The data in TABLE 1 below relates to a specific heat exchanger. A reliable
colleague has looked up an effectiveness chart and says that the effectiveness
in the given operating conditions is 0.55.
Data:
‘Hot’ fluid
‘Cold’ fluid
Mass flowrate kg s-1
0.7
0.4
Specific heat capacity kJ kg-1 K-1
1.44
4.2
Inlet temperature ֩C
130
18
Overall heat transfer coefficient 112 W m-2 K-1
Area of heat transfer surface 9 m2
Determine:
(i)
The outlet temperature of the hot fluid.
(ii)
The outlet temperature of the cold fluid.
(iii)
The heat transfer rate.
Calculate the correction factor.
Dry saturated steam at a temperature of 180ºC is to be produced in a fire tube
boiler from the cooling of 40 000 kg h-1 of flue gases from a pressurised
combustion process. The gases enter the tubes of the boiler at 1200ºC and
leave at 250ºC. The feed water is externally preheated to 180ºC before
entering the boiler. (Assume the exchanger is counter-flow)
The mean specific heat capacity of the flue gases is 1.15 kJ kg-1 K-1.
Feed water temperature = 180ºC.
(a)
Find the log mean temperature difference
(b)
Calculate the heat transfer rate
(c)
Determine the area of heat transfer required to perform this duty if the overall heat transfer coefficient based on the outside area of the tubes is given as 45 W m-2 K-1.
The tubes within the boiler are to be 38 mm inside diameter with a wall thickness of 4 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 20 m s-1 and less than 26 m s-1.
The average density of the flue gases is 1.108 kg m-3.
Calculate the minimum and maximum number of tubes in each pass
Calculate the overall length of tubes at each of these numbers of tubes
Find the minimum number of tube passes in each case, if the length of a
boiler tube is to be less than 4 metres.
