CHE 3010 - Thermodynamics Tennessee Tech University

Venkat Padmanabhan, PhD Department of Chemical Engineering

Tennessee Tech University

CHE 3010 - Thermodynamics of Chemical Processes

Lecture 35 - Turbines, Compressors and Pumps 11/29/2017

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Turbines (or expanders)

Inlet

Outlet

Ws

Combination of a nozzle and blades connected to a rotating shaft

Expansion of gas to produce high-velocity stream converts internal energy to kinetic energy

Kinetic energy converted to shaft work when steam impinges on blades

When steam is the gas, its called a turbine

When any other gas such as ammonia or ethylene is used, its called an expander

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Turbine Efficiency

The maximum possible work achievable from a turbine operating at given conditions is possible when the process is isentropic (ΔS = 0)

This is possible when the gas in the turbine expands reversibly and adiabatically

Therefore, efficiency is defined as, η =·Ws

·Ws (isen tro pic)In a good turbine design, heat transfer is negligible, inlet and exit pipes are sized to make fluid velocities roughly equal, and is horizontal

So, from energy balance, ·Ws = ΔH

and efficiency is η = ΔH(ΔH)S

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Compressors and Pumps

Inlet

Outlet

Ws

Compressors cause the pressure in the flowing fluid to increase

Compression of gases accomplished by rotating blades (for high-volume flow) or reciprocating pistons (for high pressures)

Energy equations are independent of type of equipment

Design principles are the same — potential and kinetic energy changes are negligible, and adiabatic compression is followed

Turbine operated in reverse

Turbines or expanders cause the pressure in the flowing to fluid to decrease

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Compressor Efficiency

The minimum amount of work required for compression of a gas is possible when the process is isentropic (ΔS = 0)

Therefore, efficiency is defined as, η =·Ws (isen tro pic)

·Ws= (ΔH)S

ΔH

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Example

P35.1 — A steam turbine with rated capacity of 56,400 kW operated with steam an inlet conditions of 8000 kPa and 500 oC, and discharges into a condenser at a pressure of 10 kPa. Assuming a turbine efficiency of 0.75, determine the state of the steam at discharge and the mass flow rate of the steam.

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Example

P35.2 — Saturated-vapor steam at 100 kPa is compressed adiabatically to 300 kPa. If the compressor efficiency is 0.75, what is the work required and what are the properties of the discharge stream?

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Example

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Example