Dirty water heat exchanger

A dirty water heat exchanger keeps solids from settling out while passing through the heat exchanger and includes a cylindrical furnace duct, connected at a first end to a conventional burner, having a spiral liquid heating duct along which the dirty water passes from its first end to its second end. The second end of the furnace duct is open. An exhaust tube surrounds the furnace duct and provides an annular exhaust gap for the hot gases to flow back over the outer surface of the spiral liquid heating duct. The outer end of the exhaust tube is closed to redirect hot gases into the exhaust gap. Spiral flighting along the exhaust gap spirals the exhaust gas to keep the hot gases collecting along the top of the heat exchanger. The gases pass out of the exhaust gap through a flume at the first exhaust tube end. The furnace duct and the exhaust tube are connected together at at most one axial position to accommodate different amounts of axial expansion. The heat exchanger is supported by an expansion mount at the second end of the exhaust tube and by a stationary mount at the first end of the furnace tube.

BACKGROUND OF THE INVENTION 
This invention relates to heat exchangers, more particularly to a heat 
exchanger particularly useful for heating dirty wash water. 
During many industrial cleaning processes the washing fluid, often water or 
some other solvent, is heated to enhance its cleaning effectiveness. Some 
containers, such as railroad tank cars, have large amounts of solid 
residue built up in them. This residue is usually loosened by spraying a 
heated cleaning liquid and is carried away by the wash liquid. 
In some cases it is desirable to re-use the wash liquid after removing the 
solid matter. The solids in the wash liquid are commonly eliminated by 
allowing the dirty wash liquid to remain undisturbed in a settling tank. 
However, some of the solid particles, including sand, dirt and gravel, may 
not all be removed from the cleaning liquid. These solids have a tendency 
to precipitate out while passing through a conventional heat exchanger. 
This can cause hot spots in the heat exchanger which, in addition to 
reducing the efficiency of heat transfer, may eventually cause the heat 
exchanger to buckle and fail. 
Another problem associated with heat exchangers involves the differences in 
the amount various components expand and contract according to their 
temperatures. The requirement that the designer accommodate this 
differential expansion factor often results in a design which is more 
complicated, and therefore more expensive to build, than would otherwise 
be the case. 
SUMMARY OF THE INVENTION 
The present invention solves the problem of solid matter precipitating out 
in the heat exchanger and accommodates the problems created by 
differential expansion and contraction in a simple, inexpensive, 
straightforward manner. 
The heat exchanger of the invention is particularly useful for heating 
dirty wash liquid because it prevents solids from settling out while 
passing through it. The wash liquid will often be referred to in this 
application as wash water; however, it is to be understood that wash water 
includes other cleaning liquids as well. The heat exchanger includes a 
cylindrical furnace tube surrounded by a spiral wash water heating duct 
along which the dirty water passes. The spiral heating duct extends 
substantially the entire length of furnace tube and is preferably 
integrally constructed with the furnace tube. The furnace tube and spiral 
heating duct are collectively termed the furnace duct. 
A conventional burner is mounted to the first end of the furnace duct and 
injects hot gases into the furnace duct. The second end of the furnace 
duct is open. An exhaust tube fits over the second end of the furnace duct 
and surrounds the furnace duct. The exhaust tube defines a return pathway 
for the hot gases to flow between the outer surface of the spiral liquid 
heating duct and the exhaust tube from the second end of the furnace duct 
to a position just short of the first end of the furnace duct. Exhaust 
gases pass from the exhaust tube through a flume adjacent the first end of 
the exhaust tube. The dirty water enters the spiral liquid heating duct 
adjacent the burner, that is at the first end of the furnace tube, and 
exits at the second end of the furnace tube. 
Spiral flighting between the exhaust tube and the spiral liquid heating 
duct causes the exhaust gases to flow in a spiral path as they move 
between the heat exchanger's second and first ends to keep the hot gases 
from collecting along the top of the heat exchanger. 
To compensate for the differential expansion between the furnace duct and 
the exhaust tubes, the furnace duct and the exhaust tube are connected 
together at a rigid connection at at most one point, typically the dirty 
water outlet at the end of the heat exchanger. The heat exchanger is 
preferably supported at the second end of the exhaust tube and the first 
end of the furnace duct. Assuming the furnace duct and exhaust tubes are 
rigidly connected at one point, then at least one of the heat exchanger 
supports is an expansion joint. In the preferred embodiment the first end 
of the exhaust tube is slidably mounted to and supported by a slip joint. 
A key feature of the invention is the provision of the spiral liquid 
heating duct sized to ensure adequate turbulence and velocity of the dirty 
water to be heated to keep solid material from precipitating out. Also, 
the spiral liquid heating duct heats the water more efficiently by virtue 
of the turbulence and also because the spiral flighting between the outer 
shell of the spiral liquid heating duct and the furnace tube acts as a 
heat transfer fin. 
Another significant feature of the invention is the manner in which the 
exhaust tube and the furnace duct are mounted to one another using slip 
joints to allow for difference in expansion between the two. It has been 
found that for a heat exchanger 20 feet long, the increase in length of 
the exhaust tube was about one inch greater than the increase length of 
the furnace duct under normal operating conditions. This differential in 
expansion is accommodated by rigidly coupling the furnace duct and exhaust 
tube together at not more than one axial position. Doing so permits axial 
slippage between the two due to this differential expansion and 
contraction. 
Providing the spiral flighting between the furnace duct and exhaust tube 
causes the hot gases to flow in a spiral pattern within the exhaust 
chamber. This keeps the hot gases from collecting along the top of the 
heat exchanger for increased efficiency. 
Other features and advantages of the present invention will appear from the 
following description in which the preferred embodiment is set forth in 
detail in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the figures, a heat exchanger 2 made according to the 
invention is shown with a conventional burner 4 mounted to a first end 6 
of the heat exchanger. Heat exchanger 2 includes broadly a cylindrical 
furnace duct 8 mounted within a cylindrical exhaust tube 10. Furnace duct 
8 includes a cylindrical furnace tube 12 and a spiral liquid heating duct 
14 formed integrally with and mounted rigidly to the outside of furnace 
tube 12 along the entire length of the furnace tube. A spiral liquid 
pathway 16 is defined along the length of furnace duct 8 by the outer 
surface 18 of furnace tube 12, the inner surface 20 of the outer shell 22 
of spiral heating duct 14 and the spiral flighting 24 mounted between 
furnace tube 12 and outer shell 22. A liquid inlet 26 is provided to duct 
14 at first end 6 of heat exchanger 2 while a liquid outlet 28 is 
positioned at the second or outer end 30 of heat exchanger 2. 
A burner mounting plate 32 is mounted to the first end 34 of furnace duct 
8. A ring of refractory material 36 is mounted to plate 32 within furnace 
duct 8. Plate 32 and ring 36 define a circular opening 38 through which 
flames and hot gases 40 from burner 4 pass into the cylindrical hot gas 
chamber 42 defined within furnace duct 8. 
Exhaust tube 10 extends from a position forward or in front of the second 
end 44 of furnace duct 8 to a position short of the first end 34 of 
furnace duct 8. Exhaust tube 10 is larger in diameter than furnace duct 8 
and is radially centered about the furnace duct by ten radially and 
longitudinally directed spacer plates 46, 47 at each end of exhaust tube 
10. Spacer plates 46, 47 are secured, such as by welding, to the inner 
surface 48 of exhaust tube 10. However, spacer plates 46, 47 are not 
fastened to furnace duct 8 to permit any relative axial movement of 
furnace duct 8 and exhaust tube 10 due to differences in thermal expansion 
and contraction such as occurs when heat exchanger 2 is first fired up. 
End plate 50 covers the second or outer end 52 of exhaust tube 10 and has a 
concave layer of refractory material 54 on its inner surface for directing 
hot gases 40 from hot gas chamber 42, past second end 44 of furnace duct 
8, past spacer plates 46 and into an exhaust gap 56 defined between 
exhaust tube 10 and furnace duct 8. To keep hot gases 40 from collecting 
within exhaust gap 56 along the top of heat exchanger 2, a pair of spiral 
flights 58 are wrapped around furnace duct 8 within gap 56 one complete 
turn each. This causes very little resistance to the flow of hot gases 40, 
but keeps the gases from collecting along the top of heat exchanger 2. Hot 
gases 40 are exhausted into the atmosphere through a flume 60 in exhaust 
tube 10. 
First end 6 of heat exchanger 2 is supported on a support surface 72 by a 
stationary mount 62 welded directly to furnace duct 8. Second end 30 of 
heat exchanger 2 is supported by a slip tube expansion mount 64. Mount 64 
includes a pair of inner tubes 66 mounted to exhaust tube 10 by a pair of 
brackets 68 and a pair of outer tubes 70 slidably housing inner tubes 66. 
Outer tubes 70 are supported on support surface 72 by brackets 74. 
A slip ring 76 is mounted to the first end 78 of exhaust tube 10 so to be 
in sliding, sealing engagement with the outer surface 80 of outer shell 
22. This keeps hot gases 40 from escaping through the gap between tube 10 
and duct 8 at first end 6 of heat exchanger 2. 
During operation exhaust tube 10 expands axially more than furnace duct 8 
because furnace duct 8 is kept cooler than exhaust tube 10 by the liquid 
flowing through pathway 16. This causes tube 10 to change in length more 
than duct 8. In the preferred embodiment expansion mount 64 is provided at 
second end 52 of exhaust tube 10. Also, liquid outlet 28 is, in the 
preferred embodiment, rigidly attached to both furnace duct 8 and exhaust 
tube 10 thus tying these members together at one axial position. If 
exhaust tube 10 and furnace duct 8 were not rigidly connected to one 
another, then expansion mount 64 could be replaced by a fixed or a 
stationary mount since exhaust tube 10 and furnace duct 8 would still be 
free to expand and contract. In any event, furnace duct 8 and exhaust tube 
10 should not be rigidly fastened together at two axially spaced-apart 
points since this would inhibit differential expansion. 
In use, liquid to be heated, typically dirty water containing suspended 
solid material, is pumped into inlet 26 and flows along spiral pathway 16 
which surrounds hot gas chamber 42. Flame and hot gases 40 from burner 4 
heat furnace duct 8 and thus the liquid within pathway 16 as it flows 
through the pathway. The hot gases flow the entire length of furnace duct 
8, reverse direction at the outer or second end 30 of heat exchanger 2 and 
then flow through gap 56 back along the outside of furnace duct 8 to 
continue heating the water within pathway 16. Hot gases 40, after passing 
through exhaust gap 56, pass out of heat exchanger 2 through flume 60. The 
liquid, after flowing through pathway 16, flows out outlet 28 for further 
use or processing. Because of the high speed and turbulence in pathway 16, 
solid material will not settle out along the pathway for better 
efficiency; hot spots, which can be caused by deposits of solids along a 
flow path, are also eliminated by the use of spiral pathway 16. 
Modification and variation can be made to the disclosed embodiment without 
departing from the subject of the invention as defined in the following 
claims.