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Shell Heat Transfer Fluids
Heat Transfer using petroleum fluid as a medium is a common feature in many industries for example food and chemicals. The choice of heat transfer medium is often a balance of many conflicting priorities. Where petroleum based media as preferred, mineral oils is by far the most commonly used in industries. Within the temperature ranges commonly encounter, mineral oil is by far the most versatile, cost effective media for fluid transfer in closed system.
Heat transfer systems are commonly found in:
- Petrochemical industry
- Rubber industry
- Wood industry (particle board plants)
- Food industry
- Textile industry
- Construction industry (asphalt plants)
Due to their better temperature – viscosity characteristics, paraffinic oils are the preferred hydrocarbons for heat transfer fluids. Selection criteria of heat transfer medium can be described as follows:
- Thermally stable at operating temperature ranges
- Good heat transfer coefficients.
- Low vapour pressure.
- Low viscosity to promote turbulent flow.
- Non toxic, safe and easy to handle (new as well as used)
- Readily available.
Properties of transfer fluid and how they affect heat transfer properties
- Viscosity – Lower viscosity promotes turbulent flow which is desirable for good heat transfer.
- Pour Point – Determines the lowest temperature at which the oil can still be pumped.
- Flash Point – A safety indicator but more useful as an indicator of the quality of oil. Flash point drop usually indicates thermal cracking of heat transfer fluid.
- Thermal Stability – Indicates the ability of oil to resist high temperature degradation eg. cracking.
- Oxidation Stability – Indicates the ability of oil to resist oxidation eg. when hot oil is exposed to air.
- Thermal Conductivity – Rate of heat transfer through the oil between two points of differing temperatures.
- Reynolds Number, Re - A dimensionless number defining the type of fluid Re above 10,000 is usually required for complete turbulent flow.
- Prandtl Number, Pr - A dimensionless number refining the thermal properties of the oil.
- Nusselt Number, Nu - A dimensionless number defining the thermal properties of the oil.
Care for heat transfer system
Even a premium quality thermal oil will not have a long life span in a poorly designed system. The problem is frequently compounded in mix-and-match systems relying more on convenience rather then on sound engineering principles.
Common system related heat transfer fluid problems
- Poorly sized pumps, heaters and lines leading to poor flow patterns and overheating.
- Poorly designed expansion tanks, which expose hot oil to air, accelerating oxidation.
- Heater related problems, such as those caused by flame impingement on tubes
- Poor start-up and shut down procedures.
Mineral oils are subject to two forms of degradation during use:
1. Cracking on the breaking down of hydrocarbon molecules by heat. Here a large molecule ruptures into smaller molecules. Some appear as volatile gases, while others are unstable and polymerise into non-soluble deposits. The rate of cracking is insignificant until temperatures of around 320 c are reached. Above this temperature the rate rises steeply.
2. Oxidation or the reaction of hydrocarbon oil with atmospheric oxygen. At room temperature this reaction occurs very slowly, but it accelerates rapidly with increasing temperature. Oxidation produces acids in the oil and, at a later stage, sludge. The viscosity of the oil generally increases.