B R. C. WOLF a n d J. C. BOWEN Pressure Products Industries, Inc., Hatboro, Pa.
Compressing of Gases in the Pure State to High Pressures The diaphragm compressor provides pure gas at pressures up to OOO. atm. for research and pilot plant requirements. It is especially valuable in handling toxic, explosive, flammable, or expensive gases
Single-stage compressor has maximum discharge pressure of 7500 pounds per square inch with a suction of 500 pounds per square inch in through the inlet check valve and discharged through the relief valve. Selection of Components of Diaphragm Compressor
Although this method of compressing gases is by no means new (7, Z),the literature does not tell how it works, its advantages and disadvantages, and limitations. The diaphragm can be of plastic or other elastomers, but a thin metal diaphragm was selected because of its strength, temperature stability, corrosion resistance, and impermeability. Although the diaphragm could be moved by mechanical linkage, this was ruled out because of the high stress at the point of linkage. A hydraulic medium has the added advantage that the diaphragm can be forced into intimate contact with the gas side head. Because of the relatively small deflection of a metal diaphragm compared to the diameter, this is essential, to keep the dead volume within reasonable limits. A reciprocating piston moving through packing was chosen to move the hydraulic fluid. This piston can be moved by either a crankshaft or a hydraulic cylinder, The crankshaft is by far the simplest and is capable of high speeds of operation. Advantages of the hydraulic drive are constant displacement per second (the crankshaft method produces a sinusoidal displacement) and large thrust values in a compact unit. Tha choice of driver is dependent on process requirements and economics.
The diaphragm must be thin (0.005 to 0.020 inch), to deflect easily at low stress levels. Particular attention must be given to the distribution of the hydraulic medium against the diaphragm. Efficiencies are lowered by improper distribution. The solution to the problem lies in the geometric distribution of the holes in the support plate. For maximum efficiency the liquid cavity must be free from trapped gas. To ensure this, the displacement of the piston is slightly greater than the cavity. O n the suction stroke the diaphragm meets the support plate before the piston is a t the bottom of its stroke and a small amount of the hydraulic medium is drawn in through a check valve! O n the discharge stroke the diaphragm reaches the end of its stroke first and the excess hydraulic medium is discharged through a relief valve back to the reservoir, carrying out any trapped gas. Efficient compressing of gases is made difficult by the fact that the dead volume must be kept to a minimum and no large ports or access holes are tolerable, as the thin diaphragm will be overstressed. The gas is best swept out of head cavity by a complex network of gas flow paths in conjunction with a multiple number of small discharge ports. Satisfactory check valves having low dead volume and tight shutoff qualities can be made from flat guided poppets having limited movements, either spring or magnetically loaded. Poppets having deformable seals built into them have operated for over 40,000,000 cycles without failure. Pressures
A single-stage compressor for discharge pressures u p to 30,000 pounds per square inch is in operation. This unit is capable of boosting gas from 1000 to 30,000 pounds per square inch An effiwith an efficiency of 25%. ciency of 45% was achieved with a suc-
tion pressure of 3000 pounds per square inch. For more efficient operation, two or more stages can be applied. The efficiency at a 5 to 1 compression ratio is approximately 60%. Beyond compression ratios of 15 to 1, it usually is wise to add another stage to the system. Four-stage units have been used to pressures of 10,000 pounds per square inch. Materials of Construction
As there are no moving parts in the gas side (except the diaphragm and check valves) corrosion-resistant materials can be used for all parts in contact with the gases. The use of 18-8 stainless steel has solved most problems, but Hastelloy, nickel or Inconel can be used in special cases. Safety Aspects
The relief valve in the hydraulic side of the compressor limits the maximum pressure that can be developed in the gas side. This arrangement prevents difficulty in case of a plugged line or accidental closing of a valve. When storage cylinders or batch reactors are filled, the relief valve can be used to limit the maximum pressure attained. When maximum pressure is reached, the hydraulic medium merely recirculates ’ The absence of lubricant permits the compressing of gases such as oxygen. The materials of construction chosen make it possible to compress corrosive gases without danger. Elimination of a stuffing box on the gas side gives a virtually leakproof system in which toxic or flammable gases can be easily handled. Because of the large surface areavolume ratio of a diaphragm compressor, the compressed gases do not heat up as much as with conventional piston compressors-an important factor when temperature-sensitive gases are involved,
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Four-stage compressor has maximum discharge pressure of 10,000 pounds per square inch VOL. 49, NO. 12
DECEMBER 1957
1963
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pressor was run at a constant speed and a bypass control valve used on the discharge side. Control was achieved by a differential pressure cell connected across a length of capillary tubing in conjunction with a recorder-controller positioning a control valve.
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C 0MPRESSION R AT I0 Figure 1 . Comparison of efficiency for range of heads at 300 r.p.m.
Generally, the diaphragm compressor requires more space than a piston compressor of equivalent capacity and discharge pressure, particularly in the large sizes. However, the smallest high pressure boost units not only require less space than piston compressors but are more compact than even a modest oil displacement system. cost
Very little cooling is normally required and a short length of tubing will usually suffice for inner stage cooling for small units. Subzero compression is possible so long as liquefication does not occur. As 18-8 stainless steels can be used for the gas side components, embrittlement by hydrogen can be eliminated when this gas is compressed at elevated temperatures and pressures. Volumetric Efficiency
The volumetric efficiency of a diaphragm gas compressor depends on size, speed, dead volume, compression ratio, and thermodynamic properties of the gas. As the dead volume in the diaphragm compressor can be reduced to only a certain point and does not increase proportionally in the larger units, the efficiency of larger units is higher than of smaller ones (Figure 1). For recycle work where boost requirements do not exceed 2 to 1, efficiencies of 85 to 95% can be expected. A diaphragm compressing system can be used at very high compression ratiosup to 40 to 1 in some cases-but efficiencies suffer accordingly. This advantage, rarely found in conventional compressing systems, is applicable in small operations when it is desired to utilize expensive cylinder gas fully. For example, a single-stage unit 5 inches in diameter can compress helium from 100 to 1500 pounds per square inch at an efficiency of 60%, or from 1000 to 5000 pounds per square inch at an efficiency of 80%. Applications
In atomic research the need for gases free from all contaminants has created great interest in this type of compressor, particularly as units can be produced in which the diaphragms are welded inte-
1964
grally to the heads, to eliminate all gaskets or static seals. Gas leakage from these specially designed units has been reduced to that diffusing through the thin metallic diaphragm (a fraction of 1 cc. per year). Compressed air at 2000 pounds per square inch used for skin diving must be pure. Certain food or pharmaceutical products require gases of extreme purity. Low viscosity gases of low molecular weight (hydrogen and helium), can be easily and efficiently pumped with such compressors, because of absence of stuffing boxes, piston rings, etc. In petroleum processes, these compressors are of particular value in recycle work involving gases at high temperatures containing traces of corrosive acids. Control Aspects
The capacity of the diaphragm compressor can be varied by speed, stroke, or bypass arrangement on the discharge side. The first and third methods only are considered here. Application of speed adjustment control was recently demonstrated with a single-stage unit in compressing a constant volume per minute of 5 poundsper-square-inch gas into storage cylinders, The cylinders were empty at first and the compressor had to fill them to tank pressure (400 pounds per square inch). In the beginning, because the compression ratio was low, the compressor ran at a slow speed. As the tanks became filled, the compressor had to run faster to keep up with the system producing the gas. The control element was sensitive to intake pressure and maintained it at 5 pounds per square inch by increasing the speed of the motor. A variable-speed motor has a maximum speed adjustment of approximately 7 to 1. A recent case, on a hydrogen recycle line, required a 50 to 1 ratio of controlled output of gas. The com-
INDUSTRIAL AND ENGINEERIN~CHEMISTRY
Comparison in cost is difficult, as the diaphragm compressor is usually resorted to in specific applications where conventional systems are unsatisfactory. Stainless steel construction, not applicable to other systems, adds to the cost of the unit. However, the smallest units are comparable in cost with other methods, the medium sized units are approximately 2 and the largest units 2.5 to 4 times more expensive. In general, the diaphragm method competes favorably with oil displacement systems of comparable discharge capacities; however, the oil system can be used to pressures of over 100,000 pounds per square inch, whereas the diaphragm system is limited to approximately 30,000 pounds per square inch at present. Conclusion
The diaphragm compressor fills research and pilot plant requirements for pure gas to 2000 atm. I t finds its chief value when toxic, explosive, flammable, or expensive gases are to be handled. As a general-purpose device, it is more versatile than any other currently available. literature Cited
(1) Corblin, H., U. S. Patent 1,332,806 (March 2, 1920); 1,563,166 (Nov. 24,1925). (2) Hoster, T. G., Ibid., 225,930 (March 30, 1880). (3) Ingersoll-Rand Co., New York, N. Y., Bull. 8274. ( 4 ) Perry, J. H., “Chemical Engineering Handbook,” 3rd ed., p. 1259, New York, 1950. (5) Reactions Motors, Inc., Denville, N. J., Rept. 361-F,vol. 2, Contract NOa(s) 10468 (1953). (6) Ibid., 361-Pl6, Contract NDa(s) 10462 (1952).
RECEIVED for review April 8, 1957 ACCEPTED October 7, 1957 Division of Industrial and Engineering Chemistry, High Pressure Symposium, 131st Meeting, ACS, Miami, Fla., April 1957.
