Mechanical vacuum systems Pump down

Mechanical separations, 224 Mechanical vacuum systems, 342 Applications, 352, 353 Barometric iniercondenser, 349 Evacuation times, 387 Operating range, 355 Performance curves, 386 Pump down, 380... 

Once you have successfully removed the bulk of water from the walls of the vacuum system, do not allow it to return. One easy and effective demonstration of the effect of water on vacuum is to pump a vacuum system down to some established level after it has been vented with atmosphere. Then, vent the system, filling it with dry nitrogen or argon back to atmospheric pressure. Now, repump the system back to the same vacuum as before. It should take about one-tenth the time. This example demonstrates why the ability to bake out a vacuum system improves the pumping speed by speeding up the removal (outgassing) of water vapor from the system s walls. It also demonstrates that once a vacuum system has been successfully pumped down, you do not want to re-expose it to the atmosphere. If you need to expose sections of your vacuum system to the atmosphere (for example, traps or mechanical pumps), section off these parts with valves and stopcocks so that the rest of the system can remain in a dry vacuum state. 

When shutting down a vacuum system, close off your system from the trap section. That way, as trapped compounds warm up and go into a vapor state, they will not be able to drift into the rest of the vacuum line. You should also vent your pump to the atmosphere. Many pumps do not have adequate check valves near their oil reservoirs. If they are shut off with a vacuum on the vacuum side, the mechanical pump oil can be drawn up into the system. So, to shut down a vacuum system (see Fig. 7.34), it is recommended that you ... 

Choose a mechanical vacuum pump for use in a laboratory fitted with a vacuum system having a total volume, including the piping, of 12,000 ft3 (340 m3). The operating pressure of the system is 0.10 ton, and the optimum pump-down time is 150 min. (Note 1 ton = 1 mmHg.)... 

Make a tentative choice of pump type. Mechanical vacuum pumps of the reciprocating type are well suited for system pressures in the 0.0001- to 760-ton range. Hence, this type of pump will be considered first to see if it meets the desired pump-down time. 

Compute the size of the connection pipe. In usual vacuum-pump practice, the pressure drop in pipes serving mechanical pumps is not allowed to exceed 20 percent of the inlet pressure prevailing under steady operating conditions. A correctly designed vacuum system, where this pressure loss is not exceeded, will have a pump-down time which closely approximates that obtained under ideal conditions. 

Every vacuum system consists of various components intended to ensure that the process works reliably. On the intake side, the vacuum pump is protected mechanically by a separator. This separator must work just as well in each different application. The simplest type of separator is a container that slows down the gas flow and increases the dwell time of the gaseous components (Fig. 10). 

Most of the generated vapour is condensed in spray condensers which are equipped with circulation pumps and an EG cooler. The vapour that is still uncondensed is withdrawn from the gas phase with the help of a vapour jet which is located down-stream behind the spray condenser and generates the necessary vacuum in the reaction zone. The most critical part of the spray condenser system is the end of the pipe leading the vapour from the prepolycondensation reactors and the finishers into the spray condenser. The transition from a hot to a cold environment causes deposition of solid material onto the cold walls which has to be removed manually or by means of a mechanical scraper. 

The most common problem with hydrocarbon diffusion pump oil is its fractionation into multivapor pressure components. As pump oil breaks down, it develops both lower and higher vapor-pressure characteristics. Oils with high vapor pressures can potentially drift into the system, although they are more likely to be effectively removed from the system by being trapped in the alembics of the central vertical tube, in the cold trap between the system and the diffusion pump, or in the cold trap between the diffusion pump and the mechanical pump. If not trapped, they are free to travel into the vacuum line itself or into the mechanical pump. Diffusion pump oils that collect in a mechanical pump are not likely to have any significant performance effects (as opposed to the degrading effects of mechanical pump oil collected in diffusion pumps). 

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