Marine Boiler and Steam Engineering course - Operational level (MBSO)

General considerations for design of boilers

Specific Learning Objectives:

After going through the topic student should be able to,

• Know important definitions and terms used in steam engineering
• Understand the concept of various types of heat transfer
• Understand the concept of Specific heat capacity, Latent heat capacity and Sensible heat
• Understand the use of Property diagram and Steam Table
• Understand the concept of Enthalpy and Entropy
• Understand the concept of a reversible process as applicable to the steam engine thermodynamic cycle

Introduction

Steam is a form of  water and thus available in plentiful in the world. On board a ship, surrounded by the sea, it is possible to evaporate the salt water and produce distilled water which has very low solid content. It is this water which is fed into the ship's boilers to produce steam. The heat energy comes from burning the oil in the boiler furnace. The enclosed boiler is where the  steam is collected at high pressures on top of the water inside the drum. The steam, thus produced , goes for further transformation by absorbing more heat from flue gas outside the drum to become superheated vapor which is fed to the turbines to produce work. As water is heated, boiled to steam vapors and further superheated, the steam carries progressively more heat energy as its pressures and temperatures are raised in the boiler system.

So, the real job of the water or the steam is just carrying the heat from combustion of fuel in the gas side of the boiler, and deliver that to the heat engines. Whatever heat is not able to be absorbed by the turbine engine (heat engine) is rejected with the exhausted steam to the Condenser; the exhaust steam is condensed and recycled back as feedwater to the boiler. During the passage, water or steam acts only as a carrier of heat and does not  gets spent or consumed; except for some small leakage through pipelines and  atmosphere due to evaporation and radiation. If you ignore those losses, the entire steam (in the form of recycled water) goes back to the boiler. And, the steam and water  goes round and round the plant in a cyclic process. This is called a steam cycle.

The steam is such a fluid, which when pressurized, carries increasingly more heat and it's temperature can also be raised to make it into a superheated steam outside the boiler drum, so that it can carry even more heat and then deliver that to the turbines in the form of a high temperature and pressure steam which gets expanded through the turbine stages - thereby consuming the heat brought in by the steam and convert the pressure drops and changes in velocity into work by rotating the turbine wheels, which ultimately moves the ship's propeller.

Definition and Terms

There are a number of technical terms used in connection with steam generation. Some of these commonly used terms you should know are as follows:

• Degree is defined as a measure of heat intensity.
• Temperature is defined as a measure in degrees of sensible heat. The term sensible heat refers to heat that can be measured with a thermometer.
• Heat is a form of energy measured in British thermal units (BTU). One Btu is the amount of heat required to raise 1 pound of water 1 degree Fahrenheit
• Steam means water in a vapor state. Dry saturated steam is steam at the saturation temperature corresponding to pressure, and it contains no water in suspension.
• Wet saturated steam is steam at the saturation temperature corresponding to pressure, and it contains water particles in suspension.
• The quality of steam is expressed in terms of percent. For instance, if a quantity of wet steam consists of 90 percent steam and 10 percent moisture, the quality of the mixture is 90 percent.
• Superheated steam is steam at a temperature higher than the saturation temperature corresponding to pressure. For example, a boiler may operate at 415 psig (pounds per square inch gauge). The corresponding saturation temperature for this pressure is 483°F, and this will be the temperature of the water in the boiler and the steam in the drum. This steam can be passed through a super-heater where the pressure remains about the same, but the temperature will be increased to some higher figure, called the degrees of superheat.

Fire Tube Boiler

As it indicated from the name, the firetube boiler consists of numbers of tubes through which hot gasses are passed. These hot gas tubes are immersed into water, in a closed vessel. Actually, in fire tube boiler one closed vessel or shell contains water, through which hot tubes are passed. These fire tubes or hot gas tubes heated up the water and convert the water into steam and the steam remains in same vessel. As the water and steam both are in same vessel a firetube cannot produce steam at very high pressure.

Water Tube Boiler

Watertube boiler is reverse of the fire tube boiler. In water tube boiler the water is heated inside tubes and hot gasses surround these tubes.

A water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate syeam.

The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace through a superheater to become superheated. Superheated steam is defined as steam that is heated above the boiling point at a given pressure. Superheated steam is a dry gas and therefore used to drive turbines, since water droplets can severely damage turbine blades.

Cool water at the bottom of the steam drum returns to the feedwater drum via large-bore 'downcomer tubes', where it pre-heats the feedwater supply. (In large utility boilers, the feedwater is supplied to the steam drum and the downcomers supply water to the bottom of the waterwalls).

The water tube boiler is used to generate steam having high pressure and temperature. These small internal parts produce high volume steam for high-capacity applications.

Water tube boilers are the most widely used boilers. These type has replaced many boilers, including the fire tube type, mainly because of the following reasons:

• The weight of the water tube boiler is much less than any type of boiler of equivalent size.
• Steam raising and steam generation process is much faster
• The design is custom made and flexible to include any kind of modifications
• It has very high efficiency than the rest of the boilers
• The design facilitates good natural circulation of the feed water

Uses of steam on ships

1. For propulsion uses to drive the steam ships.
2. For electric power generation on steam and few motor ships.
3. Used to drive the cargo pumps in tankers ships to transfer the oil.
4. Winch and windlass are also driven by steam in some ships like tankers.
5. For operation of cargo oil pump on most oil tankers.
6. On steam ships, this steam is used for taking or making vacuum inside the condensers.
7. The stripper pumps are driven by the steam ships. For cargo heating on both steam and motor tankers.
8. For warming up the engine in cold areas and prior to starting.
9. For operation of ships whistle on all steam ships and few oil tankers.
10. Fuel oil like heavy oil and the lubrication oil in some cases are heated with the help of steam.
11. Sea chest is blown through with help of steam.
12. One major use of steam on board ship is to soot blow on most ships.
13. Derrick on some ships is driven with the help of steam only.
14. Heating of the accommodation in colder seasons.