From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Newsgroups: sci.chem
Subject: Re: What type of OIL is used in a hot oil bath?
Date: Fri, 15 May 1998 10:16:32 GMT

"Andrew Munn" <awmunn@oakland.edu> wrote:

>    I understand that for a water bath to be used beyond 100C it has to be
>filled with oil instead... (some silly property about water at 100C).  Can
>anyone tell me what type of oil is used?

Depends what temperature you wish to operate at, for higher temperatures
( up to 315C ) then  silicone oils - such as Dow Corning 550 are available,
but are fairly expensive, but slightly cheaper are the high temperature
industrial heat transfer fluids available from oil companies, which can
be eitther refined hydrocarbons or synthetic products.

For lower temperatures, then several heat transfer fluids are available.
In most cases they will work for a short time at higher temperature, but
if heated regularly they will decompose and their flash point will
lower dramatically - creating a fire hazard. One point that is often
overlooked is the difficulty to clean the oil bath afterwards - silicones
can be a pain...

Some lower temperature choices include:-
1. Low viscosity hydrocarbon lubricant base grades with additional
   antioxidant ( such as Topanol O ) added to increase life - which
   the low cost option, and good for up to 200C. These have the
   advantage of a fairly low viscosity at ambient temperature, which
   is kind to pumps and circulators. It's important to ensure water
   does not leak into the bath, especially when cold, as it will
   sit on the botttom and can vaporise almost instantly when it
   encounters hot oil = instant stteam explosion.
2. Synthetic compounds or organic molecules with relatively high
   boiling points, from glycerine to esters and diesters to
   glycols. Usually these degrade rapidly with sustained use, but
   are OK for short term heating. Standard textbooks usually list
   several compounds that can be used, but toxicity and aerosols
   have to be considered. Some have the advantage of being water
   miscible.
3. Triglyceride oils, as used for cooking, but they do degrade
   and colour fairly rapidly, have relatively high low-temperature
   viscosity, and soon pong badly.

I prefer using a low viscosity hydrocarbon base grade with
additional Topanol 0 ( ICI product ), it's cheap and kind to
the circulator motors because of its low viscosity. Don't use
engine lubricants if circulating the fluid, because they are
too viscous at low temperatures - even synthetic like Mobil 1,
causing premature failure of circulators.

I've tended to move towards aluminium heating blocks on
hotplates - they work well up to 350C, and are far less
messy. There are also the fluidised bed alternatives,
Woods metal, and graphite flake types of heating baths
as well.

              Bruce Hamilton