Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Z0408Z7
HEAT EXCHANGER FOR A PULP DRYER
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a steam heat eycll~nger for a wood pulp dryer.
2. D~li~lion of Related Art
Pulp dryers are convelllionally used to dry wood pulp in sheet form
prior to ~hi~lllenl. Heat is applied to the pulp by heating air with heat PYch~ngers,
normally filled with steam, and blowing the air against the web. The type of heat
eYch~n~er used on many dryers built after 1980 has a fMme with a plurality of
spaced-apart, vertical copper tubes eYtPn-ling between opposite ends thereof. The
tubes extend tightly through fins, usually of aluminum, which are perpendicular to the
tubes. The tops of the tubes are connected to an inlet header which is perpendicular
to the tubes. The bottoms of the tubes are connected to an outlet header. Steam is
conventionally fed into the center of the inlet header by means of a T fitting. This
T fitting in~lreles with the connections between the tubes and the inlet header in
some prior art heat exchangers. A combination of steel and copper has been utilized
in the past to give the re~uired strength and economy of construction. However, the
life span of such prior art heat exchangers has been less than desirable. In some
cases the life expectancy has been approximately 8 years, whereas a life span of 16-
20 years is expected by many in the pulp industry. Pulp dryers have many heat
exchangers and it is expensive to replace them, both in terms of the cost of the new
heat exchangers, the labour required to replace them, and the down time of the pulp
dryer needed to carry out this m~inten~nce operation.
The failure of prior art dryers is believed to be due to a combination of
problems including the incGl~lalion of steel col~ onents in contact with the steam.
In addition, the arrangement of the inlet header appears to cause an uneven
distribution of steam in the various tubes of each heat exchanger. The tubes carrying
higher p'es~Ule steam wear faster. Finally, in some mills steam becomes
cont~min~ted with black liquor from time to time. This m~t~ri~l coats and llltim~tely
corrodes the tubes.
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SUMMARY OF THE INVENTION
It is an object of the invention to provide a heat exchanger which can be
constructed completely of copper or copper alloys, which are the ~er~lled m~tPri~
S The word "COp~l" hereinafter includes copper alloys.
It is a further object of the invention to provide a heat eYçh~nger for wood
pulp dryers which has a better distribution of steam throughout the heat eYch~nger so
that each tube thereof carries approximately the same propollion of steam.
It is a further object of the invention to provide a heat exchanger of the type
described wherein there is provision for diverting black liquor and other con~;~."in~t~d
con~len~tes from the steam before they can enter the tubes and cause corrosion.
In accor~lce with these objects, the invention provides a steam heat
exchanger for a wood pulp dryer which has a first end and a second end which is
opposite the first end. There is a plurality of tubes, each having an inlet end and an
outlet end, the tubes extending between the ends of the heat Pxch~nger in parallel,
spaced-apart relationship to each other. A plurality of spaced-apart fins are connected
20 to the tubes. There is an outlet header near the second end of the heat eYch~nger.
The outlet ends of the tubes are connected to the outlet header. A steam inlet header
is near the first end of the heat eYçh~nger. The inlet header has an inner conduit
having means at a first end thereof for conn~cting the inner conduit to a source of
steam and a plurality of openings spaced-apart therein for discharging steam from the
25 inner conduit. The inlet header also has an outer conduit which extends about the
inner conduit. The inlet ends of the tubes are connected to the outer conduit.
In a prerelled form, the inner conduit and outer conduit are elongated, straighttubes. The outer conduit has a greater cross-sectional extent than the inner conduit.
There may be a conden~te conduit which connects the inner conduit to the
outlet header.
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Preferably the tubes, the outlet header, the inner conduit and the outer conduitare of copper.
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The invention ove~ol~les the problems and limit~tions encoull~e~d in some
prior art heat eYch~ngers by allowing the use of copper for all major co~ ?onents
coming into direct contact with the steam. Furthermore, the arrangement of the
steam inlet header provides for a more even distribution of steam among the different
10 tubes of the heat eYch~n~er. Furthermore, by diverting con~ such as black
liquor and conden~te~ away from the tubes, corrosion is redllced. There is no T-fitting required on the inlet header which inlelrel~es with the connection of the tubes
thereto.
15BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Fig. 1 is a front elevation of a steam heat eYch~nger according to an
20 embodiment of the invention, shown partly in section and showing only some of the
fins;
Fig. 2 is an elevation showing three heat eYch~ngers according to
emb~iment~ of the invention and the Z~ tf~ piping and connections in a typical
25 in~Pll~tion;
Fig. 3 is a sectional view of the steam inlet header of the embodiment of Fig.
1 and a fr~mPnt of the tubes conn~ted thereto, taken along line 3-3 of Fig. 1;
30Fig. 4 is a frAgmPnt~ry bottom plan view of the inlet steam header of the
embodiment of Fig. l;
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Fig. 5 is an enlarged ~i~metric section of the inlet fitting of the steam inlet
header of the embodiment of Fig. l;
Fig. 6 is an enlarged fr~mPnt~ry section of the elbow at the bottom of the
S condPn~te conduit and the orifice plate from Fig. l;
Fig. 7 is an enlarged, di~mPtric section of the inner conduit of the inlet header
for the embodiment of Fig. l;
Fig. 8 is a top plan thereof; and
Fig. 9 is an enlarged, fra~ment~ry end view, partly in section of a heat
eY~ nger according to another embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Fig. 1, this shows a steam heat eYch~nger 10, col-llllol ly known
as a "steam coil". It has a porimeter frame 12 made of galvanized steel in this
example, though other m~teri~ls could be used. The heat exchanger has a first end
20 14, which is at the top when the heat eYch~nver is positioned for use, and a second
end 16, which is at the bottom in use. Fig. 2 shows an in~t~ tion of three heat
exchangers 10.1, 10.2 and 10.3 which are generally similar to heat exchanger 10.Like parts have like numbers with the addition of ".1", ".2" and ".3" respectively.
There is a plurality of tubes 18 which extend between the ends of the heat
exchanger in parallel, spaced-apart relationship to each other. Referring to the tube
18 which is furthest to the left in Fig. 1, it has an inlet end 20 and an outlet end 22.
The other tubes are i-len~ l The tubes are all of copper in this example although
other metals could be substi~uted. Copper is pr~f~llt;d because of its heat transfer
abilities. Silver brazing is used to connect the co~l~ponents of the heat exchanger in
this example.
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s
There is a plurality of fins 24 fitted about the tubes and which extend
perpendicularly thereto. In Fig. 1 only a few of the fins at the top and at the bottom
of the tubes are shown, but the entire area belwæn the ends of the tubes would
normally be filled with spaced-apart fins. The fins in this example are of alulllinu
S which is plc;f~led because of its high rate of heat transfer as well as its economy.
However, other metals could be used.
Heat exchanger 10 has an outlet header 26 near the second end 16 of the heat
eYch~nger. The outlet header is a straight, elongated pipe which extends
perpendicular to the tubes 18. The outlet header is of copper in this example, though
other metals could be used. The outlet ends 22 of the tubes 18 are connected to the
outlet header. The outlet header has a first end 28 and a second end 30. The outlet
header is sloped slightly towards end 30 to drain con-len~tç. There is a fitting 32
on the second end which allows the outlet header to be connected to a conden~te
return line. For example, fitting 32.1 of heat exchanger 10.1 in Fig. 2 is connected
to conden~te return line 34.1. An orifice plate (not shown) is placed belwæl fitting
32.1 and the cor~ponding flange on the conden~te return line to hold back the
steam so it conden~e~ in the heat exchanger instead of rushing through.
Heat exchanger 10 also includes a steam inlet header 36 near first end 14.
This header is elongated and perpendicular to tubes 18. The tubes are connected to
the header which supplies steam to the tubes.
As described thus far, heat exchanger 10 is generally conventional. However,
this heat exchanger includes a number of improvements over the prior art, chiefly
related to the configuration of inlet header 36 which, in the prior art, is a single
length of pipe with a T fitting at the center thereof which is connected to a source of
steam. Instead, inlet header 36 in this embodiment of the invention is comprised of
an inner conduit 38 which is located within an outer conduit 40. In the illustrated
embodiment, both conduits are straight, elongated tubes which are generally
coeYten~ive in the axial direction. The conduits are sufficiently long to extendbelween the outermost tubes 18 of the heat exchanger. In the illl~str~ted embodiment
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the inner conduit is a pipe with a smaller ~i~m~pt~pr than the outer conduit and is
radially spaced-apart thel~rlolll. In this example the conduits are concPntric~ though
this is not essenti~l. In this example the conduits are of copper, which is the
pfeî~ d m~tP.ri~l
s
Inner conduit 38 has a first end 42 and a second end 70. A flange 44 at the
first end is used to connect the inner conduit to a source of steam, such as steam pipe
46.2 shown for center heat eYc~nger 10.2 in Fig. 2.
The inner conduit has a series of opening~ 56, 58, 60, 62, 64, 66 and 68 in
the top thereof. While seven openings are shown for illustrative purposes, the
nulllber is not critical and depen~s in part on their size. As seen in Fig. 7 and 8, the
openings of this embodiment are formed by lateral slits and depressions in the top of
the inner conduit. Other types of a~llulcs could be substitutecl.
Referring back to Fig. 1, annular members 72 and 74 are located at opposite
ends of outer conduit 40 of the inlet header and are s~P~lingly connP~te~i to the inner
conduit. Therefore, space 54 between the inner conduit and outer conduit is sealed,
apart from a plurality of openings where the tubes 18 are connected. The inlet header
is sloped slightly towards member 74 to drain condPn~tP~ therein.
As shown in Fig. 3 and Fig. 4, two rows of tubes 18 and 18.4 are fitted to
outer conduit 40 of inlet header 36. Each of the tubes 18 has an upper portion 19
angled slightly, approximately 15 degrees in this embo~limPnt~ to fit within one of
openings 76 in the outer conduit. Tubes 18.4 in the second row are angled a similar
amount in the opposite direction from the vertical so their upper portions 19.4 fit
within openings 76.4 in the outer conduit. The positions of the openings 76 and 76.4
are staggered, as seen in the bottom plan view of outer conduit 40 in Fig. 4, so that
all of the tubes are exposed to the flow of air. This increases the efficiency of the
heat exchanger.
In another form of the invention shown in Fig. 9, there are two rows of tubes
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18.5 and 18.6 which have parallel tops instead of having the angled upper portions
as in the previous embodiment There is a single angled tube 18.7 closest to second
end 70 of the inner conduit, as shown in Fig. 1. The top of this tube comle~ to the
bottom of the conduit to allow condP-n~tç to drain away.
Fig. 5 is an enlarged sectional view of the first end 42 of inner conduit 38 andof flange 44. Heat eYch~nger 10 is constructed so that all of the colllponents in
contact with steam are of copper. Again, it is not es~-.l;~l that all colllponents be
made of copper, but it is p-~fel-ed for the reasons specified above. Flange 44
10 therefore has a copper lap ring 78 forming the inner portion thereof. This lap ring
is fitted within an annular lap joint flange 80 which can be constructed of a stronger
m~tPri~l, forged steel in this particular embo~iim~nt Other connectors could be
~ubsliluled.
Referring back to Fig. 1, heat exchanger 10 has a condensate conduit 82
which connects the inner conduit 38 of the inlet header 36 to the outlet header 26.
In this particular example, the condensate conduit 82 is es~-nti~lly vertical and is
conn~ted to second end 70 of inner conduit 38 and first end 28 of outlet header 26
by elbows 84 and 86, respectively. Like the other col"ponents in contact with live
steam, the conduit 82 and the elbows are made of copper in this example as is
p.c;fe -ed.
Heat Pxch~ngers 10 with conden~te conduits 82 are used at the bottom of a
vertical row of heat exchangers. Fig. 2 shows such a vertical row of heat exchangers
10.1, 10.2 and 10.3. However, in place of condPn~te conduit 82, a condens~te
conduit 83 with a plurality of fins 85 and an orifice plate 183 is used to take
condPn~te from the bottom of steam pipe 87 to condensate pipe 89 via conden~te
return line 34.3. The fins 85 may be deleted in some embo liment~. Heat exchangers
10.1, 10.2 and 10.3 have caps 91 and 92 at the ends of the headers as labelled for
eY-ch~nger 10.2 only.
There is a baffle in the outlet header 26 ~dj~nt its first end 28 in the form
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of orifice plate 88, shown best in Fig. 6. The orifice plate is ess~ lly disk-shaped
and is held in position between elbow 86 and end 28 of the header. Plate 88 has a
weep hole 90 ~ Pnt the bottom thereof.
Operation
In operation, each heat eych~nger 10 is oriçnt~ vertically with first end 14
at the top. A plurality of such heat eY~h~ng~rs are stacked vertically, one above the
other, as shown for heat exchangers 10.1, 10.2 and 10.3 in Fig. 2 . Likewise, a
10 plurality of the heat eYch~ngers are arranged side-by-side. Steam enters each heat
exchanger 10 through first end 42 of conduit 38 shown in Fig. 1. The steam leaves
the inner conduit through each of the opening~ 56-68. After the steam leaves theinner conduit through the ope-ning~, it fills the space 54 belween the inner conduit and
outer conduit. The steam then enters inlet end 20 of each of the tubes 18 through
15 openings 76 in the outer conduit. The steam then passes dow,lwardly through the
tubes and heats the fins 24. Air blown over the fins is heated before being directed
onto the pulp web to dry the pulp. Rçm~ining steam and condçn~tç leaves the outlet
end 22 of each tube and enters the outlet header 26 and eventually passes to theconden~tP return.
Rec~lse the openings 56-68 are in the top of the inner conduit 38, any black
liquor or other impurities or conden~te en~ering steam inlet header 36 passes through
the conduit towards end 70. Such m~tPri~l then passes through elbow 84, conden~tt-
conduit 82 and elbow 86 to orifice plate 88. Eventually it leaves the heat exchanger
25 and enters the condene~te return line, for example return line 34.1 shown in Fig. 2.
It will be understood by someone skilled in the art that many of the details
specified above are given by way of example only. Many ~ltern~tives and variations
are included within the scope of the invention which is to be inte~ eled with
30 reference to the following claims.
