October 5, 2020
4 MIN READ
The metallurgical sector covers a wide range of processes including alloy production, single crystal fabrication, steel degassing and annealing by heat treatment. One common factor is the use of large volume chambers, often with substantial gas loads requiring significant pumping speeds.
For annealing and steel degassing, a forevacuum pump or forevacuum pumps are sufficient. Vacuum Induction Metal furnaces - the choice for single crystal aluminium and alloy production - will require the addition of a high vacuum pump.
Rotary vane, and more particularly, piston pumps, were once the forevacuum pumps of choice. But oil usage and the cost of oil disposal, combined with reliability issues and service costs, have meant that dry screw pumps have become the default solution.
In addition, screw pumps can handle the particles frequently generated within VIM furnaces, which leads to improved uptimes, no oil back streaming, and reduced running costs. The diagram below shows the design of a dry screw pump. Compression is achieved by a variable pitch on the rotors.
Maximum speeds from a screw pump are typically 630 - 650m3/h. It is likely that additional throughput is required: this can be achieved by adding a roots pump – the mode of operation is shown below.
Impellers revolving in opposite directions
In position I and II, the intake volume is increased. Inlet pressure: pHV
In position III, a part of the volume is separated from the high vacuum side.
In position IV, this volume is opened into the direction of the forevacuum side. The gas exits the Roots pump in the direction of the backing pump. Discharge pressure: pFV
The development of a frequency convertor version allows the rotational speed of the impellers to be increased significantly. This maximum increase of rotation is dependent on the size and weight of the impeller. Thus a 700 m3/h pump operating at 50 Hz can have its rotational speed increased to 120 Hz, allowing a pumping speed of 1680 m3/h. A larger 2500 m3/h unit can have its pumping speed increased to 5000 m3/h using a frequency of 100 Hz. Pumping speeds can be tuned to optimize process requirements whilst the pumps can be set to a lower frequency standby mode, thereby reducing power consumption.
As already described, particles are a particular issue with many metallurgical processes. The chamber typically operates at an elevated temperature and high gas loads are common. Consequently, an Oil Booster pump would be an appropriate choice, especially for the pumping speed requirement. This type of pump is better able to handle these large flows in the required vacuum range of 10-4 to 10-5 mbar. Diffusion pumps exhibit lower throughput but do provide lower ultimate pressures.
The diagram below illustrates the pumping principle:
The oil booster pump operates in a similar manner to a diffusion pump, by employing oil of low pressure. The high speed jet is generated through a jet assembly and the oil is gaseous when entering the nozzles. Within the nozzles the flow changes from laminar to molecular.
Often, several jets are used in series to enhance the pumping action. The outside of the diffusion pump is water cooled, as the vapor jet hits the outer cooled shell of the diffusion pump, the working fluid condenses and is recovered and directed back to the boiler. the pumped gases continue flowing to the base of the pump at increased pressure. The oil booster pump has an additional ejector stage which increases pumping speed between 10-4 and 10-5 mbar, which is a requirement for many metallurgical processes.
Oil contamination from the pump is an issue, and baffles at the high vacuum port are recommended to minimize contamination of the product. However, these will result in a noticeable reduction in pumping speed and this needs to be factored in when determining pumpdown times.
You will also notice a small oil catch pot at the forevacuum port, which is designed to reduce carry over into the dry pump. This alone is not an issue, but particles are also carried into the pump. These can mix with the silicone oil of the oil booster pump and, if left unchecked, can act as a grinding paste, increasing the small rotor tolerances of the pump. This is in turn will result in poorer ultimate pressure over time. It is recommended that a fine inlet particle filter is used to avoid the process producing significant particles. Cleaning or replacing this filter should form part of the routine maintenance schedule.
Dry screw pumps offer an ideal solution to many metallurgical processes.
The addition of a roots pump can greatly increase pumping speeds.
Frequency convertor versions of roots pumps offer higher pumping rates whilst maintaining a smaller footprint compared to traditional types.
An oil booster pump offers high throughput in the typical 10-4 to 10-5 mbar process pressure regime.
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