US 2678532 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
B. MILLER GAS TURBINE PRocEss USING Two HEAT souRcEs May 18, 1954 A5 .sheet-sheet 1 Filed March 16, 1951 ownutlou ATTORNEY 3 Sheegs-Shee- 2 B. MILLER GAS TURBINE PROCESS USING TWO HEAT SOURCES May 18, 1954 -Filed March 1e, 1951' BY il ATTORNEY May 1954 Filed Mqrch 16, 1951 A F/c. 5
3 Sheets-.Sheet 5 )mz i ATTORNEY Patented May 18', 1954 GAS 'TURBINE PROCESS i SOURCE USING Two s `lenjarnin Miller, Ozone Park, N. Y., assigner .to
New Xgr'k inefibership corporation The 'themen 'mandante Impresa .e .a
ApplicatonMarch 16, 1951, Serial No. 216,0!73
20 Claims. l
invention relates to `eas turbn -DIOQSSS using two heat sources. This applicati@ is .a oentinuationfinepart-oi.the prior an .ic I. Gas Turbine Process With ittici` n .Qi Steam, .Sein No. 212.060, filed :February 2.1, Q51- .fIn such..earlier application there was disclosed a .gas turbine .cycle in which a substantial por: tion `of .the cooling Agas vexpanded simultaneously with the combustion products .is supplied in the ferm of steam, which .cycle makes possible substantially .lower equipment costs while retaining the thermal .efficiency characteristics .of the beSl .presently employed installation.
As a result .of further `investigation and re.-
search, it has been determined that by employ: `ing as .the cooling `,gas steam .generated `out .of
contact .with the combustion productaand .transf ferring .heat from .the expanded .gases `to such .cooling gas, ,thermal efficiencies .considerably higher .than those previously attained .can be secured while utilizing equipment of reasonable cost.
In order to .completely explain the invention, illustrative .embodiments are shownwin the ac.- companying drawings in which:
` Fie 1 is Va iiowdiagram of a simple embodimen-t of the invention-W; Y
lFig. -2 is a `flow diagram of Va modiiication -in `which the coolant is `gener-ated by Vsprayii-igWater into compressed air; `and F `3 is -a flow diagram of a modiiication in which low grade -fuel is utilized in part to generate Vsteam employed in lthe cycle.
ll-t will be observed Athat the ilow diagram of Fig. `1 somewhat resembles the conventional regenerative open cycle Agas vturbine Awithout intercooling or freheat; Vit d-iiers essentially from said conventionallcyele -in that steam is admixed with the compressed ai-r passing -to the 4regenerainir, the `steam being generated in an uniired boiler which lextracts -heat from 'the exhaust gases leaving the regeneratm, said vboiler being fed with hot Water supplied by a water heater which extracts heat `from `the exhaustgases leaving the unfired boiler.
The economic maximum `thermal eiciency of the conventional regenerative open Mcycle gas turbine Without either inter-cooling or reheat Yis about 28 percent at the present state of the art. For example, with. ambient air atlfizil pounds `per square inch absolute (p s. i. a.) (sea'level) and 8 0" E., the pressure "at the cor'npres'sor intake will be 141513. s 1.a. The 8 6 percent 4eiiieient .axial .flaw @ramasser raises .the Alefesslire .te .7l-9
(o1. en -39.05)
Z i pressing 10.900.1.I1Q1es of ,air per 11.011,1' (about *i0 per .Second-l. .it .anser-bs #i959 hsseapw r and nuts .firstname.lastname@example.org B..- t-
per heur iet@ tl air.
me anser-.the eau t.) per heur, .reisingi e m t were 15ers- .lt ser The regenerate?, ,99m-.buster tgrbipieegg'it 5.9915 nl .et te@ turbia@ is 412.2 milper hour, of which 1.2 million B. t. u. Perimeter@ .19st vte ille Strmundmdifetly r th "g` 3 io n, etogwof the remaining i. Qur, which is 15,690 horse;- absorb`sf9,95.0 to return .to
l power are available the lead.'
' AIn passing through the regenerator and giving p" `33.93' millione. jt; fil. iper" hour the exhaust .gases Vare cooled to 559MF iThe regenerator has 5231900 square feet' of "surface, or 7.5'sq11are feet per horsepower of output, and the thermal eni- .cienoy .is 28.*() per centfyfhile 63.5 per cent of the 35 turbine output recyeled .by the `compressor to the all.
The pressure .and temperature increase through .the `compressor Acan be mad or `less .than in the :exa le just givenyandf the regeneraator `:eneatil/'elless is AInain tainedcon;- stant the themel entre@ will .decrease Slightly- 'Ifhe Ethernial`.erijiiciejnxcyof this'particular convention-siecle can he increased@ using mere heat eKohalleva Surface inthe regenerator, butin general this not nornical, `because the rate vof increaseof ther a ficiency .vvithincrgase yin ets smaller and smaller.
e somewhat greater a 44,600 square feet of surface. boiler, which has 16,400 square feet of heat ex- 'change surface, the gases cool from 560 F. to "337 F., and give up 18.42 million B. t. u. per
3 Thus a gas turbine embodying the modification illustrated in Fig. 1 may have a thermal eiiciency in excess of 31.0 per cent. rIhis may be illustrated by an example in which for ease of comparison, the ambient conditions, air washer, and compressor are the same as in the previously described conventional cycle.
Considered more in detail, the installation of the invention comprises the shaft l on which the compressor 2 and turbine 3 are mounted and which shaft drives the load. The useful power driving the load ii, as is known, comprises the difference between that generated by the turbine and the power absorbed in driving the compressor.
In the novel cycle of the invention, air from the supply is passed through the air washer 1, washing water for which is provided from water supply 8 and the washed air is charged to vthe compressor. The air leaves the compressor, i mixed with steam and passed through the regenerator 9 which latter heats the steamair mixture by indirect heat exchange with hot turbine exhaust gas entering through line I0.
ter giving up some heat in the regenerator 0 the exhaust gases are utilized to generatethe steam in the steam generator li which as shown may be an unfired boiler. The exhaust gases then pass through the water heater l2 where water for the steam generator is preheated by indirect heat exchange and is charged to the steam generator through line I3. The exhaust gases then pass to the stack.
The mixture of compressed air and steam from the regenerator 9 passes to the combustor I4 Where it meets fuel entering from fuel supply l5. The products of combination are expanded into the turbine 3, thereby driving shaft l, and the exhaust gases, as explained, pass sequentially through the regenerator 9, unfired boilerv il and water heater I2.
"1n the operation air (10,000 moles per hour) leaves the compressor 2 at 440 F. and is mixed with steam supplied by the boiler il at the rate of 1100 moles per hour and generated at 305 F. and 72.2 p. s. i. a.Y The temperature of the airsteam mixture passing to regenerator 9 is 425 F.
In passing through regenerator 0 the air-steam mixture absorbs 39.02 million B. t. u. per hour and is heated up to 895 F. In combustor i4 fuel is fed at the rate of 58.68 million B. t. u. (170 moles of methane) per hour, raising the temperature to 1500 F., with a loss of 1.67 million B. t. u. per hour. The gases enter the turbine at 66.1 p. s. i. a. Since the turbine exhaust gases pass through the boiler l i and water heater l2 as well as regenerator 0, there is an additional pressure drop amounting to 04p. s. i. a. and the gases are discharged from the turbine at 15.6 p. s. i. a. IThe efficiency of the turbine being 84%, the temperature at the exit of the turbine is 1009 F. The enthalpy drop in the turbine is 45.20 million B. t. u. per hour, of which 1.35 million B. t. u. per hour are lost. Of the remaining 48.85 million B. t. u. per hour, which is 17,200 horsepower, the 4 compressor recycles 9,950 horsepower to the air,
leaving 7,250 horsepower for the load.
In passing through the regenerator and. giving up 39.02 million B. t. u. per hour the exhaust gases are cooled to 560 F. The regenerator has In the unred hour. In 'the water heater, which has 6,'600
square feet of heat exchange surface, the gases' cool to 282 F., and give up 4.50 million B. t. u. per hour.
The total heat exchange surface is 67,600 square feet, or 9.33 square feet per horsepower of output, and the thermal eiciency is 31.5 per cent, while 57.8 per cent of the turbine output is recycled to the air.
Comparing the gas turbine of Fig. 1 with the machine according to the present state of the art, it will be noted that for the same air flow rate and same compressor power, the improved machine has 26 per cent greater power output,
but requires the combustor and turbine to handle l1 per cent more volume and has two additional heat exchangers, the total heat exchange surface being 60 per cent greater for the improved machine. For the same power output the improved gas tur ine has smaller compressor, combustor and turbine, but more heat exchange surface, so that its cost is about the same as that of the conventional gas turbine, but it requires about i1 per cent less fuel.
It will be understood that the' invention can be applied with compressors causing pressure and temperature increases greater or less than those given in the example, with comparable improvement in thermal efficiency.
1t will also be understood that the ratio of steam 'to air can be varied over a considerable range. As this ratio increases from zero, the power output increases continuously, but the thermal efficiency increases at first, reaches a maximum, and then decreases. Thus the -machine can be designed for maximum output with relativeiy high steam-air ratio, and then at part ioad the steam-air ratio can be reduced and the thermal efficiency maintained or even increased. rllhe conventional gas turbines thermal emciency decreases at part load, while for many applications the machine must operate at part load most of the time, so that the fuel saving made possible by the present invention will be even greater than 11% on the average. Y
The steam utilized in the process may be generated in various ways and does not necessarily require either the unred boiler or water heater described. For example, the latent heat of evaporation of the steam may be supplied in the first instance by the sensible heat of the hot compressed air and such sensible heat may be replaced by heat transferred from the exhaust gases so that, in eect, all of the latent heat is transferred from the exhaust gases. Y
An installation embodying such a cycle is depicted in Fig. 2. As there shown, the unit comprises the shaft 2i on which the compressor 22 and turbine 23 are mounted and such shaft drives the loa-d 24. As in the modification shown in Fig. 1, from air supply 20 is cleaned in air washer 2l by water from water supply 2l and the cleaned air is passed to compressor 22.
The hot compressed air from compressor 22 is passed 'through line 20 into a spray chamber 29 where it is cooled by adiabatically evaporating water supplied thereto from the water supply through line 30 thus generating steam. The mixture of air and steam from chamber 29 passes through conduit 3i to regenerator 32 wherein it is heated by indirect heat exchange from hot exhaust gases passing through the regenerator. The heated air-steam mixture passes from regeneratcr t2 through line 33 to a second spray chamber 34 where it is cooled adiabatically by evaporating another increment of Water fed through line 35. The steam-air mixture from chamber 34 passes through line 36 to heat regenerator 3"! and is heated there by indirect heat yexchange with the hot turbine exhaust gases `charged through line 38. The hot air-steam mixture is fed from the regenerator v3'! to the combustor where it meets fuel fed from fuel supply/d. team from some other source may also `be utilized as cooling gas. In many industrial plants, such as oil refineries, substantial quantities of heat are `present invention, however, the Waste heat boiler 'supplies steam which is utilized as cooling gas in the gas turbine, thereby increasing the power output-of the gas turbine with relatively minor additional fuel consumption.
Thesteam from the other source may be added to the circuit at the entrance to the regenerator `in the modification of Fig. 1, or at the entrance to regenerator number 1 in the modification of Fig. 2.
Another advantageous feature of the invention `is that it 1may also be applied to utilize fuel not `considered suitable for introduction into the gas vturbine combustor, but suitable for use in a fired boiler. The fired boiler then supplies all or part of the steam used as cooling gas, so that the latent heat is furnished by fuel not otherwise usable in the gas turbine.
The application of such a modiiication of the invention is illustrated by Fig. 3.
This modification makes possible the `utilization of fuel having lovf form value at relatively `high thermal efficiency, While requiring relatively low cost equipment. The steam turbine is much smaller and less costly than one having the same power output but designed to exhaust to a condenser. Ilhe condenser and associated equipment 4and the `condensing water supply system are entirely eliminated, thus making this modication particularly useful in locations Where cold water is not readily available inthe volume required for a conventional steam power plant.
In the modi cation illustrated in Fig. 3, as in the earlier modications, the shaft t! mounts the compressor 42 and turbine t3 and drives the load 44. Air from air supply d5 is Washed `with water in air washer 46 the washing Water being charged from Water supply ill. The cleaned air `is passed to compressor `ft2 and the compressed air is passed through conduits d8 and i9 to the regenerator tu. ln `regenerator 50 an air-steam mixture is superheated -by indirect heat exchange with rhot exhaust` .gases `from the turbine discharged `through line 5i and passing through h eatrexchange elements 52.
'The exhaust vgases after passing through re- Agenerator-513 are used `to .generate steam in un- ,red boiler E3 by contacting het Water with indirect heat exchange surfaces 5t. Such hot `Water is introduced from Water heater 55 through the feed line tit. The water in water heater 55 is preheated by indirect heat exchange with turbine exhaust gases after which such gases are discharged'to the stach.
"Steam generated in boiler 53 passes through line-51 and f1ine"49toregenerator50 where the with latent heat supplied the load B5.
`steam charge passing to regenerator l5E! air-steam mixture is superheated. This super'- heated mixture is fed to combustor 58 through line -59 and meets the fuel fed to the combustor from fuel supply 60.
The fired boiler Si, which as has been noted can be operated with cheap fuel, is fed with preheated water from line 62 and the steam is fed to the turbine 63, the shaft 64 of which drives The exhaust from the steam turbine is passed through line 66 to mingle with airthrough line 49.
It will be appreciated that steam from the red boiler 6l may be introduced at the entrance to .combustor 58 or at the inlet nozzle of turbine 3.
Again, if desired, the exhaust steam from the steam turbine may be passed through a fired superheater which may be a separate unit or combined with the fired boiler 6l before introducing this steam into the gas turbine circuit.
.Other modifications of the cycle illustrated in Fig. 3 `may be made Within the scope of the invention. `For example, the unred boiler 5d may be omitted and all of the steam which is to be employed as cooling gas may be generated in the fired boiler 6I; or again part of the steam may be generated (as in the modification of Fig. 2) by spraying Water into the hot compressed air discharged from compressor 42. Similarly the regenerator 50 may be eliminated and all of the preheating may be done in a heater red with low grade fuel in which case all of the heat transferred from the exhaust gases of the gas turbine may be utilized to heat the feed water for `the fired boiler.
As those skilled in the art Will appreciate, the particular modification chosen will depend upon the relative availability and cost of fuel suitable for use in the gas turbine combustor and fuel not suitable for such use. It will be understood that suitability for use in the gas turbine combustor is a relative matter. Thus solid fuel such as coal may be introduced into a gas turbine combustor under certain circumstances Where special provision has been made to remove fly ash, but even Where coal is to be added, the coal may be separated into a part more suitable for introduction into a gas turbine combustor and a part less suitable, and the less suitable part may be utilized in the fired boiler to generate cooling steam.
Gas having relatively low heating value, such as producer gas or blast furnace gas, and available at atmospheric pressure or slight superatmospheric pressure, may sometimes be utilized more economically as fuel for a fired boiler or superheater, particularly when it would be necessary to cool and/or clean such gas before coi pressing it to the pressure required for introduction into the gas turbine combustor.
While the examples given to illustrate the invention have described cycles which do not use intercooling or reheat, both may be employed with the invention. The use of reheat is particularly advantageous, as the economy of using steam generated out of contact with the combustion products as cooling gas increases as the maximum pressure increases, and the gas turbine cycle with reheat employs a higher maxi-` mum pressure.
It will be understood that ie relative amount of steam which can be introduced as cooling gas is limited by the necessity to have in the ccmbustor enough oxygen to burn the fuel. So long asthe oxygen is supplied in theiorm of `atmospression process.
7 pheric air, the atmospheric nitrogen acts as cooling gas. By using oxygen or oxygen-enriched air to burn the fuel, the relative amount of steam may be increased. Thus Where oxygen or Oxygen-enriched air is available, it may be advantageous to utilize such oxygen or oxygen-enriched air for combustion in the gas turbine, and utilize a greater quantity of steam.
In particular, where a high maximum pressure is used, as in the reheat cycle, oxygen or oxygenenriched air may be economically employed even though it be necessary to prepare it specially from air, since then the nitrogen (or a substantial part of it) need be compressed only to the pressure required to operate the air separation plant.
Where the steam utilized as cooling gas comes from a waste heat boiler or a red boiler or some other similar source, the relative amount of steam which can be introduced as cooling gas is limited by the oxygen requirement, as explained above;
but where the steam is `generated entirely by heat liberated in the gas turbine combustor, the relative amount of steam which can be introduced is limited by considerations of thermal efliciency. The greater the relative amount of water used, the greater the output and the lower the temperature at which the exhaust gases pass to the stack; the lower the temperature at which the exhaust gases pass to the stack, the less the loss of heat in the form of sensible heat. However', the greater the relative amount of water used, the
greater the loss of heat in the form of latent heat. -Therefore such a ratio of water to air is used as to obtain the desired net increase in thermal efficiency, the heat loss due to latent heat being less than the reduction in loss'in the form of sensible heat as compared to the conventionalV process.
n choosing the water-air ratio the temperature at which the exhaust gases pass to the stack is an important guide. Thus in the example given above to illustrate the conventional cycle, the air leaves the compressor` at 440 F. and the exhaust gases leave the regenerator at 559 F. The improved process in the modification illustrated by Fig. l is explained with the aid of a comparable example in which the ratio of water to air is 1100 moles of water to 16,600 moles of air, and in this example the air also leaves the compressor at 440 F. but the exhaust gases are cooled at 232 F. before they pass to the stack. That is, the ratio of water to air is chosen so that the exhaust gases are cooled to a temperature lower than the temperature at which the air leaves the com presser', but still substantially higher than atmospheric.
The modification illustrated by Fig. 1 is limited to use in cycles in which the temperature at the end of the compression process is substantially higher than the saturationv temperature of steam under the pressure at the end of the com- The modification illustrated by Fig'. 2 is not limited. As noted, in some cases an 'intermediate modification will be convenient; in
ture entering the regenerator would be 197 F.
The exhaust gases would be cooled to 339 F. in
the regenerator, and further to 282 F. in the water heater.
Various other modications may be employed, each of which may be more suitable under particular circumstances, but in every case the amount of heat recovered from the exhaust gases and returned to the process, less the latent heat loss, will be greater than the amount of heat which could be recovered from the exhaust gases and returned to the process without the use of steam as cooling gas in a comparable cycle.
While preferred embodiments or the invention have been described, it is to be understood that these are given to illustrate the underlying principles of the invention and not as limiting its useful scope except as such limitations are imposed by the appended claims.
l. 1in a process for producing power by the expansion of products by combustion with abstraction of mechanical energy, wherein the combustion takes place under substantial superatmospheric pressure and the temperature of the combustion products formed is substantially in excess of the maximum safe operating temperature of the moving element of the expander and the temperature of said moving element is maintained at a safe Value by simultaneousy passing to said expander for expansion therethrough a supply of cooling gas at a pressure substantially equal to the pressure of said combustion products but at a temperature substantially lower than said maximum safe operating temperature, the improvement which comprises providing at least a substantial portion oi said cooling gas in the form of steam and generating said steam under substantial super-atmospheric pressure as saturated steam while supplying the latent heat for said steam generation from a source independent of and not derived from said combustion products.
2. in a process for producing power wherein there is a step of combustion under substantial superatmospheric pressure followed by expansion of the products of combustion with abstraction of mechanical energy, and the temperature of the moving element of the expander is maintained at a safe value by simultaneously passing to the expander for expansion therethrough a supply of cooling gas at a pressure substantially equal to the pressure of said combustion products but at a temperature substantially lower than the maximum safe operating temperature of said moving element, the improvement which comprises providing at least a substantial proportion of said supply of cooling gas in the form or" steam and generating said steam under substantial su- `peratmospheric pressure while supplying the latent heat for said steam generation by a second combustion step.
3. In a process for producing power which comprises burning fuel at superatmospheric pressure in an atmosphere containing a substantial fraction of steam to produce a mixture of gases at high temperature and pressure and expanding said mixture with abstraction of mechanical energy, the improvement which comprises generating steam out of contact with the products of said burning and mixing the steam with air to form said atmosphere before bringing said atmosphere into contact with said fuel and replacing the heat absorbed as latent heat in the steam generation step with heat from a source which is distinct from, independent of and not derived fromthe products of combustionv o f the burning of the fuel which produces the mixture of gases at` high temperature and pressure.
4.,In a process for producing power wherein combustion products and steam are simultaneously expanded through the same expander with abstraction of mechanical energy, the improvement which comprises generating the steam under substantial superatmospheric pressure and superheating said steam, before passage to said expander, by transferringY heat thereto from the exhaust of said expander and replacing the heat absorbed as latent heat in the steam generation step with heat from a source which is distinct from, independent of and not derived from the products of combustion of the burning of the fuel which produces the mixture of gases at high temperature and pressure.
5. A method of producing power by combustion of two fuels, one being more suitable than the other for utilization an internal combustion power production process, which comprises burning the less suitable fuel, transferring heat released by said burning to water to generate steam therefrom under substantial superatmospheric pressure but without bringing said steam into contact with the combustion products formed by burning said less suitable fuel, burning the more suitable fuel under substantial superatmospheric pressure, and simultaneously expanding said steam and the products of combustion of said more suitable fuel through the same expander with abstraction of mechanical energy.
6. In an internal combustion power production process wherein combustion products are expended with abstraction of mechanical energy, which process comprises the steps of compressing air, transferring heat from the expended combustion products to the compressed air, passing the heated compressed air to a combustion zone, and burning fuel therein to form said combustion products, the improvement which comprises mixing steam with said compressed air before the step of transferring heat from the expanded combustion products is completed, while supplying the latent heat for the steam generation step from a vent source which is distinct from and independent of both the compressed air and combustion products and not derived from such combustion products.
7. A process for producing power which comprises generating steam under high superatmospheric pressure by transferring to Water heat released in a rst combustion Zone but without bringing said steam into contact with the products formed in said rst combustion zone, concurrently forming combustion products under substantial superatmospheric pressure in a second combustion zone, expanding said steam in a first expander with abstraction of mechanical energy to a lower but substantially superatmospheric pressure, and expanding steam exhausted from said rst expander and the combustion products from said second combustion Zone simultaneously in a common expander.
8. The process in accordance with claim 7 in which the steam exhausted from said first expander is passed in heat exchange relationship with the exhaust from said common expander, whereby the temperature of the steam exhausted from said rst expander is increased before it is expanded through said common expander.
9. A power production process wherein an oxygen-containing gas is mechanically compressed to a pressure substantially superatmospheric, the compressed oxygen containing gas is passed to a 10 zone wherefuel of high form value is introduced and combustion takes place without increase of pressure, and the combustion products are expanded with abstraction of mechanical energy in admixture with a diluent which serves to protect the expander from excessive temperature; and including the step of increasing substantially the specific volume of the diluent before it enters the expander by transfer of heat thereto from a sec.
ond heat source which is distinct from and independent of and not derived from both the compressed oxygen-containing gas and the products of combustion of said high form value fuel therewith, the minimum pressure on said diluent during said transfer of heat thereto being not less' than Athe pressure at the entrance to the expander and the maximum temperature of said diluent during said transfer of heat thereto being substantially lower than the maximum safe operating temperature of said expander.
lo. The process of claim 9 wherein there is an excess of oxygen-containing gas over the amount required for combustion so that the excess serves as the diluent. i
l1. The process of claim 10 wherein the specific volume of the diluent is substantially increased by transfer of heat thereto from the expander exhaust and thereafter some specific volume is further substantially increasd by transfer of heat thereto from the second heat source.
l2. The process of claim 9 wherein the diluent is Water and the substantial increase in specific volume is caused at least in part by conversion of water from the liquid to the gaseous state, the latent heat for said conversion being transferred to the water from the second heat source.
13. The process of claim 9 wherein the diluent is water and wherein the specific Volume of the diluent is substantially increased by conversion of the water from the liquid to the gaseous state, the latent heat for said conversion being transferred to the water from the expander exhaust, and thereafter said specific volume is further substantially increased by transfer of heat thereto from the second heat source.
1li. The process of claim 12 wherein the specic volume of the water after conversion to the gaseous state is further substantially increased by transfer1 of heat thereto from the expander exhaust.
15. The process of claim l2 wherein the conversion of the water from the liquid to the gaseous state takes place at a pressure substantially higher than the pressure at the entrance to the expander for the combustion products, and the gaseous water is expanded with abstraction of mechanical energy from the pressure at which said conversion takes place to a pressure not lower than the pressure at the entrance to said expander for the combustion products.
16. The process of claim l5 wherein the specific volume of the diluent after the expansion to the pressure not lower than the pressure at the entrance to the expander for the combustion products, is still further increased by transfer of heat thereto from the expanded combustion products.
17. The process of claim 9 wherein the second heat source is a fuel relatively unsuitable for introduction into the Zone where combustion takes place with the compressed oxygen-containing gas.
19. The process of claim 17 wherein the second heat source is a fuel which gives rise on combustion to substances which would have a deleterious effect on the expander.
20. The process of claim 17 wherein the second heat Source is a gaseous fuel having a relatively low heating value and available at a pressure substantially lower than the pressure at the yentrance to the expander for the products of combustion of the fuel of high form value.
Name Date Zoelly Sept. 3, 1907 Number Number Name Date Dinsmore Sept. 5, 1916 Kraus May 22, 1917 Graemger Dec. 20, 1921 Bissell Jan. 12, 1932 Holzwarth Dec. 4,1934 Martnka Jan. 9, 1940 Kane Apr. 24, 1951 Sandborn Sept. 18, 1951 FOREIGN PATENTS Country Date Switzerland Jan, 16, 1947