9.70   Long stainless steel rods of 50-mm diameter are pre- heated to a uniform temperature of 1000 K before being suspended from an overhead conveyor for transport to a hot forming operation. The conveyor is in a large room whose walls and air are at 300 K.

 

 

that the coiled tube approximates a horizontal tube, and that the biological fluid has the thermophysical properties of water.

 

 

 

 

 

Tsur

 

 

(a)    Assuming the linear motion of the rod to have a negligible effect on convection heat transfer from its surface, determine the average convection coef-

 

Biological fluid

 

m• = 0.02 kg/s

Tm,i  = 25°C

 

 

Tm,o

 

 

 

 

 

 

Water bath

T¥  = 50°C

 

ficient at the start of the transport process.

(b)    If the surface emissivity of the rod is s = 0.40, what is the effective radiation heat transfer coeffi- cient at the start of the transport process?

(c)    Assuming a constant cumulative (radiation plus convection) heat transfer coefficient corresponding to the results of parts (a) and (b), what is the maxi- mum allowable conveyor transit time, if the center-

 

 

 

 

 

 

 

Dc = 200 mm

Thin-walled tube,

D = 5 mm

 

line temperature of the rod must exceed 900 K for the  forming  operation?  Properties  of  the  steel  are k = 25 W/m · K and a = 5.2 X 10-6  m2/s.

Heat transfer by convection and radiation are actu- ally decreasing during the transfer operation. Accounting for this reduction, reconsider the condi- tions of part (c) and obtain a more accurate estimate of the maximum allowable conveyor transit time.