Question

 

(a) Dry saturated steam at a temperature of 180ºC 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 1600ºC and leave at 200ºC. The feed water is externally preheated to 180ºC 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 180ºC is 2015 kJ kg–1. Feed water temperature = 180ºC. 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 mm inside diameter with a wall 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.

Answer 1