|
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
based pumps are quiet and very 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.
|
|
- Piston
based 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. If you are using a piston style pump, a good quality vacuum filter is required.
|
|
- Oil
bath pumps are a bit louder and have a tendency to emit
a light plume of oil into the air. For some users, this can
be a problem. 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.
|
|
- Rotary
vane pumps are available in oil-less and lubricated styles.
This type of pump gets 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 are using a rotary vane pump, you absolutely must use a good quality filter. Additionally, you'll need a relay to do the high current switching from the vacum controller. Why? Because rotary vane pumps draw up to 3 times more amperage at startup than the name-plate shows. Most rotary vane pumps draw 6+ amps while running. This equals 18+ amps at startup which is too much for the vacuum controller to handle on its own. These pumps are notoriously difficult to adapt to vacuum press use due to electrical issues.
|
|
- 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.
|
|
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.
|
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.
|
|
Welcome
