Air Only Data

1 INTRODUCTION

Although few reliable or universal models currently exist for predicting the pres-sure drop for gas-solid flows in pipelines, models for the single phase flow of agas are well established. Once again, although discussion will generally be interms of air, the models presented will work equally well with the appropriatevalue of the specific gas constant for the particular gas being considered. Emptyconveying pipeline pressure drop values, for air only, will provide a useful datumfor both the potential capability of a system for conveying material and the condi-tion of the pipeline. Air only pressure drop values for the conveying pipeline alsoprovide a basis for some first approximation design methods for the conveying ofmaterials.

Air supply and venting pipelines can be of a considerable length with somesystems, whether for positive pressure or vacuum systems, particularly if the airmover or the filtration plant is remote from the conveying system. In these cases itis important that the air only pressure drop values in these pipeline sections areevaluated, rather than just being ignored, for they could represent a large propor-tion of the available pressure drop if they are not sized correctly. Air flow controlis also important, particularly if plant air is used for a conveying system, or if theair supply to a system needs to be proportioned between that delivered to a blowtank and that directed to the pipeline, for example.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

180 Chapters

2 PIPELINE PRESSURE DROP

The pressure drop in the empty pipeline is a major consideration in the design of apneumatic conveying system. If a positive displacement blower is used in combi-nation with a long distance, small bore pipeline, for the suspension flow of a mate-rial, for example, it is quite possible that the entire pressure drop would be utilizedin blowing the air through the pipeline and that no material would be conveyed.The pressure drop for air only in a pipeline is significantly influenced by the airvelocity that is required for the conveying of the material. Bends and other pipe-line features also need to be taken into account.

The value of the empty line pressure drop for any pipeline will provide auseful indicator of the condition of the pipeline. If a pressure gauge is situated inthe air supply or extraction line, between the air mover and the material conveyingpipeline, this will give an indication of the conveying line pressure drop. With anempty pipeline it will indicate the air only pressure drop. If this value is higherthan expected it may be due to the fact that the line has not been purged clear ofmaterial. It may also be due to material build-up on the pipe walls or a partialblockage somewhere in the pipeline.

2.1 Flow Parameters and Properties

In order to be able to evaluate the pressure drop for the air flow in the empty pipe-line, various properties of the air and of the pipeline need to be determined.Mathematical models and empirical relationships are now well established for thissingle phase flow situation, and so conveying line pressure drops can be evaluatedwith a reasonable degree of accuracy.

2.1.1 Conveying A ir Velocity

This is one of the most important parameters in pneumatic conveying, as discussedearlier, with the air velocity at the material feed point being particularly important.If the conveying air velocity is not specified, therefore, it will usually have to beevaluated from the volumetric flow rate, pipeline bore, and the conveying linepressure and temperature, as outlined in the previous chapter.

2.7.2 Air Density

The density, p, of air, or any other gas, is given simply by the mass of the gas di-vided by the volume it occupies:

m p = lb/ft3

V

where m = mass of gas - Iband V = volume occupied - ft3

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Air Only Data 181

The Ideal Gas Law, presented earlier in Equation 5.4, applies equally to aconstant mass of a gas, as to a constant mass flow rate of a gas, and so:

mP =

144 pib/fr

V RT

where R = characteristic gas constant - ft Ibf/lb R

P

(1)

RTkg/rri (1SI)

Gas constants for a number of gases were presented earlier in Table 5.1.

A particular reference value is that of the density of air at free air conditions:

For air R = 53-3 ft Ibf/lb R and so at free air conditions ofp,, = 14-7 lbf/in2

and T0= 519 Rits density p = 0-0765 lb/ft3

It will be seen from Equation 1 that air density is a function of both pressureand temperature, with density increasing with increase in pressure and decreasingwith increase in temperature. The influence of pressure and temperature on thedensity of air is given in Figure 6.1 by way of illustration.

0

-10 -5 0 5 10 20Air Pressure - Ibf7in2 gauge

40

Figure 6.1 The influence of pressure and temperature on air density.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

182 Chapter 6

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