Thermodynamics Question 3 - State any assumptions and include the units at the e

Question

Thermodynamics Question 3
- State any assumptions and include the units at the end of each calculation.

Air flows at 120degreeC in a thin-wall stainless-steel tube with h = 65 W/m2.degreeC. The inside diameter of the tube is 2.5 cm and the wall thickness is 0.4 mm, and k = 18 W/m degree C for the steel. The tube is exposed to an environment with h = 6.5 W/m degree C and T infinity = 15degreeC. Calculate the overall heat-transfer coefficient and the heat loss per meter of length. What thickness of an insulation having k = 40 m W/m degree C should be added to reduce the heat loss by 90 percent?

Explanation / Answer

Assuming steady state conditions,

Resistance of R (air)[@120] = 1/hA in = 1/2*pi*r in *L*h

Resistance of SS tube = [ln(r out/ r in)] / 2*pi*k*L            K of steel tube is given

Resistance of environment = 1/hA out = 1/2*pi*r out *L* h(environment)

Net heat transfer rate Q = (T in - T out)/ net resistance

Net resistance = R1 + R2 + R3

To reduce the heat loss by 90% which means heat loss is 10% of Q

This means after adding the thickness resistance of the entire system increases by a factor of 10.

SO R1+R2+R3+R insulation = 10(R1+R2+R3)

R insulation = 9(R1+R2+R3)

[ln( {r out+thickness} /r out) ] / 2*pi* k* L = 9(R1+R2+R3) Here K insulation is given from above

Substituting everything gives thickness

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

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