HEAT EXCHANGERS
Heat Exchanger Heat exchanger is an apparatus or an equipment in which the process of heating or cooling occurs. The heat is transferred from one fluid being heated to another fluid being cooled. OR Heat exchanger is process equipment designed for the effective transfer of heat energy between two fluids; a hot fluid and a coolant. The purpose may be either to remove heat from a fluid or to add heat to a fluid.
Heat Exchanger Examples of Heat exchangers are: Boilers, Evaporators, Heaters and Condensers Boilers Evaporators Condensers
Heat Exchanger Examples of Heat exchangers are: Automobile radiators and oil coolers of heat engines Automobile radiator Oil Coolers
Heat Exchanger Examples of Heat exchangers are: Condensers and evaporators in refrigeration units Water and air heaters or coolers Air Heaters
Heat Exchanger Heat transferred in the heat exchanger may be in the form of latent heat (e.g. in boilers and condensers) or sensible heat (e.g. in heaters and coolers)
Classification of Heat Exchangers 1. Nature of heat exchange process Direct contact heat exchanger Indirect contact heat exchanger Regenerator Recuperator 2. Relative direction of motion of fluid Co-current (Parallel) flow Counter-current (Counter) flow Cross flow
Classification of Heat Exchangers 3. Design and constructional features (Mechanical design of heat exchange surface) Concentric tubes Shell and tube Multiple shell and tube passes Compact heat exchangers 4. Physical state of heat exchanging fluids (condensation and evaporation) Condenser Evaporator
Classification of Heat Exchangers 1. Nature of heat exchange process Direct contact heat exchanger In a direct contact or open heat exchanger the exchange of heat takes place by direct mixing of hot and cold fluids and transfer of heat and mass take place simultaneously. Direct contact or open heat exchanger
Classification of Heat Exchangers The use of such units is made under conditions where mixing of two fluids is either harmless or desirable. Examples: Cooing towers, Jet condensers, direct contact feed heaters
Classification of Heat Exchangers 1. Nature of heat exchange process Indirect contact heat exchanger In this type of heat exchanger, the heat transfer between two fluids could be carried out by transmission through wall which separates the two fluids. This type includes the following: (a) Regenerator (b) Recuperators or surface exchangers
Classification of Heat Exchangers 1. Nature of heat exchange process Indirect contact heat exchanger (a) Regenerator The regenerator are those devices in which hot and cold fluids alternately flow over the surface (through a space containing solid particles (matrix)). The heat carried by the hot fluid is accumulated in the walls of the equipment and is then transferred to the cold fluid when it passes over the surface next.
Regenerator Rotating Matrix Regenerator Fig shows a cylinder containing a matrix that rotates in such a way that it passes alternately through cold and hot gas streams which are sealed from each other.
Regenerator Fig. shows a stationary matrix regenerator in which hot and cold gases flow through them alternately. Stationary Matrix Regenerator
Regenerator Examples: I.C. engines and gas turbines, Open hearth and glass melting furnace, air heaters of blast furnace A regenerator generally operates periodically (wall or solid matrix alternately stores heat extracted from the hot fluid and then delivers it to the cold fluid). In some regenerators the matrix is made rotate through the fluid passages arranged side by side which makes the heat exchange process continuous.
Regenerator The performance of these regenerators is affected by Heat capacity of regenerating material The rate of absorption The release of heat
Regenerator Advantages of Regenerators Higher heat transfer coefficient Less weight per kw of the plant Minimum pressure drop Quick response to load variation Small bulk weight Efficiency quite high
Regenerator Dis-advantages of Regenerators Costlier compared to recuperative heat exchanger Leakage is the main trouble, therefore perfect setting is required.
Classification of Heat Exchangers 1. Nature of heat exchange process Indirect contact heat exchanger (b) Recuperator Most common heat exchangers are the recuperators in which both hot and cold fluids separated from each other by a wall, flow through the exchanger at the same time. The heat transfer process consists of convection between the fluid and wall, conduction through the wall and convection between the wall and other fluid. In case the T between wall and fluid is large, radiation heat exchange may also occur.
Recuperator Such exchangers are used where the cooling and heating fluids can not be allowed to mix. Examples: Automobile radiators Oil coolers, intercoolers, air preheaters, economizers, super heaters, condensers and surface feed heaters of a steam power plant Milk chiller of pasteurizing plant Evaporator of ice plant
Recuperator Advantages of Recuperators Easy construction More economical More surface area for heat transfer Much suitable for stationary plants
Recuperator Dis-advantages of Recuperators Less heat transfer coefficient Less generating capacity
Classification of Heat Exchangers 2. Relative direction of motion of fluid Co-current (Parallel) flow (uni-direction flow) In a parallel flow heat exchanger, as the name suggests, the two fluid stream (hot and cold) travel in the same direction. The two stream enters at one end and leaves at the other end. T between the hot and cold fluids goes on decreasing from inlet to outlet
Parallel flow Heat Exchanger
Co-current (Parallel) flow Since this type of heat exchanger needs a large area of heat transfer, therefore, it is rarely used in practice. Examples: Oil coolers, oil heaters, water heaters As the two fluids are separated by a wall, this type of heat exchanger may be called parallel flow recuperator or surface heat exchanger
Classification of Heat Exchangers 2. Relative direction of motion of fluid Counter flow In counter flow heat exchanger, the two fluids flow in opposite directions. The hot and cold fluids enter at the opposite ends. T between the two fluids remains more or less nearly constant.
Counter flow Heat Exchanger
Counter flow This type of heat exchanger, due to counter flow, gives maximum rate of heat transfer for a given surface areas. Hence such heat exchangers are most favored for heating and cooling of fluids.
Different flow configurations in cross-flow heat exchangers. Classification of Heat Exchangers 2. Relative direction of motion of fluid Cross flow In cross flow heat exchangers, the two fluids (hot and cold) cross one another in space usually at right angles.
Cross flow Hot fluid flows in the separate tubes and there is no mixing of the fluid streams. The cold fluid is perfectly mixed as it flows through exchanger. The temperature of this mixed fluid will be uniform across any section and will vary only in the direction of flow. Examples: the cooling unit of refrigeration system
Classification of Heat Exchangers In this case each of the fluids follows a prescribed path and is unmixed as it flows through heat exchanger. Hence the temperature of the fluid leaving the heater section is not uniform. Examples: radiator Automobile
Cross flow In yet another arrangement, both the fluids are mixed while they travel through the exchanger; consequently the temperature of both the fluids is uniform across the section and varies only in the direction in which flow takes place.
Classification of Heat Exchangers 3. Design and constructional features (Mechanical design of heat exchange surface) Concentric tubes In this type, two concentric tubes are used, each carrying one of the fluids. The direction of flow may be parallel or counter The effectiveness of the heat exchanger is increased by using swirling flow.
Classification of Heat Exchangers 3. Design and constructional features (Mechanical design of heat exchange surface) Shell and tube In this type of heat exchanger one of the fluids flows through a bundle of tubes enclosed by a shell. The other fluid is forced through the shell and it flows over the outside surface of the tubes. Such an arrangement is employed where reliability and heat transfer effectiveness are important
Shell-and-tube heat exchanger: The most common type of heat exchanger in industrial applications. They contain a large number of tubes (sometimes several hundred) packed in a shell with their axes parallel to that of the shell. Heat transfer takes place as one fluid flows inside the tubes while the other fluid flows outside the tubes through the shell. Shell-and-tube heat exchangers are further classified according to the number of shell and tube passes involved. The schematic of a shell and tube heat exchanger (one shell pass and one tube pass) 35
Classification of Heat Exchangers 3. Design and constructional features (Mechanical design of heat exchange surface) Multiple shell and tube passes Multiple shell and tube passes are used for enhancing the overall heat transfer. Multiple shell pass is possible where the fluid flowing through the shell is re-routed. The shell side fluid is forced to flow back and forth across the tubes by baffles. Multiple tube pass exchangers are those which re-route the fluid through tubes in the opposite direction.
Multi pass flow arrangements in shell and tube heat exchangers
Classification of Heat Exchangers 3. Design and constructional features (Mechanical design of heat exchange surface) Compact heat exchangers There are special purpose heat exchangers and have a very large surface area per unit volume of the exchanger. They are generally employed when convective heat transfer coefficient associated with one of the fluids is much smaller than that associated with the other fluid. Example: Plate fin, flattened fin tube exchangers
Plate and frame (or just plate) heat exchanger: Consists of a series of plates with corrugated flat flow passages. The hot and cold fluids flow in alternate passages, and thus each cold fluid stream is surrounded by two hot fluid streams, resulting in very effective heat transfer. Well suited for liquid-to-liquid applications. A plate-and-frame liquid-to-liquid heat exchanger.
Classification of Heat Exchangers 4. Physical state of heat exchanging fluids (condensation and evaporation) Condenser In condenser, the condensing fluid remains at constant temperature throughout the exchanger while the temperature of the colder fluid gradually increases from inlet to outlet. The hot fluid loses latent part of heat which is accepted by the cold fluid.
Classification of Heat Exchangers 4. Physical state of heat exchanging fluids (condensation and evaporation) Evaporator In this case, the boiling fluid (cold fluid) remains at constant temperature while the temperature of hot fluid gradually decreases from inlet to outlet.
THE OVERALL HEAT TRANSFER COEFFICIENT A heat exchanger typically involves two flowing fluids separated by a solid wall. Heat is first transferred from the hot fluid to the wall by convection, through the wall by conduction, and from the wall to the cold fluid again by convection. Any radiation effects are usually included in the convection heat transfer coefficients. Thermal resistance network associated with heat transfer in a double-pipe heat exchanger. 42
U the overall heat transfer coefficient, W/m 2 C The two heat transfer surface areas associated with a double pipe heat exchanger (for thin tubes, Di Do and thus Ai Ao When The overall heat transfer coefficient U is dominated by the smaller convection coefficient. When one of the convection coefficients is much smaller than the other (say, h i << h o ), we have 1/h i >> 1/h o, and thus U h i. This situation arises frequently when one of the fluids is a gas and the other is a liquid. In such cases, fins are commonly used on the gas side to enhance the product UA and thus the heat transfer on that side. 43
The overall heat transfer coefficient ranges from about 10 W/m 2 C for gas-to-gas heat exchangers to about 10,000 W/m 2 C for heat exchangers that involve phase changes. When the tube is finned on one side to enhance heat transfer, the total heat transfer surface area on the finned side is For short fins of high thermal conductivity, we can use this total area in the convection resistance relation R conv = 1/hA s To account for fin efficiency 44
Fouling Factor The performance of heat exchangers usually deteriorates with time as a result of accumulation of deposits on heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer. This is represented by a fouling factor R f. The fouling factor increases with the operating temperature and the length of service and decreases with the velocity of the fluids. Precipitation fouling of ash particles of superheater tubes 45
Points worth noting 1. The overall heat transfer coefficient depends upon the following factors: The flow rate The properties of the fluid The thickness of material The surface condition of the tubes and The geometric configuration of the heat exchanger
Points worth noting 2. The overall heat transfer coefficient U will generally decrease when any of the fluids (e.g. tars, oils or any of the gases) having low values of heat transfer coefficient, h flows on one side of the exchanger. 3. The highly conducting liquids such as water and liquid metals give much higher values of heat transfer coefficient, h and overall heat transfer coefficient, U. In case of boiling and condensation processes also, the value of U are high.
Points worth noting 4. All the thermal resistances in the heat exchanger must be low for its efficient and effective design.
Common failures in heat exchanger Choking of tubes either expected or extraordinary Excessive transfer rates in heat exchanger Increasing the pump pressure to maintain throughout Failure to clean tubes at regularly scheduled intervals Excessive temperatures in heat exchangers Lack of control of heat exchangers atmosphere to retard scaling Increased product temperature over a safe design limit. Unexpected radiation from refractory surfaces Unequal heating around the circumference or along the length of tubes
ANALYSIS OF HEAT EXCHANGERS An engineer often finds himself or herself in a position 1. to select a heat exchanger that will achieve a specified temperature change in a fluid stream of known mass flow rate - the log mean temperature difference (or LMTD) method. 2. to predict the outlet temperatures of the hot and cold fluid streams in a specified heat exchanger - the effectiveness NTU method. The rate of heat transfer in heat exchanger (HE is insulated): Two fluid streams that have the same capacity rates experience the same temperature change in a wellinsulated heat exchanger. heat capacity rate
is the rate of evaporation or condensation of the fluid h fg is the enthalpy of vaporization of the fluid at the specified temperature or pressure. The heat capacity rate of a fluid during a phase-change process must approach infinity since the temperature change is practically zero. T m an appropriate mean (average) temperature difference between the two fluids Variation of fluid temperatures in a heat exchanger when one of the fluids condenses or boils.
THE LOG MEAN TEMPERATURE DIFFERENCE METHOD log mean temperature difference Variation of the fluid temperatures in a parallel-flow double-pipe heat exchanger.
The arithmetic mean temperature difference The logarithmic mean temperature difference T lm is an exact representation of the average temperature difference between the hot and cold fluids. Note that T lm is always less than T am. Therefore, using T am in calculations instead of T lm will overestimate the rate of heat transfer in a heat exchanger between the two fluids. When T 1 differs from T 2 by no more than 40 percent, the error in using the arithmetic mean temperature difference is less than 1 percent. But the error increases to undesirable levels when T 1 differs from T 2 by greater amounts.
Counter-Flow Heat Exchangers In the limiting case, the cold fluid will be heated to the inlet temperature of the hot fluid. However, the outlet temperature of the cold fluid can never exceed the inlet temperature of the hot fluid. For specified inlet and outlet temperatures, T lm a counter-flow heat exchanger is always greater than that for a parallel-flow heat exchanger. That is, T lm, CF > T lm, PF, and thus a smaller surface area (and thus a smaller heat exchanger) is needed to achieve a specified heat transfer rate in a counterflow heat exchanger. When the heat capacity rates of the two fluids are equal
Multipass and Cross-Flow Heat Exchangers: Use of a Correction Factor F correction factor depends on the geometry of the heat exchanger and the inlet and outlet temperatures of the hot and cold fluid streams. F for common cross-flow and shell-andtube heat exchanger configurations is given in the figure versus two temperature ratios P and R defined as 1 and 2 inlet and outlet T and t shell- and tube-side temperatures F = 1 for a condenser or boiler 55
Correction factor F charts for common shelland-tube heat exchangers. 56
Correction factor F charts for common cross-flow heat exchangers. 57
The LMTD method is very suitable for determining the size of a heat exchanger to realize prescribed outlet temperatures when the mass flow rates and the inlet and outlet temperatures of the hot and cold fluids are specified. With the LMTD method, the task is to select a heat exchanger that will meet the prescribed heat transfer requirements. The procedure to be followed by the selection process is: 1. Select the type of heat exchanger suitable for the application. 2. Determine any unknown inlet or outlet temperature and the heat transfer rate using an energy balance. 3. Calculate the log mean temperature difference T lm and the correction factor F, if necessary. 4. Obtain (select or calculate) the value of the overall heat transfer coefficient U. 5. Calculate the heat transfer surface area A s. The task is completed by selecting a heat exchanger that has a heat transfer surface area equal to or larger than A s.
THE EFFECTIVENESS NTU METHOD A second kind of problem encountered in heat exchanger analysis is the determination of the heat transfer rate and the outlet temperatures of the hot and cold fluids for prescribed fluid mass flow rates and inlet temperatures when the type and size of the heat exchanger are specified. Heat transfer effectiveness the maximum possible heat transfer rate C min is the smaller of C h and C c
Actual heat transfer rate
The effectiveness of a heat exchanger depends on the geometry of the heat exchanger as well as the flow arrangement. Therefore, different types of heat exchangers have different effectiveness relations. We illustrate the development of the effectiveness e relation for the double-pipe parallel-flow heat exchanger.
Effectiveness relations of the heat exchangers typically involve the dimensionless group UA s /C min. This quantity is called the number of transfer units NTU. capacity ratio For specified values of U and C min, the value of NTU is a measure of the surface area A s. Thus, the larger the NTU, the larger the heat exchanger. The effectiveness of a heat exchanger is a function of the number of transfer units NTU and the capacity ratio c.
Effectiveness for heat exchangers.
When all the inlet and outlet temperatures are specified, the size of the heat exchanger can easily be determined using the LMTD method. Alternatively, it can be determined from the effectiveness NTU method by first evaluating the effectiveness from its definition and then the NTU from the appropriate NTU relation.
(e.g., boiler, condenser)
Observations from the effectiveness relations and charts The value of the effectiveness ranges from 0 to 1. It increases rapidly with NTU for small values (up to about NTU = 1.5) but rather slowly for larger values. Therefore, the use of a heat exchanger with a large NTU (usually larger than 3) and thus a large size cannot be justified economically, since a large increase in NTU in this case corresponds to a small increase in effectiveness. For a given NTU and capacity ratio c = C min /C max, the counter-flow heat exchanger has the highest effectiveness, followed closely by the cross-flow heat exchangers with both fluids unmixed. The lowest effectiveness values are encountered in parallel-flow heat exchangers. The effectiveness of a heat exchanger is independent of the capacity ratio c for NTU values of less than about 0.3. The value of the capacity ratio c ranges between 0 and 1. For a given NTU, the effectiveness becomes a maximum for c = 0 (e.g., boiler, condenser) and a minimum for c = 1 (when the heat capacity rates of the two fluids are equal).
SELECTION OF HEAT EXCHANGERS The uncertainty in the predicted value of U can exceed 30 percent. Thus, it is natural to tend to overdesign the heat exchangers. Heat transfer enhancement in heat exchangers is usually accompanied by increased pressure drop, and thus higher pumping power. Therefore, any gain from the enhancement in heat transfer should be weighed against the cost of the accompanying pressure drop. Usually, the more viscous fluid is more suitable for the shell side (larger passage area and thus lower pressure drop) and the fluid with the higher pressure for the tube side. The proper selection of a heat exchanger depends on several factors: Heat Transfer Rate Cost Pumping Power Size and Weight Type Materials The rate of heat transfer in the prospective heat exchanger The annual cost of electricity associated with the operation of the pumps and fans
Summary Types of Heat Exchangers The Overall Heat Transfer Coefficient Fouling factor Analysis of Heat Exchangers The Log Mean Temperature Difference Method Counter-Flow Heat Exchangers Multipass and Cross-Flow Heat Exchangers: Use of a Correction Factor The Effectiveness NTU Method Selection of Heat Exchangers