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Fire Tube versus Water Tube Boilers by V.Ganapathy
tube thick,in ext pres,psig int pres,psig
0.105 575 1147
0.120 686 1339
0.135 800 1533
0.150 921 1730
0.180 1172 2137
[2 in OD,SA178A and SA 192 carbon steel tubes at 700F]
The basic differences or features of both types of boilers are given below.
Water Tube Boilers
Suitable for high steam pressure (above 500 psig) and temperature(to 1000 F) and large capacities exceeding millions of lb/h of steam.
Extended surfaces can be used in waste heat applications to make the boilers compact if the gas stream is clean.
Various types of fuels can be fired with ease including solid,liquid and gaseous. The water cooled membrane wall construction makes an excellent furnace.
If the gas stream is dirty(as in MSW applications)provision can be made for cleaning using soot blowers or rapping mechanisms.Wide spaced tubes can be used at the gas inlet to minimize bridging of slag deposits and tube spacing can be decreased as the gas is cooled.This flexibility does not exist in fire tube designs.
Superheaters if used can be located at the optimum gas temperature region shielded by any number of screen tubes.In fire tube boiler the choice is at the gas inlet or exit.
Due to low water volume,the startup time is lesser and response to load changes is faster compared to fire tube boilers.
If the gas pressure is high,say above 5 psig,the shell/casing design gets complicated and expensive though it can be done.
Due to higher heat transfer coefficients surface area required is lesser and hence gas pressure drop is also lower.
For multiple pressure designs as in gas turbine exhaust applications,water tube is the only choice.
Fire Tube Boilers
Ideal for low pressure steam. As seen above in the table,the tube thickness increases significantly at high pressures if the pressure is applied externally.The pressure can be nearly twice in water tube designs for the same tube thickness.
Suitable for high pressures as gas is contained inside tubes.Hence you see more of them in hydrogen,ammonia plants,where the gas pressures can be in the range of 500 to 3000 psig. 
When a  large duty has to be transferred at a low log-mean-temperature-difference as in gas turbine exhaust HRSG applications,surface area required gets enormous and very long tubes are required,adding to the gas pressure drop.The shell diameter  becomes huge;hence unsuitable except in very small gas turbine Hrsgs generating low pressure saturated steam.
Economizer and superheater can be added but the location for superheater is either at the gas inlet or exit,making it difficult to come up with a good design if corrosive conditions are present.
If slagging is a concern,then fire tube designs are generally not suitable as the tube inlet can be plastered with slag. The gas inlet temperature has to be reduced through flue gas recirculation or the gas can be cooled in an external water cooled furnace,making it a difficult design.
Cleaning the tubes is easier if  there is no slagging.In the case of water tube,the deposits can be formed on both the tubes and the casing,while in the fire tube it is only inside the tubes.
A separate steam drum with internals is required if good steam purity(0.05 to 1 ppm) has to be achieved.
In general,fire tube boilers are preferred up to 50,000 lb/h of steam in fired boilers and up to 50-100,000 lb/h of gas flow in waste heat applications generating low pressure saturated steam up to 300 to 500 psig due to cost considerations.I have seen fire tube boilers generating steam at 3000 psig but the tube and tube sheet thicknesses are large and these are exceptions. When gas pressure is very high,say above 5 psig,the fire tube design is a good choice. Of course,it is possible to design water tube boilers for gas pressures as high as 1000 psig by containing the gas in shells or annular spaces but these are expensive and special designs.

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