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WHAT IS HEAT TRACING?
Heat Tracing is essentially heating a process (normally in pipe but also applicable to tanks and equipment) with an external heat source in the form of a “heat tracer”. Whether the heat energy source is electricity, steam from a boiler, hot oil from a hot oil heater or any other heating medium (or cooling medium), the energy source travels through the tracer which often spans the entire length of the pipe to ensure the process inside the piping stays at a certain temperature. This can be as simple as electric heat tracing a short run of pipe for freeze protection or as complex as a fully designed steam tracing system to heat an entire Sulfur Recovery Unit, for example. Insulation is applied over the heat tracing to ensure the heat is transferred into the process rather than lost to the surroundings.
There are many types of heat tracing from Steam Tracing, Hot Oil Tracing, Water or Water/Glycol Tracing and Electric Tracing. Even fully jacketed pipe can be considered heat tracing although its normally referred to as jacketed pipe.
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ELECTRIC TRACING
Most commonly, electric tracing is made from polymer sheathed wires that generate a specific electrical resistance along the length of the cable. When voltage and current are applied to the cable the resulting power flow is dissipated as heat, intended to replace the heat lost from the process through insulation. [All tracing should be insulated to perform the intended function]. There are many types of electric tracing systems available ranging from simple, low-temperature freeze protection systems to extremely high-temperature, Mineral-Insulated cables that can heat processes well above 500 °F.
Electric tracing does have limitations that must be understood. Electric tracing is generally intended to only maintain process temperature and is not advised for heating processes from ambient to process melting points. Because of the limited heat density of an electric tracer, it could take days or weeks to heat a product line where other types of tracing (such as steam or hot oil) can take hours. This is an important distinction that is often overlooked on a project specifying tracing. Another important consideration is that all electricity is a secondary energy source. Though electricity is assumed to be highly efficient, that may or may not be true, depending on where that electricity is made. Understanding how your electricity is generated is important in determining your carbon footprint. This should be compared to other energy sources available for tracing before a selection is made.
STEAM TRACING
Steam tracing, like electric tracing, is used to heat trace a process pipe or tank to maintain temperature. A stainless-steel tube tracer conveys steam and is in contact with the pipe, tank, and equipment. If rapid heat-up of the process is important, steam tracing offers a substantial benefit. Steam is a high-quality energy source because condensing steam transfers a large amount of energy for heating process lines. When used in conjunction with Aluminum Enhancers like QMax FTS, heat transfer can be truly maximized. Steam is normally an ideal source of energy when heat-up of the process lines is a main objective. Heating an asphalt header at a railroad offloading spur is a prime example of a process line that requires rapid heat-up.
Steam tracing does require the use of a boiler which is normally heated with natural gas or another hydrocarbon. Though these boilers use hydrocarbons to convert potential energy to heat energy to boil water, the efficiency from one BTU of energy to a BTU of usable heat can be quite high. This may mean a boiler is a better choice for reducing the carbon footprint of your facility. The steam, generated from a boiler, and used to heat the process lines, returns as condensate through steam traps. The condensate is recovered and sent to the boiler to make more steam, thus lowering emissions. Steam tracing is often used in chemical plants and oil refineries where steam is generated for use within other areas of the plants. Often referred to as “free steam”, the use of excess steam can be very beneficial in lowering the carbon footprint of an entire facility.
Steam tracing does have a specific temperature band for use because steam temperature starts at 20 psig saturated steam at 259 °F and becomes impractical above 150 psig saturated steam or 365 °F. Other strategies are available for heating processes using steam below 259 °F and above 365 °F that offer flexibility and performance.
WHICH IS MORE ACCURATE?
So, which tracing technology controls temperature most accurately, steam, or electric?
The answer is, well, both, and for different reasons. Electric tracing is normally equipped with a controller that can be adjusted for the application, even during operation. For this reason, electric tracing can be considered more accurate because the controller can be used to adjust heat input based on the process temperature, so long as the temperature sensor is functioning properly. Steam tracing, on the other hand can be more accurate if proper calculations are performed for the intended application. For example, if the goal is to heat sulfur to maintain at 280 °F with 50 psig saturated steam (298 °F), the amount of steam tracing can be calculated to maintain product temperature in the line. In this sense, steam tracing can be a “set-it-and-forget-it” system which always operates, if there are no steam or process upsets.
HOT OIL TRACING
Hot oil tracing is also used when processes need to be maintained at temperatures higher than is practical for steam. Liquid asphalt is a good example of a process that is often maintained about 365°F where steam would no longer be effective. Hot oil tracing is a similar concept to steam tracing, and the system is almost always “closed loop”. Hot oil is circulated through a hot oil heater
powered by either electricity or hydrocarbon combustion, and then pumped through a facility to heat tanks and tracing lines. In this case, steam traps are not needed, however, proper controls are required to balance all users so they each get the hot oil flow they need to operate as intended. A balancing process is normally required to start up a hot oil heated facility.
OTHER TRACING MEDIUMS
Other common types of heating mediums are glycol, water, water/glycol mixes, and less commonly hot oil vapors. Regardless of the energy source and type of tracing that is selected, it is strongly recommended to consult with a company that understands all forms of tracing. Each type of tracing has a suitable place for a given applications. Choosing the optimal course of action is where experts can help.
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Why use Aluminum to Enhance the heat transfer?
Using Aluminum Enhancers like QMax FTS in conjunction with steam, hot oil or electric tracing can have big benefits for little cost. These types of enhancers maximize the heat exchange between stainless steel tube tracers or electric tracers and what is being heated and offer consistent results. Used correctly, they transform the nature of a tracer from convection heat transfer through air to highly efficient conduction heat transfer through solids. Because of this, the heating surface area is greatly increased.
For example, a single 1/2 inch tube tracer with steam has a single point of surface contact along a pipe so the heat must be transferred through the air space for the tracer to heat the pipe. The addition of a 2 inch aluminum enhancer (shown to the right) increases the heating surface area considerably and causes the heat to transfer through aluminum instead of air. This not only offers significant heat exchange performance, but it also allows for greater predictability of the actual heat exchange.
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There are many reasons why a client would need rapid heat-up of a process inside of a tank or vessel. One reason may be to cause or control a reaction, in which case, generally fully jacketed vessels are used. Additionally, if a facility or a processing unit needs to be brought back online, rapid heat-up can cut down the time required to bring the system back to operating temperatures. Another example where rapid heat-up may be necessary is a process demand that cannot be accomplished with a traditional heat exchanger. If the process inside a vessel must be heated to a certain temperature and then maintained to replace heat that is lost naturally, then an internal or external heating system is used. There are many types of heating systems, both internal and external, using many types of energy sources.
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Tank heating solutions are required when fluid processes inside tanks need to be stored at higher temperatures to maintain a particular viscosity. When process temperatures heat or cool out of the acceptable limits, processes can freeze, become viscous, or overheat. If the process does not flow properly,
it can affect other processes up and down stream. Additionally, in industries dealing with corrosive substances, tank heating solutions serve to prevent the buildup of corrosive agents on the inner walls of tanks.
