A marine boiler is available in both fire-tube and water-tube versions. In (marine-based) industrial applications, where the required final steam temperature is above par (higher than the usual) and at premium pressure, a marine boiler with its water-tube arrangement is used, such as in marines, cargo ships, small passenger liners, and so on. However, with average to moderate level steam temperatures, marine boilers with fire-tube settings are subjected to commercial as well as industrial use.
In the present post, we shall discuss both modalities of the marine boiler in the following section.
Types of Marine Steam Boiler
Commercially, there are many designs of marine steam boilers. Some of them include:
- Single-Ended Scotch Marine Boiler (fire-tube)
- Double-Ended Scotch Marine Boiler (fire-tube)
- Yarrow Marine Boiler (water-tube)
- Babcock and Wilcox Marine Boiler (water-tube)
Single-ended Scotch Marine Boiler
Its industrial name is a scotch type of marine boiler, though, there is no Scottish background behind it.
Its construction and operation is given below:
- It consists of a robust cylindrical shell that is designed to withstand high steam pressures.
- Its design does not comprise a single furnace and combustion chamber couplet. Rather, it is provided to have one to four furnaces that have cylindrical geometry. These furnace cylinders are fabricated to be corrugated, which means they have grooves over their surface. All the furnaces are internally fired. They are surrounded by the water to be converted into steam.
- The combustion chamber lies at the back end of the furnace. It is also surrounded by plenty of water. Usually, each combustion chamber is connected to one furnace at a time. But, in many somewhat contemporary designs, two or three furnaces open up into a common combustion chamber.
- Two main circular tube plates are drilled with numerous holes for retaining fire tubes: one is known as the front tube plate, and the other as the rear tube plate. The fire tubes, which are also called the flue tubes, run from the front tube plates and end in the rear tube plate.
- For cleaning and inspection, special doors are provided.
- Since the shell of the single-ended scotch marine boiler has flat ends, they stay in place by means of stays known as longitudinal stays. Some of the smoke tubes which are screwed into the flat tube plates also work as a stay for the tube plates. The combustion chambers have flat plates which are stayed by means of supports known as screw stays. The flat tops of all CCs are regulated by stays called girder stays.
Find below in the left drawing the cross-sectional view of the Scotch marine boiler and on the right its longitudinal view.
In its operation, a draught (negative pressure or a light vacuum) is produced in the combustion chamber, due to which hot gases produced in the furnace are drawn into the combustion chamber.
These gases are now destined to pass in the smoke tubes, where they collect in the smoke-box situated at the front end of the boiler. From the smoke box, the gases pass through the uptake to exit the chimney.
It is informative to know that the walls of the combustion chamber are a great source of heating; they act as the best convective surface for heat transfer in the boiler.
Compared with the cubicle size of the boiler, the single-ended scotch marine boiler offers multiple convective surfaces for rapid and efficient heat transfer. That is, being surrounded by water, and all smoke-tubes, furnace tubes, and combustion chambers provide a large heating area for optimal convection.
Double-ended Scotch Marine Boiler
In its construction, it acts as two single-ended scotch marine boilers placed back to back.
The main parts of the double-ended scotch marine boilers are the same as that of their single-ended counterpart with a few exceptions as below:
- It has two furnaces, One at each end.
- It has one centrally shared combustion chamber.
- It has two smoke-boxes: one at each end.
In its operation, the furnaces at each end open into a centrally placed combustion chamber. Upon fuel firing, the flue gases enter the smoke-tubes. From that place, they collect into the smoke-box placed at each end and into the chimney.
For the same evaporative capacity, the double-ended type is more beneficial due to its fair economy, less foot-print, lighter weight, and so forth.
Babcock and Wilcox (B-W) Marine Boiler
It is a water-tube boiler and is known best for its efficient performance. Its longitudinal section is illustrated below:
The main parts of the B-W boiler are mentioned below:
1. Boiler Drums
In simple construction, it consists of one drum for both water and steam. However, in its different versions designed later on in the late nineteenth century, there was but one steam drum above one or two water drums. Both are axially horizontal.
At present, the W&B boiler with a single drum will be studied.
There are two headers: one is the lower and the other upper. The lower header is connected to the rear end of the drum and receives feedwater. The upper header is connected to the front end of the drum and transfers a mixture of water and steam to the drum.
The number of these double headers varies. With the help of short riser tubes, the water-steam drum is connected to a series of front-end and rear-end headers.
3. Water Tubes
To these pressed-steel headers are expanded a number of water tubes, which are oriented in an inclined fashion. These tubes are inclined at 22 degrees with the horizontal.
These tubes are made up of solid drawn mild steel. They are expanded in the header in a staggered or zig-zag manner, the purpose of which is to provide maximum surface area of tubes exposed to the burned gases rising above the furnace. It is illustrated below.
A man-hole provided in front of each tube in the header is used for cleaning and inspection of that tube. It is covered by a steel cap secured in position by a steel clamp.
Each tube is typically machined to have 4 inches diameter and 18 feet length.
4. Mud Drum
A mud drum is provided at the lowest end of the inclined water tubes for collecting the sediment in the water. It is made up of cast iron and is designed to be at the bottom of (all) lower headers.
The grate is given at the bottom of the upper headers.
6. Blow-off valve
The mud or sediment collected in the mud drum is blown off with the help of a blow-off valve. This valve is timed to operate at regular intervals for the periodic cleaning of the mud drum.
Usual mountings support it as for other boilers stated already.
It is a critical component. The hot gases, after passing through the bank of the inclined tubes (yet before the expansion of steam in the steam turbine or any other downstream equipment), travel over the superheater to raise the temperature of the saturated steam still further.
In the superheater, the saturated steam coming from the water drum (over and above water level) travels in the tubes of the superheater, which are exposed to the hot gases on their way to the chimney.
In operation, the burned gases rise above the furnace and collide with the water tubes above them. Since there are separations made in brickwork between the tubes, which are called fire-brick baffles, the burned gases moving upwards are deflected by these baffles as they change the direction of these gases.
The gases in the course of their turbulent flow, transfer heat to the water in the tubes and finally travel downward between the tubes to exit with a chimney or stack. Such an engineering design is supportive of an efficient transfer of heat even to the highest point of the water tube.
Through the feedwater inlet, the feed-water enters the front end of the drum and passes on its rear end. From there, it descends and via. downcomer tubes collect in the lower headers.
Water from these lower headers now enters into the inclined tubes. Upon receiving heat from the burned gases, it converts into steam and moves upwards due to being lighter on account of lower density and passes through upper headers and risers to the drum eventually.
Thus, the circulation of water is governed by the density difference between water and steam, which again is governed by the temperature difference between the front and rear parts of the furnace. In more technical terms, the complete circulation of water is chiefly dependent on the thermo-syphon effect.
The steam and water drum of the boiler is suspended in its place not with the help of brickwork or brick setting but by horizontal beams via. metallic chains or slings or straps, which in turn are supported on cast iron columns.
This setting is advantageous as it escapes the brickwork, which poses troubles due to expansion. However, the minimal brickwork is only for housing the furnace and provides an enclosure for the hot gases to keep them from escaping.
Yarrow Marine Boiler
It is another type of water-tube-based marine boiler that was used in marine ships with the consent of admiralty in the late nineteenth or early twentieth century. Today, it is used in the British Navy.
Its two typical designs include:
- Single-flow Yarrow Boiler
- Double-flow Yarrow Boiler
Construction & Operation
In its earlier designs, the yarrow boiler consisted of two water drums connected to one forged steam drum through straight tubes. A steel shell houses all three drums and is made resilient enough to withstand the high pressure of the steam.
An air heater is usually located in the path of the incoming air to preheat the air for combustion purposes by extracting heat from the exhaust gases, which would otherwise be lost in the atmosphere.
These water tubes were expanded and fitted in the usual manner at the respective end of the steam drum. There were downcomers attached to each of the water drums. As can be seen, each water drum, which is also known as a water pocket, is D-shaped or, in some designs semi-circular.
Its top is fitted with a flat tube plate. In view of certain technical difficulties associated with design and operation, the D-shaped geometry was replaced by the cylindrical configuration of the drum.
In early design, there were two steam drums connected through U-tubes. But, in later designs, only one superheater design was adopted. This design consisted of one large steam drum connected to three water drums via. straight tubes as shown.
Each water drum consisted of three-row, four-row, or eleven-row of water tubes. Now, the superheater consisted of another drum situated between any two of the water drums with a bank of u-tubes expanded and fitted into the superheating drum.
When a superheating control was needed, that is, a rapid change in the temperature of the superheated steam, a double-uptake was used.
The products of combustion were passed only through the path where a superheater was installed. A damper was used on the opposite side to deflect the proportion of gases in order to control steam temperature, as shown in the schematic diagram below.
1. Double Flow Yarrow Boiler
Thus, the configuration of the yarrow boiler fitted with two uptakes and a damper to restrict the flow of burned gases in order to have control over superheated steam temperature is known as double flow yarrow boiler. It is shown below.
2. Single Flow Yarrow Boiler
Meanwhile, a yarrow boiler with a single uptake and no damper, in which all the products of combustion are meant to pass over the superheater and ‘always’, is called a single-flow yarrow boiler.
I am the author of Mechanical Mentor. Graduated in mechanical engineering from University of Engineering and Technology (UET), I currently hold a senior position in one of the largest manufacturers of home appliances in the country: Pak Elektron Limited (PEL).