Steam Cycles

Vapor Power Cycles

It is defined as, "cycles in which the working fluid is alternatively vaporized and condensed".

• Max. temperature and pressure are 750℃ and 30 Mpa but with Ceramic Coating.

Rankine Cycle

• Pump  ⇔  Increase pressure of liquid.
• Boiler  ⇔  Add fuel + air → start combustion → transfer heat to steam (and combustion gases releases to environment through Stack or Chimney.
• Turbine  ⇔  Steam hitting blades cause these to rotate which is connected to Generator shaft.
• Condenser  ⇔  Transfer heat from steam to liquid and cycle starts again.

Some important points:

• Turbine blades withstand max. temp of 700-750 ℃ but with Ceramic Coating.
• Quality of steam at turbine exit should not below 85 %.
• Generally thermal efficiency of Rankine Cycle is 35-40 %.
• Pump work << Turbine work (in actual practice, it cannot be neglected).

Performance Criteria

• Thermal Efficiency  ⇔  It tells what power is generated at what input.
• Work Ratio  ⇔  Ratio of net-work output to the positive work output (helpful for cycle).
• Specific Steam Consumption OR Steam Rate  ⇔  It tells how much kg/sec of flow is required to produce unit shaft output (1 KW).
• Heat Rate  ⇔  rate of heat is KJ/sec required to produce unit shaft output (1 KW).

Effect of Operating Conditions on Rankine Cycle Efficiency

Ques: Why we are studying Operating Conditions effect?

Ans: To find what condition is desirable to our cycle but at what cost. The conditions are:

1. Superheating steam at constant pressure
2. Increasing Boiler pressure
3. Reducing Condenser pressure

Ques: How can we superheat the steam as well as having higher condenser pressure?

Ans: To accomplish this, a modified Rankine Cycle called Reheat Cycle is used.

Reheat Cycle

In a reheat cycle, the expansion takes place in two turbines. The steam expands in the high-pressure turbine to some intermediate pressure, then passes back to the boiler, where it is reheated at constant pressure to a temperature that is usually equal to the original superheat temperature.

• Limitation of Reheating  ⇔  3 times reheating is allowed.
• Reheat stages (H.P, I.P & L.P) increases maintenance cost (used after Cost & Benefit Analysis).  

Learn these topic 

• Reversible or Irreversible process 
• Quasi Equilibrium 
• Isentropic efficiency of Turbine and Pump

Heat Exchanger

It is defined as, "a device in which one fluid transfer its energy to another fluid".

Types of heat exchanger are as follows:

1. Parallel Flow Heat Exchanger

• Both fluid have same direction of flow 
• Length is greater to achieve equilibrium temperature
• Operating cost increases
• Temperature increases (at inlet and outlet) is greater

2. Counter Flow Heat Exchanger

• Both fluid have opposite flow direction
• Temperature difference (at inlet and outlet) is more or less same i.e. temperature distribution is less (less chances of thermal stresses)

Ques: Where maximum irreversibilities occur?

Ans: Maximum irreversibilities occur between d & 4, we want to reduce ∆T between them. We ideally want water to have high temperature before entering to economizer (but in actual, water from pump is saturated liquid). This type of irreversibility can be eliminated using Process of Regeneration (which increase T4).

• Hitachi Turbine  ⇔  uses Stellite Material Coating (range of cobalt-chromium alloys designed for wear resistance, may contain tungsten or molybdenum and small amount of carbon).

References:

• Material from Class Lectures + Book named Power plant Engineering by P.K Nag (4th Edition) + My knowledge. 
• Photoshoped pics are developed. 
• Some pics and GIF from Google.  
• Videos from YouTube (Engineering Sights).