PERFORMANCE OF STEAM GENERATOR

1 Evaporation Rate

The quantity of water evaporated into steam per hour is called the evaporation rate. It is expressed as kg of steam/h, kg of steam/h/m² of heating surface, kg of steam/h/m³ of furnace volume, or kg of steam/kg of fuel fired.

Equivalent Evaporation

It is the equivalent of the evaporation of 1 kg of water at 100°C to steam at 100°C. It requires 2257 kJ.

Factor of Evaporation

The ratio of actual heat absorption above feed-water temperature for transformation to steam (wet, dry, or superheated) to the latent heat of steam at atmospheric pressure (1.01325 bar) is known as a factor of evaporation.

Let h = specific enthalpy of steam actually produced

h_f = specific enthalpy of feed water

h_f (atm) = specific enthalpy of evaporation at standard atmospheric pressure

m_s = actual evaporation expressed in kg/kg of fuel or kg/h of steam

m_e = equivalent evaporation expressed in kg/kg of fuel or kg/h

Then, the equivalent evaporation = actual evaporation × factor of evaporation

where F = factor of evaporation

h₁ = h_f1 + xh_fg1 for wet steam actually generated

= h_g1 for saturated steam actually generated

= h_sup for superheated steam actually generated.

2 Performance

The performance of boiler may be explained on the basis of any of the following terms:

1. Efficiency: It may be expressed as the ratio of heat output to heat input.
2. Combustion rate: It is the rate of burning of fuel in kg/m³ of grate area/h.
3. Combustion space: It is the furnace volume in m³/kg of fuel fired/h.
4. Heat absorption: It is the equivalent evaporation from and at 100°C in kg of steam generated/m² of heating surface.
5. Heat liberated: It is the heat liberated/m³ of furnace volume/h.

3 Boiler Thermal Efficiency

It is the ratio of heat absorbed by steam from the boiler per unit time to the heat liberated by the combustion of fuel in the furnace during the same time.

where ṁ_s = mass of steam generated in kg/h

ṁ_f = mass of fuel burned in kg/h

C.V. = calorific value of fuel in kJ/kg

4 Heat Losses in a Boiler Plant

1. Heat used to generate steamQ₁ = m_s (h − h_f1)
2. Heat lost to flue gases: The flue gases contain dry products of combustion and the steam generated due to the combustion of hydrogen in the fuel.Heat lost to dry flue gases,where m_g = mass of gases formed per kg of fuelc_pg = specific heat of gasesT_g = temperature of gases, °CT_a = temperature of air entering the combustion chamber of the boiler, °C
3. Heat carried by steam in flue gases where m_s1 = mass of steam formed per kg of fuel due to combustion of H₂ in fuelh_f1 = enthalpy of water at boiler house temperatureh_s1 = enthalpy of steam at the gas temperature and at the partial pressure of the vapour in the flue gas
4. Heat loss due to incomplete combustion: If carbon burns to CO instead of CO₂, then it is known as incomplete combustion. 1 kg of C releases 10,200 kJ/kg of heat if it burns to CO, whereas it releases 35,000 kJ/kg if it burns to CO₂. If the percentages of CO and CO₂ in flue gases by volume are known, then where CO, CO₂ = % by volume of CO and CO₂, respectively, in flue gasesC = fraction of carbon in 1 kg of fuelHeat lost due to incomplete combustion of carbon per kg of fuel,  
5. Heat lost due to unburnt fuelQ₅ = m_f1 × C.V.where m_f1 = mass of unburnt fuel per kg of fuel burnt.
6. Convection and radiation losses: Heat unaccounted for due to convection and radiation losses, where Q = m_f × C.V.= heat released per kg of fuel

5 Boiler Trial and Heat Balance Sheet

There are three purposes of conducting the boiler trial.

1. To determine and check the specified generating capacity of the boiler when working at full load conditions.
2. To determine the thermal efficiency of the plant.
3. To draw up the heat balance sheet so that suitable corrective measures may be taken to improve the efficiency.

The following measurements should be observed during the boiler trial.

1. The fuel supplied and its analysis.
2. Steam generated and its quality or superheat.
3. Flue gases formed from exhaust analysis.
4. Air inlet temperature and exhaust gases temperature.
5. Volumetric analysis of exhaust gases.
6. Mass of fuel left unburnt in ash.
7. Feed-water temperature.

The heat balance sheet is a systematic representation of heat released from burning of fuel and heat distribution on minute, hour or per kg of fuel basis. A pro forma for heat balance sheet is given in Table 3.2.

 

Table 3.2 Heat balance sheet for boiler