Water tube boilers and Fire tube boilers

Water tube boiler

Definition of water tube boilers

Water tube boilers differ from shell type boilers in that the water is circulated inside the tubes, with the heat source surrounding them. Referring back to the equation for hoop stress, it is easy to see that because the tube diameter is significantly smaller, much higher pressures can be tolerated for the same stress.

Small water tube boiler may be manufactured and assembled into a single unit, just like packaged shell boilers, whereas large units are usually manufactured in sections for assembly on site.

Many water tube boilers operate on the principle of natural water circulation. This is a subject that is worth covering before looking at the different types of water tube boilers that are available. Figure 1 helps to explain this principle:

Cooler feedwater is introduced into the steam drum behind a baffle where, because the density of the cold water is greater, it descends in the ‘downcomer’ towards the lower or ‘mud’ drum, displacing the warmer water up into the front tubes.

Continued heating creates steam bubbles in the front tubes, which are naturally separated from the hot water in the steam drum, and are taken off.

However, when the pressure in the water tube boiler is increased, the difference between the densities of the water and saturated steam falls, consequently less circulation occurs. To keep the same level of steam output at higher design pressures, the distance between the lower drum and the steam drum must be increased, or some means of forced circulation must be introduced.

Advantages of water tube boilers:

They have a small water content, and therefore respond rapidly to load change and heat input.

The small diameter tubes and steam drum mean that much higher steam pressures can be tolerated, and up to 160 bar may be used in power stations.

The design may include many burners in any of the walls, giving horizontal, or vertical firing options, and the facility of control of temperature in various parts of the boiler (noi hoi). This is particularly important if the boiler has an integral superheater, and the temperature of the superheated steam needs to be controlled.

Disadvantages of water tube boilers:

They are not as simple to make in the packaged form as shell boilers (lo hoi), which means that more work is required on site.

The option of multiple burners may give flexibility, but the 30 or more burners used in power stations means that complex control systems are necessary.

Fire Tube Boiler

Defination of fire  tube boilers

These are one of the most fundamental and old designed boilers. These boilers (lo hoi) are very famous in the 18th century, and especially applicable for train engines. In this kind of boiler, combustion of heat, as well as gases, flows through the pipe enclosed by water. These boilers (noi hoi) may be high-pressure or low-pressure boilers. The measurement of these boilers can be done always with their external diameter. Generally, these boilers are intended for pressures up toward a highest of 250 psi & around 750 horsepower.

Advantages of fire tube boilers

• Designing of this boiler is simple and less maintenance price.
• It is applicable to small-scale industries.
• Fewer experts are enough for the operation.
• No pure water is necessary.
• Treatment of feed water is not very essential this boiler (noi hoi).

Disadvantages of fire tube boilers

• The efficiency of this boiler is up to 75%.
• The variation of the load cannot.
• It occupies more floor area.
• The working pressure of this boiler is maximum 20 bar.
• The handling of load fluctuations is not easy.

Difference between Fire Tube Boiler and Water Tube Boiler

The comparison between two boilers types water tube as well as fire tube mainly includes working pressure, material type, the rate of steam production, the requirement of floor area,  efficiency, load handling variation, design, operator skills, the cost for maintenance are discussed below.

• In fire tube, the flow of hot flue gases will be through pipes and enclosed with water whereas, in water tube, the flow of water will be through pipes and enclosed with hot flue gases.
• The low-pressure range of fire tube is up to 20 bar whereas in water tube up to 250 bar.
• For efficient operation, fire tubes require fewer experts whereas in water tubes require experts
• The load fluctuations in the fire tube cannot be handled whereas, in a water tube, it can be handled easily.
• This boiler occupies more floor area for output whereas, in a water tube boiler, it occupies less area.
• The design of the fire tube is simple, easy to upright, and maintenance price is low whereas in water tube, the design is complex, not easy to upright, and maintenance price is high.
• Fire tubes are large and not easy for transporting whereas, in a water tube boiler, it is not an issue.
• The size of the drum in fire tube boiler is large as well as injury caused by burst is also large whereas in water tube if any water tube gets damage then we can replace it easily.
• The efficiency of fire tubers are up t0 75% whereas in water tube boiler is up to 90%
• The flow of direction in fire tube is not in an exact direction whereas, in water tube boiler, it flows in an exact direction.

Thus, this is all about fire tube boilers, water tube boilers and its types. The applications of this boiler mainly involve in marines, railways, plants, and industries.