Vacuum systems Pressure levels

The inerts will blanket a portion of the tubes. The blanketed portion has very poor heat transfer. The column pressure is controlled by varying the percentage of the tube surface blanketed. When the desired pressure is exceeded, the vacuum system will suck out more inerts, and lower the percentage of surface blanketed. This will increase cooling and bring the pressure back down to the desired level. The reverse happens if the pressure falls below that desired. This is simply a matter of adjusting the heat transfer coefficient to heat balance the system. 

It is difficult to determine the time required to evacuate any particular vessel or process system including piping down to a particular pressure level below atmospheric. When using a constant displacement vacuum pump this is estimated by O Neil [31] ... 

Pressure/vacuum, 435, 466 Vacuum systems, 343 Absolute pressure conversions, 363 Air inleakage, 366 Calculations, 366-375 Dissolved gases release, 368 Estimated air inleakage, table, 366 Evacuation time, 371 Maximum air leakage, chart, 367 Specific air inleakage rates, 368 Temperature approach, 375 Classifications, 343 Diagrams, 380 Pressure drop, 353 Pressure levels, 343, 352 Pressure terminology, 348 Pump down example, 381 Pump down time, 380 Thermal efficiency, 384 Valve codes, 26... 

In the Langmuir free-evaporation method, the sample is suspended freely in a vacuum system with no container sunounding it. As very low levels of vapour pressure can be measured it has advantages over the Knudsen method where the lower limit is about 10" atm. (Kubaschewski et al. 1993). It is therefore more usefril in materials with high sublimation energies and therefore inherently low vapour pressiues. It has a further advantages in that there is no container with which to react, but there are more significant problems associated with temperature measurement. 

Vapors can be transferred into or out of the apparatus by applying a rough vacuum to the reservoir which draws mercury out of the U and into the reservoir. After the volatile materials are condensed into the apparatus, they are isolated from the vacuum system by bringing the mercury reservoir to atmospheric pressure and slowly bleeding mercury into the U. If it is important to know the gas volume in the apparatus, the mercury level is adjusted to some reference point, such as A in Fig 9.2, for which the volume of the apparatus has previously been determined. 

Fig. 7.2. A versatile bubbler manometer. The bubbler manometer Is securely mounted by the reservoir and attached to the vacuum system. It is then easily filled by the following process. The level of the bottom end of the vertical tube dipping into the reservoir is marked on the outside of the reservoir. Next, a calculated amount of mercury is filtered into the reservoir. With the valve between the two arms open, a vacuum is slowly drawn on the manometer. The mercury level must not drop below the mark on the reservoir, or else bubbles will enter the vertical tube and shoot mercury through the vacuum system. If the mercury level in the reservoir comes close to the mark, the manometer is brought up to atmospheric pressure and more mercury is added. When the proper amount of mercury is present in the fully evacuated manometer, the mercury level should be about 10 mm above the mark on the reservoir, and the upper meniscus should be in a region of the manometer suitable for measurement, as illustrated. Once the manometer is properly filled and evacuated, the valve is closed to isolate the reference arm at high vacuum.

The system pressure is also monitored (gauge 9). When it reaches an appropriate level (see Figure 3.8 and associated comments), valve 5 is closed and valve 4 is opened and the high vacuum valve (valve 6) is opened. 

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. 

The response of the radio frequency discharge detector was reported as 10 mV for a concentration change of 10 g/ml of methyl laureate. The noise level was reported to be 0.05 mV, which would give the minimum detectable concentration for a signal-to-noise ratio of 2 as about 6 x iQ- g/ml. This detector operated at atmospheric pressure and so no vacuum system was required. The effect of temperature on the detector performance was not reported, nor was its linearity over a significant concentration range. This detector was not made available commercially. 

It is very important to be able to measure the pressure in a vacuum system, particularly when carrying out a distillation. For low vacuum measurement a simple manometer, such as that shown in Fig. 8.3a, is commonly used and the pressure is taken by subtracting the heights of the mercury levels. Dial gauges are also useful for in-line measurements and they are particularly valuable when used with rotary evaporators. For high vacuum... 

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