Main Steam Cycle

References

• Required– Introduction to Naval Engineering (pg 41-48)

• Recommended– Principles of Naval Engineering (132-136)

Objectives

A. Comprehend the application of the laws of thermodynamics to determine the changes in state/energy which water undergoes in the basic steam cycle. B. Comprehend the purpose and safety issues of the various components and their effect on these energy and state changes. C. Comprehend the four phases of the basic steam cycle and know the components found in each.

Introduction

• What are the basic elements of a heat engine?

• Working Substance• Generation (heat source)• Expansion• Rejection• Pump

Generation Expansion

Pump

Working Substance

Basic Heat Engine

Rejection

Steam Generator or Boiler

Turbine

Pump

Working Substance

The Main Steam Cycle

Rejection

Heat Added Work Out

Heat OutWork On

Conventional Steam System

Typical Nuclear Steam Cycle

Expansion

Generation

Condensing

Feed

Background

• Cycle used is a Rankine cycle• P-v and T-s Diagrams– Trace the points of cycle– Provide graphical understanding of cycle

• Conventional vs. Nuclear

Generation Phase - Conventional• Boiler ( steam)– Watersides (steam) and Firesides (furnace)– Fuel burned to produce heat -> transferred to water which

boils to steam– Steam collects in steam drum (saturated)

• Superheater– Increase temp of steam and dries steam– Three reasons for superheating?

• Minimize erosion (dry steam better for blading)• Minimize corrosion (less chemicals entrained)• Maximize DT (Carnot efficiency)

Boiler and Superheater

Expansion Phase

• Steam travels down main steam piping• Turbines convert thermal energy -> mechanical

energy (nozzles) and then work (blading) -> turn rotor/shaft

• Pressure drops as steam goes through• Work performed on turbine blading– Main Engines (ME) -> propulsion– Ship’s Service Turbine Generators (SSTG) -> electricity

Condensation

• Vacuum 26-29” Hg– Air Ejectors - use steam to establish initial

vacuum and remove air – Maintained by condensation • volume of water << volume of steam -> contraction

– Why vacuum?• Ease of steam recovery (“pulls” steam into MC)• More work out of turbines (larger DP and DT)

Condensation

• Main Condenser– Large, indirect, cross-flow, shell-and-tube HX– Seawater used to condense steam

• Hotwell - holding area for condensate water at 80-100 F (lowest temp in cycle)

• Main Condensate Pumps - send condensate to the DFT at 20-30 psi (suction side is lowest pressure in cycle)

Main Condenser

Feed Phase

• Deaerating Feed Tank (DFT)– Direct-type HX (Aux Steam used)– Purposes

• preheats feed • storage/surge volume• removes dissolved oxygen to minimize corrosion

– FYI: RFT’s often used instead

• Main Feed Pump– Supplies feed water to Steam Generator (must be high

pressure to overcome pressure)

Deaerating Feed Tank

Rankine Cycle

4-1: Feed Phase (DQ=0, DS=0), Win 1-2: Generation Phase (DP=0), Qin

2-3: Expansion Phase (DQ=0, DS=0), Wout

3-4: Condensation Phase (DP=0), Qout

T-s Diagram

Putting It All Together

Take Away

• Draw a T-s Diagram of the Rankine Cycle, label the phases of the cycle and indicate the condition of the working fluid in each phase

• Draw and label a simple, one-line diagram of the main steam cycle

• Describe the energy conversions/phase changes occurring in each component of the steam cycle

• Describe the advantages of a super heater and deaerating feed tank in propulsion plants

Questions?