What is a positive displacement blower?

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.

What is the difference between a positive displacement blower and a centrifugal blower?

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.

What is the difference between a rotary lobe blower and a rotary screw blower?

A rotary lobe blower has no internal compression. Each pocket of air is carried to the outlet at inlet pressure and compressed by discharge air flowing back into it, which costs energy and heat. A rotary screw blower compresses the air inside the rotor cavities as they shrink toward the outlet, 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.

How much pressure or vacuum can a positive displacement blower produce?

Most rotary lobe blowers are rated to about 15 psig in pressure service and about 15 to 16 inches of mercury in vacuum service, with some tri-lobe and screw designs rated higher. The practical limit is set by discharge temperature and differential pressure rather than by flow, so the rating has to be read against the inlet temperature and altitude at your site. The manufacturer's performance curve is the authority for a specific frame.

Should I choose a bi-lobe or a tri-lobe blower?

Both work the same way. A tri-lobe rotor delivers six smaller discharge pulses per revolution instead of four, so pulsation, noise and vibration are lower and many frames carry a higher continuous pressure rating. Helical rotors and pre-inlet channels reduce 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.

How do I size a positive displacement blower?

Start with the flow the process needs at the blower inlet, in ACFM or ICFM at site temperature and altitude, rather than a catalogue SCFM figure. Then add up the 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, and choose a frame whose speed range covers the duty with the motor sized at the relief valve setting.

What is slip in a positive displacement blower?

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

Why is my blower running hot?

Discharge temperature rises with differential pressure, so anything that raises resistance heats the blower: a loaded inlet filter, a closed or throttled valve, fouled diffusers or a plugged conveying line. Other causes are hot inlet air, high altitude, worn rotors passing more slip, low oil in the gear case, or a slipping belt. Manufacturers limit both discharge temperature and differential pressure, and a discharge temperature switch is the cheapest protection on the package.

Does a positive displacement blower need a pressure relief valve?

Yes. Because the blower keeps delivering the same volume regardless of pressure, a closed discharge or blocked line drives pressure and temperature up until something fails. Every package should carry 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 a schedule; a valve that has never lifted may not.

How much maintenance does a positive displacement blower need?

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

Can a positive displacement blower run on a variable frequency drive?

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

Is the air from a positive displacement blower oil-free?

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

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Last updated 5 September 2026

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