Air Only Data
1
INTRODUCTION
Although few reliable or universal models currently exist for predicting the pressure drop for gas-solid flows in pipelines, models for the single phase flow of a gas are well established. Once again, although discussion will generally be in terms of air, the models presented will work equally well with the appropriate value of the specific gas constant for the particular gas being considered. Empty conveying pipeline pressure drop values, for air only, will provide a useful datum for both the potential capability of a system for conveying material and the condition of the pipeline. Air only pressure drop values for the conveying pipeline also provide a basis for some first approximation design methods for the conveying of materials. Air supply and venting pipelines can be of a considerable length with some systems, whether for positive pressure or vacuum systems, particularly if the air mover or the filtration plant is remote from the conveying system. In these cases it is important that the air only pressure drop values in these pipeline sections are evaluated, rather than just being ignored, for they could represent a large proportion of the available pressure drop if they are not sized correctly. Air flow control is also important, particularly if plant air is used for a conveying system, or if the air supply to a system needs to be proportioned between that delivered to a blow tank and that directed to the pipeline, for example.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
180 2
Chapters PIPELINE PRESSURE DROP
The pressure drop in the empty pipeline is a major consideration in the design of a pneumatic conveying system. If a positive displacement blower is used in combination with a long distance, small bore pipeline, for the suspension flow of a material, for example, it is quite possible that the entire pressure drop would be utilized in 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 air velocity that is required for the conveying of the material. Bends and other pipeline features also need to be taken into account. The value of the empty line pressure drop for any pipeline will provide a useful indicator of the condition of the pipeline. If a pressure gauge is situated in the air supply or extraction line, between the air mover and the material conveying pipeline, this will give an indication of the conveying line pressure drop. With an empty pipeline it will indicate the air only pressure drop. If this value is higher than expected it may be due to the fact that the line has not been purged clear of material. It may also be due to material build-up on the pipe walls or a partial blockage 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 pipeline, various properties of the air and of the pipeline need to be determined. Mathematical models and empirical relationships are now well established for this single phase flow situation, and so conveying line pressure drops can be evaluated with a reasonable degree of accuracy. 2.1.1 Conveying A ir Velocity This is one of the most important parameters in pneumatic conveying, as discussed earlier, 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 be evaluated from the volumetric flow rate, pipeline bore, and the conveying line pressure 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 divided by the volume it occupies: p
m = —
„ lb/ft3
V where m and V
= mass of gas - Ib = volume occupied - ft3
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
181
Air Only Data
The Ideal Gas Law, presented earlier in Equation 5.4, applies equally to a constant mass of a gas, as to a constant mass flow rate of a gas, and so:
P
=
m
144 p
V
RT
where R
ib/fr
(1)
= characteristic gas constant - ft Ibf/lb R
P
RT
kg/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 = p,, = and T0= its density p =
53-3 ft Ibf/lb R and so at free air conditions of 14-7 lbf/in 2 519 R 0-0765 lb/ft3
It will be seen from Equation 1 that air density is a function of both pressure and temperature, with density increasing with increase in pressure and decreasing with increase in temperature. The influence of pressure and temperature on the density of air is given in Figure 6.1 by way of illustration. 0
-10
Figure 6.1
-5
0
5 10 20 Air Pressure - Ibf7in2 gauge
The influence of pressure and temperature on air density.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
40
182
Chapter 6
0-044
__
0-042
> Ui
