OXYGEN HEAT EXCHANGER

Abstract

The present invention relates to a heat exchanger for the supply of oxygen or of a gas mixture containing at least 50% oxygen, the temperature at the outlet of the exchanger not being below 300° C., it preferably being above 400° C., the oxygen or the oxygen-rich gas feeding one or more burners of a glass melting furnace, the heat of the combustion gases being used directly or indirectly to heat the oxygen or the oxygen-rich gas in the exchanger, in which the exchange power is between 20 and 300 kW, preferably between 40 and 250 kW and particularly preferably between 80 and 170 kW.

Claims

1. A process for supplying gas to one or more burners of a glass melting furnace, comprising: supplying combustion gases from the glass melting furnace to a first heat exchanger; effecting a first heat exchange using the combustion gases to heat an intermediate heat transfer gas to form a heated heat transfer gas in the first heat exchanger; supplying oxygen gas comprising oxygen or a gaseous mixture comprising at least 50% oxygen to a second heat exchanger having an outlet; effecting a second heat exchange by heating the oxygen gas in the second heat exchanger with the heated heat transfer gas to a temperature at the outlet of the exchanger of not less than 300° C. to form heated oxygen gas; supplying the heated oxygen gas from the outlet to one or more burners of the glass melting furnace; circulating gas in the tubes carrying the oxygen gas at a rate that does not exceed 120 m/s at any point in the second heat exchanger; maintaining a pressure of the oxygen gas in the second heat exchanger below 3 bar, wherein both the first and second heat exchanges are indirect heat exchanges.

2. The process according to claim 1, wherein each second heat exchanger supplies heated oxygen gas to at most three burners of the furnace.

3. The process according to claim 1, further comprising: circulating the oxygen gas in tubes in the second heat exchanger, and contacting internal walls of the second heat exchanger with a heat transfer gas, wherein the second heat exchanger has a tubular configuration.

4. The process according to claim 3, in which the tubes in which the oxygen gas circulates are substantially straight and walls of the tubes have a thickness that is not more than 3 mm.

5. The process according to claim 3, in which a chamber enclosing the tubes is formed from several elements joined by flanges, wherein tightness is assured at these flanges by composite seals, the sealing element of which is made of material that is inert with respect to oxygen.

6. The process according to claim 5, in which the sealing element is a ring comprising compressible mineral material.

7. A process for supplying gas to one or more burners of a glass melting furnace, comprising: effecting a first heat exchange with combustion gas from the melting furnace with an intermediate heat transfer gas to form a heated heat transfer gas, passing the heated heat transfer gas to a heat exchanger having an outlet, supplying oxygen gas comprising oxygen or a gaseous mixture comprising at least 50% oxygen to the heat exchanger; effecting a second heat exchange by heating the oxygen gas in the heat exchanger via the heat transfer gas to a temperature at the outlet of the heat exchanger of not less than 300° C. to form heated oxygen gas; supplying the heated oxygen gas from the outlet to one or more burners of the glass melting furnace; each burner generating from 1 to 6 MW in the furnace, and maintaining a pressure of the oxygen gas in the second heat exchanger below 3 bar, wherein both the first and second heat exchanges are indirect heat exchanges.

8. The process according to claim 7, in which the heat transfer gas is air, nitrogen, CO.sub.2 or steam.

9. The process according to claim 7, in which the heat transfer gas is formed from combustion gases diluted by means of at least one of the gases: air, nitrogen, CO.sub.2 and steam.

10. The process according to claim 7, further comprising: heating the heat transfer gas in a recuperator, which has been heated by the combustion gases beforehand.

11. The process according to claim 1, wherein a material of surfaces in contact with the oxygen gas in the second heat exchanger is made from a metal alloy of which a sample exposed to the oxygen gas does not exhibit a weight gain of more than 0.1 mg/cm.sup.2 after 1000 cycles of exposure, wherein each cycle includes increasing the temperature of the oxygen gas to a value equal to or higher than 400° C., maintaining this phase temperature for one hour and returning to ambient temperature.

12. The process according to claim 11, in which the alloy complies with the condition of a weight gain of less than 0.1 mg/cm.sup.2 of exposed surface when the phase temperature is at least 500° C. in oxidising atmosphere.

13. The process according to claim 11, in which the alloy in contact with the oxygen gas resists the spontaneous combustion test according to standard ASTM G 124 at least up to a pressure of 3 bar.

14. The process according to claim 11, in which the alloy in contact with the oxygen gas is a ferritic steel alloy containing a percentage by weight of Cr of 12 to 30% and an Al content of 1 to 8%.

15. The process according to claim 11, in which the alloy in contact with the oxygen gas, for an oxygen temperature not exceeding 500° C., is an alloy containing a percentage by weight of chromium in the range of between 10 and 20% by weight.

16. The process according to claim 11, in which the alloy has a Ni content higher than 25% and a Cr content from 10 to 30%.

17. The process according to claim 16, further comprising bringing elements in the heat exchanger in contact with the oxygen gas to a temperature in the range of between 300° and 900° C.

18. The process according to claim 3, further comprising: placing an oxygen detector in contact with the heat transfer gas, and connecting the oxygen detector to an alarm when the oxygen content is more than 1% higher than that of the heat transfer gas.

19. The process according to claim 1, wherein a power exchanged in the second heat exchanger to heat the oxygen gas is in a range of between 40 and 250 kW.

20. The process according to claim 1, wherein a power exchanged in the second heat exchanger to heat the oxygen gas is in a range of between 80 and 170 kW.

21. The process according to claim 1, further comprising maintaining a pressure of the oxygen gas in the second heat exchanger below 2 bar.

22. The process according to claim 1, where the heated oxygen gas at the outlet is at a temperature of not less than 400° C.

23. The process according to claim 1, further comprising each burner consuming heated oxygen at a rate of between 200 to 1200 Nm.sup.3 per hour.

24. The process according to claim 1, further comprising heating the heat transfer gas to between 450° C. and 1000° C.

25. The process according to claim 11, in which the alloy complies with the condition of a weight gain of less than 0.1 mg/cm.sup.2 of exposed surface when the phase temperature reaches at least 600° C. and the oxidising atmosphere exceeds 80% oxygen.

26. The process according to claim 11, in which the alloy complies with the condition of a weight gain of less than 0.1 mg/cm.sup.2 of exposed surface when the phase temperature is at least 650° C. in oxidising atmosphere.

27. The process according to claim 7, further comprising maintaining a pressure of the oxygen gas in the second heat exchanger below 2 bar.

Description

[0062] Examples of practical details of the invention are given in the description below with reference to the set of drawings:

[0063] FIG. 1 is a schematic sectional illustration of a gas exchanger usable according to the invention to reheat oxygen or gas rich in oxygen;

[0064] FIG. 2 is a partially enlarged view of the end of the exchanger shown in FIG. 1;

[0065] FIG. 3 shows a detail of part-section A taken from FIG. 2.

[0066] The general structure of the exchanger is the conventional type for gas exchangers. It comprises a chamber 1 enclosing a bank of tubes 2. The tubes are secured inside the chamber by plates 3, 4.

[0067] The plates form a sealed wall delimiting the zone of the chamber 1, in which the heat transfer gas circulates.

[0068] The chamber is closed at its ends by two covers 5, 6. These covers are tightly secured to the chamber by means of flanges 7, 8, 9, 10 and seals. These flanges can be removed to give access to the ends of the tubes 2, where necessary.

[0069] To obtain the best possible exchange, the circulation of the heat transfer gas and the oxygen or gas rich in oxygen is advantageously conducted in reverse flow. The hot heat transfer gas passes into the chamber through conduit 11 and exits through conduit 12 after having passed through the circuit created by the baffles 13, 14, 15 inside the chamber.

[0070] The oxygen or gas rich in oxygen circulates in the tubes 2 along a substantially rectilinear course. It passes cold through end 16 and exits hot at end 17 to be conducted to the burners.

[0071] The exchange is all the more effective when circulation rates are higher. Nevertheless, the flow rate and pressure of the oxygen must be held within the limits that assure the operating safety of the device. The circulation of the oxygen must be prevented from resulting in excessive corrosion of the walls which it comes into contact with. It should also be ensured that the oxygen does not strike the walls. The use of rectilinear tubes thus restricts erosion.

[0072] The arrangement of the ends of the tubes 2 is shown in detail in FIG. 2.

[0073] To prevent turbulence at the ends of the tubes, with the risks of increased erosion at the point where these tubes connect to the plate 4 most frequently by suitable welds, these tubes terminate with a widened section. This arrangement facilitates the flow of oxygen and its expansion and subsequently some deceleration. This widening is in the shape of a truncated cone in the figure with an angle of opening α.

[0074] For the same reason, the covers, and above all cover 6 arranged at the oxygen outlet, are located at a distance from the ends of the tubes 2. In this way, the flow rate of the oxygen along the walls of the cover is substantially reduced in relation to that at the outlet of the tubes.

[0075] The general shape of this cover 6 is also chosen so that the advance of the hot oxygen encounters the wall of the cover at a low incidence, thus minimising impact. For example, the wall of the cover is at an angle of about 20 to 30 degrees relative to the direction of the tubes 2. The profile of the cover decreases progressively up to the connection with the outlet duct.

[0076] It is also advantageous to ensure that there are no sharp angles or welds in this section.

[0077] The dimensioning of the tubes and their distribution are such that the flow rate and pressure conditions indicated above are met by the delivery rates implemented.

[0078] Since the exchanger must operate continuously over very long periods, it may eventuate that a tube no longer has the necessary tightness in spite of precautions taken to prevent wear of the elements of the exchanger. The assembly of the exchanger is such that the defective tube can be blocked at these two ends. The operation requires that the covers be removed. After the defective tube has been taken out of service, the exchanger is once again usable with an efficiency that is little changed in proportion to the remaining active tubes.

[0079] The tightness at the level of the flanges of the covers 9, 10 of the exchanger or at the connection of these covers with the oxygen intake or outlet ducts is advantageously obtained by means of a metal annular seal 18 lined with a material 19, 20 resistant to oxygen. The material in question is mica or a compressible mineral material, for example. Seals of this type are produced in particular by Garlock under the brand name “Vitaflex”.

[0080] In order to determine the alloys that comply with the implementation conditions according to the invention, the inventors have conducted tests that are discussed in the following description.

[0081] For these tests, the samples are formed from 2 mm thick plates of metal alloy measuring 20×20 mm.

[0082] The condition of the surface of the samples indicates its clear importance with respect to sensitivity to oxidation. For this reason, one face of each of these plates is polished with an abrasive sheet of SiC to grain size 1200. The other face is left in its original state as produced by the industrial rolling process.

[0083] The composition by weight of the samples of alloys tested is specified in the following table:

TABLE-US-00001 Alloy Fe C Si Cr Al Ni Mn Others I 6-10 0.15 0.5 14-17 72 0.5 Cu II comp <0.04 <1.0 19-23 0.15-0.6  30-34 <1.5 0.15-0.6 Cu III comp 20 5.5 0.5 Ti 0.5 Y.sub.2O.sub.3 IV comp 1-2 24 1-2 V comp 22 5 VI comp 0.15 23-27 <1.0 1.5

[0084] The measurement of the oxidation of the samples is evaluated by the increase in their mass after testing over a thousand cycles. The results for different phase temperatures are indicated in the following table:

TABLE-US-00002 Alloy I V Temperature 550 650 800 550 650 800 mg/cm.sup.2 0.013 0.06 0.347 0.004 0.02 0.099 Alloy III IV Temperature 550 650 800 550 650 800 mg/cm.sup.2 0.097 0.10 0.232 0.026 0.05 0.103

[0085] It is evident from these tests that the oxidation is more significant when the phase temperature is higher. At 550° C. the increase in all cases remains well below 0.1 mg/cm.sup.2. At 650° C. only one sample reaches this value. At 800° C. the most resistant samples are those of alloys IV and V.

[0086] The metallographic observation of the samples shows a much lower tendency towards oxidation in the polished face of the samples.

[0087] The above measurements at the same time include the oxidation of the two faces of the sample. Since only one face is polished, the oxidation measurement obtained is thus higher than that which would be observed in practice when the surface in contact with the oxygen is polished.

[0088] Since the tests were conducted in a static manner, in other words without circulation of the atmosphere in relation to the sample, no “scale” appeared to have detached from the surface.

[0089] Analysis of the modification of the compositions at the surface, and in particular the decrease in Cr content, is a means of evaluating the risk of detachable particles forming. The presence of Cr with a content of not less than 7% guarantees the formation of a protective layer that prevents the formation of scale.

[0090] The measurements outlined in the following table show that the Cr content remains well above these values. After testing over 1000 cycles, in which the phases are at the maximum temperatures indicated, the analysis in percentage by weight of the samples at the surface (S) and at the core (C) of the product leads to the results indicated in the following table:

TABLE-US-00003 Alloy ° C. Cr Si Al Fe Ti Mn Ni III 800 S 17.9 6.3 72.7 0.4 0.1 C 19 5.6 73.6 0.5 0.0 IV 800 S 22.1 0.8 2.1 72.1 0.0 0.4 C 23.1 0.8 1.9 71.9 0.3 0.4 IV 550 S 18.6 0.7 1.8 75.8 0.0 0.3 C 23.7 0.8 1.9 71.8 0.0 0.4 V 800 S 20 0.0 6.3 71.0 0.0 0.1 C 21.5 0.0 5.6 71.4 0.0 0.1 I 800 S 10.9 0.2 8.8 79.5 C 15.7 0.2 7.9 75.5 I 550 S 15.6 0.1 7.5 75 C 15.9 0.1 8.0 74.2

[0091] Considering the nature of the anticipated atmosphere, the use of materials must comply with strict safety conditions. The risk of combustion of the material brought to elevated temperature in the presence of pure oxygen is thus evaluated in accordance with the protocol of standard ASTM G 124.

[0092] In these tests, specimens of material placed in an atmosphere of oxygen under pressure are subjected to a combustion test. The results of these tests show that at 550° C. and at a pressure of 3 bar, combustion does not occur in any of the samples.

[0093] When the pressure or temperature is increased, the tendency towards combustion increases. Alloy III was found to be the most sensitive to this test.

[0094] In general, at the temperatures envisaged above, the pressure must not exceed 10 bar, whatever alloy is selected. On this condition, the test in accordance with the standard shows that use in supply installations for gas rich in oxygen does not cause any risk of combustion.

[0095] On the basis of the results of these tests for resistance to hot oxygen, a particularly interesting point appears to be that in an exchanger according to the invention the thickness of the walls can be relatively less thick than one would assume from the prior art. Longevity simulations based on these results lead to walls for the tubes of the exchangers according to the invention that have a thickness that can be no more than 3 mm. This thickness can even be equal to or less than 2.5 mm.

[0096] The relatively low thickness of the walls of the tubes of the exchanger benefits the heat transfer and therefore increases the available power for the same exchange area.

[0097] As an exemplary embodiment, an exchanger according to the invention is configured in the following manner. It is formed by a bank of 40 tubes of Inconel 600. The outside diameter of the tubes is 17.2 mm and the thickness of the wall is 2.3 mm. The tubes have a length of 4000 mm.

[0098] The exchange area in contact with the oxygen is therefore 8.4 m.sup.2.

[0099] Coming from a first exchanger, the heat transfer gas (air with dust extracted) enters the exchanger at a temperature of 650° C. The delivery rate of the heat transfer gas is set at 750 Nm.sup.3/h. The delivery rate of oxygen is 400 Nm.sup.3/h. As it enters at ambient temperature the oxygen is heated to 550° C.

[0100] The flow rate of the oxygen in the ducts is 67 m/s and the load loss in the exchanger is less than 0.15 bar. A safety system comprising a pressure controller maintains the pressure in the exchanger at less than 1 bar.

[0101] The nominal power of the exchanger is 84 kW and per unit area is set at 9.7 kW/m.sup.2.

[0102] The exchanger supplies a burner of a glass melting furnace with a power of 2 MW with oxygen.

[0103] The full furnace is supplied with oxygen by 10 similarly dimensioned exchangers. The power of each of these exchangers is adjusted to better distribute the total power necessary to operate the furnace.