The efficiency of the Reheat Cycle can be improved by increasing the pressure and temperature of the steam entering into the turbine. As the initial pressure increases, the expansion ratio of the turbine also increases and the steam becomes quite wet at the end of expansion. This is not desirable as increase in moisture content of the steam causes corrosion of the turbine blades and increases the losses. This reduces the nozzle and blade efficiency.
In the reheat cycle, the steam is extracted from the suitable point in the turbine and is reheated with the help of the flue gases in the boiler furnace. The main purpose of reheating is to increase the dryness of the steam passing through the lower stages of the turbine. The dryness fraction coming out of turbine should not be less than 0.88. The increase in the thermal efficiency due to the reheat depends upon the ratio of the reheat pressure to original pressure of the steam. The reheat pressure is generally kept within 20% of the initial pressure of the steam. The efficiency of the reheat cycle may be less than the Rankine efficiency if the reheat is used at low pressure.
The main advantage of reheat cycle is to reduce the specific steam consumption and consequently reduce the size of the boiler and other auxiliaries for the same power.The reheat cycle is only preferred for the high capacity (above 100MW and when the pressure of the steam is as high as 100 bar).
The advantages of reheat cycle over the simple Rankine cycle for turbine-generator having the same rating and the same steam conditions at stop-valves are listed below
Advantages of Reheat Cycle in Thermal Power Plant
- There is a limit to the degree of super-heat due to the metallurgical conditions, therefore it is not possible to get all super-heat in one stage. The inevitable effect of use of higher pressure in power plants is that, the saturation line is reached which is highly undesirable. There is heavy blade erosion due to the impact of the water particles carried with the steam. Therefore, the reheating is essential in high pressure modern thermal power plants to increase the lifetime of the plant.
- The reheating reduce 4 to 5% fuel consumption with a corresponding reduction in the fuel handling
- The reheat cycle reduces the steam flow of 15 to 20% with corresponding reduction in the boiler, turbine and feed handling equipment capacities. This also reduces the pumping power in that proportion.
- The wetness of the exhaust steam with the reheat cycle is reduced to 50% of the Rankine cycle with a corresponding reduction in the exhaust blade erosion.
- Lower steam pressures and temperatures and less costly materials can be used to obtain the required thermal performance
- A reduction in the steam volume and heat to the condenser is reduced by 7 to 8%. Therefore the condenser size and cooling water requirement are also reduced by the same proportion
- The size of the Low pressure (LP) turbine blades is reduced because specific volume is reduced by 7 to 8%
The advantage claimed by the reheat cycle are higher thermal efficiency, reduced feed water pump power, smaller condenser, smaller boiler, long life of the turbine and less handling of the fuel and firing requirement.
Disadvantages of Reheat Cycle in Thermal Power Plant
- The cost of the extra pipes, equipment and controls make the cycle more expensive than the normal Rankine cycle.
- The greater floor space is reduced to accommodate the longer turbine and reheat piping
- The complexity of the operation and control increases with the adoption of the reheat cycle in thermal power plant.
- All the lighter loads, the steam passing through the last blade rows to the condenser are seriously super-heated if the same reheat is maintained. Feed water is sometimes sprayed into the low pressure cylinders as low steam flows as a precaution against over-heating of blades.
The above disadvantages are compensated in new installation by saving in the cost of the equipment for coal storage, coal handling, fuel burning, fans and dust collectors, ash disposal, boiler house structure and auxiliary electrical equipment