Heat exchanger and economizer

A heat exchanger with a housing defining a main fluid flow passage therethrough and a tube bundle access opening to the fluid flow passage, a tube bundle unit removably positioned in the housing in the fluid flow passage through the access opening, and a cover for removably closing the access opening so that heat exchange fluid flowing through the tube bundle unit is in a heat exchange relation with the fluid flowing through the housing main fluid flow passage around the tube bundle unit. Also shown is an economizer for a stream boiler incorporating a heat exchanger.

BACKGROUND OF THE INVENTION 
Many types of heat exchangers are commercially available on the market 
today. One of the primary problems with such heat exchangers, however, is 
that the tube bundle is normally fixed in the main housing of the heat 
exchanger so that it cannot be removed to easily repair any of the tubes 
in the tube bundle that may deteriorate. This means that, in order to 
repair one of these prior art heat exchangers, it is necessary to either 
remove the entire heat exchanger including the housing from its 
installation so as to gain access to the tube bundle in the heat exchanger 
to repair same or for a workman to enter the main housing and repair the 
defective tube while the heat exchanger is in place. This results in 
considerable down time of the equipment on which the heat exchanger is 
being used and further makes the repair of the particular tube that is 
deteriorated in the tube bundle extremely difficult since it cannot be 
removed from the housing to gain access thereto. If another heat exchanger 
is actually inserted into the installation while the deteriorated heat 
exchanger is being repaired, the cost of maintaining these spare complete 
heat exchangers is prohibitive from a maintenance standpoint. 
One common application for heat exchangers is an economizer which preheats 
feedwater supplied to a steam boiler. One of the problems encountered with 
the use of such economizers is that corrosive condensates are formed if 
the average temperature of the flue gases from the boiler drops below the 
dew point of the particular flue gases produced by the boiler. These prior 
art economizers have solved the condensation problem by maintaining all 
surfaces in the economizer with which the flue gases come into contact 
above the condensation temperature of the flue gases. While this prior art 
technique has prevented condensation, it has at the same time limited the 
rate at which heat can be transferred from the flue gases to the feedwater 
passing through the heat exchanger in the economizer. This is because this 
technique lowers the difference between the temperature of the flue gases 
and the temperature of the feedwater in the economizer which in turn 
controls the rate of heat transfer from the flue gases to the feedwater in 
the economizer. The net result has been that these prior art economizers 
required large heat exchangers therein in order to maximize the heat 
recovered from the flue gases by the feedwater passing through the 
economizer. 
SUMMARY OF THE INVENTION 
These and other problems and disadvantages associted with the prior art are 
overcome by the invention disclosed herein by the provision of a heat 
exchanger in which the tube bundle in the heat exchanger is selectively 
removable from the housing while the housing remains in place to permit 
easy access to the tube bundle for repair thereof. Further, this permits 
the housing to be used without the heat exchanger therein while the tube 
bundle is removed for repair to minimize the down time of the equipment to 
which it is attached. Additionally, a spare tube bundle may be 
economically placed in the housing while the defective tube bundle is 
being repaired. 
The invention disclosed herein further overcomes the problems associated 
with the prior art economizers by selectively recirculating a portion of 
the feedwater passing through the heat exchanger in the economizer in 
response to the average temperature of the flue gases after passage 
through the heat exchanger to maintain the average temperature of the flue 
gases on the exit side of the economizer above the dew point thereof. 
Alternatively, the flow of the feed water may pass through the heat 
exchanger in the economizer or bypass the heat exchanger in the economizer 
in response to the average temperature of the flue gases as they exit the 
economizer. 
The apparatus of the invention includes a heat exchanger which has a 
housing defining a primary fluid passage therethrough with a fluid inlet 
and fluid outlet on opposite sides of the housing. The housing further 
defines a pair of tube end subchambers on opposite sides thereof which 
open into the main fluid passage and which are oriented about a common 
axis normal to the central axis of the first fluid passage through a 
housing. The housing further defines a tube bundle access opening to the 
main fluid passage and the tube end subchambers which is selectively 
covered by a cover member. A tube bundle is slidably received into the 
main fluid passage and the tube end subchambers through the tube bundle 
access opening so that the tube bundle can be removed from the housing 
with the housing in place in the installation. The tube sheets which mount 
the tubes in the tube bundle serve to seal the tube end subchambers from 
the main fluid passage in the heat exchanger from bypassing the tube 
bundle. Thus, when a second fluid is passed through the tube bundle, the 
tube bundle places a second fluid in a heat exchange relationship with the 
first fluid passing through the main fluid passage. 
The apparatus of the invention as embodied in an economizer for a steam 
boiler includes the heat exchanger positioned in the flue gas duct from a 
steam boiler with a feedwater source connected to the feedwater inlet of 
the steam boiler through the tube bundle in the heat exchanger. A three 
way valve is positioned in this connection to selectively divert the flow 
of feedwater back through the tube bundle by a circulating pump in 
response to the average temperature of the flue gases after passage 
through the heat exchanger. Alternatively the three way valve may divert 
the feedwater for flow through the tube bundle in the heat exchanger to 
the feedwater inlet of the steam boiler or bypass the tube bundle in the 
heat exchanger in response to the average temperature of the flue gases as 
they pass from the heat exchanger.

These figures and the following detailed description disclose specific 
embodiments of the invention, however, it is to be understood that the 
invention may be embodied in other forms. 
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
Referring to the figures, it will be seen that the heat exchanger 10 
includes a housing 11 and a tube bundle unit 12 removably mounted in the 
housing 11. The heat exchanger 10 may be used in a number of different 
applications but is illustrated in FIGS. 2 and 3 being used in an 
economizer configuration with FIG. 3 illustrating its use on a steam 
boiler. 
The housing 11 as seen in FIGS. 1 and 2 includes a pair of generally 
parallel spaced apart side walls 20, each of which has a longitudinal 
centerline CL.sub.SW. Each of the side walls 20 has a main central section 
21 which is centered on the centerline CL.sub.SW with a length L.sub.1 and 
a height H.sub.1. Each corner of each side wall 20 is notched to form an 
end tube subchamber extension 22 with a length L.sub.2 and a height 
H.sub.2. The height H.sub.2 of each end tube subchamber extension 22 is 
less than the height H.sub.1 of the main central section 21 as best seen 
in FIG. 1 and the extension 22 is centered with respect to the centerline 
CL.sub.SW. Therefore, it will be seen that each end tube subchamber 
extension 22 on each side wall 20 will be laterally aligned with a 
corresponding subchamber extension 22 on the opposite side wall 20. The 
upper and lower ends of each end tube subchamber extension 22 is connected 
to the laterally aligned subchamber extension 22 by an end tube subchamber 
cross plate 24. Thus, the end tube subchamber cross plates 24 determine 
the overall depth of the housing 11. Each cross plate 24 has a length 
L.sub.3 as best seen in FIG. 1 so that opposite ends of the cross plate 24 
project beyond the end tube subchamber extension 22 in side walls 20. Each 
pair of laterally aligned extensions 22 in side walls 20 and the cross 
plates 24 connecting the extensions 22 define a tube bundle access opening 
25 therethrough with the height H.sub.2 and a width W.sub.1 as best seen 
in FIG. 1 which opens into the end tube subchamber 30 bounded by 
extensions 22 and cross plates 24. The opposed end tube subchambers 30 and 
their access openings 25 are centered on a common axis A.sub.ES normal to 
axis P.sub.MP and parallel to side wall centerline CL.sub.SW. The inboard 
end of each end tube subchamber 30 opens into the main fluid passage 28 as 
seen in FIGS. 1 and 2. 
The laterally aligned edges of the main central sections 21 in side walls 
20 at the notches are connected by fluid passage cross plates 26, each of 
which extends from the end tube subchamber cross plate 24 out to the top 
or bottom of the main central section 21 in the side walls 20. Thus, it 
will be seen that the main central section 21 of the side walls 20 along 
with the fluid passage cross plates 26 define the main fluid flow passage 
28 therethrough about the central axis A.sub.MP which is oriented 
generally normal to the centerline CL.sub.SW of the side walls 20 and 
generally parallel to the plane of the side walls 20. The fluid passage 
cross plates 26 and the side walls 20 define main fluid openings 29 at the 
top and bottom side of the main central sections 21 of the side walls 20 
so that the primary fluid can flow through the passage 28 in the housing 
11 from one of the fluid openings 29, here shown as the bottom opening, to 
the opposite opening, here shown as the top opening 29 as best seen in 
FIG. 2. The direction of fluid flow through the heat exchanger 10 is 
immaterial. It will thus be seen that the fluid openings 29 and the main 
fluid flow passage 28 have the same cross-sectional shape with the length 
L.sub.1 and width W.sub.2 as best seen in FIG. 1. The particular size of 
the main fluid flow passage 28 is set by the duct work which connects the 
main fluid flow to the heat exchanger while the size of the tube bundle 
access openings 25 and the end tube subchambers 30 is set by the size of 
the tube bundle unit 12 as will become more apparent. While any fluid may 
be flowed through the main fluid flow passage 28, it is illustrated as 
placing flue gases in a heat exchange relationship with the fluid in the 
tube bundle unit 12 as will become more apparent. 
Opposite ends of the main central section 21 in each of the side walls 20 
is reinforced by an upstanding post 31 with reinforcing flanges 32 about 
all of the edges of the side walls 20, the subchamber cross plates 24 and 
the main fluid passage cross plates 26 to strengthen the housing 11. Those 
reinforcing flanges 32 about the main fluid openings 29 and the tube 
bundle access openings 25 serve as mounting flanges as will become more 
apparent. 
The tube bundle access openings 25 are respectively removably closed by a 
front cover panel 40 and a rear cover panel 41 as seen in FIGS. 1 and 2. 
Each of the cover panels 40 and 41 is attached to the reinforcing flanges 
32 about the tube bundle access openings 25 using appropriate fasteners 42 
as seen in FIGS. 1 and 2. An appropriate gasket 44 is provided between 
each cover panel 40 and 41 and the reinforcing flanges 32 to which they 
are attached. The front cover panel 40 is provided with a pair of spaced 
apart header openings 45 seen in FIGS. 1 and 2 which permit the fluid 
passing through the tube bundle unit 12 to be introduced to and removed 
therefrom as will become more apparent. Each of the openings 45 is 
provided with an appropriate seal 46 to seal these openings as will become 
more apparent. 
As best seen in FIGS. 2 and 4, a tube bundle slide angle 50 is mounted on 
the lower inside of each central section 21 of the side walls 20 in 
alignment with the opposed cross plates 24. These slide angles 50 serve to 
support the tube bundle unit 12 as it slides into or out of the housing 11 
as will become more apparent. 
The tube bundle unit 12 as best seen in FIGS. 1, 2 and 4 includes a tube 
frame 55 which mounts the tube bundle 56 thereon about a longitudinal axis 
A.sub.TB. The tube frame 55 includes a pair of spaced apart tube sheets 60 
with a generally rectilinear shape. The tube sheets 60 are held in 
position by a pair of upper side members 61 and a pair of lower side 
members 62 which extend between the corners of sheets 60. The side members 
61 and 62 are oriented generally parallel to each other and define the 
side corners of the frame 55. The frame 55 has a width W.sub.4 and height 
H.sub.4 such that the frame 55 will just slidably pass through the access 
openings 25 in housing 11 as seen in FIG. 1. The tube sheets 60 are spaced 
apart so that the opposed inside surfaces 64 thereof are spaced apart a 
distance d.sub.1 substantially equal to the length L.sub.1 of the main 
fluid passage 28 in housing 11 as seen in FIG. 2. Thus, when the frame 55 
is in position in housing 11, the inside surfaces 64 of the tube sheets 60 
are in registration with the main fluid cross plates 26 so that the main 
fluid passage 28 is about the same size all the way through the exchanger 
10 when the frame 55 is in place. The ends of the side members 61 and 62 
extend outboard of each of the tube sheets 60 into the tube end 
subchambers 30 when frame 55 is in place as seen in FIG. 2. The lower side 
members 62 are supported by the lower end tube subchamber cross plates 24 
and the slide angles 50 extending between cross plates 24. Appropriate 
seals S.sub.TS may be provided along the edges of the tube sheets 60 
between side members 61 and 62 to form a seal with cross plates 24 and 
side wall extensions 22 and thus separate the end tube subchambers 30 from 
the main gas passage 28. 
The tube bundle 56 is mounted on the tube sheets 60 so that the straight 
exchange tubes 65 extend between the tube sheets 60 while the tube ends 66 
connecting tubes 65 are outboard of tube sheets 60. Thus, when the tube 
bundle unit 12 is in position as seen in FIG. 2, the heat exchange tubes 
65 extend across the main fluid passage 28 while the tube ends 66 are 
located in the tube end subchambers 30. The main fluid flowing through 
passage 28 will be placed in a heat exchange relationship with the 
exchanger fluid flowing through the heat exchanger tubes 65. The first and 
last row of tubes 65 are connected to headers 68 so that the heat exchange 
fluid can be introduced into and removed from the tube bundle 56. Each of 
the headers 68 has an inlet pipe 69 thereto which extends through the 
header openings 45 so that the tube bundle 56 can be externally connected 
when the tube bundle unit 12 and the front cover panel 40 are in place as 
seen in FIG. 2. The seals 46 seal the openings 45 to pipes 69. Usually, 
the pipes 69 are provided with screw-on flanges 70 to connect the pipes 69 
to external piping P. The screw-on flanges 70 are, of course, removed to 
permit the front cover panel to be removed when the tube bundle unit 12 is 
to be removed. 
It will be seen that the tube bundle unit 12 may be installed along its 
longitudinal axis as shown or, with appropriate relocation of cover panels 
40 and 41, installed along its lateral axis normal to the main fluid flow. 
Also, it is necessary that only one cover panel need be provided rather 
than the two panels 40 and 41 as illustrated. Further, one of the cover 
panels 40 and 41 may be fixed to the tube bundle unit 12 so that when the 
tube bundle unit 12 is in place, the cover panel on the trailing end of 
the unit 12 will appropriately seal the access opening 25 through which 
the tube bundle unit 12 is installed. 
OPERATION 
In operation, it will be seen that the housing 11 is installed in 
conventional manner in the duckwork D as best seen in FIGS. 2-4. The 
housing 11, of course, is installed without the tube bundle unit 12 in 
place in the housing 11 and without the front and rear cover panels 40 and 
41 in place. This makes the housing 11 relatively lightweight thereby 
reducing to a minimum the personnel and equipment necessary to install the 
housing 11. 
After the housing 11 is installed, the tube bundle unit 12 is then 
installed. The tube bundle unit 12 is placed in the housing 11 by picking 
same up with appropriate equipment usually available at the installation 
site and placing one end of the tube bundle unit in one of the access 
openings 25 so that the lower side members 62 rest on the lower cross 
plate 24 at that access opening. The tube bundle unit 12 is positioned so 
that its axis A.sub.TB is coaxial with the axis A.sub.ES of the 
subchambers 30 and then the tube bundle unit 12 is pushed into the housing 
11 with the lower side members 62 being supported on the end tube 
subchamber cross plates 24 and the tube bundle slide angles therebetween. 
When the tube bundle unit 12 is pushed into the housing so that the inside 
surfaces 64 of the tube sheets 60 align with the main fluid passage 28 
through the housing 11, the tube bundle unit 12 is in place. The seals 
S.sub.TS on the tube sheets 60 seal the end tube subchambers 30 from the 
main fluid passage 28. If the screw-on flanges 70 on the inlet pipes 69 to 
headers 68 are in place, they are removed and the front and rear cover 
panels 40 and 41 are attached using the fasteners 42. If accessory 
equipment such as a soot blower is to be mounted, it is usually mounted 
through an appropriate hole in one of the tube sheets 60 before the cover 
panels 40 and 41 are attached. After the cover panels 40 and 41 are 
attached, the screw-on flanges 70 are re-attached so that the piping P can 
be connected to the inlet pipes 69 on headers 68. The heat exchanger 10 is 
now ready for operation. To remove or replace the tube bundle unit 12, it 
will be seen that this process is simply reversed. 
If the repair of the tube bundle unit 12 will result in considerable down 
time of the equipment with which the heat exchanger 10 is being used, then 
the removed cover panels 40 and/or 41 may be replaced without the tube 
bundle unit 12 in housing 11. This will allow the operation of the 
equipment to continue even though the tube bundle unit 12 is not 
operational. When the tube bundle unit 12 is repaired, it can then be 
replaced and its operation continued. 
ECONOMIZER INSTALLATION 
The heat exchanger 10 is shown incorporated in a recirculating economizer 
E.sub.R in FIGS. 2 and 3. As shown in FIG. 3, the economizer E.sub.R is 
shown mounted on a conventional steam boiler B with the flue gas ductwork 
D and a feedwater inlet connection FC so that feedwater can be supplied to 
the steam boiler from a feedwater source FS as is normally required in 
steam boiler operation. The feedwater from the feedwater source FS is 
supplied to the boiler B under pressure by feedwater pump P.sub.F seen in 
FIG. 3. The heat exchanger 10 is connected to the feed water pump P.sub.F 
through the piping P and also to the feedwater connection FC on the boiler 
B. The heat exchanger 10 is plumbed in FIGS. 2 and 3 in a counterflow 
configuration so that the cooler feedwater from the pump P.sub.F enters 
the tube unit 12 on the downstream side of the main fluid passage 28 
through the heat exchanger 10. This makes the upper header 68 in FIG. 2 
the inlet header and the lower header 68 the outlet header as labelled in 
FIGS. 2 and 3. 
Pipe P.sub.1 as seen in FIG. 3 connects the feedwater water pump P.sub.F 
directly to the inlet header 68 in the heat exchanger 10 while pipe 
P.sub.2 connects the outlet header 68 to the inlet I of a three-way flow 
diversion valve V.sub.R. One outlet O.sub.1 of the diversion valve V.sub.R 
is connected by pipe P.sub.3 to the feedwater connection FC on boiler B. 
The other outlet O.sub.2 of the flow diversion valve V.sub.R is connected 
to the inlet side of a recirculating pump P.sub.R by pipe P.sub.4 as seen 
in FIG. 3 while the outlet side of the recirculating pump P.sub.R is 
connected back to the inlet header of the heat exchanger 10 through pipe 
P.sub.5. If it is desirable to continuously operate the pump P.sub.R, a 
short pipe P.sub.6 with an orifice O.sub.R interposed therein may be used 
to connect the outlet header 68 of the heat exchanger 10 directly to the 
inlet side of the recirculating pump P.sub.R bypassing valve V.sub. R with 
a very small flow of heat exchanger fluid just sufficient to keep pump 
P.sub.R cool. 
The flow diversion valve V.sub.R has an actuator A which selectively 
divides the flow of the feedwater through the valve V.sub.R from the inlet 
I so that the amount of feedwater flowing out the outlet O.sub.1 and the 
amount of feedwater flowing out the outlet O.sub.2 is selectively 
controlled. While any number of flow dividing valve constructions may be 
used, the valve V.sub.R illustrated in a pneumatically operated 
proportioning valve which proportions the flow between the outlets O.sub.1 
and O.sub.2. To supply the pneumatic control pressure to operate the 
actuator A on valve V.sub.R, an electrical-to-pneumatic transducer 
T.sub.CP may be used which produces an output pressure to the actuator A 
as seen in FIG. 2 in response to the electrical signal received by the 
transducer T.sub.CP. The control for the transducer T.sub.CP is provided 
by an averaging thermocouple T.sub.AV seen in FIG. 2 which is inserted 
into the ductwork D on the downstream side of the main fluid passage 28 
through the heat exchanger 10. The thermocouple T.sub.AV provides an 
electrical output signal that is controlled by the average temperature of 
the flue gases passing out of the heat exchanger 10 and is connected to an 
appropriate thermocouple set point controller C.sub.SP designed to operate 
with the averaging thermocouple T.sub.AV. The controller C.sub.SP produces 
an electrical output which is indicative of the average flue gas 
temperature above or below a particular set range manually set on 
controller C.sub.SP as it passes out of the heat exchanger 10. The 
transducer T.sub.CP receives this electrical signal from the controller 
C.sub.SP and converts this signal into a pneumatic output to the actuator 
A on valve V.sub.R which is responsive of the average flue gas temperature 
passing out of the heat exchanger 10. 
If the temperature of the flue gases passing out of the heat exchanger 10 
drops below the predetermined value manually set on controller C.sub.SP, 
the transducer T.sub.CP operates the valve V.sub.R so that a certain 
portion of the feedwater flowing in the inlet I of valve V.sub.R is 
diverted out the outlet O.sub.2 through the recirculating pump P.sub.R and 
back into the inlet header of the heat exchanger 10 so that the inlet 
temperature of the feedwater including the recirculated portion is raised. 
The remainder of the feedwater that does not need to be recirculated 
passes out of the outlet O.sub.1 to the feedwater connection FC on the 
boiler B. This serves to regulate the flow of the feedwater in such a way 
that the average outlet temperature of the flue gases from the heat 
exchanger 10 will be maintained above a prescribed value to prevent 
condensation of the corrosive chemical compounds in the flue gases on the 
heat exchange tubes 65 in the tube bundle 12 and the attendant corrosion 
thereof. The dew point of the flue gases from the boiler B is determined 
primarily by the sulfur content of the fuel being burned therein, and the 
sulfur content in the fuel varies between the different geographical 
sources of the fuel. Usually, the user knows the critical dew point of the 
flue gases from information provided by the source of the fuel and can 
then adjust the controller C.sub.SP each time the sulfur content in the 
fuel varies to make sure that the average outlet flue gas temperature is 
above its dew point after passage through the heat exchanger 10. 
The heat exchanger 10 is shown incorporated in a bypass economizer E.sub.B 
in FIG. 5. The economizer E.sub.B would be mounted in the flue gas 
ductwork D on a conventional steam boiler B similarly to economizer 
E.sub.R. The feedwater under pressure from the feedwater source FS is 
supplied directly to the inlet header 68 on heat exchanger 10. The outlet 
header 68 on heat exchanger 10 is connected to one inlet I.sub.1 on a 
three-way diversion valve V.sub.B whose outlet O is connected to the 
feedwater connection FC on boiler B. The other inlet I.sub.2 on valve 
V.sub.B is connected to the feedwater source FS in parallel with the tube 
bundle 56 in heat exchanger 10. The valve V.sub.B serves to selectively 
divide the flow of the feedwater from source FS between the flowing 
through the tube bundle 56 and that bypassing the tube bundle 56. The 
valve V.sub.B has an actuator A similar to that of the valve V.sub.R. 
The economizer E.sub.B also uses an averaging thermocouple T.sub.AV, a set 
point controller C.sub.SP and a transducer T.sub.CP to control the 
actuator A on valve V.sub.B. Thus, it will be seen that the controller 
C.sub.SP can be adjusted so that the feedwater flowing through the tube 
bundle 56 and bypassing the tube bundle 56 can be proportioned by the 
valve V.sub.B in response to the outlet temperature of the flue gases 
after passage about the tube bundle 56 to maintain the flue within a 
prescribed temperature range. As the load on the boiler B varies, the 
temperature of the flue gases passing through the ductwork D also varies, 
however, the output from the thermocouple T.sub.AV controls the three-way 
valve V.sub.B in such a way that the feedwater flow is controlled to 
maintain the flue gas temperature after passing through the tube bundle 56 
in heat exchanger 10 at a prescribed value to prevent condensation of the 
corrosive chemical compounds in the flue gases on the heat exchange tubes 
65 and the attendant corrosion thereof.