VacTran 3 Manual, страница 9

Therefore, even if the pump was good for 25 liters/second for itsentire range, the delivered speed at the vessel will be less because of the conductance losses.The graph on the left shows the same pump speed curve as before, with the addition of a delivered speed curveunderneath. For the particular geometry calculated here, one can see that a significant reduction in speed isoccurring. At 1 torr, for example, the delivered speed is one quarter of the speed at the pump. Therefore, we won'tpump our 100-liter vessel down to zero in 4 seconds partly because of conductance losses.© 2011 Professional Engineering ComputationsFor new vacuum technologists593) More gas keeps coming inNo matter how well we design and construct our vessel, or how well we clean our system, we can't keep allunwanted gases from entering. This ever-present addition of gas is what ultimately keeps a vacuum vessel fromreaching a pressure of zero.

This limitation is called the ultimate pressure of the system. The gases can comefrom leaks into the vessel or attached piping, permeation through seals in doors and ports, out gassing fromsurfaces, back streaming from the pump(s), or generation of gas from processes in the vessel. These sources arecollectively referred to as gas loads. The ultimate pressure can be reduced by minimizing gas loads, or byincreasing the size of the pumps.For example, if the gas load for our example system was 25 liters/second, it would exactly balance our perfectpump and the vessel pressure would never decrease. Admittedly, there probably won't be gas loads this big in a100 liter vessel, but it is apparent that any gas load will decrease rate of pump down.© 2011 Professional Engineering Computations60VacTran 3Where does this leave us? Let's review the first goal:Remove an initial gas volume from a vacuum vessel, faster than new gas enters, toachieve a target pressure in a required time period.To reach this goal, we combine pumps and conductance elements to produce a delivered speed greater than thegas loads.

How much greater should this be? That depends on the required time period, which could be 30seconds for a mass spectrometer used to support steel making operations, or several days for a spacecraft testchamber. In either case, the time requirement will drive the size of the pumps and conductance elements.

Aclassic economic trade-off will weigh the capital investment of larger pumps against the operational expense oflonger pump down times.The second goal is somewhat simpler, and at first glance doesn't look much different than the first:Remove gas from the vessel at a rate equal to the rate it enters, maintaining anoperating pressure that is acceptable to the vacuum process.The target pressure in the first goal was selected because the vacuum process in the vessel probably requires it.Once this target pressure is attained in first goal, maintaining the pressure becomes the second goal. Since we aremaintaining a low pressure in the vacuum vessel rather than trying to pump out the initial volume, the deliveredspeed needs to be equal to or greater than the gas loads at the operating pressure.The maintenance of a target pressure may require more or less pumping capacity than the initial pump down,depending on the long-term gas loads in the vessel.

A relatively clean process will usually generate a decreasinggas load over time. However, a process such as freeze-drying may generate large amounts of gas that must bepumped away.To size the appropriate pumping system, one must determine the gas loads that need to overcome or balanced.Predicting gas load behavior is among the more challenging aspects of vacuum system design.© 2011 Professional Engineering ComputationsFor new vacuum technologists5.461Basic steps vacuum system designFor certain small laboratory applications, where cost and performance of the vacuum system are not majorconstraints, vacuum systems are often bolted together with available components, with very little thought to designtradeoffs.

However, in many large-scale systems, production apparatus, and other tightly constrained environments,this method is neither practical nor acceptable. In achieving the basic goals in the previous section, the followingobjectives are usually sought:• Select the appropriate pump(s) for the job. Determining the kind and number of pumps required willdepend on a large number of often-conflicting requirements, including:ReliabilityCleanlinessStart up timeHeat generationTarget pressureAllowable spaceMounting positionVibration and noisePower consumptionControl requirementsAmbient temperatureGas load constituentsInitial capital expenseCorrosive environmentsEffective pressure rangeRequired pump down timeDecommissioning/disposalRegeneration requirementsMaintenance requirementsOperation in a magnetic fieldElectromagnetic interference (EMI)Operation in a radiation environmentThe relative importance of the above requirements will depend on the particular installation.

Although theywill not be explored at any depth in this software manual, they bear mentioning so they are not overlooked.• Minimize the conductance path to the vessel, in diameter, length, and number of bends. The pumpshould be connected to the vacuum vessel through a series of elements, which conduct the pumped gasesmost efficiently within the allowable performance, space, cost, weight, and other constraints.

Bigger pipes,along with bigger valves, traps, and fittings, usually increase space consumption, cost, and weight.• Minimize the volume and gas load sources in the vacuum vessel. The vessel is usually the biggestvolume contributor, but can also add significant gas loads attributed to its cleanliness, surface finish,materials of construction, and sealing system.VacTran has been designed to help speed the process of achieving these objectives through rapidcharacterization and evaluation of system design alternatives.© 2011 Professional Engineering Computations625.5VacTran 3Selecting design marginsAll design engineers, in all disciplines, are inevitably faced with decisions regarding the cost of additional margin vs.the risk and consequences of failure.

Margin is the additional capability or capacity of a system or componentdesign beyond what is required for operation under perfect or ideal conditions. Since such conditions don't exist inthe real world some judgment is required as to how much extra capability is needed to account for known andunknown variances from the ideal. In vacuum system design, extra pump capacity is sometimes added to a higherrisk design (perhaps many unknowns) so that unanticipated gas loads or pump inefficiencies can beaccommodated. Other ways of achieving extra margin include provision for extra pumps on the vessel, or oversizing the pump port so that a larger pipe can be retrofitted later if necessary.Due to the inherent uncertainty of vacuum system, design margin must be carefully considered before finalizing apurchase requisition for a pumping system.

Experience with similar existing systems is often valuable forcomparison. Judgment is required, specific to each situation.5.6Traditional calculation methodsThe traditional method of hand calculating the transient pump down of a simple vacuum system can be summarizedas follows: For a given vacuum vessel size, one chooses a pipe diameter and length to the pump, and selects apump based partially on the manufacturer’s performance curve of pump speed vs. time. This initial selection can bebased on experience or a recommendation from the manufacturer. Since both pipe conductance and pumpingspeed are typically highly variable over the range of pressures encountered during pump down, one must calculatepump down times for small enough pressure increments to assure valid numerical results.

One hundred incrementsare usually sufficient. At each delta pressure, determine from the calculated mean free path of the gas moleculesand line diameter whether the flow regime is molecular, transition, or viscous. Then calculate conductance for thepipe, delivered speed at the vacuum vessel, and finally pump down time for the increment. Pump down times for allincrements are added to determine the total pump down time for a system devoid of gas loads.

Hand calculatingthe effect of gas loads presents an additional challenge.Introduction to System Models demonstrates a simple example using the hand calculation method is workedsystematically.Lengthy equations are used in the described set of calculations. A given system may have to be recalculated manytimes for slightly different line dimensions, pumps, and vacuum vessel design changes. Manufacturer’s data forconductance elements and pump speeds may be provided in units other than those in the textbook equations, andmust be converted.5.7How VacTran helpsVacTran automates the calculation effort, organizing the critical vacuum system parameters in a simple, modularformat, in any common units of measure.

VacTran allows the engineer to focus efforts on design/analysis decisionsrather than on distracting and tedious hand calculations.The minimum VacTran vacuum system contains one pump connected through one conductance element to avacuum vessel with a fixed volume. Of course, more pumps and conductances can be added to the model.