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.
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 actually can 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 used in Dowfrost propylene glycol-based and Dowtherm ethylene glycol-based heat transfer fluids from Dow Chemical Co., Midland, MI, 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. Table 1 compares the corrosion effects inhibited and uninhibited glycol-based fluids and plain water may have on metals commonly found in cooling.
Trouble at the Juice Plant
A large juice manufacturer had a problem at one of its main juice 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 it prior to final packaging.The heads on the chiller's evaporator had been almost completely rusted through, and handfuls 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.
But, 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 generally is recognized as safe 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, 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 Dowfrost inhibited propylene glycol-based fluid. The heat transfer fluid is formulated with an inhibitor package that provides corrosion protection for most common metals used in cooling systems, including copper.
Just as important, Dowfrost 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.
