Shell-and-tube heat exchangers with hundreds of tubes housed in a shell are comm

Question

Shell-and-tube heat exchangers with hundreds of tubes housed in a shell are commonly used in practice for heat transfer between two fluids. Such a heat exchanger used in an active solar hot-water system transfers heat from a water-antifreeze solution flowing through the shell and the solar collector to fresh water flowing through the tubes at an average temperature of 60 degree C at a rate of 15 L/s. The heat exchanger contains 80 brass tubes 1 cm in inner diameter and 1.5 m in length. The roughness of brass tubing is epsilon 1.5 times 10^-6 m. Determine the pressure drop across a single tube. Calculate the pumping power required by the tube-side fluid of the heat exchanger. After operating for a long time. 1-mm-thick scale builds up on the inner surfaces with an equivalent roughness of 0.4 mm. For the same pumping power input determine the percent reduction in the flow rate of water through the tubes.

Explanation / Answer

solution:

1)here for calculating pressure loss is given by

dP=f(L/D)*density*V^2/2

Q=15/80=.1875 l/s or 1.875*10^-4 m3/s

D=.01 m

L=1.5 m

e=1.5*106-6 m

velocity is given by

V=Q*4/pi*D^2=2.3873 m/s

Re=VD/kinematic viscosity=23873.241

here for terbulent flow friction factor is given by

f=1.325/{ln[e/3.7D+5.74/Re^.9]}^2

so we get

f=.02510

here pressure drop is given as

dP=f(L/D)*density*V^2/2

dP=10728.74 N/m2

where pumping power is

W=Q*dP/1000=2.011*10^-3 kw

3)when scale are deposited then flow velocity is given by

reduced diameter=d1=.01-.002=8*10^-3 m

velocity=1.875*10^-4*4/pi*d1=3.7301 m/s

reynolds number is

Re1=29840.8

friction factor is for e1=4*10^-4 m

f1=.07284

pressure drop=dP1=95016.55 N/m2

for same power flow rate is

W=Q1*dP1/1000

Q1=2.1164*10^-5 m3/s

hence percent reduction in flow rate is for single tube

%reduction=(Q-Q1/Q)*100=88.75%

Related Questions

Navigate

• Browse (All)
• Browse S
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.