Vacuum systems Pump down time

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... 

You should not assume that you have a leak just because the vacuum in your system is poor. Reasons for inadequate vacuum can be carelessness (stopcock left open), anxiousness (insufficient pump-down time), or neglect (the diffusion pump was never turned on). You can save a lot of time by eliminating the various reasons why your vacuum is not performing up to expectations before you look for leaks. Factors that can prevent a system from reaching a desired low pressure include ... 

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. 

Apply the respective system factors. Studies and experience show that the calculated pump-down time for a vacuum system must be corrected by an appropriate system factor. This factor makes allowance for the normal outgassing of surfaces exposed to atmospheric air. It also provides a basis forjudging whether a system is pumping down normally or whether some problem exists that must be corrected. Table 6.33 lists typical system factors that have proven reliable in many tests. To use the system factor for any pump, apply it this way ta = tS. where ta is actual pump-down time, in min t is computed pump-down time from step 3, in min S is system factor for the type of pump being considered. 

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. 

Contamination (vacuum technology) Materials in a vacuum system that affect the pump-down time and the ultimate pressure of the system as well as the residual contamination in the system. See also Base pressure. 

Booster vacuum pumps are used to shorten the pump-down on evacuation (time) of a vacuum system before switching to the smaller vacuum pump to maintain the system opening vacuum and to handle the air inleakage to the system. 

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... 

To start a vacuum system such as that illustrated in Fig. 5.1, the clean main trap is fitted in place with an even coat of stopcock grease on the joint. A Dewar partially filled with liquid nitrogen is raised around this trap, and the fore pump is immediately turned on. CAUTION Oxygen from the atmosphere will condense in a trap heid at liquid nitrogen temperature therefore, it Is important never to leave a trap cooled to liquid nitrogen temperature exposed to the atmosphere for a significant length of time. When the line has pumped down to less than 1 torr, the... 

Then 10 to 15 g. of the anhydrous disodium monoamido-phosphate is weighed in a 50-ml. flask with a glass joint. The flask is attached to a high-vacuum system, via a stopcock and a trap filled with solid sodium hydroxide, and evacuated. The flask is then heated in an oil bath to 80° and this temperature maintained for 6 hours. The temperature is then raised, and pressure builds up from the ammonia liberated. This gas is pumped off at intervals and the temperature increased to 210°. The formation of ammonia slows down after some time, and the reaction comes to an end after about 7 days. 

Use of a carrier gas in OVPD enables deposition of the organic materials at a pressure of 10 3-10 torr (Table 9.1, no. 1) whereas the VTE system must be pumped down to high-vacuum conditions, which is a time-consuming and critical... 

The pump set on a vacuum system has to evacuate the system, starting from atmospheric pressure down to the required pressure, often in a given time. It must be able to maintain this pressure during operation of the vacuum process. Chapter 3 reviews the range of vacuum pumps available and the combinations that are used over the range from atmospheric pressure down to our current limits of measurement in the EHV range. 

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. 

To model experimentally the formation and properties of solid particles that form in the outflows of stars under realistic conditions appropriate to a circumstellar outflow, we would need a system large enough that the walls of the chamber are unimportant and the chamber itself needs to be pumped down to pressures that are several orders of magnitude less than the partial pressure of the least abundant reactant. In a typical stellar outflow SiO is present at 10-6 the abundance of hydrogen, while hydrogen is present at 1010 particles per cm3. In practical terms, this means that the experimental system must be capable of achieving a vacuum better than 10-15 atm and operate at about 10-9 atm or less. These conditions are barely achievable in terrestrial laboratories. However, for wall effects to be unimportant, the chamber radius must be several times the mean free path of the gas at 10-9 atm the mean free path is 100m. 

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). 

An extremely high vacuum (10 Torr), free of any electrically active impurity (see Chapter 20), is required for successful production of good superlattices. The inner walls usually have a cryogenic liner filled with LN2 to trap any condensable residual gases. Since pumping such a system down to the vacuum levels is a time[Pg.269]

The primary difference between a vacuum plasma roll-to-roll system and a batch-type system is that pump-down, pretreatment, and deposition time are significantly shorter for 3-D objects. The arrangement geometry of 3-D objects relative the magnetron sputtering device is also different. 

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