1.1 Boiler Terminology

 

 Boiler Terminology:

 1.     Heat transfer mediums

There are different types of heat transfer medium e.g. steam, hot water and thermal oil. Steam and Hot water are most common and it will be valuable to briefly examine these common heat transfer mediums and associated properties.

Thermic Fluid

Thermic Fluid is used as a heat transfer mechanism in some industrial process and heating applications. Thermic Fluid may be vegetable or mineral based oil and the oil may be raised to a high temperature without the need for any pressurization. The relatively high flow and return temperatures may limit the potential for flue gas heat recovery unless some other system can absorb this heat usefully. Careful design and selection is required to achieve best energy efficiency.

Hot water

 Water is a fluid with medium density, high specific heat capacity, low viscosity and relatively low thermal conductivity. At relatively low temperature e.g. 70°C -90°C, hot water is useful for smaller heating installations.

Co generation system:

There are several industries where saturated steam at the desired temperature is required for heating, drying etc. In this system exhaust team or extraction steam of turbine is utilize for process heating.

2.     Steam

When water is heated its temperature will rise. The heat added is called Sensible heat and the heat content of the water is termed its Enthalpy. The usual datum point used to calculate enthalpy is 0 °C.

When the water reaches its boiling point, any further heat input will result in some proportion of the water changing from the liquid to the vapor state, i.e. changing to steam. The heat required for this change of state is termed the 'latent heat of evaporation' and is expressed in terms of a fixed mass of water. Where no change in temperature occurs during the change of state, the steam will exist in equilibrium with the water. This equilibrium state is termed 'saturation conditions'. Saturation conditions can occur at any pressure, although at each pressure there is only one discrete temperature at which saturation can occur.

 Steam is useful heat transfer medium because, as a gas, it is compressible. At high pressure and consequently density, steam can carry large quantities of heat with relatively small volume.

 Saturated steam: It is the steam, whose temperature is equal to the boiling point corresponding to that pressure.

 Wet Steam: Saturated steam which contains moisture.

 Dry Steam: Either saturated or superheated steam containing no moisture.

Super heated Steam: Steam heated to a temperature above the boiling point or saturation temperature corresponding to its pressure.

Degree of Super heat: The increase of temperature above the saturation point.

Super Saturated steam: When steam temperature is less than the corresponding pressure and steam density is greater than the corresponding pressure. (Expansion of steam in nozzle)

Latent heat of fusion: The quantity of heat required for the transformation from solid to liquid without rise in temperature.

Latent heat of vaporization: The quantity of heat required for the transformation from liquid to vapor without rising temperature.

Sensible heat: It is the quantity of heat required to raise the temperature of one unit of water from 0⁰C to the saturation temperature at which the water begins to form steam at the given pressure.

Total heat: Heat required to convert 1 unit of water at 0 ⁰C and at constant pressure to dry saturated steam at that pressure and the corresponding temperature.

Total heat = sensible heat + latent heat

Enthalpy of combustion: It is defined as the difference between the enthalpy of product and enthalpy of reactants, when complete combustion occurs at a given temperature and pressure.

 Atmospheric pressure: The pressure due to the weight of the atmosphere. It is expressed in pounds per sq. in. or inches of mercury column or kg/cm2. Atmospheric pressure at sea level is 14.7 lbs. / sq. inch. or 30 inch mercury column or 760 mm of mercury (mm Hg) or 101.325 kilo Pascal (kPa).

Absolute pressure: The sum of the gauge and the atmospheric pressure.   For instance, if the steam gauge on the boiler shows 9 kg/cm2g the absolute pressure of the steam is 10 kg/cm2(a).

3. Types of Boiling

i. Nucleate boiling:

In initial stage of heating the water in contact with tube begins to evaporate, but as the temperature of bulk of water has not reached saturation temperature the bubbles collapse and give latent heat to surrounding water. This is sub cooled nuclear face. Further when the bulk of liquid reaches saturation temperature, the bubble will not collapse. This is called nucleate boiling.

ii. Film boiling:

Beyond nucleate boiling region the bubble collapse to form a film of superheated steam over parts or for the entire heating surface. This condition is known as film boiling.

Nuclear and film boiling in a Riser tube:

If there is a high rate of heat transfer of riser, there will be too much bubbles formation with the result that the bubbles may collapse and first form an unstable vapor film, which continually collapse and reform with a still higher heat transfer. The vapor film may be stable. Since a vapor film has a much lower thermal conductivity then a liquid film, it will offer a large thermal resistance almost blanketing the surface where it forms. As a result of which, heat absorbed and carried away, will be much less then the heat transfer to the wall. The difference will be Store in the metal of the tube with the increase in its internal energy. Consequently the temperature of the metal may exceed the melting point and the tube may rapture.

4.      Boiler 

 MCR: Steam boilers rated output is also usually defined as MCR (Maximum Continuous Rating). This is the maximum evaporation rate that can be sustained for 24 hours and may be less than a shorter duration maximum rating

Boiler Rating: Conventionally, boilers are specified by their capacity to hold water and the steam generation rate. Often, the capacity to generate steam is specified in terms of equivalent evaporation (kg of steam / hour at 100°C). Equivalent evaporation- “from and at” 100°C. The equivalent of the evaporation of 1 kg of water at 100°C to steam at 100°C.

NPHR: Net plant heat rate defined as the amount of fuel energy or boiler heat input required to generate 1 kwh.

5. Combustion

Complete combustion: The complete oxidation of the fuel, regardless of whether it is accomplished with an excess amount of oxygen or air, or just the theoretical amount required for perfect combustion.

Perfect combustion: 

The complete oxidation of the fuel, with the exact theoretical (psychometric) amount of oxygen (air) required.

6. Types of Circulation in Boiler

Natural circulation:

The natural convection current is induced to water due to difference in density resulting from difference in temperature. The baffle separates out the heated riser from the unheated down comer and therefore create a temperature difference between the two tube systems. Saturated water flow from the unheated down comer and receives heat in the risers where upon a part of it gets converted into steam. The difference in density of saturated water in the down comer and steam water mixer in the riser brings about natural circulation. It is applicable only in sub critical boiler. (176 Kg/cm2 / 221.2 Bar)

Forced circulation:

At high pressure circulation head cause by density difference is too low to cause effective circulation. Then working fluid is forced through the boiler circuit by an external pump. This circulation is called positive or forced circulation. (Above 182.7 Bar / 2650 psi)

Circulation ratio = weight of water feed to the steam generating circuit / the steam actually generated

= kg water: kg steam

Circulation ratio: It is the ratio of flow rate of saturated water in down comer and flow rate of steam released from the drum.

Circulation ratio in any tube should not be less than 6 otherwise tube will get overheated and fail prematurely and CR not greater than 25 for effective utilization of the tube in steam generation.

 Re-circulation ratio: It is the ratio of mass flow rate of the working fluid with re-circulation (Gr) to the mass flow rate of the working fluid without re-circulation (G).

R = Gr/G

 7. Boiler Efficiency

Efficiency: In the boiler industry there are four common definitions of efficiency:

 i. Combustion efficiency

Combustion efficiency is the effectiveness of the burner only and relates to its ability to completely burn the fuel. The boiler has little bearing on combustion efficiency. A well- designed burner will operate with as little as 15 to 20% excess air, while converting all combustibles in the fuel to useful energy.

ii. Thermal efficiency

 Thermal efficiency is the effectiveness of the heat transfer in a boiler. It does not take into account boiler radiation and convection losses – for example from the boiler shell water column piping etc.

 iii. Boiler efficiency

 The term boiler efficiency is often substituted for combustion or thermal efficiency. True boiler efficiency is the measure of fuel to steam efficiency.

 iv. Fuel to steam efficiency

 Fuel to steam efficiency is calculated using either of the two methods as prescribed by the ASME (American Society for Mechanical Engineers) power test code, PTC 4.1. The first method is input output method. The second method is heat loss method.

8. Blow down: 

The removal of some quantity of water from the boiler in order to achieve an acceptable concentration of dissolved and suspended solids in the boiler water. Please ref. Blow down page.

9. Boiler turn down Ratio

 Boiler turn down is the ratio between full boiler output and the boiler output when operating at low fire. Typical boiler turn down is 4:1. The ability of the boiler to turn down reduces frequent on and off cycling. Fully modulating burners are typically designed to operate down to 25% of rated capacity. At a load that is 20% of the load capacity, the boiler will turn off and cycle frequently.

10. Voidage

Voidage is defined as the fraction of the total volume which is free space available for the flow of fluids, and thus the fractional volume of the bed occupied by solid material is depending upon the nature of the porous medium, the voidage may range from near zero to almost unity. The higher the value of voidage, the lower is the resistance to flow of a fluid.

 11. Thermal Gradient:

Thermal gradients are caused by differences and fluctuations in temperatures between the inlet steam on the inside of the header and the outside surface on the fire-side and boiler room-side.

 12. Heat Transfer Coefficient:

The heat transfer coefficient or film coefficient, or film effectiveness, in thermodynamics and in mechanics is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat.

The general definition of the heat transfer coefficient is:

h = q / ΔT 

Where:

q = heat flux, W/m²; i.e., thermal power per unit area, q = dQ/dA

h = heat transfer coefficient, W/ (m²•K)

ΔT = difference in temperature between the solid surface and surrounding fluid area, K

 13. Attrition:

The attrition process causes the size reduction of the mother particle and the production of fine particles, as long as the particle size dia is larger than the critical size dcrit , below which attrition does not happen.

 14. Agglomeration

Agglomeration is an ash generated problem that occurs due to sticky ash which is related to the alkali loading and the combustion temperature in the boiler. The particles of the bed sand are stuck together forming new larger particles, so called agglomerates. This phenomenon is called adhesion. Agglomeration occurs mainly in fluidized beds since the bed sand is a key factor for trigging the agglomeration process.  Slagging of fuel ash can also occur during combustion on a grate (without any presence of bed sand).

 15. Superficial Velocity:

Superficial velocity (or superficial flow velocity), in engineering of multiphase flows and flows in porous media, is a hypothetical (artificial) flow velocity calculated as if the given phase or fluid were the only one flowing or present in a given cross sectional area. Other phases, particles, the skeleton of the porous medium, etc. present in the channel are disregarded.

Superficial velocity can be expressed as:

u_s = Q/A

Where:

u_s - superficial velocity of a given phase, m/s

Q - volume flow rate of the phase, m³/s

A - Cross sectional area, m²

 16. Excess Air:

There is a theoretical amount of fresh air that when mixed with a fixed amount of fuel and burnt will result in perfect combustion. In this situation all of the fuel will have been properly burnt and all of the oxygen in the air will have been consumed. In this circumstance there will be no excess air and combustion efficiency will be maximized.

In the real world, perfect combustion is not possible. The theoretical amount of fresh air would provide insufficient oxygen for complete combustion and some of the carbon in the fuel would be converted into carbon monoxide rather than carbon dioxide. A lack of air can lead to dangerous levels of carbon monoxide being formed and smoke being produced.

Therefore it is usual to adjust the combustion process so that a level of excess air is present to give margin safety. This level is set to account for any likely process variable, e.g. the variability of the fuel supply, changes in atmospheric pressure, changes in wind direction etc.

 Note:  A boiler operating at low load conditions can cycle as frequently as 12 times per hour or 288 times per day. With each cycle, pre and post purge airflow removes heat from the boiler and sends it out the stack. Keeping the boiler on at low firing rates can eliminate the energy loss. Every time the boiler cycles off, it must go through a specific start-up sequence for safety assurance. It requires about a minute or two to place the boiler back on line. And if there is a sudden load demand the start up sequence cannot be accelerated. Keeping the boiler on line assures the quickest response to load changes. Frequent cycling also accelerates wear of boiler components. Maintenance increases and more importantly, the chance of component failure increases.

 Boiler(s) capacity requirement is determined by much different type of load variations in the system. Boiler over sizing occurs when future expansion and safety factors are added to assure that the boiler is large enough for the application. If the boiler is over sized the ability of the boiler to handle minimum loads without cycling is reduced. Therefore capacity and turn down should be considered together for proper boiler selection to meet overall system load requirements.

 Oxygen trims sensor measures flue gas oxygen and a closed loop controller compares the actual oxygen level to the desired oxygen level. The air (or fuel) flow is trimmed by the controller until the oxygen level is corrected. The desired oxygen level for each firing rate must be entered into a characterized set point curve generator. Oxygen Trim maintains the lowest possible burner excess air level from low to high fire. Burners that don’t have Oxygen Trim must run with Extra Excess Air to allow safe operation during variations in weather, fuel, and linkage.