Boilers MCQ Quiz - Objective Question with Answer for Boilers - Download Free PDF

Concept:

We use the principles of boiler thermodynamics to determine the equivalent evaporation, which represents the amount of water that would be evaporated under standard conditions (100°C and 1 atm) using the same heat input as the actual boiler operation.

Given:

• Steam production rate = \( 2000 \, \text{kg/hr} \)
• Enthalpy of steam, \( h_{steam} = 2426 \, \text{kJ/kg} \)
• Feedwater temperature = \( 40°C \)
• Enthalpy of feedwater, \( h_{fw} = 168 \, \text{kJ/kg} \)
• Enthalpy of vaporization at 100°C, \( h_{fg} = 2258 \, \text{kJ/kg} \)

Step 1: Calculate heat added per kg of steam

\( \text{Heat added} = h_{steam} - h_{fw} = 2426 - 168 = 2258 \, \text{kJ/kg} \)

Step 2: Compute equivalent evaporation

\( \text{Equivalent Evaporation} = \frac{\text{Steam production} \times \text{Heat added}}{h_{fg}} = \frac{2000 \times 2258}{2258} = 2000 \, \text{kg/hr} \)

Explanation:

1. Lancashire Boiler (a):

• Type: Horizontal double fire tube boiler
• The Lancashire boiler is a horizontal, stationary, fire-tube boiler with two large fire tubes running along its length. It is commonly used in industries for generating steam at low to medium pressures. The boiler consists of a cylindrical shell containing water, through which the fire tubes pass to heat the water and generate steam.
• Correct Match: 2 (Horizontal double fire tube)

2. Cornish Boiler (b):

• Type: Horizontal single fire tube boiler
• The Cornish boiler is similar to the Lancashire boiler but has only one fire tube running through its length. It is a simple, horizontal, stationary boiler used for producing low-pressure steam. The single fire tube makes it less efficient than the Lancashire boiler but more compact in design.
• Correct Match: 5 (Horizontal single fire tube)

3. La Mont Boiler (c):

• Type: High-pressure water tube boiler
• The La Mont boiler is a high-pressure water tube boiler known for its use in power plants. It uses forced circulation of water and is designed to produce large quantities of steam at high pressures and temperatures. The boiler's efficiency is enhanced by the use of water tubes, which provide a larger heating surface area.
• Correct Match: 1 (High pressure water tube)

4. Cochran Boiler (d):

• Type: Vertical multiple fire tube boiler
• The Cochran boiler is a vertical, stationary fire-tube boiler commonly used in small industries and workshops. It consists of a cylindrical shell with multiple fire tubes arranged vertically. The compact design and ease of operation make it suitable for applications requiring low-pressure steam.
• Correct Match: 3 (Vertical multiple fire tube)

Explanation:

Mechanical Draught in Modern Boilers

• Mechanical draught in boilers refers to the use of fans, blowers, or other mechanical devices to control the flow of air into the combustion chamber and the removal of flue gases. This is a critical aspect in modern boilers to ensure efficient combustion and proper evacuation of exhaust gases.

This option correctly identifies the reasons why stacks alone cannot create enough mechanical draught in modern boilers. Let’s analyze the factors:

(A) The rate of air supply as well as the rate of flue gas removal is high:

• Modern boilers are designed to operate at high capacities, requiring a significant amount of air for combustion. Likewise, the removal of flue gases must match the rate of combustion to prevent backpressure and ensure uninterrupted operation. The natural draught created by stacks is insufficient to handle these high rates, necessitating mechanical aids.

(B) Various heat exchangers are used on the way cause large pressure drop:

• Modern boilers incorporate various heat exchangers, such as economizers, air preheaters, and superheaters, to recover heat from flue gases and improve thermal efficiency. These components create resistance to the flow of gases, resulting in a substantial pressure drop. Natural draught alone cannot overcome this resistance, making mechanical draught essential.

Explanation:

Cooling Tower in a Steam Power Station

• A cooling tower in a steam power station is a heat rejection device that is designed to lower the temperature of the cooling water used in the condenser. This is achieved by dissipating the waste heat into the atmosphere, either through evaporation or by direct heat exchange. Cooling towers play a critical role in maintaining the thermal efficiency of power stations by providing cooled water to the condenser, which in turn improves the vacuum and enhances the overall efficiency of the cycle.

Working Principle: The cooling tower works on the principle of heat transfer. Warm water from the condenser is pumped to the cooling tower, where it is distributed and exposed to the ambient air. Through direct contact or evaporation, the heat is rejected to the atmosphere, and the cooled water is collected at the bottom of the cooling tower. This water is then recirculated back to the condenser for reuse. The efficiency of the cooling tower is influenced by factors such as the design of the tower, the flow rate of water, the temperature difference, and the wet-bulb temperature of the surrounding air.

Types of Cooling Towers:

• Natural Draft Cooling Towers: Use natural air convection to remove heat. These towers are typically large and require no external power for air circulation.
• Mechanical Draft Cooling Towers: Use fans to force or draw air through the tower, which enhances the cooling efficiency and allows for more compact designs.
• Wet Cooling Towers: Rely on the evaporation of a portion of the water to reject heat, making them highly effective in reducing water temperature.
• Dry Cooling Towers: Use air to cool the water without any direct contact, eliminating water loss due to evaporation but typically less efficient.
• Hybrid Cooling Towers: Combine wet and dry cooling methods to achieve a balance between water conservation and cooling efficiency.

Explanation:

Statement 1: Pulverized fuel gives high and controlled burning rate.

• This statement is correct. Pulverized fuel refers to finely ground or powdered fuel, typically coal, which is used in many thermal power plants. The fine particles have a larger surface area per unit mass, allowing for better mixing with air and leading to a high and controlled burning rate. This results in efficient combustion and better control over the combustion process.

Statement 2: Insufficient air causes excessive smoking of exhaust.

• This statement is also correct. Combustion requires a proper air-fuel ratio to ensure complete burning of the fuel. If the amount of air supplied is insufficient, it leads to incomplete combustion, resulting in the formation of carbon particles and other unburned substances. These unburned particles cause excessive smoke in the exhaust gases.

Statement 3: Excess air is provided to control the flue gas temperature.

• This statement is incorrect. Excess air is provided primarily to ensure complete combustion of the fuel, especially when the air-fuel mixture is not perfectly balanced or when there are variations in fuel quality. While it can indirectly influence the flue gas temperature, its primary purpose is not to control the temperature but to achieve complete combustion and minimize the formation of carbon monoxide, soot, and other pollutants. In fact, excessive amounts of excess air can lead to heat loss and reduced efficiency.

Statement 4: Effect of sulfur in fuel is to give high heat transfer rate.

• This statement is incorrect. Sulfur in fuel does not enhance the heat transfer rate. Instead, sulfur can have detrimental effects, such as forming sulfur dioxide (SO₂) and sulfur trioxide (SO₃) during combustion. These compounds can combine with water vapor in the flue gases to form sulfuric acid, which can corrode equipment and reduce the efficiency of heat exchangers. Sulfur is generally considered an undesirable component in fuel due to these adverse effects.

Explanation:

Boiler:

• According to A. S. M. E. steam generating unit or boiler is defined as "A combination of apparatus for producing, furnishing or recovering heat together with the apparatus for transferring the heat". So made available to the fluid being heated and vapourised.

Types of boiler:

Water Tube Boiler:

• When water is contained inside the tubes (called water tube) which are surrounded by flames and hot gases from outside, then the boilers are named as water tube boilers.
• They are safe, quick steaming, flexible in construction and operation.
• These boilers are extensively used because they can be built for high pressure and large evaporative capacities.

Types of Water Tube Boilers

• Babcock and Wilcox boiler
• Stirling boiler
• Lamont boiler
• Benson boiler
• Loeffler boiler, etc.

Fire Tube Boilers:

• When the flames and hot gases, produced by combustion of fuel, pass through the tubes (called multitube) which are surrounded by water, then the boilers are named as fire tube boilers.

Types of Fire Tube Boilers:

• Simple vertical boiler
• Lancashire boiler
• Cochran boiler
• Locomotive boiler

Supercritical boiler:

• A large number of steam generating units are designed between working ranges of 125 atm and 510°C to 300 atm and 660°C. These are basically characterised as subcritical and supercritical. Subcritical consists, preheater, evaporator and superheater while supercritical boiler requires only preheater and superheater.
• Supercritical boilers operate at a pressure greater than 22 MPa and also referred to as Once through boilers since the feed water circulates only once through boiler in each steam cycle.

Concept:

Fire Tube Boiler: In this boiler, the hot flue gases are present inside the tubes and water surrounds them. They are low-pressure boilers. The operating pressure is about 25 bar.

Example to fire tube boilers:

• Cornish boiler
• Cochran boiler
• Locomotive boiler
• Lancashire boiler
• Scotish marine boiler

Water Tube Boiler: In this boiler, the water is present inside the tubes and the hot flue gases surround them. They are high-pressure boilers. The operating pressure is about 250 bar.

Examples of water tube boilers:

• Stirling boiler
• Babcock and Wilcox boiler
• Yarrow boiler
• La mont boiler
• Loeffler boiler
• Velox boiler

In a locomotive boiler, the draught is produced by a steam jet.

The draught is one of the most essential systems of the thermal power plant which supplies the required quantity of air for combustion and removes the burnt products from the system.

Natural draught: A draught produced by a chimney due to the difference of densities between the hot gases inside the chimney and cold atmospheric air outside it.

Induced draught: The air pressure at the fuel bed is reduced below that of the atmosphere by means of a fan placed at or near the bottom of the chimney.

Steam jet draught:

a)Induced steam jet: The draught produced by a steam jet issuing from a nozzle placed in the chimney.

b)Forced steam jet: The draught produced by a steam jet issuing from a nozzle placed in the ashpit under the fire grate of the furnace. Example: Locomotive boiler

Balanced draught: It is a combination of induced and forced draught.

Explanation:

A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various process or heating applications including water heating central heating, boiler-based power generation, cooking, and sanitation.

A fusible plug is a safety device used in fire-tube boilers for the following reasons:

• In a fire-tube boiler, hot gases generated from the combustion of fuel pass through tubes containing water, which is being heated to produce steam. These tubes can sometimes become overheated due to various factors, such as a low water level in the boiler or a malfunction in the burner system. If the temperature inside the boiler exceeds a safe limit, it can lead to catastrophic failures like a boiler explosion.
• To prevent such dangerous situations, a fusible plug is installed in the crown sheet (the top portion) of the fire-tube boiler. This plug is typically made of a material with a lower melting point than the boiler's metal. If the water level drops too low or the temperature becomes excessively high, the fusible plug will melt, creating an opening. This allows water to enter the combustion chamber, reducing the risk of a catastrophic explosion by cooling down the boiler and preventing direct contact between the hot gases and the boiler's metal.

Therefore, a fusible plug is an essential safety device in fire-tube boilers to protect against overheating and potential boiler explosions, making option 3, "Fire-tube boiler," the correct choice.

Additional Information

Boiler Mountings

• Boiler mountings are components used for ensuring the safety of boiler operation.
• These are generally mounted on the surface of the boiler.
• Control fluid parameters at the inside of the boiler shell.
• The mountings are an essential part of a boiler, without which boiler operation is impossible.
• Examples: Pressure gauges, Water level indicators, Safety valves, Stop valve, Fusible plug, Blow-off cock, etc.

Boiler Accessories:

• Accessories are the auxiliary items required for the proper operation of the boiler and to improve its boiler efficiency of it.
• These are integral parts of the boiler, but not mounted on it.
• Control fluid parameters outside of the boiler.
• These are not essential parts of the boiler, without which the boiler can operate at lower efficiency.
• Examples: Superheater, Feed Pump, Injector, Economizer, Steam Separator, Air preheater, etc.

Note: A fusible plug must be used primarily for safety purposes in Fire-tube boilers.

Concept:

Explanation:

Equivalent evaporation is the amount of water evaporated "from and at 100°C" into dry and saturated steam at 100°C at a standard pressure of 1.01325 bar.

The equivalent evaporation from and at 100°C is given by:

The equivalent evaporation = \(\frac{{Total\;heat\;required\;to\;evaporate\;feed\;water}}{{2257}}\)

Total heat required to evaporate 'm' kg of water:

\(E = m\left( {h - {h_f}} \right)\)

Where hf = sensible heat of feed water or fluid enthalpy

Explanation:

• A Steam Trap is an automatic drain valve which distinguishes between steam and condensate.
• A steam trap holds back steam & discharges condensate under varying pressures or loads.
• The steam traps should have a good capacity to vent out air and other non - condensable gases quickly while holding back the live steam.
• A steam trap automatically drains away the condensed steam from the steam pipes, and steam jackets without permitting it to escape.

Explanation:

Spring loaded safety valve:

• One of the common type of safety valve is the spring loaded safety valve, which uses spiral spring to apply the closing or spring force.
• It is used to protect the vessels, pipes, container from over pressure.
• The spring force rises as a pressure relief valve starts to lift. Thus, for lift to continue, system pressure must be increases.
• The pressure relief valves are given an over-pressure allowance to achieve full lift because of this. 
• In unfired systems, this permitted over-pressure for valves is typically 10%.
• One helical spring is used in a spring loaded safety valve.

Explanation:

Air preheater:

• Air preheaters are used in large boilers found in thermal power stations producing electric power.
• An air preheater or air heater is a general term to describe any device designed to heat air before another process (for example, combustion in a boiler) with the primary objective of increasing the thermal efficiency of the process.
• The purpose of the air preheater is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas.

Superheater:

• A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam
• Superheated steam is used in steam turbines for electricity generation, steam engines, and in processes such as steam reforming
• While the temperature of the steam in the superheater rises, the pressure remains the same as that of the boiler.
• Superheater is placed in the path of hot flue gases from the furnace.

Economiser:

• An economiser is a mechanical device that is used as a heat exchanger by preheating a fluid to reduce energy consumption.
• In a steam boiler, it is a heat exchanger device that heats up fluids or recovers residual heat from the combustion product i.e. flue gases in a thermal power plant before being released through the chimney.
• Economiser is situated before air pre-heater in the flue gas path. 
• Economiser increases the efficiency of the power plant.

Injector:

• It is a device used to feed the water into the boiler.
• It is usually employed when space is less and a feedwater pump cannot be employed.
• It is used for vertical and locomotive boilers.
• Steam from the boiler is accelerated by a nozzle and as the pressure at the exit is low, therefore it enters into feedwater.

Explanation:

According to the method of circulation of the water, boilers are classified

• Natural circulation: If the circulation of water takes place due to the difference in density caused by the temperature of the water, then it is called a natural circulation boiler. 
• Forced circulation: If the circulation water takes place with the help of an external pump, then it is called a forced circulation boiler.

Boiler with the natural circulation of water

• Lancashire boiler
• Babcock-Wilcox boiler
• Cochran boiler
• Locomotive boiler

Boiler with forced circulation of water

• La Mont
• Benson
• Velox