Question 1. A fuel gas consists of 75% butane (C₄H₁₀), 10% propane (C₃H₈) and 15% butene (C₄H₈) by volume.

It is to be fed to the combustion chamber in 10% excess air at 25°C, where it is completely burnt to carbon dioxide and water. The flue gases produced are to be used to generate 5 bar steam from water at 90°C.

(a) Write balanced equations for the combustion of each component of the fuel gas.

(c) Determine the actual fuel: air ratio

  (i)    by volume

  (ii)  by mass.

(d) Calculate:

  (i) the net calorific value (CV) per m³ of the fuel/air mix at 25°C

  (ii) the net calorific value (CV) per kmol of the fuel/air mix at 25°C.

(e) Determine the composition of the flue gases by volume (assuming the inlet air is dry):

  (i) on a wet basis

  (ii) on a dry basis.

(f) Determine the maximum flame temperature.

(g) State how varying the amount of excess air may affect the flame temperature.

(h) Determine the 'furnace efficiency' if the flue gases leave the boiler at 300°C.

(i) If 5% of the heat available for steam production is lost to the atmosphere, determine the amount of steam raised per hour when the total flow of flue gases is 1400 kmol h^-1.

(j) Determine the dew point temperature assuming that the flue gas pressure is 1.00 bar and the inlet air:

(i) is dry

(ii) contains 0.8 kg water per kmol of air at the temperature of the inlet air.

(k) If the flue gases exiling the boiler are used to preheat the water fed to the boiler from a temperature of 28°C to 90°C and assuming:

• a mean specific heat capacity for water over this temperature range to be 4.2 kJ kg^-1K^-1
• a mean molar heat capacity for the flue gases up to 300°C to be 31 kj kmol^-1 K^-1
• 10% of the heat required to heat the water is lost in the heat exchanger
• all water entering the system is converted to steam

determine the final outlet temperature of the flue gas and state if the dew point will be reached in both of the cases given in part (j).

DATA

Net calorific value (MJ m^-3) at 25°C of:

Butane (C₄H₁₀) = 111.7 MJ M^-3

Butene (C₄H₈) = 105.2 MJ M^-3

Propane (C₃H₈) = 85.8 MJ M^-3

Air is 21% oxygen, 79% nitrogen by volume and 23.3% oxygen and 76.7% nitrogen by mass.

Atomic mass of C = 12,0 = 16, N=14 and H = L.

Question 2:

(a) Process water with a specific heat capacity of 4.182 kJ kg^-1 K^-1 flows at a rate of 0.050 kg s^-1 through a heat exchanger where its temperature is increased from 16°C to 85°C. Heat is supplied by exhaust gases (mean specific heat capacity 1.075 kJ kg^-1 K^-1) which enter the heat exchanger at a temperature of 420°C. If the mass flowrate of the exhaust gases is 0.044 kg s^-1, determine their outlet temperature.

(b) The heat exchanger in Question 1 (a) above is of the double-pipe type, and the fluids are in counter flow. If the overall heat transfer coefficient is 35 W m^-2 K^-1, calculate the size of the heat transfer surface.

(c) What would be the new heat transfer area if the fluids were in parallel flow?

Question 3:

(a) Dry saturated steam at a temperature of 180°C is to be produced in a fire tube boiler from the cooling of 50 000 kg h^-1 of flue gases from a pressurised combustion process. The gases enter the tubes of the boiler at 1600°C and leave at 200'C. The feed water is externally preheated to 180°C before entering the boiler.

The mean specific heat capacity of the flue gases is 1.15 kJ kg^-1 K^-1. The latent heat of vaporisation of the water at 180°C is 2015 kJ kg^-1. Feed water temperature = 180°C.

Determine the amount of steam produced per hour, if the total heat loss is 10% of the heat available for steam raising.

(b) The overall heat transfer coefficient based on the outside area of the tubes is given as 54 W m^-2 K^-1. Determine the area of heat transfer required to perform this duty.

(c) The tubes within the boiler are to be 25 rnm inside diameter with a all thickness of 3 mm. The average flue gas velocity through the tubes to maintain the overall heat transfer coefficient value and to minimise pressure losses is to be more than 22 m s^-1 and less than 28 m s^-1.

Assuming that the average density of the flue gases is 1.108 kg m^-3, calculate:

(i) the minimum and maximum number of tubes in each pass

(ii)  the overall length of tubes at each of these numbers of tubes

(iii) the minimum number of tube passes in each case, if the length of a boiler tube is to be less than 5 metres.