Powder Transfer Vacuum System
In many manufacturing operations, a very significant compressed air use is pneumatic conveying of many types of materials such as cement, fly ash, starch, sugar, salt, sand, plastic pellets, oats, feeds, etc. Often these are systems that use high-pressure air (100 psig class) reduced to lower pressures (15 psig, 45 psig). This creates an air savings opportunity.
These processes are often complex and engineered by the manufacturer to utilize high-pressure inlet air. To convert them to low pressure may sometimes be a daunting job requiring significant changes in controls, valves, piping, regulators, orifices, etc., so plants are often resistant to approach these opportunities as a serious project.
Our area of expertise does not lie in the design, application, selection or operation of pneumatic conveying equipment. It does lie in the area of identifying the operating cost of compressed air as an energy source and opportunities to reduce plant operating cost by effective management of compressed air power.
Dilute Phase Systems
The chart at left graphically shows the magnitude of opportunity that exists in electrical energy savings with compressed air produced at the appropriate lower operating pressure instead of using high-pressure air volume (scfm) reduced to a significantly lower pressure (psig).
The two most distinct categories of pneumatic conveying are low-pressure or high-pressure systems (there are other names “in between” such as medium phase, lean phase, positive pressure, etc.). The first category is low-pressure systems, also referred to as dilute phase pneumatic conveying systems. These systems utilize entry air pressure under 15 psig and use either positive or negative pressure to push or pull materials through the conveying line at relatively high velocities and volume. They are described as low pressure/high velocity systems and have a high air-to-material ratio.
A typical low-pressure system using a rotary air lock feeder will use a high pick-up velocity of around 2,500 fpm at the beginning of the system, and about 6,000 fpm at the end. The conveying line pressure is under 15 psig at the beginning and near atmospheric pressure at the end.
Low-pressure systems should use a low-pressure positive displacement blower as the primary compressed air source. Often, the abrasive nature of product being transported precludes the use of dilute phase and the associated very high velocities, which will create significant system piping and valve erosion.
Dense Phase Systems
The second category, high pressure systems, are known as dense phase systems. These systems utilize air pressure above 15 psig (up to 50 psig) in the pipe and use positive pressure to push materials through the conveying line at relatively low velocities (from 100 fpm to a maximum of 1000 fpm) much like extruding. They are described as high pressure/low velocity systems and have a low air-to-material ratio.
Dense phase conveying uses smaller amounts of air to move large amounts of bulk solid material in slugs through the conveying line.
Generally the product being transported is often abrasive in nature. The Dense Phase system is selected to eliminate or minimize transfer pipe erosion, by maintaining the design’s moderate velocities and still deliver the appropriate pounds per hour of production. Generally, this type system requires an air compressor of the appropriate size and pressure (35 – 100 psig)