Heat-tracing systems can be divided into two broad categories, fluid and electric. Fluid heat-tracing systems utilize heating media at elevated temperatures to transfer heat to a pipeline. The fluid is usually contained in a tube or a small pipe attached to the pipe being traced. If steam is the tracing fluid, the condensate is either returned to the boiler or dumped. A number of desirable features made steam the original heat-tracing system of choice to maintain process temperature and provide freeze protection. Steam’s high latent heat from vaporization is ideal for heat-transfer applications. Only a small quantity is required for a large heating load; and it can heat a line quickly, condense at constant temperature, and flow to the point of use without pumping. Steam is universally available and non-toxic.
If an organic heat-transfer fluid is employed, it is returned to a heat exchanger for reheating and re circulation. In general, heating of tracing fluids can be provided by waste heat from a process stream, burning of fossil fuels, steam, or electricity.
Electric heat-tracing systems convert electric power to heat and transfer it to the pipe and its contained fluid. The majority of commercial electric heat-tracing systems in use today are of the resistive type and take the form of cables placed on the pipe. When current flows through the resistive elements, heat is produced in proportion to the square of the current and the resistance of the elements to current flow. Other specialized electric tracing systems make use of impedance, induction, and skin conduction effects to generate and transfer heat.
The following excerpts from Fluid Controls Institute (FCI) Tech Sheet #ST 102 identify the benefits of Steam Tracing vs. Electric Tracing.
Many articles have been written concerning the advantages of steam over electricity (or vice versa) for tracing applications. A valid case can probably be made for either, depending on the method being promoted by the particular author. These promotional articles have generally been based on strictly economical considerations which represent only part of the story. This article is intended to provide insights into inherent practical advantages of steam which may have been overlooked in the past. Here are some points and simplified piping schematics to consider.
Trace heating is a vital element in the reliable operation of pipe lines, storage and process vessels throughout the process industry. Anti-frost protection for water pipelines is needed both in the processing industries as well as in commercial and domestic buildings. Of equal importance is the tracing of process pipelines which carry liquids that can be pumped only at temperatures that are well above the freezing temperature of water. For example, many oil and fat lines have to be heat traced. In the chemical process industry a multitude of products such as asphalt and sulfur can only be moved through pipe lines at specific temperatures.
Which Tracing Method
The amount of heat energy required to maintain the desired temperatures may influence the type of tracing to be used. (The heat load calculation methods are beyond the scope of this discussion). The choice of tracing method will usually lie between steam, hot liquid or electrical tracing. Hot water is often used in the food industry where relatively low temperatures are desirable, such as keeping chocolate in a molten state, and hot oils are sometimes used in chemical processing especially when high temperatures are required. For the majority of applications, however, the choice falls between steam tracing and electrical tracing.
A hybrid of the temperature regulator is referred to as an Ambient Sensing Steam Tracing Valve. These valves sense the temperature of the air around the actuator and stroke the valve open as temperature approaches the set point. They are generally used for the steam tracing of oil, gas, or chemical lines that must be kept from freezing.
In oil refineries, petroleum processing plants, and any industry handling heavy liquids, there are often times when it is necessary to heat a product before it can be moved. Viscous fluids become extremely difficult, and sometimes impossible, to pump if they cool below a certain temperature. To avoid this, it is necessary to supply heat to the product to keep it fluid enough to flow without too much work being expended by the pumps.
As you may know, steam tracing is one of the most practical methods used to provide supplemental heat to process lines to protect the product, instruments, valves, and other automatic controls from low process- fluid temperatures. Steam tracing can be used to prevent freezing, solidification, or separation, particularly important in lines that cannot be flushed, blown down, or drained while the process unit or equipment is on stream.
In its simplest terms, steam tracing consists of applying steam to the areas requiring such protection by means of tubing or small sized pipe that follows the contours of the product line or equipment to provide the level of heat required.
There are three general types of steam tracing:
In the external method, pipe or tubing of small bore carrying steam is either wrapped around the product line or vessel, or else parallel runs are placed outside the product line, with the transfer of the heat taking place through the pipe wall. This is the more common method of the three, as it is the simplest and cheapest, it’s easy to repair or alter, and there is no possibility of cross-contamination between the product and the steam or condensate. The disadvantages are that the heat transfer rate between the tracer and the pipe is often unpredictable, consequently there is usually a slow heat up rate if the product is allowed to cool, and a high temperature drop is required between the tracer and the pipe, which often results in uneven temperature distribution.
If more than one tracer line is used on the outside of the pipe, then it is recommended that they be on different traps. This will ensure that some heating will still take place if one trap becomes clogged or stuck in the closed position.
Also, for more uniform temperatures, two parallel tracer lines along a product pipe should, if possible, be fed in opposite directions so that a drop in temperature along the tracer lines will cancel each other out and maintain a more uniform total effect on the product line.
When insulation is placed over the traced line, it requires larger diameter insulation (often 1″ larger or more than normally would be used). This often leads to odd sizes being required, which in turn increases the price of the insulation.
On external tracing, if there is too much heat transfer to the product, it is usual to place some blocking between the tracer and the product line, or else place a thin layer of insulating paper between them. It is essential that the tracer be spaced uniformly along long pipe runs, and it should be fastened properly so as to permit some movement, without danger of it working loose or pulling away from the product line.
Internal steam tracing is considerably more difficult. It requires some complicated fittings where the steam line enters and leaves the product pipe. This often requires packing glands or stuffing boxes, and makes repairs more challenging.
There is also the danger of cross contamination between the steam and the product. A much faster heat- up is obtained, however, and a much better heat transfer is obtained than with the external method, especially if the tracer line is coiled within the product line.
The steam heating is provided from outside the product line, and there is much more heating surface available than with either of the other two methods. There is a lower temperature drop possible between the tracer medium and the product with this system due to the greater amount of transfer surface. The heat transfer rate is more predictable, and the heat-up from cold is faster. However, the cost is higher than either of the other two methods, it is difficult to repair, and there is still danger of cross-contamination.