Rankine Cycle with Closed Feed Water Heaters
Rankine cycle with closed feed water heaters are having its benefits and is most commonly used in all modern power plants. Closed feed water heater employs indirect mode of heat transfer, i.e extracted steam or bleed steam from the turbine transfers its heat indirectly to feed water in shell and tube heat exchanger. Since the steam and water are not mixing directly, so both steam and water circuits are at different pressures. Close feed water heater in a cycle is represented on T-s diagram as shown below in Fig:1.
Theoretically or ideally heat transfer in closed feed water heater should be in such a way that the temperature of the feed water should be increased to that of the saturation temperature of the extraction steam (heating the feed water).
But in actual plant operation the maximum temperature which feed water can attend is normally slightly less than that of the saturation-temperature of the steam. The reason may be few degrees temperature gradient is required for the effective and efficient heat transfer.
This condensate or condense steam from the heater shell shall be transferred to next heater (low- pressure) in the cycle or sometimes to the condenser.
Differentiate Between Open and Closed Feed Water Heater
The open and closed feed water heaters can be differentiated as follows:
|Open feed water heater||Closed feed water heater|
|Open and simple||More complex in design|
|Good heat transfer characteristics||Less effective heat transfer|
|Direct mixing extraction steam and feed water temperature in a pressure vessel||In-direct mixing feed water and steam in a shell and tube type heat exchanger.|
|Pump is required to transfer the water into next stage in the cycle.||Closed feed water pumps don’t require pump and can operate with the pressure difference between the various heaters in the cycle.|
|Requires more area||Requires less area|
|Less expansive||More expensive|
All modern day power plants are employing the combination of open and closed feed water heaters to maximise the thermal efficiency of the cycle.
Engineering thermodynamics looks at converting the valuable form of energy (heat) to work. In power plants, this is done by transferring it to the working fluid called water. So the purpose is to avoid the wastage of heat of steam in the steam turbine condensers. This is possible if find the means to use the low-pressure steam going into the condenser.
Cogeneration is the concept of utilizing the heat of the steam for a useful purpose, rather than wasting it (currently wasted in the condensers).
Cogeneration means Combined Heat and Power (CHP) that is the generation of heat and power simultaneously for the industry requiring process heating steam. In cogeneration plant, both heat-and-power are judiciously utilized so the efficiency of it can be as high as 90% or more. Co-generation offers energy savings.
Cogeneration offers the reduction in wasting of large amount of steam and the same can be utilized in many devices in the form of heat. Most of the industries like paper and pulp, chemical, textile and fiber and cement are depending upon co-generation plant for process heating steam. Process heat steam requirement in above industries are in the order of 4 to 5 kg/cm2 at temperature around 150 to 180oC.
Paper, chemical and textile industries require both electric power and process steam to accomplish their objective. So this requirement can be easily meet through by installing cogeneration power plant.
Temperature in inside the boiler is of the order of 800oC to 900oC and the energy is transferred to the water to produce steam of pressure 105 bar and temperature around 535oC for co-generation power plants. Steam at these parameters are considered as of very good quality source of energy and is thus first utilized in steam turbine for producing power and the turbine exhaust (low quality energy) is used to meet the requirement of process steam.
Cogeneration plant is known for meeting the requirement of power while meeting the process steam requirement of Industrial processes.
Ideal steam-turbine co-generation is shown in the figure 2 above. Let us say that the process heat requirement Qp is at 5.0 Kg/cm2 at around 100 KW. In order to meet the process steam requirement at 5.0 Kg/cm2 steam is expanded in the turbine till the pressure of the steam drop to 5.0 Kg/cm2 and thus produces the power around 20 KW. The condensate from process heater is recycled backed to boiler for cyclic operation. Pump work required to raise the pressure of the feed the water in the cycle is considered as small so not considered.
All energy transferred to the working fluid in the boiler is used either in steam turbine or in process plant, thus utilization factor of the cogeneration plant is:
Qout Heat rejected in the.
Thus in the absence of the condenser the heat utilization factor of the cogeneration plant is 100%.