The invention relates to a pressure vessel intended to be fitted as part of a pressurised gas pipe and arranged for heating the flowing pressurised gas, comprising two concentric tubes inside the pressure vessel, an inlet for pressurised gas to the gap between the tubes, and an outlet from the pressure vessel, wherein the gap between the tubes has its outlet in the pressure vessel and the inner tube has a heating unit for heating the tube from inside.

The invention also relates to a method of heating a flowing pressurised gas in a pipe to a high temperature by leading the gas through a small gap between two tubes fitted in a pressure vessel, wherein the inner tube is heated from the inside and the heated gas is allowed to flow from the gap freely out into the pressure vessel and on to the outlet of the pressure vessel.

US 2,797,297 shows a heater that can heat pressurised gas to a high temperature. The gas flows between the walls of an outer pressure vessel and an inner tube and then back through this inner tube along heating coils. EP 089 998 shows a heater that has an annular gap between two tubes and a burner in the inner tube that must thus be pressure-classified. Yet another heater adapted to heat pressurized gas to a high temperature is known from US 1 985 280. Further fluid heaters are known from patent applications US2527013A and DE19610593A1.

An object of the invention is to provide at relatively low cost a gas heater for high pressure and high temperatures that is easily constructed, easy to maintain and easy to adapt to different conditions.

The object of the invention is achieved by the method as claimed in claim 1 and the pressure vessel as claimed in claim 4. According to the invention the inner tube is open towards the flow path of the gas in the pressure vessel for pressure equalisation between the inside and the outside of the inner tube without the inner tube being part of the flow path of the gas, and the inner tube has an electric element for heating the tube from inside by radiant heat. The two tubes will thereby have roughly the same pressure on their outside and their inside and they do not need to be pressure-approved. The tubes are therefore interchangeable without this affecting the pressure vessel approval. It is only the outer pressure vessel that has to be approved. The electric element is simply interchangeable and is separated from the flow path of the gas. For the process industry, the tube quality can therefore be selected freely and the tubes adapted to the process gas in question. For example, powder-metallurgically manufactured tubes or ceramic tubes that do not tolerate high pressures can be used. With normal tubes, a catalytic effect on the gas can be obtained and carbon deposition occure, for example, if the gas is a reduced gas containing an H₂ and/or CO. The Sandvik Kanthal APM tube (ferritic iron-chromium-aluminium tube) is an example of a tube that can be used. The invention is defined by the claims.

• Figure 1 shows a section through a gas heater as an example of the invention.
• Figure 2 shows an enlarged inlet part of the heater shown in figure 1.
• Figure 3 shows an enlarged outlet part of the heater shown in figure 1.
• Figure 4 corresponds to figure 2, but shows an alternative embodiment.
• Figure 5 corresponds to figure 2 and shows another alternative embodiment.

Figures 1-3 show a gas heater in the form of a pressure vessel, the outer casing of which consists of a tube 11 with ends 12, 13. The end 12 can be bolted firmly to a pipe, for example, or directly to a reactor vessel in a process industry in order to supply heated gas at a high pressure. The entering process gas at a high pressure, for example 100 bar, that is to be heated to a high temperature, for example 1000 degrees Celsius, is supplied through the end 13. The tube 11 is insulated internally by an insulation 14 that is adapted to the high temperature that shall be reached. The insulation can be a ceramic insulation or a fibre insulation, for example. Different sections of the tube 11 can have different insulations adapted according to the temperature, which increases towards the outlet. The insulation can be created in layers with different properties.

Inside the insulation's cavity 15, two concentric tubes 16, 17 are put in as is best shown by figures 2 and 3. The upper ends of the tubes are joined together in a sealing manner, for example welded together or bolted together, and the gap 18 formed between the tubes has an inlet 19 through the end 13 for the gas that is to be heated, which is clearest from figure 2. The gap 18 is maintained by control projections, which are not shown, on the inner tube. The gap is open towards the cavity 15 in the insulation and towards the tapering outlet 20 from the pressure vessel that is formed by this cavity, which is shown best by figure 3. The inner tube 17 has a closed end 21 at the outlet 22 of the gap 18. The tubes 16, 17 are kept in place at the inlet 19 and the tubes can expand freely in a longitudinal direction upon heating.

The inner tube 17 is open towards the end 13 and has electric elements in the form of heating coils 23, 24 along its length. The electric elements have their electric leads 25-28 led in a sealing manner through the end 13. The inner tube 17 is thus heated only by radiant heat from inside and the inner tube does not participate in the flow through the gas heater, which means that the electric coils are not exposed to chemical or catalytic reactions to such an extent. The reaction risk can be reduced further by having a small continuous supply of buffer gas to the inside of the inner tube. In figures 2 and 3, a supply line 30 for buffer gas is shown that extends down towards the closed end 21 of the inner tube 17.

Between the insulation 14 and the outer tube 16 is a gap 31 that provides pressure equalisation between the inside and outside of the inner tube 17, since the inside of the inner tube here remains open towards the gap outlet 22 and thereby towards the part 32 of the insulation cavity 15, i.e. open towards the outlet 20 of the pressure vessel. The part 32 takes up the longitudinal expansion of the tubes 16, 17.

The first coil 23 seen in the flow direction has a tighter winding and greater power than the second coil 24 and the power of the coils can be varied respectively so that the power supplied per unit of length of tube reduces when the gas becomes hotter. The first part of the flow path can have power that is three times as great per unit of length as the last part, for example. The temperature of the electric coils is limited thereby. It is possible to have more than two zones with different power. The gas that flows through the gap 18 acquires a large increase in volume due to heating and pressure reduction. The pressure gradient and heat transfer can be optimised by having a varying gap along the length of the tubes.

Figure 4 shows an alternative embodiment in which a separating wall 34 seals between the pressure vessel tube 11 and the tube 16. Instead of the inner tube 17 communicating with the outlet side of the flow path of the gas in the pressure vessel, it communicates with the inlet side through an opening 35. The embodiments are otherwise the same.

Figure 5 shows another alternative embodiment in which the pressure vessel tube 11 has a flange 36 that is directly bolted to a flange 37 on the inlet tube 38 for the pressurised gas that is to be heated. The inner tube 17 is thus open towards the pressurised inlet side of the flow path of the gas in the pressure vessel. The gap 18 has its inlet 39. Only one, 25, of the electric connections is shown.

The pressure vessel/gas heater can be manufactured in various sizes and as an example of a typical size it can be said that the outer tube 16 can have a length of 3.5 m and a diameter of 140 mm, and the pressure vessel tube 11 can have an outer diameter of 600 mm.