US3555845A

US3555845A - Forced-flow evaporator for compression refrigeration equipment

Info

Publication number: US3555845A
Application number: US757648A
Authority: US
Inventors: Zbigniew Ryszard Huelle
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1967-09-06
Filing date: 1968-09-05
Publication date: 1971-01-19
Anticipated expiration: 1988-01-19
Also published as: NO123736B; ES357801A1; AT289164B; FR1599763A; GB1233924A; DK127136B; JPS4821982B1

Abstract

THE INVENTION RELATES TO A FORCED-FLOW EVAPORATOR ASSEMBLY FOR COMPRESSION TYPE REFRIGERATING EQUIPMENT. IN CONNECTION WITH PREVENTING REFRIGERANT IN LIQUID FORM FROM PASSING FROM THE EVAPORATOR TO THE COMPRESSOR, THE EVAPORATOR TUBING IS PROVIDED WITH LIQUID ADSORBING MEANS WHICH COMPRISE A CAPILLARY SYSTEM. THE LIQUID ABSORBING MEANS IS FORMED WITH A GAUZE FABRIC WHICH OCCUPIES LESS THAN ONE-HALF THE CROSS-SECTIONAL AREA OF THE TUBING.

Description

Jan. 19, 1971 z. R, HUE'LLE 3,555,845

FORCED-FLOW EVAPORATOR FOR COMPRESSION REFRIGERATION EQUIPMENT Filed Sept. 5, 1968 United States Patent 3,555,845 FORCED-FLOW EVAPORATOR FOR COMPRESSION REFRIGERATION EQUIPMENT Zbigniew Ryszard Huelle, Sonderborg, Denmark, assignor to Danfoss A/S, Nordborg, Denmark, a company of Denmark Filed Sept. 5, 1968, Ser. No. 757,648 Claims priority, application Ggrmany, Sept. 6, 1967,

Int. cl. F2 51 41 /04 US. Cl. 62-225 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a forced-flow evaporator for compression refrigeration equipment incorporating a temperature sensor which is arranged in the vicinity of the evaporator output point and which controls an element that influences the flow of refrigerant, for example a thermostatic expansion valve fitted ahead of the evaporator.

In refrigerating equipment of this kind liquid refrigerant must be prevented from passing from the evaporator into the compressor or its case, since otherwise trouble could occur. As soon as the temperature sensor determines that the evaporation temperature, rather than the superheating temperature, obtains at the point of measurement, i.e. that liquid refrigerant has still to be evaporated, the flow of refrigerant must be throttled or interrupted. For reasons of safety, the element that influences the flow of refrigerant must, how ever, respond if a prescribed superheating temperature is not reached, and should not respond only when the evaporating temperature is attained. Practice has shown that the superheating temperature at which response occurs must be higher than the evaporating temperature, even if, for precautionary reasons, a re-evaporation run is provided to enable only refrigerant vapour to return to the compressor. This fact, however, runs counter to the requirement that evaporation should be caused, as far as possible, to take place over the entire length of the evaporator, since the evaporator and the refrigerating system are best utilized in this way. Optimally, such control of the interposed expansion valve, by the temperature sensor, should be such that just enough liquid refrigerant is introduced into the evaporator for it to be completely evaporated exactly at the end of the evaporator.

In the case of absorption refrigerating machines, it is known to use a heat-exchanger wherein a corrugated and helically wound wire gauze is inserted in a tube in order to increase the turbulence of the refrigerant vapour flowing through and thereby to increase the heat-transfer by conducting heat along the wire gauze to the wall of the tube. Also, the gauze is able to retain liquid refrigerant by capillary action until it is evaporated. Such an ar rangement is not suitable for compression refrigerating equipment since the wire gauze, which occupies the entire cross-section of the tube, would cause a drop in pressure, greatly reducing the efficiency of the refrigerating equip- 3,555,845 Patented Jan. 19, 1971 ment, at the high flow velocities, which can be as high as 5 metres/sec. in the case of the refrigerant vapour.

Also known is an evaporator for absorption refrigerating equipment, wherein a tubular surface is provided with a longitudinal groove and with closely spaced circumferential channels of such size that liquid can be drawn into them by capillary action. In this way, the surface of the evaporator tube is intended to be uniformly wetted even if the liquid refrigerant, subjected to no pressure, drips on to it only at one or a few places.

The object of the present invention is to provide a forced-flow evaporator for compression refrigerating equipment, the control system of which responds to a small excess-heat temperature and wherein it is nevertheless ensured that no fluid refrigerant reaches the compressor.

According to the invention, this object is achieved by providing at least part of the wall of the evaporator passage with means for increasing wetting by liquid refrigerant, which means, however, leave the major part of the cross-section of the passage open.

During evaporation in a forced-flow evaporator, vapour is formed which flows through the piping together with the liquid. The liquid proportion progressively decreases until an evaporation end-point is reached beyond which only vapour is present. It is known that even in the case of constant flow, the evaporation end-point moves to and fro, in periods of up to 3 sec., over a considerable length of the evaporator piping, e.g., 50 cm. The invention is based upon the knowledge that the excess-heat temperature, hitherto required for reasons of safety, can be considerably reduced if the migration of the evaporation end-point along the evaporator piping can be curtailed.

Such curtailment of the migration is achieved by the means for increasing the wetting. These lead to larger quantities of liquid adhering to the wall of the passage, i.e. to a larger surface of the passage wall being wetted. This results in improved heat-transfer and more rapid evaporation. Furthermore, consequent upon the measurement surface tension, a large part of the liquid adheres to the wall of the passage and is not so readily displaced by the pressure-difference that exists. This applies particularly in the case of the ring of liquid that forms on the wall of the passage on completion of evaporation. For all this, the cross-section of the passage is substantially open, so that the remaining flow, particularly that of the vapour, is not adversely afiected.

In view of these effects, the means for increasing the wetting should be located at least in the zone near the point at which evaporation ends, i.e., in the portion in front of the sensor. They need only extend over roughly the second half of the length of the evaporator, for example. In many cases, particularly where the system is to be fitted at a later stage, it is of advantage if the main part of the evaporator is provided with an added pipe, to the end of which the sensor is fitted, and the means for increasing the wetting extend only along this pipe.

In a preferred form of construction, the means for reinforcing the wetting action take the form of a capillary system. The liquid is rapidly spread over a large surface of the wall of the passage by the capillary system and is efiiciently held by surface tension.

A particularly simple form of the invention is characterized by an inserted wall, containing orifices and extending parallel with the wall of the passage at a short distance therefrom and supported thereon, so that a space is created between the inserted wall and the passage wall, this forming the capillary system. In this way, a relatively thick layer of liquid is spread over and retained on the wall of the passage.

The inserted wall and its supporting means are preferably formed from a gauze fabric which lies on the passage wall, e.g. the wall of the tube. Such a gauze material can be readily inserted into a tube. Since the liquid can flow on both sides of the gauze, particularly good wetting of the wall of the passage results. The cross-section of the passage is not, however, interfered with to any considerable extent at the centre. The wire gauze can be readily provided with a capillary mesh so that liquid is retained therein in a particularly efficient manner.

It is of advantage to provide an axial gap in the gauze fabric. By compressing the gap, the gauze fabric can be readily introduced into the flow-passage of the evaporators. In the case of evaporators in which oil is likely to be deposited, the gap should be located at the bottom of the cross-section of the passage, so that the oil can drain off.

The invention will now be explained in more detail by reference to the drawing, wherein:

FIG. 1 shows, schematically, a first embodiment of the invention,

FIG. 2 is a cross-section through the evaporator tube in the region of the line A-A, and

FIG. 3 is a schematic illustration of another embodiment.

FIG. 1 shows a normal tubular evaporator, which consists of a tube 1, at the front of which is fitted a thermostatic expansion valve 2 and at the outlet of which is fitted a temperature sensor 3. By means of this sensor and with the help of the expansion valve 2 the flow of refrigerant is so adjusted that, whatever the circumstances, all the liquid refrigerant is evaporated at the point 4. In known evaporators, the point at which evaporation ends ranges over quite a considerable length of the evaporator, for example between the

points

4 and 5. At the moment at which evaporation used to be completed at point 5, the refrigerant vapour is considerably over-heated along the stretch running to the sensor 3. Below this excess-heat temperature, the

thermostatic system

2 and 3 should not respond, since when the supply of refrigerant is correspondingly increased, the end point 4 moves beyond the sensor 3 and liquid refrigerant would thus reach the compressor.

According to the invention, a cylindrical gauze element 6 is inserted in the smooth-walled evaporator tube 1. In the embodiment illustrated, this gauze extends over approximately the second half of the length of the evaporator, i.e. over about 40-60% of the length. By this measure, the point at which evaporation ends is restricted to moving backwards and forwards over a very much smaller distance, for example, between the

points

4 and 7. The possible overheating between point 7 and the feeler 3 is considerably smaller than hitherto. Consequently, the thermostatic system may respond at a lower excess-heat temperature, without there being any danger of the liquid refrigerant reaching the compressor.

FIG. 2 shows the cylindrical gauze element 6 inserted in the evaporator tube 1, the wires 8 of this element also forming supports 9 (indicated schematically). The gauze contains a longitudinal slot 10 which, on the one hand, can serve for the discharge of deposited oil and, on the other hand, enables the gauze to be easily pushed into the tube 1 in front of the bend of this tube. The gauze lies around the smooth inner wall of the tube 1 and forms a space 11 which is of such size that refrigerant can be retained therein by capillary action or surface tension. The refrigerant is, of course, driven along the tube by the compression pressure. On account of the uneven surface of the wire gauze and the capillary adhesive force, the layer of liquid refrigerant flowing in this area wets the wall of the passage very efliciently, and this contributes to a reduction in the fluctuation of the point at which evaporation terminates. Due to this wetting, the mobility of the ring of liquid is reduced. Furthermore, the heat-transfer is improved and evaporation is thus accelerated. This acceleration of the evaporation also reduces the fluctuations in the point at which evaporation ends, since the small quantity of liquid finally remaining is evaporated over a shorter portion of the tube.

In the embodiment shown in FIG. 3, there is shown a normal plate evaporator 12 in which the evaporator pipe 13 has been made by the roll-bond process. In order to be able to apply the concept of the invention while still utilizing this simple manufacturing process, a tube 14 is added to the plate evaporator 12, a gauze 15, equivalent to the gauze 6 shown in FIG. 2, being inserted in this tube. The sensor is then fitted at the end of the tube 14.

The mesh-size of the gauze will depend upon the particular circumstances. Mesh-size number (DIN 4189), for example, has proved to be particularly suitable. A capillary system can also be obtained by machining the inside wall of the pipe; the insertion of a sock-like gauze element is of advantage, however, both for reasons of simpler manufacture and for reasons of its superior functioning.

What is claimed is:

1. A forced-flow evaporator assembly for compression type refrigerating equipment, comprising, tubing means having inlet and outlet ends, valve means at said inlet end and temperature sensing means at said outlet end for controlling said valve means, liquid absorption means in said tubing means comprising a capillary system for absorbing liquid refrigerant, said liquid absorption means occupying less than one-half the linear length of said tubing means, said tubing means defining an internal wall surface, said liquid absorption means comprising a perforated wall internally spaced from said wall surface to define an annular space between said wall surface and said wall, said annular space forming a portion of said capillary system.

2. A forced-flow evaporator assembly according to claim 1 wherein said perforated wall comprises a gauze fabric spaced for said wall surface.

3. A forced-flow evaporator assembly according to claim 2 wherein said gauze fabric is a heat conducting material.

4. A forced-flow evaporator assembly according to claim 1 wherein said perforated wall defines a longitudinally extending channel.

5. A forced-flow evaporator assembly according to claim 1 wherein said liquid absorption means is disposed between said temperature sensing means and said valve means and in juxtaposition to said temperature sensing means.

6. A forced-flow evaporator assembly according to claim 5 wherein about one-half the length of said tubing,

means is equipped with said liquid absorption means.

References Cited UNITED STATES PATENTS 2,446,763 8/1948 Haymond 62-527 2,702,460 2/1955 Gaugler 62527X 2,720,763 10/1955 Doekeli 62527 2,983,107 5/1961 Forrest 6'2527X MEYER PERLIN, Primary Examiner US. Cl. X.R. 62-527