final heat transfer

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By: Trisharaj (110 month(s) ago)

Heat transfer is such a complicated chapter in books but this presentation made it even simplier to understand and study. Its such a wonderful slide. Thank you

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HEAT TRANSFER : 

HEAT TRANSFER Prepared by : Mr. Jitendra L. Patel

Introduction : 

Introduction Many pharmaceutical process involves heating of material. E.g. distillation, evaporation, drying, crystallization etc. At ab zero temp substance have zero heat content. Heat is form of energy. Unit is joules. The study of heat transfer required in designing the plant efficiently and economically.

HEAT TRANSFER : 

HEAT TRANSFER In any heat transfer process, the heat energy can only travel in one direction, hot to cold. Objects heated by the transfer cannot be heated higher than that of the flame or heat source, unless of course the object being heated ignites.

Sources of Heat : 

Sources of Heat Sun Hot interior of earth Chemical action : Burning of fuel, Oxidation of food in body Mechanical energy of friction. Heat is produced by electrical energy. Heat energy is produced by a nuclear energy. Steam.

STEAM AS A HEATING MEDIUM : 

STEAM AS A HEATING MEDIUM Most commonly used in pharmaceutical industry as a heating medium. Also used for sterilization of material.

Advantages : 

Advantages Steam has very high heat content. Steam give up heat at constant temp. The raw material for steam is cheap & plentiful. High pressure steam is used to generate electric power and low pressure steam is used for heating process. Steam is easy to generate, distribute, control. Steam has a high thermal capacity, i.e. absorbing and giving off heat makes it ideal for heating.

STEAM AS A HEATING MEDIUM : 

STEAM AS A HEATING MEDIUM Steam has 3 diff type of heat 1. sensible heat (h) of water. 2. the latent heat (L) of steam. 3. superheated steam.

1. Sensible heat (h) of water. : 

1. Sensible heat (h) of water. Sensible heat is potential energy in the form of thermal energy, or heat, and refers to the heat that is added or removed from the air and dry bulb temperature changes without water vapor content change. Sensible heat (h) = mass of water * sp capacity * temperature rise. E.g. 1 g of water bring to boil (ts=100 C) from ice cold water (o degree Celsius). Water have sp heat 1.0 calorie, then Sensible heat = 1 g * 1 * (ts-0) = 1 g *1 * (100-0)

2. The latent heat (L) of steam. : 

2. The latent heat (L) of steam. The expression latent heat refers to the amount of energy released or absorbed by a chemical substance during a change of state that occurs without changing its temperature, meaning a phase transition such as the melting of ice or the boiling of water. The specific latent heat is the amount of energy required to convert 1 kg (or 1 lb) of a substance from solid to liquid (or vice-versa) without a change in temperature is known as the specific latent heat of fusion for that substance. Likewise, the amount of energy required to convert 1 kg (or 1 lb) of a substance from liquid to gas (or vice-versa) without a change in temperature is known as the specific latent heat of vaporization for that substance.

2. The latent heat (L) of steam. : 

2. The latent heat (L) of steam. If more heat is added, a fraction of the water (q) will be vaporized. If the latent heat of vaporization is L cal/g, then further heat added is q.L cal/g. Heat content of wet steam = sensible + latent. Hs = h + L = h + q.L q is known as the dryness fraction of the wet steam, i.e. ratio of amount of steam to total wet steam (water + steam), it is expressed as % or decimal part of one.

2. The latent heat (L) of steam. : 

2. The latent heat (L) of steam. As further heat is added to steam water mixture, all water is converted to steam, and dryness fraction q become 1 and steam is said as a dry steam. The volume occupied by one Kg of dry steam is known as sp. Volume.

3. Superheated steam. : 

3. Superheated steam. Superheated steam is steam at a temperature higher than its boiling point at a given pressure. For superheating to take place one of two things must occur. Either all of the liquid water must have evaporated or, in the case of steam generators (boilers), the saturated steam must be conveyed out of the steam drum before superheating can occur, as steam can not be superheated in the presence of liquid water

3. Superheated steam. : 

3. Superheated steam. There are three stages of heating to convert liquid water to superheated steam. First the liquid water’s sensible (the heat that can be measured with a thermometer) heat is raised. Then latent heat (this heat does not raise the temperature of the fluid) is added. After all of the liquid is evaporated or the saturated steam is taken from the steam drum sensible heat is again added superheating the steam.

3. Superheated steam. : 

3. Superheated steam. H total = Hs + H sup Hs = h + L H sup = Cpm (tsup – ts) Hs = Heat content of saturated steam Hsup = Heat content of superheated steam h = Sensible heat L = latent heat Cpm = Mean sp. heat of steam t sup = temp in degree Celsius of superheated steam t s = temp in degree Celsius sat. steam at corresponding pressure

Generation of steam : 

Generation of steam Generally steam is generated in a central boiler house at high pressure. High pressure steam used to drive a turbine to generate electric power, while low pressure exhaust steam for process heating. High pressure steam have high temp. More steam is stored in the boiler if high pressure are used. Expansion of the high pressure steam to low pressure at the plant will help to dry the steam. The pressure provides the driving force for distribution of the steam.

Generation of steam : 

Generation of steam The best feed water for the boiler is clean condensate. All the dissolved solids are deposited in the boiler, forming scale, and hinder heat transfer. In addition to dissolved solids main water supplies also contains dissolved air, when such water is added to the boiler the dissolved air comes off in the steam. Even 1 % of air can reduce the heating efficiency of the steam by 10 %. Air will collected at the point of condensation and it should be passed out through the trap. If not removed, air will remain in the jacket or coil and stop heat transmission.

Distribution of Steam. : 

Distribution of Steam. Steam from boiler distributed through pipe. Pipe should be adequate size to carry the required quantities of the steam and should be as short as possible to minimize heat losses. To reduce heat loss, pipe should be lagged. As there is a some heat losses, there should be arrangement to remove condensate. E.g. Steam dryer or water separator.

Use of steam in the plant : 

Use of steam in the plant Steam can be used directly or indirectly in plant. In direct use the steam is blown directly into material. It have no boundary greater efficiency of heat transfer. Disadvantage : condensate enters the material. It have application in steam distillation and for sterilization.

Use of steam in the plant : 

Use of steam in the plant When indirect heating there is barrier between steam and the material. This can be achieved by means of a jacket or by steam coil or tubes. Jacket is not suitable for large vessel as it provides limited heating area. A steam coil or tubes permit the use of larger areas.

Condensate removal : 

Condensate removal If a drainage is alone was fitted to the jacket then steam would issue under pressure and be lost. The problem is solved by fitting a steam trap to exit. Steam trap retains the steam until it has condensed and periodically the trap opens to allow the condensate to be expelled, closing when steam reaches the trap. Steam trap normally fitted at the end of long pipes.

Condensate removal : 

Condensate removal In direct heating steam must be in closed systems to maintain steam pressure and prevent loss of steam. Steam must condense the latent heat can pass into the liquid being heated. The condensate forms will accumulate. Water conducts heat between the steam and metallic pipe and more condensation, & no more steam could enter. There should be some arrangement make for condensate removal.

The Purpose of a Steam Trap : 

The Purpose of a Steam Trap A steam trap is used to remove condensate (water), air and carbon dioxide from the steam lines. Once the steam has released its desired latent heat and condensed, the hot condensate is removed by the steam trap to ensure maximum heat transfer of the steam to the heating element.

Why Do We Need Steam Traps? : 

Why Do We Need Steam Traps? When using steam to exchange heat to a specific process water will accumulate from the condensed steam and this water will cut down on the heat transfer. Any air in the system will occupy the volume that would otherwise be occupied by steam Carbon Dioxide will build up and form carbonic acid, which may corrode piping and equipment.

Steam Trap : 

Steam Trap There is no one steam trap to solve all of our problems, we prevent our specific problems by selecting the proper trap and installing it properly. Once the correct steam trap has been chosen, if there is a problem there are guidelines for diagnosis.

Classification of steam trap : 

Classification of steam trap A) Mechanical (Float type) B) Thermostatic (Expansion type) Works on the principle of physical diff, between steam and water, that is between vapor and liquid. E.g. Float type Bucket type Works on the principle of temp as the condensate lose sensible heat at a lower temp than steam. E.g. balanced pressure steam trap.

Basic Types of Steam Traps : 

Basic Types of Steam Traps Float Trap Inverted Bucket Trap Thermodynamic Trap Thermostatic Metallic-Expansion Trap Balanced-Pressure Thermostatic Trap Bimetallic Trap etc. Inverted Bucket Trap

Float Trap : 

Float Trap The simplest type of mechanical steam trap. the float (B) is attached to the end of a rod (C). The opposite end of the rod is attached to a discharge valve (D). When condensate fills the body of the trap the float rises, gradually opening the discharge valve. This trap is seldom used today without a thermostatic or bi-metallic plate to control discharge.

Float Trap : 

Float Trap Float traps can be used where large condensate loads demand immediate removal and where steam locking occurs. Applications: Heat Exchangers, Reboiler Jacket Pans, Batch Operations that require frequent startups Limitations: Susceptible to damage by water hammer (water traveling at high velocity), it cannot be used in super heated steam, the float can be damaged by water freezing.

Inverted Bucket Trap : 

Inverted Bucket Trap The most common type of trap is the Inverted Bucket Trap. The Steam and water enter at "E" and initially the steam will push the inverted bucket "A" up. When the trap fills with water, the inverted bucket "A" will drop and the water will be blown out into a condensate line at "B". The bucket then rises again and the hole at "B" closes. The cycle then repeats itself.

Inverted Bucket Trap : 

Inverted Bucket Trap Inverted Bucket Traps are useful because they can withstand water hammer and they can operate at very high pressures. Applications: High-Pressure Steam Mains, Shipboard Systems, Continuous Processes with Small Quantities of Non condensable. Limitations: The small discharge hole in the top of the bucket discharges air slowly, the water seal must always be maintained to ensure proper operation, it will always lose steam.

Thermostatic traps : 

Thermostatic traps They can be set to open at a particular temp appropriate to the steam pressure. Disadvantage: slight variation in the steam pressure may cause the trap to stay open or closed until the setting has been altered to meet the new conditions. This difficulty is overcome by balanced pressure expansion trap.

Balanced Pressure Steam Trap : 

Balanced Pressure Steam Trap The operating element is a capsule containing a special liquid and water mixture with a boiling point below that of water. In the cold conditions that exist at start-up, the capsule is relaxed. The valve is off its seat and is wide open, allowing unrestricted removal of air. This is a feature of all balanced pressure traps and explains why they are well suited to air venting.

Balanced Pressure Steam Trap : 

Balanced Pressure Steam Trap As steam passes through the balanced pressure steam trap, heat is transferred to the liquid in the capsule. The liquid vaporizes before steam reaches the trap. The vapor pressure within the capsule causes it to expand and the valve shuts. Heat loss from the trap then cools the water surrounding the capsule, the vapor condenses and the capsule contracts, opening the valve and releasing condensate until steam approaches again and the cycle repeats.

Balanced Pressure Steam Trap : 

Balanced Pressure Steam Trap This trap is useful because it is small, light and has a large capacity for its size. It is fully open on start-up, allowing air and other non-condensable gases to be discharged freely and giving maximum condensate removal when the load is greatest. Limitations: In common with all other thermostatic traps, the balanced pressure type does not open until the condensate temperature has dropped below steam temperature (the exact temperature difference being determined by the fluid used to fill the element).

Pressure reduction : 

Pressure reduction When saturated steam condenses on a surface, a specific heat of steam must be given up before condensation take place. Sp heat does not given up by a steam as gas are bad conductor of heat. Hence steam has to be superheated a little to prevent condensation in steam lines. Process plant uses steam at a pressure 1.7-2 Bars, but boiler produce steam at 6-8 Bars pressure.

Pressure reduction : 

Pressure reduction The pressure of the spring attempts to open the valve against the high pressure steam. The closing of the valve is caused by the low pressure steam , so that when this reaches a predetermined value, a balanced will be reached in which the low pressure steam acting on the diaphragm close the valve against the spring pressure. Equilibrium is set up in which the valve is open slightly.

Expansion of the steam : 

Expansion of the steam When pressure is reduced by passage through a reducing valve the process is ISOENTHALPIC. Isoenthalpic means the heat content of the steam remained unchanged by its expansion. At lower pressure there is a high value of latent heat of vaporization so by expansion drying of the steam take place. This process is known as THROTTLING or WIRE-DRAWING. The steam may be superheated if expanded to too low pressure.

Slide 38: 

Common Problems of Steam Traps Steam Leakage: Valve seat of a steam trap is subjected to erosion and corrosion. When seat is damaged the valve will not seat completely and the trap may leak live steam. If steam trap is oversized, it may waste significant amounts of steam. A properly working thermodynamic trap may fail to close if the condensate pressure is too low.

Slide 39: 

Common Problems of Steam Traps Improper Sizing: A trap that is undersized will cause condensate to interfere with the heat transfer efficiency because the condensate forms a thin film on the heat-transfer surfaces. Traps are typically specified several times larger than required using a safety factor to calculate the trap capacity. A trap having too much excess capacity wastes money, acts sluggishly and generates high back-pressure that may significantly reduce life of the trap.

Slide 40: 

Common Problems of Steam Traps Dirt: Steam condensate often contains particles of scale and corrosion products that can erode trap valves. If the particles are big enough, they can plug the discharge valve or jam it open. To prevent this, a strainer is installed upstream of each trap.

Slide 41: 

Common Problems of Steam Traps Air Binding: When the trap is connected to the plant by a long length of small horizontal pipe, condensate holds up in the stream and cannot flow to the trap. To prevent air binding, the piping to the traps should be a larger diameter pipe or shorter length, which allows for higher flow rate. Another method to prevent air binding is to put a vent valve at a high point in the system.

Slide 42: 

Common Problems of Steam Traps Steam Locking: When the trap is connected to the plant by a long length of small horizontal pipe, steam may prevent the condensate from reaching the trap. Condensate cannot get to the trap unless it can displace the steam. To prevent steam locking, it is necessary to put the trap as close as possible to the equipment or line to be drained.

Slide 43: 

Common Problems of Steam Traps Water Hammer: Condensate lying in the bottom of a steam line can cause water hammer. Water Hammer- Steam traveling at very high velocities produces waves as it passes over this condensate. If condensate quality increases, the high-velocity steam pushes the condensate and creates a dangerous slug when it changes direction. To prevent water hammer, robust traps like thermodynamic or inverted bucket traps may be used, or the pipe may be re-oriented.

Slide 44: 

Common Problems of Steam Traps Freezing: If the steam trap is shut down with significant amount of condensate remaining in the trap, and the ambient temperature falls below 0oC, the freezing inside the trap will occur. Float traps and balanced-pressure thermostatic traps are destroyed by freezing. If freezing is likely, thermodynamic or bimetallic traps, which are not affected by freezing, should be used. Opening the drain valves after shutdown is another option.

Slide 45: 

Common Problems of Steam Traps Loss of Prime: This problem is only related to inverted bucket traps. The inverted bucket trap starts functioning only when there is some quantity of water inside the trap. If there is a sudden pressure drop or superheated steam enters the trap, the prime is lost and trap won’t operate. To prevent loss of prime a check valve may be put in the trap inlet line.

Slide 46: 

MODES OF HEAT YRANSFER

Heat transfer : 

Heat transfer

Heat Transfer : 

Heat Transfer There are three ways in which heat can be transferred from one object to another: Conduction – when two objects are in physical contact. k = thermal conductivity Q = heat transferred A = cross sectional area t = duration of heat transfer L = length DT = temperature difference between two ends In a hot oven the air and the metal rack are at the same temperature, but which one feels hotter and why?

Convection and Radiation : 

Convection and Radiation Convection – when heat is carried by a moving fluid Example: heat house with radiator Gulf stream transports Heat from Caribbean to Europe Radiation – when electromagnetic waves (radiation) carry heat from one object to another. Example: heat you feel when you are near a fire Example: Heat from the sun

Conduction : 

Conduction The heat transfer occurs by transmission of momentum of individual molecules. Here, no mixing occurs, so conduction is limited to solids and to fluids. Heat transfer occur when there is a temp gradient. The molecules at the hot end vibrate more rapidly than molecules at the cold end. This vibration is imparted to the neighboring molecules which vibrate faster and hence heat up. There is no actual movement or flow of the material. The molecule vibrate in just one place, but there energy is transmitted.

Conduction Of Heat Through A Material : 

Conduction Of Heat Through A Material

Conduction : 

Conduction

Conduction : 

CONDUCTION is the heat transfer to another body or within a body by DIRECT CONTACT. As a solid wall is heated from one side, the heat is carried from the hot surface to cooler parts of the solid by conduction. The rate of heat transfer is simply the quantity of heat imparted to the wall per unit time. Rate = driving force / resistance Conduction

Conduction : 

The driving force is the temp drop across the solid surfaces. The greater the temp drop, the greater will be the rate of flow. The flow of heat also depends on the conductivity of materials through which it is flowing. Conduction of heat is faster through iron road than wooden log. Resistance = thickness of surface (m) mean proportionality constant (W/m.k) * area of the surface (m2) Conduction

Fourier’s Law : 

Fourier’s Law It states that the rate of heat flow through a uniform material is proportional to the area and the temp drop and inversely to the length of the path of flow. Rate of heat flow α Area (m2) * temp diff (∆t) thickness (m) Where Km = mean proportionality constant, W/m.k.

Conduction : 

Rate of heat transfer by conduction, Q/t through the length, L across the cross-sectional area, A is given by the following equation, where k is the thermal conductivity and ΔT is the temperature difference between the two ends. SI Unit of Thermal Conductivity: J/(s · m · C°) Conduction

Derivation : 

Derivation Fourier’s law can be applied to a metal wall through which the conduction of heat is taking place. Area of the wall = A, m2 Thickness of the wall = L, m Face of the wall is maintained at a uniform, definite and higher temperature = t1, K Face of the wall is maintained at al Lower, but uniform temperature = t2, K The flow is right angles to the plane A and is assumed to be in steady state.

Derivation : 

Derivation Consider a thin section of thickness dL at an intermediate point in the wall. This section is parallel to the plane A. For this low Fourier’s Law may be applied as Where Q= heat transfer, J Ѳ= time, s K= proportionality constant, W/m.k t= temperature

Derivation : 

Derivation The constant, k is a function of temp, but independent of length. The minus sign indicates the decrease in temp in the direction of flow. dt/dL represents the temp gradient. Heat transfer equation for steady state Where q= Rate of heat transfer, J/s or W.

Derivation : 

Derivation The temp diff in intermediate section is not known. We know the temp at two faces of wall. The area, A, may vary with L, but is independent of temp. So by separating variable we can write

Derivation : 

Derivation Integrating equation between the limits L = 0 when t = t1 and L = L (total thickness) when t =t2 Rearranging the equation Where Km = mean proportionality constant

Derivation : 

Derivation In steady state heat transfer q remains constant. ∆t indicates driving force. Comparing the equn with rate = driving force/resistance Resistance = L / Km.A. Fourier’s law thus used to define the resistance quantitative term.

Applications : 

Applications The coefficient of thermal conductivity is quantity of heat that flows across a unit surface area in unit time, when the temp drop is unity. The thermal conductivity depends on the materials used and temp. Thermal conductivity of liquids and gases is small compared to solids so the resistance is high. In case of steam jacketed vessel the kettle ( inner surface) must have good conductivity so that max amt of heat passes from steam to the contents. Copper have high thermal conductivity.

Slide 64: 

High conductivity High conductivity High conductivity

Applications : 

Applications The metal used for jacket must have poor thermal conductivity to prevent heat loss. Thermal conductivity values are useful to construct the evaporator and heat exchanger. Heat transfer by conduction is often important in equipment such as fluidized beds, rotary kilns, spray driers.

Compound resistance in Series : 

Compound resistance in Series Consider a flat wall constructed of a series of layers. Thickness of the three layers = L1, L2, L3 Area of the entire wall = A, m2 Temp drop across three layers = ∆t1, ∆t2, ∆t3, K. Resistance of 3 layers = R1, R2, R3. Total temp drop ∆t= ∆t1 + ∆t2 + ∆t3.

Compound resistance in Series : 

Compound resistance in Series The rate of flow of heat through several resistance series is analogues to the current through several resistance in series. There fore, R = R1 + R2 + R3. As per Fourier's law R = L/K.A There fore, R1 = L1/K1.A1, R2 = L2/K2.A2, R3 = L3/K3.A3 Now, Entire heat must pass through a resistance series , heat q = q1 = q2 =q3.

Compound resistance in Series : 

Compound resistance in Series ∆t= ∆t1 + ∆t2 + ∆t3.

Compound resistance in Series : 

Compound resistance in Series The contribution of temp drop to the total temp and individual resistance can be expressed mathematically as ∆t : ∆t1 : ∆t2 : ∆t3 :: R : R1 : R2 : R3.

Compound resistance in Parallel : 

Compound resistance in Parallel When several solids are placed side by side with their edges touching, and direction of heat flow is perpendicular to plane of exposed face surfaces. We can assume that temp drop taken place across each solids is constant. ∆t = ∆t1 = ∆t2 = ∆t3 The total surface area available for heat transfer is the sum of individual area of solids. The total heat transfer is sum of individual heat transfer. q = q1 + q2 + q3.

Compound resistance in Parallel : 

Compound resistance in Parallel This is the final equation for heat transfer through a resistance in parallel.

Heat transfer through a Cylinder : 

Heat transfer through a Cylinder In heat exchanger, hot fluid or steam is passed through the circular pipe. The hot fluid transfers the heat to inner surface of the pipe wall. Further heat transfer takes place by conduction through a pipe wall. Consider the heat is flowing from inside to outside of the hollow cylinder. Consider a very thin cylinder at the centre of the pipe.

Heat transfer through a Cylinder : 

Heat transfer through a Cylinder Mean thermal conductivity of material of cylinder= Km, W/mk Temp of inside surface, higher =t1, K Temp of outside surface, lower =t2, K Radius of the cylinder = r, m Thickness of the thin section = dr, m Radius of inner wall= r1, m Radius of outer wall= r2, m Length of hollow cylinder= N, m

Heat transfer through a Cylinder : 

Heat transfer through a Cylinder The heat flow is considered as parallel and the rate of heat transfer can be written as: Where 2ПrN is the area of the heating surface, i.e., the interior of the cylinder. The mean surface area may be written as circumference multiplied by length of cylinder. Separate the variable radius and temp.

Heat transfer through a Cylinder : 

Heat transfer through a Cylinder Integrating the equation within the limits of r = r1, when t = t1 and r = r2, when t = t2. This equation is used To calculate the rate of Flow through a thick Walled cylinder.

Heat transfer through a Cylinder : 

Heat transfer through a Cylinder The rate of heat flow can be expressed as By comparing equation

Heat transfer through a Cylinder : 

Heat transfer through a Cylinder L is the thickness , it is related to thickness of the tube, i.e.,(r2-r1) of the cylinder. This is substituted for substituted for L and rearranged to obtain A (mean area of a cylinder) A may be considered as 2 ПrN. From the equation mean radius may be considered as

Heat transfer through a Cylinder : 

Heat transfer through a Cylinder rm = is called logarithmic mean radius. Logarithmic mean is less convenient than the arithmetic mean. The arithmetic is sufficiently accurate, if the tube is thin walled. The value r2/r1<3.20 reflects that the wall is thick. If arithmetic mean radius is used, the result will be within 10% of that obtained by equation that uses logarithmic mean. The value r2/r1<1.5 reflects that the wall is thin. If arithmetic mean d is diameter used, the result will be within 1% of that obtained by equation that uses logarithmic mean.

Conduction through fluids : 

Conduction through fluids Conduction in liquids usually small and this presents a considerable obstacles for heat transfer. Conduction in fluids is because of eddies set up by the changes in density with temp, which is observed in the boiling of liquids. Conduction through fluids rarely occurs in practice, except when heat flows through thin films. Here the thickness of the film is not exactly known so above eqn can not be applied. This difficulty can be over come by use of surface coefficient.

Slide 80: 

CONVECTION

Convection : 

Convection When heat flow is achieved by actual mixing of warmer portions with cooler portions of the same material, the process is known as convection. The heat transfer in fluids occurs on account of actual mixing of its layers. Two type 1. Forced convection 2. Natural convection.

Forced Convection : 

Forced Convection Mixing of fluid may be obtained by the use of a stirrer or agitator or pumping the fluid for recirculation. Such a process in heat transfer is designated as forced convection. E.g. some type of tube evaporators, the evaporating liquid is forced through the tubes under pressure.

Natural Convection : 

Natural Convection Mixing of fluid may be accomplished by the currents set up when body of fluid is heated. Such a process is known as natural convection. E.g. in pan evaporators, convection currents are set up in the evaporating liquids.

Natural Convection : 

Natural Convection When heat is passed through tube, stagnant films are determining the rate of heat transfer. When fluid exhibits viscous flow, the velocity is zero at the actual surface of the wall. It means that the layer of fluid adjacent to the wall acts as a stagnant film. A comparatively stagnant film can be observed even in turbulent flow. At the centre, the fluid is turbulent, while at the centre the fluid exhibits viscous flow. When steam gives up latent heat, water will condense on the surface of vessel. The heat must be conducted through this water film.

Natural Convection : 

Natural Convection For heat transfer in a tube, heat must pass through the stagnant film by conduction. Normally, thermal conductivity of fluids are low. The conductivity of the stagnant film will be still less E.g. A film of water has a resistance of about 500 times and that of air film is about 13,000 times greater than a copper sheet of the same thickness. Thus, the resistance offered by these films is large for the heat flow.

Forced Convection- Individual heat transfer coefficient : 

Forced Convection- Individual heat transfer coefficient If mixing of fluid is obtained by means of stirrer or agitator or pumping the fluid for recirculation, is known as forced convection. E.g. some type of tube evaporators, the evaporating liquid is forced through the tubes under pressure. The heat transfer coefficient depends up on many variables.

Forced Convection- Individual heat transfer coefficient : 

Forced Convection- Individual heat transfer coefficient Consider a case of heat flowing from a hot fluid through a metal wall in to a cold fluid. At specific point, the variation of temp on each side of metal wall is shown in figure.

Forced Convection- Individual heat transfer coefficient : 

Forced Convection- Individual heat transfer coefficient The characteristics of metal wall : HH and CC represents the boundaries of the film in viscous flow on the hot and cold sides of the metal wall. The temp gradient through the line tctd is caused by the flow of heat purely by conduction through the metal whose thermal conductivity is known. Metal wall thickness is L.

Forced Convection- Individual heat transfer coefficient : 

Forced Convection- Individual heat transfer coefficient The characteristics of Hot fluid side : To the right of HH, the fluid is in turbulent flow on the hot side. ta is the maximum temp in the hot fluid. tb is the temp at the boundary on the hot side (turbulent and viscous flow junction). tc is the temp at the actual interface (between fluid and solid surface).

Forced Convection- Individual heat transfer coefficient : 

Forced Convection- Individual heat transfer coefficient The characteristics of Hot fluid side : Curve ta, tb, tc represents the temp gradient from the bulk of the hot fluid to the metal wall. This is caused by the flow of heat in forced convection. t1 is the average temp on the hot fluid side represented by the line MM. In general, for heat transfer calculation, average temp is important. This can be obtained by taking its temp after mixing.

Forced Convection- Individual heat transfer coefficient : 

Forced Convection- Individual heat transfer coefficient The characteristics of cold fluid side : To the left of CC, the fluid is in turbulent flow on the cold side. tf is minimum temp on the cold fluid. te is the temp at the boundary on the cold side. (viscous and turbulent flow junction) td is the temp at the actual interface.(between fluid and solid)

Forced Convection- Individual heat transfer coefficient : 

Forced Convection- Individual heat transfer coefficient The characteristics of cold fluid side : Curve td, te, tf represents the temp gradient from the metal wall to the bulk of the fluid. This is caused by the flow of heat in forced convection. t2 is the average temp on the cold fluid side represented by line NN. In general, for heat transfer calculation, average temp is important. This can be obtained by taking its temp after mixing.

Forced Convection- Individual heat transfer coefficient : 

Forced Convection- Individual heat transfer coefficient Surface or Film coefficient : In forced convection, the forced, the stagnant films are of great importance in determining the rate of heat transfer. the films are thin but the resistance offered by them is large. The turbulence also brings about rapid equalization of temp. Film coefficient is the quantity of heat flowing through unit area of film for unit drop in temp. It is conductive capacity of stagnant film for the transfer of heat.

Forced Convection- Individual heat transfer coefficient : 

Forced Convection- Individual heat transfer coefficient Surface or Film coefficient : The thickness of the film can not be known precisely so it is difficult to determine the thermal resistance of the fluid films. The thickness of the film depends on the viscosity and forced convection. So we can not calculate the resistance offered by the films directly. There fore indirect method of computation of surface coefficient is used.

Slide 95: 

Surface or Film coefficient : let q watt of heat is flowing from hot fluid to cold one. So same amt of heat pass through stagnant film on the hot side, through metal wall and through the stagnant film on the cold side. Consider, the area of metal wall on hot side = A1, m2 The area of metal wall on cold side = A2, m2 The average area of metal wall = Am, m2 Surface or Film coefficient on the hot side (h1) = amt of heat flowing (w) Area (m2)*diff in temp (K)

Slide 96: 

Surface or Film coefficient : is analogous to L/KA l is the resistance term for metal wall the term 1/h1A1 is known as thermal resistance on hot side. The thermal resistance is due to combined effect of viscous film HH and turbulent core. This resistance caused the difference in temp, ta-tb. Surface or Film coefficient on the cold side (h1) = amt of heat flowing (w) Area (m2)*diff in temp (K) 1/h2A2 is known as thermal resistance on cold side.

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