The Screw Compressor: A Built-In Ratio, and the Cost of the Wrong One

US 2,111,568 shows two rotors, 10 and 12, in a casing 14 — *"compressors of the rotary screw type in which two or more cooperating rotors"* provide compression spaces whose volume varies as they turn. Each rotor's lobes form pockets with the casing; a male pocket and a female pocket are *"brought into communication with each other and thereafter the two spaces work together as a common compression space"* until the outlet port opens. The discovery is the next sentence: the reduction in volume of one pocket *"commences prior to"* that of its partner. So the two are at different pressures when they meet. The cure: a **pressure relief passage** that keeps the early pocket from compressing on its own. In the preferred form the end wall is shaped so that pocket P *"remains in communication with the suction space"* until the pockets join, so that *"no compression is effected prior to the time of direct communication"* and the two then compress together. The claims also cover a passage connecting the two pockets directly *"before direct communication between the pockets is established"*. Find on the drawing: the two rotors, the inlet and outlet, and the shaped end wall that keeps pocket P open to suction. That porting is the invention; the screw compressor itself was already known.

Before choosing any compressor, find the pressure the work needs — not the pressure the compressor can make. Fit a pressure gauge at the far end of the air line, right at the tool, through a tee. Run each tool you use and read the gauge while it works. Most air tools are rated for about 6 bar (90 psi) at the tool; many jobs run happily below that. Then read the pressure at the compressor at the same moment: the difference is what the hoses, couplings and filters lose, and it is often more than a bar on a long, narrow line. Now set the regulator at the compressor down, step by step, until the hardest-working tool starts to slow. The pressure just above that, plus the line loss, is your real system pressure. Converted to an absolute ratio (add 1 bar to the gauge reading), it is the number the notebook's table wants — and every bar you did not need is work the compressor was doing for nothing.

**US 2,111,568, 'Rotary Compressor', Alf Lysholm of Stockholm and Gustav Karl William Boestad of Lidingö, Sweden; application filed 10 February 1936 (Serial 63,160), claiming a Swedish filing of 12 February 1935; granted 22 March 1938.** The specification also refers to related US applications, Serial 44,935 of 14 October 1935 and Serial 183,664 of 6 January 1938, filed as a continuation in part. Lysholm worked on rotary machines at the Ljungström steam-turbine company in Stockholm, and his rotor-profile work there is the root of the twin-screw compressor. The early machines ran dry, with timing gears keeping the rotors from touching. Injecting oil into the compression space came later: the oil seals the clearances, carries the heat of compression away and lets one rotor drive the other directly. The oil-flooded screw is now the standard factory and workshop compressor above a few kilowatts. Its relatives here are the Roots blower (two lobed rotors that move gas but do not compress it inside — all the compression happens as the pocket opens to the pipe), the gear pump, and the scroll compressor, which shares the fixed built-in ratio. **Honest limits.** The built-in ratio is fixed, so it is efficient over a narrow band of system pressures. The rotor profiles are difficult to cut and to measure. Oil-flooded machines need an oil separator and still carry some oil into the air. And it is poor at part load unless it has a variable-speed drive — running unloaded still costs power.