4.1 INTRODUCTION
A heat recovery steam generator (HRSG) is a heat exchanger or series of heat exchangers that recovers heat from a hot gas stream and uses that heat to produce steam for driving steam turbines or as process steam in industrial facilities or as steam for district heating as shown in Fig.(4.1)
The Cairo North Power Station combined cycle consists of four gas turbines (with associated generator), four HRSGs and two steam turbine (with associated generator).The Power Station HRSGs are a triple pressure level HRSGs with a reheat system.
4.1.1 HRSG Process
Exhaust gas from Gas Turbine enters the HRSG, heats water/steam, and leaves the HRSG through the stack. An isolating damper system (also called a bypass damper) with seal air fans is required in these units to ensure that hot gases do not leak to the fan when the gas turbine is running and that maintenance can be performed on the gas turbine when the fresh air fan is operating. Bypass dampers are also used in some units to ensure that the gas flow to the HRSG can be modulated in order to match steam generation with steam demand.
Different types of tube bundles:
1. Economizer: Water is heated up to almost saturation temperature.
2. Evaporator and Drum: Slightly sub cooled water enters the drum. Then it is circulated to the evaporator, where it is heated and transformed into a water/steam mixture. This mixture is returned to the drum, where water and steam are separated.
3. Superheater: Saturated steam leaving the drum reaches the maximum heat exchange temperature with the hottest exhaust gas leaving the gas turbine. From the last stage of the steam turbine the steam is led to a condenser where it is condensed to water. From the condenser the condensate is pumped to the HRSG, passing the gland steam condenser. Make-up water is added to compensate for any water losses. Condensate then flows through the condensate preheater (normal situation) and is fed to the deaerator/feed water tank.
From the deaerator/feed water tank, water is pumped to the LP drum using the LP feed water pump. From the deaerator the combined IP/HP feed water pumps bring the feed water to the IP and HP drum, passing the respective economizer before the water is fed to the drum. From the drum boiler water circulates through the evaporator system by means of natural circulation. In the evaporator a part of the water is evaporated. The steam/water mixture returns from the evaporator to the drum where steam and water are separated. Saturated steam flows from the drum to the superheaters. Superheated steam leaves the HRSG via the main stream line and is supplied to the steam turbine as well as to the feed water tank/deaerator. Cold reheat steam from the outlet of the HP stage of the steam turbine returns to the reheater of the HRSG in operation. The cold reheat flow is mixed with superheated IP steam and passes through the reheater, in which the temperature of the steam is increased. The hot reheated steam leaves the HRSG via the hot reheat steam line. The HRSGs of the Cairo North Power Station II operates at three pressure levels. Figure (4.2) shows a triple pressure system with 3(economizer, drum, evaporator and superheater).
4.1.2 T-Q Diagram
Figure (4.3) shows the most useful representation of the combined cycle heat transfer, a TQ diagram. It shows the decreasing exhaust gas temperature and the increasing water/steam temperatures of a single pressure system in relation to the amount of heat transferred. The process data is represented as a set of energy flows, or streams, as a function of heat load (kW) against temperature (deg C). These data are combined for all the streams in the plant to give composite curves, one for all hot streams (releasing heat) and one for all cold streams (requiring heat). The point of closest approach between the hot and cold composite curves is the pinch point (or just pinch) with a hot stream pinch temperature and a cold stream pinch temperature
.
Pinch Point Temperature
Difference between steam outlet temperature and exhaust gas outlet temperature in the evaporator.
Lower pinch point = More heating surface, more steam generated and more energy taken from exhaust gases.
Normal values between 8 and 15 ºC
Approach Temperature
Difference between saturation temperature in the drum and water temperature at the economizer outlet.
Lower approach = More heating surface more steam generated and higher risk of evaporation in economizer (problem known as “steaming”)
Normal values between 5 and 12 ºC.
.
A heat recovery steam generator (HRSG) is a heat exchanger or series of heat exchangers that recovers heat from a hot gas stream and uses that heat to produce steam for driving steam turbines or as process steam in industrial facilities or as steam for district heating as shown in Fig.(4.1)
The Cairo North Power Station combined cycle consists of four gas turbines (with associated generator), four HRSGs and two steam turbine (with associated generator).The Power Station HRSGs are a triple pressure level HRSGs with a reheat system.
4.1.1 HRSG Process
Exhaust gas from Gas Turbine enters the HRSG, heats water/steam, and leaves the HRSG through the stack. An isolating damper system (also called a bypass damper) with seal air fans is required in these units to ensure that hot gases do not leak to the fan when the gas turbine is running and that maintenance can be performed on the gas turbine when the fresh air fan is operating. Bypass dampers are also used in some units to ensure that the gas flow to the HRSG can be modulated in order to match steam generation with steam demand.
Different types of tube bundles:
1. Economizer: Water is heated up to almost saturation temperature.
2. Evaporator and Drum: Slightly sub cooled water enters the drum. Then it is circulated to the evaporator, where it is heated and transformed into a water/steam mixture. This mixture is returned to the drum, where water and steam are separated.
3. Superheater: Saturated steam leaving the drum reaches the maximum heat exchange temperature with the hottest exhaust gas leaving the gas turbine. From the last stage of the steam turbine the steam is led to a condenser where it is condensed to water. From the condenser the condensate is pumped to the HRSG, passing the gland steam condenser. Make-up water is added to compensate for any water losses. Condensate then flows through the condensate preheater (normal situation) and is fed to the deaerator/feed water tank.
From the deaerator/feed water tank, water is pumped to the LP drum using the LP feed water pump. From the deaerator the combined IP/HP feed water pumps bring the feed water to the IP and HP drum, passing the respective economizer before the water is fed to the drum. From the drum boiler water circulates through the evaporator system by means of natural circulation. In the evaporator a part of the water is evaporated. The steam/water mixture returns from the evaporator to the drum where steam and water are separated. Saturated steam flows from the drum to the superheaters. Superheated steam leaves the HRSG via the main stream line and is supplied to the steam turbine as well as to the feed water tank/deaerator. Cold reheat steam from the outlet of the HP stage of the steam turbine returns to the reheater of the HRSG in operation. The cold reheat flow is mixed with superheated IP steam and passes through the reheater, in which the temperature of the steam is increased. The hot reheated steam leaves the HRSG via the hot reheat steam line. The HRSGs of the Cairo North Power Station II operates at three pressure levels. Figure (4.2) shows a triple pressure system with 3(economizer, drum, evaporator and superheater).
4.1.2 T-Q Diagram
Figure (4.3) shows the most useful representation of the combined cycle heat transfer, a TQ diagram. It shows the decreasing exhaust gas temperature and the increasing water/steam temperatures of a single pressure system in relation to the amount of heat transferred. The process data is represented as a set of energy flows, or streams, as a function of heat load (kW) against temperature (deg C). These data are combined for all the streams in the plant to give composite curves, one for all hot streams (releasing heat) and one for all cold streams (requiring heat). The point of closest approach between the hot and cold composite curves is the pinch point (or just pinch) with a hot stream pinch temperature and a cold stream pinch temperature
.
Pinch Point Temperature
Difference between steam outlet temperature and exhaust gas outlet temperature in the evaporator.
Lower pinch point = More heating surface, more steam generated and more energy taken from exhaust gases.
Normal values between 8 and 15 ºC
Approach Temperature
Difference between saturation temperature in the drum and water temperature at the economizer outlet.
Lower approach = More heating surface more steam generated and higher risk of evaporation in economizer (problem known as “steaming”)
Normal values between 5 and 12 ºC.
.
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