The universe of vacuum
Vacuum is a relative term, and usually refers to a gaseous environment whose pressure is less than atmospheric pressure. Typical applications using vacuum systems will encounter gas densities that vary 10 orders of magnitude or more. Outer space is a vacuum, except where massive bodies of matter, such as planets, create a gravitational field 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 the technology, vacuum system design can appear to be an exercise in black magic; particularly as practiced by those who use rules of thumb or gut feel to design systems. It’s pretty amazing how many expensive and inefficient pumping 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 help predict performance, vacuum system design can look pretty reasonable. However, those new to vacuum system design should be forewarned that to some extent, this is an inherently inexact discipline. Although much has been done over the past 50 years to mathematically characterize the phenomena of low-pressure flow, the real world of vacuum technology is plagued with uncertainties, which must be characterized or minimized to achieve predictable results. Dirt, grease, humidity, virtual leaks unique geometries, and a host of other culprits will conspire to render calculations useless, if not fully considered. Proceed with caution.
Once one understands the basic phenomena that affect vacuum systems, and the governing equations that help predict performance, vacuum system design can look pretty reasonable. However, those new to vacuum system design should be forewarned that to some extent, this is an inherently inexact discipline. Although much has been done over the past 50 years to mathematically characterize the phenomena of low-pressure flow, the real world of vacuum technology is plagued with uncertainties, which must be characterized or minimized to achieve predictable results. Dirt, grease, humidity, virtual leaks unique geometries, and a host of other culprits will conspire to render calculations useless, if not fully considered. Proceed with caution.
Basic goals of vacuum systems
Basic steps in vacuum system design
For certain small laboratory applications, where cost and performance of the vacuum system are not major constraints, vacuum systems are often bolted together with available components, with very little thought to design tradeoffs. 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 following objectives are usually sought:
• Select the appropriate pump(s) for the job. Determining the kind and
number of pumps required will depend on a large number of often-conflicting
requirements, including:
Reliability
Cleanliness
Start up time
Heat generation
Target pressure
Allowable space
Mounting position
Vibration and noise
Power consumption
Control requirements
Ambient temperature
Gas load constituents
Initial capital expense
Corrosive environments
Effective pressure range
Required pump down time
Decommissioning/disposal
Regeneration requirements
Maintenance requirements
Operation in a magnetic field
Electromagnetic interference (EMI)
Operation in a radiation environment
The relative importance of the above requirements will depend on the
particular installation. Although they will 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 pump should be connected to the vacuum vessel through
a series of elements, which conduct the pumped gases most 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 biggest volume 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 rapid characterization and evaluation of system design alternatives.
• Select the appropriate pump(s) for the job. Determining the kind and
number of pumps required will depend on a large number of often-conflicting
requirements, including:
Reliability
Cleanliness
Start up time
Heat generation
Target pressure
Allowable space
Mounting position
Vibration and noise
Power consumption
Control requirements
Ambient temperature
Gas load constituents
Initial capital expense
Corrosive environments
Effective pressure range
Required pump down time
Decommissioning/disposal
Regeneration requirements
Maintenance requirements
Operation in a magnetic field
Electromagnetic interference (EMI)
Operation in a radiation environment
The relative importance of the above requirements will depend on the
particular installation. Although they will 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 pump should be connected to the vacuum vessel through
a series of elements, which conduct the pumped gases most 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 biggest volume 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 rapid characterization and evaluation of system design alternatives.
Selecting design margins
All 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 component design beyond what is required for operation under perfect or ideal conditions. Since such conditions don't exist in the real world some judgment is required as to how much extra capability is needed to account for known and unknown variances from the ideal. In vacuum system design, extra pump capacity is sometimes added to a higher risk design (perhaps many unknowns) so that unanticipated gas loads or pump inefficiencies can be accommodated. Other ways of achieving extra margin include provision for extra pumps on the vessel, or over sizing 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 a purchase requisition for a pumping system. Experience with similar existing systems is often valuable for comparison. Judgment is required, specific to each situation.
consequences of failure. Margin is the additional capability or capacity of a system or component design beyond what is required for operation under perfect or ideal conditions. Since such conditions don't exist in the real world some judgment is required as to how much extra capability is needed to account for known and unknown variances from the ideal. In vacuum system design, extra pump capacity is sometimes added to a higher risk design (perhaps many unknowns) so that unanticipated gas loads or pump inefficiencies can be accommodated. Other ways of achieving extra margin include provision for extra pumps on the vessel, or over sizing 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 a purchase requisition for a pumping system. Experience with similar existing systems is often valuable for comparison. Judgment is required, specific to each situation.
How VacTran helps
VacTran automates the calculation effort, organizing the critical vacuum system parameters in a simple, modular format, in any common units of measure. VacTran allows the engineer to focus efforts on design/analysis decisions rather than on distracting and tedious hand calculations.
The minimum VacTran vacuum system contains one pump connected through one conductance element to a vacuum vessel with a fixed volume. Of course, more pumps and conductances can be added to the model. Once such a configuration is defined, pump down time can be calculated and graphed. If the results are undesirable, parameters are interactively changed, and calculations are repeated within seconds.