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CHARACTERISTICS OF AN IDEAL WORKING FLUID

    CHARACTERISTICS OF AN IDEAL WORKING FLUID

We all know any thermodynamic power cycle depends on some working fluid for the transfer of heat from source to the point where it is converted to some useful work. Picking a working fluid for any vapour power cycle is not a random decision, since in case of any malfunction caused by it can lead to danger to human life and can cause huge operation and maintenance cost. While choosing a working fluid for a vapour power cycle one should keep following points in his mind. These points suggests the properties of ideal working fluid.

1.  The working fluid should have high critical point temperature so a working fluid with high critical point temperature can provide the opportunity of using considerably lower pressure to reach the higher temperature.

2.  The saturation pressure at the temperature of heat rejection should be above atmospheric pressure.

3.  The working fluid should have lower pressure at higher temperature.

4.  The specific heat of working fluid should be less so that little heat transfer is required to reach the boiling point.

5.  The freezing point of the liquid should be below room temperature such that they don’t get solidified in the pipelines.

6.  The saturated vapour line of T-S diagram should have steep enough such that the turbine expansion line is almost parallel to it and turbine exhaust point doesn’t become very wet.

7.  The liquid should be chemically stable and should not contaminate the material of construction at any temperature.

8.  The fluid should be non-toxic, non-corrosive, not excessively viscous and it should low in cost.

             

REGENERATIVE CYCLE EXPLANATION

                       REGENERATIVE CYCLE

This section will introduce one such cycle - the ideal regenerative Rankine cycle, which increases the fluid average temperature during the heat addition process. To save fuel, efforts are constantly made to improve the efficiency of the cycle on which steam power plants operate. The general idea is to increase the fluid average temperature during heat addition and decrease the fluid temperature during heat rejection.

Increasing the temperature of the feedwater (water leaving the pump and entering the boiler) can be considered. This is accomplished by extracting stream from the turbine to heat the feedwater. This process is called regeneration and the heat exchanger where heat is transferred from steam to feedwater is called a regenerator, or a feedwater heater. There are actually two main types of feedwater heaters. If the steam mixes with the compressed water from the pump, it is an open feedwater heater. If the steam does not mix with the compressed water from the pump, it is a closed feedwater heater.

schematic of a power plant running an ideal regenerative rankine cycle with feed water heater.

                               

T-S diagram of ideal regenerative rankine cycle with feed water heater

                                                 

 In an ideal condition, the water leaves the heater as a saturated liquid at the heater pressure. The schematic of a steam power plant with one open feedwater heater is shown on the left. In an ideal regenerative Rankine cycle with an open feedwater heater, steam from the boiler (state 1) expands in the turbine to an intermediate pressure (state 2). At this state, some of the steam is extracted and sent to the feedwater heater, while the remaining steam in the turbine continues to expand to the condenser pressure (state 3). Saturated water from the condenser (state 4) is pumped to the feedwater pressure and send to the feedwater heater (state 5). At the feedwater heater, the compressed water is mixed with the steam extracted from the turbine (state 2) and exits the feedwater heater as saturated water at the heater pressure (state 6). Then the saturated water is pumped to the boiler pressure by a second pump (state 7).The water is heated to a higher temperature in the boiler (state 1) and the cycle repeats again.

Note that the mass flow rate at each component is different. If 1 kg steam enters the turbine, m kg is extracted to the feedwater heater and (1-m) kg continues to expand to the condenser pressure. So if the mass flow rate at the boiler is M, then the mass flow rate from other components are:
      Condenser: M (1-m)
      Pump 1: M (1-m)
      Feedwater Heater: Mm +M(1-m) = M
      Pump 2 : M
For convenience, heat and work interactions for regenerative Rankine cycle is expressed per unit mass of steam flowing through the boiler. They are:
      Heat Input: Q IN = h₁ – h₇ 
      Heat Output: Q OUT = (1 - m)(h₄ – h₃)
      Work Output: W TURBINE OUT= (h₁ – h₂) + (1 - m)(h₂ – h₃)
      Work input: W PUMP IN  = (1 - m)(h₅ – h₄) + (h₇ – h₆)

Open feedwater heaters are simple and inexpensive, and can also bring the feedwater to saturated state. However, each feedwater needs a separate pump which adds to the cost.

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