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© Copyright 2014Operator Generic Fundamentals Components – Heat Exchangers and Condensers.

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1 © Copyright 2014Operator Generic Fundamentals Components – Heat Exchangers and Condensers

2 © Copyright 2014Operator Generic Fundamentals 2 Heat Exchangers and Condensers Principles of Operation Transfer heat from one fluid to another –Fluids must be at different temperatures –Fluids must come into thermal contact –Heat flows from hotter to cooler fluid Maintains separation between the two fluids Intro

3 © Copyright 2014Operator Generic Fundamentals 3 Terminal Learning Objectives At the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of ≥ 80% on the following areas: 1.Describe the purpose, construction, and principles of operation for each major type of heat exchanger. 2.Describe the purpose, construction, and principles of operation of condensers. Intro

4 © Copyright 2014Operator Generic Fundamentals 4 Heat Exchanger Construction & Operation Heat exchangers transfer heat from higher energy to lower energy system Conduction and convection heat transfer methods Allows systems to maintain physical separation of processes Heat exchangers use different flow designs, including counter and cross flow TLO 1 TLO 1 – Describe the purpose, construction, and principles of operation for each major type of heat exchanger.

5 © Copyright 2014Operator Generic Fundamentals 5 1.Describe the construction, effectiveness, and operation of the following types of heat exchangers and their components (tubes, tube sheets, baffles and shells): a.Tube and shell b.Plate 2.Describe hot and cold fluid flow paths in the following types of heat exchangers: a.Parallel flow b.Counter flow c.Cross flow 3.Describe the difference between the following types of heat exchangers: a.Single-pass versus multipass heat exchangers b.Regenerative versus nonregenerative heat exchangers Enabling Learning Objectives for TLO 1 TLO 1

6 © Copyright 2014Operator Generic Fundamentals 6 4.Describe the operation of a typical heat exchanger to include the following: a.Startup and shutdown b.Control of temperature c.Effects and control of fouling 5.Given the necessary data, calculate flow rates, and temperatures for various types of heat exchangers. 6.Explain the consequences of heat exchanger tube failure. Enabling Learning Objectives for TLO 1 TLO 1

7 © Copyright 2014Operator Generic Fundamentals 7 Types of Heat Exchangers Heat exchanger construction falls into one of two categories: –Tube and shell –Plate Each type has advantages and disadvantages ELO 1.1 ELO 1.1 – Describe the construction, effectiveness, and operation of the following types of heat exchangers and their components (tubes, tube sheets, baffles, and shells): tube and shell and plate.

8 © Copyright 2014Operator Generic Fundamentals 8 Types of Heat Exchangers Tube and Shell Most basic and most common type Consists of tubes in a container called a shell Fluid is separated from the shell-side fluid by the tube sheet(s) Tubes can withstand higher pressures than shells Support plates act as baffles, directing the flow of fluids Figure: Tube and Shell Heat Exchanger ELO 1.1

9 © Copyright 2014Operator Generic Fundamentals 9 Types of Heat Exchangers Plate Heat Exchanger Consists of plates instead of tubes to separate the hot and cold fluids Fluids alternate between each of the plates Baffles direct the flow of fluid between the plates Plates have a large surface area and provide each of the fluids with an extremely large heat transfer area ELO 1.1 Figure: Plate Heat Exchanger

10 © Copyright 2014Operator Generic Fundamentals 10 Heat Exchanger Applications Heat exchangers are found in most chemical or mechanical systems Common applications are: –Ventilation and air conditioning (HVAC) systems –Radiators on internal combustion engines –Boilers –Condensers –Preheaters or coolers ELO 1.1

11 © Copyright 2014Operator Generic Fundamentals 11 Preheaters and Feedwater Heaters Some heaters, such as feedwater, heat in stages verses all heat transfer in one large heat exchanger –Increases the plant's efficiency –Minimizes thermal shock stress to components compared to injecting ambient temperature liquid into a boiler (reactor) or other device that operates at high temperatures –In a steam system, a portion of the processed steam is tapped off and used as heat source to preheat the feedwater in preheater stages ELO 1.1

12 © Copyright 2014Operator Generic Fundamentals 12 Preheaters and Feedwater Heaters Below is an example of construction and internals of a U-tube feedwater heat exchanger found in a preheater stage of a power plant Figure: Feedwater Heater ELO 1.1

13 © Copyright 2014Operator Generic Fundamentals 13 Radiator A heat exchanger is any device that transfers heat from one fluid to another. Some equipment uses air-to-liquid heat exchangers An example of an air-to-liquid heat exchanger is a car radiator –Coolant flowing in engine picks up heat from engine block and carries it to radiator Hot coolant flows into tube side of radiator (heat exchanger) Relatively cool air flowing over the outside of the tubes picks up the heat, reducing temperature of coolant Fins on the outside of the tubes increase surface area for heat transfer and maximize heat transfer efficiency ELO 1.1

14 © Copyright 2014Operator Generic Fundamentals 14 AC Evaporator and Condenser AC units contain at least two heat exchangers, usually called evaporator and condenser In each of these, refrigerant fluid flows into heat exchanger and transfers heat, either gaining or releasing it to cooling medium, which is commonly air or water In the condenser, the hot, high-pressure refrigerant gas condenses to a subcooled liquid In evaporator,subcooled refrigerant flows into heat exchanger, heat flow is reversed, cool refrigerant absorbs heat from hotter air flowing on the outside of tubes Cools the air and boils refrigerants ELO 1.1

15 © Copyright 2014Operator Generic Fundamentals 15 Condenser Condenser is a type of heat exchanger used to condense a substance from a gaseous state to a liquid state by cooling. Condenser removes latent heat from the condensing fluid and transfers it to coolant Normally, tube and shell heat exchanger serves as a condenser Baffles usually added at inlet to prevent tube impingement from incoming gas or steam Frequently use large steam condensers as heat sinks for steam system ELO 1.1

16 © Copyright 2014Operator Generic Fundamentals 16 Knowledge Check In a tube and shell heat exchanger, the fluid flowing ________ the tubes is called the tube-side fluid and the fluid flowing _________ of the tubes is the shell-side fluid. A.around; inside B.around; outside C.inside; outside D.outside; inside Correct answer is C. Types of Heat Exchangers ELO 1.1

17 © Copyright 2014Operator Generic Fundamentals 17 Heat Exchanger Classification Parallel Flow Tube-side and shell-side fluid flow in the same direction Fluids enter from same end with large temperature difference Heat transfers from hotter to cooler –Temperatures of two fluids approach each other –Hottest cold-fluid temperature is always less than the coldest hot-fluid temperature ELO 1.2 – Describe hot and cold fluid flow paths in the following types of heat exchangers: parallel flow, counter flow, and cross flow. Figure: Parallel-Flow Heat Exchanger ELO 1.2

18 © Copyright 2014Operator Generic Fundamentals 18 Heat Exchanger Classification Counter Flow Fluids flow in opposite directions and enter at opposite ends Cooler fluid will approach inlet temperature of the hot fluid Generally most efficient type of heat exchanger Hottest cold-fluid temperature can actually be greater than the coldest hot-fluid temperature Figure: Counter-Flow Heat Exchanger ELO 1.2

19 © Copyright 2014Operator Generic Fundamentals 19 Heat Exchanger Classification Cross Flow Fluid flows perpendicular Usually used when one of the fluids changes phase Large volumes of vapor may be condensed Most efficient when comparing heat transfer rate per unit surface area Average difference in temperature (∆T) between two fluids over the length of the heat exchanger is maximized Figure: Cross-Flow Heat Exchanger ELO 1.2

20 © Copyright 2014Operator Generic Fundamentals 20 Heat Exchanger Comparison Compare average temperature difference across heat exchanger Heat exchanger with largest average temperature difference is more efficient Mean (average) temperature for a heat exchanger calculated using this equation: ELO 1.2

21 © Copyright 2014Operator Generic Fundamentals 21 Heat Exchanger Comparison ELO 1.2

22 © Copyright 2014Operator Generic Fundamentals 22 Heat Exchanger Comparison ELO 1.2

23 © Copyright 2014Operator Generic Fundamentals 23 Heat Exchanger Comparison ELO 1.2

24 © Copyright 2014Operator Generic Fundamentals 24 Heat Exchanger Comparison ELO 1.2

25 © Copyright 2014Operator Generic Fundamentals 25 Classification by Flowpath Correct answer is A. ELO 1.2

26 © Copyright 2014Operator Generic Fundamentals 26 Knowledge Check – NRC Bank The rate of heat transfer between two liquids in a heat exchanger will increase if the… (Assume specific heats do not change.) A.inlet temperature of the hotter liquid decreases by 20°F. B.inlet temperature of the colder liquid increases by 20°F. C.flow rates of both liquids decrease by 10 percent. D.flow rates of both liquids increase by 10 percent. Classification by Flowpath Correct answer is D. ELO 1.2

27 © Copyright 2014Operator Generic Fundamentals 27 Other Heat Exchanger Characteristics Most large heat exchangers are not purely parallel-flow, counter-flow, or cross-flow Combination types maximize heat exchanger efficiency Having two fluids pass each other several times within a single heat exchanger increases efficiency ELO 1.3 – Describe the difference between the following types of heat exchangers: single-pass versus multipass heat exchangers and regenerative versus nonregenerative heat exchangers. ELO 1.3

28 © Copyright 2014Operator Generic Fundamentals 28 Other Heat Exchanger Characteristics Single-pass heat exchanger –Fluids pass each other once Multipass heat exchanger –Fluids pass each other more than once –Reverses flow in the tubes by use of one or more sets of U-bends in the tubes Baffles on the shell side of the heat exchanger direct fluid back and forth across the tubes to achieve the multipass effect Figure: Single-Pass and Multipass Heat Exchangers ELO 1.3

29 © Copyright 2014Operator Generic Fundamentals 29 Other Heat Exchanger Characteristics Heat exchangers are classified by their function –Regenerative –Nonregenerative Regenerative heat exchanger –One fluid is both the cooling fluid and the cooled fluid –Usually found in high-temperature systems –Improves efficiency Nonregenerative heat exchanger –Hot fluid is cooled by fluid from a separate system –Energy (heat) removed is not returned to the system ELO 1.3

30 © Copyright 2014Operator Generic Fundamentals 30 Regenerative and Nonregenerative Heat Exchanger Comparison ELO 1.3 Figure: Regenerative Heat ExchangerFigure: Nonregenerative Heat Exchanger

31 © Copyright 2014Operator Generic Fundamentals 31 Knowledge Check In a ________________ heat exchanger, heat from the main process flow is ______________ the system. A.regenerative; rejected from B.regenerative; returned to C.nonregenerative; stored in D.nonregenerative; returned to Correct answer is B. Other Heat Exchanger Characteristics ELO 1.3

32 © Copyright 2014Operator Generic Fundamentals 32 Heat Exchanger Startup & Operation Startup Filled with fluid on both sides Cold fluid first Hot fluid slowly to minimize thermal shock Vent air and noncondensable gases (these can effectively reduce surface area and heat transfer) ELO 1.4 – Describe the operation of a typical heat exchanger to include startup and shutdown, control of temperature, and the effects and control of fouling. ELO 1.4

33 © Copyright 2014Operator Generic Fundamentals 33 Shutdown Hot fluid normally stopped first Cold fluid stopped Never isolate from overpressure protection (either a discharge valve left open or a relief valve installed) ELO 1.4 Heat Exchanger Startup & Operation

34 © Copyright 2014Operator Generic Fundamentals 34 Temperature Control Control the flow of either the cooled fluid or cooling fluid Example – Cooling fluid flow reduced: –Cooling flow rate slowed, less heat transfer, heat exchanger outlet is higher (less cooling of system fluid) ELO 1.4 Figure: Operating Water Cleanup System

35 © Copyright 2014Operator Generic Fundamentals 35 Temperature Control Refer to the drawing: Valves are identical and are initially 50 percent open To raise the temperature at point 7, the operator can adjust valve D (the cooling water throttle valve) in the closed or shut direction ELO 1.4 Figure: Operating Water Cleanup System

36 © Copyright 2014Operator Generic Fundamentals 36 Temperature Control Cooled fluid –Increasing cooled fluid flow will lower the outlet temperature –Reverse will happen if the cooling fluid flow is slowed ELO 1.4

37 © Copyright 2014Operator Generic Fundamentals 37 Fouling of Heat Exchange Surfaces Foreign material (algae, scale, or debris) accumulates in a heat exchanger Lowers the efficiency Removal by –Hydrolancing –Chemical cleaning –Backwashing Maintaining minimum flow through heat exchanger can prevent deposits Turbulent flow aids in heat transfer by agitating laminar film Chemicals can be added ELO 1.4

38 © Copyright 2014Operator Generic Fundamentals 38 Knowledge Check Refer to the drawing of an operating lube oil heat exchanger below. Increasing the oil flow rate through the heat exchanger will cause the oil outlet temperature to _________ and the cooling water outlet temperature to __________. (Assume cooling water flow rate remains the same.) A.decrease; decrease B.decrease; increase C.increase; decrease D.increase; increase Correct answer is D. Heat Exchanger Startup & Operation ELO 1.4

39 © Copyright 2014Operator Generic Fundamentals 39 Heat Exchanger Calculations ELO 1.5 – Given the necessary data, calculate flow rates and temperatures for various types of heat exchangers. ELO 1.5

40 © Copyright 2014Operator Generic Fundamentals 40 Heat Exchanger Calculations ELO 1.5

41 © Copyright 2014Operator Generic Fundamentals 41 Heat Exchanger Calculations ActionFormula Determine heat transferred across heat exchanger to or from one of the fluids Determine log mean temperature difference between two fluids if necessary Once heat transfer is known, solve for flow or temperature difference of other fluid Use the following table to determine flow or temperature difference of heat exchanger fluids: ELO 1.5

42 © Copyright 2014Operator Generic Fundamentals 42 Heat Exchanger Calculations ELO 1.5

43 © Copyright 2014Operator Generic Fundamentals 43 Heat Exchanger Demonstration ELO 1.5 StepFormulaSolution

44 © Copyright 2014Operator Generic Fundamentals 44 Correct answer is A. Heat Exchanger Calculations A.135°F B.140°F C.145°F D.150°F ELO 1.5

45 © Copyright 2014Operator Generic Fundamentals 45 Heat Exchanger Tube Failure Most common failure is a breach of the pressure boundary Tubes can be worn or eroded over time due to: –High flow rates –Particulate in the fluids passing through Vibration –Caused by irregular flow pattern or flow is throttled ELO 1.6 – Explain the consequences of heat exchanger tube failure. ELO 1.6

46 © Copyright 2014Operator Generic Fundamentals 46 Heat Exchanger Failure Vibration could compromise seal between tubes and tube sheet or sealing surfaces between fluids Failure of the heat exchanger will allow two fluids to mix –Higher-pressure fluid forces into lower-pressure system –Contaminated fluid is generally of low-pressure side Instrumentation shows an equalization of fluid temperatures at some mid-temperature Lower-pressure system level should rise and increase the level in an expansion tank and the higher-pressure system level should decrease ELO 1.6

47 © Copyright 2014Operator Generic Fundamentals 47 Heat Exchanger Failure Example Refer to the drawing of an operating cooling water system below. What occurs when a tube fails in the heat exchanger? Figure: Cooling Water System ELO 1.6

48 © Copyright 2014Operator Generic Fundamentals 48 Heat Exchanger Failure Example Refer to the drawing of an operating cooling water system below. What occurs when a tube fails in the heat exchanger? High-pressure fluid from the tubes would force into shell- side of heat exchanger Low-pressure system pressure would rise and high-pressure system surge tank level would lower as fluid lost High-pressure fluid being cooled would also add heat to low-pressure system ELO 1.6 Figure: Cooling Water System

49 © Copyright 2014Operator Generic Fundamentals 49 Knowledge Check – NRC Bank Borated water is flowing through the tubes of a heat exchanger being cooled by fresh water. The shell-side pressure is less than tube-side pressure. What will occur as a result of a tube failure? A.Shell-side pressure will increase and the borated water system will be diluted. B.Shell-side pressure will decrease and the borated water inventory will be depleted. C.Shell-side pressure will increase and the borated water inventory will be depleted. D.Shell-side pressure will decrease and the borated water system will be diluted. Correct answer is C. Heat Exchanger Failure ELO 1.6

50 © Copyright 2014Operator Generic Fundamentals 50 TLO 1 Summary Now that you have completed this TLO, you should be able to do the following: Describe the purpose, construction, and principles of operation for each major type of heat exchanger. TLO 1

51 © Copyright 2014Operator Generic Fundamentals 51 Some important points concerning heat exchangers are as follows: Two methods of constructing heat exchangers are plate type and tube type Heat exchangers can be classified by the following types of flow: Parallel flow — hot fluid and coolant flow in same direction Counter flow — hot fluid and coolant flow in opposite directions Cross flow — hot fluid and coolant flow perpendicularly The four heat exchanger parts are as follows: –Tubes/plates –Tube sheet –Shell –Baffles TLO 1 Summary TLO 1

52 © Copyright 2014Operator Generic Fundamentals 52 Single-pass heat exchangers have fluids that pass each other once Multipass heat exchangers have fluids that pass each other more than once by using U-tubes and/or baffles Heat exchangers should be vented when starting Colder fluid is supplied first to a shutdown heat exchanger Regenerative heat exchangers use same fluid for heating and cooling Nonregenerative heat exchangers use separate fluids for heating and cooling Heat exchangers are often used in the following applications: –Preheater –Radiator –Air conditioning evaporator and condenser –Steam condenser TLO 1 Summary TLO 1

53 © Copyright 2014Operator Generic Fundamentals 53 NRC Exam Examples Refer to the drawing of an operating water cleanup system. All valves are identical and are initially 50 percent open. To lower the temperature at point 7, the operator can adjust valve __________ in the open direction. A.A B.B C.C D.D Correct answer is D. TLO 1

54 © Copyright 2014Operator Generic Fundamentals 54 NRC Exam Examples Refer to the drawing of an operating lube oil heat exchanger. Increasing the oil flow rate through the heat exchanger will cause the oil outlet temperature to __________ and the cooling water outlet temperature to __________. A.increase; increase B.increase; decrease C.decrease; increase D.decrease; decrease Correct answer is A. TLO 1

55 © Copyright 2014Operator Generic Fundamentals 55 NRC Exam Examples Refer to the drawing of an operating water cleanup system. Valves A, B, and C are fully open. Valve D is 80 percent open. If valve D is throttled to 50 percent, the temperature at point... A.3 will decrease. B.4 will increase. C.5 will increase. D.6 will decrease. Correct answer is B. TLO 1

56 © Copyright 2014Operator Generic Fundamentals 56 Now that you have completed this TLO, you should be able to do the following: 1.Describe the construction, effectiveness, and operation of the following types of heat exchangers and their components (tubes, tube sheets, baffles and shells): tube and shell, and plate. 2.Describe hot and cold fluid flow paths in the following types of heat exchangers: parallel flow, counter flow, and cross flow. 3.Describe the difference between the following types of heat exchangers: single-pass versus multipass heat exchangers and regenerative versus nonregenerative heat exchangers. 4.Describe the operation of a typical heat exchanger to include the following: a.Startup and shutdown b.Control of temperature c.Effects and control of fouling 5.Given the necessary data, calculate flow rates and temperatures for various types of heat exchangers. 6.Explain the consequences of heat exchanger tube failure. TLO 1 Summary TLO 1

57 © Copyright 2014Operator Generic Fundamentals 57 Condenser Construction and Operation Type of heat exchanger that condenses substance from a gaseous state to a liquid state by cooling Removes latent heat from condensing fluid and transfers it to the coolant Tube and shell heat exchanger normally used as condenser Baffles added at inlet, prevents tube impingement from incoming gas or steam Industrial plants frequently employ large steam condensers as heat sinks for steam system TLO 2 – Describe the purpose, construction, and principles of operation of condensers. TLO 2

58 © Copyright 2014Operator Generic Fundamentals 58 Enabling Learning Objectives for TLO 2 1.State the purpose of a condenser. 2.State the definitions of hotwell and condensate depression. 3.State the reason(s) why condensers in large steam cycles operate at a vacuum. 4.State the definition of thermal shock. 5.Describe the relationship between condenser vacuum and backpressure. 6.Explain the process of forming a vacuum within a condenser. TLO 2

59 © Copyright 2014Operator Generic Fundamentals 59 Purpose of a Condenser Main Condenser Steam condenser is a major component of steam cycle in power generation facilities Purpose of the condenser is to: –Provide a heat sink for turbines to exhaust to give up latent heat of vaporization –Operating in a vacuum provides lowest heat sink, maximizing available heat energy transfer –Deareates condensate and feedwater, improving corrosion protection ELO 2.1 ELO 2.1 – State the purpose of a condenser.

60 © Copyright 2014Operator Generic Fundamentals 60 Purpose of a Condenser Gives up latent heat of vaporization –Converts used steam into water for return to S/G –Increases cycle's efficiency, allowing largest possible ΔT and ΔP between heat source and heat sink Figure: Single-Pass Condenser ELO 2.1

61 © Copyright 2014Operator Generic Fundamentals 61 Called latent heat of condensation Specific volume decreases drastically Creates a low pressure, maintaining vacuum Increases plant efficiency Purpose of Condenser – Thermo Review ELO 2.1 Figure: Condenser

62 © Copyright 2014Operator Generic Fundamentals 62 Purpose of Condenser – Thermo Review Thermodynamic Cycle Liquid delivered to condensate pump, then feed pump where its pressure is raised (point 1) to the saturation pressure corresponding to steam generator temperature High-pressure liquid is delivered to steam generator where cycle is repeated Figure: Thermodynamic Cycle ELO 2.1

63 © Copyright 2014Operator Generic Fundamentals 63 Purpose of a Condenser Condensed steam (saturated liquid) continues to transfer heat as it falls to the bottom of the condenser, called subcooling –Subcooling prevents condensate pump cavitation –Too much subcooling lowers efficiency Typically uses a single-pass, cross-flow condenser ELO 2.1

64 © Copyright 2014Operator Generic Fundamentals 64 Knowledge Check Condensers increase cycle efficiency by... A.allowing the cycle to operate with the largest possible ΔT. B.allowing the cycle to operate with the smallest possible ΔT. C.allowing the condensate to operate with the largest possible ΔT. D.allowing the condensate to operate with the smallest possible ΔT. Correct answer is A. Purpose of a Condenser ELO 2.1

65 © Copyright 2014Operator Generic Fundamentals 65 Condenser Terms ELO 2.2 – State the definitions of hotwell and condensate depression. Figure: Condenser Cross-Section ELO 2.2

66 © Copyright 2014Operator Generic Fundamentals 66 Condenser Hotwell As steam comes in contact with tubes, it cools and condenses Series of baffles redirects steam to minimize direct impingement on the cooling water tubes Condensed steam falls to the bottom of condenser Bottom area is a reservoir where condensate collects and is called the hotwell ELO 2.2

67 © Copyright 2014Operator Generic Fundamentals 67 Condensate Depression As the condensate falls towards the hotwell, it subcools (goes below T SAT ) as it comes in contact with tubes lower in the condenser Amount of subcooling is “condensate depression” T SAT – T HOTWELL = the amount of condensate depression Figure: T-v Diagram for Typical Condenser ELO 2.2

68 © Copyright 2014Operator Generic Fundamentals 68 Knowledge Check After the steam condenses, the saturated liquid continues to transfer heat to the cooling water as it falls to the bottom of the condenser, or hotwell. This is called ____________ and is _______________. A.subcooling; desirable B.subcooling; undesirable C.latent heat; desirable D.latent heat; undesirable Correct answer is A. Heat Exchanger Failure ELO 2.2

69 © Copyright 2014Operator Generic Fundamentals 69 Condenser Vacuum Steam's latent heat of condensation is passed to water flowing through tubes Condenser usually operated at a vacuum Vacuum helps increase plant efficiency by extracting more work from the turbine ELO 2.3 – State why condensers in large steam cycles are operated at a vacuum. ELO 2.3

70 © Copyright 2014Operator Generic Fundamentals 70 Condenser Vacuum Steam turbines designed to exhaust into a condenser that operates within a specific vacuum range If pressure increases above these limits, physical damage will occur to turbine blades When exhausted steam is condensed, its specific volume decreases, which helps maintain vacuum If condensate level is allowed to rise over lower tubes of condenser, fewer tubes are exposed for heat transfer, reducing heat transfer area If condenser is operating near design capacity, a reduction in effective surface area results in difficulty maintaining condenser vacuum Temperature and flow rate of cooling water through condenser control temperature of condensate –This in turn controls saturation pressure (vacuum) of condenser ELO 2.3

71 © Copyright 2014Operator Generic Fundamentals 71 Condenser & Noncondensable Gases If noncondensable gases are allowed to build up in condenser, vacuum decreases and saturation temperature at which steam condenses increases Noncondensable gases also blanket tubes of condenser, thus reducing surface area for heat transfer in condenser Reduction in heat transfer surface has same effect as a reduction in cooling water flow ELO 2.3

72 © Copyright 2014Operator Generic Fundamentals 72 Vacuum & Noncondensable Gases Condenser vacuum should be maintained as close to 29 inches of Mercury (Hg) as practical Allows maximum expansion of steam and therefore maximum work If condenser perfectly airtight and no air or noncondensable gases present in exhaust steam, only necessary to condense steam and remove condensate to create and maintain vacuum Figure: Condenser Cross-Section ELO 2.3

73 © Copyright 2014Operator Generic Fundamentals 73 Vacuum & Noncondensable Gases Sudden reduction in steam volume as it condenses creates a vacuum –Pumping water from condenser as fast as it forms maintains vacuum Not possible to prevent entrance of air and other noncondensable gases into condenser Some method also needed to create initial vacuum in condenser –Necessitates use of an air ejector and vacuum pump to establish and help maintain condenser vacuum ELO 2.3

74 © Copyright 2014Operator Generic Fundamentals 74 Knowledge Check During normal nuclear power plant operation, a main condenser develops an air leak that decreases vacuum at a rate of 1 inch of Hg/minimum. Which of the following will increase because of this condition? A.Extraction steam flow rate B.Condenser hotwell temperature C.Low-pressure turbine exhaust steam moisture content D.Steam cycle efficiency Correct answer is B. Condenser Vacuum ELO 2.3

75 © Copyright 2014Operator Generic Fundamentals 75 Thermal Shock ELO 2.4 – State the definition of thermal shock. Thermal shock is the severe stress produced in a material upon experiencing a sudden, unequally distributed change in temperature. Heat exchangers and condensers experience pressure stress and thermal stress due to function of transferring heat To reduce effects of these stresses, condensers should be as close as possible to operating temperatures prior to admitting steam from main turbine If equipment is not properly preheated, severe damage can occur to condenser tubes and the turbine ELO 2.4

76 © Copyright 2014Operator Generic Fundamentals 76 Thermal Shock Large temperature differences between two fluids in a heat exchanger or between a fluid and vapor in a condenser are good from a thermodynamic perspective However, they should be controlled in the system to prevent thermal shock to components Prevention Hotter fluids or vapors should be slowly admitted When out of service, heat exchangers are maintained, filled, and pressurized –Condensers steam side vented, waterside filled ELO 2.4

77 © Copyright 2014Operator Generic Fundamentals 77 Thermal Shock Prevention Main condensers must be at operating pressures (vacuum) prior to admitting steam Colder fluid should be supplied to heat exchanger first, followed by hotter fluid When securing a heat exchanger or condenser hot fluid, vapor is stopped first Colder fluid is allowed to operate for a period of time to cool down component(s) and reduce stress ELO 2.4

78 © Copyright 2014Operator Generic Fundamentals 78 Thermal Shock Heat exchangers and condensers should not be isolated in such a manner that they do not have relief valve protection Liquid isolated within the heat exchanger could warm up due to surrounding air temperature Increased temperature would lead to expansion of liquid and damage to heat exchanger if not protected from overpressure ELO 2.4

79 © Copyright 2014Operator Generic Fundamentals 79 Knowledge Check The major thermodynamic concern resulting from rapidly cooling a reactor vessel is... A.thermal shock. B.stress corrosion. C.loss of shutdown margin. D.loss of subcooling margin. Correct answer is A. Thermal Shock ELO 2.4

80 © Copyright 2014Operator Generic Fundamentals 80 Vacuum Versus Backpressure Pressure and Vacuum Relationship If pressure is below that of atmosphere as in case of a condenser, it is a vacuum Vacuum, although a negative pressure, is normally expressed as a positive value ELO 2.5 – Describe the relationship between condenser vacuum and backpressure. ELO 2.5

81 © Copyright 2014Operator Generic Fundamentals 81 Vacuum Versus Backpressure Pressure is a measure of force exerted per unit area on boundaries of a substance (or system) Collisions of molecules of substance with boundaries of system cause force Pressure is frequently expressed in units of lbf/in 2 (psi) in the English system of measurement Pressure can also be measured using equivalent columns of liquid, such as water (H 2 O) or mercury (Hg) –These scales use units of inches of H 2 O or Hg Height of column of liquid provides a certain pressure that can be directly converted to force per unit area ELO 2.5

82 © Copyright 2014Operator Generic Fundamentals 82 Vacuum Versus Backpressure Perfect vacuum would correspond to absolute zero pressure Gauge pressures are positive if they are above atmospheric pressure and negative if they are below atmospheric pressure Vacuum, although a negative pressure, is normally expressed as a positive value ELO 2.5

83 © Copyright 2014Operator Generic Fundamentals 83 Vacuum Versus Backpressure Pressure and Vacuum Relationship Figure: Comparison of Pressure Ranges ELO 2.5

84 © Copyright 2014Operator Generic Fundamentals 84 Vacuum Versus Backpressure ELO 2.5

85 © Copyright 2014Operator Generic Fundamentals 85 Vacuum Versus Backpressure PSIAPSIGInches of HgVInches of Hg 14.70029.90 13.72.0327.87 12.7-2.04.0625.84 11.7-3.06.0923.81 Table shows the relationship between pressure measurements associated with a condenser 29.9 inches of Hg pressure equals zero (0) inches of mercury vacuum (HgV) and zero (0) inches of Hg equals 29.9 inches of HgV ELO 2.5

86 © Copyright 2014Operator Generic Fundamentals 86 Vacuum Versus Backpressure Given that a condenser pressure is 3 inches of Hg, determine the corresponding vacuum by: 29.9 – 3 = 26.9 inches of Hg vacuum ELO 2.5

87 © Copyright 2014Operator Generic Fundamentals 87 Knowledge Check A turbine has a design backpressure of 5 inches of Hg. The main condenser is operating at 28 inches of HgV. What is the margin to design for the turbine? A.24.9 inches of Hg B.3 inches of Hg C.3.1 inches of Hg D.24.9 inches of HgV Correct answer is C. Vacuum Versus Backpressure ELO 2.5

88 © Copyright 2014Operator Generic Fundamentals 88 Drawing a Vacuum ELO 2.6 – Explain the process of forming a vacuum within a condenser. To prepare plant for startup, a vacuum must be drawn in the main condenser –Allows recirculation of feedwater and condensate in order to clean up and deaerate these systems –Allows warming of the main turbine –Allows identification of any condenser tube leaks prior to placing the turbine in service ELO 2.6

89 © Copyright 2014Operator Generic Fundamentals 89 Vacuum is a condition where all molecules are removed Allows steam cycle to exhaust to lowest possible heat sink, providing largest enthalpy drop through the main turbine Consists of isolating air in-leakage paths, establishing cooling water flow, then removing air from the condenser shell Mechanical vacuum pump initially removes air from condenser, then shifts to air ejectors Air removal during operation is essential for efficient operation If air leaks into the condenser, pressure increases, temperature increases, and plant efficiency decreases because the turbine exhaust enthalpy increases Drawing a Vacuum ELO 2.6

90 © Copyright 2014Operator Generic Fundamentals 90 Action StepHowDiscussion Establish cooling flowStartup support cooling systems Startup circulation water system Startup condensate system Various systems provide cooling for components necessary for vacuum pull (condensate pumps, air compressors, mechanical vacuum pumps) Isolate all air inleakage paths to the main condenser Complete valve lineup checks Close vacuum breakers Establish steam seal on turbines All possible leakage paths must be isolated to prevent loss of condenser pressure and temperature control. Air leakage across turbine shaft seals is removed via gland exhaust system. Establish turbine sealsStartup steam sealing system Startup gland exhaust system Turbine shafts are sealed using labyrinth seals and low-pressure steam to prevent high-pressure steam from leaking out and air from leaking in. Remove air from the condenser Startup the mechanical vacuum pump (also known as a hogger) Shift to the air ejectors when vacuum reaches approximately 26 inches of HgV Mechanical vacuum pump removes air out of main condenser. As air molecules are removed, pressure decreases. Drawing a Vacuum ELO 2.6

91 © Copyright 2014Operator Generic Fundamentals 91 Knowledge Check During normal nuclear power plant operation, why does air entry into the main condenser reduce the thermodynamic efficiency of the steam cycle? A.The rate of steam flow through the main turbine increases. B.The condensate subcooling in the main condenser increases. C.The enthalpy of the low-pressure turbine exhaust increases. D.The air mixes with the steam and enters the condensate. Correct answer is C. Drawing a Vacuum ELO 2.6

92 © Copyright 2014Operator Generic Fundamentals 92 TLO 2 Summary Now that you have completed this TLO, you should be able to do the following: Describe the purpose, construction, and principles of operation of condensers. TLO 2

93 © Copyright 2014Operator Generic Fundamentals 93 Condenser is type of heat exchanger used to condense a substance from a gaseous state to a liquid state by cooling Condenser removes latent heat from fluid and transfers it to coolant Condenser removes latent heat of vaporization, condensing vapor into liquid Hotwell is at the bottom of condenser where condensed steam is collected and pumped back into feedwater Condensate depression is amount condensate is cooled below saturation (degrees subcooled) Condensers operate at a vacuum to ensure temperature (thus the pressure) of steam is as low as possible, maximizing ΔT and ΔP between source and heat sink, ensuring highest cycle efficiency Thermal shock is stress produced in a body or in a material as a result of undergoing a sudden change in temperature TLO 2 Summary TLO 2

94 © Copyright 2014Operator Generic Fundamentals 94 Now that you have completed this TLO, you should be able to do the following: 1.State the purpose of a condenser. 2.State the definitions of hotwell and condensate depression. 3.State the reason(s) why condensers in large steam cycles operate at a vacuum. 4.State the definition of thermal shock. 5.Describe the relationship between condenser vacuum and backpressure. 6.Explain the process of forming a vacuum. TLO 2 Summary TLO 2

95 © Copyright 2014Operator Generic Fundamentals 95 Heat Exchangers & Condensers Summary This module covered types of heat exchangers and condensers, their applications and advantages, proper methods for operation, and system responses. Heat Exchangers The type of flow classifies different heat exchangers –Parallel flow — hot fluid and coolant flow in same direction –Counter flow — hot fluid and coolant flow in opposite directions –Cross flow — hot fluid and coolant flow perpendicularly Single-pass heat exchangers have fluids that pass each other once Multipass heat exchangers have fluids that pass each other more than once through using U-tubes and/or baffles Regenerative heat exchangers use same fluid for heating and cooling Nonregenerative heat exchangers use separate fluids for heating and cooling

96 © Copyright 2014Operator Generic Fundamentals 96 Condensers Condensers perform an important function in any heat cycle. They provide a heat sink that allows the cycle to operate at maximum efficiency. Condensers remove latent heat of vaporization, condensing vapor into a liquid Condensers operate at a vacuum to ensure temperature (thus pressure) of steam is as low as possible, maximizing ΔT and ΔP between source and heat sink, ensuring highest cycle efficiency Heat Exchangers & Condensers Summary

97 © Copyright 2014Operator Generic Fundamentals 97 Now that you have completed this module, you should be able to do the following: 1.Describe the purpose, construction, and principles of operation for each major type of heat exchanger. 2.Describe the purpose, construction, and principles of operation of condensers. Heat Exchangers & Condensers Summary


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