Waste heat recovery system

These and other objects of the present invention are achieved in a heat recovery system to recover the heat potential and including a plurality of heat exchanger assemblies from exhaust or waste gas at a temperature of from 500 to 2500.degree. F. wherein the heat exchanger assemblies include induced draft fans for controlling the flow of exhaust or waste gas therethrough as a heating medium for diverse fluids passing in indirect heat transfer relationship at descending temperature levels.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention relates to the recovery of waste heat, and more particularly 
to an apparatus for the recovery of heat from high temperature gas stream. 
(2) Description of the Prior Art 
Cogeneration and heat recovery systems normally recover heat from hot gases 
leaving a gas turbine, gas engine, or other source of hot gas by 
generating steam which is used for process or space heating, for driving a 
steam turbine and generating electricity or for driving a pump or 
compressor. Infrequently, a high temperature hot oil stream is used to 
recover heat and transfer the heat to other streams via heat exchangers. 
A frequent application of heat recovery systems is one which hot gases 
leaving a gas turbine in the range of 800.degree. to 950.degree. F. 
produces power at a steady rate which is sold to a local power company, 
split between the power company and operating company or used by the 
operating company. Generally, there is an excess of heat available or the 
heat uses vary depending on plant operation, and therefore, some of the 
hot gases by-pass the heat recovery system via expensive and somewhat 
unreliable large by-pass stack damper. An expensive and complex control 
system is required to automatically modulate the opening of such damper, 
and in the case of failure of the by-pass system, steam pressure will rise 
to elevated pressures and/or the heat transfer oil will rise to an 
elevated temperature requiring automatic shutdown of the gas turbine or 
the source of the hot gases. 
In a hot oil recovery unit, heat transfer design is very complex and 
expensive because oil will break down if its maximum film temperature 
(730.degree. to 800.degree. F.) is exceeded due to oil flow interruptions, 
gas flow interruptions or overfiring. Therefore, the capital cost is as 
high as is operating cost to replace or replenish the heat transfer oil. 
In U.S. Pat. No. 4,257,579 to Bruhn et al. and assigned to the same 
assignee as the present invention, there is disclosed a waste heat 
recovery system using at least two heat exchange recovery systems 
utilizing intermediate heat transfer mediums for a process operation 
producing an exhaust or waste gas at temperatures of from 500.degree. to 
2500.degree. F. wherein the heat exchanger assemblies may be fabricated 
using conventional materials of construction and wherein heat may be 
recovered at levels substantially higher than with the use of a single 
intermediate heat transfer medium, however, such systems depend upon using 
two or more heat recovery systems and each system has to have the ability 
to fully use all the heat that is recovered in each system. 
OBJECTS OF THE PRESENT INVENTION 
It is an object of the present invention to provide an improved apparatus 
for recovering heat from a high temperature gas stream. 
Another object of the present invention is to provide an improved apparatus 
for recovering heat from a high temperature gas stream obviating any 
by-pass dampers requirements for balancing user requirements. 
Still another object of the present is to provide an improved apparatus for 
recovering heat from a high temperature gas stream at reduced capital 
costs. 
Yet another object of the present invention is to provide an improved 
apparatus for recovering heat from a high temperature gas stream of 
greater reliability. 
A further object of the present invention is to provide an improved 
apparatus for recovering heat from a high temperature gas stream 
permitting wide ranges of operational flexibility of heating loads. 
SUMMARY OF THE INVENTION 
These and other objects of the present invention are achieved in a heat 
recovery system to recover the heat potential and including a plurality of 
heat exchanger assemblies from the exhaust or waste gas at a temperature 
of from 500.degree. to 2500.degree. F. wherein the heat exchanger 
assemblies include draft fans for controlling the flow of the exhaust or 
waste gas therethrough as a heating medium for diverse fluids passing in 
indirect heat transfer relationship at descending temperature levels.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawing, a hot exhaust or waste gas stream at a 
temperature of from 500.degree. to 2500.degree. F. in line 10 is passed in 
indirect heat transfer relationship through a plurality of heat exchanger 
assemblies, generally indicated as 12, 14 and 16, including induced draft 
fans 18, 20 and 22, respectively, to recover the heat potential of the hot 
gas stream in line 10. After recovery of the heat potential in the heat 
exchanger assemblies 12, 14 and 16, a residual gas stream in line 24 may 
be passed through a heat exchanger 26 prior to venting to the atmosphere 
by line 28. The heat potential is recovered in the heat transfer 
assemblies 12, 14 and 16 by heating process streams therein at diverse 
temperature levels. It will be understood that generally only a portion of 
the gaseous flow in line 10 is to be passed through the heat exchanger 
assemblies 12, 14 and 16 as more fully hereinafter discussed. 
The heat exchanger assembly 12 includes a heat exchanger 30 for passing in 
indirect heat transfer relationship a gas stream in line 32 in fluid flow 
communication with the line 10 with a process fluid, such as a cold heat 
transfer fluid introduced by line 34 into the heat exchanger 30. A heated 
fluid is withdrawn by line 36 from the heat exchanger 30 and is passed to 
an in-plant processing unit (not shown). The gas stream at a lower 
temperature level of from 650.degree. to 750.degree. F. is withdrawn by 
line 38 from the heat exchanger 30 and passes to the suction side of the 
induced draft fan 18 under the control of controller assembly 40 and 
returned by line 42 to line 10. The controller assembly 40 may control 
,the speed of a motor 44 driving the induced draft fan 18, or alternately 
modulate gas flow in the inlet line 38 or the outlet line 42 to and from 
the induced draft fan 18 by a damper 46 or 48, respectively positioned in 
lines 38 or 42. 
The heat exchanger assembly 14 includes a heat exchanger 50 for passing in 
indirect heat transfer relationship a gas stream in line 52 in fluid flow 
communication with the line 10 with a process fluid, such as a cold heat 
transfer fluid introduced by line 54 into the heat exchanger 50. A heated 
process fluid is withdrawn by line 56 from the heat exchanger 50 and is 
passed to an in-plant processing unit (not shown). The gas stream at a 
lower temperature level of from 490.degree. to 590.degree. F. is withdrawn 
by line 58 from the heat exchanger 50 and passes to the suction side of 
the induced draft fan 20 under the control of controller assembly 60 and 
returned by line 62 to line 10. The controller assembly 60 may control the 
speed of a motor 64 driving the induced draft fan 20, or alternately 
modulate gas flow in the inlet line 58 or the outlet line 62 to and from 
the induced draft fan 20 by a damper 66 or 68, respectively, positioned in 
line 58 or 62. A line 70 under the control of valve 72 is connected to the 
line 62 and to line 10 upstream of the line 52 to provide recirculation 
capability as more fully hereinafter discussed. 
The heat exchanger assembly 16 includes serially disposed heat exchangers 
74, 76 and 78 for passing in indirect heat transfer relationship a gas 
stream in line 80 in fluid communication with the line 10 with a process 
fluid, such as water introduced by line 82 into the heat exchanger 78 and 
thence for serial passage through the heat exchangers 74 and 76. A heated 
process fluid, such as superheated steam is withdrawn by line 84 from the 
heat exchanger 74 and is passed to in-plant processing unit(s) (not 
shown). The gas stream at a lower temperature level of from 310.degree. to 
410.degree. F. is withdrawn by line 86 from the heat exchanger 78 and 
passes to the suction side of the induced draft fan 22 under the control 
of controller assembly 88 and returned by line 90 to line 10. The 
controller assembly 88 may control the speed of the motor 92 driving the 
induced draft fan 22, or alternately modulate the gaseous flow in the 
inlet line 86 or the outlet line 90 to and from the induced draft fan 22 
by a damper 94 or 96, respectively, positioned in line 86 or 90. 
The gas stream in line 24 at a much lower temperature level, i.e. 
310.degree. to 410.degree. F. as hereinabove discussed may be passed in 
indirect heat transfer in heat exchanger 26 with a process stream in line 
98, such as boiler feed water, steams, or super heated steam to recover 
the residual heat potential of the gas in line 24 prior to venting to the 
atmosphere. 
EXAMPLE OF THE INVENTION 
Operation of the apparatus of the present invention is illustrated in the 
following example which is intended to be merely illustrative and the 
invention is not to be regarded as limited thereto. 
EXAMPLE I 
A hot gas stream (500,000 lb/hr.) at a temperature of 950.degree. F. is 
passed through the heat exchanger 12, 14 and 16 in indirect heat transfer 
relationship to heat fluids, as set forth in Table I, below: 
______________________________________ 
Recovered 
Heat Heat Pro- 
Exchanger 
Potential cess Inlet Return 
Quantity 
Assembly 
M Btu hr. Fluid T(.degree.F.) 
T(.degree.F.) 
(10.sup.3 -#/hr.) 
______________________________________ 
12 21.6 *htf 950 750 400 
14 10.8 *htf 790 590 200 
16 20.25 Water 710 410 250 
______________________________________ 
*htf = heat transfer fluid. 
The gas stream is line 24 at a temperature of 560.degree. F. may be 
subjected to further heat transfer in the heat exchanger 26. 
In accordance with the present invention, the heat potential of a gas 
stream may be readily recovered in a facile manner with the following 
advantages: 
1. If the source of the hot gaseous stream is a gas turbine, the gas 
turbine may operate more efficiently since there will be negligible back 
pressure on the gas turbine exhaust; 
2. The heat exchangers recovering the heat potential are readily controlled 
by controlling the speed of the motor driving the induced draft fan, or 
alternately by controlling gas flow to or from the induced draft fan; 
3. A heat exchanger assembly may be taken out of service without closing 
down the source of the hot gas stream, e.g., gas turbine; 
4. Greater flexibility since the heat exchanger may function in a once 
through or recirculation capacity mode; and 
5. In a recirculation mode, any heat exchanger assembly may be provided 
with higher gas flow at a lower temperature than once through which would 
be of significance with temperature sensitive materials. 
While the apparatus of the present invention has been described with three 
different forms of heat transfer assemblies, i.e. once through, 
recirculation and serially arrayed process heat exchangers, it will be 
understood that the heat exchanger assemblies may be of like form as 
understood by one skilled in the art.