Operating a Thermic-Fluid System – Part 1: System Components

Operating a thermic-fluid(hot oil heat transfer) system can be easy if one understands the system components, general operation, start-up and shut-down procedures. A few other things any operator should know are procedures of drainage, recharge and subsequent start-up with new oil. These series of articles will provide the basic knowledge needed to operate a hot oil heat transfer fluid system.

System components and their function – The system comprises a pump that pushes the thermic fluid through an insulated ridged or flexible piping system to a heater and on to the process equipment. An expansion tank in the system allows the oil to expand and contract as it heats up and cools down. To remove particulates from the system, some systems have an in-line or side-stream (preferred method) filtration units. The in-line filtration filters 100% of the flow, while the side-stream filters takes care of 10% or less of the system flow rate.

Two types of pumps are used in thermic fluid systems. Gear pumps, which are not commonly used and centrifugal pumps, which are the preferred because of their higher flow rates which cause turbulent flow to happen within the heater. The pump keeps the thermic fluid flowing and if it ever stops, there will be major problems with the system.  Therefore, we must ensure that the pump keeps up the flow as long as the heater is running. A battery/generator backup system to keep the pump running is mandatory to prevent  flow stagnation during any electrical outage or during rapid shut down for any reason.

There are many types of thermic fluids. The most preferred thermic fluid is non-hazardous, non-toxic and organic petroleum-based. Most systems running on this type of fluid is very efficient. Disposal of this type of fluid is also the same as that as used motor oil or hydraulic oil. This type of fluid prevents rusting of the system from the inside.

Synthetic thermic fluids are required to meet EPA standards and regulations for operation and disposal. In comparison, organic fluids provide the user with a safe and efficient alternative at a less demanding standards. However, there are certain heat transfer applications where only a synthetic thermic fluid will meet the requirements.

Piping is like the veins in our bodies. It carries fluid from one location to another in the most direct path possible. The preferred method for pipe joints is welding them together because the viscosity of the fluid is so low at elevated temperatures that it can find its way past threads and seep out of the system. Flexible pipes and hoses can also be used, but if a perfect seal does not form at connections, there will be some seeping of the oil. For organic fluids, there are three materials that cannot be used; copper, aluminum and brass. These materials act as catalysts for oxidation. Oxidation breaks down organic heat transfer fluid. Steel or stainless steel are the preferred materials for organic oil systems. Rubber or plastic may be used, but care must be taken to ensure that they can handle the low viscosity of the oil as well as high temperatures.

Like all the other thermic fluid system components, there are many types of heaters. Heaters are classified by the amount of heat produced and the fuel that is used; such as electric, gas, oil and wood. A heater OEM can provide specific recommendations to meet the needs of the application. Some systems are smaller in size and are packaged with a pump, heater and expansion tank so that all the hoses can be quickly installed and started up easily. Large capacity systems are not as easy as smaller systems since components need to be chosen, purchased and installed separately along with the piping.

Hot oil systems are used in many processes and application which may include heating dies, cooling dies, heating molds, cooling molds, heating reactors, heating vats, heating process machines, heating rolls, heating storage tanks and so on. It is important to know that each application has its own specific requirements and each system is designed to meet those requirements. The smaller systems are relatively simple, but the larger systems can become very complicated. The larger systems require design work from engineering or consulting firms and can take many months to design as well as install.

The final component of a hot oil system is the expansion tank. This tank is critical to the operation of the system. The expansion tank allows the volume of the hot heat transfer fluid to increase in a controlled space and also allows the cooler oil to be drawn back and recirculated into the system. It also has a built-in reserve tank for the system, which compensates for leaks and keeps the system full. It is important to monitor the fluid level of the expansion tank on a daily basis. If the level drops from its normal position, it is an indication that the system has developed a leak.

A rule of thumb is to fill the expansion tank to its 1/3 capacity when the system is cold. When the system is running hot, it would then be 2/3 to 3/4 full due to expansion of the oil. There are usually two pipes or legs that connect to an expansion tank and during system operation, one leg needs to be closed to prevent thermal currents from running into the expansion tank and heating up the fluid. The temperature of the fluid in the expansion tank should be less than 60°C (140°F). This prevents oxidation of the fluid due to the presence of air inside the tank. If the expansion tank is required to run hotter than 60°C (140°F), a nitrogen blanket should be installed on the head of the tank to remove any trace of oxygen and prevent oxidation.

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