GB2241320A

GB2241320A - Nucleate boiling devices

Info

Publication number: GB2241320A
Application number: GB9004409A
Authority: GB
Inventors: John Edward Hesselgreaves
Original assignee: UK Secretary of State for Trade and Industry
Current assignee: UK Secretary of State for Trade and Industry
Priority date: 1990-02-27
Filing date: 1990-02-27
Publication date: 1991-08-28
Also published as: GB9004409D0

Abstract

An enhanced surface for a nucleate boiling device has a removable shroud (14) shaped to have first portions (16) which contact a prime surface (13) and second portions (18) which stand proud of the prime surface (13). First sets of holes (15) are arranged in the first portions (16) and second sets of holes (17) in the second portions (18). in use liquid passes through the first sets of holes (15) to cavities (19) between the second portions (18) and the prime surface (13) where it is entrapped whilst receiving heat which causes it to vaporise. Bubbles of vapour then exit through the second sets of holes (17). <IMAGE>

Description

NUCLEATE BOILING DEVICES The present invention relates to nucleate boiling devices, and in particular to enhanced surfaces for such devices.

Nucleate boiling devices such as reboilers and tome industrial refrigeration equipment operate at low temperature differences, which might be as low as l C or less. Many nucieat boiling devices therefore have heat exchange surfaces treated in some way in order to improve heat transfer. Such treated surfaces are usually referred to as enhanced surfaces, and the most usual form of enhanced surface incorporates some form of re-entrant cavity. Material having enhanced surfaces for use in nucleate boiling devices and having re-entrant cavities of various forms are produced commercially, examples of the types of re-entrant cavity being porous bonded or sintered surfaces (produced by, for example, Union Carbide), grooved surfaces with regular holes (Hitachi, Thermoexcel) and T-shaped fins (Gewa T).Similar surfaces also occur in heat pipe evaporators, where the surface can consist of a fine wire mesh, in which the capillary effect is important. In all these enhanced surfaces it is considered essential that the material providing the enhancement is at the main metal temperature so that an incipient vapour bubble forming in liquid is surrounded by slightly superheated liquid.

These known types of enhanced surfaces are expensive to manufacture, and many of them also suffer from the disadvantage that their efficiency can be reduced as a result of contaminants in boiler fluids clogging the re-entrant cavities.

There is, therefore, a requirement for a form of enhanced surface for nucleate boiling devices which is cheaper and easier to manufacture, and which can preferably also be decontaminated.

According to the present invention an enhanced surface for a nucleate boiling device includes a removable shroud shaped to have first portions and second portions such that when the first portions contact a prime surface the second portions stand proud of the prime surface, there being a first set of holes in each of the first portions and a second set of holes in each of the second portions, the configuration of the first portions and the first sets of holes being such that there is fluid access through these holes from one side of the shroud to cavities between the second portions and the prime surface.

In use liquid will pass through the first set of holes into the cavities. The structure of the second portions inhibits natural convection of superheated liquid away from the prime surface, thus providing ideal conditions for bubble growth, and vapour so formed escapes through the second set of holes. The type of re-entrant cavity provided by the present invention acts by confinement only, rather than, as with the prior art, surrounding an incipient vapour bubbiby slightly superheated liquid. It is not necessary that the shroud should, as with the prior art, be at the same temperature as the metal of the component providing the prime surface, and the shroud may advantageously be formed from a material having low heat conductivity such as stainless steel or plastic such that there is little heat lost by conduction through the shroud.When the shroud is formed from plastic it may be shaped by a moulding process.

The shroud is preferably formed such that liquid supply and vapour escape routes follow regular paths.

In one form of the invention the shroud is corrugated, with the first set of holes being formed in the bottoms of the corrugations and the second set of, preferably larger, holes being in the top of the corrugations.

In another form of the invention a shroud has slots formed therein by, for example, milling, with the first set of holes being formed through the unslotted regions and the second sets of holes leading from the slots.

In yet another form of the invention a series of tapered holes may be formed in the shroud.

Shrouds may be adapted to fit plane prime surfaces, external or internal cylindrical prime surfaces, or any other shape of prime surface.

Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, of which Figure 1 shows an elevation, in section, of one embodiment of the invention, Figure 2(a) shows a plan view of a second embodiment of the invention, Figure 2(b) shows an elevation in section along line A-A of Figure 2(a), Figure 2(c) is a perspective view of a detail of the embodiment of Figures 2, Figure 3 is a plan view of a third embodiment of the invention, and Figure 3(b) is an elevation in section along line B-B of Figure 3(a).

A. nucleate boiler device (Figure 1) includes a sheet of conducting material 10 separating a heating medium 11 from a liquid 12 to be vapourised. The side of the sheet 10 adjacent the liquid 12 constitutes a prime surface 13 on which is positioned a shroud 14. The shroud 14 is a corrugated shape and has a first set of holes 15 at each of first portions 16 of the corrugations which contact the prime surface 13 and a second set of holes 17 in each of second portions 18 of the corrugations which stand proud of the prime surface 13. The holes l7of the second set are larger than the holes 15 of the first set. The first sets of holes 15 and second sets of holes 17 are preferrably symmetrically arranged with the same number of holes in each set.

In use liquid passes through the first set of holes 15 into cavities 19 between the second portions 18 of the shroud 14 and the prime surface 13, where it is entrapped whilst receiving heat which causes it to vapourise. Bubbles of vapour then exit through the second set of holes 17. When the first and second sets of holes 15,17 are symmetrically arranged and there are an equal number of holes in each set liquid will be supplied through the first set of holes 15 and vapour will escape through the second set of holes 17 in regular paths.

In another embodiment of the invention (Figure 2(a) (b) (c)) a shroud 24 is formed from a sheet of material milled or otherwise machined to have a series of slots 29 alternating with flanges 26.

The first sets of holes 15 are drilled through the flanges 26, the diameter of the holes 15 being slightly greater than the width of the flanges 26. The second sets of holes 17 lead from cavities formed by the slots 29. In use this embodiment of the invention operates in an identical manner to that described with reference to the embodiment illustrated in Figure 1.

In a third embodiment of the invention (Figure 3(a) (b)) a shroud 34 has series of frusto-conical apertures 35 formed therein, the tips of the apertures 35 providing second sets of holes 37.

First sets of holes 35 are drilled with each hole 35 leading into two adjacent frusto-conical apertures 35. In use this embodiment of the invention operates in identical fashion with the two previously described embodiments.

It will be realised that while the invention has been described with reference to nucleate boiling devices having plane prime surfaces 13 it can be readily adapted for use with other shapes of prime surface 13. For example shrouds similar to those illustrated at 14, 24, 34 can be designed to fit external or internal prime surfaces 13 of cylindrical shape. Such shrouds may be resilient and have discontinuities therein (such as axial splits in a cylindrical shrouds)so that they can be deformed to allow them to be fitted and then allowed to reform so as to engage the prime surfaces.

It will be realised that the detailed design of the sleeve and the size and distribution of the sets of holes 15, 17, 35, 37, will affect the performance of the invention, and that optimisation of the various parameters will be dependent on the characteristics of the various operating fluids and working conditions.

In a simple form of the invention an enhancement factor of 2 has been achieved before optimisation of performance was attempted.

Claims

What is claimed is: 1. An enhanced surface for a nucleate boiling device including a removable shroud shaped to have first portions and second portions such that when the first portions contact a prime surface the second portions stand proud of the prime surface there being a first set of holes in each of the first portions and second set of holes in each of the second portions, the configuration of the first portions and the first sets of holes being such that there is fluid access through these holes from one side of the shroud to cavities between the second portions and the prime surface.
2. An enhanced surface as claimed in Claim 1 wherein the shroud is formed of a material having low heat conductivity.
3. An enhanced surface as claimed in Claim 2 wherein the material is stainless steel.
4. An enhanced surface as claimed in Claim 2 wherein the material is a plastic material.
5. An enhanced surface as claimed in Claim 4 wherein the shroud is shaped by a moulding process.
6. An enhanced surface as claimed in any one of Claims 1 to 5 wherein the shroud is so formed that, in use, liquid is supplied through the first set of holes to the cavities, and vapour escapes from the cavities through the second set of holes, the liquid supply and vapour escape routes following regular paths.
7. An enhanced surface as claimed in any one of Claims 1 to 6 wherein the shroud is corrugated with the first set of holes being formed in bottoms of corrugations and the second set of holes formed in tops of corrugations.
8. An enhanced surface as claimed in any one of Claims 1 to 6 within the shroud has slots formed therein, the first set of holes being formed through unslotted regions and the second set of holes leading from the slots.
9. An enhanced surface as claimed in any one of Claims 1 to 6 wherein the cavities and second set of holes comprise a series of tapered holes formed in the shroud.
10. An enhanced surface as claimed in any one of Claims 1 to 9 wherein the holes of the second set of holes are larger than the holes of the first set of holes.
11. An enhanced surface as claimed in any one of Claims 1 to 10 wherein the prime surface is plane.
12. An enhanced surface as claimed in any one of Claims 1 to 10 wherein the prime surface is the inside of a cylinder.
13. An enhanced surface as claimed in any one of Claims 1 to 10 wherein the prime surface is the outside of a cylinder.
14. An enhanced surfaces substantially as herein described.
15. An enhanced surface substantially as herein described with reference to Figures 1 to 3 of the accompanying drawings.