GB2193534A

GB2193534A - Multi-stage positive displacement gas-moving apparatus

Info

Publication number: GB2193534A
Application number: GB08715577A
Authority: GB
Inventors: John Frederick Groves
Original assignee: Peabody Holmes Ltd
Current assignee: Peabody Holmes Ltd
Priority date: 1986-07-18
Filing date: 1987-07-02
Publication date: 1988-02-10
Also published as: JPS6329086A; GB8617612D0; GB8715577D0

Abstract

The apparatus, for example for discharging bulk cargo from a vehicle, has two mechanically identical Roots blowers connected together in series and driven from a single prime mover (a hydraulic motor) through a mechanical drive transmission 42-44 which is arranged to give the required volumetric displacement ratio between them. In the preferred arrangement, the pressure ratio in passing through the upstream rotors (Pu) and the downstream rotors (Pd) is within the range 0.92 to 1.08. The upstream rotors are driven at a faster angular velocity than the downstream rotors. The apparatus may include first and second cooling chambers each in thermal contact with parts of both blowers. <IMAGE>

Description

SPECIFICATION Gas moving device This invention relates to an improved positive displacement gas-moving Roots-type machine and in particular to a positive displacement air blower designed for use as a pneumatic discharge device for fluent material such as a cargo discharge device for a bulk transport vehicle.

Positive displacement gas-moving devices find many applications included among which may be mentioned the conveying of fluent material (e.g. flour, sugar, salt, dry cement powder, coal, plastics and wood chips), the generation of sub- or supra-atmospheric pressures in storage or process chambers and the supply of large volumes of air for water treatment plants.

A traditional Roots-type machine utilises twin impellers mounted on parallel shafts contained within a casing and driven in opposite directions. The shape of each impeller is such that a small accurately gauged clearance is maintained at all times between the impellers and between the impellers and the casing. As the impellers rotate, the gas to be moved is drawn through an inlet opening in the casing, into the space between the impellers and the casing where it is trapped as the tip of each impeller in turn passes the respective edge of the inlet opening. As each impeller continues to rotate, its opposite tip clears the respective edge of an outlet opening in the casing, allowing the erstwhile trapped gas to be pushed through the outlet opening. This action is repeated twice for each 360" revolution of each impeller.

One of the impellers is driven by a prime mover connected to one end thereof and the second impeller is driven from the other end of the first impeller via an intermeshing pair of timing pinions. A typical single-stage Roots blower can subject up to 30000 cubic metres of gas per hour (m3/HR) to a pressure rise of some 800 m.bar. Under good operating conditions a pressure rise of some 1250 m.bar is achievable with a single-stage machine.

In a number of uses of Roots-type machines and in particular in the application of bulk cargo discharge from a vehicle, there is a requirement for increased pressure operation which is not obtainable by any yet apparent redesign of the machine.

Pressures of up to 1600 m.bar at flow rates of 650 m3/HR would be desirable for transport vehicle applications since with these operating parameters significantly shorter discharge times could be achieved. A rotary vane compressor can generate pressures up to about 1750 m.bar but these gas-moving machines are an undesirable substitute for a Roots-type machine in the case of bulk transport vehicle unloading devices where the operating conditions of the machine are controlled by what can be a driver impatient to make the return leg of a delivery. The sliding vanes, which characterise a rotary vane compressor, can stick in their innermost position causing serious loss of performance and the rate of vane wear is unacceptable for good intervals between machine services.

It has previously been proposed to increase the discharge pressure in transport vehicle applications by using two Roots-type blowers in series, although this makes for a space-wasting unit and increases the tare weight of the equipment since not only are two blower casings and their drive equipment required, but it may be necessary to include intercooling between the stages, otherwise the operating temperatures of the downstream blower becomes excessive.

In the case of the creation of sub-atmospheric pressures it has been known to locate two Roots blowers end-to-end in a single casing and to arrange for the downstream pair of impellors to receive the gas they move directly from the output side of the upstream pair.

According to a first aspect of this invention a positive displacement gas-moving machine comprises two mechanically identical Roots blowers with the outlet of the first leading into the inlet of the second, whereby the blowers are adapted to produce a two-stage gas pressure rise, the two blowers being driven from a single prime mover through a mechanical drive transmission arranged to give the required volumetric displacement ratio between them.

Preferably the mechanical drive transmission is arranged so that the rotors of the upstream blower are driven at a faster angular velocity than the rotors of the downstream blower. It is preferred that the pressure ratio achieved in air passing through the upstream rotors (Pu) and the downstream rotors (Pd) lies in the proportion 40:60 to 60:40. Desirably, the ratio Pu:Pd lies within the range 0.82 to 1.22 and preferably within the range 0.92 to 1.08.

In a preferred construction, the two pairs of rotors of the two Roots blowers are housed within a single casing and the prime mover comprises a motor mounted on the casing.

The motor may be a hydraulic motor bolted directly on to the casing.

According to a second aspect of the invention two pairs of Roots blower type impellors are mounted side-by-side in a casing with two sets of gearing one at each end of the impellors, the first set of gearing transmitting driving motion to one of the impellors of each pair and a second set of gearing transmitting driving motion from one impellor to the other of each pair.

According to a third aspect of the invention a positive displacement gas-moving machine comprises two Roots blowers with the outlet of the first leading to the inlet of the second, whereby the blowers are adapted to produce a two stage gas pressure rise, there being first and second cooling chambers each in thermal contact with parts of both blowers.

Preferably the two Roots blowers are mounted side-by-side and the cooling chambers comprise oil containing gear boxes arranged one at each end of the assembly of two Roots blowers, one gearbox providing a control for the relative speeds of rotation of the two Roots blowers and the other providing the timing gears for synchronising the two Roots blower impellors of each pair.

According to a fourth aspect of the invention a positive displacement Roots type blower comprises a single casing containing four identical impellors driven from a single motor attached to the casing, an upstream pair of impellors being driven synchronously at a faster angular velocity than the downstream pair.

According to a fifth aspect of the invention a bulk carrier vehicle comprises a cargo discharge system which includes an hydraulically powered Roots type air blower characterised in that the air blower includes two pairs of impellors mounted side-by-side (in series) in a single casing.

It will be appreciated that preferred features of the first aspect of the invention can be combined with any one of the other aspects of the invention.

The construction of a two stage pneumatic pump and its employment in a bulk carrier, will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a bulk transport vehicle according to the invention pneumatically discharging cargo, Figure 2 is a sectional side elevation of the blower used in the cargo discharge system of Figure 1, taken on the line ll-ll in Figure 3, Figure 3 is a schematic end elevation of the blower taken on the line Ill-Ill in Figure 2 and showing the drive gearing, Figure 4 is a detail section on the line IV-IV in Figure 2, Figure 5 is a schematic end elevation of the blower taken on the line V-V in Figure 2, showing the timing gearing, and Figure 6 is a non-dimensional performance curve for the blower shown in Figures 2 to 5.

Referring to Figure 1, a tanker vehicle 10 is shown discharging cargo pneumatically from its tank 12 into a delivery pipe 14, the air flow for the pneumatic discharge being created by a Roots type positive displacement blower 16 powered from a pressurised hydraulic system driven by the vehicle engine.

The blower 16 is illustrated in detail in Figures 2 to 5. It comprises a casing 20 having a central region 22, which includes an inlet 23 on one side and an outlet 24 on the other side (see Figures 3 and 5). The inlet 23 is in communication with an air filter and optionally also with a silencer (neither of which are shown) and the outlet 24 is connected by ducting to a relief valve (not shown) a nonreturn valve (not shown) and then to the lower end of the tank 12.

Located within the central region 22 of the casing 20, are four identical Roots blower type impellors 30, 32, 34 and 36. The impellors 30, 32 constitute an upstream pair and the impellors 34, 36 a downstream pair and each impellor is driven in the direction of the arrow marked thereon in Figures 3 and 5. The impellors of each pair mesh together and coact with the wall of the central region 22 of the casing in the manner well known in the art of Roots blowers, and this will not be further described here. However, there is no inter-meshing between the impellors of the pair 30; 32 and the impellors of the pair 34; 36-these two pairs being arranged side-byside in the casing but not physically in contact with each other at any time.

It is to be noted that the four impellors are all of the same dimensions (which is advantageous from a manufacturing point of view) so that there are within the one casing two mechanically identical Roots blowers.

The lower impellors 30 and 34 are driven from a hydraulic motor 40 bolted directly on to the casing 42 of a drive gearbox 44 at one end of the casing 20. The motor 40 has a splined drive shaft 41 which drivably engages within a bore in a primary drive gear wheel 42. This gear wheel 42 is in meshing engagement with first and second stage driven gear wheels 43 and 44 respectively in driving engagement with the shafts of the lower impellors 30 and 34. The ratios between the gear wheels 42 and 43 on the one hand and between the gear wheels 42 and 44 on the other hand are chosen to set the required speed ratio between the impellors of the two stages provided by the two Roots blowers. In the instance where identical impellors are used in both stages the gear ratio between the driven wheels 43 and 44 is chosen to give a first stage blower speed (angular velocity) up to fifty per cent greater than that of the second stage blower. A speed twenty five per cent greater (i.e. a ratio of 5:4) is particularly convenient.

The gear wheel 43 is taper mounted on the shaft 31 of the impellor 30 and the gear wheel 44 is similarly mounted on the shaft of the impellor 34.

The three gear wheels 42 43 and 44, the bearings of the adjacent ends of the four impellors (only two of these bearings are shown at 46, 47) and the oil seals 45 are located in an input gearbox 50 bolted to one end of the central section 22 of the casing.

A timing gearbox 52 is bolted on the other end of the central section 22 of the casing and houses two identical pairs of timing, gears 54, 56 and 58, 60 (see Figure 5). The timing gears transmit the drive to the top rotor of each pair and ensure that each pair of impellors remains properly in synchronism during the operation of the machine. It will be noted that there is no mechanical interlinking between the impellor pair 30; 32 and the impellor pair 34; 36 at the timing gear end of the casing. As with the gearbox 50, the gearbox 52 includes bearings 62, 64 and oil seals 61.

Each gearbox 50, 52 contains oil which, when the machine is not operating, fills each gearbox to the level shown by the line 63.

This oil is splashed around inside the gearboxes by the gear wheels and has the effect of transferring heat away from the face of the gearbox which is clamped to the central region 22. Thus, each gearbox has an important function in reducing the temperature rise occurring in the central region of the casing particularly in that part which defines the second stage of the compression, i.e. the part housing the impellors 34 and 36. Finning on the central region 22 of the casing 20 also assists in the removal of heat from the blower.

A particular form of the illustrated pump is rated at 1600m. bar and delivers between 600 and 680m3/hr of air for pneumatic conveying.

The use of identical impellors for both stages in a common casing is important and results in manufacturing economies. To achieve efficient compression, the second stage has to be driven more slowly than the first because the gas volume decreases as the pressure increases. Ideally, compression in both stages should be isothermal, but this is not practicable. What is desirable however is to have the pressure ratios of the two stages identical to obtain optimum efficiency. Knowing the overall and stage pressure ratios enables the designer to calculate the volume of gas which is to be "passed on" to the second stage. The proportional decreasing speed (angular velocity of the impellors) at the second stage to accept this flow rate can then be selected.

The behaviour of the two stage Roots machine illustrated is very similar to a single stage machine. Both have the "positive displacement" characteristics which are very desirable for pneumatic conveying applications.

Figure 6 illustrates, non-dimensionally, that throughput is very nearly linear with input speed, and only slightly reduced by the pressure load.

The power consumption of the two stage machine with both stages in a common casing is not inherently lower than that of a more conventional layout with separate cylinders.

However, since the separate cylinder layout involves connecting pipework with a significant pressure drop, power is wasted. In practice therefore, the integral design illustrated in Figures 2 to 5, is expected to have a small power advantage, and considerable manufacturing and design advantages.

It would also be possible to construct a multi-stage (three or more stages) Roots blower in accordance with the invention. For instance, three pairs of rotors could be mounted in a casing at approximately 1200 spacings around the axis of the common driving shaft (motor shaft 41 in Figure 3). In that case, there would be three driven gear wheels at the driving end of the blower and these would be of differing sizes to provide a high speed first stage, medium speed second stage and low speed third stage.

Claims

1. A positive displacement gas-moving machine comprising two mechanically identical Roots blowers with the outlet of the first leading into the inlet of the second whereby the blowers are adapted to produce a two stage gas pressure rise, the two blowers being driven from a single prime mover through a mechanical drive transmission arranged to give the required volumetric displacement ratio between them.

2. A positive displacement gas-moving machine as claimed in Claim 1, in which the mechanical drive transmission is arranged so that the impellors of the upstream blower are driven at a faster angular velocity than the impellors of the downstream blower.

3. A positive displacement gas-moving machine as claimed in Claim 2, in which the pressure ratio achieved in air passing through the upstream impellors (Pu) and the downstream impellors (Pd) lies in the proportion 40:60 to 60:40.

4. A positive displacement gas-moving machine as claimed in Claim 3, wherein the ratio Pu:Pd lies within the range 0.82 to 1.22.

5. A positive displacement gas-moving machine as claimed in Claim 4, wherein the ratio Pu:Pd lies within the range 0.92 to 1.08.

6. A positive displacement gas-moving machine as claimed in any one of Claims 1 to 5, wherein the two pairs of impellors of the two Roots blowers are housed within a single casing and the prime mover comprises a motor mounted on the casing.

7. A positive displacement gas-moving machine as claimed in Claim 6, in which the motor is a hydraulic motor bolted directly on to the casing.

8. A positive displacement gas-moving machine, in which two pairs of Roots blower type impellors are mounted side-by-side in a casing with two sets of gearing one at each end of the impellors, the first set of gearing transmitting driving motion to one of the impellors of each pair and the second set pf gearing transmitting driving motion from one impellor to the other of each pair.

9. A positive displacement gas-moving ma chine comprising two Roots blowers with the outlet of the first leading to the inlet of the second, whereby the blowers are adapted to produce a two stage gas pressure rise, there being first and second cooling chambers each in thermal contact with parts of both blowers.

10. A positive displacement gas-moving machine as claimed in Claim 9, in which the two Roots blowers are mounted side-by-side and the cooling chambers comprise oil containing gearboxes arranged.one at each end of the assembly of two Roots blowers, one gear box providing a control for the relative speeds of rotation of the two Roots blowers and the other providing the timing gears for synchronising the two Roots blower impellors of each pair.

11. A positive displacement Roots-type blower comprising a single casing containing four idential impellors driven from a single motor attached to the casing, an upstream pair of impellors being driven synchronously at a faster angular velocity than the downstream pair.

12. A bulk carrier vehicle comprising a cargo discharge system which includes an hydraulically powered Roots-type air blower characterised in that the air blower includes two pairs of impellors mounted side-by-side (in series) in a single casing.

13. A bulk carrier vehicle comprising a cargo discharge system including a positive displacement gas-moving machine in accordance with any one of Claims 1 to 12.

14. A positive displacement gas-moving machine constructed and arranged substantially as herein described with reference to Figures 2 to 5 of the accompanying drawings.