Thermal fluid heating is a form of indirect heating. The basic premise is that the heat transfer medium is heated and circulated within a loop system. This distributes heat to one or more sources within the loop. Heat transfer fluids are commonly used due to their low maintenance, low corrosion, environmental safety, and precise temperature controls. The most common thermal fluids include oils, water, glycol, and glycol-water mixtures. Picking the right thermal fluids and heating equipment for your specific process will maximize efficiency and keep costs lower. This guide will help you identify the right thermal fluids and thermal heating equipment for your application.
Thermal fluid heating is used in a wide variety of industries and applications. The following are a few of the most common thermal fluid heating applications:
Thermal fluid heating can use liquid or vapor as the heat transfer medium. Liquid thermal fluid heating generally uses a closed loop system. The liquid remains as a fluid as it travels through the system. In vapor thermal fluid heating, the medium starts off as a fluid and is vaporized either within the system or externally through a flash drum. The main advantage of vapor is that it provides more uniform heat throughout the system.
The four most common thermal fluids are water, glycol-water mixes, glycol, and oils. Each is best for specific conditions. These fluids must be viscous so as to easily move throughout the system and must have a high enough thermal capacity for its process.
The main advantage of water as a thermal fluid is the low cost and environmentally friendliness. The main drawback is that water has a lower boiling point than its alternatives so it is not suitable for all applications.
Glycol-water mixes can be used to speed up the heating process and provide a higher boiling point than water on its own. It is less costly than using pure glycol fluids and can operate at higher temperatures than water on its own.
Glycol thermal fluids can operate at a higher temperature than glycol-water mixes, or water on its own. However, it is more costly than either.
Oils offer the highest boiling point and are highly versatile. Glycol-water mixes are generally effective up to about 300°F, whereas oils can be effective up to 800°F. Mineral oils are used in residential, industrial, and process heating. Silicon and transformer oils are used for process and as an insulator for high power electric transformers.
The fluid selected must be capable of meeting the temperature requirements for the heating process. You must consider the maximum bulk temperature, the minimum operating temperature, and the minimum start-up temperature. Glycol-water mixtures are sometimes used when water does not meet the minimum start-up temperature. The addition of glycols helps to speed up the process.
Pumpability is an important factor in the selecting the right thermal fluid. This is not only a consideration of viscosity but of functional operating temperatures. If your application is operating in freezing, or sub-zero, environments the fluid must be able to resist freezing so that it can be pumped through the system.
A low viscosity correlates with greater heat transfer coefficients for moderate temperatures. When operating at a low to moderate temperature low viscosity fluids will provide more efficient heating. However, for high temperatures, you may want a more viscous fluid or a secondary fluid added.
Flow rate and watt density are two of the main sizing considerations for thermal fluid heating equipment. Sizing considerations should be made based on the watt density of the fluid. Typical applications use 6 inch and 8-inch flange heaters but there are many that go up to 12 and 14-inch flanged heater and vessels. Flow rate becomes a major factor when considering the size of the heater that you are looking for. The majority of projects that involve fluids use skid mounted systems that have all encompassing material such as expansion tanks, heater, controls, pump and various other pieces of equipment. Pressure and fluid gravity are factors that should be considered as well. Pumps should be sized to accommodate pressure drops in the system. The size of the expansion tank will be affected by the operating temperature, fluid selection, and total system volume.
The components used for thermal fluid heating equipment will be made mostly based on thermal fluid selection, process, and application. Some of the components to consider include:
Piping for thermal fluid heating systems, for example, will generally be 3 to 6 inches. This allows for a good flow with minimal heat loss. The pipes are typically steel. Steel is a good choice because it offers corrosion protection, excellent heat transfer qualities, and lower costs. For these same reasons steel is commonly used for flanges in thermal fluid heating processes.
The type of fluid used is an important safety consideration. For one, the fluid must suit the temperature requirements to prevent any issues. Also, fluids should be ideally non-poisonous and stable, this is for environmental and employee safety.
Proper fluid flow will ensure a longer lifespan and better system integrity. If the flow is too low it may result in overheating, fluid degradation, or heater failure. To avoid these issues the pump must be designed for use with its thermal fluids at operating temperature and should be tested for proof of flow.
Appropriate temperature controls must be used to ensure the safety of operators, the medium, and the heating system itself. Fluid excess temperature protection should be put into place, as well as stack excess temperature limits.
Whether you are selecting thermal fluids or a thermal fluid heating equipment, the Wattco team is ready to help. Contact a Wattco representative today to find the right thermal fluid and heating equipment for your process.
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