Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relates to an atmospheric vaporizer
suitable for vaporizing cryogenic liquids on a substantially
continuous basis at greatly improved operating efficiency.
Atmospheric gases such as oxygen, nitrogen and
argon find wide use in a variety of applications. These
gases are typically produced by means of air separation
plants. Users of large quantities of such gases may have
air separation units at the site of gas usage, while users
of small quantities of such gases generally find it con-
venient to purchase their requirements in cylinders. Users
of intermiedate or moderate amounts of such gases generally
do not have suff~cient usage to ~ustify an on-site air
separation plant, but generally their requirements are large
enough to make purchasing of gas in cylinders uneconomical.
Typically, moderate users of gases will find it convenient
to maintain on-site cryogenic liquid storage tanks and vapor-
ize the liquid as requirements dictate. The gas is then
piped to the use locations. The usage requirements may be
intermittent or continuous.
An atmospheric vaporizer is a device which vaporizes
cryogenic liquids by employing heat absorbed from the ambient
air. Atmospheric vaporizers have been employed by users of
intenmediate ~uantities of gases as means of vapori~ing the
stored cryogenic liquid when the user's gas requirements are
intenmittent, but generally not when the user's gas require-
ments are continuous. The reason why atmospheric vaporizers
are not generally uæed for continuous service iB because ice
and froæt build up on the outside surfaces of the atmospheric
vaporizer, rendering the unit inefficient after a sustained
perlod of use.
Typically, an atmospheric vaporizer is comprised
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of one or more pssses vertically positioned and piped
together. The passes are comprised of a center tube through
which the liquid passes, and the tube generally has one or more
fins attached to it to increase the heat transfer area. The
passes are spaced about 9.25 inches, centerl~ne to cente~line,
from each other. The cryogenic liquid enters at the bott~m
of one pass, passes up through it and then through a oonnec-
tion to the top of another pass through which it descends.
This flow pattern is repea~ed through other passes as condi-
tions such as usage and ambient temperature dictate. As thecryogenic liquid passes through the atmospheric vaporizer,
the liquid is vaporized and the gas then further heated by
heat transferred from the ambient air to the fluid through
the vaporizer. The g8S exits the atmospheric vaporizer at
the required flow rate and exit temperature.
As the fluid passes through the vaporizer and as heat
is exchanged from the ambient air, the moisture in the air
condenses and freezes on the surfaces of the vaporizer. This
frost and ice continues to build up during operation of
the vaporizer resulting in decreasing efficiency until a
steady state condition is attained. As the efficiency of the
atmospheric vaporizer decreases,either the exit flow rate
or the exit temperature or both must be decreased. Depending
on the relative importance of these parameters, one or both
of them are decreased until the 8teady state condition is
achieved. Typically, this steady state condition is achieved
at about 20 percent of the capacity of the vaporizer without
the frost buildup.
When the user' 8 requirements are intermittent, the
fro8t builtup i8 generally not a problem because whatever
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frost bulld up does occur during ~per~t~on melts off or can
be essily removed while the unit is not in operation. Under
these conditions, the vsporizer is oper~ting efficiently,
and the above-mentioned low efficiency steady state is
generally not encountered.
However, when the atmospherlc vaporizer is operating
in the continuous mode, the frost and ice do not get a chance
to melt, and the vaporizer is soon operating inefficiently.
For this reason, atmospheric vaporizers are generally not
preferred for continuous vaporization of stored cryogenic
liquids. Instead, a vaporizer is employed which utilizes a
source of heat other than or in addition to ambient heat.
This source of heat or energy is normally obtained from steam
or electricity. Due to the escalating cost of energy, it is
desirable to reduce or eliminate the need for a supplementary
heat source of the vaporizer. It would be desirable to
vaporize stored cryogenic liquid continuously without en-
countering the heretofore unavoidable drsstic decrease in
operating efficiency characteristic of atmospheric vaporizers
of the prior art.
OBJECTS
Accordingly, it is an ob~ect of this invention to
provide an improved atmospheric vaporizer for cryogenic
liquids.
; It is another ob~ect of this invention to provide
an inproved atmospheric vaporizer that is suitable for con-
tinuous operation.
It 18 another ob~ect of this invention to provide
an improved atmospheric vaporizer that 18 sultable for con-
tinuous operation while substantially avoiding the drasticreductlon ln operating efficiency chsracterlstic of prior art
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atmospheric vsporizers.
SUMMARY OF THE INVENTION
The above and other ob~ects which will be apparent
to those skilled in the art are achieved by the present in-
vention which comprises an apparatus for cont~nuously
vaporizing a cryogenic liquid by employing heat absorbed
from the ambient air, said apparatus comprising at least
three substantially vertically positioned passes which are
piped together, each pass being eomprised of a center tube
having an outside diameter of from 0.5 to 1.5 inches
and provided with 3 to 8 fins substantially equally spaced
around said tube, each fin having a radial length of from
1.5 to 7 times the outside diameter of said tube and extending
longitudinally along substantially the entire length of said
tube, each pass having a length of from 5 to 20 feet and a
ground clearance of from 1 to 4 feet, and wherein the ratio
of the distance between ad~acent fin tips to radial fin
length is from 1 to 5.
THE DRAWINGS
; 20 Figure 1 illustrates a typical atmospheric vaporizer
pass.
Figure 2, Figure 3, and Figure 4 are each a plan
view of a vaporizer pass. Figure 2 illustrates a pass
having four fins, Figure 3 illustrates a pass having eight
fins, and Figure 4 illustrates a pass having three fins.
Figure 5, Figure 6, and Figure 7 are each a plan
view of a vaporizer pass array. Figure 5 lllustrates a
~qusre array of four-finned passes, ~igure 6 lllustrates a
triangular array of four-finned passes, and Figure 7 lllus-
trates a square array of three-finned passes.
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Figure 8 illustrates one embodiment of the atmDs-
pheric vaporizer of this inventlon.
Figure 9 is a graphical representation of steady
state flow rate versus sp~cing ratio.
DETAILED DESCRIPTION OF THE INVENTION
The atmospheric vaporizer of this invention is
comprised of three or more finned passes piped together
and arranged in an array having defined spacing in between
individual passes.
Figure 1 illustrates a typical pass 10 comprising
a center tube 11 snd fins 12. The center tube has an out-
side diameter of from 0.5 to 1.5 inches (1.27 to 3.81 cm),
preferably about 1 inch (2.54 cm),and is of sufficient
thickness to contain the fluid at the requisite supply
pressure. The cryogenic fluid passes through the center
tube as it passes through the vaporizer. The fins 12 extend
from the tube in radial fashion and extend longitudinally
~- along the tube for substantially the entire length of the
tube. The fins normally have a radial length of from 1.5 to
7 times the diameter of the tube, preferably about 3.5 times
the diameter of the tube. The fin thickness is not critical,
and is generally sufficient to obtain adequate mechanical
strength to permit support of the unit in an upright posi-
tion with suitable br~ckets and leg supports. Generally,
the fin thickness is from 1/16 to 1/8 inch (0.16 to 0.32 cm);
a convenient thickness is 1/10 inch (0.25 cm). The number
of fins can range from 3 to 8 fins per pass. Figure 1
illustrates a four-fin arrangement. The fin8 are spaced
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substantially equidistant around the center tube. Thus,
for the four-finned tube 20 shown in Figure 2 the fins 22
are arranged around the center tube 21 at about 90~ from
each other. For the eight-finned tube 30 shown in Figure
3, the fins 32 are arranged around the center tube 31 at
about 45 from each other. And for the three^finned tube
40 shown in Figure 4, the fins 42 are arranged around the
center tube 41 at about 120 from each other.
Each pass has a length of from 5 to 20 feet (1 52
to 6.08 m), preferably from 8 to 16 feet (2.44 to 4.88 m),
most preferably about 12 feet (8.66 m). The fins extend
along the length of the center tube substantially from end
to end; however, there is a small length at both the top and
bottom of the tube where the fins do not extend so as to
permit connecting of the passes together. The passes are
connected together in any suitable manner; a convenient
method of connecting is by use of U-bend joints.
The passes may be constructed from any material
having good heat transfer characteristics. Aluminum is the
preferred material. When high pressure is required, the
center tube may be fabricated from stainless steel or monel
and fitted with the aluminum fins.
The passes are usually set on the ground, positioned
vertically and held in place by suitable leg supports.
Normally, the passes have a ground clearance of from about
1 to 4 feet (0.3 to 1.2 m), preferably from about 1.5 to 3
feet (1.46 to 0.92 m). By ground clearance it is meant the
verticfll distance from the ground, or other platform such
as a concrete pad, to the bottom edge of the radial fins.
The higher ground clearances would be better suited for
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clim~tes having significant accumulations of snow so th~t
the vaporizer can clear the level of ~now.
The passes may be piped in series or in a combina-
tion of series and parallel confi~urations. The top of one
pass is piped to the top of an adjacent pass, and the bottom
of a pass is piped to the bottom of an ad~acent pass. Thus,
cryogenic fluid enters one pass at the bottom, travels up the
pass and over through the connection to an adjacent pass,
down that pass and over through the connection to another pass,
and so on until it exits as a gas at the temperature appro-
priate for the end use. The number of passes the fluid flows
through, and the path of the fluid, i.e. series or combina-
tion series and parallel, will depend on various factors,
such as end use temperature and flow rate requirements~
ambient temperature,heat transfer characteristics, pressure
drop fsctors and other considerations which are known to
those skilled in the art.
The passes of the atmo~pheric vaporizer of this
invention are spaced apart from one another such that the
ratio of the distance between fin tips to the fin length,
herein also called the spacing ratio, is from 1 to 5. For
example, one embodiment cf the atmospheric vaporizer of this
invention employs 1 inch (2.54 cm) diameter passes spaced
13.25 inches (33.66 cm) apart centerline to centerline or
I2.25 inches (31.12 cm) apart edge to edge with fins having
8 radial length of 3.5 inches (8.9 cm). In this case, the
distance between fin tips or the gap between fins is the
distance between passes minus the fin radlal length on each
p~ss, or 12.25 - 2(3.5) - 5.25 inches (13.34 cm) and the
6pacing ratio i8 5.25/3.5 ~ 1.5. This spacing ratio is
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calculsted with reference to the two closest fins on
ad~æcent passes.
As previously mentioned, the spacing ratio may
be from 1 to 5; preferably the spacing ratio is from 2 to
4 and most preferably it is about 3. There is a sharp
increase ln the ~perating efficiency of the atmospheric
vaporizer at the lower end of the defined spacing ratio
range. The efficiency gradually increases and then evens
out as the upper limit of the range is approached and
attained. This phenomenon is shown more clearly in ~igure
9 and in the Examples.
Heretofore, atmospheric vaporizers typically
employed a spacing ratio o~ less than 1 and generally the
spacing ratio was about 0.4 or less. This is because it
was felt that atmospheric vaporizers would operate at better
efficiency, i.e., better heat transfer could be attained,
when the passes were closer together. This invention com-
prises the discovery that moving the individual passes
further apart than heretofore thought prudent will result,
surprisingly, in an increase in efficiency when the spacing
ratio is increased to 1, and that the efficiency continues
to increase until the spacing ratio is increased to 5.
The passes of the atmospheric vaporizer of this
invention are arranged in a square, rectangular or triangular
array. Figure 5 illustrates an array 50 of four-finned
passes. Here one square array is demarcated by individual
passes 51, 52, 53 and 54 at the corners of the square.
Figure 6 illustrstes an array 60 of four-finned passes.
Here one triangular array is demarcated by individual passes
61, 62 ~nd 63. Another triangular array is demarcated by
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ind~vidual passes 62, 63 and 64. Figure 7 illustrates an
array 70 of three-finned passes. Here one square array is
demarcated by individual passes 71, 72, 73 and 74.
Figure 5 may be used to illustrate some typical
modes of operating for the atmospheric vaporizer of th ~
invention. For example, a cryogenic liquid may enter the
bottom of pass 51, travel up through it and over to pass 52,
down this pass and so on down the line until it is discharged
as a gas from pass 55. This illustrates operation in series.
Similarly, the fluid may enter pass 53 and exit pass 56, or
enter pass 58 and exit pass 57. Alternatively, the inlet
fluid could be combined so that the inlets of pass 51, 53
and 5~ would be combined and connected to the liquid source
and the oùtlet of pass 55, 56 and 57 would be combined so
that the exit gas would all be combined for the end use.
This illustrates operation in combined series and parallel
flow patterns. Those skilled in the art will readily see
additional ways of operating the atmospheric vaporizer of
this invention by altering the fluid path simply by appro-
priate piping and valving.
Figure 8 illustrates a typical stmospheric vaporizer
of this invention. The front supports are shown in broken
view in order to more clearly show the inlet and outlet
connections. As shown in Figure 8, liquid cryogen enters
the vapor~zer through inlet 81, travels up pass 82 and
~cross piping connection 83 to pass 84, down pass 84 and
across piping connect~on 85 to pass 86, up pass 86 and
across piping connection 87 to pass 88, and down pass 88
and out outlet 89 in the form of a gas.
The passes which comprise the stmospheric vaporizer
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of this invention m~y be deployed in any suitable spatial
configuration consi~tent wlth the spacing ratio defined
previously. It i8 expected that the array of three or more
passes will define an enclosed air space between them, i.e.
that the passes will not all be in a stra~ght line. Pre-
ferred arrays are rectangular or square arrays as shown in
Figures 5 and 7 and triangular arrays as shown in ~igure 6.
The atmospheric vaporizer of this invention may
also be provided with one or more control devices. One such
device is to control the flow rate of the fluid. By regulat-
ing the flow of fluid, one can compensate for changes in
ambient air temperature and/or system heat transfer efficiency
in order to keep the exit gas temperature constant. Such an
arrangement is most useful when the particular usage to which
the gas is put requires a specific gas temperature or tempera-
ture range.
This invention, by employing the critical pass
spacing ratio and the other defined features provides an
atmospheric vaporizer which is capable of vaporizing cryo-
genic liquids on a continuous basis at an efficiency con-
siderably higher than is achievable by the use of atmospheric
vaporizers of the prior art.
The following examples serve to further illustrate
preferred embodiments of the stmospheric vaporizer of this
invention, and the greatly improved results attainable by
its use over those obtained with conventional atmospheric
vaporizers. The examples sre intended to illustrate the
invention, and are not ~ntended to limit the scope of the
invention.
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Exsmples 1-4
An atmospheric vsporizer having four passes in a
square array was employed to vaporize liquid oxygen and~was
evaluated to determine its flow capacity at steady state.~
The evaluation was conducted in a climate control h~use ~o
that ambient temperature and humidity were relatively con-
~tant throughout the evaluation. The vaporizer passes were
7.5 feet (2.29 m) in height, each had a ground clearance of
1.5 feet (0.46 m)~ each had a center tube diameter of one
inch (2.54 cm) and each was provided with eight fins. Each
fin had a radial length of 3.5 inches (8.9 cm) and a thick-
ness of 0.1 inch (.025 cm). The pass spacing, centerline to
centerline, was 13.25 inches (33.66 cm) between adjacent
passes and the spacing gap, i.e. the space from fin tip to
fin tip was 5.25 inches (13.3 cm). Thus, the spacing ratio
was 5.25/3.5 - 1.5.
The ambient temperature was kept at about 35F
(1.7~C) throughout the evaluation and the relative humidity
was kept about 100 percent. The gas exit temperature was
m~intained relatively constant throughout the evaluation at
about 5F (-15C) by periodically ad~usting the fLDw through
the vaporizer. As the ice and frost blanket continued to
increase, the flow was adju~ted downward to maintain the gas
exit temperature es~entially con6tant. After about six days
of operation the vaporizer operating conditions reached
steady state ~nd the flow rate was measured. m e results
appesr in T~ble 1 under Example 1.
The procedure described above was repeated except
that the distance between ad~acent passes was lncreased to
18 inches (45.7 cm) giving a spacing gap of 10 inches (25.4 cm)
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and 8 spacing raeio of 2.9. The vaporizer was evaluated
for steady state flow rate and the re~ults a~e shown in
Table 1 under Example 2.
The above-described proc2dure was repeated a third
time. The pass spacing again was 18 inches. However, in
this example the number of fins used on esch pass was four
instead of eight. The vaporizer was evaluated for steady
state flow rate. The results are reported in Table 1 under
Example 3.
For comparative purposes the above-described pro-
cedure was repeated, but without employing the atmospheric
vaporizer of this ~nvention. The vaporizer employed herein
was identical to the eight-fin vaporizer described above
except that the pass spacing was only 9.25 inches (23.5 cm).
The spacing gap was only 1.25 inches (3.2 cm) and the spacing
ratio was only 0.4. This pass spacing i8 representative of
the pass spacing characteristic of heretofore available
atmospheric vaporizers. m is vaporizer was also evaluated
for steady state flow rate and the results are also reported
ln Table 1 under Example 4.
TABLE 1
Example 1 2 3 4
Number of Fins 8 8 4 8
Pass spacing 13.25 18 18 9.25
(Lnches) (cm) (33.66) (45.7) (45.7) (23.5)
Spacing Gap 5.25 10 10 1.25
i (~nches) (cm~ (13.3) (25.4) (25.4) (3.2)
Spacing Ratio 1.5 2.9 2.9 0.4
Flo~ Capacity 385 465 350 245
(m3/hr) (35.8) (43.2) (32.5) (22.8)
A8 8hown in Table 1 there i8 a shrap increa8e in
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steady stste flow rate when the atmospheric vaporizer of
this invention is employed over that obtalned in the com-
parative example wherein the atmospheric vap~rizer of th~s
lnvention was not employed. Thls increase in steady state~
flow rate is indicative of the increase in operating effi-
ciency.
Comparison of Example 1 with Example 4 demonstrates
that the increase in pass spacing ratio from 0.4 to 1.5
results in an increase ln steady state flow capacity of 57
percent. Comparison of Example 2 with Example 4 demonstrates
that the increase in pass spacing ratio from ~.4 to 2.9
results in an increase in steady state flow capacity of 90
,percent, Comparison of Example 3 with Comparative Example 4
demonstrates that even though the fin number per pass was
reduced from 8 to 4 resulting in a decrease in the heat trans-
fer area of about 50 percent, an increase in pass spacing
ratio from 0.4 to Z.9 results in an increase in steady state
flow capacity of 43 percent.
The results of these examples are shown graphically
in Figure 9; curve E represents the results employing the
vaporizer provited with 8 fins per pass and curve F represents
the results employing the vapor~zer provided with 4 fins
per pass.
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