Turbo- or Roots-Blowers?
Pneumatic Ship Unloaders
The characteristic behaviour of turbo- and roots- blowers in pneumatic ship unloading plants is discussed and the advantages and disadvantages are compared in detail.
This journal has already dealt in detail with pneumatic ship unloading plants in general, as well as with their components and properties (bulk solids handling, Vol. 2....) That is why we will focus here on the compressor for producing the conveying air, since the advantages and disadvantages of the different aggregates have lately been discussed intensively because of the additional use of turbo-blowers for ship unloading plants with a higher capacity.
In order to convey a material flow in a pipeline, a certain air volume flow is necessary that shows a pressure drop over the pipeline length. Volume flow and decrease of pressure will be determined by the type of material to be conveyed, the througput, the pipeline and the pipeline length. It is part of the know-how of various manufacturers for pneumatic ship unloading plants to design a plant in an optimal way. Two basically different types of blowers can be chosen to generate the air volume flow: the turbo-blower, or the rotary piston blower, also called the roots-blower. Both types can offer the required data, such as volume flow and differential pressure, in comparable dimensions.
2. Turbo Blowers
The design of turbo-blowers is well-known. They essentially consist of a fixed housing and a driven blade wheel. The air will be drawn into the centre of the blade wheel and accelerated to the outside due to the centrifugal action. In the adjacent spiral housing, the kinetic energy is transformed into static energy to a large part.
The amount of differential pressure is a function of the circumferential speed of the blade wheel and of the number of stages, while the amount of the volume flow is a function of the size. For the mobile GSD-types and the Multiports, NEUERO mainly uses 2 single-stage turbo-blowers of its own design which are connected in series in order to generate the necessary differential pressure (Fig. 1) In connection with the development of turbo-blowers for the charging of diesel engines, there are also much more expensive turbo-blowers on the market nowadays, producing an intake pressure of 0.5 bar absolute in one stage. The efficiency of thoroughly designed turbo-blowers comes up to 80%. Fig. 2 shows the characteristic curve of a turbo blower at different revolutions.
The characteristic curves run very flat, i.e. turbo-blowers can be used over a broad range of volume flow at a rather constant differential pressure. In the case of the expensive turbo-blowers, this range will be further extended by an adjacent diffuser blade wheel; thus, a volume flow of 45 to 100% can be continuously regulated there, at a constant differential pressure and with equally high efficiency. The power requirement increases with an increasing volume flow.
Roots-blowers mainly consist of two double-bladed pistons of particular cross-section rotating synchronically in a casing, so that a constant and tight clearance between the pistons and towards the casing is attained.
These blowes are valveless displacement engines, in which no internal compression takes place. From the lower pressure level in the intake, the air reaches the working room and will be driven by the blades of the piston into the range of higher pressure in the exhaust opening. The air will only be compressed once a connection between working room and higher pressure level is made. Then, air with higher pressure flows back into the working room, with the effect that the piston must be moved against this higher pressure. The back-flow of the higher compressed air causes the typical flow frequency noise of the rotary piston compressor. Although the gaps between the pistons and towards the casing can be made very tight (normally 0.2 to 0.6 mm), there the so-called gap leakages occur which become bigger with increasing pressure differential. The efficiency of roots-blowers is lower than that of thoroughly designed turbo-compressors and is approx. 70%.
Fig. 3 Characteristic curves of a roots blower
The characteristic curve runs very steep ...(Fig. 3, characteristic curve at different rpm) i.e., a nearly constant volume flow is produced ove a wide differential pressure range, in case the gap leakages are neglected. The volume flow depends on the size and the revolutions per minute which are limited by a max. admissible circumferential speed at the piston. If the volume flow is altered, a different number of revolutions at the roots-blower must be chosen. This can, however, only be achieved with a considerable expenditure at the drive. The power demand rises linear with the differential pressure.
How do the two types of blowers behave when being used in a ship unloading plant?
4. Operational Behaviour
The operational behaviour of the blowers can be explained in a simple way in connection with the characteristic curve of the plant. Fig. 4 shows the characteristic curve of the plant when only air is conveyed and when conveying a material flow, as well as the characteristic curve of turbo-blower and a roots-blower, chosen for a determined state. The intersection of the characteristic curve or the plant during the conveyande of material with the characteristic curve of the blower poduces the point of operation of the plant. This point of operation must be chosen in such a way that the air speed will be as low as possible to save energy costs, and that it will also-on the other hand - be high enough to avoid possible cloggings in the conveying pipe.
Fig. 4 Characteristic curves of conveying plant and blowers
What will happen now when less or no material is conveyed? The point of operation of the turbo-blower moves to point P1 on the characteristic curve of the blower, this point P1 being the intersection with the plant characteristic curve only for air conveyance. On account of the characteristic flat curve o the turbo-blower, there will be a higher intake volume flow which leads to a higher power demand of the blower.
In order to avoid the increased no-load power demand, NEUERO installs a so-called air-flow control (Fig. 5) that provides for the intake volume flow to remain nearly constant also in the partial-load range and under no-load operation, i.e. that the point of operation of the plant moves from P0 to P2. This air flow control consists of an inclined flat plate being exposed to the air flow; this flap has a position of lowest differential pressure in the full-load range. The torque of air flow acting on the flap will be compensated by a counterweight attached at a lever crank. If the material flow decreases, the air volume flow will increase. This can, however, be avoided by automatically inclining the flap, so that an increased differential pressure is created that - under no-load operation - exactly corresponds to the distance P0 to P2 = Ap valve. It is also possible to control the flap by means of the vacuum ahead of the turbo-blower.
Due to this control, the point of operation does not move under full-load, nor in the range of partial load, i.e. consequently the power demand in the range of partial load as well as of no-load will approximately correspond to that under full-load operation, and the air speed in the suction nozzle remains constant in the entire range, from no-load to full-load.
Compared with the operation of a roots-blower, unnecessary wear at the conveyor pipe and possible damage of sensitive conveying materials are avoided.
Fig. 5 NEUERO - air flow control
Another advantage of the air-flow control consists in the fact that by altering the counterweight at the control, different air speeds in the conveyor pipe can be adjusted; thus, the suction plant can be used for sensitive goods like malt and brewing barley at low air speeds and, if necessary, for other goods with higher air velocity. The air flow control provides a simple manner for finding the optimal air speed for a plant at which a conveyance will still just be possible, so that also an optimal use of the plant will be achieved concerning the energy demand.
Under no-load operation only, in case a material-conveyance is not needed, a throttle valve can compulsorily be shut; thus, the conveyance of air will be reduced except for a small amount of scavenging air required for cooling the blower. By that means, the power demand of the blower will be reduced to an approximately comparable dimension to that of the roots-blower.
Comparisons in the practical applications show that the power demand under full-load will be up to 10% lower for turbo-blowers, on account of their better efficiency, than for plants with roots-blowers; a plant in North Germany could reduce the average specific energy kWh/t during the conveyance of tapioka by 40% by replacing a roots-blower with a turbo-blower, and because an optimal air velocity was chosen by means of the diffuser blade wheel.
The point of operation of the roots-blower moves from P0 to the point P3 on the characteristic curve of the blower. The intake volume flow remains nearly constant, but the necessary differential pressure falls, so that the necessary power demand decreases as well. This can certainly be advantageous in the partial-load range.
On the other hand, the velocity in the conveyor pipe increases, as the differential pressure in the conveyor pipe is very small and as the number of gap leakages in the roots-blower is reduces. An air velocity at the nozzle of 25 m/s at full-load can be raised to 50 m/s at no-load; this results, in the range of partial-load, in an undesired additional wear on the conveyor pipe and in damage to sensitive conveying goods. A further disadvantage when using a roots-blower consists in the fact that a supplementary modification of the conveying speed or an adaption of the conveying speed to different conveying goods will not be possible unless very expensive adjusting gear for modifying the number of revolutions of the blower is installed.
The roots-blower is advantageous only when the same conveying material is unloaded with the plant; when the velocity of the conveying air flow is chosen correctly, the plant will often be used for partial-load operation and the increased air velocity in the range of partial load will be of secondary importance.
5. Summary of the Advantages and Disadvantages
At the end, the advantages and disadvantages of both types of blowers are summarized once again for the purpose of a better comparison:
5.1 Turbo Blower
The advantageous use of turbo-blowers in pneumatic ship unloading plants can be seen from this list of advantages and disadvantages; therefore, NEUERO has always chosen this way and used the single stage radial high-capacity blower as major components of its conveying installations.