Plasma spray gun with cooling fin nozzle and deionizer

A plasma spray gun nozzle with fins for cooling. The fins are designed in the region radially outward of the point where the arc strikes the nozzle to have a base width and a slot width and slot depth having advantageous dimensions. Coolant is forced through the slots at a rate to achieve optimum heat transfer. Accordingly, the design for the nozzle used in its desired manner provide a plasma flame spray gun which will operate for an appreciably longer time than could prior art plasma flame spray guns before a gun failure is likely to occur.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of plasma flame spray guns and 
particularly to a plasma flame spray gun nozzle with a thin nozzle wall 
and an annular coolant passage having cooling fins therein, which 
increases the nozzle life over that previously achieved with prior art 
designs. 
In typical plasma flame spraying systems, an electrical arc is created 
between a water cooled nozzle (anode) and a centrally located cathode. An 
inert gas passes through the electrical arc and is excited thereby to 
temperatures of up to 30,000.degree. F. The plasma of at least partially 
ionized gas issuing from the nozzle resembles an open oxy-acetylene flame. 
A typical plasma flame spray gun is described in U.S. Pat. No. 3,145,287. 
The electrical arc of such plasma flame spray guns, being as intense as it 
is, causes nozzle deterioration and ultimate failure. One cause for such 
deterioration is the fact that the arc itself strikes the nozzle/anode at 
a point thereby causing instantaneous melting and vaporizing of the nozzle 
surface. Deterioration is also caused by overheating the nozzle to the 
melting point so that part of the nozzle material flows to another 
location which may eventually cause the nozzle to become plugged. 
There are varying degrees and rates associated with each cause for nozzle 
deterioration. Experience has shown that wall erosion, ultimately causing 
the coolant to burst through the nozzle wall, is another cause of nozzle 
failure. When the jacket bursts, coolant water is released into the arc 
region, resulting in a locally intense electrical arc, causing parts to 
melt. Once a meltdown has occurred, gun repair can be very costly. The 
nozzle deterioration and failure problem is particularly severe at high 
power levels. 
In seeking to overcome this problem, plasma flame spray guns have been 
designed with easily changed water cooled nozzles. During operation, water 
coolant is forced under pressure through passages in the nozzle to cool 
the nozzle walls. Even so, gradual, or sometimes rapid, deterioration 
occurs, and as a precaution against failure, the nozzles are usually 
replaced after a given number of hours of service. This practice of 
replacing the nozzle periodically, however, is quite costly, because the 
interchangable nozzles are fairly expensive and many nozzles with 
considerable remaining life are thereby discarded. 
Many factors are involved in determining the rate of deterioration and 
ultimate failure of a plasma flame spray gun nozzle. For the most part, 
nozzle operating conditions and geometry, gas type and flow rate influence 
the nozzle life, as well as does nozzle cooling. 
The prior art generally recognizes that cooling the nozzle wall is 
necessary and has the above-noted effect on nozzle life. The prior art, 
however, does not recognize the optimum design for nozzles and cooling 
passages, including cooling fins in plasma spray guns, thus leaving the 
designer to endless experimentation in attempting to determine the optimum 
design for maximum nozzle life. 
Some installations of plasma spraying equipment have included deionizers in 
the coolant system which, as indicated by recent studies, has enhanced the 
life of the nozzle. The reason for the nozzle life enhancement apparently 
arises from a reduction of scale formation within the coolant passages of 
the nozzle. However, under more severe operating conditions, e.g. high 
power level, use of a deionizer alone is not sufficient to significantly 
enhance nozzle life. 
Therefore, it is the primary objective of the present invention to provide 
a plasma flame spray system designed to maximize nozzle life. 
It is a further objective of the present invention to provide a nozzle for 
a plasma flame spray gun, which includes cooling fins and is designed to 
maximize the operational life thereof. 
It is still a further objective of the present invention to provide a 
nozzle for a plasma flame spray gun with a coolant passage therein 
designed to maximize heat removal from the nozzle wall. 
It is yet a further objective of the present invention to provide a nozzle 
for a plasma flame spray gun having a wall thickness which maximizes the 
nozzle life as defined by the equation: 
EQU Life=(T.sub.start -T.sub.min)/R 
where T.sub.start is the initial wall thickness, T.sub.min is the wall 
thickness at failure and R is the erosion rate in depth per unit time. 
Another objective of the present invention is to provide a nozzle for a 
plasma flame spray gun having a wall thickness, a coolant passage therein, 
and cooling fins in the coolant passage all designed to minimize melting 
and flow of nozzle material to thereby reduce failure by plugging of the 
nozzle. 
BRIEF DESCRIPTION OF THE INVENTION 
In achieving the foregoing and other objectives of the present invention, 
the plasma spray gun system of the present invention has a nozzle designed 
for long life. The nozzle has a plurality of fins disposed within the 
coolant passage and extending radially outward from the nozzle wall, the 
inner side of which is subjected to the plasma flame. When the coolant 
flows between and around the fins, heat is removed from the fin sides as 
well as from the nozzle wall located at the base of the slot between 
adjacent fins. The range of dimensions for base width of the fins, the 
width of the slot between the fins, the depth of the slot and the nozzle 
wall thickness are selected to maximize nozzle life. 
In addition, the plasma spray gun system of the present invention may 
include means to remove ions and dissolved gases from the coolant. Tests 
have demonstrated that removal of certain ions and trapped gases from the 
coolant has the advantageous effect of increasing nozzle life. In 
combination with the optimally designed nozzle with a thin nozzle wall and 
with cooling fins in the coolant passage, the nozzle life is extended 
beyond what could be expected, considering the nozzle life improvement 
achieved with the optimal nozzle design by itself and the nozzle life 
improvement achieved using a deionizer and/or a dissolved gas remover 
alone.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a nozzle shell is illustrated generally at 10. This 
shell 10 is generally annular in shape and includes a central opening 12 
which extends through the nozzle shell 10 and is symmetrically located 
with respect to the center line 14. The nozzle shell has a radially 
extending flange portion 16 with a forward facing surface 18 and a rear 
facing surface 20. When the nozzle, according to the present invention, is 
installed in a plasma flame spray gun such as a Type 3MB or Type 7MB 
manufactured by METCO Inc., Westbury, N.Y., the forward facing surface 18 
bears against the rear surface of a holding ring (not shown) which 
attaches by threads, or the like, to the gun. The rear surface 20 of the 
flange 16 engages an O-ring (not shown) which bears against the forward 
surface of the gun, so that when the holding ring is tightened, the 
O-ring, which is in contact with the surface 20, is compressed to provide 
a seal between the nozzle shell and the gun body. 
The nozzle shell 10 includes an annular-shaped opening 22, which provides a 
passage for a liquid coolant, such as water, to be distributed evenly 
around the nozzle shell 10 when it is operatively coupled to the body of a 
plasma spray gun. The shell 10 additionally includes an annular slot 24 
located in the inner wall 26. This slot 24 also provides a means for 
evenly distributing cooling fluid around the center line 14 of the nozzle 
shell 10 when it is operatively coupled to a nozzle as shown in FIG. 2. 
Communicating between the slot 24 and the passage 22 is a plurality of bore 
holes 28 which are provided in the nozzle shell 10 in order to permit 
cooling fluid to pass between the slot 24 and the passage 22. 
A second annular slot 30 is located between the portion having a 
cylindrical wall 26 and that portion having a cylindrical wall 32. The 
slot 30 is provided to receive an O-ring (not shown) to form a coolant 
seal. This coolant seal will be described in greater detail later. 
The nozzle shell 10 additionally includes three set screws 34 (one being 
shown), each of which is located in a threaded bore such as 36, that are 
spaced evenly around the shell 10. The tip thereof 38 extends through the 
wall 26 for engaging, as illustrated in FIG. 4, the rear surface of the 
flange 60 to hold the nozzle 50 into the nozzle shell 10. 
Referring now to FIG. 2, a nozzle is illustrated generally at 50. The 
nozzle 50 has an entrance portion with a substantially cylindrical wall 52 
and an exit portion also having a substantially cylindrical wall 4. The 
diameter of the cylinder having wall 54 is smaller than the diameter of 
the cylinder having wall 52. Accordingly, the nozzle 50 includes a tapered 
portion having a tapering wall 56 which communicates between the wall 52 
and the wall 54. 
Disposed near the forward end of the nozzle 50 is a radially projecting 
flange 60 which completely encircles the nozzle at a point close to its 
forwardmost end. The outer surface 62 of the flange 60 is designed to 
cooperate with the slot 30 and the surface 26 so that a portion of the 
surface 62 bears against the surface 26 to in part provide a coolant seal. 
In addition, the surface 62 bears against an O-ring, which is located in 
the slot 30. This O-ring in the slot 30 (not shown in FIG. 1) additionally 
provides a seal between the coolant passage of the assembled nozzle and 
the exterior of the assembled nozzle. 
As is readily understood, the nozzle wall temperature is a major 
contributing factor to nozzle life, and particularly the temperature at 
the point where the arc strikes the nozzle wall. Reducing the sidewall 
temperature of the nozzle has the effect of increasing the nozzle 
strength, reducing melting migration, reducing erosion rate and increasing 
the nozzle life. Such a nozzle wall temperature reduction can be achieved 
by reducing the wall thickness between the coolant passages in the nozzle 
and the arc/plasma passages. When the wall temperature goes down, the 
erosion rate also goes down; however, there is a trade off to be made 
between structural integrity and the reduced erosion rate. The reduced 
temperature due to the reduced wall thickness must lower the erosion rate 
fast enough to compensate for the reduced depth of tolerable erosion. 
The body of the nozzle 50 comprises the anode of the plasma flame spray gun 
and is designed with a wall thickness of T in the region where the arc is 
likely to strike the anode. The body 50 is made of substantially pure 
copper (preferably at least 98% pure) and has a wall thickness T in the 
range of about 1.9 to 2.8 mm (0.075 to 0.110 inches). 
Copper (substantially pure) is the preferred material for many parts of the 
nozzle because of its electrical and thermal properties. That is, copper 
is a good electrical and thermal conductor and yet has a relatively high 
melting point. Those of skill in the art will recognize that other metals 
or alloys with electrical and thermal properties substantially like those 
of copper can be used for the nozzle, although the dimensions may need to 
be somewhat different in order to optimize nozzle life. 
In the region 66, the nozzle 50 has a plurality of fins 68 which are formed 
on the exterior surface of the nozzle 50. The fins 68 are shown in greater 
detail in FIG. 3 and extend radially outwardly from the surface 70 of the 
nozzle. Each such fin 68 has an outer surface 72 which, when the nozzle 50 
is nested into the shell 10 as illustrated in FIG. 4, preferably does not 
bear against the tapered surface 74 of the nozzle shell 10, but has a gap 
therebetween of up to 2.5 mm (0.100 inches), with the preferred range 
being 0.127 to 2.0 mm (0.005 to 0.080 inches), while Applicant prefers 
using about 0.25 mm (0.010 inches). 
As illustrated in FIG. 3, each of the fins 68 is spaced equidistant from 
each other fin by a slot 76, which has a width W at the base of the slot 
and a depth indicated by the doubleheaded arrow D. Each of the fins have a 
base width B. The dimensions of the slot and the fin are important in 
assuming long life for the nozzle as these dimensions control the extent 
to which heat can be removed from the nozzle during operation of the 
plasma flame spray gun. 
It has been found that the dimensions herein are important at a point 
radially outward of the point where the arc of the gun strikes the nozzle 
50. This is determined by first making a nozzle 50 of the desired shape 
and running it under the desired operating conditions for a short time. 
The place of maximum erosion will identify the location where the arc 
strikes the nozzle. The fin and slot dimensions radially outward of the 
point where the arc strikes are then decided on. 
The fin base B should be as thin as possible to provide maximum heat 
transfer away from teh axis of the nozzle, but the thinness is limited by 
the need for longitudinal heat flow and for structural strength. The slot 
width W similarly should be as small as possible but should not be so 
small as to restrict the turbulent water flwo or to allow blockage by 
bubbles or small particles of debris that inadvertently may be in the 
cooling system. 
It has been determined that the fin base B should be in the range of 
between about 0.127 to 6.35 mm (0.005 to 0.250 inches), although it is 
preferred to be in the range of 0.25 to 1.27 mm (0.010 to 0.050 inches). 
The slot width W at the base of the slot should be in the range of between 
about 0.127 to 3.8 mm (0.005 to 0.150 inches), although it is preferred to 
be in the range of 0.025 mm to 1.78 mm (0.010 to 0.070 inches). The depth 
of the slot D should be in the range of between about 0.127 to 7.6 mm 
(0.005 to 0.300 inches), although the preferred range is from 0.25 to 2.5 
mm (0.010 to 0.100 inches). The Applicant's preferred dimensions are 1 mm 
(0.040 inches) for B and W and D=2.3 mm (0.090 inches), although fins and 
slots falling in the preferred range will give similar performance. 
Computer simulation suggests that another slightly different set of 
dimensions may also give excellent nozzle life. They are B=0.03 inches, 
W=0.02 inches and D=0.08 inches. 
The exact fluid used for cooling the nozzle according to the present 
invention is not critical, although it is desirable to have a fluid which 
can rapidly absorb the heat flowing through the nozzle 50 from the intense 
heat zone in the region of the arc to the cooler zone in the region of the 
thin annular passage. The rate of fluid flow is preferably sufficient to 
prevent the fluid in the thin annular passage between the nozzle 50 and 
the shell from boiling due to contact with the exterior surface of the 
nozzle 50. The principle reason for this is that preventing boiling of the 
fluid also reduces scale formation on the exterior surface of the nozzle 
50, which therefore promotes longer useful life of the nozzle. A high 
coolant flow rate also reduces the extent of gases which become dissolving 
in the coolant which has the beneficial effect of improving nozzle life. 
The rate of flow through the passages should have a Reynolds Number in the 
range of 2000 to 100,000, while the preferred range is between 5000 and 
50,000. Testing has shown that a Reynolds Number of 10,000 works very 
well. These figures are achieved with a flow rate for water through the 
slots in the range of 0.76 to 46 meters per second (2.5 to 150 feet per 
second), with the preferred range being between 3 to 18 meters per second 
(10 to 60 feet per second). Actual coolant speed of about 6 meters per 
second (20 feet per second) has given good results. This coolant speed 
translates to about 0.25 liters per second (4 gallons per minute) of water 
through a nozzle having dimensions in the preferred range. 
Referring now to FIG. 4, the shell 10 of FIG. 1 and the nozzle 50 of FIG. 2 
are shown interfitted with each other, as well as inserted into the gun 
body of a plasma spray gun such as the Type 3M or 7M manufactured by 
Metco, Inc. of Westbury, N.Y. The body of the gun 100 has an internal 
passage indicated generally by the arrow 102 which couples with the 
opening 22 and permits a cooling fluid, such as water, to be pumped into 
the passage 22 from an external source. The coolant then can flow through 
the bore 28 and the passage 24 to the forward end of the slots formed 
between adjacent fins 68 on the nozzle 50. The cooling fluid then passes 
through the slots between the fins 68 and exits into the passage 104 
formed between the wall of the nozzle 50 and a cylindrical wall 106, which 
forms part of the gun. The coolant then passes through the wall 106 
through a passage (not shown) indicated by the arrow 108 and is thereafter 
either discarded or placed into a reservoir for recirculation back through 
the nozzle. 
Those of skill in the art will readily recognize that the specific design 
may take other forms. For example, the screws 34 and O-ring 30 may be 
omitted and replaced with silver soldered joints between the shell 10 and 
the nozzle 50. Alternatively, the shell 10 may be made in two halves with 
holes therethrough so that they can be screwed or bolted together to from 
the coolant passages between the shell and nozzle. Such an arrangement is 
similar to that described in U.S. patent application Ser. No. 292,763, 
filed on Aug. 14, 1981, entitled "Heavy Duty Plasma Spray Gun". That 
application also describes a gun which can use a nozzle of the present 
invention. 
Referring now to FIG. 5, the cooling system for the nozzle according to the 
present invention may take the form shown therein or it may comprise a 
simple system wherein a source of water is coupled to the annular-shaped 
opening 22 and the fluid exiting from the passage 104 is simply allowed to 
be discharged. The system of FIG. 5, however, is a closed loop system 
which offers, among other advantages, a means for reducing cost of coolant 
water used by the system. 
The water exiting from the plasma flame spray gun 110 is at a higher 
temperature than that entering the gun and exits the gun through the 
passage 104 via a conduit 108 and eventually reaches a heat exchanger 112 
which may comprise a conventional heat exchanger arrangement. Once the 
temperature of the cooling fluid is reduced, the fluid then passes through 
a pump 118, which raises the fluid pressure on the output side of the pump 
to a sufficient level so as to provide the desired cooling fluid flow rate 
through the nozzle. The cooling fluid then passes through a deionizer 114 
which removes ions from the cooling fluid by means of an ion transfer 
resin contained in the deionizer 114. A suitable resin for this purpose is 
known as Red Line mixed bed resin and is manufactured by Crystalab. 
After exiting the deionizer 114, the cooling fluid then passes through a 
dissolved gas remover 116, which may be of the resin type, having a 
suitable resin for removing dissolved oxygen from the cooling fluid. An 
alternative approach for removing dissolved gases is to use a pressure 
reducer of the type used by electrical utilities companies. In the process 
of reducing the pressure of the cooling fluid, dissolved gases within the 
fluid are released. If a pressure reducer is used in the configuration of 
FIG. 5, the position of the pump 118 and the gas remover 116 must be 
reversed. Dissolved gas in the cooling fluid has the effect of diminishing 
nozzle life and, by removing such gas from the cooling fluid, nozzle life 
improves. 
The output of the gas remover 116 communicates via a pipe 102 to the spray 
gun 110. This allows the cooling fluid to recirculate through the nozzle 
and ultimately back to the heat exchanger 112. It should be noted that it 
is preferable to locate the deionizer 114 and the gas remover 116 as close 
as is practical to the fluid input of the plasma spray gun. 
While the arrangement shown in FIG. 5 includes a heat exchanger 112, a 
deionizer 114 and a gas remover 116, each with a specific function, it is 
possible to operate the plasma flame spray gun of the present invention 
including a nozzle of the type illustrated in FIGS. 1-4 with a closed loop 
cooling system including only a heat exchanger 112 and a pump 118. These 
two elements are necessary to assure sufficient coolant flow through the 
nozzle to prevent melting. 
As indicated above, however, the deionizer 114 does have an advantageous 
effect in that it has been shown that deionizing the cooling fluid has the 
effect of improving nozzle life. Test results of the present system 
indicate, however, that adding a deionizer 114 to the system including a 
thin wall nozzle with fins disposed in a thin annular passage as 
illustrated in FIGS. 1-4 results in a product life improvement which is 
greater than one would expect, considering the nozzle life improvement 
achieved by the thin annular passage nozzle of FIGS. 1-4 by itself and the 
nozzle life improvement achieved by a deionizer by itself. Accordingly, it 
is advantageous, though not necessary, for systems according to the 
present invention to include a deionizer of the type described. 
The system of FIG. 5 also includes a gas remover 116 which, as already 
indicated, may comprise a pressure reducing device of the type used in the 
electrical utility industry, although other pressure reducers or other 
means, such as an oxygen removing resin may be used. As indicated above, 
the gas remover 116 is not an essential element of the present invention, 
but may be used in cooperation with other system elements to achieve an 
increase in nozzle life. 
An alternative approach is to use a single cannister in the coolant path 
located close to the coolant entry to the nozzle. The cannister has a 
layer of deionizer resin, a layer of deoxygenator resin and a layer of 
charcoal. This single cannister arrangement sreves to remove ions, oxygen 
and other dissolved gases from the coolant before it enters the nozzle. 
The foregoing and other modifications to the system illustrated in FIGS. 
1-5 may be made without departing from the spirit and scope of the present 
invention as defined in the following claims.