INTRODUCTION:

When vapor power cycles in which the working fluid is alternatively vaporized and condensed. We also consider power generation coupled with process heating called cogeneration. The continued quest for higher thermal efficiencies has resulted in some innovative modifications to the basic vapor power cycle. Among these, we discuss the reheat and regenerative cycles, as well as combined gas–vapor power cycles. Steam is the most common working fluid used in vapor power cycles because of its many desirable characteristics, such as low cost, availability, and high enthalpy of vaporization. Therefore, this chapter is mostly devoted to the discussion of steam power plants. Steam power plants are commonly referred to as coal plants, nuclear plants, or natural gas plants, depending on the type of fuel used to supply heat to the steam. However, the steam goes through the same basic cycle in all of them. Therefore, all can be analyzed in the same manner.

Furthermore talking about steam power cycle the main application we can find is the steam turbine.A steam turbine is a mechanical device that converts thermal energy in pressurized steam into useful mechanical work. Reciprocating pistons have now been almost totally replaced by the steam turbine because the steam turbine has a higher thermodynamic efficiency and a lower power-to-weight ratio and the steam turbine is ideal for the very large power configurations used in power stations. The steam turbine derives much of its better thermodynamic efficiency because of the use of multiple stages in the expansion of the steam.This is a in closer approach to the ideal reversible processes.

Steam turbines are made in a variety of sizes ranging from small 0.75 kW units used as mechanical drives for pumps, compressors and other shaft driven equipment, generally 1,500,000kW turbines are used to generate electricity.  Steam turbines are widely used for marine applications for vessel propulsion systems.  In recent times gas turbines, as developed for aerospace applications, are being used more and more in the field of power generation once dominated by steam turbines.

Main Components for a Basic Cycle of Steam Generation and Power Production:

Boiler - Steam generatorTurbine - Steam expander-produces work and drive the electric generatorCondenser - Exchanges heat from low pressure steam to cooling waterPump - Pushes liquid water from low condenser pressure to high boiler pressure.

The basic process behind steam power generation is the “Rankine Cycle”. Water is heated until it is a saturated liquid. From there, it is compressed into steam. The steam is transferred to a turbine where the pressure of the steam is reduced (usually to sub atmospheric pressures) by expansion over the turbine blades. This process produces electricity. The low pressure steam is condensed back to a liquid. The water, now referred to as return water, is mixed with new water, referred to as “feed water”, and pumped back to the boiler. The figure below shows a common diagram used to describe the Rankine Cycle.

Many of the impracticalities associated with the Carnot cycle can be eliminated by complimenting with a cycle phenomena the Rankine cycle, which is the ideal cycle for vapor power plants. The ideal Rankine cycle does not involve any internal irreversibility’s and consists of the following four processes:

1-2 isentropic compression in a pump2-3 Constant pressure heat addition in a boiler3-4 isentropic expansion in a turbine4-1 Constant pressure heat rejection in a condenser

-Heat flow rateDiagram of a Rankine cycle

-Mass flow rate

-Mechanical power

Rankine thermal efficiency =h3−h 4h3−h1

Before this Rankine cycle there was a cycle called Carnot cycle for the applications and analysis of thermodynamics. But due to practical impossibilities that Carnot cycle was refused by industry and learning objectives.

Hence the Rankine cycle was introduced as a solution for those practical problems and then those problems could be overcome by this cycle properties.

Diagram of a Carnot cycle

A figure of a steam power cycle of a thermal power plant

TYPES OF BOILERS:

Fire-tube boiler was more common in the 1800s. It consists of a tank of water perforated with pipes. This boiler consists of a series of straight tubes that are housed inside a water-filled outer shell. The tubes are arranged so that hot combustion gases flow through the tubes. As the hot gases flow through the tubes, they heat the water surrounding the tubes. The water is confined by the outer shell of boiler.

Most modern fire tube boilers have cylindrical outer shells with a small round combustion chamber located inside the bottom of the shell. Depending on the construction details, these boilers have tubes configured in one, two, three, or four pass arrangements.

Water tube boilers are designed to circulate hot combustion gases around the outside of a large number of water filled tubes. In the older designs, the tubes were either straight or bent into simple shapes. Newer boilers have tubes with complex and diverse bends Small water tube boilers, which have one and sometimes two burners, are generally fabricated and supplied as packaged units. Because of their size and weight, large water tube boilers are often fabricated in pieces and assembled in the field. In water tube or “water in tube” boilers, the conditions are reversed with the water passing through the tubes and the hot gases

passing outside the tubes.

These boilers can be of a single- or multiple-drum type. They can be built to any steam capacity and pressures, and have higher efficiencies than fire tube boilers.

TYPES OF TURBINES:

Condensing turbine - The maximum amount of energy is extracted from the steam by passing the exhaust steam into a condenser. The steam is condensed by surface contact with bundles of tubes through which cooling water is passing. As the steam condenses, its volume, on changing to water, decreases by about 1800 times. This great decrease in volume causes a vacuum to form in the condenser. Due to this, the pressure drop across the turbine and therefore the turbine power is maximized.Non-condensable gases include both air and a small amount of the corrosion byproduct of the water-iron reaction, hydrogen.

Back Pressure Steam turbine - The turbine speed is controlled by the steam input. In this type of turbine, the exhaust must be maintained at a constant pressure by a PCV control system downstream of the turbine exhaust to prevent changes in the exhaust pressure that would affect the turbine speed by changing the pressure drop across it. The governor would be fighting against these pressure fluctuations and speed control would be erratic.50, 150 and 250 psig are the most typical pressure levels for steam distribution systems. The lower pressures are most often used in small and large district heating systems, and the higher pressures most often used in supplying steam to industrial processes.

Condensing turbine

TYPES OF CONDENCERS:

Surface condenser - This turbine is the commonly used term for a water-cooled shell and tube heat exchanger installed on the exhaust steam from a steam turbine in thermal power stations .These condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric pressure. Where cooling water is in short supply, an air-cooled condenser is often used. An air-cooled condenser is however significantly more expensive and cannot achieve as low a steam turbineexhaust pressure

Jet condenser - In a jet condenser, cooling water and exhausted steam are together. Therefore, the temperature of cooling water and condensate is the same when leaving the condenser. Advantages of this type of condenser are: low initial cost, less flow area required, less cooling water required and low maintenance charges.Disadvantages are: condensate is wasted and high power is required for pumping water. In a surface condenser, there is no direct contact between cooling water and exhausted steam.

Energy balance of overall system

The total Energy of the overall process should be balanced according to the first law of thermodynamics. When consider the energy transformation,

Where, is the heat flow rate in the system

is the mechanical power consumed

(work )

h is the specific enthalpy

s is the entropy of the system

Energy balance of the turbine

In this cycle useful output is electrical energy that produces by turbine. Kinetic energy drop, temperature

drop, pressure drop are the other energy changing method inside the turbine. As a result of vapour partially convert to the water volume reduce. Then speed of fluid which come out of turbine is low than the entering speed. Part of this energy reduction cause to rotate the turbine.

The power output is then calculated as the mass flow rate of the steam multiplied by the difference in enthalpy between the inlet and outlet, or:

Where Wt

the power output by the turbine is, m

is the mass flow rate of the

steam, and h1 and h2 are the specific enthalpies at the inlet and outlet, respectively.

ME 2842

STEAM POWER CYCLE