Superheaters

Superheaters Superheated steam increases the plant’s capacity since each pound of steam contains a higher energy content (BTU) per pound than saturated steam Superheated steam reduces condensation in steam lines Superheated steam reduces the engines steam consumption Superheated steam eliminates erosion of turbine blading by insuring that only dry steam enters the turbine.

Superheaters Superheated steam minimizes the possibility of carryover since the steam leaving the dry pipe must pass through the superheater before entering the engine. Superheated steam reduces the size of the boiler, turbine and connecting piping for a given output.

Superheaters

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

Superheaters Enthalpy of the steam leaving the steam drum is? hg = 1203.7 Btu/lbm Enthalpy of the steam leaving the superheater is? hS.H. = 1409.2 Btu/lbm

Superheaters Energy added per lbm Q = hS.H. – hg Q = 1409.2 – 1203.7 Q = 205.5 Btu/lbm Total Q = 90,000 lbm/hr * 205.5 Btu/lbm Total Q = 18,495,000 Btu/hr

Types of Superheaters Radiant Type Convection Type Radiant-Convection Type

Radiant Type Superheater The radiant type of superheater receives its heat by radiation in the furnace area of the boiler. An increase in load on a boiler increases the rate of steam flow through the superheater tubes. To maintain a constant superheater temperature the heat input to the superheater must also increase.

Radiant Type Superheater Since radiant heat is proportional to the furnace temperature, and the furnace temperature remains fairly constant with an increase in the number of fires or firing rate the amount of heat entering the superheater per pound of steam flow will decrease. Therefore, with an increase in load with a radiant type superheater, the outlet steam temperature decreases

Radiant Type Superheater Underdeck Superheater

Convection Type Superheater The convection type superheater is located in the path of the combustion gas flow and receives its heat from the convective flow of these hot combustion gases past the tubes. With an increase in the load the rate of steam flow through the superheater increases. To support the load increase more fuel is burned and more air is used, increasing the amount of combustion gases, and increasing the convective flow of heat to the superheater.

Convection Type Superheater This increase in the convection air flow is greater than the increase in steam flow, hence the amount of heat entering the superheater per pound of steam increases. Therefore, with the convection type superheater, an increase in load causes the outlet temperature of the superheater to increase.

Convection Type Superheater Overdeck Superheater

Radiant-Convection Type Superheater The radiant-convection type superheater receives its heat both by radiation and convection. For this reason it is located in flow path of the combustion gases near enough to the furnace so that it may also receive heat by radiation. By proper positioning of the superheater the best characteristics of the other types of superheaters may be blended.

Radiant-Convection Type Superheater This results in a constant superheat temperature throughout the load range of the boiler. On certain type of multi-drum boilers a separately fired radiant-convection superheater may be employed.

Radiant-Convection Type Superheater Interdeck Superheater

Radiant-Convection Type Superheater

Types of Superheaters Underdeck (Radiant Type) Overdeck (Convection Type) Interdeck (Radiant-Convection Type) Separately Fired (Radiant-Convection Type)

Superheaters

Steady Superheater Temperature Turbines are designed to operate most efficiently at a constant inlet temperature as well as at a constant inlet pressure A rise in superheater temperature above the normal operating temperature may cause undue expansion and over stressing of turbine parts. A wide variation in superheater temperature would cause expansion and contraction in the steam lines, valves, and machinery which could result in the development of leaks.

Superheat Temperature

Causes of Change in Superheater Outlet Temperature Excess Air Change in Feedwater Temperature Soot Accumulation Waterside Deposits Carryover

Superheat Temperature Control Control Desupeheater External Attemporator Internal dampers

Superheat Temperature Control

Superheat Temperature Control

Superheater Protection Desuperheater (Auxiliary and Control) Superheater Safety Valve Superheater Vent Superheater Protection Steam

Superheater Protection

Superheater Protection

Auxiliary Desuperheater The purpose of the auxiliary desuperheater is to insure a continuous flow of steam through the superheater by supplying the auxiliary machinery during all operating conditions. The desuperheater is located in the lower half of the steam and water drum below the normal water level. It consists of an inlet and outlet header which is connected to either a bank of tubes or a tube(s) with several “U” bends.

Auxiliary Desuperheater The relatively cooler water in the drum removes some of the heat that the steam acquired in the superheater and thus lowers the superheater temperature of the steam. The desuperheater has a small pressure differential to the pressure in the steam and water drum.

Types of Desuperheaters Auxiliary Desuperheater Control Desuperheater External Desuperheater Attemporator