|
Question: Why does my new vacuum pump not produce the same vacuum as listed on its data sheet?
Answer:
There are many issues that can prevent a new pump from obtaining its' ultimate pressure. Assuming the pump is new and just recently installed, the most common cause is a small air leak somewhere in the pumps fore line/chamber. Quite simply, if the air leakage coming into your vacuum system is greater than your pumps' ability to remove it, then you will have trouble maintaining your desired pressures. As an example, If the pump is operating at 10.13 mBar and you have a 1 cfm atmospheric (1013 mBar) air leak, the air entering the pump's fore line will expand proportionally (100x) to it absolute pressure or 100 cfm. If the vacuum pump does not have enough pumping capacity (large enough) to handle this additional air in-leakage, it will not be able to maintain the vacuum level you desire. There are many more reasons for loss of vacuum-Call U.S. Vacuum for assistance.
 |
Question: If I have a leak, how can I find it?
Answer:
When dealing with vacuum leaks, you must first determine if the leak is a "Real" or "Virtual" leak. A real leak is one that is coming into the vacuum chamber from an outside
source. A virtual leak is a gas load evolving from inside the vacuum system from outgassing. Outgassing comes from any liquid (water, solvents, etc.) that is vaporizing inside the vacuum
chamber either off the chamber walls or out of a product/substance placed into the vacuum chamber for processing. A simple means of determining if you have a real or virtual leak is to
do a "rate of rise test". To begin, simply reduce the pressure in the vacuum chamber to its' maximum obtainable vacuum level. With a calibrated/highly accurate Torr gauge, measure
the rise in pressure over time in the chamber. If the pressure eventually rises to atmospheric pressure, then the leak is real. If the pressure rises and stops at some pressure below
atmospheric in a 24 hour period, then the leak is virtual.
For real leaks the use of a Helium leak detector or ultrasonic leak detector maybe used
depending on what size of leak you are looking for. Also, you can check for real leaks by spraying a small amount of acetone around the suspect leak areas, when the acetone enters
the vacuum system there will be a spike in the pressure you will see on your vacuum gauge.
Virtual leaks can also be verified by measuring the vacuum in the system with a Mcleod
gauge (partial pressure gauge). Since a Mcleod gauge can only measure permanent gases (Air, nitrogen) and not vapors (water vapor/solvents) then a low pressure reading with a
Mcleod gauge and a higher pressure reading with a total pressure gauge (Torr or Thermocouple gauge) will indicate a virtual leak. Conversely, a high pressure reading on
both the Mcleod & Torr/Thermocouple gauge will indicate a real leak.
|
Question: How is vacuum measured and what is a Torr, Micron, mm HG.
Answer: Basically, one standard atmosphere at standard conditions will support a column of
mercury 29.92" high. This is where the linear measurement in vacuum comes into play. 29.92" can be also measured in mm Hg (760mm = 29.92") and microns (760,000 microns =
760 mm = 29.92"). Depending on what vacuum level you require, you will use a different unit of measure for the vacuum measurement. When measuring vacuum below 1 micron, we go
to scientific notation (Example: 1 x 10-3 mm Hg)
|
Question: What is the difference between SCFM & ACFM?
Answer:
SCFM stands for Standard Cubic Feet per Minute. SCFM is the air volume measured at "standard conditions" which refers to atmospheric pressure (760 m Hg), at sea
level elevation and a temperature of 60 Deg F. ACFM stands for Actual Cubic Feet per Minute and represents air volume pumped at the pressure, elevation and temperature your
particular system is operating at. Many vacuum pump performance curves shows ACFM to indicate the actual volume of process gas pumped at various inlet pressure to the pump.
|
Question: What is an Absolute vacuum gauge?
Answer:
An absolute pressure gauge is one that will measure your vacuum system without regard to and independent of local barometric pressure. Many dial (Bourdon) gauges and
electronic Transducers reference local barometric pressure as their base measurement. However, since these devices are calibrated at SEA LEVEL conditions, operation of these
devices above sea level will cause an erroneous reading. Either the gauge/transducer must be recalibrated for the higher elevation use or an absolute pressure gauge would need to be used.
|
Question: What is a Torr gauge?
Answer:
A Torr gauge is an absolute pressure gauge and operates on the principle of an altimeter. The Gauge case is evacuated by the vacuum from the process and exerts a
negative pressure on a hermetically sealed capsule. The lowering of the pressure in the gauge case causes the capsule to expand thereby causing the gauge movement to turn the
pointer. The Torr gauge is highly sensitive and accurate in the lower pressure regions (0-100 mm Hg).
|
Question: What Is the best vacuum pump for my application?
Answer: There is no one vacuum pump that is best for all applications. However, there are
some general guidelines to remember for your selection.
Oil Lubricated Rotary Pumps are used in applications requiring fairly deep vacuum (< 1
mmHg) and pumping relatively clean gases (Air/N2). Oil lubricated pumps are available in
Single stage & Dual stage depending on what vacuum level you need. Additionally, all the oil
lubricated pumps are available in Belt drive or Direct drive configurations. Belt drive is
preferred in applications where pump longevity and durability is desired because of the low pump rpm (<700 rpm) and their high oil holding capacity which also guards against
premature wear from oil breakdown. Direct drive pumps are preferred because of their low cost, compactness and portability.
Dry Vane pumps; are used when a pump is required that does not require lubricating oil
because of the objection to oil vapor discharge from the pump and filling/disposal issues with
oil. Rotary vane dry pumps however are capable of only maximum vacuum of approximately 25" Hg and can only pump clean DRY air. Any presence of moisture in the gas being
pumped can lead to the pump rusting because of the absence of lube oil.
Rotary Screw Dry Pumps are used in applications where a high vacuum is required (up to 0
.010 mm Hg) and the process gas is not compatible with lubricating oil in oil sealed rotary pumps. These pumps are fairly expensive and are used where a lubricated oil sealed pump
or liquid ring pump is not desired. Call U.S. Vacuum for more details.
Liquid Ring Pumps are used in applications where the process gas may contain a sizable
amount of condensable vapors (water, solvents, acids, etc.) that will react negatively with the
lubricating oil in Rotary Vane pumps, thereby causing pump damage. Being that a liquid ring
pump is a centrifugal unit, the sealing medium can be water, oil or any other fluid compatible
with the process. Liquid ring pumps are relatively inexpensive and can use any sealing fluid (water, oil, ethylene glycol, solvents, etc.) that is compatible with the process.
|
Question: I am thinking of purchasing a dry pump for my needs.
How do I know if it is right for me?
Answer: Dry pumps are useful for many applications, but some precautions must be taken
when selecting them. Typically, dry pumps have uncoated internal parts made of cast or ductile iron. Being that these uncoated components can easily rust and oxidize in the
presence of moisture or corrode in the presence of stronger reactive gases, care must be
taken to protect the pump. Since dry pumps do not have a sealing fluid (oil) between it's
moving internal parts, like rotary oil sealed pumps, dry pumps must rely on very tight internal
clearances (0.001"-0.003") to obtain their low ultimate pressures. If the uncoated internal
parts start to corrode/oxidize from the presence of condensable vapors the formation of rust/flaking deposits can cause the pump to lock-up (because of the pumps' very tight
operating tolerances). Proper shut-down procedures incorporating purge gases (N2) can help in removing these "wet" type of gases. Additionally, any process that will allow the
incoming gas stream to accumulate on the internal rotors/screws of the pump in the form of
crystals or paste can also cause clearance issues…steam cleaning would be required in these applications. For additional assistance on Dry pumps call U.S. Vacuum.
|
Question: What is "gas ballast"?
Answer:
A gas ballast is a regulated in-bleed of a dry gas (usually Air/Nitrogen) into the
compression portion of the pumping cycle of the vacuum pump. The gas acts as a stripping
agent that will saturate with the contaminating vapors present in the pump and expelled out
the discharge of the pump. Gas ballasts are usually installed as a standard component on all
oil lubricated rotary vacuum pumps to aid in the removal of condensable vapors from the vacuum pump oil.
U.S. VACUUM OFFERS ONSITE TAILORED
VACUUM TRAINING SEMINARS FOR YOUR COMPANY
CALL FOR INFORMATION REGARDING
OUR TRAINING PROGRAM: 888-416-7366
|
