Stepwise turndown by closing heat exchanger passageways responsive to measured flow

In a cryogenic plant utilizing a liquid sparged into a cold vapor in heat exchange relationship with hotter vapor, variations in the load in terms of the volume of hot vapor are compensated for by a stepwise complete closing of a uniformly-spaced-apart fraction of the cold vapor passageways.

BACKGROUND OF THE INVENTION 
This invention relates to heat exchangers using sparged liquid refrigerant 
in cryogenic plants. 
It is known to utilize a heat exchanger employing a sparged liquid into a 
cold vapor, the resulting mixture being passed in heat exchange 
relationship with a hotter vapor as shown by Young, U.S. Pat. No. 
3,895,676 issued July 22, 1975, the disclosure of which is hereby 
incorporated by reference. Such heat exchangers are particularly useful in 
liquefying gases as, for instance, in the production of liquefied natural 
gas. In such operations there is a problem of maintaining good heat 
transfer during variations in the loading (throughput). As a result of 
various factors such as partial shutdown for repairs and maintenance, 
changes in the production rate of the natural gas, and the like, the heat 
load on the heat exchangers in terms of the volume of hot vapor may vary 
by as much as a factor of 10. In an ordinary heat exchanger, such 
variation may be tolerated. However, in heat exchangers employing as a 
refrigerant a cold liquid sparged into a cold vapor, a decreased heat load 
reduces the volume of refrigerant which causes a maldistribution of the 
liquid phase into the parallel heat exchange passageways and ultimately 
insufficient vaporization of the cold liquid and flooding and blockage of 
some of the refrigerant passageways and no liquid in others. This causes a 
severe loss of efficiency in the heat exchange operation with recognition 
and positive action being required to correct such a condition. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide for stepwise turndown of a 
cryogenic plant; 
It is a further object of this invention to avoid liquid accumulation in 
passageways of a heat exchanger utilizing a liquid sparged into a vapor; 
It is yet a further object of this invention to maintain maximum efficiency 
of heat transfer in a cryogenic plant during variations in the load. 
In accordance with this invention, uniformly spaced apart fractions of 
passageways carrying liquid sparged into cold vapor are completely closed 
off in response to a significant decrease in the load.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The apparatus of this invention is applicable in any cryogenic heat 
exchange operation and is of particular utility in connection with a 
liquefied natural gas plant. 
Referring now to the drawings, particularly FIG. 1, there is shown an 
elongated heat exchanger 2 having a core comprised of a stack of elongated 
longitudinally extending platelike passages. As can be seen from FIG. 2, 
there are alternately spaced first and second fluid passages 4 and 6, 
respectively, only a single passage 4 being directly visible in FIG. 1, 
passage 6 being shown by cutout. Each passage is formed by interposing fin 
material 44 (see FIG. 4) between two spaced metallic plates 16 (see FIG. 2 
or 3). The formation of such passages is well known to those skilled in 
the art. The composite of these platelike passages may then be brazed 
together as an integral unit. 
Communicating with first fluid passages 4 are first inlet and outlet 
headers 8 and 10, respectively. Communicating with second fluid passages 6 
are second inlet and outlet headers 12 and 14, respectively. Hot vapor 
carried by a line 30 is conveyed by means of gas compressor 18 to first 
inlet header 8 and thence through first fluid passageways 4, collected by 
first outlet header 10 and removed by hot vapor outlet line 36 (i.e., 
means to removed the thus cooled fluid). Cold vapor is conveyed by line 32 
to second inlet header 12 and thence through second fluid passages 6, 
collected by second outlet header 14, and removed via refrigerant outlet 
line 38. Liquid from liquid refrigerant inlet line 34 is sparged into the 
vapor at the entrance to the second fluid passages by means such as that 
shown in detail in FIG. 3. Other conventional sparging means can also be 
used. Once distributed within a passage, the heat exchange fluid moves 
longitudinally through and around longitudinally extending perforated fin 
material 44 (see FIG. 4) to the opposite end of the passage. The fin 
material can be formed from corrugated sheet or otherwise fabricated metal 
such as solid or perforated aluminum as with apertures 46 which are shown 
in detail in FIG. 4. These apertures may comprise from about 10 to 20 
percent of the total sheet metal surface. The fluid material is confined 
within the heat passageway by end bars 48. 
Referring now specifically to FIG. 3, there is shown a preferred means for 
sparging the liquid into the cold vapor comprising conduit 22 positioned 
in each of the second fluid passages 6 communicating with second inlet 
header 12 and lines 32 and 34 as shown in FIG. 1. Each of the conduits 22 
has a plurality of fluid exit openings 24 formed along its length and 
opening into the respective fluid passageways 6 for passing and 
distributing a liquid from the conduit into the second fluid passages 6. 
Referring now to FIG. 5, there is shown a heat exchanger 2 in more 
diagrammatic form employing the turndown control of this invention. As can 
be seen, hot vapor enters via line 30 and is distributed by means of 
header 8 to alternate passageways 4. Header 10 collects the resultant 
fluid and passes same from the heat exchanger via line 36. Cold vapor 
enters via line 32 and liquid via line 34 and the two phase stream is 
distributed by header 12 to the other alternate passageways 6, the warmed 
refrigerant exiting by header 14 and line 38. Flow controller 42 operates 
valve 50 one half of header 12 in response to a measured rate of flow of 
the hot vapor. This operation is carried out by linear flow transmitter 40 
which transmits a signal representative of the flow rate in conduit 30. 
When the rate of flow is reduced to a preset level or lower, flow 
controller 42 closes valve 50. This shuts off line 12a to one-half of the 
second inlet header system 12, thus shutting off the cold vapor and liquid 
being sparged into alternate second passageways 6. As can be seen, every 
other passageway 6 is shut off, this being every fourth passageway since 
half of the passageways are hot vapor passageways. It is essential that 
the fraction of the refrigerant passageways shut off be substantially 
uniformly spaced apart. In this way the entire heat exchanger core is 
always used to its maximum efficiency. This avoids an entire section being 
unused since even the passageways shut off still conduct heat as a result 
of their proximity to the active passageways. Thus, the sequence is a hot 
vapor passageway 4, a closed off cold vapor passageway 6, a hot vapor 
passageway 4, and an open cold vapor passageway 6, etc. Similarly, a 
turndown ratio in increments of successive one-thirds can be effected by 
shutting off every third refrigerant passageway and a turndown ratio in 
increments of successive one-fourths can be achieved by completely 
shutting off every fourth refrigerant passageway. 
Referring now to FIG. 6, there is shown a schematic representation of a 
heat exchanger identical to that of FIG. 5. FIG. 5 is duplicated in order 
to introduce another shorthand form for depicting the hot vapor 
passageways 4 and the alternating, parallel, countercurrent cold vapor 
passageways 6. As is apparent, this does not represent the actual spaced 
relationship of the passageways but is a conventional shorthand form 
depicting passageways in a complex multipass heat exchanger. For instance, 
passageway 4 depicted in FIG. 6 is actually a plurality of passageways 
having passageways 6 sandwiched therebetween. 
FIG. 7 is a schematic representation of a heat exchanger similar to FIG. 6 
except that the flow transmitter measures the flow rate of cold liquid and 
in response to a decreased flow, the controller shuts off a refrigerant 
exit line 38a rather than an inlet line, thereby both the measurement and 
control loci are different from FIGS. 5 and 6. In order to have a more 
accurate measurement, it is essential to measure a reliably single phase 
flow rate. Accordingly, the preferred measured stream is that as shown in 
FIGS. 5 and 6, where the flow rate of hot vapor is measured. However, in 
certain plants such as liquefied natural gas plants where the liquid is 
produced by compressing and cooling hot vapor, the flow rate of liquid is 
proportional to or at least correlatable with the incoming flow rate of 
hot vapor. 
FIG. 8 is similar to FIG. 7 except that there are provided two flow 
controllers 42a and 42b operating valves 50a and 50b, respectively, so as 
to shut off completely uniformly-spaced-apart refrigerant feed lines in 
increments of 1/3 of the total number in the heat exchanger. Also, in this 
embodiment, the refrigerant liquid flow rate coming into the heat 
exchanger is utilized to regulate when the stepdown is to occur. Thus, 
when the flow rate of liquid is reduced to a preset level of less than 2/3 
but more than 166 of the normally expected flow rate, flow controller 
42a, in response to the comparison of its signal from linear flow 
transmitter 40 with the 2/3 flow rate set point thereto, shuts off valve 
50a, thus completely shutting off the uniformly-spaced-apart 1/3 fraction 
of the refrigerant lines. When the liquid flow rate is further reduced to 
a second preset level (less than 1/3 normal flow), flow controller 42b 
shuts off valve 50b, thus shutting off a second bank of refrigerant lines 
leaving only a single uniformly-spaced-apart 166 fraction of the 
refrigerant lines in operation. 
FIG. 9 is a schematic representation of three parallel trains of heat 
exchangers as would be employed in a cryogenic plant. In this embodiment, 
the liquid flow into the system is measured for control purposes, however, 
hot vapor flow rate measurement could be employed by utilizing only 1/2 
turndown ratio in each of the three parallel trains and by shutting down 
one train completely after first having shutdown one 
uniformly-spaced-apart 1/2 of the refrigerant lines therein, it is 
possible to achieve a turndown ratio of 1/6, 1/3, 1/2, 2/3, or 5/6. By 
utilizing the 1/3 increment turndown system of FIG. 8 in each of the 
parallel trains of FIG. 9, it would be possible to operate in increments 
ranging from 1/9 to 100 percent of the total capacity. Minor variations in 
the load on the heat exchange plant can be tolerated and thus with 
turndown ratios such as those depicted herein, two-phase-refrigerant heat 
exchangers can be operated at high efficiency as the actual material being 
processed varies from 100 percent to as little as about 10 percent of the 
designed capacity of the plant, a particularly useful feature when 
starting up and when plant throughput is being changed. Specifically, 
again referring to FIG. 9, there is shown three parallel trains of 
physically-separated heat exchangers 2, 2a and 2b. Linear flow transmitter 
40, in response to the volume of total refrigerant liquid flow through 
line 34, operates flow controller 42c through g. Controller 42 can be five 
separate controllers with the five setpoints representing successively 
5/6, 4/6, 3/6, 2/6 and 1/6 of scale or in a preferred direct digital 
control system, the controller can be represented by one equation in a 
computer with five different setpoint values, one being applied to each 
control loop with the controller being switched periodically as desired, 
outputting control signals to all values in sequence. As the liquid flow 
rate drops to the first preset level, valve 50c is actuated, thus giving a 
turndown ratio of 1/2 in exchanger 2 or a turndown ratio of 1/6 for the 
entire plant. When the measured liquid flow rate reaches a second preset 
level, valve 50d is actuated to shut down one-half of the 
uniformly-spaced-apart refrigerant lines in exchanger 2a. When the liquid 
flow rate is reduced to the third preset value, the valve 50e is actuated 
to shut off the uniformly-spaced-apart half of the refrigerant lines in 
exchanger 2b to give a total plant turndown ratio of 1/2. On reaching the 
fourth preset value of liquid flow rate, valve 50f is closed, thus 
completely shutting off exchanger 2b to give a total turndown ratio of 
2/3. At this point of successive turndown, with all refrigerant shut off 
to exchanger 2b, valve 51b on the hot vapor stream to 2b is also shut off 
to prevent by-passing of hot vapor around coolers 2 and 2a (still being 
refrigerated at half-capacity each). On reaching the final setpoint, valve 
50g is actuated shutting off completely exchanger 2a to give the total 
turndown ratio of 5/6. Valve 51a is also actuated to shut off hot vapor to 
2a for the aforementioned reason. Alternatively as desired, both halves of 
a single exchanger could be turndown prior to shutting down 1/2 of the 
second parallel exchanger with appropriate shut off of hot vapor flow. 
Since hot vapor distribution among the parallel exchangers would become 
disproportionate with the refrigerant supply, for example at the turndown 
ratio of 1/6, valve 50c being closed, only 1/6 of the previous total 
refrigerant supply (1/5 of the present supply) would be allotted to 
exchanger 2 while 1/3 of the hot vapor would pass therethrough. In the 
event of partial condensation rather than mere coolings of the hot vapor, 
it might become desirable to additionally manipulate the distribution of 
the hot vapor supply for the purpose of obtaining approximately the same 
degree of cooling and possible partial condensation of the hot vapor 
whereby mixing steps (to achieve uniform temperatures) applied to the 
cooled hot vapor stream and the vaporized-heated refrigerant stream may be 
avoided. Thus, potential uniformity problems which might plague further 
processing steps (phase separations, further heat exchange, etc.) may be 
avoided. Instrumentation of the character previously described may thereby 
be applied to the hot vapor stream, as well upon recognition of a problem. 
Referring now to FIG. 10, there is shown a schematic representation of one 
form of control apparatus. Herein orifice or similar head metering device 
52 in hot vapor line 30 allows communication between the line carrying 
vapor, the flow of which is to be measured, and differential pressure 
transmitter 54. This transducer transmits a signal representative of the 
square of the flow rate which is fed to square root computing-scaling 
relay 56. Differential transmitter 54 and square root computing relay 56 
together make up a linear flow transmitter. Other conventional automatic 
control devices, preferably combining these functions in one unit, may be 
employed. This signal is then fed to a series of flow controllers 42j, k 
and l to close off the refrigerant exit lines 38a. Controller 42 is set to 
close valve 50j when the measurement signal (its pneumatic analog) falls 
below 3/4 of normal flow rate, which is represented by the setpoints given 
below. Similarly, flow controllers 42k and 42l receive setpoints 
representing, respectively, one-half and one-fourth of normal flow rate. 
This then depicts the usual method of flow metering by orifice where the 
differential pressure is representative of the flow rate squared. The 
standarized pneumatic instrumentation scale range is 3 to 15 psi (20.7 to 
30.4 kilopascals) air pressure full scale. In this example, the orifice 
and transmitter are sized and scaled whereby a 13 psi (89.6 kPa) signal 
indicates expected maximum throughput. Thus, a 10.5 psi (72.4 kPa) 
measurement signal indicates a 3/4 rate of operation, necessitating a 1/4 
turndown, an 8 psi (55.2 kPa) signal similarly necessitates a 1/2 turndown 
and a 5.5 psi (37.9 kPa) signal necessitates a 3/4 turndown so that when 
the flow rate drops slightly below the 10.5 psi (72.4 kPa) signal, 
controller 42j (adjusted to a high gain setting) closes valve 50j on one 
of four parallel passes of the two-phase-refrigerant heat exchanger. Valve 
closure can be delayed or lagged as desired by conventional means to avoid 
pressure shock. Either electrical or pneumatic signals can be utilized 
between flow transducers, square root computing relay and flow controllers 
and the control configuration may be totally analog or partially digital 
in character as known to those skilled in this art. 
Turbine flow meters or other types of measuring devices could be used which 
give a signal directly related to the refrigerant flow rate and thus do 
not require a square root calculation. While electrical differential 
pressure transmitter relay and controllers could be used, pneumatic 
equipment is preferred so as to avoid any explosion hazard such as may be 
present in liquefied natural gas processing. 
Typical inlet temperature ranges for liquefied natural gas plant using 
three stages, each utilizing the turndown arrangement of this invention 
are as follows: 
__________________________________________________________________________ 
1st 2nd 3rd 
__________________________________________________________________________ 
Hot vapor inlet 
50.degree. to 80.degree. F. 
-25.degree. to -40.degree. F. 
-65.degree. to -80.degree. F. 
Cold vapor inlet 
-35.degree. to -50.degree. F. 
-75.degree. to -90.degree. F. 
-180.degree. to 190.degree. F. 
Liquid inlet 
-35.degree. to -50.degree. F. 
-75.degree. to -90.degree. F. 
-180.degree. to -190.degree. F. 
__________________________________________________________________________ 
Calculated Illustrative Embodiment 
A liquefied natural gas separations plant is operated utilizing an 
exchanger as shown in FIG. 8, which has a pneumatic system so as to allow 
shutting down uniformly spaced 1/3 or 2/3 fractions of the total number of 
cold vapor passageways. 
The heat exchanger dimensions and stream flow conditions for the exchanger 
are as follows: 
__________________________________________________________________________ 
Hot Vapor Passageway 
17 passages having a thickness of 0.20" (5.1 mm) 
and having 0.20" (5.1 mm) high perforated fins 
therein, 14 fins/inch .012" (0.3 liquefied fin thickness, 
each passage 32" (0.813 m) wide and 5 ft. (1.52 m) 
long, total free cross section 0.58 ft..sup.2 (0.054 
m.sup.2) 
Cold Vapor Passageway 
18 passages having a thickness of 0.25" (6.4 mm) 
and having 0.25" (6.4 mm) high 1/8-inch (3.2 mm) lanced 
fins therein, 15 fins/inch .012" (0.3 mm) fin thick- 
ness, each passage 32" wide (0.813 m) and 3 ft. 
(1.52 m) long, total free cross section 0.78 ft..sup.2 
(0.0725 m.sup.2) 
Stream Properties 
flow rate 30,000 lb/hr (13608 Kg/hr) 
Hot Vapor Stream 
composition 89.5% CH.sub.4 10.5% H.sub.2 
Inlet temperature = 0.degree. F. (-18.degree. C.), Inlet 
pressure = 
505 psia (3481.9 k Pa) 
Outlet temperature = - 133.degree. F. (-91.7.degree. C.), 
(CH.sub.4 totally 
condensed) 
Outlet pressure - 500 psia (3447.4 k Pa) 
Cold Vapor Stream 
flow rate 26850 lb/hr (12179 Kg/hr) 
composition 100% CH.sub. 4 
Inlet temperature = -205.degree. F. (-131.7.degree. C.), 
inlet 
pressure = 96 psia (661.9 k Pa) 
Liquid Stream 
flow rate 21,500 lb/hr (9752 Kg/hr) liquefied 
methane 
Inlet temperature = -205.degree. F. (-131.7.degree. C.), 
inlet 
pressure = 100 psia (609.48 k Pa) 
liquid injected into cold vapor stream through two 
(one from each side) 7/16" (11.1 mm) OD .020 
(5.1 mm) wall 16" (0.406 m) long tubes/passage. 
Total of 36 tubes per heat exchanger flow rate/tube 
= 596 lb/hr (270 Kg/hr). Tube having 0.035" 
(0.9 mm) diameter orifices on 1/4 inch (6.4 mm) 
centers through which liquid passes at a velocity 
of 15 ft/sec. 
__________________________________________________________________________ 
While this invention has been described in detail for the purpose of 
illustration, it is not to be construed as limited thereby but is intended 
to cover all changes and modifications within the spirit and scope thereof 
.