Boilers

Definition

A boiler (steam generator) is a closed pressure vessel in which water is heated by the heat of burning fuel (or another heat source) to generate steam at a pressure higher than atmospheric.

Applications

• Power generation: steam for turbines in thermal power plants.
• Process heating in industries: sugar mills, textile, paper, dairy, chemical and food industries.
• Space heating and hot-water supply in hospitals, hotels and large buildings.
• Steam locomotives and ships (mobile boilers).
• Sterilisation, cooking and cleaning (autoclaves, laundries).
• Driving steam engines and steam hammers.

Factors governing selection

1. Working pressure and quantity of steam required (kg/h).
2. Quality of steam: wet, dry saturated or superheated.
3. Type and cost of fuel available (coal, oil, gas, biomass, husk).
4. Floor area and head room available.
5. Load pattern: steady or fluctuating; quick steaming needed or not.
6. Quality of feed water available.
7. Initial cost, running cost, maintenance cost and ease of inspection.
8. Efficiency and statutory safety requirements.

In a fire tube boiler the hot flue gases pass through tubes surrounded by water. In a water tube boiler water flows inside the tubes and the hot gases pass outside them.

Examples: Fire tube: Lancashire, Cornish, Cochran, locomotive boiler. Water tube: Babcock and Wilcox, Stirling, Yarrow.

A Lancashire boiler is a horizontal, internally fired, natural-circulation fire tube boiler with two large furnace tubes (flues) running through the shell. It is a stationary boiler for medium pressure.

Construction

• A cylindrical shell about 2 to 3 m in diameter and 6 to 9 m long, set in brickwork, about two-thirds full of water.
• Two large-diameter fire tubes pass lengthwise through the shell. The grate is at the front end of each tube.
• Brick-work forms a bottom flue under the shell and two side flues along its sides.
• Typical rating: pressure up to about 16 bar and steam output up to about 8 t/h.

   Steam stop valve   Safety valves
         \            /
   ______[=====steam space=====]____
  |  front <---- water ---- rear   |
  |  ===== furnace tube 1 =====>   |
  |  ===== furnace tube 2 =====>   |
  |___________  shell  ____________|
   grate        |  bottom flue  |
   ----> hot gas path:           |
   tubes (1st pass) -> bottom flue (2nd pass,
   to front) -> side flues (3rd pass) -> chimney

Working

1. Fuel burns on the grates inside the furnace tubes.
2. Hot gases travel to the rear through the furnace tubes (first pass), heating the water.
3. At the rear they turn downward into the bottom flue and flow forward under the shell (second pass).
4. At the front they divide into the two side flues and flow backward (third pass), then reach the chimney through a damper.
5. Steam collects in the steam space and is taken through the stop valve (a steam separator or anti-priming pipe can be fitted).

Mountings

Water level gauge, pressure gauge, safety valve (dead-weight or lever type), fusible plug, steam stop valve, feed check valve, blow-off cock, manhole and an Hoffmann-type damper.

Merits: simple, reliable, large water reserve, easy to operate and clean. Demerits: slow steaming, large floor space, low pressure and capacity.

The Babcock and Wilcox (B&W) boiler is a horizontal-drum, longitudinal, inclined water tube boiler with natural circulation, used for high-pressure steam.

Construction

• A horizontal steam and water drum at the top, about half filled with water.
• A bank of water tubes inclined at about 15 degrees to the horizontal, joined at the front to an uptake header and at the rear to a downtake header.
• Headers connect to the drum by short vertical tubes.
• A mud box at the lowest point collects sediment; a furnace with grate below the tubes; baffles direct the gas across the tubes; a superheater is fitted in the gas path.

        steam outlet
           |   _____________
   ______ [  / steam drum   \ ]______
  |uptake|  \_______________/ |down- |
  |header|      water         |take  |
  |      | \\ \\ \\ \\ \\ \\ |header|
  |      |  inclined tubes   |      |
  |______| baffle   baffle   |______|
        FURNACE (grate)   mud box

Working

1. Hot gases from the grate rise, cross the tubes upward and downward between baffles (three passes) and leave to the chimney.
2. Water in the tubes heats up, becomes lighter and rises through the uptake header into the drum. Cooler, denser water falls through the downtake header, giving natural circulation.
3. Steam separates in the drum and passes through the superheater to the main stop valve.
4. Feed water enters the drum; sludge settles in the mud box and is blown down.

Advantages over Lancashire boiler

• Higher pressure (up to about 40 bar) and larger capacity.
• Quick steam raising because of small water content.
• Occupies less floor space and can be extended by adding tubes.
• Flexible to load changes and gives higher efficiency.
• Any tube can be replaced easily; the sections can be transported separately.

An ideal boiler should satisfy the following.

1. Safety: high factor of safety with reliable mountings (safety valves, water gauge, fusible plug) and low explosion risk.
2. Quick steaming: small water content and good circulation so steam is raised quickly and the boiler responds to load changes.
3. High efficiency: large heating surface, good combustion and low heat losses to produce maximum steam per kg of fuel.
4. Dry steam: adequate steam space and separators so the steam delivered is dry.
5. Compact: small floor area and weight for the output, with ease of erection and transport.
6. Simple construction: easy to inspect, clean, repair and maintain; all parts accessible.
7. Low cost: low initial, operating and maintenance cost.
8. Fuel flexibility: able to burn the available fuel with good combustion and low smoke.
9. Free circulation and expansion: good water circulation and provision for unequal expansion, without leakage.
10. Meets statutory codes for design and safety.

Boilers are classified in the following ways.

Note: a boiler may belong to several classes at once. For instance the Babcock and Wilcox boiler is a water tube, horizontal-drum, externally fired, stationary, natural-circulation boiler.

Data from steam tables

At 10 bar: $h_f = 762.7$ kJ/kg, $h_{fg} = 2014.4$ kJ/kg. Feed water at 40 °C: $h_{fw} = 167.6$ kJ/kg.

Enthalpy of wet steam:

$$h = h_f + x\,h_{fg} = 762.7 + 0.95 \times 2014.4 = 2676.4\ \text{kJ/kg}$$

Heat added per kg of steam $= h - h_{fw} = 2676.4 - 167.6 = 2508.8$ kJ/kg.

(a) Equivalent evaporation

$$E = \frac{\dot m_s\,(h - h_{fw})}{2257} = \frac{4500 \times 2508.8}{2257} = 5002\ \text{kg/h}$$

(b) Factor of evaporation

$$F_e = \frac{h - h_{fw}}{2257} = \frac{2508.8}{2257} = 1.112$$

(c) Boiler efficiency

$$\begin{aligned} \eta &= \frac{\dot m_s (h - h_{fw})}{\dot m_f \times CV} = \frac{4500 \times 2508.8}{600 \times 29000}\\ &= \frac{11289480}{17400000} = 0.649 \end{aligned}$$

(d) Equivalent evaporation per kg of coal

$$\frac{E}{\dot m_f} = \frac{5002}{600} = 8.34\ \text{kg steam/kg coal}$$

(Actual evaporation is $4500/600 = 7.5$ kg/kg coal.)

Answer: (a) about 5002 kg/h; (b) 1.112; (c) about 64.9 %; (d) about 8.34 kg/kg of coal.

Enthalpies (steam tables)

• Superheated steam at 15 bar, 300 °C: $h = 3038.3$ kJ/kg.
• Feed water at 50 °C: $h_{fw} = 209.4$ kJ/kg.

(a) Heat supplied to water and steam

$$Q = \dot m_s (h - h_{fw}) = 2000 \times (3038.3 - 209.4) = 5657706\ \text{kJ/h}$$

(b) Efficiency

Heat released by fuel $= 250 \times 32000 = 8000000$ kJ/h.

$$\eta = \frac{5657706}{8000000} = 0.707$$

(c) Factor and equivalent evaporation

$$F_e = \frac{h - h_{fw}}{2257} = \frac{3038.3 - 209.4}{2257} = 1.253$$ $$E = F_e \times \dot m_s = 1.253 \times 2000 = 2507\ \text{kg/h}$$

Answer: (a) $5.66 \times 10^{6}$ kJ/h; (b) 70.7 %; (c) $F_e = 1.253$, $E \approx 2507$ kg/h.

Heat utilised in steam

Steam per kg of coal $= 6400/800 = 8$ kg.

At 12 bar dry saturated: $h_g = 2783.8$ kJ/kg. Feed water at 35 °C: $h_{fw} = 146.7$ kJ/kg.

$$Q_{steam} = 8 \times (2783.8 - 146.7) = 21097\ \text{kJ/kg coal}$$

Unaccounted loss (by difference)

$$Q_{un} = 30000 - (21097 + 5775 + 1500 + 700) = 928\ \text{kJ/kg coal}$$

Heat balance sheet (basis 1 kg coal, CV = 30000 kJ/kg)

Answer: Boiler efficiency = 70.3 %; radiation and unaccounted losses = 928 kJ/kg coal (3.1 %).