PISTON EXPANDER-COMPRESSOR UNIT HAVING SELF-ACTING GAS DISTRIBUTION SYSTEMS

Abstract

plants [1]. Operation of such expanders at a low initial air pressure (as low as 0.8 MPa) to produce moderate temperatures is inefficient due to impairment of specific energy parameters. This is associated primarily with forced gas distribution systems used in piston (reciprocating) expanders. In a forced valve drive (external and internal), there is a large number of highly passive (sluggish) moving parts, because of which the rotation speed of piston expanders is low (as low as 500 rpm with an external drive and 1000 rpm with an internal drive) [1]. One way of refining designs of piston expanding machines is to replace forced gas distribution systems by self-acting valves [2–4]. In the piston expander cylinder, the gas may be made to move by a concurrent flow or by a nonconcurrent flow scheme: the concurrent flow scheme provides for a normally open inlet valve and an exit port at the end of the piston stroke (at the lower dead spot); the nonconcurrent flow scheme provides for a normally open inlet and a closed outlet valve. Self-acting valves, by virtue of their low inertia, allow one to raise the rotation speed of the expander crankshaft to the level of the rotation speed of modern high-speed piston compressors, which makes it possible to combine a compressor and an expander into an expander-compressor unit (ECU) with a common crannkshaft. Expander-compressor units designed to obtain moderate temperatures and operating at a low pressure can be built on unified compressor bases with a rated piston force of the order of 2.5–16 kN. In recent years, several designs have been developed and investigations have been carried out into the processes that occur in piston expanders having self-acting valves [5]. Let us examine a piston ECU having an expander operating in accordance with the concurrent flow [6] and nonconcurrent flow [7] gas distribution schemes. The experimental investigations of the ECU were carried out on a bench built on the basis of the vertical two-stage two-set ship compressor 20K1 which has one-way operating differential pistons: cylinder diameters 0.1 and 0.035 m respectively, piston stroke 0.1 m, rated rotation speed 500 rpm, air suction (intake) pressure of the compressor atmospheric, and compression ratio 4–8. The compressor 20K1 was converted to an ECU as follows. The valve heads of the cylinders with a diameter 0.035 m of the second stage of the compressor were replaced by valve heads with self-acting valves that ensure gas distribution of the function of the expander that operates at a low initial pressure (as low as 1 MPa). In the lower part of one of the cylinders (diameter 0.035 m), a circular slit was made on the perimeter for the exit of the expanded cooled air. Chemical and Petroleum Engineering, Vol. 37, Nos. 9–10, 2001