General Operation
Heat Transfer Fluid 8
8-5
adequate ventilation system must be used if hazardous or bothersome vapors are
produced.
Warning
Fluids at high temperatures may pose danger from BURNS,
FIRE, and TOXIC FUMES. Use appropriate caution and safety
equipment.
Fluids may be flammable and require special fire safety equipment and procedures. An
important characteristic of the fluid to consider is the flash point. The flash point is the
temperature at which there is sufficient vapor given off so that when there is adequate
oxygen present and an ignition source is applied the vapor will ignite. This does not
necessarily mean that fire will be sustained at the flash point. The flash point may be
either of the open cup or closed cup type. Either condition may occur in a bath situation.
The open cup flash point is measured under the condition of vapors escaping the tank.
The closed cup flash point is measured with the vapors being contained within the tank.
Since oxygen and an ignition source is less available inside the tank the closed cup flash
point will be lower than the open cup flash point.
Environmentally hazardous fluids require special disposal according to applicable federal
or local laws after use.
Cost
Cost of bath fluids may vary greatly, from cents per gallon for water to hundreds of
dollars per gallon for synthetic oils. Cost may be an important consideration when
choosing a fluid.
Commonly Used Fluids
Below is a description of some of the more commonly used fluids and their
characteristics.
Water
Water is often used because of its very low cost, its availability, and its excellent
temperature control characteristics. Water has very low viscosity and good thermal
conductivity and heat capacity, which make it among the best fluids for good control
stability at lower temperatures. Temperature stability is much poorer at higher
temperatures because water condenses on the lid, cools and drips into the bath. Water is
safe and relatively inert. The electrical conductivity of water may prevent its use in some
applications. Water has a limited temperature range, from a few degrees above 0°C to a
few degrees below 100°C. At higher temperatures evaporation becomes significant.
Water used in the bath should be distilled or deionized to prevent mineral deposits.
Consider using an algicide chemical in the water to prevent contamination.
Note
Water used at temperatures greater than 75°C (167°F) may have stability
problems.
Ethylene Glycol
The temperature range of water may be extended by using a solution of one part water
and one part ethylene glycol (antifreeze). The characteristics of the ethylene glycol-water
solution are similar to water but with higher viscosity. Use caution with ethylene glycol
since this fluid is very toxic. Ethylene glycol must be disposed of properly.
Mineral Oil
Mineral oil or paraffin oil is often used at moderate temperatures above the range of
water. Mineral oil is relatively inexpensive. At lower temperatures mineral oil is quite
viscous and control may be poor. At higher temperatures vapor emission becomes
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