[0001] The present invention relates to a process for the production of a heat transfer
element for boiling a liquid, to the heat transfer element produced in this way, and
to an apparatus for carrying out the process.
[0002] It is well known that the heat transfer co-efficient for boiling a liquid may be
improved by coating the heat transfer surface of a heat exchanger with a matrix of
small heat-conductive particles which produce a network of linked re-entrant cavities.
The cavities act as nucleation centres for the production of bubbles of vapour.
[0003] So called "high flux" (trademark) tubing having a high co-efficient of heat transfer
is available but its production is complex and the tubing is correspondingly expensive.
High flux tubing is generally produced by coating a tube with petroleum jelly and
depositing particles of a cupro-nickel powder onto the tube so that the particles
become embedded in the petroleum jelly. The cupro-nickel powder is made up of two
components having different melting points. The coated tube is then heated in a furnace
to a temperature such that one of the components just melts and adheres the other
non-melted powder to the tube. In this way, a matrix of metal particles firmly adhered
to the tube is produced.
[0004] There have in the past been various attempts to provide a high heat transfer surface
by plasma spraying liquified metal onto a substrate, but generally speaking, results
have not been consistent.
[0005] European Patent application 88307468.4 describes a method of coating heat transfer
surfaces by spraying a particulate mixture of metal and a plastics material onto a
thermally conductive surface to form a coating comprising particles of plastics material
embedded in metal, and heating to a temperature sufficient to volatilise the plastics
material, thereby forming pores in the metal coating.
[0006] Our earlier European Patent application 91310265.3 describes the production of a
surface coating comprising porous carbon particles embedded in a matrix of metal particles
produced by plasma spraying a mixture of particles of metal and carbon onto a substrate
in an inert gas atmosphere.
[0007] US Patent 3,384,154 describes a method of coating heat transfer surfaces with small
metallic particles which are subsequently sintered onto the substrate material.
[0008] US Patent 4,753,849 describes a process of coating an evaporator tube which involves
arc spraying two dissimilar metals onto the tube and then etching out one of the metals.
[0009] European application 80101983.7 describes the production of a porous boiling surface
by spraying liquified aluminium onto a substrate under inert gas according to certain
specified conditions.
[0010] The reasons why particular types of coating give high co-efficients of heat transfer
are complex. The important parameters include particle size, pore size, range of pore
sizes, activity of the particles, surface tension of the liquid being boiled and the
angle of contact between liquid and particle surface in the presence of vapour.
[0011] Generally speaking, the present invention is based on the discovery that co-spraying
liquified metal and liquid carbon dioxide results in a surface having good heat transfer
properties.
[0012] Most specifically, the present invention provides a process for the production of
a heat transfer element for boiling a liquid which comprises:
- spraying a substrate with liquified metal particles; and
- concurrently spraying the substrate with liquid carbon dioxide:
such as to form a heat transfer element having a matrix of metal particles attached
to the substrate. The invention also provides a corresponding apparatus.
[0013] The term "spraying" is to be interpreted broadly to include processes wherein substantially
semi-solid or liquid metal is impacted onto a substrate.
[0014] Whilst the liquid carbon dioxide and metal may be sprayed onto the substrate from
closely spaced adjacent locations (e.g.nozzles) it is preferred to move the substrate
relative to the liquified metal spray (by moving the spray locations, the substrate,
or both), such that the liquid carbon dioxide is sprayed onto a given area of the
substrate slightly before the liquified metal particles impinge on that area of the
substrate. This has the effect of pre-cooling the substrate and also cooling the liquified
metal particles somewhat so that the particles may become skinned or semi-solid. In
this way, the particles have sufficient adherent nature to adhere firmly to the substrate,
yet are not so liquid that they flow and lose their particle-like nature when they
hit the substrate.
[0015] It is particularly advantageous if the liquid carbon dioxide is sprayed into a shroud
which directs the liquid carbon dioxide first onto the substrate and then the cold
carbon dioxide is directed (usually sideways) into the spray of liquified metal particles.
[0016] Liquified carbon dioxide has been found to be uniquely effective in the practising
of the invention. It is possible that some thermal decomposition of the carbon dioxide
takes place leading to the production of carbon. However, this is merely a hypothesis
and should not be used to limit the generality of the present invention. On spraying,
the liquid carbon dioxide forms a powder which then vapourises to provide a cooling
effect. The formation of the fine powder and its subsequent vapourisation may assist
in the formation of pores in the matrix. Thus, it is found that the good results of
the present invention are not obtained by spraying other liquids such as water, liquid
nitrogen, or sulphur hexafluoride.
[0017] The metal particles are preferably formed of a heat conductive material such as aluminium
or other high conductivity metal or alloy known in the art. The particle size is usually
in the range 1 to 100 microns, particularly 10 to 60 microns, and the thickness of
the layer is preferably less than 250 microns. Thus, on average, the layer will usually
be about 2 to 5 metal particles deep.
[0018] Preferably, the substrate is grit-blasted prior to spraying. Usually, the substrate
is in the form of a heat transfer tube but may also be a plate or other heat transfer
element. The substrate is usually of metal, for example cupro-nickel, copper, steel
or stainless steel. Generally, it is not possible to produce "high flux" tubing from
stainless steel according to conventional technology.
[0019] The heat transfer element of the invention allows heat exchangers for boiling liquids
to be fabricated, which have high co-efficients of heat transfer. The heat transfer
element finds particular use in the refrigeration field.
[0020] An embodiment of the invention will now be described by way of example only in conjunction
with the attached drawings wherein;
Figure 1 is a schematic cross section through an arrangement used for carrying out
the process of the present invention;
Figure 2 is a test rig for comparing the heat conductivity of a tube B according to
the invention with two other tubes for comparison;
Figure 3 is a cut away section through a heat transfer tube equipped with a heater
for use in the test rig; and
Figure 4 is a graph of temperature difference (delta T) verses power P for the three
tubes.
[0021] A heat transfer tube according to the invention was produced as follows. A cupro-nickel
tube (90% copper and 10% nickel) was cleaned by grit blasting and was then mounted
in the apparatus shown in Figure 1. The tube 2 was mounted in a lathe for rotation
about its longitudinal axis. Around the tube is mounted a spraying device 1 which
is arranged so as to be moveable along the length of the tube in the direction indicated
by the arrow for concurrently spraying the tube with metal particles and liquid carbon
dioxide. It comprises a plasma spray gun 4, equipped for spraying aluminium metal
in a flow of inert gas onto the tube, and a shroud 6 having a central aperture 10
for receiving the tube and an inlet nozzle 8 for spraying liquid carbon dioxide into
the shroud and onto the tube. The shroud is arranged slightly upstream of the plasma
spray of aluminium particles, such that the liquid carbon dioxide first cools the
pipe surface and then exits from the shroud in a direction towards the plasma spray,
so that the cold carbon dioxide then cools the plasma spray also. The flow rate of
liquid carbon dioxide and the spacing of the shroud from the plasma stream is chosen
such as to provide good adhesion of the aluminium metal particles 12 to the tube surface
and to provide a high co-efficient of heat transfer.
[0022] The coating of aluminium particles was typically around 100 microns thick.
[0023] The heat transfer ability of the tube was then tested in an apparatus as shown in
Figures 2 and 3. The heat transfer tube 2 was fitted with a heater 34 as shown in
Figure 3, and a low melting point alloy Ostalloy 158 (trademark) was poured into the
space 39 between the heater and the tube to provide good thermal contact. The tube
was equipped with a surface thermocouple pair 36 to measure the temperature at the
surface of the tube.
[0024] Figure 2 shows a test rig for measuring temperature difference between the tube surface
and a boiling liquid, in this case refrigerant R11 (trichlorofluoromethane). A sealed
container 10 is filled with liquid R11. The surface temperature of the tube is measured
by thermocouple 36 and the temperature of the refrigerant R11 measured by a series
of thermocouples (not shown) placed in the bulk of the liquid. The liquid is boiled
by three heated tubes A, B and C each equipped with a heater and variable power supply.
Each power supply is fed through a varistor 22. A watt meter 20 is fitted to measure
the power drawn by the heater. The vapour produced is condensed using cooling water
supplied to a cooler via inlet 12 and returned via outlet 14.
[0025] Heat transfer tube B is provided with an aluminium particle coating according to
the invention, as described above. Tube A is a shot blasted cupro-nickel tube, and
tube C is a high flux tube, for comparison purposes. The temperature difference between
the tube surface and the R11 liquid was measured.
[0026] Figure 4 shows the test results. The shot blasted cupro-nickel tube A shows a temperature
difference of typically around 6 to 9°C depending on the power. The temperature difference
of the tube B according to the present invention is relatively low, indicated good
heat transfer, and is comparable to the conventional high-flux tube C. However, the
tube of the present invention is simpler and cheaper to produce.
1. A process for the production of a heat transfer element for boiling a liquid which
comprises:
- spraying a substrate with liquified metal particles; and
- concurrently spraying the substrate with liquid carbon dioxide;
such as to form a heat transfer element having a matrix of metal particles attached
to the substrate.
2. A process according to claim 1 wherein the liquid carbon dioxide is sprayed onto a
given area of the substrate such that the substrate is precooled before liquified
metal particles are sprayed onto said area.
3. A process according to claim 2 wherein the liquified metal particles and the liquid
carbon dioxide are sprayed onto the substrate from spaced adjacent locations, the
substrate being moved relative to the liquified metal spray; such that the liquid
carbon dioxide is sprayed onto a given area of the substrate before the liquified
metal particles.
4. A process according to claim 3 wherein the liquid carbon dioxide is sprayed into a
shroud which directs the liquid carbon dioxide first onto the substrate, and then
into the spray of liquified metal particles.
5. A process according to any preceding claim wherein the substrate is in the form of
a tube.
6. A process according to any preceding claim wherein the substrate is formed of a metal
selected from the group consisting of cupro-nickel, copper, steel and stainless steel.
7. A process according to any preceding claim wherein the metal particles are formed
of aluminium.
8. A process according to any preceding claim wherein the matrix of metal particles has
a thickness less than 250 microns.
9. A process according to any preceding claim wherein the liquified metal particles have
a particle size in the range 10 to 60 microns.
10. An apparatus for the production of a heat transfer element for boiling a liquid which
comprises:
- means (4) for spraying a substrate (2) with liquified metal particles; and
- means (8) for concurrently spraying the substrate with liquid carbon dioxide;
such as to form a heat transfer element having a matrix of metal particles (12) attached
to the substrate.
11. An apparatus according to claim 10 which further comprises means for moving said liquified
metal particles, spray means and said liquid carbon dioxide spray means relative to
the substrate, such that the liquid carbon dioxide is sprayed onto a given area of
the substrate before the liquified metal particles.
12. An apparatus according to claim 10 or 11 which further comprises a shroud (6) into
which the liquid carbon dioxide is sprayed, which shroud directs the liquid carbon
dioxide first onto the substrate, and then into the spray of liquified metal particles.