Superheaters. Superheaters ► Superheated steam increases the plant’s capacity since each pound...

Superheaters

SuperheatersSuperheaters

►Superheated steam increases the Superheated steam increases the plant’s capacity since each pound of plant’s capacity since each pound of steam contains a higher energy content steam contains a higher energy content (BTU) per pound than saturated steam(BTU) per pound than saturated steam

►Superheated steam reduces Superheated steam reduces condensation in steam linescondensation in steam lines

►Superheated steam reduces the engines Superheated steam reduces the engines steam consumptionsteam consumption

►Superheated steam eliminates erosion Superheated steam eliminates erosion of turbine blading by insuring that only of turbine blading by insuring that only dry steam enters the turbine.dry steam enters the turbine.


►Superheated steam minimizes the Superheated steam minimizes the possibility of carryover since the possibility of carryover since the steam leaving the dry pipe must pass steam leaving the dry pipe must pass through the superheater before through the superheater before entering the engine.entering the engine.

►Superheated steam reduces the size of Superheated steam reduces the size of the boiler, turbine and connecting the boiler, turbine and connecting piping for a given output.piping for a given output.


►Calculate the Energy added by Calculate the Energy added by superheating the steam in a 600 psia superheating the steam in a 600 psia boiler with a evaporation rate of boiler with a evaporation rate of 90,000 lbm/hr and having the steam 90,000 lbm/hr and having the steam leave the superheater outlet at 580 leave the superheater outlet at 580 psia and 800 psia and 800 °F.°F.


►Enthalpy of the steam leaving the Enthalpy of the steam leaving the steam drum is?steam drum is?

► hhgg = 1203.7 Btu/lbm = 1203.7 Btu/lbm

►Enthalpy of the steam leaving the Enthalpy of the steam leaving the superheater is?superheater is?

► hhS.H.S.H. = 1409.2 Btu/lbm = 1409.2 Btu/lbm


►Energy added per lbmEnergy added per lbm►Q = hQ = hS.H.S.H. – h – hgg

►Q = 1409.2 – 1203.7Q = 1409.2 – 1203.7►Q = 205.5 Btu/lbmQ = 205.5 Btu/lbm► Total Q = 90,000 lbm/hr * 205.5 Total Q = 90,000 lbm/hr * 205.5

Btu/lbmBtu/lbm► Total Q = 18,495,000 Btu/hrTotal Q = 18,495,000 Btu/hr

Types of SuperheatersTypes of Superheaters

►Radiant Type Radiant Type ►Convection TypeConvection Type►Radiant-Convection TypeRadiant-Convection Type

Radiant Type SuperheaterRadiant Type Superheater

►The radiant type of superheater The radiant type of superheater receives its heat by radiation in the receives its heat by radiation in the furnace area of the boiler. An increase furnace area of the boiler. An increase in load on a boiler increases the rate of in load on a boiler increases the rate of steam flow through the superheater steam flow through the superheater tubes. tubes.

►To maintain a constant superheater To maintain a constant superheater temperature the heat input to the temperature the heat input to the superheater must also increase. superheater must also increase.

Radiant Type SuperheaterRadiant Type Superheater

►Since radiant heat is proportional to the Since radiant heat is proportional to the furnace temperature, and the furnace furnace temperature, and the furnace temperature remains fairly constant temperature remains fairly constant with an increase in the number of fires with an increase in the number of fires or firing rate the amount of heat or firing rate the amount of heat entering the superheater per pound of entering the superheater per pound of steam flow will decrease. steam flow will decrease.

►Therefore, with an increase in load with Therefore, with an increase in load with a radiant type superheater, the outlet a radiant type superheater, the outlet steam temperature decreasessteam temperature decreases

Radiant Type SuperheaterRadiant Type SuperheaterUnderdeck Superheater

Convection Type SuperheaterConvection Type Superheater►The convection type superheater is The convection type superheater is

located in the path of the combustion located in the path of the combustion gas flow and receives its heat from the gas flow and receives its heat from the convective flow of these hot combustion convective flow of these hot combustion gases past the tubes. With an increase gases past the tubes. With an increase in the load the rate of steam flow in the load the rate of steam flow through the superheater increases. through the superheater increases.

►To support the load increase more fuel To support the load increase more fuel is burned and more air is used, is burned and more air is used, increasing the amount of combustion increasing the amount of combustion gases, and increasing the convective gases, and increasing the convective flow of heat to the superheater. flow of heat to the superheater.

Convection Type SuperheaterConvection Type Superheater

►This increase in the convection air flow This increase in the convection air flow is greater than the increase in steam is greater than the increase in steam flow, hence the amount of heat flow, hence the amount of heat entering the superheater per pound of entering the superheater per pound of steam increases. steam increases.

►Therefore, with the convection type Therefore, with the convection type superheater, an increase in load superheater, an increase in load causes the outlet temperature of the causes the outlet temperature of the superheater to increase.superheater to increase.

Convection Type SuperheaterConvection Type SuperheaterOverdeck Superheater

Radiant-Convection Type Radiant-Convection Type SuperheaterSuperheater

►The radiant-convection type The radiant-convection type superheater receives its heat both by superheater receives its heat both by radiation and convection.radiation and convection.

► For this reason it is located in flow path For this reason it is located in flow path of the combustion gases near enough of the combustion gases near enough to the furnace so that it may also to the furnace so that it may also receive heat by radiation. receive heat by radiation.

►By proper positioning of the By proper positioning of the superheater the best characteristics of superheater the best characteristics of the other types of superheaters may be the other types of superheaters may be blended. blended.


►This results in a constant superheat This results in a constant superheat temperature throughout the load temperature throughout the load range of the boiler. range of the boiler.

►On certain type of multi-drum On certain type of multi-drum boilers a separately fired radiant-boilers a separately fired radiant-convection superheater may be convection superheater may be employed.employed.


Interdeck Superheater

Types of SuperheatersTypes of Superheaters

►Underdeck (Radiant Type)Underdeck (Radiant Type)►Overdeck (Convection Type)Overdeck (Convection Type)► Interdeck (Radiant-Convection Type)Interdeck (Radiant-Convection Type)►Separately Fired (Radiant-Convection Separately Fired (Radiant-Convection

Type)Type)

Steady Superheater Steady Superheater TemperatureTemperature

►Turbines are designed to operate most Turbines are designed to operate most efficiently at a constant inlet efficiently at a constant inlet temperature as well as at a constant temperature as well as at a constant inlet pressureinlet pressure

►A rise in superheater temperature above A rise in superheater temperature above the normal operating temperature may the normal operating temperature may cause undue expansion and over cause undue expansion and over stressing of turbine parts.stressing of turbine parts.

►A wide variation in superheater A wide variation in superheater temperature would cause expansion and temperature would cause expansion and contraction in the steam lines, valves, contraction in the steam lines, valves, and machinery which could result in the and machinery which could result in the development of leaks.development of leaks.

Superheat TemperatureSuperheat Temperature

Causes of Change in Causes of Change in Superheater Outlet TemperatureSuperheater Outlet Temperature►Excess AirExcess Air►Change in Feedwater TemperatureChange in Feedwater Temperature►Soot AccumulationSoot Accumulation►Waterside DepositsWaterside Deposits►CarryoverCarryover

Superheat Temperature Superheat Temperature ControlControl

►Control DesupeheaterControl Desupeheater►External AttemporatorExternal Attemporator► Internal dampersInternal dampers

Superheat Temperature Superheat Temperature ControlControl

Superheater ProtectionSuperheater Protection

►Desuperheater (Auxiliary and Control)Desuperheater (Auxiliary and Control)►Superheater Safety ValveSuperheater Safety Valve►Superheater VentSuperheater Vent►Superheater Protection SteamSuperheater Protection Steam

Superheater ProtectionSuperheater Protection

Auxiliary DesuperheaterAuxiliary Desuperheater►The purpose of the auxiliary The purpose of the auxiliary

desuperheater is to insure a continuous desuperheater is to insure a continuous flow of steam through the superheater flow of steam through the superheater by supplying the auxiliary machinery by supplying the auxiliary machinery during all operating conditions. during all operating conditions.

►The desuperheater is located in the The desuperheater is located in the lower half of the steam and water drum lower half of the steam and water drum below the normal water level.below the normal water level.

► It consists of an inlet and outlet header It consists of an inlet and outlet header which is connected to either a bank of which is connected to either a bank of tubes or a tube(s) with several “U” tubes or a tube(s) with several “U” bends. bends.

Auxiliary DesuperheaterAuxiliary Desuperheater

►The relatively cooler water in the drum The relatively cooler water in the drum removes some of the heat that the removes some of the heat that the steam acquired in the superheater and steam acquired in the superheater and thus lowers the superheater thus lowers the superheater temperature of the steam. temperature of the steam.

►The desuperheater has a small The desuperheater has a small pressure differential to the pressure in pressure differential to the pressure in the steam and water drum.the steam and water drum.

Types of DesuperheatersTypes of Desuperheaters

►Auxiliary DesuperheaterAuxiliary Desuperheater►Control DesuperheaterControl Desuperheater►External DesuperheaterExternal Desuperheater►AttemporatorAttemporator

Superheaters. Superheaters ► Superheated steam increases the plant’s capacity since each pound...

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