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PROTECT YOUR
COOLING SYSTEM AGAINST CORROSION
Taken from "Process Cooling
& Equipment USA, November 2000."
Although attractive in intial price, uninhibited glycols can
lead to maintenance expenses and reduced system life.
An industrial secondary heating and cooling
system's overall performance, longevity and long-term cost may well hinge on the type of heat transfer fluid selected.
Glycol-based heat transfer fluids are widely used in food and beverage cooling systems for several reasons:
- They provide excellent low temperature heat transfer
properties.
- They have a wide operating temperature range.
- They are economical to use.
- They do not require extensive or elaborate handling
precautions.
Glycol-based fluids provide both efficient freeze
point depression when mixed with water and low viscosity to temperatures as low as -29°C (-20°F). In addition,
they are practically odorless and colourless (although they can be dyed to ease detection of system leaks). But,
a glycol-based fluid without inhibitors may not provide adequate protection.
Avoid Uninhibited Glycols
Uninhibited or plain glycols provide freeze and burst protection at a relatively low initial cost. But, freeze
protection is not the only consideration in choosing fluids. Corrosion
presents an ongoing threat to water-based system components.
If left unchecked, heat, oxygen, chloride, sulfates, metallic impurities and other contaminants can increase the
rate of corrosion in a heat transfer system. Corrosion can lead to unscheduled shutdowns, high maintenance expenses
and reduced system life.
Because they lack corrosion inhibitors, plain
glycols can actually increase the threat of corrosion.
Glycols produce organic acids as they degrade, especially when heated. If left in solution, these acids will lower
the fluid's pH. With no corrosion inhibitors to buffer these acids and protect the metals in the system, the corrosion
rate of a plain ethylene or propylene glycol solution can be greater than plain water - a highly corrosive fluid
in its own right.
The industrial inhibitor packages needed are specially formulated to help prevent corrosion in two ways. First,
they treat the surfaces of the metal to make them less susceptible to corrosion. Second, the inhibitors buffer
the organic acids formed as a result of glycol oxidation to keep the fluid from becoming acidic. Thus, inhibited glycol-based heat transfer fluids
provide corrosion protection without reducing a system's heat transfer efficiency by fouling.
Trouble at the Juice Plant
A large juice manufacturer had a problem at one of its main processing plants. Corrosion had seriously damaged
an ammonia chiller, one of five such chillers linked together in a system used for cooling pasteurized juice concentrate,
then freezing prior to final packaging.
The heads on the chiller's evaporator had been almost completely rusted through, and hands full of corroded metal
lay in the bottom of the heads. Damage to the evaporator and its components was so extensive that the entire evaporator
had to be replaced.
Damage was not limited to one evaporator in one chiller. Further examination revealed significant damage from corrosion
in all five chillers in the system, and not just to the evaporators. Corrosion of the evaporator tubes had allowed
ammonia refrigerant to leak into the glycol solution. When the dissolved ammonia in the fluid came into contact
with the copper process equipment, rapid and severe corrosion occurred. The company was forced to shut down the
system for extensive repairs and rebuilding just four years after it had been installed.
The source of the problem was use
of uninhibited USP-grade propylene glycol. The juice manufacturer had decided to run it with its ammonia chillers
after learning that USP-grade propylene glycol is generally recognized as safe (GRAS) for use in foods when used
in accordance with U.S. Food and Drug Administration (FDA) good manufacturing practices (GMP). The company had
reasoned that in addition to saving some modest initial fluid costs, using such an uninhibited fluid would ensure
the safety of its end juice products should the fluid accidentally come into contact with them.
In fact, uninhibited USP-grade propylene glycol is not suitable for use as a heat transfer fluid in food processes.
Because it is uninhibited, it can cause heavy corrosion - enough to do substantial damage to expensive process
equipment. Furthermore, corrosion damage is not restricted to specific parts but can be a problem for the entire
system.
The company decided to replace the USP-grade fluid with inhibited propylene glycol-based fluid. The heat transfer
fluid was formulated with an inhibitor package that provides corrosion protection for most common metals used in
cooling systems, including copper.
Just as important, inhibited fluid is considered chemically acceptable by the U.S. Department of Agriculture (USDA),
and its ingredients are generally recognized as safe by the FDA for use where incidental contact with food is possible.
A conversion was made to a solution of 40% heat transfer fluid and deionized water. The company also put in its
own deionizing system to ensure quality water for the system.
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