amplifiers previously described in the amateur literature
have used water as the cooling fluid. This is probably
because water has the highest specific heat capacity of
all commonly available fluids and it is inexpensive and
easy to get. The specific heat rating (the amount of heat
needed to raise the temperature of one gram of a substance
by one degree) of water is 1.0 and other cooling fluids
considered here have lower ratings. However, a closer
look at using water for the cooling fluid shows that there
are tradeoffs necessary in the implementation.
can be conductive. This is a problem when using water
to cool a tube anode that is energized at a very high
voltage. Distilled or de-ionized (they are not the same)
water helps, but the water must routinely be replaced
because it becomes contaminated by ions leached from the
cooling system hardware or from the atmosphere. Because
of the molecular structure of water, it has a good ability
to form strong bonds to dissolved ions.
high voltage leakage current detector will indicate when
the dissolved ion count becomes too high and it is time
to change the water, but this requires additional hardware.
also supports life. Of course this means that things
can grow in the water. The common green algae that is
visible in aquariums is a familiar reminder of this. Using
bleach or other simple chemicals as a remedy is a big
mistake because they increase the conductivity of the
water and make it unsuitable. Water is an efficient and
inexpensive cooling fluid, but the continuous replacement
and maintenance requirements justifies a look for a reasonable
liquids like transformer oil have very high dielectric
strength. The tradeoff here is the specific heat of these
types of fluids is lower than water with about a .4 to
.5 value. However, transformer oil can sometimes be hard
to obtain in small quantities of a couple of gallons and
the increased viscosity over water implies it is harder
oil is not a good alternative because it can become contaminated
suitable alternative fluid that is easily obtained, inexpensive,
has high dielectric strength, high flash point, and easy
to use is common automotive automatic transmission fluid
(ATF). The MSDS information for Pennzoil
Dextron/Mercon ATF reveals these properties:
strength: 35 kV
Flash point: 399 degrees F
Weight: 7.25 lbs/gal
biggest disadvantage of ATF is a lower specific heat rating
when compared to water. This means it is a less efficient
cooling fluid although this is not a hard problem to overcome.
A more efficient cooler and/or larger pump can make up
for the cooling efficiency difference of the two fluids.
by design also contains friction additives that may degrade
pump seals over the long term. This potential problem
can be eliminated through the correct choice of pump type.
A magnetic drive pump has no seals.
What about transformer oil?
(polychlorinated biphenyl) was banned in the USA and Canada
in 1976. This product was of course used in high voltage
transformers, capacitors, and heat exchangers. Some modern
replacements for PCB are fluids made with terphenyl. Their
properties include high temperature and good thermal stability.
66 transformer oil appears to be widely used and
seems to be an excellent transformer/heat exchanger oil.
I suggest that you avoid the LT version since it has a
low flash point of about 134 degrees F. The 66 version
has a rating of 363 degrees F. The specific heat rating
is .409 which is similar to many insulating oils.
would choose this fluid for an oil-cooled amateur tube
amplifier or dummy load application provided it was reasonably
priced and was easy to get.