Heat Transfer, Refrigeration and Air Conditioning — Practice

Question 1

Write the critical radius of insulation for a cylinder.

Accepted Answer

r_c = k/h, with k the insulation conductivity and h the outer convection coefficient.

Question 2

State the Stefan-Boltzmann law for a black body.

Accepted Answer

E = sigma T^4, with sigma = 5.67e-8 W/m2 K4 and T in kelvin.

Question 3

A wall has 0.2 m brick (k 0.8) and 0.1 m insulation (k 0.04) with a 30 K difference. Find the heat flux.

Accepted Answer

R = 0.25 + 2.5 = 2.75 m2K/W, so q = 30/2.75 = 10.9 W/m2.

Question 4

In a counterflow exchanger hot fluid goes from 100 to 60 C and cold fluid from 20 to 50 C. Find the LMTD.

Accepted Answer

Terminal differences 100 - 50 = 50 and 60 - 20 = 40. LMTD = 10/ln(1.25) = 44.8 K.

Question 5

What does Bi below 0.1 imply and what model follows?

Accepted Answer

Internal conduction resistance is negligible against surface convection, so the temperature is uniform and the lumped capacitance model with time constant rho V c over hA applies.

Question 6

How much does one thin shield of the same emissivity reduce radiation between two large parallel plates?

Accepted Answer

It halves the exchange. With n identical shields the exchange falls to 1/(n + 1) of the original.

Question 7

A vapour-compression cycle has h1 = 400, h2 = 450 and h4 = 250 kJ/kg. Find COP and the refrigeration effect.

Accepted Answer

Refrigeration effect = h1 - h4 = 150 kJ/kg. Work = 50 kJ/kg. COP = 150/50 = 3.

Question 8

Which international instrument phases down HFCs?

Accepted Answer

The Kigali Amendment to the Montreal Protocol.

Practice — Heat Transfer, Refrigeration and Air Conditioning

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