VacTran 3 Manual, страница 8

Gasloads are not part of this simple tool.To access this function from the tool buttons, use the following short cut:© 2011 Professional Engineering ComputationsFirst lookVolume area calculatorThis tool calculates volume and area for common shapes used as vacuum vessels.To access this function from the tool buttons, use the following short cut:© 2011 Professional Engineering Computations4748VacTran 3Special cases:4-way intersectionOnly includes the intersection of four pipes (also called a Steinm etz solid)Volume = 16/3 r3Surface Area = 16 r2© 2011 Professional Engineering ComputationsFirst look4-way crossIncludes the intersection of four orthogonal pipes and (A) length of pipe from the intersection pointVolume = Volume of 4 cylinders (A long) - Volume of 4-way intersectionSurface Area = Surface Area of 4 cylinders (A long) - Surface Area of 4-way intersection© 2011 Professional Engineering Computations4950VacTran 36-way intersectionOnly includes the intersection of six pipes (also called a Steinm etz solid)Volume = (16 - 8*20.5) r3Surface Area = 3 * (16 - 8*20.5) r2© 2011 Professional Engineering ComputationsFirst look6-way crossIncludes the intersection of six orthogonal pipes and (A) length of pipe from the intersection pointVolume = Volume of 6 cylinders (A long) - 2 * (Volume of 6-way intersection)Surface Area = Surface Area of 6 cylinders (A long) - 2 * (Surface Area of 6-way intersection)© 2011 Professional Engineering Computations5152VacTran 3Pump curve digitizerPump models can be created by copying and pasting pump curve images into this tool, and then digitizing the curvepoint by point after setting up the scaling points.

This is considerably faster than trying to read points and enteringthem manually.To access this function from the tool buttons, use the following short cut:© 2011 Professional Engineering ComputationsFirst lookBasic math calculatorThis tool provides basic math and trigonometric functions.To access this function from the tool buttons, use the following short cut:© 2011 Professional Engineering Computations5354VacTran 3Graph galleryRecent graphs are saved on demand or automatically, at the current user settings (picture format and resolution).To access this function from the tool buttons, use the following short cut:© 2011 Professional Engineering ComputationsFor new vacuum technologists555For new vacuum technologistsThis section is for people who are new to vacuum technology.

It covers basic concepts, but is not intended to be acomprehensive tutorial. Please seek further direction from the many excellent texts, journals, and classes availablefrom the American Vacuum Society, local institutions or recognized vacuum companies. Experienced people canskip this section.See also:The American Vacuum SocietyThe universe of vacuumBasic goals of vacuum systemsBasic steps vacuum system designSelecting design marginsTraditional calculation methodsHow VacTran helps5.1The American Vacuum SocietyThe American Vacuum Society, now simply called the AVS, is a national organization dedicated to fostering animproved understanding of vacuum, materials processing, and related technologies. It encourages the exchangeand dissemination of information pertinent to these fields through a variety of events and activities such as technicalshort courses, symposiums, equipment exhibitions, and newsletters.The AVS has local chapter organizations throughout the US, teaching short courses at many times and locationsduring the year.

These courses are highly recommended for initial or advanced study in vacuum technology, and willenhance the effective use of VacTran.For more information,visit http://www.avs.org or contact the AVS,Telephone (212) 248-0200,Fax (212) 248-0245,Email avsnyc@avs.org5.2The universe of vacuumVacuum is a relative term, and usually refers to a gaseous environment whose pressure is less than atmosphericpressure. Typical applications using vacuum systems will encounter gas densities that vary 10 orders of magnitudeor more.

Outer space is a vacuum, except where massive bodies of matter, such as planets, create a gravitationalfield great enough to capture a relatively dense atmosphere. Achieving a vacuum in an enclosed, sealed container,or vacuum vessel, is performed on Earth routinely in a variety of industries. To those who are new to thetechnology, vacuum system design can appear to be an exercise in black magic; particularly as practiced by thosewho use rules of thumb or gut feel to design systems. It’s pretty amazing how many expensive and inefficientpumping systems are still designed by in-house experts using the “this worked last time” method.Once one understands the basic phenomena that affect vacuum systems, and the governing equations that helppredict performance, vacuum system design can look pretty reasonable.

However, those new to vacuum systemdesign should be forewarned that to some extent, this is an inherently inexact discipline. Although much has beendone over the past 50 years to mathematically characterize the phenomena of low-pressure flow, the real world ofvacuum technology is plagued with uncertainties, which must be characterized or minimized to achieve predictableresults. Dirt, grease, humidity, virtual leaks, unique geometries, and a host of other culprits will conspire to rendercalculations useless, if not fully considered.

Proceed with caution.© 2011 Professional Engineering Computations565.3VacTran 3Basic goals of vacuum systemsBasic goals of vacuum systemsVacuum systems are designed to achieve of one or both of the following goals. We want to......Remove an initial gas volume from a vacuum vessel, faster than new gas enters, to achieve a targetpressure in a required time period.Or...Remove gas from a vacuum vessel at a rate equal to the rate it enters, maintaining an operatingpressure that is acceptable to the vacuum process.VacTran has been designed to mathematically simulate these situations, forming the basis of sizing vacuumpumping systems.Let's take a closer look:The first goal is typically sought when pumping a gas (typically air) out of a vessel the first time.

Forexample, start with a 100-liter vacuum vessel filled with a gas that must be removed. It would appearto be simple matter of using a 25 liters/second vacuum pump to take exactly 100/25 = 4 seconds toreach a pressure of zero Torr. After all, this logic would be perfectly valid for pumping water out of adrum, so it would seem reasonable for pumping gas out of a container.Not exactly.We never reach the target pressure of zero Torr. Not in 4 seconds, and not ever.

There are three main reasons forthis:1.Pump speed is not constant2.Pump speed is degraded by losses3.More gas is always coming in.© 2011 Professional Engineering ComputationsFor new vacuum technologists571) Pump speed is not constantPump Speed vs PressureP u m p S p e e d (L it e rs / S e c o n d )25201510510-210-1101001Pressure (Torr)102103The volumetric speeds of many pumps vary non-linearly with inlet pressure. Our pump speed rating of 25 liters/second is most likely the peak pumping speed at the optimum pressure for this type of pump. Since our pump'sinlet pressure varies continuously as we pump down the vessel, the pumping speed will probably also vary. Mostpump manufacturers publish a curve for each pump they sell, showing the variation of pumping speed with pressure.A typical pump performance curve might look like the one shown to the right. We won't pump our 100-liter vesseldown to zero in 4 seconds partly because the actual pumping speed is less than 25 liter/second during part of thepump down range.© 2011 Professional Engineering Computations58VacTran 32) Pump speed is degraded by lossesDelivered Speed vs PressureSpeed at pumpDelivered speedD eliv er ed S peed ( Liter s /S ec ond)25201510510-210-1101001Pressure (Torr)102103Although our pump is nominally rated at 25 liters/second, it is probably connected to the vacuum vessel through avalve, and perhaps a trap and some piping.

Some plumbing is usually necessary because pumps often need to besome distance from the vessel due to vibration or cleanliness considerations. Traps, devices that inhibit the flow ofpump oil toward the vessel, are sometimes required to minimize vessel contamination. Valves isolate the pumpfrom the vessel for a number of reasons. Each element that connects the pump to the vessel is called aconductance element that provides resistance to gas flow. Each conductance element that the pumped gas hasto travel through contributes to a conductance loss in effective pumping speed. This reduction results in adelivered speed at the vessel. Delivered speed at the vessel is always less than the rated speed of the pump,because it accounts for the conductance loss.