David Corson
September 2, 2023
Positive displacement blowers provide metal plants with considerable savings in terms of both fuel and supplies and make the entire production process more streamlined and organized.
PD blowers operate under the time-tested volumetric principle. Their two rotors mesh via synchronization gears and are protected by a casing to form an effective blower unit. Read on to learn more about these powerful machines and their parts.
What is a PD Blower?
Positive displacement blowers (PD blowers) are industrial machines that employ the volumetric principle of fluid displacement to compress air, providing a simple and economical solution for pneumatic conveying, wastewater aeration (which you can learn about here) in wastewater treatment facilities, fluidization processes or filter cleaning that require low gas compression levels. PD blowers have been in use since 1867 when Roots blowers with two lobes first made their debut at a Paris exhibition.
They feature an elegant yet simple design consisting of one or more rotors in an oblong body fitted with both inlet and outlet ports on either end, intermeshing through synchronizing gears to produce friction-free rotation inside an enclosed housing, driving gas through its inlet into its outlet port without friction or lubrication - the rotary action being what makes PDs such an efficient and cost-effective choice.
These machines provide several distinct advantages over industrial fans or air compressors in terms of operating at high discharge pressures while using minimal power consumption, thanks to producing continuous flows of gas without surges or cycles that cause overpressure issues. Thus, they make ideal tools for heavy duty conveying applications such as pneumatic or vacuum conveying systems.
What Are Vacuum Pumps?
A basic PD pump typically employs a dual figure-8-shaped impeller that features two or three lobes to trap air between rotor parts and casing pieces, creating tight spaces within them where air pockets form between these components and system pressure. Every time one of the rotors spins, one of these air pockets is expelled through its discharge port against system pressure causing shock loads that place undue strain on downstream systems and exert an immense amount of energy onto them.
Positive displacement blowers offer an effective solution to combat shock load and pulsation caused by airflows: an internal design which gradually pressurizes pockets until discharge conditions are reached with each rotation, helping reduce both its impact on both it and any system it is installed into. A variety of machines such as twin lobe and tri-lobe models exist, while using a helical lobe design may further decrease noise levels and pulsation levels.
Momentum transfer pumps are another form of positive displacement blowers. Their operation relies on fluid dynamics law which states matter moves differently under various pressures and volumes, involving different vacuum pump products and maintenance tasks. As their rotors spin, gas molecules are forced into tight spaces between rotors and casing to form pockets of gas which overcome system pressure before leaving through the pump.
Positive displacement rotary lobe blowers can be utilized for pneumatic conveying, surface cleaning, dairy milking and other applications that require higher pressure or vacuum levels than can be provided by regenerative PD blowers. They're also built to operate faster speeds while being cooled either by air (dry lobe pumps) or oil (oil-cooled or liquid ring pumps).
What are the Other Parts of a PD Blower?
A PD blower comprises two rotors (often known as impellers) connected by gears and enclosed in a casing for protection and sound absorption. Depending on its type, this casing may also house a filter screen and sound absorber. Rotor material typically made of high-quality ceramic or steel to ensure long-term reliability and performance.
As the rotors rotate, they create pockets of air between themselves and the blower casing that are then forced through to the discharge side of the machine to overcome any line pressure and deliver desired amounts of air or vacuum.
Positive displacement blowers differ from centrifugal fans by offering consistent volumetric flow regardless of changes to pressure, making it ideal for pneumatic conveying applications (source: https://ecolonomics.org/what-are-positive-displacement-pd-blowers/). These machines can efficiently handle large volumes of air due to their rotary structure. Unfortunately, this means they require more maintenance and have lower capacities compared with centrifugal compressors or similar air moving machinery.
A PD blower's rotors are specifically engineered to reduce air leakage as they rotate. This eliminates the need for internal air seals and maintains peak operating efficiency of the machine - not to mention that PD blowers are smaller than other industrial air compressors.
A PD blower is an efficient machine designed to withstand high-pressure environments. As such, they're an adaptable choice that can be utilized across a variety of industries; however, it is crucial that businesses understand how PD blowers operate before investing in one for their operations.
Liquid filled pressure gauges equipped with pulsation snubbers are recommended when monitoring PD blowers to help reduce fatigue failure of their Bourdon tube pressure gauge and to protect its metering system from vibration exposure, which could cause premature wear-and-tear. Installation should take place in a well-ventilated space for maximum airflow and ventilation.
