Module 10. Steam generators

Lesson 24

CLASSIFICATION AND PERFORMANCE PARAMETERS OF BOILERS/ STEAM GENERATORS

24.1  Introduction

Steam generator is a device or equipment which burns the fuel and facilitates the exchange of heat produced to the water to generate required quantity and quality of steam. Thus it is a heat exchanger which has the place for burning of fuel and flow of hot flue gases produced and also has space for storing of water and steam. As steam is produced & stored at high pressure than the atmospheric pressure, steam generator is also a pressure vessel. To handle the hot flue gases and to keep high pressure steam, certain other mountings and accessories are also required for its safe and efficient operation. In this way steam generator is not simply a vessel to boil water but it is a complete unit performing the complete task of producing & handling the high pressure steam by burning of the fuel and exhausting the flue gases efficiently and safely. Most of the boilers are actually a type of shell & tube type heat exchangers.

24.2  Classification of Steam Generators

Depending on the construction and operation, steam generators or commonly named as boilers are classified on the basis of:

(a) Alignment of axis of boiler:  Boilers are classified as Horizontal, Vertical and Inclined type boiler. Horizontal boiler occupies more floor area but is easily accessible for inspection and maintenance. Vertical boiler occupies less floor area but difficult to access and clean. In a dairy processing plant, we generally find horizontal type boiler.

(b) Flow of water and hot flue gases through the boiler: Boilers are broadly classified as fire tube and water tube boiler. This is the first major classification of boilers. Most of the boilers are actually shell and tube type heat exchangers in which one fluid i.e. either water or hot gas flows through the shell and other through the tubes. In a boiler, if fire flows through tubes and water remains in the shell, it is called fire tube boiler. Conversely if water flows through tubes and fire or hot flue gases pass through the shell it is called water tube boiler. In a dairy processing plant, fire tube boilers are generally used due to low pressure requirement, comparatively safe, less operational cost and easy to maintain type characteristics.

(c) Location of furnace or burner: Boilers are classified as: Externally fired and internally fired. If fuel is burnt outside the boiler and after burning only, hot flue gases are forced to flow through boiler, it is called externally fired. e.g. Babcock & Wilcox boiler, Sterling boiler etc.

If the furnace is located inside the boiler itself, it is called internally fired boiler e.g. Cochran boiler, Lancashire boiler etc.

(d) Mode of water circulation: Boilers are classified as forced circulation and natural circulation boilers. If the water flows through boiler by the force of pump, it is called forced circulation boiler. If water flows due to natural current produced, it is called natural circulation boiler.

(e) Pressure of steam produced: Boilers are classified as high pressure and low pressure boilers. Above 80 bar pressure, boilers are called high pressure boilers and below this limit, boilers are called low pressure boilers. In a dairy processing plant where steam is used only for heating purpose and not to produce any mechanical work, low pressure boilers are used. High pressure boilers are used in applications where steam is used as a working agent like in thermal power plants etc. High pressure boilers are Benson, Babcock & Wilcox, and Lamont etc. Low pressure boilers are Cochran, Cornish, Lancashire & Locomotive boilers.

Table 24.1 Comparison between fire tube and water tube boilers

S.No	Fire Tube Boilers	Water Tube Boilers
1.	Hot gases flow through tubes or flues passing through water stored in the shell	Water flows through tubes passing through the passage of the flue gases.
2.	Steam production rate is less i.e. upto 9 tons / hour	Steam production rate is high. It can be upto 450 tones/hour.
3.	Steam pressure is limited to 25 bar approx.	Steam pressure is generally in the range of 125 bar or even more in high pressure or super critical boilers.
4.	Due to low pressure and less production rate of steam, their use is generally limited to processing plant or small size power plants used in private industries.	Due to high pressure and high production rate of steam, these are commonly used in large capacity power plants like thermal plants for power generation.
5.	Chances of bursting are less but in case of bursting, a great damage to the life & property occur. It is because the main shell is under pressure and whole shell bursts.	Chances of bursting are more due to high pressure, but the damage is not so severe as in case of fire tube boilers.
6.	Feed water treatment is necessary but not so critically required because small deposit in the boiler shell does not deteriorate much the performance of boiler.	Feed water treatment is critical and highly essential as even a small deposit in boiler tube may cause overheating and bursting.
7.	Construction is more complicated. It occupies more floor area. Transportation is difficult. Because it comes in assembled position. Hence overall cost is high for a unit steam production rate.	Construction is simpler. It occupies less floor area. Transportation is not difficult because it can be easily dismantled & assembled. Overall cost is less for a unit steam production rate.
8.	Overall efficiency is upto 75%.	Overall efficiency with economizer is 90%
9.	It can cope up with fluctuating load.	It can also bear fluctuating load but only for a shorter period.
10.	It takes time to produce steam and increase the steam pressure at a slower rate.	It produces steam fast and so increases the steam pressure at a faster rate.

24.3  Cochran Boiler

It is a best example of vertical fire tube boiler and has a very simple construction. It is internally fired multi tubular and natural circulation boiler. Its working can be well understood with the help of a animated diagram Fig. 24.1.

The various parts & mountings of Cochran boiler are also shown in Fig. 24.1

Description: Cochran boiler has a vertical cylindrical shell with dome shaped top used as steam space. Furnace is a single piece construction situated at the bottom of the shell. The fuel (coal) is burnt on the grate by supplying air naturally through fire hole. Hot flue gases produced are directed through fire brick lining to the horizontal fire tubes surrounded by water filled in the shell. After exchanging their heat with water flue gases are directed to atmosphere through smoke box and chimney. Ash collects below the grate in the ash pit from where it is periodically removed. The various boiler mountings are also installed over the boiler shell.

24.4  Babcock & Wilcox Boiler

It is a simple water-tube boiler and used where higher steam pressure and higher steam production rate is required. It can be used for stationary and marine purposes. The construction is as shown in Fig. 24.2

It consists of a horizontal shell supported on the masonry structure. The shell is connected to two headers on both ends, which are at different level and connected by a number of inclined tubes. When shell remains filled with water, up to the desired level, the headers and tubes also remain filled with water. The tubes are inclined at an angle of 15° and remain suspended/ supported inside the closed furnace/flue gases space. The flue gases produced from burning of the fuel are directed over the tubes in between the baffles forming the zig-zag flame passage. Steam is produced inside the inclined tubes and lifts through the top header and collects in the steam space of the boiler shell. Water from the bottom header comes in place of steam. Water level is checked by level indicator and maintained by feed pump.

The boiler shell & tubes are hung by means of wrought iron girders supported on pillars. This arrangement allows the drum and tubes to expand or contract freely.

24.5  Boiler Terms

(i) Boiler Shell: It is a cylindrical shaped structure fabricated with steel plates rolled & riveted or welded together.

(ii) Grate: It is a Cast Iron platform in the furnace of boiler on which fuel is burnt. Fuel is coal or wood or any other solid fuel which rests on the perforated surface of grate made of C.I. bars. Air can easily pass through perforated surface from the bottom and also after burning, ash can fall down itself easily.

(iii) Furnace/Fire box: It is an enclosed space where fuel is burnt and hot flue gases accumulate. From the furnace, flue gases are directed to flow through the boiler.

(iv) Water Space: Volume occupied by water in the boiler is termed as water space which is maintained at a constant level with the help of water level indicator fitted on boiler shell.

(v) Steam Space: It is volume occupied by steam over the water surface in boiler. As steam is light, it lifts up and remains in the steam space at the top end of the boiler.

(vi) Boiler Mountings: Various fittings on the boiler like pressure gauge, safety valves etc which are necessary for its safe and efficient operation are called Boiler Mountings.

(vii) Boiler Accessories: The integral parts of boiler which are required to enhance its efficiency or for overall performance are called accessories e.g. super heater, economizer, feed pump etc.

(viii) Foaming: The formation of bubbles on the surface of boiling water is called foaming.

(ix) Scale: The deposits of water salts and foreign particles on the heating surface in the form of a hard layer or film is called scale.

(x) Blowing off: The suspended impurities in the boiler water usually settle down and are thrown out through a cock due to pressure difference inside and outside the boiler. This process is called blowing off and the cock used is called blow-off-cock.

(xi) Lagging: The insulation block mode or rope made of asbestos or magnesia wrapped or fixed outside the boiler shell and steam pipe are called lagging.

(xii) Refractory: The fire bricks or clay having low thermal diffusivity are used in the furnace walls and other passages of flue gases where flue gases should retain their heat i.e. where heat loss of flue gases is to be prevented.

24.6  Boiler Start-up procedure

i. Check maximum permissible working pressure which is indicated by a red line on pressure gauge fitted.

ii. Check and close drain valves.

iii. Open water and steam cocks of water gauge glass and shut its drain cock.

iv. Clean and examine filter in feed water system.

v. Open feed water tank outlet valve, and ensure water level in tank is at least half the gauge glass.

vi. Check fuel oil daily service tank level and drain off any water present.

vii. Clean and examine filters in fuel oil system.

viii. Shut main steam stop valve and open air vent.

ix. Switch on power supply. Feed water pump will automatically supply water to boiler till approximately half gauge glass.

x. Fuel oil pump will automatically start. Fuel oil, if heavy diesel oil, will circulate through fuel oil heater till the desired operating temperature is reached.

xi. The blower motor will start and purge furnace for at least 1-2 minutes to clear any combustible gas accumulated in the furnace and avoid a blow back.

xii. The burner will be ignited according to an automatic sequence, initially at low fire rate.

xiii. When steam comes out form air vent, shut off air vent and switch to high fire rate. Steam pressure will gradually increase to normal working pressure.

xiv. Check gauge glass, safety valve, and all automatic safety devices to ensure they are functioning normally.

xv. Open steam valve very slowly to permit warming through and draining of the cold steam pipes to avoid damage due to water hammer.

24.7  Performance of Boilers

24.7.1  Equivalent evaporation

It is defined as the weight of saturated water at 100°C evaporated to dry and saturated steam at 100°C by utilizing heat at the same rate as would have been used under the actual working conditions.

If H = Total heat of steam at given working pressure in kJ/kg

H_w1 = Total heat of feed water in kJ/kg

L = Latent heat of steam at atmospheric pressure = 2257 kJ/kg

W_a = Weight of steam produced per hour at given working pressure per kg of fuel  

W_e = Equivalent evaporation in kg per kg of fuel

Then as per the above definition, W_e × L = W_a (H – H_w1)

Or

……… (Eq. 24.1)

= W_a .F, where F = = Factor of Evaporation ……… (Eq. 24.2)

24.7.2  Factor of evaporation

It is a quantity which when multiplied by the actual amount of steam generated at a given pressure from water at given temperature, gives the equivalent evaporation.

Equivalent Evaporation = F x Actual Evaporation

Thus factor of evaporation may also be defined as ratio of Equivalent Evaporation to actual Evaporation.

24.7.3  Boiler efficiency

It is the ratio of heat actually utilized in generation of steam in a given period to the heat supplied by fuel in the same period or it is the ratio of heat utilized in generation of a given quantity of steam to the heat supplied by fuel burnt to produce this steam.

……… (Eq. 24.3)

Where C = calorific value of fuel in kJ/kg and W_f = weight of fuel per hour.

Boiler efficiency is always less than 1 because of some loss of heat through hot gases escaping to atmosphere and also directly to atmosphere by conduction convection and radiation.

24.7.4  Boiler horse power

This term is although not justified in the boiler as it is not producing any mechanical work/power; even then it is adopted by American Society of Mechanical Engineers as standard of measurement of capacity of boiler.

One boiler horse power means evaporation of 15.653 kg of water per hour from and at 100^oC into dry saturated steam at 100℃. Or a boiler generating 15.653 kg of equivalent evaporation per hour means developing one boiler H.P.

……… (Eq. 24.4)

24.8  Numerical Problems

24.8.1 A boiler evaporates 3.6 kg of water per kg of coal burnt into dry saturated steam at 10 bar pressure .The temperature of feed water is 32^oC. Find the equivalent evaporation from and at 100^oC as well as factor of evaporation.

Solution

Mass of steam produced, m_s = 3.6 kg/kg of coal

Initial temperature of feed water, t_w = 32°C

So, Enthalpy of feed water, H₁= m.c_p.t_w = 3.6 × 4.187 × 32 = 482.34 kJ

Enthalpy of dry saturated steam produced

H₂ = m. h_g

At 10 bar pressure, the value of h_g from steam tables is = 2776.2 kJ/kg

So, H₂ = 3.6 × 2776.2 = 9994.3 kJ

Hence the heat supplied to steam per kg of coal burnt in the boiler is

H₂ – H₁= 9994.3 – 482.3 = 9512 kJ

Equivalent Evaporation of Boiler                 


(Ans)

24.8.2 The following readings were obtained during a boiler trial of 6 hour duration

                   Mean Steam Pressure = 12 bar

                   Mass of steam generated = 40000Kg

                   Mean dryness fraction = 0.85

                    Mean feed water temp = 30^o C

                    Coal Used = 4000kg

                    Calorific value of coal = 33400 kJ/kg

 Calculate: (1) Factor of equivalent evaporation

                  (2) Equivalent evaporation 

                  (3) Efficiency of boiler     

Solution

Mass of steam produced in 6 hours duration = 40000 Kg

Mass of coal burnt in 6 hours duration = 4000kg

Thus mass of steam produced per kg of coal

Now specific enthalpy of feed water at temperature 30^oC

h₁ = C_pt = 4.187 × 30 = 125.6 kJ/kg

Specific enthalpy of steam produced at 12 bar pressure and dryness fraction 0.85

h₂ = h_f + x h_fg

Now at 12 bar pressure value of h_f and h_fg from Steam Tables are

h_f = 798.4 kJ/kg

h_fg = 1984.3 kJ/kg

Putting in above equation

h₂= 798.4 + 0.85 × 1984.3 = 2485 kJ/kg

= 1.045                            (Ans)

Equivalent Evaporation = mass of steam x Factor of Evaporation

                                    = 10 x 1.045 = 10.45 kg/kg of coal burnt        (Ans)

Efficiency of boiler,