Selecting a Heat Transfer Fluid
- Fewer refrigerant leaks due to
substantially less refrigerant piping.
- As much as 75 to 80% reduction in the primary refrigerant charge.
- Fewer service calls.
- Stable process temperature.
But, if the heat transfer fluid is improperly selected or poorly maintained, it will reduce the system's efficiency, causing a loss in production and revenue. Fortunately, the process engineer has several options to choose from when selecting secondary coolants in operating temperature ranges from -170 to 32°F (-112 to 0°C). They include:
- An aqueous solution of organic compounds such as alcohols or glycols. These are water solutions of various concentrations including methanol/water, ethanol/water, ethylene glycol/water and propylene glycol/water.
- An aqueous solution of inorganic salts such as sodium chloride and calcium chloride.
- Chlorinated or fluorinated hydrocarbons including methylene chloride, trichloro-ethylene, R-11, hydrofluoroethers and perfluoropolyethers.
- Hydrocarbon-based fluids, including aliphatic, aromatic and terpene.
- Freezing point.
- Viscosity at low temperatures.
- Thermal properties (specific heat and thermal conductivity).
- Flammability (flash and fire points).
- Degree of corrosivness to materials of construction.
- Environmental concerns.
- Service life.
- A relatively high freezing point, which limits its use to approximately 14°F(-10°C).
- High corrosivity, requiring inhibitors that must be checked on a regular basis and replenished to prevent an acid condition from occurring in the system.
A calcium chloride aqueous solution also can be used as a circulating brine. Similar to sodium chloride, this nonflammable, nontoxic solution has a lower freezing point and can be used at temperatures down to -35°F (-37°C). This solution's main disadvantages include high corrosivity, reduced heat transfer coefficient below -4°F (-20°C), and its incompatibility for use in direct contact with foods.
Considering HalocarbonsBelow -40°F (-40°C), salt-based brines and glycol- or alcohol-based solutions do not perform well because of their high viscosity. Certain halocarbons such as methylene chloride, trichloroethylene and fluorocarbons can be used at these temperatures. Nonflammable and noncorrosive under normal operating conditions, chlorinated compounds such as methylene chloride or trichloroethylene are toxic and regulated by the Environmental Protection Agency (EPA). These fluids will be removed from existing systems in the next few years.
Fluorinated compounds such as hydrofluoroethers and perfluorocarbon ethers have unique properties that make them suitable for use in low temperature heat transfer fluid applications. Nonflammable and nontoxic, some fluorinated compounds have zero ozone-depleting potential and other environmental properties. Additionally, these fluids have low freezing point and low viscosity at low temperatures. However, they are expensive, and due to their extremely low surface tension, leaks can develop around fittings. Also, fluorocarbons posses a lower boiling point than many other heat transfer fluids. Therefore, they are not suitable for applications where both low and high temperatures are desired. Typical fluorocarbon-based fluid applications are in the pharmaceutical and semiconductor industries within a temperature range from -148 to -238°F (-100 to 150°C).
Looking at HydrocarbonsAromatic hydrocarbons such as diethyl benzene are common low temperature heat transfer fluids in the temperature range from -94 to 500°F (-70 to 260°C). The low temperature heat transfer characteristics and the thermal stability of aromatic compounds are good. However, these alkylated benzene compounds cannot be classified as nontoxic. They also have a strong odor that can be irritating to personnel. Few aromatic compounds have a freezing point lower than -112°F (-80°C), so most are used at temperatures above -94°F (-70°C) in closed, airtight systems typically found in chemical processing and industrial refrigeration.
Paraffinic and iso-paraffinic type aliphatic hydrocarbons also are used in some systems as a low temperature heat transfer fluid. Many petroleum-based aliphatic compounds meet FDA and USDA criteria for incidental food contact. In addition, these petroleum-based fluids do not form hazardous degradation byproducts. Most have an indiscernible odor and are nontoxic. Even with these advantages, though, they are not commonly used in low temperature applications because of their high viscosity at low temperatures. Also, thermal stability of aliphatic compounds is not as good as aromatic compounds. Some of the iso-paraffinic based fluids (with 12 to 14 carbons) can be used from -76 to 392°F (-60 to 200°C). They are preferred in food and pharmaceutical applications where toxicity is an issue.
Another class of low temperature heat transfer fluids is based on naturally derived terpenes such as d-limonene. D-limonene is the major component in citrus fruit oil and is present in trace quantities in orange juice. It is recovered in commercial quantities by distilling orange oil, which is obtained from citrus peels. Being derived from the citrus industry, d-limonene is considered a safe and environmentally friendly heat transfer fluid and is preferred in many food and pharmaceutical processes. The published melting point of d-limonene is approximately -140°F (-96°C). But, testing has shown that below -108°F (-78°C), it becomes a thick white, gel-like substance that is impossible to pump. In addition, at elevated temperatures, d-limonene oxidizes rapidly in the presence of air. This oxidation triggers the acidification and polymerization of the molecules.
Choosing SiliconesAnother popular class of low temperature heat transfer fluids is dimethyl polysiloxane or silicone oil. Because this class is a synthetic polymeric compound, molecular weight and thermophysical properties can be adjusted by varying the chain length. Silicone fluids can be used at temperatures as low as -148°F (-100°C) and as high as 500°F (260°C). These fluids offer a long service life in closed systems in the absence of oxygen, and with low toxicity and essentially no odor, silicone fluids are known to be workplace friendly. However, with low surface tension, these fluids have the tendency to leak around pipe fittings - although this low surface tension improves the wetting property. Because silicone fluids have low toxicity, they are used most often in the pharmaceutical industry.
Several factors can affect the decision-making process when choosing a low temperature heat transfer fluid. However, proper selection can reduce the overall process cost in the long run. In addition to the fluids discussed, there are many more secondary coolants on the market that may be more efficient and environmentally friendly.
Selecting a low temperature heat transfer fluid for a given application depends on a number of considerations. Choosing a particular fluid is invariably a compromise solution that best satisfies the specific application and economics. Strike a balance between several criteria:
The heat transfer fluid should posses a freezing point at least 20°C lower than the lowest operating temperature to avoid freezing on the wall. It also should have low vapor pressure or a high boiling point to avoid system pressurization at elevated temperatures. Additionally, a high flashpoint and autoignition temperature are desired so the fluid is less susceptible to ignition.
Good thermophysical properties are required to obtain high heat transfer coefficients and the low pumping power needed for the fluid to flow through a pipe with a particular flow rate. Therefore, high heat capacity and thermal conductivity is desirable in addition to low viscosity at low temperatures.
Apart from thermophysical properties, the fluid also should exhibit sufficient stability toward oxidative degradation. In the presence of air, most organic fluids oxidize at high temperatures and can form acidic and polymerization products in the system, which can initiate corrosion and fouling. This can severely affect the heat transfer system's efficiency. Corrosion also can be caused by a salt brine or glycol/water solution if proper inhibitors are not used.
The ideal heat transfer fluid would be nontoxic, environmentally friendly, classified as food grade and able to satisfy Food and Drug Administration (FDA) and United States Department of Agriculture (USDA) criteria for incidental food contact. Addition-ally, its vapor should neither contribute to the greenhouse effect nor ozone layer depletion. None of the currently used heat transfer fluids satisfy all of these requirements. A few satisfy most but are expensive.
Comparing Aqueous SolutionsCommonly used as an antifreeze, ethylene glycol is used in refrigeration service and process cooling applications at lower temperatures. Ethylene glycol is colorless, practically odorless and miscible with water. When properly inhibited, it has relatively low corrosivity - an advantage when compared to salt-based brines. Ethylene glycol solutions can be used as a refrigeration system brine at temperatures as low as -40°F (-40°C). However, due to its high viscosity at low temperatures, ethylene glycol is most effective at 14°F (-10°C) or above. Ethylene glycol also is toxic, so it is not suitable for open baths or in food and pharmaceutical applications.
The water quality used to prepare a glycol solution is important. Typically, water with low chloride and sulfate ion concentration (<25 ppm) is recommended. Also, a schedule should be maintained to ensure that inhibitor depletion is avoided and solution pH is consistent. Once the inhibitor has been depleted, it is recommended that old glycol be removed from the system and a new charge be installed.
In its inhibited form, propylene glycol has the same low corrosivity advantages shown by ethylene glycol. In addition, propylene glycol has low toxicity, and certain grades can be used in food applications. Other than low toxicity, it has no advantages over ethylene glycol, being higher in cost and more viscous. Due to its high viscosity at low temperatures, propylene glycol is used at 14°F (-10°C) or above.
Methanol/water is an antifreeze solution used in refrigeration services and ground source heat pumps. Similar to glycols, this solution can be inhibited to stop corrosion. It can be used at temperatures as low as
-40°F (-40°C) owing to its relatively high heat transfer rate in this temperature range. Its main disadvantages as a heat transfer fluid are its toxicological considerations. It is considered more harmful than ethylene glycol and consequently is used only for process applications located outdoors. Methanol also is flammable and, as such, introduces a potential fire hazard where it is stored, handled or used.
Ethanol/water is an aqueous solution of denatured grain alcohol. Its main advantage is that it is nontoxic. Therefore, it is used in applications in breweries, chemical plants, food freezing plants and ground source heat pumps. As a flammable liquid, it requires certain handling and storage precautions.
Selecting Salt BrinesSodium chloride water solutions are used for refrigeration services and other low temperature applications. Because salt brines have low toxicity and are nonflammable, they can be used in applications involving contact with foods and in open systems. A high heat transfer coefficient can be obtained with this type of fluid due to its good thermophysical properties. However, it has two main drawbacks:
- Freezing point.