I/EC
Equipment & Design
A Cascade Refrigeration System for - 8 5 ° C. Process Cooling In this cascade refrigeration system ethane absorbs process heat, then rejects it to Freon-22. The F-22 in turn rejects heat to cooling water. Flooded evaporators, nonlubricated compressors, and automatic control are unusual features by Carlyle S. Herrick, Research Laboratory,
POOLING
FACILITIES for
tempera-
tures down to about —40° C. require only a single stage of refrigerating equipment. Below this point more complex arrangements are needed. Two stage systems can be devised for temperatures as low as —100° C. Two alternative approaches are available; use a multistage compressor or use two separate single stage systems in cascade (series) arrangement. Process objectives determine which choice is made. This cascade system was designed
General Electric Co.
to meet the process-cooling requirements of a diborane pilot plant described previously [I/EC 52, 105 (February I960)]. In the process purification step diborane was distilled under pressure. This required a condenser temperature of —85° C. and a capacity of 12,000 B.t.u. per hour. Cooling the take-off piping and product receiver required an additional 2000 B.t.u. per hour, a total requirement of 14,000 B.t.u. per hour. To fit in with the pilot plant 8-hour operating cycle the refrigera-
tion system had to pull down from ambient temperature to —85° C. in 1 hour. Satisfactory distillation column performance requires stable temperatures so the condenser was designed to operate on the shell side as a flooded ethane evaporator. This ensures constant condenser temperature in event of varying demand. A flooded evaporator has the advantage of boiling heat transfer rates, is smaller and more adaptable to shell-side operation than a "dry" evaporator. It is inherently stable and positively prevents liquid refrigerant from reaching the compressor suction. Stable operation of a flooded evaporator does however require careful control of any nonvolatile components in the refrigerant such as lubricating oil. Nonlubricated carbon ring compressors were used to avoid lubricant entrainment in the refrigerant with attendant problems of plug-up in low temperature expansion valves, accumulation in evaporator, changes in effective evaporator heat transfer surface, maintaining proper lubricant level in compressor, pitching suction lines to drain lubricant, and scrubbing compressed refrigerant to remove lubricant. The sum of these requirements determined the choice of a cascade system for this application. Flowsheet
This is the heart of the refrigeration system. Freon 22 and ethane condensers a r e at the lower left. Compressors for these streams a r e at the lower right
The flowsheet for this system shows compressed Freon-22 condensed by cooling water then boiling to condense compressed ethane. Liquid ethane boils to remove heat from the distillation column condenser. Both the F-22 and ethane VOL. 52, NO. 8
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EQUIPMENT AND DESIGN circuits use flooded evaporators with liquid level control floats acting as expansion valves. Each evaporator is followed by a vapor separator to return entrained liquid refrigerant to the evaporator. In the F-22 circuit condensed liquid is cooled by heat exchange with cold vapor returning from evaporator to compressor. In the ethane circuit compressed gaseous refrigerant is heat exchanged with cold returning vapor to help limit the start-up load on the ethane condenser. Identical nonlubricated
compressors are used, each provided with an aftercooler on the discharge side. The particular controls provided to assist rapid start-up and safe steadystate operation are not common to refrigeration systems. Each circuit has a pressure controller operating to limit the upper value of compressor suction pressure and indirectly to limit the discharge to rated pressure. Also, each evaporator has a pressure controller acting on a compressor discharge to suction bypass valve to
limit the lower value of compressor suction pressure. This prevents suction pressure from falling below one atmosphere during periods of partial load. It is especially important in the ethane circuit where refrigerantair mixtures may be combustible. The ethane circuit has a pressure controller sensing discharge pressure acting on the bypass control valve through a relay. It limits the discharge pressure by increasing evaporator pressure to control the heat load. (Continued on page 45 A)
Here is the cascade f l o w d i a g r a m f o r handling e t h a n e — F r e o n - 2 2 which . . .
. . . w a s o p e r a t e d o n this s t e a d y s t a t e t h e r m o d y n a m i c cycle
44 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
EQUIPMENT A N D DESIGN Summary of Design Heat Load Calcu ations Phase 1 Heat load in F-22 circuit, B.t.u./hr. Evaporator Heat exchanger Compressor Aftercooler Condenser Heat load in ethane circuit, B.t.u./ hr. Evaporator Heat exchanger Compressor Aftercooler Condenser F-22 circulation rate, lbs./hr. Ethane circulation rate, lbs./hr. Theoretical work by F-22 compressor, hp. Theoretical work by ethane compressor, hp.
Phase 2
Phase 3
Steady State
38,500 3,530 11,300 9,550 40,200
38,300 3,510 11,300 9,500 40,000
31,700 2,900 9,300 7,810 33,100
14,300 1,700 4,220 3,670 14,900
30,000 12,200 11,900 38,500
38,000 10,700 15,500 15,300 38,300
34,500 17,300 15,000 17,900 31,700
14,300 6,840 8,740 8,740 14,300
453 187
450 238
372 238
166 05
0
4.45
4.43
3.66
1.66
4.78
6.07
5.90
3.44
flooded operation. Internal tube area was 3 square feet. Double tube sheets were used to remove the possibility of ethane contamination of the process and vice versa. In both circuits the aftercoolers and heat exchangers were assembled from commercial units consisting of copper concentric tube coils having a 1-inch o.d. inner tube, a 1.5-inch o.d. outer tube, and 5.2 square feet of surface. The F-22 aftercooler used a single unit and the ethane aftercooler used two units in series. The F-22 heat exchanger used 4 units in two parallel banks while the ethane heat exchanger used 6 units also in two parallel banks.
Operation System Design
The 1-hour pull-down requirement without process load dominated system design considerations. A heat load profile covering the startup period was calculated for each piece of equipment using the thermodynamic cycle. Thermodynamic data on F-22 were supplied by Kinetic Chemical Co. and ethane data are from Barkelew, Valentine, and Hurd [Trans. Am. Inst. Chem. Engrs. 43, 25 (1947) ]. The maximum heat load was used for a tentative design of each item. Then the sensible heat in equipment and insulation, which constitutes the pull-down load, was determined. Pull-down time was calculated as the time required to move this load through the evaporator. Because evaporator capacity changed according to the load profile this calculation was done by increments. The entire procedure was repeated twice to arrive at a satisfactory design. Heat load profiles were calculated point by point at four recognizable phases of the operation: 1. Maximum compressor discharge pressure, evaporator at condenser temperature, no heat exchange 2. Maximum discharge pressure, evaporator at condenser temperature, heat exchanger down to temperature 3. Maximum discharge pressure, evaporator down to pressure at which suction throttle valve opens wide, 19 p.s.i.g.
4. Steady state operation with 14,000 B.t.u. per hour process heat load in ethane evaporator at —85° C. These calculations are summarized in the table. Equipment Details
Identical nonlubricated carbonring 5 X 5 double acting compressors were used in the two refrigerant circuits. Both operated at 500 r.p.m. (15 hp.) during the start-up period; however, the F-22 compressor had a two-speed motor and operated at 250 r.p.m. at steady-state conditions. In the F-22 circuit refrigerant condensed on the tube side of a 148 square-foot steel condenser, 9.5 feet high, 13 inches o.d. with a 6 foot long tube bundle. Space was allowed within the shell but below the bottom tube sheet to store 14.5 gallons of refrigerant. F-22 evaporated on the shell side of the 127 square-foot stainless steel ethane condenser. The float-type expansion valve kept the liquid refrigerant level near the top for full flooded operation. The shell was 13 inches o.d. and 7.5 feet high with a 4 foot long tube bundle. Space for refrigerant storage in the lower part of the shell amounted to 14.5 gallons. In the ethane circuit the compressed cooled liquid refrigerant evaporated in the distillation condenser shell side. This vessel of stainless steel was a 4-inch O.D., 2foot high shell and tube exchanger equipped with a float valve for
In operation this system met all requirements. Time to pull down to temperature from a warm start was between 55 and 60 minutes. Start-up operation required only starting the F-22 compressor, opening two block valves used to isolate the compressors during shut down periods, starting the ethane compressor, and adjusting pressure controller set points. At steady state the F-22 compressor speed was reduced to the lower value. No attention was needed as the process heat load varied between zero and full value. System steady-state capacity fully met the process design rating. The flooded evaporators gave stable thermal operation at the expected high transfer rates and the float valves were free of plugging problems. Alumina dryers provided for water removal were normally not needed. System maintenance consisted of adding make-up refrigerants, adjusting compressor packing glands, inspecting compressor carbon rings, and checking evaporators for accumulation of nonvolatile materials.
Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, D.C. Letters will be forwarded and answered promptly. VOL. 52, NO. 8
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