Thermodynamic Cycles

Question 1

Define thermodynamic cycle.

Answer 1

A thermodynamic cycle is a
recurring series of thermodynamic processes
having the same starting and ending point used
for the transformation of energy to produce a
useful effect.

Question 2

Explain the difference between a process and a cycle.

Answer 2

A thermodynamic process is when the working fluid of a system goes through a change of state, An ideal thermodynamic process can change all available heat into useful work, but it cannot do so continuously.

A thermodynamic cycle is a recurring series of thermodynamic processes. An ideal thermodynamic cycle cannot change all available heat into work. However, it is
capable of changing part of this heat into useful work, and it can do so continuously.

Question 3

Define thermodynamic cycle efficiency in terms of

net work produced and energy applied.

Answer 3

Thermodynamic cycle efficiency is defined as the ratio of net work (W) or energy output of the system divided by the heat energy added to the system. The greater the percentage of energy input converted to work, the greater the cycle efficiency.

Question 4

Draw a Carnot cycle using a temperature-entropy (T-s) diagram.

Answer 4

Question 5

What is the efficiency formula in a Carnot Cycle?

Answer 5

Question 6

Draw a Rankine cycle using a temperature-entropy (T-s) diagram

Answer 6

Question 7

What is Rankine Cycle efficiency.

Answer 7

Question 8

Describe the effects of moisture on turbine integrity, efficiency, and design.

Answer 8

• Because the velocity of moisture particles is less than that of the steam in expanding through the turbine, less energy is converted to work. This implies that turbine efficiency is reduced as the moisture content of the steam is increased.
• The moisture in the steam will also result in increased impingement and erosion of the turbine blades and nozzles.

Question 9

State the advantages of moisture separator reheaters for the steam cycle.

Answer 9

The two main purposes of an MSR are to improve efficiency by reducing moisture losses and to prevent erosion damage to the LP turbine blading.

The superheated steam allows more energy to be removed by the low pressure
turbine prior to the moisture content becoming so high that turbine damage may occur in the latter stages of the turbine before the steam is exhausted to the main condenser.

Question 10

State the advantage of feedwater heaters for the steam cycle.

Answer 10

The heat rejected is not lost to the cooling water in the condenser. In this
case, the heat rejected is used to preheat the feedwater in the feedwater preheaters. Therefore, the total heat rejected from the cycle
is reduced, and plant efficiency increases.

Question 11

Explain how changes in secondary system parameters affect plant efficiency.

Answer 11

• Increasing condenser vacuum – the lower the pressure in the main condenser, the greater the work done by the turbine and, hence, the greater the overall plant efficiency.
• Increasing circulating water system flow rate – a higher circulating water system flow rate reduces condenser temperature.
• Lowering circulating water system inlet temperature – lower circulating water system inlet temperature increases the heat transfer rate in the condenser.
• Reducing condensate depression – the less heat energy rejected to the circulating water system by excessively subcooling the condensate, the more efficient the plant.
• By raising feedwater temperature to near T_sat for the existing boiler pressure by using feedwater heaters and the heat energy of low quality steam in the steam extraction system and high temperature water in the drain collection systems that would normally be lost to the system, plant efficiency is increased.
• Increasing steam temperature at the turbine entrance – higher inlet steam temperature increases the available work that can be extracted from the turbine.