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Project: E-Vac Pump Driven System
Selecting
A Vacuum Pump
Speed
and Maximum Pressure
 Vacuum
pumps are often rated on the flow capacity which is stated as CFM (cubic
feet per minute). It references the speed at which the pump is capable
of moving or removing air and is most often measured at zero
pressure. A pump rated at 1 CFM will be fine for flat panel work on
vacuum bags up to 4' x 4'. For bags up to 4' x 8' a 3 CFM or greater
pump is needed.
If your
vacuum press will be pulling down curved veneer forms or bent laminations,
the general rule of thumb is that the pump must be three times faster.
This means that a 3 CFM or greater pump is best for vacuum bags up to
4' x 4' for this type of use. Why? Because there is, on average, 3 times
more free air inside of a bag containing a bent lamination than there
is inside a bag containing a flat panel.
Vacuum
pumps are also rated by their maximum achievable vacuum pressure at
sea level which is often expressed as inches of mercury or "Hg".
For vacuum veneering, the minimum acceptable level of vacuum is 18".
The ideal vacuum pressure though is 21" of Hg. At the high end,
the maximum level of vacuum pressure for veneer work is 25.5" of
Hg. Anything over this amount is not only overkill, it's also harder
on the pump and once in a while, extreme vacuum levels can cause the
veneer to develop small pustules of glue on the veneer face.
Type
of Vacuum Pumps
The
next consideration when building a pump based vacuum press system is
the pump style. Here are the some of the options:
- Diaphragm pumps are very quiet and durable. This type of pump is oil-less and usually has a small footprint. They are ideal pumps for a vacuum system. Typical CFM rating is 1 to 3.
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- Piston pumps are not as quiet as diaphragm pumps, but are just as durable. They are almost always oil-less and also work well for a vacuum press. Typical range of CFM is 2 to 5.
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- Oil bath pumps are less noisy than piston pumps but not as quiet as diaphragm pumps. This type of pump has a tendency to emit a plume of oil into the air. For most users, this can be a huge issue that causes problems with the finish that is applied to the veneered project or any other unfinished project in the vicinity of the pump. This type of pump requires occasional oil changes and can draw a large amount of amperage. Usually, these pumps range from 3 to 6 CFM.
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- Rotary vane pumps are available in oil-less and lubricated styles. They are generally maintenance free but they get very hot during use. This can create a serious issue if the thermal protection circuit kicks on while your press is in use. It's a good idea to have a standby unit on hand in the event that the rotary vane pump overheats. Typical CFM range for this style of pump is from 5 to 20. If you choose this style pump, you may need a relay to do the high current switching for the vacum controller. These pumps are notoriously difficult to adapt to vacuum press use due to electrical issues.
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- Refrigerant compressors can also be used for limited runs of vacuum press work. They are quiet, but slow to pull a full vacuum. Typical CFM is less than 1.
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Vacuum
Pump Manufacturers
The most common names in vacuum pump apparatus are Gast
Manufacturing and Rietschle
Thomas (pronounced 'rich-ley thomas'). Both companies offer very refined pumps in a wide range of models. Other manufacturers
include Alcatel, Edwards, Sargent, Welch, Busch and Leybold. These particular manufacturers offer high end, industrial, or scientific-use vacuum
pumps.
Restart
Pressure
 The
next consideration is for a pump based vacuum source is "restart"
pressure. Many smaller pumps have a zero pressure restart rating. This
means that if you turn off the vacuum pump and turn it on again without relieving the back pressure, the
pump will
not start. Some rotary vane vacuum pumps and refrigerant compressors
can restart regardless of the pressure in the system. The good news
is that there is a simple fix that will allow any vacuum pump to work
in a cycling mode. There's more about that on the next page.
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Testing
Your Vacuum Pump's Restart Pressure
 1.
Plug up the air intake on the pump (I use my finger).
2. Turn the pump on for a few seconds to let it build up full
pressure.
3. Turn the pump off and then turn it on again.
Did
it restart without hesitation?
If it did, you won't need to unload the vacuum pressure using
a sub-reservoir and Mac valve as described on the next page.
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Welcome
