Steam Properties and Unit Conversion
Question (Maharashtra BOE 2023, 5 marks)
Convert the units as mentioned:
(a) 1 Gallon = ____ litres
(b) 9900F = ____ 0C
(c) 2 sq. inch = ____ sq. mm
(d) 1 kg = ____ Pound
(e) 1 ATA = ____ Bar
Answer
(a) 1 US gallon = 3.78541 litres
Hence, 1 Gallon = 3.785 L
(b) \(C = \frac{5}{9}(F - 32)\)
Putting values,
9900F = 532.22°C
(c) 1 inch = 25.4 mm → 1 sq. inch = 25.4×25.4 = 645.16 mm²
2 sq. inch = 2×645.16 = 1290.32 mm2
(d) 1 kg ≈ 2.20462 lbs
(e) 1 ATA = 1 standard atmosphere (atm)
1 atm ≈ 1.01325 bar
Question (Jharkhand BOE 2024, 5 marks)
Convert it:
1. 1 bar = ____ Torr.
2. 860 kilocalorie = ____ Btu.
3. 1 metric hp = ____ Watt.
4. 1 mm WC = ____ Pascal.
5. 10C = ____ 0F
Answer
(a) 1 bar = 750.062 Torr
(b) 1 kcal = 3.96567 Btu
860 kcal×3.96567 Btu/kcal≈3410.48 Btu
(c) 1 metric hp (PS) = 735.499 W
(d) 1 mm H₂O (Water Column) = 9.80665 Pa
(e) 10°C = 50°F
Hint:\(C = \frac{5}{9}(F - 32)\)
Question (Kerala BOE 2021, 5 marks)
Explain the term ‘quality of steam’. Differentiate between
wet, dry saturated and superheated steam.
Answer
The quality of steam refers to the ratio of the mass of steam to the total mass of the mixture of steam and water present in a steam system.
Steam quality (often expressed as a percentage) indicates how "dry" or "wet" the steam is. It is calculated using the formula:
\(Quality = \frac{{{m_{steam}}}}{{{m_{steam}} + {m_{water}}}}\)
A quality of 100% means the steam is completely dry, while a quality of less than 100% indicates the presence of liquid water in the steam.
Question
Fill in the blanks
(i) 1 J is equivalent of ____ Nm of work.
(ii) T = t + ____, where T = Temperature expressed in centrigrade absolute scale, (K).
(iii) Absolute pressure = ____ + Atmospheric pressure.
(iv) Volume of wet steam = x × ____
where x = dryness fraction.
(v) Enthalpy of wet steam = Enthalpy of liquid + ( ____ × dryness fraction).
(This also applies to Entropy and Internal Energy)
(vi) 1 N/m2 = ____ bar = 0.10197 kgf /m2 of pressure.
Answer
(i) 1
(ii) 273
(iii) Gauge pressure
(iv) Vs = Specific volume of saturated Steam
(v) Latent heat
(vi) 10-5
Fuels, Combustion and Fuel Gases
Question (Karnataka BOE 2022, 2 marks)
What are the harmful effects of sulphur present in coal when it is used as a fuel in boiler?
Answer
- Acid Rain: When coal is burned, sulfur dioxide (SO₂) is released into the atmosphere. This gas can react with water vapor to form sulphuric acid, contributing to acid rain, which can harm ecosystems, soil, and water bodies.
- Air Pollution: Sulfur emissions contribute to air quality issues and can lead to respiratory problems in humans and animals.
- Corrosion: High sulfur content can lead to increased corrosion of boilers and other equipment used in coal combustion. This can result in higher maintenance costs and reduced operational lifespan of equipment.
- Clinker Formation: Sulfur can contribute to the formation of clinker in boilers, which can impede heat transfer and efficiency.
- Combustion Efficiency: Coal with high sulfur content may burn less efficiently than low-sulfur coal. This inefficiency can lead to increased fuel consumption and higher greenhouse gas emissions per unit of energy produced.
Question (Haryana BOE 2019, 1 mark)
What is fixed carbon?
Answer
Fixed carbon contents of coal is the carbon found after volatile matter is removed. It is determined by removing the mass of volatile matter from the original mass of coal sample. This fixed carbon differs from the ultimate carbon contents of coal because some carbon is lost with volatiles.
Question (Haryana BOE 2019, 1 mark)
What is the proximate analysis.
Answer
In this analysis, the range of constituents like moisture, volatile matter, ash and carbon is measured.
Question (Haryana BOE 2019, 1 mark)
What should be the value of the polydispersity coefficient for pulverised coal?
Answer
n > 1.
In this case, the coal dust will contain a small concentration of very fine fractions and a small concentration of coarse fractions.
Its importance lies in its characterization of the structure of dust and its particle size distribution.
Question (Karnataka BOE 2023, 4 marks)
Explain cyclone combustion, flame combustion and rapid combustion.
Answer
Cyclone combustion uses a spiral airflow to promote efficient burning of fuel, while flame combustion is the general process of rapid oxidation with heat and light. Rapid combustion, also known as burning, is a fast combustion process releasing heat and light quickly, exemplified by burning fuels like LPG or kerosene.
Question (Chhattisgarh BOE 2021, 2 marks)
Why do liquid fuels with lower specific gravities have higher heating values per unit mass?
Answer
Liquid fuels with lower specific gravities (meaning they are lighter for the same volume) tend to have higher heating values per unit mass. This is because lower specific gravity often correlates with a higher proportion of lighter, more volatile hydrocarbons, which typically burn more readily and release more energy per unit mass during combustion.
Example: Gasoline (specific gravity 0.74) has a higher heating value (46 MJ/kg) than heavy fuel oil (specific gravity ~0.95, heating value ~40 MJ/kg) because gasoline contains lighter hydrocarbons with more hydrogen and fewer heavy impurities.
Question (Jharkhand BOE 2024)
What is clinker?
Answer
In grate firing, during the process of combustion, fuel ash melts and turns jellylike, which attaches coal and ash particles and also fines turning into lumps called clinker. Towards the end of combustion, such small clinker formation is quite natural. When this process starts in the middle of combustion, the clinker does not stop at small lumps but grows to form big stone-like masses disturbing the bed and combustion. the same place. Clinker formation is a nightmare to any stoker operator.
Question (Assam BOE 2022, 2 marks)
Why are different types of chimneys in practice? Explain the basis of their selection.
Answer
- Steel Chimney. This type of chimney is preferred for short exhaust stacks where the draft is created by fan. To increase the life of the chimney, these are lined with bricks. The erection period of this type of Chimney in Power Plant is less compared to that of other types. This type of chimney is constructed by welded section and erected by horizontal welding joints (or) by riveting.
- Site constructed chimneys. Site constructed chimneys of brick (or) concrete are used where very tall chimneys are necessary for power stations (or) where the life requirement is more than 30 years. Nowadays perforated radial bricks are used, instead of common bricks, as they give better results. The perforations aid the structural stability. The crushing strength of perforated radial brick is more than that of the design value required in the actual practice.
- Plastic chimneys. These Chimney in Power Plant are made of glass fiber reinforced plastic. They cannot be operated at high temperature. Because at high temperature, the flue gas catches fire and leads to disintegration of chimney. This kind of chimneys plays a vital role where a low stress, low temperature chimney is required for highly corrosive effluents.
Question ((Karnataka BOE 2023, 2 marks)
What is stalling of fans?
Answer
Stall is a phenomenon that occurs when a fan is pushed beyond its normal performance limits and the flow of air around the blades becomes disrupted. This can be caused by the angle at which the air hits the blades being too steep, causing the air to separate from the surface of the blades. When this happens, the air is thrown outward towards the edges of the blades and a build-up of pressure occurs at the tips of the blades. If this pressure is not released, the fan can become unstable and start to vibrate.
Question (Assam BOE 2022, 2 marks)
What does black smoke from the stack of a gas-fired boiler indicate?
Answer:
- Inadequate combustion leading to excess fuel consumption and disintegrated carbon particles being released without being completely burnt in the furnace.
- Carryover of coal fines due to excess of its percentage in the coal feed.
- Low supply of secondary air that would otherwise complete the combustion.
Question (Assam BOE 2023, 2 marks)
What is a flue gas scrubber?
Answer
A dry scrubbing is shortly, a dosing of a chemical powder in the flue gas, (ex.: sodium-bi-carbonate), followed by a reaction chamber, and a bag house filter. The powder will chemically react with the acids in the flue gas in the reaction chamber and in the bag house filter.
A wet scrubber is shortly, spraying water in the flue gas, so it condensates and mixes/reacts with the chemical in the flue gas.
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