US2186877A - Heat exchanger - Google Patents

Description

Jan. 9, 1940. Q w, G, NOACK 2,186,877

HEAT EXCHANGER Filed Feb. 3, 1959 32;, Frizz, P YW,

Patented Jan. 9 1940 UNITED STATES PATENT "OFFICE 2.136.877 near EXCHANGER Application February 3, 1939, Serial No. 254,512

In Germany February I, 1938 I Claims. In recent years metal alloys have been made available which are capable of withstanding high temperatures. By the use of such alloys it is now possible to replace heat exchangers, such as 5 hot blast heaters made of refractory ceramic material and generally designed to operate intermittently, by metallic regenerators or recuperators designed to operate continuously. These alloys, however, do not permit the use of gases 10 at the highest temperatures attainable by the combustion of fuel with a slight excess of air.

The temperature of the hot combustion gases.

' sometimes is inconvenient and the use of a large excess of air in the combustion is uneconomical because the large volume of waste gases carry away heat. It is desirable therefore andis an object of the present invention to provide a system and a method of operating it whereby the temperature of the gases entering the heat exchanger is lowered to the desired extent without the necessity for-using a large excess of air in the combustion of the fuel or generating steam or an otherwise utilizing the gases at their highest temperature.

According to the present invention the hot gases, 'e. g., combustion gases. before entering the heat regenerator or recuperator are mixed :5 with heating gas which has already passed through the regenerator and given up'part of its heat content. .The temperature of the hot gases is thus lowered to the desired extent without the heat loss incident to burning the fuel 40 with a large excess of air and without generating steam or cooling the combustion gases. in someother manner which may be inconvenient or uneconomical. This expedient solves the principm problem involved in the use of metals or 46 alloys in heat regenerators or recuperators. An-

' other problem, however, is the high cost of the metal or alloy used in the construction of the device. In order to reduce the size and cost of the recuperator one may resort to passing the 60 gases at high velocity. However, if the heatexchange surfaces were made as small as is possible for the necessary heat transfer when the combustion gases alone were passed at high velocity, the passages in the recuperator would be so small ll that therecuperator would bediflcult to manufacture. If, however, the hot combustion gases are mixed with a portion of the gases leaving the recuperator the volume of the mixture is sufficient, even when passed at high velocity, towarrant a construction of the recuperator which is 5 easy to manufacture and still economical in theuse of the expensive metal or alloy.

The recuperator may be operated on the Velox principle, i. e., by passing the gases through it not only at high velocity but also at high pres- 10 sure. Such operation calls for the service of a compressor which may be driven by a gas turbine which in turn may be driven by the hot gases themselves. In this method of operation, inorder to avoid too high temperatures in the re- 15 cuperator it may appear to be. desirable to employ a steam boiler of the Velox type in the path of the gases on their way to the recuperator in order ,to use the heat of the gases at their highest temperature for generating steam. Accord- 2g ing to the present invention, however, the use of a steam generator for lowering thetemperature of the gases on their way to the regenerator or recuperator is avoided by the return of a portion of the gases leaving the recuperator to the 25 stream of gases on their way to the recuperator.

Specifically these partially cooled gases may be withdrawn from the stream of gases passing from the recuperator to the gas turbine. At this point the gases are at such a temperature that 3 they may be handled without cooling and their recompression to the pressure of the gases entering the recuperator requires only the small amount of increase in pressure corresponding to thefiow resistance of the recuperator.

'The invention will be more fully understood by. reference to the accompanying drawing in which Fig. 1 is a diagrammatic showing of a source of combustion gases, a recuperator and means for returning a portion of the gases leav- 40 big the recuperator to the gases entering the same and Fig. 2 is a diagrammaticshowing of a system designed for burning the fuel .under pressure.

' In Fig. l, lis a combustion chamber, 2 is the as supply of fuel and air to the combustion chamber, 3 is the heat exchanger, for instance a hot blast heater designed as a recuperatonl and 5 are the inlet and outlet respectively of the combustion gases into and out of the heat ex- 50 changer, 6 and I are the inlet and outlet respectively. of the gas to be heated by heat exchange withthe hot combustion gases, 8 is a motor and ii a fan driven bymotor 8 for returning aportion of the gases leaving the regenerator 3 tp the combustion chamber I. The movement the gases is indicated by arrows. As appears, he stream of gases leaving the regenerator 3 divides and a portion only of it is returned to the combustion chamber for admixture with the hot combustion gases passing to the regenerator while the remainder is disicrhaziged to waste or to other means for utiliz- In Fig. 2 the combustion chamber I is operated on the Velox principle with both the air and fuel supplies under pressure. Fuel and air are supplied under pressure by the blowers or pumps l0 and II. The hot. combustion gases from the combustion chamber pass into the second stage heat exchanger l2 as indicated at 4 and pass out at 5. Here the stream is divided, part of it passing to the fan or blower 9 which delivers it to the combustion chamber for admixture with the hot products of combustion thereby producing a gas mixture having a sufficiently lowered temperature to handle in the heat exchanger. The other portion of the gasesleaving the second stage heat exchanger passes to the gas turbine l 3 and finally through the first stage of the heat exchanger l4. Turbine I3, driven by the waste gases, drives the fans 9, l0 and II. I5 is an auxiliary motor, e. g., an electric motor which may supplement the turbine 13. The gases to be heated enter the first stage .heat exchanger 14 at 6 and then pass to the second stage heat exchanger I2 and are delivere therefrom at 1.

The temperature of the gas delivered to the heat exchanger may be varied at will without loss of emciency. When the amount of gas in circulation is increased, its speed is increased and the heat transfer is thereby increased, the heat transfer surface remaining constant. The increased heat transfer compensates for the lower temperature of the gas passing through the re- 7 generator.

The great advantage of the method of operation described is that it leaves the combustion chamber operation entirely independent of the temperature limitations of the metallic heat exchanger and permits the combustion of the fuel at maximum efliciency. The gases returned to the combustion chamber from the regenerator may even serve to cool the walls of the combustion chamber and thus lengthen its life.

The invention is of course applicable to any hot gases from which heat is to be recovered and with respect to whicha lowering of their temperature and an increase in their volume is desirable.

1. Apparatus for hat' exchange comprising a source of hot heating gasesunder high pressure, a metallic heat exchanger, means for delivering the hot heating gases from the source thereof 3. Apparatus as defined in claim 1 comprising a combustion chamber designed to be operated under pressure, compressors for supplying fuel and air to said combustion chamber under pressure, a turbine connected to drive said compressors, a gas pump driven by said turbine connected to deliver gases into the hot heating gases pro duced in the combustion chamber, means for delivering a portionof the heating gases leaving the heat exchanger to said gas pump and means for delivering the remainder of. said gases to drive the gas turbine.

4. Apparatus for heat exchange comprising a combustion chamber designed to operate under pressure, compressors for delivering fuel and air to said chamber under pressure, a second stage recuperator having heat transfer walls made. of metal, means for passing hot heating gases from the combustion chamber through said second stage recuperator, a gas pump, means for delivering a portion of the heating gases leaving said second stage recuperator to said gas pump, means for delivering saidportion of the heating gases high pressure combustion chamber, a heat exchanger having metallic. heat transfer walls, means for delivering hot heating gases from said combustion chamber to said heat exchanger and means for delivering a portion of theheating gases after passage through said heat exchanger into contact with the walls of said combustion chamber and then into admixturewith the hot heating gases leaving said combustion chamber.

WALTER GUSTAV NOACK.

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