How can we increase the efficiency of the boiler in Rankine Cycle?

How can we increase the efficiency of the boiler in Rankine Cycle?

How can we increase the efficiency of the boiler in Rankine Cycle?

1) Introduction to boilers

Background

A boiler is an enclosed system that is designed to provide means for combustion and transfers heat to water until it becomes hot water then steam. Under pressure, the steam is then used for transferring the heat to a process. The boiler system is made out of three different systems:

  • Feed Water System: this system is responsible for providing water to the boiler and regulating it automatically to meet the steam demand.
  • Steam system: this system is responsible for collecting and controlling the steam that is produced in the boiler.
  • Fuel system: the fuel system, is the combination of equipment necessary for generating Heat.

Types of Boilers

Based on their structure and use, there are two general types of Boilers: Fire tube boilers,
Water tube boilers:

  • Fire Tube Boilers: combustion gas passes through the inside of the tubes while the water is surrounding the outside of the tubes. The main advantage of these boilers includes their relatively simple structure and ease of constructing them and their less rigid water treatment requirements. Their disadvantages are the excessive weight-per-pound of steam generated and the excessive time required in order to raise steam pressure. Fire tube boiler is usually used in the facilities of heating applications.
  • Water tube Boilers: unlike Fire Tube Boilers, in the water tube boilers the water flows inside the tubes and combustion gas passes around the outside of the tubes. The advantage of water tube boilers as compared to fire tube boilers is their lower unit weight-per-pound of generated steam and less time needed to raise steam pressure; however, they also have some disadvantages. These disadvantages include their high initial capital cost and their cleaning process. They are mostly used in facilities that use large steam.

2) Introduction to Rankine cycle

what is a Rankine Cycle?

The Rankine cycle is the process used for modeling and predicting the performance of a steam turbine system.  Rankine cycle is also extensively used in studying the reciprocating steam engine and their performance. The term Rankine cycle is referred to the ideal cycle for a steam power plant, so it can be used to facilitate the modeling of an ideal system and not a real one.

Rankine Cycle

how does the Rankine Cycle work?

A Rankine Cycle basically describes how a heat engine that is based on steam generates power. The Power that is generated depends on the temperature difference between a heat source and a cold source. In other words the higher the difference is, the more mechanical power can be efficiently extracted out of heat energy.

  • The heat sources: Heat sources are usually in the form of a nuclear fission or the combustion of fossil fuels such as coal, natural gas, and oil, or concentrated solar power. And obviously the higher the temperature is, the better.
  • The cold sources: They are usually in the form of cooling towers and a large body of water such as a river or a sea. And obviously the colder the better.

the four processes in the Rankine cycle

In the Rankine Cycle there are four processes:

  • Process 1–2: The working fluid is pumped from low to high pressure.
  • Process 2–3: The high-pressure liquid enters the boiler, and is converted into a dry saturated vapor through heating at a constant pressure by an external heat source.
  • Process 3–4: The dry saturated vapor then later expands through a turbine, thus generating power. This procedure decreases the temperature and the pressure of the vapor, and some other condensation may occur.
  • Process 4–1: The wet vapor then finally enters a condenser, where it is going to be condensed at a constant pressure in order to become a saturated liquid.

Four processes in the Rankine cycle

3) How to increase the efficiency of the boiler in Rankine cycle?

There are plenty of ways to increase the efficiency of a boiler by increasing the efficiency of the Rankine Cycle, here are some examples:

  • Decreasing the condenser pressure: The overall effect of lowering the condenser pressure is an increase in the cycle’s thermal efficiency. It should be mentioned that it is possible to increase the average temperature at which heat is added to the steam without increasing the pressure of the boiler.
  • Fuel Quality: the Quality of the fuel is of prime importance as far as boiler efficiency is concerned. If the fuel has a high moisture content, the enthalpy losses taking place will be much higher and thus the efficiency of the boiler will without a doubt come down. For solid fuel-fired boilers, drying the fuel before combustion can prevent losses of enthalpy which results in improving the boiler’s efficiency.
  • Replacing the pressure valves with small turbines: it is possible to increase the efficiency of a boiler by simply changing the pressure reducing valves also known as PRV’s with small turbines. But how does that work? In boilers and other machines that produce steam, steam operates at high pressures that’s why it is necessary to reduce its pressure. So in order to reduce its pressure, Steam passes through the pressure reducing valves (PRVs) or also known as letdown valves at various locations in the steam distribution system. In order to let down or in other words reduce its pressure. However, even though that the PRVs effectively reduce steam pressure, they also waste valuable energy. However, by replacing these pressure reducing valves with small turbine generators it is possible to avoid energy loss thus increasing the overall efficiency of the boiler by simply reducing the pressure of the steam while capturing the waste energy.

pressure valves with small turbines

4) Conclusion

In conclusion, there are plenty of ways in order to improve the efficiency of the Rankine cycle, such as improving the quality of the fuel or cleaning the tubes, however replacing the pressure reducing valves with a small turbine generator may be the ideal solution, generally speaking of course.

One Comment

  1. Kenneth Cummings
    May 26, 2018 Reply

    it has been a long time since I took steam thermo in college. Is usubg the spent steam to heat the feed water used?

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