Pump down times

The pump-down time 0 for evacuating a vessel in the absence of air in-leakage is given approximately by... 

Installation arrangements, 351 Pump-down time, 380 Selection procedure, 374 Specification form, 377 Specifications, 373 Steam jet comparison, 356 Types of loads, 359 Ejectors, 346 Applications, 353 Barometric condenser, 249, 376 Booster, 370 Calculations Actual air capacity, 362 Air equivalent, 360... 

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... 

Because of the factors described above, an assessment of the pump-down time must be basically different for the evacuation of a container in the rough vacuum region from evacuation in the medium- and high vacuum regions. 

In this case the required effective pumping speed Sg,f, of a vacuum pump assembly is dependent only on the required pressure p, the volume V of the container, and the pump-down time t... 

Beginning at 1013 mbar at time t = 0, the effective pumping speed is calculated depending on the pump-down time t from equation (2.32) as follows ... 

The ratio V/Sgj, is generally designated as a time constant x. Thus the pump-down time of a vacuum chamber from atmospheric pressure to a pressure p is given by ... 

The dependence of the factor from the desired pressure is shown in Fig. 2.75. It should be noted that the pumping speed of single-stage rotary vane and rotary piston pumps decreases below 10 mbar with gas ballast and below 1 mbar without gas ballast. This fundamental behavior is different for pumps of various sizes and types but should not be ignored in the determination of the dependence of the pump-down time on pump size. It must be pointed out that the equations (2.32 to 2.36) as well Fig. 2.75 only apply when the ultimate pressure attained with the pump used is by several... 

Determination of pump-down time from nomograms... 

The pump-down times of rotary vane and rotary piston pumps, insofar as the pumping speed of the pump concerned is constant down to the required pressure, can be determined by reference to example 1. 

In general. Roots pumps do not have constant pumping speeds in the working region involved. For the evaluation of the pump-down time, it usually suffices to assume the mean pumping speed. Examples 2 and 3 of the nomogram show, in this context, that for Roots pumps, the compression... 

In the medium vacuum region, the gas evolution or the leak rate becomes significantly evident. From the nomogram 9.10 in Section 9, the corresponding calculations of the pump-down time in this vacuum region can be approximated. 

In many applications it is expedient to relate the attainable pressures at any given time to the pump-down time. This is easily possible with reference to the nomogram 9.7 in Section 9. 

Pgnd p = 3 lO mbar. From these figures the corresponding time constants in the nomogram can be determined from there, the pump-down time tp can be found by calculating the pressure reduction R on the left side of column 5. The result is curve c in Fig. 2.77. 

The magnitude of the allowable temperature for the bake-out process in question will, indeed, be determined essentially by the material in the chamber. Precise pump-down times can then be estimated by calculation only if the quantity of the evolving and pumped vapors is known. Flowever, since this is seldom the case except with drying processes, a quantitative consideration of this question is abandoned within the scope of this publication. 

The backing pump must be suitably rated with regard to the volume of the vessel and the desired pump-down time. This pump-down time is arranged according to the desired process duration if the process is to be finished after 12 -15 h, the pump-down time should not last longer than 1 h. The size of the backing pump may be easily calculated according to Section 2.3.1. 

Naturally it is possible to improve this ultimate pressure, should it be insufficient, by using a larger-capacity pump (e.g. the TURBOVAC 151) and at the same time to reduce the pump-down time required to reach ultimate pressure. 

Column (D Pump-down time tp in (top right) seconds or (center left) minutes or (bottom right) hours. Column Right ... 

Pressure pg, Q in millibar at the END of the pump-down time if the atmospheric pressure PsTART (Pn " prevailed at the START of the pump-down time. The desired pressure pg, Q is to be reduced by the ultimate pressure of the pump p Up and the differential value is to be used in the columns. If there is inflow Ppyj,, the value Pend - Puitp - PpV,in > be Used in the... 

Fig. 9.7 Nomogram for determination of pump-down time tp of a vessei in the rough vacuum pressure range...

Fig. 9.10 Determination of pump-down time in the rttedium vacuum range taking into account the ouigasing from the watts...

As an example, consider the time required to pump a large air lock with a volume of 50 L from a pressure of 760 to 0.5 torr. The free-air capacity of a widely available pump is 140 L/min, but three-fourths of this is a more reasonable average pumping speed for the above pressure range. Insertion of these numbers into Eq. (3) gives a pump-down time of 3.5 min. This value may somewhat underestimate the pumping time, but the calculation illustrates a method for approximately matching the pump to the patience of the experimentalist. 

As already described, the close coupled showerhead in OVPD enables homogeneous distribution of the organic gas phase and two-dimensional scalability (Table 9.1, no. 2) suitable for deposition on larger substrates. Conventional VTE operates with a point source, which is placed about the same distance from the substrate as the substrate diagonal to achieve the desired layer uniformity. This three-dimensional approach in VTE dramatically increases the equipment size and pump-down time to high vacuum, obviously limiting the scalability of VTE equipment. 

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. 

To use the pump-down factor, apply this relation t = VF/d, where t is pump-down time, min Vis system volume, ft3 F is pump-down factor for the pump dis pump displacement, ft3/min. 

Thus, if a 1200-ft3/min single-stage roughing pump were used to reduce the pressure to 15 torr, its pump-down time would be t = 12,000(4.0)/1200 = 40 min. The total pump-down time for the combination would then be 40 + 69 = 109 min, using the time computed above for the two pumps in combination. 

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. 

Thus, using the appropriate system factor for each pump, the actual pump-down time for the single-stage mechanical pump is ta = 130(1.5) = 195 min. For the compound mechanical pump, ta = 114(1.25) = 142.5 min. For the combination mechanical booster pump, ta = 109(1.35) = 147 min. 

Choose the pump to use. B ased on the actual pump-down time, either the compound mechanical pump or the combination mechanical booster pump can be used. The final choice of the pump should take other factors into consideration—first cost, operating cost, maintenance cost, reliability, and probable future pressure requirements in the system. Where future lower pressure requirements are not expected, the compound mechanical pump would be a good choice. However, if lower operating pressures are anticipated in the future, the combination mechanical booster pump would probably be a better choice. 

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