Chemical Technology April 2015

PUMPS & VALVES

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Loading a vacuum furnace.

5. Correct heater adjustment according to the selected driving fluid 6. Standby operation/intelligent process management Pump design features It is obvious that the total diffusion pump must already be designed to minimise consumption. To save energy and consumables, modern diffusion pumps are optimised with some design features: • Heaters are positioned inside the boiler • Reduction of heat-transfer losses, thereby lowering power demand • Grooved pump body with built-in cooling-water coils • Reduction of weight and therefore total energy for initial heating • Optimisation of contact surface/heat-transfer efficiency, thereby reducing cooling-water demand • Water-cooled cold-cap baffle at inlet • Reduction of fluid backstreaming into the vacuum chamber, thereby reducing fluid losses • Water-cooled fore-vacuum baffle at outlet • Reduction of fluid losses into the fore-vacuum line Electronic regulation of heater power  One general statement to first consider: More heating power does not automatically result in more suction speed! As previously described, the main driving force for the pumping mechanism is the partial-pressure difference

Reaching the cooled pump housing, the vaporised pump fluid condenses on the cold wall. The liquid fluid runs downward and returns into the boiler from where it will be evaporated again. Why does this principle pump at all? Figure 2 shows a detailed view into the pumping mecha- nism of the diffusion pump. We can see the vapour jet on its way from the jet system toward the water-cooled wall. The jet consists of ‘degassed’ fluid vapour, which means it only contains extremely low partial pressure of the gases that should be pumped. The area above the vapour jet, therefore, contains a much higher partial pressure of the gas. Driven by partial-pressure difference, the gas from above diffuses into the jet stream to compensate. The jet stream will push the gases toward the cooled wall and the next nozzle stage of the pump. Finally reaching the exhaust side of the pump, the gases will be pumped away by the backing pump. The vacuum inside the boiler will ensure that the reboiling fluid will contain the lowest partial-pressure of gases again. Energy-saving measures The following measures have been identified as having the highest influence on the power consumption of a diffusion pump: 1. Pump design features 2. Electronic regulation of heater power 3. Regulation of cooling water 4. Housing insulation

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Chemical Technology • April 2015