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Watt-Flex® is a Good Fit !!!
Fit
is a critical issue for effective operation of cartridge heaters.
Fit is the difference between the O.D. of the cartridge heater and the I.D. of the bore
into which it is inserted. Wall-to-wall contact provides for conduction heat transfer,
which is the most effective means of moving heat from heater to host metal.
Conventional cartridge heaters make a "point" contact
along their profile as shown in the left illustration below. There
is a layer of air around the heater and inefficient
radiant heating limits effective heat transfer. This cartridge
heater depends on a fairly tight fit for effective heat transfer
... the down side is that it will often not come out of the bore.
The Watt-Flex cartridge heater is inserted as shown in the right
illustration and
expands under load, as shown in the center, to affect conduction
heating on both
sides, doubling the contact surface for superior heat transfer.
The Watt-Flex heater
is manufactures .005" undersized with a tolerance of +/- .002" on
the diameter.
The general rule for fit is to allow .005" for the heater
to expand. Using a 1/2" heater
as an example, it is manufactured at .495". The limit of the
tolerance brings it to .497"
and .005" fit allowance will give us a minimum bore of .502".
Since there must be a tolerance to bore drilling, the bore may be specified as .502 to
.505". The split sheath design will allow the heater to expand into this bore and supply
effective heat transfer and long life.
Factors that influence the specification of the bore are:
- Higher watt density in small diameter, short heaters should
have tighter bores.
- Short, large diameter heaters will not expand as much as
a 15"+
long heater.
- Longer heaters will be more tolerant of over-sized bores.
The fit issue is quite important and, if you are involved with specification of a newly drilled bore, a Dalton Electric
Regional Manager will be glad to assist.
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Friday Funnies
A WELL PLANNED LIFE????
Two women met for the first time since graduating from high school.
One asked the
other, "You were always so organized in school. Did you manage
to live a well planned life? "
"Yes," said her friend. "My first marriage was to
a millionaire banker; my second marriage was to an actor; my third
marriage was to a preacher; and now I'm married to an undertaker."
Her friend asked, "What do those marriages have to do with
a well planned life?"
The response: "One for the money, two for the show, three
to get ready, and four to go."
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