Underground power cables sharing space with other utilities, such as steam lines, can build up detrimental hot spots. If these hot spots are not mitigated by external cooling, the cables may have to be derated, resulting in a loss of revenue and operating flexibility. Their service life could also be shortened by overheating. Thus, a careful evaluation of the thermal impact and suitable mitigation methods must be considered.

Consolidated Edison Co. of New York (Con Ed), New York, U.S., began considering these matters in 1994 when it built a 30-inch (762-mm) steam main in downtown Manhattan, New York. The main crossed several 69- and 135-kV oil-impregnated, paper-insulated, underground cables installed in 6- to 8-inch (152- to 203-mm) dia high-pressure gas-filled steel pipes.

Initial thermal studies of the cable crossings indicated that the steam main (even though covered with a 5-inch or 127-mm thick foam insulation wrap and encased in a concrete housing) would raise the ambient earth temperature to above 100oC (212oF), exceeding the recommended Association of Edison Illuminating Companies' maximum conductor temperature for these cables. And, at several crossings the clearance between cable pipes and steam main was less than 2 ft (0.6 m). Earlier-used vent chambers over cable crossings were unsatisfactory because of road debris, road salts and loose grills.

Underground cable hot spots can be treated by: - Use of low-thermal-resistivity corrective backfills. - Insulating the heat sources near the cables. - Internal cooling by circulating insulation fluids in cables. - Forced circulation of fluid around the cables in pipes. - External cooling: either forced-cooling or passive-cooling systems installed adjacent to the cables in the hot-spot zone.

Mitigation of hot spots may require installing external cooling systems if the use of corrective thermal backfill and internal cooling by circulation of insulation fluid is not adequate. Forced circulation is feasible only for fluid-filled pipe-type cables and only if the system has an adequate pumping facility. Passive cooling systems, such as heat pipes, are well suited for treating local hot spots without requiring any operation or maintenance.

The cable crossings could be cooled only by external means with a passive system, an alternative to the conventional vent chamber application. Geotherm Inc., Uxbridge, Ontario, Canada, proposed the passive method of cooling the hot spots by heat pipes, which Con Ed adopted. This application is believed to be the first using heat pipes to mitigate a number of hot spots in underground electric cable in the United States. Heat pipe is similar to a sealed heat exchanger. The heat transport takes place through a cycle of phase changes of coolant from gas to liquid and liquid back to gas. Its heat-exchange cycle (vaporization, vapor flow, condensation and liquid return flow) repeats itself without requiring any mechanical devices. It is also referred to as a "thermo syphon" (Fig. 1).

The rate of evaporation and condensation of the coolant (working fluid) in the heat pipe is governed by the thermal regime in the surrounding media. The vapor phase transport of the coolant from the warm to cool end depends on the vapor pressure gradients set up in the sealed system, while the return flow of the condensed coolant in the liquid phase depends on the capillary tension in the wick system or gravity head at the condensing end.

A variety of working fluids and pipe materials have been used for the heat-pipe construction. Table 1 shows typical operating characteristics. Heat pipes have been successfully used for cooling hot spots in cable splices, terminations and duct banks.

Cooling the Cable Crossings Treating the hot spots in the feeder cable crossings consisted of: - Insulating the steam-main housing with a rigid foam insulation to reduce the thermal impact on the cables. In this case, insulation alone was not adequate to fully mitigate the hot spot. - Designing and installing heat pipes to fully mitigate the impact of heat from the steam main on the cables. - Installing a low thermal resistivity fluidized thermal backfill (FTB) around the cables and heat pipes to conduct the heat efficiently from the hot spot to the surrounding soil and ambient air.

Use of FTB not only facilitated the installation of heat pipes but also ensured good mechanical protection for them. The above mitigating measures are shown in Fig. 2.

Heat-Pipe Design The heat pipes selected were similar to the one shown in Fig. 1. They were made of 1.5 inch (38 mm) dia and 8 ft (2.4 m) long copper pipes. Their working fluid was a mixture of water and methanol with a working temperature range of -20øC-150øC (-4øF-302øF), covering the expected temperature range in the vicinity of cables crossing over the steam main. The working fluid has been found to be suitable for up to 40 years of continuous operation at 100oC (212oF).

From thermodynamic formula, the heat-transport capacity of a 1.5 inch (38.1 m) dia, 8 ft (2.4 m) long heat pipe (assuming an effective length of 5 ft [1.5 m]) was estimated to be 75 W. To verify this amount, a calibration test was performed in the laboratory. The heat source at one end was simulated with an electric heater coil and a heat sink at the other end, open to atmosphere. The intermediate section was insulated with glass wool. An average heat-transfer capacity of 60-100 W was measured over a temperature range of 50oC-90oC (122oF-194oF) at the heat source.

Thermal Analysis Requirements The optimum amount of external insulation over the steam main housing, the layout and number of heat pipes required at each crossing were based on a detailed thermal analysis of the hot-spot region and the space constraints caused by other underground services. A two-dimensional thermal analysis was performed using a heat-transfer code in which various layers of insulation, soil and backfill along with the boundary conditions were modeled in detail. The steady-state thermal regime corresponding to the steam temperature of 245oC (473oF) in the steam main and 20oC (68oF) ambient air temperature was superimposed on the steady-state thermal regime due to the heat generation from the feeder cables at each crossing. The heat generation from the feeder cables was based on a 30-hr emergency rating. The resultant heat flux and the temperatures at the cable depth were calculated.

Con Ed adopted the following strategy to determine the number of heat pipes that were required at each crossing: - The heat pipes should have a minimum heat-transfer capacity equal to the upward heat flux at the cable depth from the steam main. - The heat pipes should be capable of removing the excess heat from the steam main over the crossing area, so as to limit the maximum cable-pipe temperatures to 70oC (158oF).

For most of the hot spots treated, 20 heat pipes of 8 ft (2.4 m) length (10 on either side and perpendicular to the cable pipes) were found to be adequate. Figure 4 shows a typical layout of heat pipes. Figure 5 shows the temperature profiles across the cables for a typical steam main crossing with and without the heat pipes, as predicted by a two-dimensional heat-conduction analysis.

The Installation At each of the seven crossings, as determined by the thermal analysis, the steam-main's reinforced concrete housing was insulated with two layers of 2 inch thick (51 mm), rigid foam. The foam, manufactured by Dow Chemical Co., Midland, Mich-igan, U.S., has an R value of 5, a 200oC (392oF) temperature rating and 25-psi (172 kPa) strength. Both the top and sides of the steam-main housing were insulated 12 ft (3.7 m) on either side of the cable pipes as in Fig. 3.

The heat pipes were installed with their evaporator end located just below the cable pipes and the condenser end at a slightly higher elevation of 4-6 inches (102-152 mm) above the cable pipes as in Fig. 6. The heat pipes were spaced uniformly in the 8 ft (2.4 m) wide construction trench. At some locations the heat-pipe layout had to be adjusted to fit the space between various buried services such as gas and water mains, to prevent other heat sources from interfering with their performance.

The trench was backfilled with a low-thermal-resistivity FTB (Fig. 6). This backfill was specially formulated to have a free-flowing consistency without segregation during installation and to have a thermal resistivity of no higher than 70oC-m/W (158oF) in its totally dried condition. FTB was supplied by a local ready-mix concrete company whose source materials were tested, and a suitable mix was designed by Geotherm Inc. to achieve the flow, strength and thermal requirements. Field supervision and quality control were carried out to ensure proper installation of heat pipes and FTB.

With the use of FTB, the backfilling, the removal of trench shoring and the restoration of pavement at the busy street crossings of downtown Manhattan were achieved in the shortest possible time. Thermal sand or other corrective backfills would not have met these critical requirements.

Thermocouples were installed on the cable pipes and a few of the heat pipes at each crossing. These are being monitored on a regular basis to evaluate the performance of heat pipes in mitigating the hot spots. Maximum temperatures of the cable pipes were recorded during August 1995, when the ambient air temperatures exceeded 33oC (91oF) and some of the feeders were fully loaded. Comparing these temperature readings with the predicted maximum values from the thermal analysis, the cables appear to be running 2-15oC (36-59oF) cooler (Table 2).

Conclusions Heat pipes have been known for their high performance as passive heat-transferring devices and have found diverse applications ranging from thermal energy recovery to cooling electronic and electrical equipment. Their application for cooling the hot spots in Con Ed's underground power cables demonstrated some unique advantages over other conventional methods. These advantages are simplicity, efficiency, cost effectiveness and adaptability for different configurations, requiring no auxiliary devices or maintenance. The detailed thermal analysis of the hot spots helped optimize the design, while the use of FTB simplified installation and ensured good heat transfer from the heat pipes to the ambient. Field supervision of installation and a quality assurance program carried out jointly by Con Ed and Geotherm were also a key factor in the success of this project.

E.W. Ng is senior engineer with Consolidated Edison Co. of New York, which he joined in 1982. He has the MS degree from Polytechnic University. His responsibilities include underground steam piping and equipment design. He is a member of the American Society of Mechanical Engineers. Ng is a registered professional engineer in New York.

A.D. Wong is an engineer with Consolidated Edison Co. of New York, which he joined in 1992. He has the BSEE degree from the Polytechnic Institute of New York, Brooklyn, New York. He is a member of the Institute of Electrical and Electronic Engineers (IEEE).

H.S. Radhakrishna is vice president of Geotherm Inc., which he joined in 1993. He has the PhD in civil engineering from the University of Waterloo, Ontario, Canada. His responsibilities include marketing and directing R & D projects. He is a member of IEEE, the American Society for Civil Engineers, and the Canadian Geotech-nical Society (CGS).

D. Parmar is president of Geotherm Inc., which he joined in 1978. He has the BSC degree from Woolwich Polytechnic, London, U.K. His responsibilities include administration and marketing. He is a member of IEEE, CGS, the Insulated Conductor Committee, and the Canadian Society for Civil Engineers.