05.08.2008 | Editor: Manja Felgentreu
There is a wide selection of heat transfer oils out there to choose from, and each product has its own specific properties. But how do you find the right one for your particular application? A few key criteria will help you make the best choice.
By its very nature, the process industry tends to generate a lot of heat. It is hard to achieve useful results without heating and cooling. Water and steam are the most commonly used heat transfer media. However, at temperatures above 200 °C, water quickly reaches its limits. The systems have to be pressurized, and it takes a considerable amount of monitoring effort to ensure safe operation. Water also causes corrosion on pipes and other parts of the system, and this of course is another big disadvantage. Mineral and synthetic heat transfer oils are a viable alternative. These products are suitable for the temperature range between 200 and 400 °C, and the systems do not have to be pressurized at temperatures up to 350 °C.
Heat transfer oil has to meet a number of criteria depending on the specifics of the application. Some of the key criteria are low viscosity, good thermal stability, high flash point, good heat transfer properties and ease of waste oil disposal. The oil should also be non-corrosive, non-toxic and non-flammable. No heat transfer oil can meet all of these criteria equally well, and as a result every decision is a compromise. However, some key parameters can help make the selection easier. Viscosity should be low especially at the lower end of the temperature range, because this affects the lowest temperature at which the system can be started up. This can be particularly important when the systems are located outdoors or no auxiliary heating is available.
Operating viscosity affects the flow properties in the pipes, potentially causing friction losses. Correct viscosity and the associated turbulent flow enhance good heat transmission. Thermal stability makes a crucial contribution to pump efficiency and safe operation of the heat transfer system. Heat transfer oils are normally mixtures. The constituents with a low boiling point evaporate during normal operation, and they have to be removed, because they have a major influence on the key performance characteristics of heat transfer oil. They reduce the viscosity and lower the flash point. If the temperature continues to rise, the constituents with a high boiling point are cracked, producing a high viscosity substance which leaves coke-like deposits on pipes and heating surfaces.
When you are choosing a heat transfer oil, you of course would like to know how long you can use it. Depending on the type and quality, today’s heat transfer fluids can be used for several years. Their aging characteristics depend on a number of factors. Because organic heat transfer oil reacts with oxygen, long service life depends on good resistance to oxidation. Organic acids, which increase the risk of corrosion, are produced when saturated hydrocarbons react with oxygen. Polymerization products are also produced, which create sludge and sticky deposits. Copper and contamination including dust, water and rust increase the oxidation rate, making it important to avoid the use of large copper surfaces.
Besides all of these criteria, the heat transmission characteristics are of course also a major consideration. Heat conductivity gives a good indication of how well heat will be transferred from the film on the pipe wall to the flowing fluid. The heat transfer medium must also be suitable for the particular system. Vapor pressure at the high end of the temperature range gives a good indication of whether the system can be run without pressurization. The thermal expansion coefficient can be used to determine whether the heat transfer medium is compatible with the size of the expansion tank.
Summary: You have to consider a number of factors when you are selecting a heat transfer medium. Making the right choice will enhance efficiency and increase the life of your heat transfer system. To help you make the best decision, PROCESS has summarized the key data for a number of heat transfer oil products in the table on this page.
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