Pneumatic Conveying Terms

Actual gas velocity is the volume flow rate at pressure and temperature conditions per unit cross-sectional area of the empty pipe, normally expressed in distance/time. Actual gas velocity varies throughout the entire length of the pipeline.

The average (also called mean) gas velocity of a system is usually defined as the mean of the beginning (of pick-up) gas velocity and the terminal gas velocity.

The choking velocity of a material is the actual gas velocity in a vertical pipeline at which particles in a homogeneous mixture with the conveying gas settle out of the gas stream.

The conveying pressure for any system is that pressure required to overcome resistances in the system caused by interactions between the conveying as the material being conveyed, the pipeline, and other system components. It is also referred to as “pressure drop”. The conveying pressure is the difference measured between the beginning and the end of the pneumatic system and is applicable to both positive pressure and vacuum (negative pressure) systems.

A dense phase system is any pneumatic conveying system for which the conveying gas velocity is generally below the saltation velocity of the material being conveyed.

A dilute phase system is any pneumatic conveying system for which the conveying gas velocity is generally equal to or above the saltation velocity of the material being conveyed.

The flotation velocity at which material will be suspended in air. Knowing flotation velocity is critical to determining “enclosure velocity”, which is the upward velocity of gas in a filter receive or bin vent. This term is typically used in the design of baghouses and dust collection systems.

The mass of material conveyed over a specified period of time, usually expressed in tons/hour or lbs./minute. Material mass flow rate is also called conveying rate or system capacity.

A parameter used by pneumatic system designers. It is the ratio of the mass of material conveyed to mass of conveying as used. It is also referred to as “phase density,” “solids loading ratio,” and “mass flow ratio.”

The material velocity is the velocity of the material itself, which is somewhat lower than the gas velocity. Material velocity is usually specified as either average (or mean) velocity or terminal velocity. There are no reliable means, at the present time, for measuring the actual material velocity, and only an estimate can be made.

The minimum conveying velocity is the lowest gas velocity that can be used to insure stable conveying conditions. Since the minimum conveying velocity occurs at the material feed point in the system, it is also known as the "pick-up" velocity. These terms are generally applied to dilute phase systems.

The saltation velocity of a material is the actual gas velocity in a horizontal pipeline at which particles in a homogeneous mixture with the conveying gas will begin to fall out of the gas stream.

The terminal gas velocity in a pneumatic conveying system is the velocity of the gas as it exits the system. It is also known as the ending gas velocity and conveying line exit velocity.

All bulk solid materials pneumatic conveying systems operate on a two-phase flow principle. That is, a solid phase (the materials being conveyed) and the gaseous phase (the conveying gas).

The user should be aware that there are several different terms used when considering volumetric gas flow. The volumetric gas rate during conveying is expressed as "free air delivered" or FAD. Most air movers, such as blowers and compressors, are specified in terms of FAD. measured in standard cubic feet per minute (SCFM). FAD is the volumetric gas flow at the suction port of a positive pressure blower or compressor or at the discharge port of a vacuum blower or compressor or at the discharge port of a vacuum blower or vacuum pump. SCFM is the volumetric gas flow rate at standard atmospheric conditions (i.e.. barometric pressure at sea level, 68° F. and 36% relative humidity).

Actual cubic feet per minute (ACFM) or inlet cubic feet per minute (ICFM) is the volumetric gas flow at the actual conditions that will be experienced where compressor or blower is located. The ADFM or IDFM must be calculated from the SCFM. taking into account elevation of the location and maximum summertime ambient conditions.

In the case of vacuum systems, the pressure drop of the system must also be taken into account when calculating the gas flow at the inlet of the blower.