Page 282 - Fluid mechanics, heat transfer, and mass transfer

CONVECTIVE HEAT TRANSFER 263

➢ No regulatory constraints. Preferably it should be have a 50/50 or 60/40 glycol to water ratio. Ethylene
of food grade quality and satisfy the criteria of glycol is extremely toxic. Food grade propylene
incidental food contact. glycol/water mixtures can be used in a single-walled
➢ Favorable economics. heat exchanger. Most glycols deteriorate at very high
temperatures. One must check the pH value, freezing
. What are the commonly used heat transfer fluids in the
chemical process industry, other than steam? point, and concentration of inhibitors annually to
determine whether the mixture needs any adjust-
& Heat transfer fluids include glycol–water solutions,
ments or replacements to maintain its stability and
hydrocarbon oils, silicone oils, molten salts, and
effectiveness.
liquid metals.
& Hydrocarbon Oils: Hydrocarbon oils have a higher
& Hydrocarbon oils are very commonly used fluids in
viscosity and lower specific heat than water. These
the chemical process industries, either petroleum-
oils are relatively inexpensive and have a low freez-
based or synthetic, which are available for use over
ing point. The basic categories of hydrocarbon oils
a wide range of operating temperatures. Some are
are synthetic hydrocarbons, paraffin hydrocarbons,
suitable for low-temperature operation and others for
and aromatic refined mineral oils.

good thermal stability at temperatures up to 400 C.
& Synthetic hydrocarbons are relatively nontoxic and
& Proprietary heat transfer fluids like Dowtherm and
require little maintenance. Paraffin hydrocarbons
Therminol fluid products.
have a wider temperature range between freezing
. What are the particularly important requirements for and boiling points than water, but they are toxic.
low-temperature heat transfer fluids? Aromatic oils are the least viscous of the hydrocar-
& Low freezing point and low viscosity. bon oils.
. Name some low-temperature heat transfer fluids? & Aromatics: Aromatic hydrocarbons such as diethyl-
& Air: Air will not boil or freeze and is noncorrosive. benzene are very common low-temperature heat
However, it has a very low heat capacity and tends to transfer fluids in the temperature range from 70 to

leak out of equipment. 260 C. Their low-temperature heat transfer charac-
teristics and thermal stability are excellent. However,
& Water: Water is nontoxic and inexpensive. With a
these are not nontoxic. They have a strong odor that
high specific heat, and a very low viscosity, it is easy
can be irritating to the personnel handling them, and
to pump. Unfortunately, water has a relatively low
very few aromatic compounds have freezing points
boiling point and a high freezing point. It can also be
lower than 80 C. Hence, these are used above

corrosive if the pH (acidity/alkalinity level) is not
70 C in closed airtight systems in chemical pro-

maintained at a neutral level. Water with a high
cessing and industrial refrigeration.
mineral content (i.e., hard water) can cause mineral
deposits to form on heat transfer surfaces and reduce & Aliphatics: Paraffinic and isoparaffinic aliphatic
heat transfer rates. hydrocarbons are used in some low-temperature
systems. Many petroleum-based aliphatic com-
& Glycol–Water Mixtures: Ethylene and propylene
pounds meet the criteria for incidental food contact.
glycol are antifreezes and used as heat transfer fluids
These fluids do not form hazardous degradation by-

in the temperature range of 121–40 C.
products, and most have an insignificant odor and are
& Trichloroethylene (TCE): Has also been used
nontoxic in case of contact with skin or ingestion.
extensively in the past because of its low viscosity
& Because of their high viscosity at low temperatures,
at low temperatures and nonflammability. Glycols
these fluids are not very common in low-temperature
and TCE are inexpensive, but they have several
applications. Also, the thermal stability of aliphatic
limitations. The most important drawback is the
compounds is not as good as aromatic compounds.
environmental concerns associated with their use.
Ethylene glycol and TCE are considered toxic, so & Some of the isoparaffinic fluids (with 12–14 carbons)

they cannot be used in many processes. Propylene can be used from 60 up to 150 C. These fluids are
glycol, although nontoxic, has very high viscosity at preferred in food and pharmaceutical applications
low temperatures, so it is normally used at tempera- where toxicity is a major issue.
tures above about 23 C. Glycols also degrade over & Refrigerants/Phase Change Fluids: Refrigerants

time and form glycolic acid, which can contribute to are well-known examples of heat transfer fluids.
the corrosion of metals in the system. These are commonly used as the heat transfer fluid
& Glycols are usually added towater to control freezing in refrigerators, air conditioners, and heat pumps.
point and viscosity of water. Glycol/water mixtures They generally have a low boiling point and a high