Introduction to pneumatic conveying and the guide:Definitions and Solids loading ratio

Definitions

To provide a uniform approach to the work, basic definitions of conveying phases, velocities, operating pressures and conveying conditions are given here for reference. The most important point is that the dilute and dense are the only conveying phases that are recognized in this Guide and to which reference is made. This is primarily a function of material properties. The vast majority of materials are capable of being conveyed in dilute phase, or suspension flow, but only certain materials are capable of being conveyed in dense phase, or non-suspension flow, in a conventional pneumatic conveying system.

Solids loading ratio

Solids loading ratio, cf, is the ratio of the mass flow rate of the material conveyed to the mass flow rate of the air used for conveying, as presented in Equation (1.3). It is used by pneumatic conveying engineers to describe the nature of the gas–solid flow in a pipeline. Other terms used include phase density, mass ratio and mass flow ratio. It is a useful dimensionless quantity since its value does not vary with the conveying air pressure and so its value remains constant throughout the pipeline.

Dilute phase conveying

Dilute phase conveying occurs when a material is conveyed in suspension in the flowing air.

Note: The dilute phase mode of conveying is sometimes referred to as lean phase or suspension flow. To keep the material in suspension in the pipeline it is necessary to maintain a minimum value of conveying line inlet air velocity that, for most materials, is of the order of 13–15 m/s.

Dense phase conveying

Dense phase conveying occurs when materials are conveyed with air velocities lower than those required for dilute phase over all or part of the pipeline.

Note: The nature of dense phase flow is very varied, for it depends upon the prop- erties of the material being conveyed, the solids loading ratio and the conveying air velocity. Typically it includes flow over a deposited layer, which may itself be moving slowly, and flow in discrete or separate plugs of material. In terms of solids loading ratio the appropriate range, for most materials, is normally above about 15, provided that the conveying line inlet air velocity is below that required for dilute phase con- veying of the material.

Low pressure and negative pressure (vacuum) conveying Low pressure conveying systems are those that operate with air pressures below about 1 bar gauge.

Note: These systems cover the normal operating range of positive displacement blowers and conventional low pressure rotary valve systems. Low pressure is not syn- onymous with dilute phase conveying. If a material is capable of being conveyed in dense phase, a low pressure, or vacuum system, could be used to convey the material in dense phase, since for these materials it is only a function of pressure gradient, as illustrated in Figure 1.1.

High pressure conveying

High pressure conveying systems are those that operate with air pressures above about 1 bar gauge.

Note: High pressure is not synonymous with dense phase conveying. It is only pos- sible in conventional conveying systems with materials having appropriate properties, and then only if the pressure gradient is sufficiently high, since conveying distance can have an over-riding effect.

Free air conditions

Free air conditions are specified as those at which p = 101.3 kN/m2 absolute (standard atmospheric pressure) and t =15°C (standard atmospheric temperature).

Note: Free air conditions are generally used as the reference conditions for the specification of blowers and compressors.

Superficial air velocity

This is the velocity of the air disregarding the presence of the solid particles or porous media.

Note: In a pipeline it is the air velocity based upon the cross-sectional area and neglecting the space occupied by the conveyed material. For flow across a membrane or filter it is the open duct velocity normal to the surface. Air velocity, for a given mass flow rate, is dependent upon both pressure and temperature. When conveying air velocities are evaluated at any point in the system, the local values of pressure and temperature at that point must be used.