PORTABLE PLASMA TORCH SYSTEM WITH COOLING SYSTEM FOR THE ELECTRONIC COMPONENTS USING A SECOND MEDIA

The present invention relates to a portable plasma torch, that is a plasma torch not being directly connected to external sources for electrisity and/or compressed air. More specifically, the invention relates to a cooling system for portable or detached plasma torches.

s Plasma torches are used for plasma cutting or shearing. Plasma cutting is a process mainly used to cut work pieces of steel or other types of metal. Pressured air or gas under high pressure is blown with high velocity through a mouth piece. Inside the mouth piece, there is an electrode. The work piece will function as anode, via a ground lead. By means of an electrical field, an electric arc is formed between the electrode and io the work pieve and an ionisation of the air or gas arise, so that plasma is formed. The plasma has a sufficiently high temperature to melt the metal and has a velocity sufficiently high enough to blow the metal away.

Because plasma cutting is a process demanding high amounts of energy, parts of the electronic components will have a high consumption of power. A part of the electric or is electronic components therefore demand separate cooling. This applies among others to thyristors, resistors or transistors. The usual way to cool such components in a commercially available torch with an external supply of air and electrisity, is to mount such components on a heatsink. The heatsink is cooled by forced convection by means of a ventilator blowing air over the heatsink. Systems like that are relatively heavy and

20 can weigh 4-5 kg for a machin with a typical output power of 6 kW.

On a portable plasme torch, a battery arrangement will attend to supply of electrisity, while an air bottle will attend to supply of compressed air or gas. For the torch to have a practical utility value, the batteries and air bottle should have a capasity of at least 5 minutes of continous cutting. However, torches with a longer cutting time is 25 envisageable, such as, for example from 5 to 10 minutes or more, while one at the same time in some embodiments may find cutting times from 5 to 3 minutes adequate.

The above mentioned elements, battery arrangement, gas supply etc. will give an additional weight to the portable plasma torch exciding normal weight for a stationary plasma torch. In addition, a portable plasma torch is provided with some sort of carrying 30 arrangement, for example harnesses and/or a frame and others. The complete torch should have a total weight allowing a person to relatively easily carry it on the back over a certain distance, in some cases it should also be carried in rugged terrain, that is, it should have the lowest weight possible.

In addition to weight, is it an advantage that the portable torch has a design and size and a handleability being acceptable to the user and preferable simplifying the use of the torch.

By using prior art cooling of the electrical components, such as with ventilators, one will obtain a portable plasma torch unit with all necessary equipment having a weight of around 30 kg.

It is an object of the present invention to provide a plasma torch with a minimal weight, It is also an object to provide a portable plasma torch being easy to handle by the user.

To obtain this, a plasma torch according to the present invention will be provided with an improved cooling system.

The invention will now be further explained and described by means of non-limiting examples of embodiments of a portable plasma torch and cooling system for the plasma torch. In the figures is shown:

Fig. 1 shows a first perspective view of a portable plasma torch.

Fig. 2 shows a second perspective view of the portable plasma torch.

Fig. 3 shows a perspective view of a first embodiment of a cooling system according to the invention.

Fig. 4 shows a perspective view of the cooling system according to the first embodiment.

Fig. 5 shows a perspective view of a second embodiment of a cooling system according to the invention.

Fig. 6 shows a perspective view of the cooling system according to the second embodiment.

Fig. 7 shows a perspective view of the cooling system according to a third embodiment.

Fig. 8 shows a perspective view of a fourth embodiment of the invention. The plasma torch is in the description described as "portable". Plasma torches connected to a supply network is also portable over a limited distance, depending on the length of the wires or cables connecting the plasma torch to the supply system. However, in this application, with "portable", it is meant a plasma torch that can be used by the user without being connected to any external supply system. Another word for "portable" in the meaning used in the application could be "detached".

Figure 1 shows a portable plasma torch 1 according to the invention. The plasma torch 1 comprises a carrying arrangement 2, 3. In the shown embodiment, the carrying arrangement comprises a carrying frame 2 with harnesses 3. A power source in form of a battery arrangement 4, a source of compressed air or gas and an electronics housing 6 are mounted to the carrying frame 2.The plasma torch is furtermore provided with a ground lead 7 with earth clamp 8 and torch cable 9 with torch nozzle 10. At least one electric cable 11 pass between battery 4 and electronics housing 6. A reduction valve 24 is provided in connection with the compressed gas source 5. An pressure regulator 13 is provided in connection with the electronics housing 6. The reduction valve 24 and pressure regulator 13 will be further explained below. It should however be noticed that the reduction valve 24 can also be arranged on the upper side of the plasma torch 1, if this is desirable. This will typically be the case if also the compressed air/gas source 5 is turned the other way around, that is 180° in relation to the direction given in the embodiment shown in the figures. In the same way, is it possible to arrange the gas pressure regulator 13 other places on the electronics housing, if desirable, but this will be clearer from the following part of the description.

Figure 2 shows a second perspective view of the plasma torch 1. Electronics housing, compressed air source, power source and other equipment is mounted on a carrying frame 2. The earth clamp and torch nozzle will under transport be arranged in suitable holding devices (not shown). Harnesses 3 are attached to the frame, so that the arms of the user are free when he is carrying the plasma torch. This will be an advantage if the plasma torch is to be made operational quickly, as the user do not need to take the arrangement off the back to activate the torch, that is, to attach the earth clamp, open the gas supply etc. At the same time, the harnesses make it possible for the user to hold and/or bring other equipment, when the plasma torch is not in use, but is merely placed on the back of a user. Figure 3 shows a portable plasma torch 1 with a first embodiment of a cooling < _. arrangement 12 in accordence with the invention. In the shown embodiment, a battery arrangement 4 run the plasma torch. The batteries should preferably have a capasity to give the plasma torch an operating time before recharging of at least 5 minutes before they are discharged, and should also stand quick discharge. Furthermore, it is neccesary for the batteries to have a sufficently high voltage to create an arc. The battery arrangement in the shown example is a number of batteries connected in series.

An example of a suitable battery is cell no. ANR26650M1 commersially available at the time of filing, from the manufacturer Al 23 Systems, Watertown, Massachusetts, USA. The battery is a rechargeable cell unit with normal voltage of 3,3 V and normal capacitance of 2,3 Ah. Internal impedance at 1 kHz AC is typically 8 mΩ. The batteries can be delivered with an amperage of up to 7OA at continuous discharge, and at 10 sec pulsated discharge, they can delive an amperage of up to 120A. The batteries are connected with the electronics chamber via at least one electric cable 11.

The compressed air/gas source in the embodiment is a composite gas cylinder with a pressure of 300 bar and a volume of about 6 litre, of common type. This type of cylinders are e.g. commercially available at the time of filing, from Drager Safety AG & Co. KGaA, Lϋbeck, Germany. The gas cylinder 5 is provided with a pressure regulator 24 for controlled relase of the gas or air. The gas will have a considerable pressure fall through the pressure regulator and will probably have a pressure of about 8 bar after the regulator. During this pressure fall, the temperature of the gas will fall, as explained further below. The gas is lead throug a gas channel 25 to a gas pressure regulator 13. The gas pressure regulator 13 controls that the resulting pressure of the gas to be lead further to the torch nozzle is correct. Compressed air or gas under a relatively high pressure, that is normally around 4-5 bar, is lead further from the gas pressure regulator via a gas cable to the cooling arrangement 12 and further from the cooling arrangement 12 via a gas cable 15 to the torch cable 9 and via the torch cable to the torch nozzle 10.

The cooling arrangement 12 in accordence with the first embodiment of the invention is a radiator. In the shown embodiment, the radiator is a radiator of general type, where a fluid is lead through one or more channels provided with cooling ribs. An example of this type of radiator is the modell 21908ERL commersially available at the time of filing, from Earl's Performance, Rancho Dominguez, California, USA. However, it is possible that the cooling arrangement or radiator can be of a more basic type, such as a tube, e.g a lightweight metal tube made of for example aluminium, magnesium or other, going in a loop over a plate, e.g, a lightweight metal plate made of for example aluminium, magnesium or other, being tightly connected to the plate for temperature transfer between the cold gas in the tubes and the electronic components in need of cooling, for example being conected to the plate on the opposite side of the plate.

The radiator is cooled by using the fall of temperature in the compressed gas due to a fall of pressure in the compressed air/gas, when the air or gas leaves the compressed gas source 5. This will be further explained below, but a simple explanation on this is as follows.

The compressed gas source or gas supply in the shown example is an air bottle or gas cylinder with a typical pressure of 300 bar. A reduction valve 24 and a regulator 13 regulate the pressure out of the cylinder down to proper work pressure for the plasma torch. The working pressure is in order of magnitude 5 bar. When the gas is regulated from a pressure of up to 300 bar down to around 5 bar, one will get an expansion of the gas from the gas cylinder. This expansion, together with a throttling process in the reduction valve, makes the gas considerably refrigerated. This will be further explained below under the heading "Free expansion and throttling process in air". For a gas cylinder with gas such as air pressurized to 300 bar and a typical air consumption for a plasma torch, this process will lead to an air temperature in order of magnitude of -10 ⁰ C after one minute and order of magnitude -20°C after 3 minutes.

This cold gas, being eventually used in the cutting process, is led through the cooling arrangement 12 in the form of a radiator, a cooling loop, a pressure tight housing or other, where the gas is first used to cool down electronic components being a part of the plasma torch arrangement, before it is led further to the torch nozzle 10.

By not having a separate cooling system, but integrate this in the system of compressed gas , it will be possible to reduce the weight of the cooling arrangement considerably, from around 3-5 kg in a traditional system with a ventilator or the like, to approx. 1 kg or more in a gas cooled system according to the invention. Electronic components with a need for cooling, e.g. thyristors, resistors, transistors or the like, are mounted on a cooling plate 17 and the cooling plate 17 is connected to a cooling arrangement 12, such as a radiator. Gas is brought into the radiator arranged after the reduction valve 24 and regulator 13, passing through the cooling arrangement or radiator and out via a solenoid valve 18, controlling the of supply gas further to the torch cable 9. The solenoid valve is controlled by an activate button 19 on the torch nozzle 10. The same activate button can, and will in general, also activate the electronic components.

On a plasma torch with a cooling arrangement according to the first embodiment, the radiator will be cooled by forced convection and the electronic components will be cooled via the cooling plate by conduction.

Figure 4 shows a more detailed view of the cooling arrangement in accordence with the first embodiment of the invention. A radiator 12 is mounted on one side of a cooling plate 17. Electronic components 16 being in need of cooling during use of the plasma torch, are mounted on the other side of the cooling plate 17. The radiator 12 is of a type being cooled by means of gas flowing into the radiator 12 through an inlet 20 and out of the radiator through an outlet 21. The inlet 20 is connected with the first gas cable 14 being connected to the compressed air source 5. Between the inlet 20 and the compressed air source 5, there will be a reduction valve (see e.g. fig. 3) and an air pressure regulator 13 to regulate the gas supply to the cooling arrangement 12 and the torch nozzle 10. The outlet 21 is connected to a second gas cable 15 leading the gas through a solenoid valve 18 and further to the torch cable 9.

On at least one of the gas cables 14, 15, there is arranged a solenoid valve 18 or other valve suitable for being controlled by an easily accessible switch arrangement. In the shown embodiment, the valve 18 is mounted between the second air cable 15 and the torch cable 9. The valve 18 can suitably be opened and closed by means of a start button 19 arranged on the torch nozzle, as shown on fig. 1. The start button may also be used to start the arc by activating the electrical system of the plasma torch. The start button 19 can be arranged on any available place on the portable plasma torch after the requirements in force for the use of such plasma torches.

Figures 5 and 6 shows the cooling arrangement in accordence with a second embodiment. This embodiment uses the same principle as in the first example, i.e. the use of the cold air flowing out of the compressed air source towards the torch nozzle and works in short as follows: The electronic components with a need for cooling are mounted in a gas tight housing, where small cooling ribs can be mounted on the components in question. Air is brought into the housing after the regulator, the air passes through the housing and out via a solenoid valve, leading the air further to the torch cable in the same way as in the first shown embodiment. The solenoid valve is controlled by the activate button on the torch nozzle, also being decribed above. In this second embodiment, the electronics are cooled by forced convection.

Figure 5 shows a portable plasma torch 1 with the second embodiment of a cooling system according to the invention. The plasma torch is provided with a carrying frame 2, harnesses 3, batteries 4 and a compressed air source 5 as described above. Pressurized air or other gas under pressure is lead from the compressed air source 5 through a reduction valve 24 further through a gas channel 25 to an air pressure regulator 13 and further to a gas cable 14 leading the pressirized air or gas in to a cooling arrangement in the form of a cooling housing 6. The pressurized air is then lead further from the cooling housing via a second gas cable 15 to the torch cable 19. The plasma torch 1 is provided with a reduction valve 14, solenoid valve 18 and activate button 19, as described above in relation to the first example of embodiment.

Figure 6 shows, partly sectionally and with more details, the cooling arrangement according to the second embodiment. Gas, such as air, under pressure is led from the compressed air source via a reduction valve and gas cable (not shown) and further past the pressurised air regulator 13, through the first gas cable 14 and in through the inlet 20 to the cooling arrangement 12 in the form of a gas tight housing. The gas tight housing 12 is partly taken away in figure 5 to show the components inside the housing. Electronic components 16 with a need for cooling, as mentioned above, are arranged on a plate 22 such as for example a circuit board or print card 22. The components 16 can furthermore optionally be provided with cooling ribs 23. Compressed air or pressurised gas flow over the cooling ribs and the electronic components in the housing 12, out of the outlet 21, through the second gas cable 15, past the relay controlled valve 18 to the torch cable 9 and further out of the torch nozzle (not shown).

Fig. 7 shows a perspective view of the cooling system according to a third embodiment. Fig. 8 shows a perspective view of a fourth embodiment of the invention. As a further possibility, one could use phase change materials as a cooling agent in the cooling arrangement. This is shown in the third and fourth embodiment. A phase change material (PCM) is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy. When thermal energy is withdrawn from a liquid or solid, the temperature falls. When heat energy is added the temperature rises. However, at the transition point between solid and liquid (the melting point), extra energy is required (the heat of fusion). To go from liquid to solid, the molecules of a substance must become more ordered. For them to maintain the order of a solid, extra heat must be withdrawn. In the other direction, to create the disorder from the solid crystal to liquid, extra heat must be added. Hence heat is absorbed or released when the material changes from solid to liquid and vice versa; thus, PCMs are classified as latent heat storage units.

PCMs latent heat storage can be achieved through solid-solid, solid-liquid, solid-gas and liquid-gas phase change. However, the normal phase change used for PCMs is the solid- liquid change. When PCMs reach the temperature at which they change phase (their melting temperature), they absorb large amounts of heat at an almost constant temperature. The PCM continues to absorb heat without a significant raise in temperature until all the material is transformed to the liquid phase. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its stored latent heat.

A large number of PCMs are available in any required temperature range from -5 ⁰ C up to 190 ⁰ C . There are several materials that can be used as PCMs, either organic materials, normally paraffin and fatty acids or inorganic materials, normally salt hydrates. In some cases organic and inorganic PCMs are combined.

In the embodiment shown in fig. 7, a container 26 with PCM is connected to the electronic components 16 and the cooling plate. As the cooling plate is heated by the electronic components, parts of the heat will be absorbed by the container 26 as a further cooling means in addition to the radiator provided with cooled gas.

Fig. 8 shows a perspective view of a fourth embodiment of the invention. The electronic components 16 are connected to one side of the container 26 containing PCM. When the components 16 gets hot during use, the PCM in the container will absorb energy in form of heat, thereby keeping an acceptable temperature for the electronic components. As a further cooling measure, the gas flowing from the gas cylinder towards the torch nozzle, is led through a cooling loop passing over one side of the container 26, thereby absorbing heat from the container and keeping the temperature of the container down.

It is possible that a container of PCM will provide sufficiently cooling effect for the electronic components necessary to run the plasma torch. However, by introducing the use of PCM as cooling material in a plasma torch in addition to a cooling arrangement using low-temperature gas, one will obtain a highly efficient and compact overall cooling arrangement.

In the shown examples, cooling ribs are arranged in the assumed direction of the air flow through the cooling arrangement in the form of an air chamber 12. As an example, one can use a rib arrangement of the type SKl 06 commersially available at the time of filing, from Fischer elektronik, Lϋdenscheid, Germany. These are extruded rib units with 7 ribs per 40 mm width and 27 mm height.

However, a person skilled in the art will be free to choose any direction, height, width and number of ribs to obtain the best possible cooling. In other words, the ribs can also be arranged transverse to the assumed direction of flow or current, or in any angle between 0 and 180° to the direction of the flow. Neither must the ribs stand paralell, but can be arranged in any desired random or predefined pattern. If this is desirable, it is also possible to omit the ribs, so that the cold gas flows directly over the electronic components.

Furthermore, it can also be possible to use more than one gastight cooling chamber or housing and distribute the electronic components with a need for cooling, in a number of chambers or housings, for example so that the ones with the highest need for cooling are arranged in a first housing, while components with a descending need are arranged in the following one or more housings before the gas is lead to the torch nozzle.

As the gas flow out of the compressed air source, it can obtain very low temperatures. It can be of interest to lead only a part of the gas through the cooling arrangement to obtain a sufficient, but not too hard cooling of the electronic components. If this is desirable, a thermocontrolled valve can be arranged in the gas flow before the cooling arrangement, where the thermocontrolled valve at a given temperaure leads a part of the gas past the cooling arrangement through a third gas cable connected to the solenoid valve and/or further to the torch cable and the torch nozzle.

It should be noted that even if it in the introduction is said that a portable plasma torch with the use of prior art will weight around 30 kg, this is only an assumption, as it is not known to make such portable or detached plasma torches. A normal plasma torch connected to a supply system weighs at the time of filing from around 9 kg up to around 44 kg. A portable plasma torch will in addition comprise carrying arrangements and portable sources for power and compressed air. Therefore, it is likely that a portable plasma torch with the use of prior art cooling will have an even higher weight and quite possibly up towards 40 kg or that the weight will be somewhat under 30 kg. This will however be without importance for the invention, as any saving of the weight for this type of equipment will be of great value for the user and will considerably increase the extent and range of the use.

It should also be noted that even if it in the description is given examples of compressed air sources such as gas cylinders with 300 bar, it is obvious for a skilled person that gas cylinders with other pressures can also be relevant, as long as the pressure is sufficiently high to provide the needed amount of gas. It can e.g. be cylinders from 300 bar and down towards 50 bar and even lower or, if desirable, cylinders with a higher pressure than 300 bar, i.e from 300 bar and up towards 600 bar and even higher, for example up to 1000 bar or more. It can also be used other types of cylinders than composite cylinder, such as steel cylinders, light metal cylinders, containers in fibre reinforced polymer materials and other.

Furthermore it should be noted that in the shown examples, a battery arrangement with a number of smaller connected units is shown as a power source. However, it can also be possible to use one single battery being of such quality that it in itself gives sufficient power and discharge rate. It is obvious that this will also fall under the scope of the invention. The batteries can be of a rechargeable type or disposable batteries. Furthermore, the power source can be any suitable compact transportable power source, and other poser sources such as a small compact light weight power generator, for example a generator made of components known from radio telecontrolled airplanes or the like, could also be used. This will also fall under the scope of the invention, as it is stated in the appended claims.

The cooling effect from pressurised air flowing from a compressed air source to a torch nozzle will now be further explained.

FREE EXPANSION AND THROTTLING PROCESSES IN AIR Free expansion and real gases

The description of cooling by free expansion of gas and cooling by means of the Joule- Thomson effect following below is found in Fredrick Reif. Fundamentals of Statistical and Thermal Physics. McGraw-Hill, 1965. Joule-Thomson or the throttling effect for air is previously described and quantified in J. R. Roebuck. The Joule-Thomson Effect in Air. Physics, Proc. N. A. S., 12:55.58, 1925. In J. R. Roebuck and H. Osterberg. The Joule-Thomson Effect in Nitrogen. Phys. Rev. ,48:450.457, September 1, 1935 presents curves and tables being used further below and especially fig. 1 in J. R. Roebuck and H. Osterberg. The Joule-Thomson Effect in Nitrogen. Phys. Rev. ,48:450.457, September 1, 1935 where isenthalpic curves from experimental data are plottet.

First, we look at free expansion of a gas. We thnk of a container divided into two chambers, A and B being separated by a closed valve. At first, the gas is in chamber A and there is a vacuum in chamber B. We open the valve so that gas flows from A to B. Equilibrium is obtained with the same pressure in both chambers. Will the temperature change during expansion?

We assume that the container is undeformable and thermally isolated from the surroundings so that the heat trasport Q to the surroundings is ignorable, the last can be written as

Q = O (1.1)

As the walls of the containers are rigid, no mechanical work is performed on the surroundings

W= O (1.2)

From the first law of the thermodynamics Q = AE +JF it follows that the total energy kept for this process AE = Q (1.3)

For simplisity it is assumed that the container has an ignorable heat capacity. Thereby, it will not absorb any heat (in the real world steel containers are often used and they have a high heat capacity). The experiment with a gas container as described above and lowered into water was performed by Joule. The temperature of the water before and after the expansion was measured and there was not registered any changes. In retrospect, you could say that the precision of the measurements was not very high, but it indicates at least the result expected from an ideal gas. Generally, we have the inner energy preserved

ΔE = 0 => E(T ₂ , V ₂ ) = E(T ₁ , F ₁ ). (1.4)

We make a distinction between ideal and real gases. For an ideal gas, the energy resulting from interaction between the molecules is ignorable and the inner energy is only dependent on temperature, E=E(T). For this approach it applies that:

E(Tx) = E(T ₂ ) => T ₁ = T ₂ , (1.5)

The temperature remains the same before and after expansion. This will not necessarily relate to a real gas.

Non-ideal gasses

For a real gas on must consider the molecular interaction. They can repel and attract each other. Approximately, one can say that at high temperatures, the molecules will repel each other, at low temperatures they will retract each other. The state equation for a gas can in general be written as

p = kT(n + B ₂ (T) _n ² + B ₃ (T)H ³ + ^••• ) (2.1)

where the coefficients B ₂ , B ₃ , ... are called viral coefficients. For an ideal gas, these are ignorable. The weak interaction (attraction) between molecules being at great distance appears to be significant at low temperatures, where the cinetical energy of the molecules is small. This results in a smaller distance between the moleculed than without the interaction. The attractions results in a lowered pressure. In this case is B ₂ (T) < 0. In a higher temperature span, the kinetical energy is higher and repulsive short distance forces will dominate. Since you get repulsion as the temperature increases, SBj(T)I δT> 0 and Bj(T) will be positive. This makes both cooling and heating by free expansion possible depending on which temperature and state the gas is in.

Van der Waal introduced the following emprical equation for a real gas:

(p+^)(v~b) = RT, (2.2)

where v = Vl v is the molar volume, a and b are positive constants specific for the actual gas. For a verified gas is a « v and b « v, and (2.2) is reduced to the state equation for an ideal gas PV= vRT.

We are intending to look at the effect of the the deviation from ideal gases, for example the deviation in inner energy compared to ideal gas. For an ideal gas E(T, v) = E(T). For example, what will the change hi inner energy E(T, v) - E(TQ, V ₀ ) be for a van der Waals gas. By deriving (2.2) we get

^' dp\ R

^dTj _n υ-b ^' (2.3)

From thermodynamics we have

«-MS-^ = CV*Γ ₊ (Γ(|) -P)-( -T ₊ (P) iv (2.4)

where we used the Maxwell relation

This gives

'dE\ (dE\ (dp\ βτ) _v = ^Cv ' W) _τ = ^T \&τ) _v - ^p - (2-6)

We introduce inner energy per mol, ε and can write

\dυj _τ v ² We also have that

dv) _τ - ^τ {&r-z) _v - ^τ {w) _υ { ^~ b)-°- (2.8)

Heat capacity c _v is only a function of temperature for a van der Waal gas. For the inner energy/mol we can, according to (2.4) write

dε = c _υ (T)dT + ~dυ (2-9) υ ² which integrated gives

ε(T, υ) - ε(T ₀ , V ₀ ) = f ^(T^dT' - a ( - - -Y (2.10)

At sufficiently low temperature changes c _v can be regarded as constant and (2.10) gives which shows that for a free expansion, the gas is cooled (T < TQ).

Joule-Thomson process

We will now look at what is happening when the gas is flowing through an obstacle or valve. The pressure on the upstreamside is higher than the pressure on the downstreamside, pi >p ₂ . This will lead to an expansion of the gas. In the Joule- Thomson process it is assumed that gas is flowing towards the valve at constant pressure and temperature and likewise out of the valve. We will ignore the heat loss to/from surroundings and assume that all quantities are constant over the time span necessary to obtain thermodynamical equilibrium. We assume that the process is approximately adiabatic.

Look at the gas on the upstream side of the valve. The work necessary to transport a mass M=/>2^ ₂ through the valve is pzVi- On the downstream side, the work performed by the mass M is correspondingly/? _! V\. Net work becomes

W=P ₂ V ₂ -PiV ₁ . (3.1) the change in inner energy becomes

AE = E(T ₂ , P ₂ ) -E(T ₁ , _Pl ). (3.2)

According to the assumption of an adiabatic process, no heat is absorbed by the mass M, Q = O, and

ΛE +F= β = 0 => H(T ₂ , p ₂ ) = E ₂ +p ₂ V ₂ = E ₁ +P ₁ V ₁ = H(TuP ₁ ). (3.3)

The enthalpien His constant for this process.

For an ideal gas the enthalpy can be written as

H= E +pV= E(T) + vRT (3.4)

This is only a function of the temperature. In this case H(T ₂ ) = H(Ti) and T _\ = T ₂ applies. The temperature is constant, for a real gas, the Joule-Thomsnon provess will lead to a change in temperature that we will look at below. The quantity μ = (ST/δp)π is called the Joule-Thomson coefficient and this is an important parametre in the description of the process, μ > 0 gives a cooling while μ < 0 givies heating. The change in enthalpy can be written as

dH= d(E + PV) = TdS + VdP: (3.5)

Since there is no change in the enthalpy, we get

Thereby, we have the following equation for the Joule-Thomson coefficient

'-(SL- ^T (SX ⁺ From the Maxwell relations we get

Here, α is the volume expansion coefficient. Thereby, we can write

μ = -(Ta - I). (3-9)

For an ideal gas a = T ^l and μ = 0. A truncated version of the expansion (2.1) can be written as

p = ^ {l + ^B ₂ ) = ^ (kT + _P B ₂ ) . (3.10)

Joule-Thomson coefficient will in this case become

At low temperatures, where the molecules are in an attractive state, you will have i? ₂ < 0 and δ5 ₂ /δr> 0 so thatμ > 0. In this case the Joule-Thomson process will lead to cooling. It is worth noticig that there are gases, for example He, which have the transition μ = 0 at very low temperatures, 34K, while for N ₂ (air) this happens at 625K. For higher pressures the stage μ = 0 is lowered. We notice that air mainly consists of oxygen O ₂ and nitrogen N ₂ . Both are biatomic and therefore behaves similarly thermodymically. Therefore, air can be simulated well by clean N ₂ gas. Isenthalpic curves ranging over a large temperature and pressure range is given in fig. 1 in R. Roebuck and H. Osterberg. The Joule-Thomson Effect in Nitrogen. Phys. Rev. ,48:450.457, September 1, 1935.

Air stream from a bottle and through a reduction valve

Two effects will manifest themselves here. The one is the expansion of the gas as the gas cylinder is emptied, and the other is passing of the air through the reduction valve. Both processes will lead to a cooling of the gas. We have measured the development over time of temperature and pressure over the reduction valve. The development over time of the pressure inside the valve was also measured. The development of the temperature of the gas inside the bottle can then be estimated from the isenthalpic curves of the gas. In our case (0 <p < 300 Bar, -30°C < T< 20°C), all isenthalpic curves are "parallel", so that a wanted isenthalpic curve can be obtained by shifting an other along the temperature axis. We have picked the isenthalp going through the point (-3 ⁰ C, 20 bar). This is shown in Diagram 1 below. We must rememeber that the use of isenthalpic curves applies approximately under adiabatic conditions. For any moment, it applies that

H(T ₁ , _Pϊ ) = H(T ₂ , P ₂ ): (4.1)

Ti can be decided from the measured quantities p _\ , T ₂ , p ₂ . We read T _\ for measured/^ on the isenthalpic curve passing through (p ₂ , T ₂ ), see Diagram 1.

40

30

20

10

υ

-10

-20

-30

-40

50 100 ISO 200 250 300 350

Bar

Diagram 1: Isenthalpic curve through thepointt (-3 ⁰ C, 20 Bar). The curve applies for air

Description of the invention

Divergence from adiabacy is the largest source of errors in the expansion and throttling prosess. Measured values shows that the temperature in air after the reduction valve varies with the pressure after the reduction valve (p ₂ ). According theory on throttling, the temperature should fall with reduced pressure, while the measurements show the opposite. This is shown in Diagram 2. The diferences are relatively large.

By increasing the pressure, the temperature is lowered. Measurements were done at p ₂ = 2 bar, 5 bar, 10 bar, where a pressure of 10 bars give the lowest temperature. According theory being based on constant enthalpy, it should be the opposite. The fall in temperature with falling pressure estimated from the isenthalpic curve is 2-3 °C. This is not consistent with what is actually measured. The measured temperature fall is as much as 10-15°C, where the temperature fall increases with increased pressure. Our theoretical results are based on the assumption of adiabaticy. The discrepancy described above can be explained by the process not being adiabatic. This is in conformity with the observed fact that the valve and the surrounding metal is considerably cooled down. This heat is provided to the gas and thereby the gas become varmer than that given by the idealised theory. It is therefore essential to measure the cooling that is in fact obtained to find ot if it is sufficient for the purpose.

5 bat 5 bar ^; 2 bar. 2 bar 10 • 10

60 120 180 240 300 360 420 480 540 Sβkunder

Diagram 2: Measured temperatures at different pressures after the reduction valve

Air is a compressible medium. By increasing the pressure after the valve from 2 to 10 bar, the density will change by a factor of 5 at 10 bar, (p = pi RT). The present difference in temperature is of minimal significance for the result. The heat capacity C for the gas is 5 times higher at 10 bar than at 2 bar. For heat and temperature we can approximately write AT= AQ/C. Since || ATT || « T we can write AT _2bar lAΥ _mar ~ Ciobar I C ₂ bar = 5. Consequently we can expect a higher temperature at 2 barthan at 10 bar provided that there is a heat transport to the surrounding components. Both temperatures are higher than the ideal temperature Υ _ad resulting from adiabatic conditions. We have a number of measurements and it is on principle possible to calculate Υ _a d CCo ₂ TTo ₂ —- OOi 1io0pTT- ¹ iIiOo 11 . C5 n

C ^i2 —— C ^iiQo 4 ⁴

We see that T ₁₀ Tad when Cw 00.

Example: With T ₂ = -10 ⁰ C, T ₁₀ = -27°C, you have T _ad = -32°C. By reading on the matching isenthalpic curve, we get a final temperature inside the cylinder of approx. - 15°C after the pressure is redused to 100 bar, that is after approx. 100 sec. After this, it semm that the curves are flattening.

The temperature fall for a composite cylinder and cable comparet to steel cylinder and cable is shown in Diagrams 3 and 4. The heat capacity and the thermal conductivity of the composite material is lower than that of steel. We can suggest that for steel cylinders, there will be a considerable loss of heatfrom the steel cylinder to the gas (when the temperature of the gas is lower than the temperature of the steel). This leads to s considerable deviation from adiabaticy. In a sensible calculation of the cooling of the gas/air, one must consider this. By measuring pressure and temperature after the reduction valve simultaneously with the pressure in the cylinder, the temperature inside the cylinder can nevertheless be calculated (as we have seen above).

A lower temperature of air is obtained after the reduction valve for composites than for steeel and it takes a longer time before the composite cylinder reaches it's lowest temperature (measured outside the cylinder). This is shown in Diagrams 3 and 4 and is consistent with the expected. Since cylinder, reduction valve and metal parts surrounding it are cooled down, the assumption of adiabacy is not correct. However, the measurements shows that you still gets the plenty cooling. For a composite cylinder a temperature below -3O ⁰ C is obtained after 120 sec at 10 bar cable pressure. Suppose that the pressure inside the cylinder is 30 bar at this time. Then we can read from the isenthalpic curve a temperature on the short side of -20 ⁰ C inside the cylinder. (etfer i ; verιtll)=

Sekunder

Diagram 3: Measured temperature and pressure development for a steel cylinder and cable

Conclusion: Expansion of air in a gas cylinder together with the throttling process in a reduction valve gives large amounts of cooled air for cooling of electronics in a plasma torch. Temperatures below -30°C are obtained, available for cooling of electronics and temperatures below -20° C is obtained inside the gas cylinder. It can be discussed wether it is necessary with a starting pressure of 300 bar in the gas cylinder. The Joule- Thomson is much less efficient at high pressure, as the isethalpic curve is flattening and the Joule-Thomson index μ is approaching zero.

-♦-Temperatur slangs (βtter ventil) :

^■ -•- Temperatur flaske :

: Trykk slaπge (etler ventil)

Sekundor

Diagram 4: Measured temperature and pressure development for a composite cylinder and cable

The invention is described with reference to the use of air as a cooling gas. Air is normally cinsidered as a gas mixture of about 80% N ₂ and about 20% O ₂ or 100% N ₂ .

Based on what is explained in relation to expansion of compressed air expanded through a nozzle, it will be possible for the skilled person, without unnecessary experiments, to arrive at fluids or fluid mixtures behaving in a similar way. Examples of other fluids or fluid mixtures one could think of as usable, could be CO ₂ , O _2, N ₂ and/or mixtures of these. In fact all fluids with the exeption of helium, hydrogen and neon, could probably be used, but due to the easy and cheap availability of air, this gas/gas mixture is preferred.