Heat Pipe Thermal Resistance

V. Kiseev, N. Pogorelov, A Study of Loop Heat Pipes Thermal Resistance , in Proceedings of the 11 IHPC, September 21-25, 1997, Stuttgart, Germany, Sessions Al, A2,p. 45. 

Typically, the temperature drop between the evaporator and condenser of a heat pipe is of particular interest to the designer of heat pipe thermal control systems and is most readily found by utilizing an analogous electrothermal network. Figure 12.4 illustrates the electrothermal analogue for the heat pipe illustrated in Fig. 12.1. As shown, the overall thermal resistance is composed of nine different resistances arranged in a series-parallel combination. These nine resistances can be summarized as follows ... 

By virtue of its chemical and thermal resistances, borosilicate glass has superior resistance to thermal stresses and shocks, and is used in the manufacture of a variety of items for process plants. Examples are pipe up to 60 cm in diameter and 300 cm long with wall tliicknesses of 2-10 mm, pipe fittings, valves, distillation column sections, spherical and cylindrical vessels up 400-liter capacity, centrifugal pumps with capacities up to 20,000 liters/hr, tubular heat exchangers with heat transfer areas up to 8 m, maximum working pressure up to 275 kN/m, and heat transfer coefficients of 270 kcal/hz/m C [48,49]. 

Conducted heat is that going in through cold store surfaces, tank sides, pipe insulation, etc. It is normally assumed to be constant and the outside temperature an average summer temperature, probably 25-2/°C for the UK, unless some other figure is known. Coldroom surfaces are measured on the outside dimensions and it is usual to calculate on the heat flow through the insulation only, ignoring other construction materials, since their thermal resistance is small. 

For the case of heat loss to the atmosphere from a lagged steam pipe, the thermal resistance is due to that of the condensate film and dirt on the inside of the pipe, that of the pipe wall, that of the lagging, and that of the air film outside the lagging. Thus for unit length of a lagged pipe ... 

Person 1 Calculate the log-mean heat transfer area for the stainless steel pipe, then use this to calculate the thermal resistance through the pipe. 

Fig. 10-2 Double-pipe heat exchange (a) schematic (b) thermal-resistance network for overall heat transfer...

Discussion Note that the thermal resistance of the pipe is too small relative to the other resistances and can be neglected without causing any significant error. Also note that the temperature drop across the pipe is practically zero, and thus the pipe can be assumed to be isothermal. The resistance to heat flow in insulated pipes is primarily due to insulation. 

Consider the flow of oil at 20°C in a 30-cm-diameter pipeline at an average velocity of 2 m/s (Fig. 8-24). A 200-m long section of the horizontal pipeline passes through icy waters of a lake at 0°C. Measurements indicate that the surface temperature of the pipe Is very nearly 0°C. Disregarding the thermal resistance of the pipe material, determine (a) the temperature of the oil when the pipe leaves the lake, (b) the rate of heat transfer from the oil, and (c) the pumping power required to overcome the pressure losses and to maintain the flow of the oil in,the pipe. 

The thermal resistance network associated with this heat transfer process involves two convection and one conduction resistances, as shown in Fig. 11-7. Here the subscripts i and o represent the inner and outer surfaces of the inner tub d. For a double-pipe heal exchanger, the Ihennul resistance of the tube wall is >... 

A double pipe (shell-and-tube) heat exchanger is constructed of a stainless steel [k = 15.1 W/m O inner lube of inner diameter O/ = 1.5 cm and outer diameter 1.9 cm and an outer shell of inner diameter 3,2 cm. The convection heat transfer coefficient is given to be h,- = 800 W/m °C on the inner surface of the tube and h = 1200 W/m °C on the outer surface. For a fouling factor of f f, - 0.0004 m °C/W on the tube side and Ri =- 0.0001 m °C/W on the shell side, determine (a) the thermal resistance of the heat exchanger per unit iength,and (6) the overall heat transfer coefficients, Ujand U based on the inner and puter surface areas 0) the tube, respectively. 

Consider a double-pipe heat exchanger with a lube diameter of 10 cm and negligible tube thickness. The total thermal resistance of the heat exchanger was calculated to be 0.025°CAV when it was first constructed. After some prolonged use, fouling occurs at both the inner and outer surfaces with (he fouling factors 0.000 15 CAV and 0.00015 °C/W, re-... 

M. M. Yovanovich, C. H. Tien, and G. E. Schneider, General Solution of Constriction Resistance Within a Compound Disk, Heat Transfer, Thermal Control, and Heat Pipes, AIAA Progress in Astronautics and Aeronautics, Vol. 70, pp 47-62,1980. 

If there is any contact or bond resistance present between the fin and tube or plate on the hot or cold fluid side, it is included as an added thermal resistance on the right side of Eq. 17.5 or 17.6. For a heat pipe heat exchanger, additional thermal resistances associated with the heat pipe should be included on the right side of Eq. 17.5 or 17.6 these resistances are evaporator resistance at the evaporator section of the heat pipe, viscous vapor flow resistance inside heat pipe (very small), internal wick resistance at the condenser section of the heat pipe, and condensation resistance at the condenser section. 

To determine the heat transfer area of a heat exchanger from Eq. (13.7), an overall heat trans fer coefficient is required. It can be estimated from experience or from the sum of the indi vidual thermal resistances. For double-pipe and shell-and-tube heat exchangers, the area fo heat transfer increases across the pipe or tube wall from the inside to the outside surface. Ac cordingly, the overall heat transfer coefficient is based on the inner wall, i, the outer wall, o or, much less frequently, a mean, m. The three coefficients are related by... 

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