US2868590A

US2868590A - Pneumatic conveyors and the like for powdered and granular materials

Info

Publication number: US2868590A
Application number: US649249A
Authority: US
Inventors: Allen Robert William; Taylor Alan Wilson
Original assignee: Henry Simon Ltd
Current assignee: Henry Simon Ltd
Priority date: 1956-04-30
Filing date: 1957-03-28
Publication date: 1959-01-13
Anticipated expiration: 1976-01-13

Description

R. w. ALLEN ETAL 2,868,590 PNEUMATIC CONVEYORS AND THE LIKE FOR Jan. 13, 1959 POWDERED AND GRANULAR MATERIALS '2 Sheets-Sheet 1 Filed March 28, 1957 Jan. 13, 1959 R. w. ALLEN ETAL 2,868,590

PNEUMATIC CONVEYORS AND THE LIKE FOR POWDERED AND GRANULAR MATERIALS 2 Sheets-Sheet 2 Filed March 28, 1957 United States Patent() PNEUMATIC coNvnYonsANn THE LIKE FOR POWDERED AND GRANULAR MATERIALS Robert William Allen, Hazel Grove, and Alan Wilson Taylor, Heaton Chapel, Stockport, England, assignors to Henry Simon Limited, Stockport, England, a British company In pneumatic conveying systems for .the conveyance ofpowdered or granular material wherein a stream of conveying gas (hereinafter for convenience of reference termed .air) passing along a duct entrains the material to be conveyed and carries it" to a terminal point :where it is separated from the air stream, variations in the re sistance to air flow through the duct, caused for example by variations in the quantity of material being entrained by the air, result in variations in therate of air flow in the duct. Thus an increase in the quantity of material being entrained in the air passing through the duct will causea corresponding reduction in the rate of air flow throughjthe duct.. On the contrary, a decrease in the quantity of material entrained in the air will cause a' corresponding increase in the rate of air flow through the duct. It will be appreciated that the air flow rates before referred to are related to the characteristics of the air impelling. means (e. g. a compressor) by which the air flow is produced. It follows therefore that where the quantity of material available for entrainment by the air may vary from time to time, the variable air flow condition above indicated applies and is objectionable because it may cause inefficient operation of any separator for the .material fromthe air, which is located at the outlet from the conveying duct but is particularly objectionable when a single air impeller serves a plurality of conveying ducts to which it is coupled by a suitable manifold an increase in air flow in one duct, due .to a reduction in the quantity of material being entrained by the air in that duct, will result in a reduction in the air flow through one or more. ducts in parallel operation wherethere has not beena corresponding reduction in the quantity of material being entrained by the air, The latter ducts may thus become overloaded and perhaps choked unless a large and uneconomic air flow capacity. is provided in excess of balanced requirements.

In connection with fluidising conveyors, such as are used in containers for powdered or granular materials, which have fluidising means by which air is diffused into the material through porous surfaces which are inclined towards the discharge position in order to render the material free-flowing so that it will flow down the inclined surface towards the discharge position by gravitational forces, the objections before indicated in connection with pneumatic conveying systems apply, because varying resistance to the diffusion of air into the mate.

rial due to the heights of the material above the diffusing means causes a variation in the rate of air flow into the material and where there are several containers supplied with fluidising air from a common source or impeller, an increase in air fiow at a discharge point, where resistance is low, will engender corresponding re" duction in the quantity of air available for fiuidising the material in other containers which will cease to discharge unless a large and uneconomic air flow capacity is provided in excess of balanced requirements.

Various proposals of a mechanical nature have been made to deal with the objectionable conditions before re- 2,868,590 Patented Jan. 13, 1959 2 ferred to but these have involved relatively expensive parts which have tobe accurately set and maintained.

The object of the present invention is to provide simple and effective means for overcoming the objectionable conditions before described in pneumatic conveying and discharge apparatus.-

In accordance with our present invention we provide in the air duct leading to the point where material. is entrained in the air or to the air diffusing elements, an air nozzle coaxial with and forming part of the. duct, which nozzle at its inlet or upstream side converges rapidly to the nozzle throat which has a cross sectional area so related towthe desiredmass flow rate through the nozzle that in operationthe air velocity through the throat is approximately. equalto the velocity of sound in the air at the temperature and pressure existing at the throat, the nozzle diverging from its throat at a small angle preferably less than 12, the cross sectional area at the delivery or downstream end: of the nozzle being equal or substantially equal to that of the air duct.

When the" air velocity through the nozzle throat is equal onsubstantially equal to that of sound as before indicated, decrease in the downstream pressure beyond the nozzle does not ellect the flow rate through the nozzle. The absolute pressure at thethroat under the conditions before stated is a fixed proportion of the absolute pressure of the air source. When the gas passing through thelnozzle is actually air, the critical ratio of the absolute throat-pressure to the absolute source pressure is about 0.55.

With the use of our invention, we find that the pressure downstream of the nozzle can be varied between the lowestlpractical limits and a value only slightly less than the upstream pressure without affecting the mass flow through the nozzle, due to the fact that the energy transformed from pressure to kinetic energy in the converging section of the nozzle, as reflected in the fall in absolute pressure atthe throat to produce the critical ratio,is regained in the form of pressure energy in the downstream divergent section of the nozzle, this pressure regain being limited only bythe need to maintain a smallppressuredillerentiai across the nozzle to balance friction losses. it will be apparent that the mass flow rate in an air duct according to our invention will be substantially; constant irrespective of variations in the quantity of materialtwhich is being conveyed or fluidised. Moreovenjwhere two or more air ducts each provided with a suitably proportional nozzle of this type are operating in parallel, the control exercised by the nozzle ensures that each duct operates individually and substantially no interaction between ducts due to unbalanced loads can occur.

3 exceeds. the downstream pressure by a minimum amount (e;: g.- .2 lbs t per square inch) determined by the frictional losses in the nozzle, therefore the mass flow rate in an air duct in accordance with this invention can be adjusted by altering the absolute pressure of the air upstream of the nozzle and to this end we may provide at a position immediately upstream of the nozzle a variable pressure regulating means whereby air from a higher pressure source can be reduced in pressure to suit the operational requirement of its associated air duct.

It will be appreciated that the object of our invention is achieved by a simple device involving no moving parts and requiring no attention in service, the device functioning in accordance with known scientific principles and performing a very valuable practical commercial service.

Referring to the accompanying explanatory drawings:

Figure 1 is a diagrammatic view of a pneumatic conveying system for powdered or granular materials incorporating our invention.

Figure 2 is a detail sectional view on the line 22 of Figure 3 drawn to an enlarged scale showing one of the nozzles which in theconveying system shown in Figure 1 determine the mass flow rate of the air substantially irrespective of normal variations in the quantity of material which is being conveyed or fluidised.

Figure 3 is a detail view partly in section drawn to an enlarged scale of part of Figure 1.

In Figure 1, there are shown two main compressed air supply pipes a and b (which may lead from a .common pipe), and these pipes lead to branches 0, d, e, f, g and h in each of which is located a nozzle 1 of a construction as shown in detail in Figure 2. From the noz,

zles i in the branches 0, d' and e the compressed air leads to and passes through a rotary valve 1', as shown in detail in Figure 3, which receives the grain or powder to be conveyed from a feed hopper z and as it rotates delivers such grain into the air stream. The rotary valve comprises a number of radial division pieces k, between which are pockets to receive the material from the hopper z. As the valve is rotated or turned, each pocket is brought to a position between the conveying air inlet and delivery branches m: and n respectively which results in the air clearing the contents of the pocket and delivering it to a separator 0, which may be of the cyclone type, in which the grain or powder is separated from the air and delivered through a valve p to a dis charge pipe q whilst the separated air is delivered by the branch r.

The nozzles i in the branches 1, g and h deliver compressed air to spaces as s between a porous base if over which the material flows from the hopper z to the rotary valve j. Such air delivered through the porous base fluidises the material so that it flows freely from the base of the hopper and does not become blocked or clogged up.

There are pressure regulating valves v in the branches 0, d and e in advance of the nozzles i and pressure gauges w and x are associated with each nozzle i in said branches, the gauge w indicating the pressure difierence across its nozzle and the gauge x indicating the pressure immediately upstream or in advance of the nozzle rel-' ative to atmospheric pressure.

Each nozzle i has, as before stated, its throat so proportioned with relation to the desired mass flow rate through the nozzle, that in operation the air velocity through the throat is approximately equal to the velocity of sound in the air at the temperature and pressure existing at the throat. The nozzle diverges from its throat at an angle which is preferably less than 12 and joins up with the air duct into which it delivers its air without disturbance.

With the arrangement shown, and the regulating valves v adjusted having regard to the readings of the gauges w and x to ensure correct conveying at maximum capacity, any reduction of pressure in the air duct into which a nozzle delivers its air, such as would occur for example with a reduction of the rate of flow of material into the air stream from the valves whilst causing a change in the indication given by a gauge w, would not cause any change in the indication given by a gauge x. It will be appreciated that the optimum setting of each valve v is the one which provides the lowest pressure in advance of its associated nozzle 1 which will remain constant for all conditions of conveying up to maximum capacity.

The nozzles i in the branches 3, g and h, ensure that for a given air pressure at the inlet side of the nozzle, there will be a constant mass flow rate through the nozzle notwithstanding considerable variations in the pressure at the delivery side of the nozzle due tovariations in the resistance of the material on the porous base to the flow of the fluidising air therethrough.

We claim:

1. A pneumatic conveying system for the conveyance in several separate air ducts of streams of powdered or granular material in which in advance of the positions in which the air mixes with the material, there is provided in each of the several air ducts a convergent-divergent nozzle which at its inlet or upstream end converges rapid ly to the nozzle throat which has a cross sectional area so related to the desired mass flow rate through the nozzle that in operation the air velocity through the throat is approximately equal to the velocity of sound in the air at the temperature and pressure existing at the throat, the nozzle diverging from its throat at a small angle (e. g. not more than 12), the cross sectional area at the delivery or downstream end of the nozzle being equal or substantially equal to that of the air duct, whereby a substantially constant mass flow rate through the several nozzles is obtained notwithstanding variations in the resistances of the materials to the air issuing from the several nozzles.

2. A pneumatic conveying system as claimed in claim 1 for the conveyance of powdered or granular material, in which the several streams of compressed conveying air after passing through the convergent-divergent nozzles.

entrain the material to be conveyed and carry it'to terminal points where it is separated from the air streams.

3. A pneumatic conveying system as claimed in claim 1 for the conveyance of powdered or granular material in which the several streams of material travel over separate porous bases through which compressed air is delivered in order to fluidise the material, characterised in this that at a position in advance of each point where the air enters the porous base, it is passed through a nozzle as specified in claim 1, the material leaving each porous base passing into a duct which receives compressed air through a nozzle as specified in claim 1. I

4. A pneumatic conveying system as claimed in claim 1 in which the throat dimension of each nozzle results in the absolute pressure at the throat being about 0.55 of the absolute pressure at the nozzle inlet.

Weller June 22, 1954 Atkinson Oct. 16, 1956