connected to each of the heat exchangers whereby coolant fluid of the recirculation line exchanges heat with process fluid in the fluid bypass line thereby cooling the process fluid for return to the process fluid line.

Preferably the one or more heat exchangers are plate heat exchangers. More preferably the one or more heat exchangers are arranged as a bank of plate heat exchangers in parallel.

Preferably the heat exchangers) is retrofitted to the fluid bypass line.

Preferably the process fluid is a refrigerant Alternately the process fluid includes oil or a liquid hydrocarbon.

Preferably the coolant fluid is water or a refrigerant. More preferably the water is chilled water.

Preferably the coolant fluid recirculation line is a chilled water line. More preferably the chilled water line is operatively coupled to a refrigeration circuit designed to cool the chilled water line. Alternatively the coolant fluid recirculation line is a refrigerant circuit.

Preferably the supplementary cooling system also comprises a process fluid controller operatively coupled to each of the process fluid line and the fluid bypass line for controlling the relative portion of process fluid flowing in each of the bypassed portion of the process fluid line and the fluid bypass line. More preferably the fluid bypass line and the bypassed portion of the process fluid line each include a fluid control valve, operatively coupled to the process fluid controller.

Preferably the supplementary cooling system also comprises a coolant fluid controller operatively coupled to the coolant fluid recirculation line for selective actuation to control flow of the coolant fluid. More preferably the coolant fluid controller is coupled to a coolant valve connected to the coolant recirculation line and configured to open or close in response to predetermined conditions.

Preferably the supplementary cooling system further comprises a temperature and/or pressure sensor connected to the process fluid line and configured to actuate the coolant fluid controller at predetermined temperature and/or pressure, respectively.

Generally the supplementary cooling system is a standalone system.

According to another aspect of the invention there is provided a method of supplementary cooling of a process fluid, said method comprising the steps of: diverting at least a portion of the process fluid from a process fluid line to a fluid bypass line; exchanging heat from the process fluid in the fluid bypass line with a coolant fluid; and returning the cooled process fluid to the process fluid line.

According to yet another aspect of the invention there is provided a supplementary cooling system comprising: one or more heat exchangers adapted to connect to a refrigerant line; and a chilled water line connected to each of the heat exchangers whereby chilled water in the chilled water line exchanges heat with refrigerant of the refrigerant line thereby cooling the refrigerant .

Preferably the supplementary cooling system also includes a fluid bypass line adapted to bypass a portion of the refrigerant line.

Preferably the one or more heat exchangers are plate heat exchangers. More preferably the one or more heat exchangers are arranged as a bank of plate heat exchangers in parallel.

Preferably the heat exchanger(s) is retrofitted to the refrigerant line.

Preferably the supplementary cooling system also comprises a refrigerant controller operatively coupled to each of the refrigerant line and the fluid bypass line for controlling the relative portion of refrigerant flowing in each of the bypassed portion of the refrigerant line and the fluid bypass line. More preferably the fluid bypass line and the bypassed portion of the refrigerant line each include a fluid control valve, operatively coupled to the refrigerant controller.

Preferably the supplementary cooling system also comprises a chilled water controller operatively coupled to the chilled water line for selective actuation to control flow of the chilled water. More preferably the chilled water controller is coupled to a chilled water valve connected to the chilled water line and configured to open or close in response to predetermined conditions.

Preferably the supplementary cooling system further comprises a temperature and/or pressure sensor connected to the refrigerant line and configured to actuate the coolant fluid controller at predetermined temperature and/or pressure, respectively.

Generally the supplementary cooling system is a standalone system.

According to still another aspect of the Invention there is provided a method of supplementary cooling of a refrigerant by exchanging heat from the refrigerant with chilled water.

According to still yet another aspect of the invention there is provided a method of controlling a supplementary cooling system designed to cool a process fluid, said method comprising the steps of: determining at least one of the following process parameters:

(a) if the ambient temperature exceeds a predetermined value;

(b) if the temperature of the process fluid exceeds a predetermined value; (c) where the process fluid is a refrigerant in a refrigeration system

{i) if, for a first time, head pressure of the process fluid exceeds a first predetermined value; and

(ii) if, for a second time, following a time delay the head pressure of the process fluid exceeds a second predetermined value; (d) where the process fluid Is a refrigerant in a refrigeration system

0) if. for a first time, the superheat of the refrigerant after expansion exceeds a first predetermined value; and

(il) if, for a second time, following a time delay the expanded refrigerant exceeds a second predetermined value; and where said at least one of the process parameters is satisfied, selectively actuating a coolant fluid controller operatively coupled to a coolant fluid line to control flow of coolant fluid for cooling of the process fluid.

Preferably the step of determining if the temperature of the process fluid exceeds a predetermined value includes the step of comparing the temperature of the process fluid between a compressor and a condenser of the refrigeration system with the predetermined value.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

One or more preferred embodiments of a supplementary cooling system of the invention will be described with reference to the accompanying drawings.

Fig. 1 An embodiment of the supplementary cooling system.

Fig. 2 An example of the use of the supplementary cooling system.

Fig. 3 Another example of the use of the supplementary cooling system.

Fig. 4 An example of the algorithm used by the coolant fluid controller.

DETAILED DESCRIPTION OFTHE PREFERRED EMBODIMENTS

The present invention relates to _n a supplementary cooling system and a method of thermal management of a process fluid under adverse operating conditions. Rg. 1 shows an embodiment of the supplementary cooling system. The supplementary cooling system generally comprises a fluid bypass line 12 adapted to connect to and bypass a process fluid line 11 , one or more heat

exchangers 13 connected to the fluid bypass line 12, and a coolant fluid recirculation line 14 connected to each of the heat exchangers 13.

A process fluid 15 generally flows in the process fluid line 11. A portion or all of the process fluid 15 may be diverted to flow in the bypass fluid line 12, bypassing at least a portion of the process fluid line 11. A coolant fluid 16 may flow in the coolant fluid recirculation line 14. The coolant fluid 16 in the coolant fluid recirculation line 14 may exchange heat with the process fluid 15, which is therefore cooled, via the heat exchangers 13. After being cooled the process fluid 15 may return to the process fluid line 11. The heat exchangers 13 may be plate heat exchangers, or arranged as a bank of plate heat exchangers in parallel.

In some embodiments, a fluid control valve 17 in the bypassed portion of the process fluid line 11 may be present for controlling the flow of the process fluid 15 through the bypassed portion of the process fluid line 11. Similarly a fluid control valve 18 in the fluid bypass line 12 may also be present for controlling the flow of the process fluid 15 through the fluid bypass line 12. The fluid control valves 17 and 18 may be separately or jointly controlled such that the relative amounts of the process fluid 15 flowing through the process fluid line 11 and the fluid bypass line 12 can be regulated, thereby controlling the level of supplementary cooling. In other embodiments the bypassed portion of the process fluid line 11 may be removed altogether, thereby diverting all the process fluid 15 to go through the fluid bypass line 12.

The supplementary cooling system, as shown in Rg. 1 , has four external connections which are:

• an inlet for the process fluid line;

• an outlet for the process fluid line;

• an inlet for the coolant fluid recirculation line; and

• an outlet for the coolant fluid recirculation line.

-Examples of how the supplementary cooling system may be connected to other external systems are now described.

Fig. 2 shows an example of the use of the supplementary cooling system. The supplementary cooling system is shown in Fig. 2 as block 23, which is a simplified representation of the whole of

Fig. 1. The block 23 is shown to connect to four lines, which correspond to the inlet and outlet of the process fluid line and the inlet and outlet of the coolant fluid recirculation line of the supplementary cooling system.

In this example, the process fluid line of the supplementary cooling system 23 is a primary refrigerant circuit 21 in which the process fluid is a refrigerant 25. In other uses, the process fluid line may contain oil as the process fluid, for example, for use in cooling applications in a substation for electrical power transformers or used in hydraulic motors. In this example, the coolant fluid recirculation line of the supplementary cooling system 23 is a chilled water line 24 in which the coolant fluid is chilled water 26. The chilled water 26 may be chilled by a secondary refrigerant system 29 or other cooling means.

The supplementary cooling system 23 may be retrofitted to the primary refrigerant circuit 21.

In this example, the refrigerant 25 when leaving a compressor 28 is in a compressed vapour form. The pressure of the refrigerant 25 in this form is known as the head pressure. The refrigerant 25 is then cooled and condensed in a condenser 27. The condensed refrigerant 25 then enters the supplementary cooling system 23 and may exchange heat with, and hence receiving further cooling by, a coolant fluid 26 in the coolant fluid recirculation line 24 at the heat exchangers within the supplementary cooling system 23. The refrigerant 25 is then passed through an expansion device 22 where the refrigerant returns to vapour form known as a suction gas before re-entering the compressor 28.

Figure 3 shows another example of the use of the present invention, where the supplementary cooling system is shown as block 33. As in the example shown in Fig.2, the process fluid line in this example is also a primary refrigerant circuit 31. But unlike that in Fig. 2, the coolant fluid recirculation line in this example is a secondary refrigeration circuit 34, which forms part of a secondary refrigeration system 39. The secondary refrigeration circuit 34 may be separated from the primary refrigerant circuit 31. The coolant fluid, in this example, is a refrigerant 36. The secondary refrigeration circuit 34 may be any type of refrigeration system, including those based on conventional mechanical compressors, or based on physical adsorption or liquid absorption.

In some embodiments, such as the one shown in Fig. 2, flow of the coolant fluid 26 in the coolant fluid recirculation line 24 may be controlled by a controller 41. The supplementary cooling system may also comprise a coolant valve 42 connected to the coolant fluid recirculation line 24 and controlled by the controller 41. It should be noted that both the controller 41 and coolant valve 42 may be part of the supplementary cooling system. They are drawn to locate outside the supplementary cooling system 23 in Rg. 2 only for clarity.

In some embodiments, the controller 41 may also control flow of the process fluid 15 by controlling the fluid control valves 17 and 18. In other embodiments, there may be a separate controller for controlling each of the flow of the coolant fluid 16 and the flow of the process fluid 15. For example, a process fluid controller may be dedicated to controlling the fluid control valves 17 and 18. Likewise, a coolant fluid controller may be dedicated to controlling the coolant valve 42.

The controller 41 may include a programmable microprocessor or a programmable logic circuit capable of controlling the coolant valve 42 and the fluid control valves 17 and 18 without human intervention.

In some embodiments, the controller 41 may open the coolant valve 42 at predetermined conditions, such as temperature and/or pressure. The controller 41 may also open or close the fluid control valves 17 and 18 at the predetermined conditions or other different conditions. For example, the coolant fluid controller 41 may be triggered or actuated to open the coolant valve 42, close fluid valve 17, and open fluid valve 18 by a signal transmitted from a temperature and/or pressure sensor. The opening and/or closing of the coolant valve 42 and fluid valves 17 and 18 may, for example, be triggered when:

• the ambient temperature, typically measured near the condenser (for example, by a temperature sensor 54), exceeds a predetermined value; and/or

• the process fluid temperature', typically measured (for example, by a temperature sensor 55) at the refrigeration line 21 where the refrigerant 26 has passed through the condenser 27 but before passing through the supplementary cooling system 23, exceeds a predetermined value; and/or

• if the process fluid line is a refrigeration circuit, the head pressure of the refrigerant, typically measured (for example, by a pressure sensor 56) on the condenser side of the refrigeration circuit exceeds a predetermined value for a sufficiently long duration; and/or

• if the process fluid line is a refrigeration circuit, the superheat of the suction gas, typically measured (for example, by a temperature sensor 59) after the refrigerant passes through the expansion device, exceeds a predetermined value.

In the embodiment shown in Fig. 3, the coolant fluid controller 41 may directly control the operation of the secondary refrigeration system 39, by controlling, for example, the on/off status of a compressor 37 of the secondary refrigeration system 39.

Figure 4 provides an example of an algorithm that may be used by the controller 41 for selective actuation of the controller 41. The relevant temperature and/or pressure measured by the temperature and/or pressure sensors 54, 55, 56 and 59 may be used in the algorithm to determine whether a signal is to be transmitted from the coolant fluid controller 41 to the coolant valve 42 for the opening of the coolant valve 42. The algorithm starts at logic gate 44.

• At logic gate 44, if the ambient temperature is determined to exceed a predetermined value, a signal is transmitted to the coolant valve 42 to open the coolant valve 42. Otherwise, the algorithm may proceed to logic gate 45.

• At logic gate 45, if the temperature of the refrigerant 25 in the primary refrigerant circuit 21 is determined to exceed a predetermined value, a signal is transmitted to the coolant valve 42 to open the coolant valve 42. Otherwise, the algorithm may proceed to logic gate 46.

• At logic gate 46, if the head pressure of the refrigerant 25 on the condenser 29 side of the primary refrigerant circuit 21 is determined to exceed a predetermined value, a time delay 47 is introduced, for example by a timing circuit or counter, before the algorithm proceeds to logic gate 48, where if the head pressure of the refrigerant 25 on the condenser 29 side of the primary refrigerant circuit 21 is again

determined to exceed a predetermined value, a signal is transmitted to the coolant valve 42 to open the coolant valve 42. Otherwise, the algorithm may proceed to logic 49.

• At logic gate 49, if the superheat of the suction gas, typically measured after the refrigerant has passed through the expansion device, is determined to exceed a predetermined value, a time delay 50 is introduced, for example by a timing circuit or counter, before the algorithm proceeds to logic gate 51 , where if the superheat of the suction gas is again determined to exceed a predetermined value, a signal is transmitted to the coolant valve 42 to open the coolant valve 42. Otherwise, the algorithm may terminate, or re-start at logic 44 immediately or after a certain time delay.

Although the algorithm described below only refers to the coolant valve 42, the algorithm may also be used to determine whether a signal or signals are to be transmitted to the fluid valves 17 and 18 for the opening and closing of the fluid valves 17 and 18. Similarly, the algorithm may also be used to determine whether a signal is to be transmitted to the compressor 37 shown in Rg. 3 for controlling the operation of the secondary refrigeration system 39.

The reasons for including a time delay after logic gate 46 are now explained. There may be times when the head pressure can rise momentarily prior to tripping of the compressor 28 and then subside. To avoid premature opening of the coolant valve 42, the coolant valve controller may include a timing circuit or a counter to ensure the coolant fluid controller 41 only actuates the coolant valve 42 if the head pressure exceeds a predetermined value for a sufficiently long duration.

The reasons for including the time delay after logic gate 49 are now explained . If there is sudden increase in the load on the refrigeration circuit, for example, when restacking the refrigerator with uncooled fresh load or when there is a sudden ingress of people in an air conditioned space, the superheat of the suction gas will tend to increase. Although the expansion devices used in refrigeration systems may partially alleviate this, an additional cooling capacity will allow stabilisation of operating conditions much faster if the liquid refrigerant is subcooled by a supplementary cooling system.

Generally the supplementary cooling system is a standalone system.

Now that several preferred embodiments of the present invention have been described it will be apparent to those skilled in the art that the supplementary cooling system has the following advantages:

• The present invention is not limited to supplementary cooling of a refrigerant but is equally applicable to other process fluids, such as oil used in cooling transformers in a power substation, where the process fluid needs further cooling when a primary cooling system is not sufficient, for example, on days when the ambient temperature is too high. Oil cooling in mining equipment on sufficiently hot days is another situation where the supplementary cooling system is well suited.

• The coolant fluid line may operate as required. This reduces the operating expenditure since the coolant fluid line may be switched off when supplementary cooling is not required, for example on a sufficiently cool day.

• The use of a supplementary cooling system may reduce the capacity requirements of the primary system with most refrigeration plants being sized for 40 ⁰ C condensation although such conditions may not prevail for over 270 days a year. Thus, for most part of the year the cooling plants tend to be over sized. It will be possible to downsize the primary system to operate at lower condensing temperatures and use the present invention to supplement cooling needs on days when the primary system is not able to meet the demands. This may also result in smaller condensers and compressors of the primary system and smaller real estate foot prints.

• The supplementary cooling system is capable of automatic operation when, for example, a coolant fluid controller with an algorithm for determining actuation of the coolant fluid controller is used.

• If chilled water is used as the coolant fluid, it needs to be supplied at no cooler than 20 ⁰ C even for a typical 5 ⁰ C temperature rise through the heat exchanger. If a

direct expansion cooling is adopted, the evaporating temperature cart be as high as 20 ⁰ C.

> The supplementary cooling system can also provide a cooling capacity that is scalable to at least hundreds of kilowatts.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. For example, the coolant fluid controller is not limited to actuate the opening of the coolant valve in a chilled water line. The coolant fluid controller may also be used to activate the operation of a secondary refrigerant system refrigerant if the coolant fluid line is a refrigeration circuit. The algorithm used by the coolant fluid controller for selective actuation of the coolant fluid controller may also comprise steps in an order other than that shown in Fig. 4. Some of the steps of the algorithm may also be bypassed. The supplementary cooling system may also be made to come into operation not only on high ambient days but also when off design operation conditions are encountered even during normal operation. Examples of such conditions are when there is a sudden increase in the load, such as restacking of supermarket shelves with fresh produce or uncooled soft drinks, or when there is a sudden ingress of people or unfiltered air in an air conditioned space.

All such variations and modifications are to be considered within the ambit of the present invention the nature of which is to be determined from the foregoing description.