VacTran 3 Manual (779748), страница 22
Текст из файла (страница 22)
This model will have aconductance vs. pressure curve that is the same as the series of conductances using the “From” diameter and thecurrent start and stop pressure settings. The raw data conductance file can be used later in system models.See also:Activating a conductance studyCalculations in conductance studiesHow to calculate backing pump speedExample backing pump problem© 2011 Professional Engineering Computations21210.4VacTran 3How to calculate backing pump speedFor some type of pumps, such as diffusion pumps and roots pumps (also known as roots blowers), the ratedpumping speed given by the manufacturer is dependent on a minimum backing pump speed.
In other words, thesetypes of pumps cannot operate without an additional pump between them and the atmosphere.The minimum backing pump speed would be a simple requirement to design for if the backing pump was connecteddirectly to the main pump without losses. In this case, one would simply select the appropriate pump by looking upthe manufacturer's rated pumping speeds at the operating pressure.However, the backing pump cannot always be directly connected.
A conductance loss is usually introduced due tothe pipes, elbows, traps that connect the backing pump to the main pump, and the backing pump may be hundredsof feet away. This conductance loss results in a delivered speed at the outlet of the main pump, which is less thanthe rated speed of the backing pump.
If we do not consider the conductance loss, the effective pump speed mayactually be undersized for the backing speed required. At best, the result will be a main pump that does notoperate efficiently. Far worse is the potential for damage or self-destruction of the main pump, which was notdesigned to operate across a high-pressure differential.Backing pump delivered speed is given by:whereSb = Backing pump delivered speedSp = Speed at backing pump inletC= Conductance between backing pump and main pumpSee also:Activating a conductance studyCalculations in conductance studiesHow to calculate backing pump speedExample backing pump problem© 2011 Professional Engineering ComputationsConductance Studies10.5213Example backing pump problemAssume that as the designer, you determine that the most appropriate type of pump to achieve a given system'starget pressure requirement of 10-6 torr is a 20 inch (inlet) diffusion pump, perhaps because of the type and volumeof process gas loads.
The particular pump model is selected based on its pumping speed and ultimate pressure.However, since diffusion pumps cannot pump a system from atmospheric pressure, a different type of pump mustbe used before we "cross over" to the diffusion pump. In addition, the manufacturer suggests that the diffusionpump should be backed by a pump with a 400 cfm capacity at 10-2 Torr, because diffusion pumps do not workacross a high pressure differential. After some research, you determine that the best type of pump for the job is aroots blower, because of its high pumping capacity at 10-3 Torr.
However, like the diffusion pump, the roots blowercannot pump the system down from atmosphere, although it operates at higher pressures than the diffusion pump.This particular roots blower works best starting at 10 Torr. It too needs a backing pump, with a capacity of 75 cfm.A mechanical pump is selected which meets this requirement, and is capable of pumping down the system fromatmospheric pressure to the 10 Torr crossover point for the roots blower.To quickly review the problem, we have determined that three types of pumps are required for this particularpumping system.
Do not be concerned with the details of pump selection at this point. The purpose of thisexample is to show how pumps back one another.A typical pumping sequence (with some details omitted) used by this type of pump combination is described asfollows: In stage 1, mechanical pump is used initially alone, pumping the system down to about 10 Torr, which isthe optimum crossover point recommended by the manufacturer for this example roots blower. For stage 2, at 10Torr, the roots blower turns on, backed by the mechanical pump.
For stage 3, at 10-2 Torr, the diffusion pump turnson, backed by the roots blower, which is still backed by the mechanical pump. The diffusion pump brings thevessel down to its target pressure of 10-6 Torr. Valves are used to manage the flow at each stage.© 2011 Professional Engineering Computations214VacTran 3The following table and figure illustrate the desired sequence of pumping:Stage 1Stage 2Stage 3Mechanical pump760 to 10 torrbacking rootsblowerbacking rootsblowerRoots bloweroff10 to 10-2 torrbacking diffusionpumpDiffusion pumpoffoff10-2 to 10-6 torrDiffusionpumpStage 3Roots blower Stage 2Gas flowMechanicalpumpStage 1atmosphereNote that the mechanical pump and roots blower each play a dual role. Each initially is used for pumping thevacuum vessel, as shown in stages 1 and 2 below, and is then valved to become a backing pump.
Each of the tworoles involves a conductance loss that must be accounted for.We can divide the problem of calculating the backing pump speed into two separate problems, one for backing thediffusion pump and one for backing the roots blower. Attacking the diffusion pump problem first is most appropriatebecause it may result in a different roots blower than first anticipated. Consequently, a different backing pump forthe new roots blower will be required.© 2011 Professional Engineering ComputationsConductance Studies215Backing the diffusion pumpOur problem at hand is to find the optimum pipe size to meet the backing speed requirement for the diffusion pumpof 400 cfm at 10-2 torr, using the roots blower as the backing pump. This pump is provided in the Examplesdirectory.Step 1) Start a new studyActivate a Series Conductance Study by clicking on thespeed button below the main menu.
The following dialog will be activated:Click once here, then click on theplus button to add a pump model.Or press Ctrl-A to do the samething.© 2011 Professional Engineering Computations216VacTran 3Step 2) Add a roots blower to the studyClick once on the pump list. Add a new pump by clicking on theExamples directory from the file dialog:button. Select the roots blower in the© 2011 Professional Engineering ComputationsConductance Studies217Select by double clicking on the pump name directly, or click once on the pump name and then click on OK.
Thepump is displayed in the pump list below:© 2011 Professional Engineering Computations218VacTran 3Step 3) Add a conductance element to the studyClick once on the conductance list. The Conductance Palette will come to the front.Click once here to bring theconductance palette into view.If the palette is not visible, click on thespeed button under the main menu to bring it to the front. Add a pipeby clicking on the pipe button using the conductance palette as shown below:Click here to add a pipe tothe model.© 2011 Professional Engineering ComputationsConductance Studies219For our example, assume that facility requirements locate the roots pump 25 feet from the diffusion pump.
Enter 25feet in the pipe dialog, and select “feet” units. Note that the diameter field is dimmed, because it will be a variable inthe conductance study.For simplicity, we assume no exit or entrance loss for this pipe and uncheck the corresponding options as shown.For more information on exit and entrance losses see Choosing entrance and exit loss options.© 2011 Professional Engineering Computations220VacTran 3After clicking OK, the pipe will be added to the Conductance Study.© 2011 Professional Engineering ComputationsConductance Studies221step 4) Set the diameter rangeNow set the From: variable field to 2 inches, the To: variable field to 16 inches, and the # Curves field to 8.
TheConductance Study should now look like the figure:step 5) Create Delivered Speed vs. Pressure curveClick on the Delivered Speed button to generate the following curve:Delivered Speed vs PressureSpeed at pump{s) Diam=2.00E+00 In Diam=4.00E+00 In Diam=6.00E+00 In Diam=8.00E+00 InD eliv er ed S peed ( C u F eet/M inute)600Diam=1.00E+01 In Diam=1.20E+01 In Diam=1.40E+01 In Diam=1.60E+01 In50040030020010010-410-31010-2-1Pressure (Torr)100101The top-most curve is that for the pump with no losses.
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