Heat exchange – With coated – roughened or polished surface
Patent
1982-08-12
1985-05-07
Davis, Jr., Albert W.
Heat exchange
With coated, roughened or polished surface
165180, 261112, 423274, 5253337, 5253338, 5253339, 525344, 525345, F23F 1302
Patent
active
045152101
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a heat exchanger which utilises a plastics film material as a membrane through which heat is to be transferred between fluids, especially between two liquids or between a liquid and a gas or vapour.
BACKGROUND ART
It has been proposed to use plastics film as the membrane through which heat is to be transferred between fluids in a heat exchanger because, compared with the metals which are conventionally used, it is inexpensive, resistant to corrosion and comparatively easy to replace.
The low price of polyolefin and polyester films makes them attractive for this use, but they suffer from the fact that their surfaces are not naturally wettable. This means that heat transfer coefficients are reduced because liquids flowing over the surfaces of the film do so as droplets rather than as the desired liquid film. Various treatments have been given to polyethylene film in particular to make it more wettable, including subjecting it to ionising radiation from a corona discharge and coating it with silica, but to our knowledge this has not resulted in any treated films which combine cheapness with retention of wettability over extended periods of use.
DISCLOSURE OF INVENTION
According to this invention, a heat exchanger for exchanging heat between fluids and having a heat exchange membrane through which heat is transferred from one fluid to another is characterised in that the membrane comprises a polyolefin or polyester film of which one or both surfaces have been given increased wettability by a treatment with an oxidising agent selected from the group consisting of sulphur trioxide, ozone, oleum, chlorosulphonic acid and chromosulphuric acid.
The polyolefin may be a homopolymer or copolymer, for example low density or high density polyethylene, polypropylene, copolymers of ethylene and propylene with each other and/or other olefin monomers, or silylated ethylene copolymers such as those made by Dow Corning Corporation under the trade name "Sioplas E". The polyester may be a homopolyester or copolyester and preferably comprises poly(ethylene terephthalate).
The oxidising agent used to treat the film may be gaseous or liquid. The preferred agent is gaseous sulphur trioxide. With such powerful oxidising agents the desired wettability can be imparted with treatment times as short as a minute or less. More extended treatments are unnecessary in relation to wettability, and, as the film is progressively degraded as the treatment proceeds, it is prudent to keep the treatment time to the minimum required.
With sulphur trioxide, oleum, chlorosulphonic acid and chromosulphuric acid, the treatment can be carried out at room temperature; any elevation of temperature tends to make the reaction of the oxidising agent with the plastics film too vigorous, so that control of the treatment is more difficult. With ozone, higher temperatures are preferred to give a reasonably rapid reaction rate leading to satisfactory results after short treatment times, 60.degree. to 80.degree. C. being a suitable range.
The heat exchange membrane may be in any form which is suitable for plastics film but is preferably in the form of a tube or sheet. The treatment of the tube or sheet may be carried out as a continuous process by passing the tube or sheet in continuous form through a vessel containing the strong oxidising agent. Both faces of the film may be treated where the heat exchange is to be between liquid phases, but it is advantageous to leave one face untreated and therefore less wettable if it is to be contacted by a condensing vapour.
With a tube, it is comparatively easy to treat the outer surface only, for example directly after extrusion, in which case the outer surface will be of greater wettability than the inner surface. Such tubes are useful in heat exchangers in which a liquid is passed over a tube bundle to be heated by a vapour condensing within the tubes. One process of this type involves passing a film of brine or other aqueous liquor over the tube bundle, compressing the wat
REFERENCES:
patent: Re24062 (1955-09-01), Horton
patent: 3231490 (1966-01-01), Fry
patent: 3289754 (1966-12-01), Erb et al.
patent: 3426814 (1969-04-01), Johnson
patent: 3551393 (1960-12-01), Muzyczko
Boiston David A.
Smith John
Courtaulds PLC
Davis Jr. Albert W.
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