Positive Displacement Blowers for Every Process Air System

A positive displacement blower is a rotary machine that traps a fixed volume of air or gas between its rotors and casing on every revolution and carries it to the discharge. Flow depends on speed rather than on downstream pressure, so the blower delivers a near-constant volume as system resistance changes. Rotary lobe blowers, often called Roots-type blowers, and rotary screw blowers are the two main families. Both deliver oil-free air because the rotors never touch and run in a chamber sealed off from the lubricated bearings.

A centrifugal blower accelerates air with an impeller and turns velocity into pressure, so its flow falls as pressure rises and it can surge if pushed too far up its curve. A positive displacement blower moves the same volume per revolution whatever the pressure, within its rating. Positive displacement suits duties where resistance varies, such as aeration, pneumatic conveying and vacuum service. Multistage centrifugal and high-speed turbo blowers suit large, steady flows where efficiency at one design point matters most.

A rotary lobe blower has no internal compression. Each air pocket reaches the outlet at inlet pressure and is compressed by discharge air flowing back into it, wasting energy and generating heat. A rotary screw blower compresses air inside shrinking rotor cavities, so it uses less power at higher pressure ratios and runs cooler and quieter. Screw blowers cost more and pay back in continuous duty above roughly 8 psig.

Most rotary lobe blowers are rated to about 15 psig in pressure service and about 15 to 16 inches of mercury in vacuum service. Some tri-lobe and screw designs rate higher. The practical limit comes from discharge temperature and differential pressure, not flow, so read the rating against your site inlet temperature and altitude. The manufacturer's performance curve governs a specific frame.

Both work the same way. A tri-lobe rotor produces six smaller discharge pulses per revolution instead of four, lowering pulsation, noise and vibration, and many frames carry a higher continuous pressure rating. Helical rotors and pre-inlet channels cut pulsation further. Bi-lobe blowers cost less to buy and rebuild and remain common on vacuum trucks and older conveying systems. New wastewater aeration and pneumatic conveying packages are mostly tri-lobe.

Start with flow needed at the blower inlet, in ACFM or ICFM at site temperature and altitude, not a catalogue SCFM figure. Then add discharge pressure: static head plus pipe, valve, diffuser or receiver, and silencer losses. Check discharge temperature at that differential, add margin for slip, filter loading and future demand, then choose a frame whose speed range covers the duty with the motor sized at the relief valve setting.

Slip is air leaking back from discharge to inlet through running clearances between rotors and casing. It rises with differential pressure and matters more on small blowers, where clearances are large relative to displacement. Published performance curves already account for slip. Worn or damaged rotors pass more slip, showing up as lost flow and a hotter discharge at the same speed.

Discharge temperature rises with differential pressure, so any added resistance heats the blower: a loaded inlet filter, closed or throttled valve, fouled diffusers, or a plugged conveying line. Also check hot inlet air, high altitude, worn rotors passing more slip, low gear-case oil, or a slipping belt. Manufacturers limit discharge temperature and differential pressure. A discharge temperature switch is the cheapest protection on the package.

Yes. Because the blower delivers the same volume regardless of pressure, a closed discharge or blocked line drives pressure and temperature up until something fails. Every package should include a relief valve sized to pass the blower's full flow at the set pressure, a check valve to stop reverse flow and reverse rotation when the blower stops, and an inlet filter. Test the relief valve on schedule; a valve that has never lifted may not.

Daily checks: oil level, discharge pressure and temperature, and any noise or vibration change. Weekly: filter restriction indicator, belt condition, leaks. On the manual's interval: oil changes, belt tension and sheave alignment, relief valve test. At overhaul, replace bearings, seals and timing gears and measure rotor clearances. Two rules cover most failures: never run without the inlet filter, and never run against a closed discharge.

Yes, and speed control is the usual way to match a blower to aeration or conveying demand. Flow is nearly proportional to speed, so a variable frequency drive turns the blower down without wasting air through a bypass. Stay within the manufacturer's minimum and maximum speed for the frame at design pressure, confirm the motor is inverter-rated, and check that motor and blower cooling remains adequate at the lowest speed.

The rotors never touch each other or the casing, and shaft seals separate the air chamber from oil-lubricated gear and bearing housings, so delivered air carries no lubricant by design. It is not dry or sterile: moisture follows inlet humidity, and inlet filtration handles dust. Worn shaft seals can let oil migrate into the air stream, so seal condition belongs on the maintenance schedule.