2.  Emptying a pipe after it is used. Arranging the piping such that itdrains itself when not inuse， can be an effective method of avoiding the need for heat tracing. Some infrequently used lines can be pigged or blown out with compressed air. This technique is not recommended for commonly used lines due to the high labor requirement.

3.  Arranging a process such that some lines have continuous flow can eliminate the need for tracing these lines. This technique is generally not recommended because a failure that causes a flow stoppage can lead to blocked or broken pipes.

Some combination of these techniques may be used to minimize the quantityof traced pipes.However， the majority of pipes containing fluids that must be kept above the minimum ambient temperature are generally going to require heat tracing.
Types of Heat-Tracing Systems
Industrial heat-tracing systems are generally fluid systems or electricalsystems. In fluidsystems， a pipe or tube called the tracer is attached to thepipe beingtraced， and a warm fluid is put through it. The tracer is placed under the insulation. Steam is by far the most common fluid used in thetracer， although ethylene glycol and more exotic heat-transfer fluids areused. In electricalsystems， an electrical heating cable is placed against the pipe under the insulation.
Stream Tracing Systems
Steam tracing is the most common type of industrial pipe tracing. In1960， over 95 percent of industrial tracing systems were steam traced.By1995， improvements in electric heating technology increased the electric share to30 to 40 percent， but steam tracing is still the most common system. Fluid systems other than steam are rather uncommon and account for less than 5% of tracing systems.
Half-inch (12.7 mm) copper tubing is commonly used for steam tracing.Three-eighths-inch (9.525 mm) tubing is also used， but the effective circuit length is then decreased from 150 feet (50 meters) to about 60 feet (20 meters).In some corrosive environments， stainless steel tubing is used， and occasionally standard carbon steel pipe (1坦2 -1 inch) is used as the tracer.
In addition to the tracer， a steam tracing system as seen in Figure 12-12， consists of steam supply lines to transport steam from the existing steamlines to the tracedpipe， a steam trap to remove the condensate and hold back thesteam， and in most cases a condensate return system to return thecondensate to the existing condensate return system. In thepast， a signifi-cant percentage of condensate from steam tracing was simply dumped todrains， but increased energy cost and environmental rules have caused almost all condensate from new steam tracing systems to be returned. This has significantly increased the initial cost of steam tracing systems.
Applications requiring accurate temperature control are generally limitedto electric tracing. For example， chocolate lines cannot be exposed to steamtemperatures or the product will degrade， and if caustic soda is heated above 150 0F (66 0C)， it becomes extremely corrosive to carbon steel pipes.
For some applications， either steam or electricity is simply not available and this makes the decision. It is rarely economic to install a steam boiler
To Other Tracers 
Condensate 
Return 
Pressure Reduction 
(If required) 
Steam Main 
Steam 

Condensate Return Main Trap
Assembly Steam tracer length limited Low pressure Small diameter Elevation changes
Figure 12-12. Steam tracing system.
just for tracing. Steam tracing is generally considered only when a boiler already exists or is going to be installed for some other primary purpose. Additional electric capacity can be provided in most situations for reason-able costs. It is considerably more expensive to supply steam from a long distance than it is to provide electricity. Unless steam is available close to thepipes being traced， the automatic choice is usually electric tracing.
　
中國航空網 www.k6050.com
航空翻譯 www.aviation.cn
本文鏈接地址：燃氣渦輪工程手冊 Gas Turbine Engineering Handbook 2(77)