Heat transfer surface

Details Heat transfer surface Heat transfer surface

1999-06-18

2003-06-03

Bennett, Henry (Department: 3743)

Heat exchange

With coated, roughened or polished surface

C428S408000, C427S580000

Reexamination Certificate

active

06571865

ABSTRACT:

The present invention relates to a heat transfer surface, to a heating element or condenser comprising said heat transfer surface, to a method of applying said heat transfer surface and to a method for carrying out dropwise condensation employing said heat transfer surface.
During heating of a fluid via a heat transfer surface the surface will readily become coated with various deposits (eg. CaCO
3
, CaSO
4
and MgSO
4
) which are dissolved or partially dissolved in the fluid. Users of kettles in hard water areas will be well acquainted with this problem. Such deposits have an adverse effect on the heat transfer coefficient of the surface, which is reduced. This, in turn, leads to a decreased working efficiency of the heat transfer surface and/or a reduction in its service life.
There is therefore a need for a heat transfer surface which alleviates the prior art problem of undesirable build-up of deposits.
Condensation processes employ the use of a condenser, also having a heat transfer surface. Due to the nature of conventional metal materials used in the construction of heat exchangers, filmwise condensation occurs in industrial plants, for example in electric utility condensers and commercial distillation plant condensers [1]. Filmwise condensation results in a film of liquid forming over the heat transfer surface, the liquid typically being water which is a very poor conductor of heat. This film then acts as an insulator and impedes efficient heat transfer across the heat transfer surface.
Dropwise condensation has received much attention in the past because of the inherent order of magnitude higher (eg. up to 20 times higher) steam-to-condensing wall heat transfer coefficients that can be achieved compared with filmwise condensation.
To date, however, suitable dropwise condensation surfaces have not been developed to the satisfaction of condenser designers [2]. In this respect, it is a prerequisite for achieving dropwise condensation that the condensing surface possess a low surface energy [2]. Known metal surfaces possess a high surface energy with the result that steam condenses filmwise on such surfaces. To obtain dropwise condensation on a metal surface, the surface energy must be reduced and the following methods have been tried [3]:
(i) to apply organic promoters to a condenser surface;
(ii) to coat the surface with organic polymer; and
(iii) to electroplate a layer of silver or gold on the surface.
Referring to (i), a solution of organic promoter (ie. non-wetting agent) may be wiped, brushed or sprayed onto the surface. However, such promoters wash off within a few hours of use, and dropwise condensation on the surface changes to filmwise condensation [4].
Referring to (ii), many attempts have been made to produce long-term dropwise surfaces by coating with organic polymers [5]. However, because the polymers such as PTFE (Teflon®) have a very low film conductivity, most of the benefits of dropwise condensation are offset by an increase in the thermal resistance of the polymer coating itself.
Referring to (iii), thin coatings of silver and gold have been shown to promote dropwise condensation. However, such coatings are impracticable from an economic point of view.
There is therefore a need for a heat transfer surface which may be used to achieve dropwise condensation.
The above prior art problems are alleviated by the present invention which, according to a first aspect, provides a heat transfer surface comprising a layer of tetrahedral amorphous carbon and/or diamond-like carbon, and/or a composite thereof.
Tetrahedral amorphous carbon (ta-C) is a substantially hydrogen-free form of diamond-like carbon with an sp3 bonding fraction that can be up to 90% or higher. The hydrogen content of ta-C is preferably less than 20% (w/w), and the sp3 bonding fraction is typically at least 50%, preferably at least 60%, more preferably at least 70%. The properties of ta-C are similar to those of crystalline carbon, with a short range tetrahedral order and a density approaching that of diamond. It has a similar hardness, chemical inertness and thermal stability to that of diamond. Large compressive stresses (up to 10 GPa) can be associated with the growth of high sp3 fraction films, and these stresses can reduce the adhesion of such films at higher film thicknesses. Improved adhesion is conveniently obtained by depositing an intermediate reactive metal or metal oxide layer to bond the film and the substrate together, allowing the deposition of ta-C onto any smooth surface material. Adhesion may also be promoted by depositing ta-C films using FCVA techniques on to a substrate which has been negatively biased (typically between −500 to −5000 V). Such biasing creates an interface surface which interacts well with the top surface layers of the substrate, thereby providing much improved adhesion. In addition, ta-C has important electrical properties. The band gap of ta-C is around 2 eV (versus 5 eV for crystalline diamond) making it a suitable candidate for use as a semi-conducting material for a large area and for high temperature device applications.
Diamond-like carbon (DLC), unlike for example a-Si:H, has a nearly state-free optical gap without hydrogenation. It has a hydrogen content of typically 15-60% (w/w), and an sp3 bonding fraction which is typically less than 50%.
In preferred embodiments of the invention, the coating comprises or consists of ta-C. It is particularly preferred when temperatures in excess of 100° C. are likely to be encountered. Hydrogen atoms are very mobile and escape at elevated temperatures leading to a disruption in the structure of the carbon layer. With ta-C, the film is typically substantially hydrogen-free and any such structural disruption is minimised.
The term “composite” means a combination (eg. an alloy) of ta-C or DLC with one or more additional components, typically metal components. The effect of the additional component is generally to modify a physical property of the selected carbon source, such as to increase the hardness and/or hydrophobicity of the coating.
Examples of additional components include gold, silver, nickel, chromium, halides (preferably chloride or fluoride), Teflon® and Teflon®-like organic compounds. Where the additional component is a metal, that metal should preferably have a low surface energy. Where the additional component is Teflon® or Teflon®-like, films of greater thickness can be employed due to the lower stress in such composites. Such films generally have excellent toughness and adhesion properties, and improved thermal stability properties, compared with metal-containing composites.
Composites are preferably employed when high temperatures (eg. 400° C. and above) are encountered.
According to a preferred embodiment the carbon-containing and/or composite layer has a maximum thickness of 1 &mgr;m, more preferably a maximum thickness of 0.5 &mgr;m. The thicker the layer is, the longer the service life will be. However, if the layer is excessively thick, the heat transfer coefficient may be compromised. For this reason, the carbon and/or composite layer is most preferably 0.06 to 0.1 &mgr;m thick.
The carbon and/or composite layer should be preferably clean and uniform in thickness. In this respect, a defect in the layer, especially at the surface, can provide a high stress zone and/or a site for nucleation of scale. The macroparticle content of the coating is preferably zero or close thereto, whilst in practice it is difficult to avoid microparticle (less than 1 &mgr;m) deposits.
According to a preferred embodiment of the present invention, a heating element is provided having a heat transfer surface, said heat transfer surface comprising a layer of tetrahedral amorphous carbon and/or diamond-like carbon, and/or a composite thereof. Such a heating element has a prolonged service life and requires less frequent cleaning to remove deposits which may otherwise cause a reduction in the heat coefficient of the element and an increase in the con

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