POWER EQUIPMENT INSTRUCTOR: ROBERT A. MCLAUGHLIN ZAILI THEO ZHAO

1. POWER EQUIPMENT INSTRUCTOR: ROBERT A. MCLAUGHLIN ZAILI THEO ZHAO FUNDAMENTALS OF HEAT EXCHANGERS

2. Learning Objectives • Understanding of various types of heat exchangers used in power production. • Identify operating principles, features, applications and limitations of various types of heat exchangers. • Discuss the Second Law of Thermodynamics and how it applies to heat exchangers. • Identify the three (3) modes of heat transfer and the “gremlins” that prevent the process. • Identify the two rules pertaining to efficient main condenser operation and examine and identify the main heat exchangers used in the Rankine Cycle. • Determine method of calculating heat transfer in tube type exchangers.

3. First Law of Thermodynamics • Internal energy from one body will equal that energy taken by a second body. • Any thermodynamic system in an equilibrium state possesses a state variable called the internal energy (E) • Between any two equilibrium states, the change in internal energy is equal to the difference of the heat transfer into the system and work done by the system.

4. 2nd Law of Thermodynamics • Heat flows from high temperature regions (called the source) to low temperature regions (called the sink). • There exists a useful thermodynamic variable called entropy (S). • A natural process that starts in one equilibrium state and ends in another will go in the direction that causes the entropy of the system plus the environment to increase for an irreversible process and to remain constant for a reversible process.

5. Transfers in three ways • Conduction • Direct contact heat exchange • Convection • Heat exchange within a fluid • Two types • Free convection where density difference is created within the fluid, causing movement within the fluid. • Heated water or air moves within the fluid due to the density difference. • Forced convection where a pressure difference created by pump or fan causes the fluid motion. • Radiation • Heat transfer by waves.

6. Heat transfer through pipe

7. Heat Exchangers • A mechanical device that allows the transfer of heat from one substance to another. • Heat transfer is the flow of thermal energy form one location to another • T signifies the temperature difference of the cooling fluids in a heat exchanger • T of the cooling fluid is the difference in temperature entering and leaving a heat exchanger • T of the cooled fluid is the temperature difference between the entering and the exiting heat exchanger. • When a phase change takes place, there may not be a T across the heat exchanger – latent heat transfer • That means that temperature is not and indicator of heat transfer, but is only an indicator of potential for heat transfer.

8. Heat Exchangers • The fluid flow through a heat exchanger includes • Counter flow • The two fluids flow in opposite directions

9. Heat Exchangers • Parallel flow • Rarely used because of the length of time it would take to create the necessary heat exchange • – making the transfer surface impossible big

10. Heat Exchangers • Cross flow • Primarily used where the main purpose is to remove latent heat – like in a main condenser. • The Temp. variable tendency curves same as parallel

11. Heat Exchangers • Heat exchangers fall into one of three categories • Shell and tube also known as surface type heat exchangers.

12. Heat Exchangers • Plate type

13. Heat Exchangers • Plate type • Small & high heat load • Expandable – retractable • Easy to clean & maintain inspection • Small passage can easy clog • Easy to damage while assembling • Generally limited • 290 PISG and lower • 300 °F and less

14. Heat Exchangers • Direct contact heaters • When the two mediums are in direct contact

15. Water to Steam • The heat-o-meter would be calibrated in Btu • One British thermal unit is the heat required to raise one pound of water one degree Fahrenheit • As the water temperature reaches 212℉, the heat-o-meter reads 180Btu.

16. Water to Steamwater into steam

17. SATURATED PRESSURE • Molecules at the liquid surface are in equilibrium between the liquid and vapour phases; the vapour at the liquid surface is said to be saturated. • At any given liquid surface temperature, the vapour exerts a pressure known as the saturation pressure. • More molecules escape as the temperature increases and this increases the vapour pressure

18. RANKINE CYCLE • Rankine cycle is a heat engine with vapor power cycle. • The common working fluid is water. • The cycle consists of four processes: • 1 - 2: Isentropic expansion (Steam turbine) • 2 - 3: Isobaric heat rejection (Condenser) • 3 - 4: Isentropic compression (Pump) • 4 - 1: Isobaric heat supply (Boiler)

19. BOILER • Start at the boiler (the heat source) • Saturated steam is produced in the boiler then superheated • The turbine uses thermal energy then exhausts or rejects some thermal energy to the condenser. • That is energy the turbine cannot use.

20. CONDENSER • The condenser is a shell and tube heat exchanger (also known as a surface) that uses water as a cooling medium. • Salt water for ships • Fresh water for shore plants • The cooling medium flows through the tubes and the exhaust steam surrounds the tubes

21. CONDENSER • The condenser takes away the latent heat of the steam causing it to collapse to water or condensate. • The condensate temperature should be at or near the saturation temperature • If it is below the saturation temperature, then the condensate has been subcooled or it is depressed.

22. CONDENSER–RULE NO.1 • Temperature Rise is the difference between the circulating water temperature inlet to the condenser and the overboard discharge temperature. • On a single pass, marine type condenser, the temperature rise should be about10ºF. • There is a direct relationship between temperature rise and condensate depression, and as an operating engineer this is one of the fundamental relationships that you have control over and should clearly understand.

23. CONDENSER–RULE NO.2 • Condensate depressionis the difference in temperature between the saturation temperature of the steam entering the condenser and the temperature of the condensate being removed from the hotwell of the condenser and represents inefficiency in the plant. • Condensate depression should be maintained at only a few degrees (2-4ºF) as it is heat that we will have to add back into the process otherwise.

24. CONDENSER • The condenser consists of a shell, a hotwell, tube sheets and tubes. • The hotwell acts a storage basin for condensate. • The tubes are arranged to form steam lanes • Steam lanes direct the steam to the hotwell then back into the tubes • This is called a reheating hotwell, and it helps keep the condensate depression to a minimum. • The shell is an air tight barrier

25. CONDENSER - VACUUM • A vacuum is maintained within the shell of the condenser primarily due to steam collapsing and turning into condensate. • To begin the process and assist in maintaining vacuum inside the condenser, either an air ejector or vacuum pump is used. • Vacuum is maintained between 25˝ of mercury and 29 ˝ of mercury • By maintaining a vacuum, increase the efficiency of the cycle • The bigger the difference the higher the overall efficiency • The saturation temperature of steam at 29 ˝ of vacuum is 76oF. At 25 ˝ of vacuum it is 133oF. • The lower the vacuum, the more efficient the plant will be.

26. CONDENSER - VACUUM • Most of the vacuum created in a condenser is created by the condensing of the steam. • As steam collapses from a large volume as steam to a smaller volume as condensate it leaves behind a void or vacuum. • To help create vacuum initially, and to help maintain the vacuum by removing non- condensable gases, either an air ejector assembly is used, or a vacuum pump is used. • An air ejector assembly consists of two jet nozzles, each using high pressure steam as the medium to create a vacuum.

27. CONDENSER • A main condenser is being properly operated when: • Condensate depression is no more than 2 - 4oF. • The T temperature rise of the cooling medium is no more then 10oF. • A dirty condenser would be identified by: • A drop in the T • A drop in vacuum

28. MAIN CONDENSATE PUMPS • Take suction from the condenser hotwell • Condensate is the cooling medium for the air ejector condensers. • They provide the first heating for the condensate.

29. DC HEATERS • Condensate is atomized through spray nozzles. • Steam at about 30 psi enters the shell, surrounding the atomized water. • It causes a very rapid rise in water temperature. • This causes and dissolved gasses, • like oxygen, CO2, and Nitrogen to come out of solution. • The gases are vented through a vent condenser • which allows gasses to escape and condenses any steam that tries to escape.

30. DC HEATERS • The main functions of a DC heater • Provides for a reservoir of water for the cycle operation. • Provides for a positive suction head for boiler feed pumps. • Causes condensate to give up dissolved non – condensable gases by heating the condensate very rapidly. • Acts as a stage of feed water heating.

31. DC HEATERS • The DC heater will be located at the highest point in a shipboard engine room. • The water temperature leaving the DC heater is approximately 250oF to 270oF. • At these temperatures, water could easily flash at the pump suction causing cavitation, or worst causing the suction eye to be completely filled with steam, with no water flow through the pump. • The DC heater is located high enough above the pump suction so that flashing will not normally occur.

32. GPM Temperature Measurement

33. T4 T3 T6 T5

34. Calculation Sheet • Heat Exchanger: Parallel Flow or Counter Flow • Steam side: Inlet Temp. __Gauge 264 (T6 240)__°F • Outlet Temp. ________T4 160_____ °F • Inlet Pressure ________20_____ Psig • Water side: Inlet Temp. _______ T3 53___ °F • Outlet Temp. _________ T5 98___ °F • Flow Rate _________3__ _ GPM • Time period of experiment? _______1____ Minutes • Steam Quality at inlet? Saturated • Is condensate above or below saturation? Below • Calculate the heat transfer occurring in your experiment.

35. Calculation Sheet – steam side • Steam in @ 264 ˚F 20 Psig + 14.7 = 34.7 Psia • The inlet steam is Saturated • Condensate down to 160 ˚F @ 14.7 Psia • The outlet water is Subcooled • Heat transfer • From Properties of Saturated Steam Temperature Table • 240˚F @ 24.968 Psia & Enthalpy 952.1 Btu/lb • Steam (240 ˚F - 240 ˚F)  ½ Btu/lb = 0 Btu • Latent heat 1 Btu/lb 240 ˚F @24.968 Psia = 952.1 Btu/lb • Sensible heat (240 - 160˚F)· 1 Btu/lb = 80 Btu/lb • 1.2 lb  (0 + 952.1 +80) Btu/lb = 1238.88 Btu

36. Calculation Sheet – water side • Cooling water flow rate is 3 GPM • Outlet T = 98 ˚F • Inlet T = 53 ˚F • 1 gal water = 8.34lbs • 3 GPM = 25.03 lbs/min • 1 Btu/(lb˚F) × 25.03 lbs/min × (98-53)˚F × 1 min = 1,126.35 Btu • Because of some of heat releases to atmosphere, the heat of steam side is larger than the heat received by water.

37. THANK YOU