2. a)

i)

ii)

b) i)

A thin-walled concentric tube heat exchanger has a uniform overall heat transfer

coefficient, and operates under the following conditions in Table 1.

ii)

Cold fluid

Hot fluid

Mass flow Specific heat

rate

[kg-s¹]

capacity

[J-kg-¹-K¹]

3850

1900

Inlet

Outlet

temperature temperature

[°C]

[°C]

0.2

0.25

Table 1: Heat exchanger conditions for question 2a.

25

240

110

Calculate the maximum possible heat transfer rate between the fluids,

describing fully the assumptions you must make.

[7 marks]

Calculate the hot outlet temperature, assuming the heat exchanger operates in

the mode that maximises the heat transfer rate.

[5 marks]

Write a chemical equation for the complete combustion of 1 kmol of cetane

(C16H34) in air assuming stoichiometric conditions. Take air to be 79%

nitrogen and 21% oxygen by volume.

Substance

Carbon Dioxide

Water

Oxygen

Pentane

5 kilomoles of pentane (CsH12) are burned completely in pure oxygen under

stoichiometric conditions with temperature and pressure being constantly

maintained at standard conditions of 25°C and 105 Pa respectively. Calculate

how much energy is available for external heating.

Enthalpies of formation are available in Table 2 and you may take the

chemical equation for the combustion of pentane to be

C5H12 +80₂5 CO₂ + 6H₂0

Enthalpy of Formation [kJ.kmol ¹]

-393,522

[5 marks]

-241,827

0

-146,800

Table 2: Enthalpy values for question 2b.

[8 marks]