Note: Descriptions are shown in the official language in which they were submitted.
:~LZ'~5~3
COUNTERFLOW TUBULAR HEAT EXCHANGER HAVING
CENTER PIPE CONSTRUCTION
_
BACKGROUND OF INVENTION
This invention pertains to a tube type counterflow heat
exchanger arranged for operation at elevated temperature and
pressure conditions. It particularly pertains to such heat
exchanger having an elongated removable center pipe construction
for handling a hot gas inlet stream, so as to minimize exposure
of the pressure stressed metal walls to the high temperature
conditions.
As energy conservation has become increasingly important,
particularly in the chemical processing industry, improved
tubular type heat exchangers have been developed suitable for ~
high temperature service, such as for inlet gas temperatures
above about 1000-F. However, a major problem with such heat
exchangers for high temperature service is minimizing the
differential thermal expansion between the tube passes and
reducing as much as possible those portions of the pressurized
boundary walls of the exchanger which are subjected to the higher
temperature ~7ithin the unit. To overcome these problems, the
heat exchanger configuration of the present invention was
developed and it ic principally intended for use in hydro-
deal}cylation (HDA) process units to exchange heat between the
reactor feedstream and the reactor hot effluent gas stream, so as5 to provide more reliable and trouble-free operations.
SUMMARY OF INVENTION
The present invention provides an improved tube type
counterflow heat exchanger for high temperature service. The
heat exchanger unit includes a tube bundle of parallel tubes
within an outer pressurizable shell, and contain6 an elongated
center pipe for a hot gas inlet stream. The central pipe is
arranged in flow communication with the single pass tube bundle
and i~ capable of operating at a high tubeside temperature, such
as up to about 1250-F., and to minimize exposure of stressed
metal parts to high temperature gas flows. More specifically,
the heat exchanger comprises a pressurizable outer shell having a
83
-- 2
head attached at each opposite end, said heat at the inlet end
being attached to the shell by a bolted flange type connection;
and the head at the opposite end heing welded to the shell;
multiple parallel tubes provided longitudinally within said outer
shell, said tubes being located outwardly from a centrally-
located parallel inner shell and sealably connected to a tube
sheet at each en~; with the inlet end stationary tube ~heet being
also connected to ~aid inl.et head; a central pipe extended
longitudinally within 6 aid inner shell so as to provide a narrow
annular space between the central pipe and the inner shell; and a
metal bellows attached at one end to Eaid inner shell stationary
tube sheet and attached at the other end to said inner 6hell, so
as to accommodate dif~erential thermal expansion between the
stationary tube sheet and the inner shell.
A metal therma~ sleeve is advantageously prov.ided within the
annular space between the center pipe and inner ehell and
attached to the center pipe to minimize exposure o~ the inner
shell to the temperature of the hot inlet gas stream passing
through the center pipe. To protect the heat exchanger tubes
from the effect of excessive differential temperature, the metal
bellows i~ provided between the stationary tubesheet and the
lnner shell of the exchanger. Also, the inlet end removable head
is made generally conical shaped and has an annular-shaped baffle
th~rein arranged to direct the flow of coolsd gas exiting the
tubes past the bellows an~ past mo~t of the metal inner surfaces
in the end he~d
Advantages of the heat exchanger construction provided by
the present invention include minimizing the temperature of
stressed metal parts exposed to the high primary unit pressure,
thus reducing the quantity of expensive steel alloy material
required for the exchanger, and also minimizing stresse6 usually
caused by differential thermal expansion of the tubes relative to
the inner shell.
~2'~3~1~3
BRI EF DESCRI PTI ON OF DR~WI NGS
FIG. 1 shows a longitudinal sectional view of the tube type
counterflow heat exchanger constructed according to the present
invention.
FIG. 2 is an enlarged partial sectional view ~howing details
of the central pipe and thermal sleeve construction for the heat
exchanger.
DETAI LED DESCRI PTI ON OF I NVENTI ON
The various features provided by a preferred embodiment of
the present invention are generally shown in FIG. 1, in which
heat exchanger assembly 10 has an outer pres6urizable shell 12
which i6 flange connected to the tube side inlet and outlet bead
as6embly 14 and i~ welded to the shellside outlet head 16.
Multiple parallel tube6 20 are provided in a removable tube
bundle 22 within the outer 6hell 12, the tubes being sealably
attached at one end to stationary tube sheet 23 and at the
- opposite end to floating tubesheet 25. Tube bundle 22 is also
provided with an inner cylindrical shell 26 which is flexibly
attached to the tube sheet 23 by bellows 40 and is rigidly
attached to the floating tube6heet 25. Attached to the
removable inlet head assembly 14 i6 a center pipe 30 which i6
attached to flange 13 and i8 provided concentrically within inner
shell 26, and extends past the stationary tubesheet 23 to the
floating t~b~sh~et 25. The center pipe 30 conduct6 the hot
inlet gas directly from the tube 6ide inlet nozzle 13 to the
floating head 24 of the heat exchanger bundle 22 from where it
flows baak through multiple tubes 20 to head 14. Central pipe 30
is made of a stainle6s steel material suitable for the high
temperature service up to about 1250 F., 6uch as AISI Type 347
6 teel.
From the hsat exchanger floating head 24 end, the hot gas
then flows back mainly ll:hrough the tubes 20, but a small portion
of the hot ga~ is also distributed through an annulus 32 formed
between inner shell 26 and a thermal 61eeve 34 attached to the
outer surface of center pipe 30. From the floating head 24 end,
the small bypass gas stream enters the annular space 32 and
pa6ses between the thermal sleeve 34 and the inside diameter of
the inner shell 26, and thereby flows parallel to the gas flow~ng
through multiple tubes 20. The annulus 32 is of such cross-
6ectional area that the small flow of hot bypasR gas therethrough
will re6ult in the temperature for inner shell 26 being
approximately that of tubes 20, so as to minimize the
differential temperature between the inner ~hell 26 and tubes 20
and thereby minimize the difference in thermal growth between
shell 26 and the tubes 20. Thus, because a substantially "dead
air" space effectively exists in the annular space 32 between the
thermal 61eeve 34 and center pipe 30 to provide an insulating gas
layer therebetween, the temperature profile along the inner shell
26 6ubstantially matches the temperature profile of the tubes 20.
If desirea, the thermal sleeve 34 can be provided as multiple
~leeves 35 arranged in serie6 and, each attached at one end to
center pipe 30, as is shown in FIG. 2.
The center pipe assembly 30 is separately attached to head
assembly 1~ and bolted flange 18. By this construction
arrangement, the central pipe assembly can be advantageously
removed rom within shell 26 of tube bundle 22 of the heat
exchanger 10, to permit removal of carbon deposits within
passageway 32.
The basic design concept for the heat exahanger of the
present invention is to avoid or minimize elevated temperatures
for those metal parts subject to the high primary unit pressure,
such as above about 500 psig. The utilization of the center pipe
30, which immediately conducts the hot inlet gas stream entering
at flange 13 to those parts subjected only to differential
pressure 80 that the hot gas i6 cooled immediately by the
counteraurrent gas flow exiting from tubes 20, aids in
accomplishing this desired result. Thus the gas flow streams are
arranged so as to minimiz0 exposure of the pressure stressed
,. ~
,.,
~lL2'~9~33
metal walls to the high temperature inlet gas and to expose as
much as po~sible of the shell 12 and channel head assembly 14 to
the lower temperature proces~ gas stream within the shell and
tube side of the exchanger. Also, the heat exchanger i~ ar~anged
so as to avoid insofar as possible differing temperatures across
the diameter of the exchanger. In addition, the tube sheet
bolted joints at 27 and 28 located at opposite ends of tube
bundle 22 are designed to accommodate unexpected strains and
movements that may occur in the tube bundle 22 dua to rapid
temperature changes such as during start-up or shut-down
operation of the heat exchanger unit.
To minimize the temperature level6 to which the heat
exchanger pres6urized outer 6hell 12 and flow ohannel head
assembly 14 are exposed, the hot effluent stream entering the
channel head assembly is conducted via center pipe 30 directly to
the floating head 24, which serves to isolate the hot gas
temperature from the high pressure boundary shell 12 and the
boundary channel head assembly 14 of the exchanger. The shell
side gas flow enters at nozzle 17 and flows around multiple
parallel tubes 20 and then through the annular space 24a around
the floating head 24 to exit at centrally located nozzle 19.
This flow pattern tends to keep the temperatures uniform around
the inner floating head 24, avoids distortions of flange joint
28, and prevents the shell side outer head lS from overheating
due to it~ proximity to the higher temperature floating head 24.
In order to additionally protect the tubes 20 and inner
shell 26 from damaging differential temperature stresses during
process ~tartup or any upset conditions, a metal bellows
expansion -~oint 40 is provided at the channel head assembly end
between the inner shell 26 and the fixed tubesheet 23. The
cooled gas exiting from the tubes 20 within the head assembly 14,
which gas has been lowered in temperature by heat exchange with
the feedstream flowing in the shellside, flow~ past the bellows
expansion joint 40 and thereby cools the ~oint to a temperature
33
level where reasonable allowable metal stresses can be used in
the design of the joint.
Another important feature of the heat exchanger of the
invention is the use of an annular channel provided by conical-
shaped f~ow baffle 42 located within inlet head assembly 14 and
attached at its base to head assembly 14, as shown in FIG. 1.
Baffle 42 serves to direct the flow exiting from tubes 20
towards the inlet point for the reactor hot gas feedstream in the
central pipe 30, and then annularly through the annular flow
channel ~4 to exit at 15. This gas return flow in the annulus 44
formed by ~affle 42 and the inner wall of head assembly 14 cools
this head a~sembly, which restricts the elevated metal
temperature to that area immediately adjacent to the hot gas
15 entry pipe 30. Thu6, the gas flow exiting from tubes 20 is
deflected by annular baffle 42 through the annular flow channel
44 towards the location of tube side exit 15, which flow path
cools the head assembly 14 and minimizes the metal temperature at
the point where the hot gas stream enters the heat exchanger.
The tube bundle 22 is preferably made removable from the
outer shell 12, which can be accomplished by removal of head
assembly 14 at outer bolted type flange 29. The bolted joints at
27 and 28 are ring type joints designed with a clamp ring to
enable the use of longer bolts 37 than if the fixed tube sheet
and floating cover head 24 were each bolted directly to their
ad~acent mating pieces. The use of an oval-shaped ring gasket 38
in these flanged ~oints 27 and 28 permit6 adequate joint rotation
to accommodate strains and distortions in the joint, while the
additional strain available in the longer bolts 37 aids in
maintaining a tight joint during transient as well as normal
temperature conditions.
As explained above, to limit the amount of heat transmitted
from the hot inlet gas s~tream through nozzle 13 to the
pressurized inner shell 26, the center pipe 30 is preferably
fitted with a plurality of thermal sleeves 35 extending from pipe
~,
~2'~
free end 30a to a point beyond the internal expansion joint 40,
as best shown by FIG. 2. Also, short lengths of tubes or
ferrules 21 having an outside diameter slightly smaller than the
inside diameter of tubes 20 are preferably provided attached in
the floating head end of tubes 20, so as to provide gas pockets
at 21a within the tubes 20 to absorb the differences in
temperature at this location.
The heat exchanger of this invention is suitable for high
temperature service at high pressure, such as for 1000-1300-F
temperature and 500-1000 psig pressure. By using this heat
exchanger design configuration, the temperature of the two
counterflowing gas streams can be brought close together and even
crossed if desired, so as to maximize the heat recovery from the
hot gas stream provided by the exchanger. This heat exchanger is
typically used for exchanging heat between a feedstream to a
reactor entering at connection 17 and the reactor hot effluent
stream being passed first through the center pipe assembly 30 and
then back through the multiple tubes 20 to heat the feedstream
passing through the exchanger shell side and around the tubes to
near the reactor inlet temperature, and thereby reduce furnace
heating costs for the feedstream.
Although this heat exohanger invention has been described
broadly and in terms of a preferred embodiment, it will be
understood that design modifications and variations to the
exchanger can be made within the spirit and scope of the
invention, which is definad by the following claims.
',,~
