US20040159423A1

US20040159423A1 - Apparatus and method for improving the performance of an evaporator

Info

Publication number: US20040159423A1
Application number: US10/475,934
Authority: US
Inventors: Hakan Brannmark; Per Nyqvist
Original assignee: Emerson Energy Systems AB
Current assignee: Vertiv Sweden AB
Priority date: 2001-05-10
Filing date: 2002-05-02
Publication date: 2004-08-19
Also published as: WO2002090847A1; SE0101636D0

Abstract

The present invention relates to the improvements of the performance of an evapo-rator, specifically adapted for a natural convection system. The evaporator com-prises tubes (11) intended to contain a refrigerant to be evaporated. An apparatus (13) is arranged inside at least one of the tubes and comprises means (21) for disin-tegrating vapour bubbles (27′) of a gaseous phase of the refrigerant inside the tube (11) during operation of the evaporator. Small vapour bubbles can much easier move through the liquid compared to big vapour bubbles, which easily get stuck in the tube thereby obstructing the movement of other vapour bubbles. Thus, by breaking up big bubbles into smaller bubbles the flow through the tubes (11) is in-creased as well as the heat emitting surfaces of the vapour bubbles (27″).

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and an evaporator according to the preambles of

claims

1 and 2, respectively.

BACKGROUND OF THE INVENTION

Electronic equipment housed in closed cabinets require correct climate conditions to work properly, e.g. temperatures, humidity etc. Heat dissipated by the equipment tend to rapidly increase the temperature within the cabinet, and may, if the equipment is not cooled, jeopardise the function of the electronic equipment. A number of ways of cooling electronic equipment housed in closed cabinets are known in the art, such as natural/forced convection liquid/air cooling.

Natural convection cooling is a less complicated and a less noisy way of cooling compared to forced convection cooling, where fans, pumps or compressors are required to maintain a necessary cooling flow. Liquid cooling provides, in contrast to air cooling, a more efficient cooling due to better heat carrier capabilities.

Thus, a simple, quiet and effective cooling system would be a natural convection liquid cooling system.

This system may imply the use of an evaporator and a condenser, where the evaporator is arranged inside the cabinet and the condenser is arranged outside the cabinet, even though other arrangements are conceivable. Pipes connecting the evaporator and the condenser create a closed loop system. During operation, when the system is partly contained with a refrigerant, a natural convection system is created, where heat is absorbed at the evaporator side and is dissipated at the condenser side of the system. The condensation creates a suction, which maintains a circulation of the refrigerant in the system.

An evaporator for a natural convection system preferably comprises vertical tubes, in which the evaporation of the contained refrigerant is intended to occur. Vertical tubes facilitate the natural convection of an evaporated refrigerant within the system, from evaporator to condenser, compared to e.g. horizontal tubes.

During the evaporation, vapour bubbles of the refrigerant are formed at inner surfaces of the vertical tubes, and they will gradually break away from the surfaces and rise upwards to the liquid surface, i.e. the liquid-free parts of the evaporator, where they can continue towards the condenser. However, a newly broken away vapour bubble is quickly growing, in dependence of surface tension at the liquid-vapour interface as well as temperature and pressure, thus taking up a lot of space in the evaporator tube. Hereby, the bubble will encounter difficulties to move through the liquid, obstructed by other growing vapour bubbles and friction against the inner surfaces. This behaviour will suppress the degree of efficiency of the evaporator. Still, the degree of efficiency of the evaporator needs to be high, since the trend within this field of technology is towards high-density electronics, which means that cabinets will be filled with more and more heat generating equipment, which in turn require more effective cooling systems.

OBJECT OF THE INVENTION

The object of the present invention is to provide a more efficient evaporator, specifically an evaporator intended for a natural convection system.

SUMMARY OF THE INVENTION

This object is achieved according to the present invention by means of a method, an evaporator and an apparatus, respectively, as initially defined and having the features mentioned in the characterising portions of the independent claims.

By disintegrating vapour bubbles of a gaseous phase of the refrigerant inside the vertical tube during operation of the evaporator, i.e. when the contained refrigerant absorbs heat conducted through the walls of the tube, smaller bubbles are formed. These smaller bubbles can much easier move through the liquid, while they are not as easily obstructed by friction as bigger bubbles and do therefor not get stuck in the same extent as bigger bubbles. Hereby the flow through the evaporator tube is in creased, whereby the performance of the evaporator is improved. Additionally, a vapour bubble that is disintegrated into smaller vapour bubbles having the same volume as the bigger bubble, nevertheless have a bigger heat emitting surface in comparison to the heat emitting surface of the bigger bubble. Thus, the heat transfer between gas and liquid is increased, which in turn also improves the performance of the evaporator.

Preferably, said disintegration of vapour bubbles is accomplished by means of a first apparatus to be arranged inside at least one vertical tube of the evaporator, wherein said first apparatus comprises a shaft being substantially concentrically arranged with the longitudinal axis of the tube, and wherein said shaft has a plurality of radially extending, peripherally separated bristles. Alternatively, the disintegration is accomplished by means of a first apparatus being provided with means having a grid-like structure. Accordingly, inexpensive and easily mounted apparatuses for in creasing the flow through the vertical tubes and thereby the performance of the evaporator is achieved.

Advantageously, a second apparatus is arranged downstream the first apparatus, wherein said second apparatus comprises means for rotation of the refrigerant about the longitudinal axis of the tube during operation of the evaporator. By also rotating the contained refrigerant about the longitudinal axis of the tube, after disintegrating the gaseous vapour bubbles, a liquid phase of the refrigerant will be radially separated from a gaseous phase of the refrigerant due to centrifugal action. This is possible since the liquid phase and the gaseous phase have different densities. Thus, the liquid phase of the refrigerant will be forced against the inner walls of the tubes, while the gaseous phase of the refrigerant will move to the centre of the tube. Hereby, a liquid-free space is obtained in the centre of the tube, whereby the distance the vapour bubble has to travel before it reaches liquid-free space is substantially shortened compared to the case where a vapour bubble has to rise all the way to the liquid surface. Thus, the flow through the evaporator is further increased, which still more improves the performance of the evaporator. Moreover, since the liquid phase is forced against the inner surfaces of the tube, non-evaporated refrigerant is constantly wetting the heat emitting surfaces, which improves the heat exchange between gas and liquid and still further increases the degree of efficiency of the evaporator. As a consequence, evaporators can be made smaller with maintained performance.

DRAWING SUMMARY

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, on which: [0013]
FIG. 1 shows a schematic perspective view of a natural convection system for a closed cabinet housing heat generating, electronic equipment; [0014]
FIG. 2[0015] a shows a cross-sectional side view of a vertical tube in accordance with a first embodiment of the present invention;
FIG. 2[0016] b shows a cross-sectional top view of the vertical tube in FIG. 2a;
FIG. 3[0017] a shows a cross-sectional side view of a vertical tube in accordance with a second embodiment of the present invention;
FIG. 3[0018] b shows a cross-sectional top view of a vertical tube in FIG. 3a;
FIGS. 4[0019] a-b show cross-sectional side views of a vertical tube in accordance with an alternative embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 depicts a [0020] cabinet 1 housing not shown electronic equipment, which during operation generate heat. An evaporator 3 is arranged inside the cabinet 1 and is in tended to absorb heat from the electronic equipment. A condenser 5 is located outside the cabinet 1 and is intended to emit the absorbed heat to an external cooling flow 7, e.g. air, oil, water, etc. Between the evaporator 3 and the condenser 5 are pipes 9 arranged, so as to connect them and to create a closed cooling system. The system is partly filled with a not shown refrigerant, such as water, and a vacuum is created, so as to achieve a low evaporation temperature. The evaporator 3 comprises vertical tubes 11 in which the evaporation occurs. When the refrigerant is evaporated it is expanding and is moving towards the condenser 5. The flow in the system is indicated by arrows. When the evaporated refrigerant, i.e. the gaseous phase reaches the condenser 5 heat is emitted and the gaseous phase is transformed back to the liquid phase. The liquid phase is returned to the evaporator 3, where the evaporation process is repeated. The condensation process keeps up the free-circulation in the system.
FIGS. 2[0021] a-b show a cross-sectional side- and top view, respectively, of a vertical tube 11 intended for an evaporator 3 as described with reference to FIG. 1. A bottle-brush-like apparatus 13 is arranged inside the vertical tube 11. The apparatus comprises a shaft 15 being concentrically arranged with a longitudinal axis 17 of the vertical tube. The shaft 15 is provided with sets/ tufts 19, 19′, 19″ of bristles 21, which sets are axially separated along the shaft 15. The bristles 21 in each set protrude from the shaft in basically radial directions towards inner surfaces 23 of the tube, even though other directions also are conceivable. The bristles 21 are also peripherally separated about the shaft creating a free space 25 between two adjacent bristles.
A [0022] vapour bubble 27 that is formed at the inner surface 23 will be dissipated in the liquid L after breaking away from the inner surface, whereby the size of the bubble increases as it rises. When a bubble 27′ reaches the lower set 19 of bristles the bubble may be to big to get through. If that is the case the bubble is broke up by the bristles 21 into smaller bubbles 27″, as shown in FIG. 2a. These bubbles 27″ will in turn start to grow, but a second set 19′ of bristles will break up or disintegrate them as well, if they are to big. A third set 19″ of bristles is shown, even though the number of sets, as well as the distance between them, may vary.
FIGS. 3[0023] a-b show a second embodiment of the inventive apparatus. In stead of a bottle brush, the apparatus comprises discs 29 having a grid-like structure. The discs are axially separated along the longitudinal axis of the tube, and provide for the same result as the first embodiment. That is, a bubble 27′ is disintegrated into smaller bubbles 27″ when it is forced through the meshes 31.
FIG. 4[0024] a shows a cross-sectional side view of an alternative vertical tube 11 intended for an evaporator 3 as described with reference to FIG. 1. A helically shaped apparatus 33 in the form of a twisted strip 33 having fixed screw formed walls 35, is arranged substantially in the centre, and in parallel with the longitudinal axis 17 of an upper portion of the vertical tube 11.
Below the twisted strip, i.e. “upstream”, is the [0025] apparatus 13 according to the first embodiment of the invention arranged.
Thus, after the disintegration of the vapour bubbles, as described with reference to FIGS. 2[0026] a-b, the bubbles will reach the twisted strip 33 where they will be guided, through the liquid by the helical shape of the twisted strip creating an overall rotating motion of the refrigerant inside the vertical tube. As indicated in FIG. 4b the liquid phase L of the refrigerant is forced towards the inner surfaces of the wall, while the gaseous phase G remain in the centre of the vertical tube, thus creating a liquid-free space in the centre of the tube. This radial separation, indicated by the dotted line 35, of the two phases is accomplished by means of centrifugal action due to the density differences between the liquid L and the gas G. The line 35 is somewhat in-lined, indicating that the presence of the gaseous phase G is dominating in the upper portion of the tube, while the liquid phase L is dominating in the lower portion of the tube.
Of course, it is conceivable to use evaporators not having vertical tubes, e.g. horizontal or inclined tubes. [0027]

Claims

1. Method for increasing the performance of a natural convection system evaporator (3) comprising tubes (11), in which tubes a contained refrigerant is intended to absorb heat during operation of the evaporator, whereby vapour bubbles (27′) of the refrigerant are formed,

characterised by the step of:

disintegrating said vapour bubbles (27′) inside the tubes (11) of the evaporator (3) into smaller vapour bubbles (27″).

2. Natural convection system evaporator comprising tubes (11), in which tubes a contained refrigerant is intended to absorb heat during operation of the evaporator whereby vapour bubbles (27′) of the refrigerant are formed,

characterised by:

a first apparatus (13) being arranged inside at least one of said tubes (11), said apparatus (13) comprising means (21, 31) for disintegrating said vapour bubbles (27′) inside the tubes (11) of the evaporator (3) into smaller vapour bubbles (27″).

3. Natural convection system evaporator according to claim 2, wherein said first apparatus (13) has a bottle-brush like shape.

4. Natural convection system evaporator according to claim 2-3, wherein said first apparatus (13) comprises a shaft (15) being substantially concentrically arranged with the longitudinal axis (17) of the tube (11), said shaft having a plurality of radially extending, peripherally separated bristles (21).

5. Natural convection system evaporator according to claim 2, wherein said first apparatus (13) has a grid-like structure.

6. Natural convection system evaporator according to any of claims 2-5, wherein a second apparatus (33) is arranged downstream the first apparatus (13), wherein said second apparatus (33) comprises means (35) for rotation of the refrigerant about the longitudinal axis (17) of the tube (11) during operation of the evaporator (3), so as to radially separate a liquid phase (L) from a gaseous phase (G) of the refrigerant due to centrifugal action.

7. Natural convection system evaporator according to any of claims 2-6, wherein said tubes are vertical.