8.63   The problem of heat losses from a fluid moving through a buried pipeline has received considerable attention. Practical applications include the trans-Alaska pipeline, as well as power plant steam and water distribution lines. Consider a steel pipe of diameter D that is used to trans-

 

port oil flowing at a rate m· o  through a cold region.  The

 

pipe is covered with a layer of insulation of thickness t and thermal conductivity ki and is buried in soil to a depth z (distance from the soil surface to the pipe center- line). Each section of pipe is of length L and extends between pumping stations in which the oil is heated to ensure low viscosity and hence low pump power require- ments. The temperature of the oil entering the pipe from a pumping station and the temperature of the ground

 

above the pipe are designated as Tm,i

 

 

Â
Â

and Ts, respectively,

and are known.

 

Consider conditions for which the oil (o) properties may be approximated as po = 900 kg/m3, cp,o = 2000

 

m• = 18 kg/h

 

J/kg · K,  vo

 

= 8.5 X 10-4  m2/s,    ko

 

= 0.140 W/m · K,

 

 

 

Pro = 104; the oil flow rate is m· o = 500 kg/s; and the pipe diameter is 1.2 m.

 

(a)    Expressing your results in terms of D, L, z, t, m· o,

 

Tm,i, and Ts, as well as the appropriate oil (o), insula- tion (i), and soil (s) properties, obtain all the expres- sions needed to estimate the temperature Tm,o of the oil leaving the pipe.

 

(b)  If  Ts = -40°C,  Tm,i = 120°C,  t = 0.15 m,  ki = 0.05

 

W/m · K, ks = 0.5 W/m · K, z = 3 m, and L = 100 km, what is the value of Tm,o? What is the total rate of heat transfer q from a section of the pipeline?

 

The operations manager wants to know the tradeoff between the burial depth of the pipe and insulation thickness on the heat loss from the pipe. Develop a graphical representation of this design information.