Suction of Water Vapour From Vermi Compost Extract

Sounds to me that you need a very cold vacuum trap before the vacuum pump. This can consist of a glass tube (ideally with long path length) cooled by either liquid nitrogen (-196 C) or solid carbon dioxide in acetone or isopropanol (about -77 C). The latter is cheaper and easier to obtain. The trap will condense out any volatiles in the vapour, preventing them from contaminating the vacuum pump oil and giving you a much better vacuum i.e. lower pressure. The condensed volatiles remain in the trap until its full. Isolating valves either side enable the trap to be emptied, coolant replenished without destroying the vacuum in the heated chamber.

Dear susheeldespande, now you have mentioned that extraction is 90% water (vapor). You have not yet mentioned what is the volume flow rate of extraction. So 90% of total volume handled by vacuum pump is water vapor. Water get mixed with vacuum pump oil and reduce its performance. So it is better to trap it before entering in to vacuum pump, it will solve the back pressure problem also.

I do not recommend liquid nitrogen or dry ice or isopropanol trap for your application as suggested by some members because the volume of water vapor is too high. Installing a heat exchanger along with a moisture trap before the vacuum pump for condensing the water vapor shall be sufficient. Coolant can be water or chilled water if you need more condensing.

Is the vacuum pump outlet is let to atmosphere?

What kind of vacuum pump do you have?

If it is a rotary vane pump or a piston in cylinder pump running with oil circulation to lubricate it then the water vapour will certainly condense in the oil. Your problem may be more to do with that water evaporating in the expansion chamber, so reducing the sucking effect. The lowest pressure you can develop is the vapor pressure of water at the temperature of the oil - probably over 80°c.

Why are you using a vacuum pump to remove the vapor? Is it to limit the temperature of the compost? A centrifugal pump (like a vacuum cleaner) or dry pump (very expensive) sounds a better bet than any pump with oil in it.

If you use a vapor trap you may well need mechanical refrigeration with a capacity at least equal to the heat input of your composter plus the heat generated in the process. Drawback is that you have to keep empying the trap and if it is freezing the water vapor that includes time to thaw and drain the ice. Remember this trap will pump 90% of your vapor, leaving the vacuum pump to only handle the 10% of non-condensable vapors, therefore the vacuum pump needs a capacity only 10% of total vapor generated and the pipe between the composter and the vapor trap will be some nine times the cross section of the pipe between the trap and the pump. (the vapor trap becomes much more of a pump than the vacuum pump itself.)

Do you want to collect the vapors? If not a centrifugal pump to atmosphere might suffice if it can drop the pressure to what you need. A lot simpler and cheaper to run.