Comparison between fire tube boiler and water tube boiler?
The reason is to be found in thermodynamic nature. To get a high efficient water-steam-cycle a high temperature and pressure of the steam is required. The higher pressure and temperature the more effective process you get.
Water tube boilers are better suited for the high pressures and temperatures requirements. Higher pressures require greater wall thickness, but this reduces heat transfer. The practical limit is about 16bar (240psi).
Fire / smoke tube boilers are also more prone to catastrophic failure. The water - and energy - in a Scotch boiler, say at a modest pressure of 10 bar, is enough to launch the boiler through bulkheads and accident in the boiler room.
Water tube boilers lend themselves to modern modular construction techniques more than fire tube boilers. Even in field-erected water-tube boilers, most of the fabrication - for instance, water-wall panels - can be done in a workshop under controlled conditions or using automated equipment. This improves quality and reduces construction time and costs.
Fire tube boilers are due to their big shell diameter not suitable for high pressure and temperature.
A fire tube boiler would have to be very large, and all that energy in the water is just sitting there. If there were a long seam burst, the deluge of water and steam would be immense and could injure many people. Also, due to its mass, and fact that they are floor mounted, they create a lot of pressure on the foundation. So, the nature of most fire tube boilers is for small to medium manufacturing facilities. Many plants have 2 or more and these can be distributed around the plant where needed.
Water tube boilers by comparison carry very little water and can be often started up quickly as they do not have a large amount of water should they have a failure, it will likely be minimal and mainly localized in a wall tube. Water tube Boilers, in large plants are suspended from the ceiling and expand downwards to the floor, but don't touch the floor. The building structure supporting the boiler has the boiler mass distributed over a larger area. Water tube boilers can also create superheated steam with the use of a super heater, and have a much better economizer and / or air heater arrangement as there is much more room for these ancillaries. This makes the water tube boiler much more efficient. The main downside is the size and cost of a water tube boiler often prohibits an extra standby boiler. So, in the event of a tube failure, the plant must often shut down for repairs.
The water tube boiler designs have a number of technical advantages even in this range but are very much more expensive. The Fire Tube Boiler is also much more forgiving then the Water Tube Boiler due to their larger thermal mass. The major limitation is boiler cleaning frequency if the gas is dirty which then becomes an assessment of down-time against initial cost.
The main advantage, at these low steaming rates, of the fire tube boiler is its ease of fabrication and lower price since the water tube boiler requires a steam drum, a mud drum, bending of the boiler bank tubes and refractory lining. In contrast the fire tube arrangement requires one single shell, the heads , the tube sheets and a simple arrangement of tubes.
A small list of why the water-tube boiler is used for high pressure and temperature rather than fire tube arrangement is as follows:
1) Safety.
An explosion in the furnace of a water-tube boiler can be mere puff that can warp the
furnace tubes or bend buck stays but not much more. An explosion into the tightly contained gas passages of a fire-tube arrangement can probably blow off the tube sheets turning them into missiles.
2) Size
A fire-tube boiler would have to be enormous to generate say 100 t/h of steam. So big it would prove very difficult to install.
3) Superheat
Imagine you try to superheat the 100 t/h in the already gigantic fire tube boiler to 400 deg C. You end up with a real leviathan.
4) Fuels
In the good old times, the flue gases were generated in a separate furnace. There was this chap with a shovel that shoveled coal into the furnace and from there, flue gases went to the fire-tube boiler. The fire tube boiler was more of a heat recovery device.
The modern water-tube arrangements have a steam generating water-wall furnace that can fit spreader stokers, dump grates, Dutch ovens, sloping grates, etc, etc to fire whatever your heart wishes. Furthermore, the fluidized bed design can be adopted into any type of water tube boiler.
5) Steam temperature and pressure
In this energy thirsty world, utilities are resorting more and more to the critical and supercritical water-wall steam generators, that are always of the once-through type. They can superheat and reheat to more than 500 deg C. This could have only been achieved with the once-through water tube design
Fire tube boilers have limitation of pressure & temperature as only saturated steam can be obtained, for power generation high pressure superheated steam is needed for turbine which is possible in water tube boilers with integral super heaters. Water tube boilers are good for cyclic loads as the steam drum has holding capacity more than fire tube design steam space. Fire tube boilers have a constraint of fuel selection such as oil / gas. There are integral furnaces fire tube boilers for solid fuels too but the overall efficiency of this design is poor. The water tube boilers can be fired with any fuel and has a varied scope of fuel selection. With very efficient modes of firing such as FBC, CFBC etc. water tube boilers are safer to operate than fire tube boilers.
All designs have their respective limitations, Fire Tube design is limited to low pressure, low capacity output only since it is design and constructed that much only.
Water tube boiler is the fast steam generation and low water holding capacity boiler so we can design compact type boiler than fire tube boiler and water tube boiler has more heating surface than fire tube boiler.
Water tube boilers are suitable for high pressure and temperature due to their small tube and header diameters.
Fire tube boilers cannot be designed for high pressure and steam generation capacity. This is due to limitation in shell thickness which is not feasible beyond a limit.
Water tube Boilers has forced circulation while it is not there in case of fire tube Boilers.
Most modern packaged fire tube boilers can achieve between 85 to 90% efficiency (LCV) without any problem; this efficiency can then be enhanced by a further 4 to 5% by the addition of an economizer. Also if there is an need for a hot water supply and the products of combustion are suitable, then an additional condensing economizer can be utilized to recover the latent heat from the moisture in the flue gas, which in some cases will take the LCV efficiency up to 100%.