A furnace is typically run at temperature between 500 o C- 1200 o C. Because of such elevated temperatures, furnaces require a lot of safety precautions in their normal operations. Because of requirement of better products many times it is essential that heating inside furnace be done under vacuum. Such a furnace is called a vacuum furnace.
Advantages
of vacuum heat treatment of components-
1. No unwanted
carburization takes place.
2. Bright
and smooth surface finish because of non-oxidation of components.
3. Low
distortion due to high temperature uniformity inside the vacuum furnace.
4. Results
are easily reproducible because of defined temperature guidance by load
thermocouples.
5. Complete
documentation possible.
6. Completely
automated procedure.
Vacuum
in furnace not only prevents oxidation of heated material but also allows
controlled cooling under neutral atmosphere. At the same time addition of
vacuum also mandates additional safety precautions to be observed so that there
is no damage to equipment or chance of injury to people operating the
equipment.
Because
furnaces have been in operations since very long time so most engineers are
aware of basic precautions for safe running of the furnaces. About vacuum
system however not much literature is readily available. This blog post
primarily addresses issues related to vacuum system for running the vacuum
furnace safely.
Major precautions are given below-------
1. In a
vacuum system having diffusion pumps, the diffusion pump must never be touched
or opened till it has cooled down to room temperature because during operations
diffusion pump oil is heated to a temperature of about 250 o C. Add to it the latent heat of
vaporization and we have a dangerous situation at hand if mishandling occurs.
2. Leakage
in vacuum systems can result in cold air suddenly rushing into the heated
furnace. To prevent this, following needs to be done.
a) O
Rings and oil seals must be replaced from time to time.
b) Oil
seals and o rings must be smeared with high vacuum silicone grease which prevents damage to these and also provides additional
vacuum sealing.
c) O
rings and oil seals should always be placed properly in the groove provided for
them. Otherwise they can get sucked in under vacuum.
d) Flanges
where o ring are fitted must have good surface finish. A coarse surface finish
may result in gas leaking through the seal.
e) All
joints must be tightened so that o rings /oil seals are squished to about 80%
of their original cross section. Less than this will result in vacuum leak and
greater than this will result in damage to o ring.
f) Good
quality neoprene, viton or silicone o rings should be used.
g) O
rings must never be removed with a screw driver or any other sharp object as
this can damage the groove of o ring.
3. At
the start of the cycle, for a minute or two, outlet of rotary pump throws out oil mist. Either
provision should be made to exhaust this mist outside or a mist eliminator must
be installed.
4. Care
should be taken to change the vacuum oils periodically.
5. Diffusion pump oil must be topped up as soon as it falls below the minimum level. This
prevents over heating of the diffusion pump.
6. In
case leakage is observed then proper leak detection procedure should be
followed. For details on leakage detection please see http://supervacoils.com/leakage-detection-and-prevention-in-vacuum-systems/ .
7. During
heating in a furnace out gassing increases so vacuum pump of sufficient
capacity must be selected. Inadequate pumping capacity of vacuum system will
not be able to remove gases.
8. At
least once in a month furnace must be run on a dry run where empty furnace is
heated to a temperature of 1000 o C for at least 30 minutes under
vacuum.
9. Faulty
vacuum reading can result in accident. So vacuum gauges must be checked from
time to time.
10.
To
prevent back streaming of rotary vacuum pump oil, rotary pumps must be fitted
with solenoid valves.
Solenoid Valve
11. Sequencing
valves in vacuum system must be properly maintained.
12. Moisture
is detrimental to creation and maintenance of vacuum. When furnace is not in
operation, as far as possible furnace doors must be kept closed to prevent
moist air entering the chamber.
13. Using good quality silicone diffusion pump oil rather than hydrocarbon diffusion pump should be used to minimize
the risk of explosion.
14. Outlet water temperature from diffusion
pump cooling coils must be monitored and never allowed to go beyond for 40 o C.
15. If a human is present inside the chamber
and vacuum is switched on the person will die almost instantly. Proper
interlocking and fool proofing must be observed to prevent this at all costs.
16. Always use
silicone diffusion pump oils instead of hydrocarbon/mineral oils for your
diffusion pump as silicone oils are inert and more resistant to process gases
present in the vacuum furnace. Mineral oils are likely to deteriorate or catch
fire when in contact with certain process gases e.g., oxygen.
I
am sure that by observing these precautions (which are primarily about the
vacuum system of the furnace) you will get safe and trouble free operation of
your vacuum furnace.
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Written by: Anshuman Punj.
Go to Supervac Industries Website
