RAVI KUMAR

VAPOUR POWER CYCLES

1796-1832

First Law of Thermodynamics

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Boundary work

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Shaft Work

Vapour Power Cycle

It is an Ideal Cycle. In this cycle, the fluid

evaporates and condenses.

Which processes here would cause problems?

Limitations?

Rankine Cycle

Effect of Condenser Pressure

Effect of Vapour Superheat

Effect of Boiler Pressure

11-08

Reheat Cycle

Regenerative Cycle

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Deviation of Actual Cycle From Ideal Cycle

1. Turbine Losses 2. Pump Losses3. Piping Losses4. Condenser Losses

Turbine and Pump Losses

Friction resulting from the flow of the working fluid through the turbine

Heat loss from the turbine to the surroundings

Leakage loss

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43

43

=

Piping Losses

Requirement of Ideal Working Fluid

Ample amount should be available at low cost.

Chemically stable at the maximum temperature

Critical temperature should be higher than the

metallurgical limits.

Reasonable saturation pressure at the max.

temperature as per the metallurgical limit.

Steep saturation vapor line.

Low liquid specific heat.

Non-toxic and non-corrosive

Freezing point should be much below the atmospheric

pressure

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Numerical-1

Determine the efficiency of a Rankine Cycleusing steam as the working fluid in which thecondenser pressure is 10 kPa. The boilerpressure is 2 MPa. Steam leaves the boileras saturated vapour.

Numerical 2

In a reheat cycle steam leaves the boilerand enters the turbine at 4 MPa pressureand 400 oC temperature. After expansion inthe turbine to 400 kPa pressure, the steamis reheated to 400 oC temperature and thenexpanded in the low pressure turbine to 10kPa. Determine the cycle efficiency andspecific steam consumption.

Numerical 3

In a regenerative cycle steam leaves theboiler and enters the turbine at 4 MPapressure and 400 oC temperature. Afterexpansion in the turbine to 400 kPa pressure,some of the steam is extracted from theturbine to heat the feed water in a feed waterheater. The pressure in feed water heater is400 kPa, and water leaving feed water heateris saturated liquid at 400 kPa. The steam notextracted is expanded up to 10 kPa.Determine the cycle efficiency.

Numerical 4

A steam power plant operates on a cyclewith pressure and temperature asdesignated in the following Figure. Theefficiency of turbine is 86% and theefficiency of pump is 80%. Determinethermal efficiency of the cycle.

Binary Vapour Cycle

Thermal efficiency of Rankine cycle can be increased by:

Increasing the average temperature of heat addition.

Decreasing the average temperature of heat rejection.

binary vapour cycles

The limitation for maximum temperature are asfollows for steam:

Critical temperature of steam is equal to 374 0Cand critical pressure is 221.2 bar. It is not possibleto work at this pressure.

Latent heat of vaporization decreases as thepressure increases.

When high pressure steam is expanded, highdegree of moisture content will be present at theend of process.

At the normal pressure of 12 bar, the saturationtemperature for water and mercury are 187oC, 560oC, respectively. The highest temperatureachieved in a power plants is about 550 600oC.Therefore mercury is a better working fluid in thehigh temperature range, because its vaporizationpressure is relatively low. Mercury vapour at hightemperature with low pressure which avoid thedifficulties connected with high pressure.

Advantages of Mercury Cycle

Moderate vapor pressure at high temperature.

Stable

Liquid mercury has high density. Easy to separate in

the boiler and feeding through hydrostatic head is

possible.

Specific heat is only 0.13 kj/kg-K. Steep liquid line.

Specific enthalpy of mercury vapour is low resulting in

low jet velocity in turbine.

Thermal Efficiency is higher than Rankine cycle.

Disadvantages of Mercury Cycle

High cost and limited supply

Toxic in nature

Pervasive, thus, tends to leak through joints.

Latent heat is low, large amount is requiredfor same heat utilization.

The entire boiler operates under hightemperature

It does not wet surface and, therefore, heattransfer is poor.

Numerical

Calculate the ideal efficiency of a binary vapourcycle. The steam cycle operates betweenpressures of 30 and 0.04 bar, uses a superheattemperature of 450 oC. The mercury cycleworks between pressures of 14 and 0.1 bar, themercury entering the turbine in a dry saturatedcondition.

Supercritical Rankine Cycle The Supercritical Rankine Cycle is basically a

superheat Rankine Cycle in which the operatingpressure of the boiler is greater than the criticalpressure of the working fluid.

This method generally provides a bigger increasein the efficiency than the superheat cycle whilestill maintaining high quality in the turbine effluent.

The drawback is that the boiler and turbine mustbe built to withstand high pressure and hightemperatures and this can be quite expensive.

CogenerationCogeneration or Combined Heat and Power (CHP) is

defined as the sequential generation of two different

forms of useful energy from a single primary energy

source, typically mechanical energy and thermal energy.

Mechanical energy may be used either to drive an

alternator for producing electricity, or rotating

equipment such as motor, compressor, pump or fan for

delivering various services.

Thermal energy can be used either for direct process

applications or for indirectly producing steam, hot

water, hot air for dryer or chilled water for process

cooling.

Steam Cogeneration System