Table of Contents
Advertisement
always the lower of the two. The closed cup represents the contained vapors in-
side the tank and the open cup represents the vapors escaping the tank. Oxygen
and an ignition source will be less available inside the tank.
Environmentally hazardous fluids require special disposal according to applica-
ble federal or local laws after use.
8.1.9
Cost
Cost of bath fluids may vary greatly, from cents per gallon for water to hun-
dreds of dollars per gallon for synthetic oils. Cost may be an important consid-
eration when choosing a fluid.
8.1.10
Commonly Used Fluids
Below is a description of some of the more commonly used fluids and their
characteristics.
8.1.10.1
Water
Water is often used because of its very low cost, availability, and excellent tem-
perature control characteristics. Water has very low viscosity and good thermal
conductivity and heat capacity which makes it among the best fluids for 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 pre-
vent 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.
8.1.10.2
Ethylene Glycol
The temperature range of water may be extended by using a solution of 50%
water and 50% 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 the fluid is very toxic. Ethylene glycol must be dis-
posed of properly.
8.1.10.3
Methanol
Methanol is a relatively inexpensive fluid with a low temperature range. Pure
methanol has a temperature range from its freezing point of about –96 °C to
near its flash point at 54 °C. Vaporization is significant above 25 °C so its use
above this temperature is not recommended. Methanol at low temperatures
tends to condense and absorb water from the air. This is generally an advan-
tage, however, since small amounts of water (less than 11%) mixed with metha-
nol decreases the freezing point. The resulting low temperature capability can
be well below –100°C but the viscosity can become excessive at such low tem-
peratures. The ideal methanol/water solution to be used to –100°C should be
8 General Operation
Bath Heat Transfer Fluid
29
Advertisement
Table of Contents
