FIELD OF THE INVENTION
The present invention in general relates to the field of heat exchangers and in particular to double tube and shell type heat exchanger. More particularly, the invention relates to a device for injecting a sublimating substance in double tube and shell type heat exchanger to enhance heat transfer. The invention further relates to a method for injecting the sublimating substance using the device for enhancement of heat transfer in a double tube and shell type heat exchanger.
BACKGROUND OF THE INVENTION
Heat transfer from hot fluid to cold fluid using double tube and shell type heat exchangers are widely used in many process plants such as chemical industries, thermal power plants, fertilizers, petrochemical industries etc. In these heat exchangers, one fluid flows in shell side and the other in tube side. In order to maintain purity, some heat exchangers applications demand protection from mixing of the two fluids within the heat exchanger in the event of tube puncture. To ensure non-mixing of fluids in case of such events, an additional concentric outer tube is provided over the inner tube of the heat exchanger. In case of leakage from any side of the tube, the leaked fluid comes into an annular space and ultimately at the lowest part of the double tube compartment of the heat exchanger. Leak detectors are fitted at that place to detect the leakage so that the problem can be addressed.
Inner tube is manufactured with low fin and multiple spiral grooves to provide better surface contact between the two tubes as well as passage for movement of the leaked fluid in the event of tube puncture. This arrangement has one

inherent limitation of having poor heat transfer due to lack of hundred percent contact between two tubes. Heat transfer in the annular section filled with air is reduced as air is a poor conductor of heat Because of poor heat transfer the surface area required for the heat exchange increases. The present invention addresses these limitations.
Patent No.95054-3299 (US) describes a process to increase heat transfer between a wafer and heat sink in a semiconductor processing chamber.
Patent No. 9273413B2 (US) describes injecting a fill gas into gaps between wafers and wafer carrier for enhanced heat transfer and uniform temperature of the wafers.
OBJECTS OF THE INVENTION
It is an object of the present invention to propose a method for injecting a sublimating substance in double tube heat exchanger to enhance heat transfer.
Another object of the present invention is to propose a method for injecting a sublimating substance in double tube heat exchanger to enhance heat transfer, which allows higher heat transfer with less surface area of the heat exchanger.
A still another object of the present invention is to propose a method for injecting a sublimating substance into an enclosed annular space of a double tube and shell type heat exchanger.

A further object of the invention is to propose a device for injecting the sublimating substance for enhancement of heat transfer in a double tube and shell type heat exchanger, in which the sublimating substance sublimes with increase in temperature.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a device for enhancing heat transfer across two fluids in a double tube and shell type heat exchanger by filling the annular gap between concentric tubes of heat exchanger with a substance having higher thermal conductivity than that of air. The substance which is selected as the conducting medium has unique property. It is sublimating and once injected remains in either solid or in vapour from inside the space, The state of sublimating fluid depends on temperature. Under normal operation the substance remains in vapour form and increases rate of heat transfer by thermal convection. In another aspect of the present invention, there is provided a method for injecting the sublimating substance using the device in double tube and shell type heat exchange for enhancement of heat transfer.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Figure 1 illustrates an arrangement of a double tube and shell type heat exchange or cooler.
Figure 2 is perspective view of a double tube assembly used in a cooler.
Figure 3 is a side view of a double tube where outer tube is longitudinally cut along the axis.

Figure 4 describes a device to inject a sublimating substance inside the annular space of a double tube assembly disposed inside a double tube and shell type heat exchanger.
DETAILED DESCRIPTION OF THE INVENTION
A general arrangement of a double tube and shell type heat exchanger or cooler (10) used for the purpose of cooling hot fluid is shown In figure 1 inner tube (1) receives a cold fluid from cold fluid inlet (3) and sends the fluid to a cold fluid outlet (4). An outer tube (2) is in contact with a hot fluid which enters the heat exchanger from a hot fluid inlet (5) and leaves at a hot fluid exit (6). Baffle plates (8) are arranged to give direction to the hot fluid. During operation of the heat exchanger, the hot fluid rejects heat to said outer tube (2) which conducts it to said inner tube (1) and ultimately to the inner tube cold fluid. The outer tube (2) and inner tube (1) are concentrically arranged but lack complete metal to metal contact due to groove formation on either the outer tube (2) or the inner tube (1). This results in formation of a spiral annular gap (9) between the two tubes assembly (30). Gaps (9) are provided for smooth passage of cold or hot fluid in case of leakage Figure 2 and Figure 3 show the gaps (9) on the outer surface of the inner tube (1). The leaked fluid gets collected at a leak detector port (7), where a device (not shown in accompanying figure) detects the leakage, and the process is stopped for rectification. In the present situation, the leak detector port (7) is also used for injection of a sublimating substance inside the cooler (10). Figure 2 is the perspective view of the double tube assembly (30) which is used in a double tube and shell type heat exchanger. Figure 3 depicts a view of the double tube where the outer tube (2) is cut along axis longitudinally. When the heat enhancing substance is injected inside the annular gaps (9) of the assembly shown is figure 2 and figure 3, the substance settles

inside the gaps and remains either in solid or in vapour form.
Figure 4 describes a device (40) for injecting a sublimating substance inside the annular gaps of a double tube heat exchanger (10). The substance is poured into a cylinder (11) of the device through a hopper (15) which is connected to a cylinder top inlet (12) with a plurality of pipes (16); an inlet valve (17); and an inlet pipe (18). A cylinder bottom outlet (13) is connected to an outlet pipe (19). A outlet valve (16) is connected with said outlet pipe (19) to controls outflow of the substance. A first pipe (21) and a second pipe (22) are connected in series to a outlet valve (20). An electric heating pad (26) is wrapped at the lower portion of the cylinder (11). An air inlet pipe (23) is connected at an air inlet (14) of the cylinder (11) to control inflow of compressed air. A through bolt (27) is fitted at the end of the second pipe (22) which connects to the leak detector port (7) of the cooler (10) during injection of the substance. Leak detector is under removed condition during this operation.
When the poured substance is heated with the heating pad (26), the inlet valve (16), the outlet valve (20) and an air regulating valve (24) remain in closed condition. Slowly the air regulating valve (24) is opened and compressed air starts flowing via a third pipe (25) into the cylinder (11). The subliming substance remains in liquid condition under pressure. Ensuring that the liquid substance temperature is well above the melting point temperature, slowly the outlet valve (20) is opened. Liquid, under pressure, gets injected inside the annular gaps (9) of the large number of double tube assemblies. Since the fluid is under high pressure, and once the outlet valve (20) is opened, the fluid starts flowing inside the cooler (10) and again the fluid tends to move outside of the cooler (10) into the environment, the bolt (27) fitted over the second pipe

(22) is tightened at the leak detector port (7), The complete fluid is injected and allowed to get solidified inside the gaps (9) or a fraction of it remains in vapour form. Slowfy the air regulating valve (24) is closed and pressure is released by loosening the bolt (26).
When the heat exchanger (10) is in operation, the sublimating substance gets vaporized which enhances heat transfer between the two fluids due to convective currents inside the annular space,

WE CLAIM :
1. A device for injecting a sublimating substance in between annular gaps (9) of double tube assembly (30) in a double tube and shell type heat exchanger (10) for enhancement of heat transfer between a first fluid medium flowing inside the inner tube (1) and a second fluid medium flowing outside the outer tube (2), the device comprising :-(i) a cylinder (11) having one each top inlet (12), bottom outlet (13), and
side air inlet (14); (ii) an inlet pipe (18) connected to said top inlet (12) of said cylinder (11); (iii) an inlet valve (17) connected to said inlet pipe (18); (iv) a plurality of pipes (16) connected to said inlet valve (17); (v) a hopper (15) connected to said pipe (16); (vi) a first outlet pipe (19) connected to said bottom outlet (13) of said
cylinder (11); (vii) a outlet valve (20) connected to said first outlet pipe (19); (viii) a first pipe (21) connected to said outlet valve (20); (ix) a second pipe (22) connected to said first pipe (21); (x) a through bolt (27) over the end of said second pipe (22); (xi) a compressed air inlet pipe (23) connected to said side way (14) of
said cylinder (11); (xii) an air regulating valve (24) connected to said compressed air inlet
pipe (23) (xiii) a third pipe (25) connected to said air regulating valve (24), and (xiv) a heating pad (26) overlapping at lower portion of said cylinder (11).

2. A method for injecting the sublimating substance in a device for enhancement of heat transfer in a double tube and shell type heat exchanger, the method comprising the steps of :-
a) connecting the through bolt (27) to a leak detector port (7) of the heat exchanger (10) after removing a leak detector from the port;
b) connecting the third pipe (25) to the compressed air source maintaining the inlet valve (17), outlet valve (20) and air regulating valve (24) in closed position;
c) opening the inlet valve (17) and pouring the sublimating substance into the hopper (15) which gets settled into the cylinder (11);
d) closing the inlet valve (17) and switching on the heating pad (26) to
allow melting of the substance, keeping the air regulating valve (24)
opened;
e) opening the outlet valve (20) and injecting the molten substance inside the annular gap (9) of the double tube assembly (30);
f) closing the air regulating valve (24) to allow solidification of the sublimating substance inside the gaps (9); and
g) loosening the through bolt (20), disconnecting the device and re-fitting the leak detector on the port (7).