Views: 5 Author: Site Editor Publish Time: 2016-01-07 Origin: Site
No single vacuum pump can meet all the above criteria completely. Let us now see some of the widely used pumps for distillation along with their limitations.
Ring type pumps (e.g., water ring pump, oil ring pump)
These pumps use water and sometimes oil as the pumping medium. For this reason, with ring type pumps, the ultimate vacuum achieved gets limited to the vapour pressure of the pump fluid at the working temperature. Owing to the above, even an efficient double stage water ring pump would stall at around 50 torr abs. (710 mm Hg), since vapour pressure of water at 300C is about 30 torr.
They have further disadvantage of being highly energy inefficient, because most of the power is lost in friction losses of moving the pump fluid inside the pump. This restricts the water ring (or oil ring) pump to relatively modest volumetric pumping capacities. Water ring pumps are widely used in food processing and pharmaceutical industry, since any other fluid contamination is not acceptable. This restricts the process capabilities – as working vacuum and speed get restricted. Another disadvantage of ring pumps is that the working fluid often has to be treated before it can be discharged or reused – as it contains the carry-over of condensed product.
Steam ejectors
Steam ejectors can produce low pressures (when used in multiple stages with suitable condensers) and have very high volumetric speeds. However, they require the maintenance of a complete steam generation facility confirming to IBRA regulations and inspection. They are generally not available as stand-alone installations – but can be found where process steam is easily available.
Relatively large barometric condensers are required to handle the ejector steam. For the obvious reason, large amount of soft-demineralised water is required, which is an additional recurring expense.
Rotary vane and piston pumps
These types of pumps have high power to capacity ratios, and are therefore, not available in large volumetric capacities. They are effective for pumping non-condensable loads but have a limitation of not being able to pump large and regular quantities of water vapour (and other vapours) released in lowpressure vacuum processes. Various precautions have to be taken – if they are used for food grade applications to avoid contamination of process material by the pump oil or back streaming of oil vapours.
