Assignments Contents : Heat exchanger basic concept Working principle LMDT Method NTU MethodTypes of heat Exchanger based on; Working Principle Construction Configuration Construction MaterialStandards use for H.E..(i.e TEMA…) Refernces
HEAT AND MASS TRANSFER LABHEAT EXCHANGER A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air.Principle Of Heat Exchanger Heat exchangers work because heat naturally flows from higher temperature to lower temperatures. Therefore if a hot fluid and a cold fluid are separated by a heat conducting surface heat can be transferred from the hot fluid to the cold fluid. Two fluids of different temperatures are brought into close contact but are prevented from mixing by a physical barrier. The temperature of the two fluids will tend to equalize. By arranging counter-current flow it is possible for the temperature at the outlet of each fluid to approach the temperature at the inlet of the other. The heat contents are simply exchanged from one fluid to the other and vice versa. No energy is added or removed. Heat transfer depends upon following factors: Type of the material between fluids Thickness of material Surface Area of material Type of fluid Gravity of fluid Flow rate of fluid
HEAT AND MASS TRANSFER LABBasic Equation defining the Heat Exchanger Principle: Heat exchanger theory leads to the basic heat exchangerdesign equation: Q = U A ΔTlm, where Q is the rate of heat transfer between the two fluids in the heat exchanger in But/hr, U is the overall heat transfer coefficient in Btu/hr-ft2-oF, A is the heat transfer surface area in ft2, ΔTlm is the log mean temperature difference in oF, calculated from the inlet and outlet temperatures of both fluids.Log mean temperature difference(LMDT): The log mean temperature difference (LMTD) is used to determine the temperature driving force for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger. The larger the LMTD, the more heat is transferred. The use of the LMTD arises straightforwardly from the analysis of a heat exchanger with constant flow rate and fluid thermal properties. We assume that a generic heat exchanger has two ends (which we call "A" and "B") at which the hot and cold streams enter or exit on either side; then, the LMTD is defined by the logarithmic mean as follows: where ΔTA is the temperature difference between the two streams at end A, and ΔTB is the temperature difference beween the two streams at end B. This equation is valid both for parallel flow, where the streams enter from the same end, and for counter-current flow, where they enter from different ends. Once calculated, the LMTD is usually applied to calculate the heat transfer in an exchanger according to the simple equation: Where Q = Heat transfer
HEAT AND MASS TRANSFER LAB Once calculated, the LMTD is usually applied to calculate the heat transfer in an exchanger according to the simple equation:Where Q is the exchanged heat duty (in watts), U is the heat transfer coefficient (in watts per kelvinper square meter) and A is the exchange area. Note that estimating the heat transfer coefficient maybe quite complicated.Number of Transfer Units (NTU) method: The Number of Transfer Units (NTU) Method is used to calculate the rate of heat transfer in heat exchangers (especially counter current exchangers) when there is insufficient information to calculate the Log-Mean Temperature Difference (LMTD). In heat exchanger analysis, if the fluid inlet and outlet temperatures are specified or can be determined by simple energy balance, the LMTD method can be used; but when these temperatures are not available The NTU or The Effectiveness method is used.U = Overall heat transfer co-efficientA= surface areaC min =specific heat capacity of cold fluidEffectiveness of heat exchanger is calculated as; Classification of Heat Exchangers: Types w.r.t Flow: There are two primary classifications of heat exchangers according to their flow arrangement. Parallel-flow heat exchangers: The two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side. Counter-flow heat exchangers: The fluids enter the exchanger from opposite ends. The counter current design is
HEAT AND MASS TRANSFER LAB most efficient, in that it can transfer the most heat from the heat (transfer) medium.Cross-flow heat exchangers: In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger. Single and Multi-pass Heat Exchanger In actuality, most large heat exchangers are not purely parallel flow, counter flow, or cross flow; they are usually a combination of the two or all three types of heat exchangers. This is due to the fact that actual heat exchangers are more complex than the simple components shown in the idealized figures used below to depict each type of heat exchanger. The reason for the combination of the various types is to maximize the efficiency of the heat exchanger within the restrictions placed on the design. That is, size, cost, weight, required efficiency, type of fluids, operating pressures, and temperatures, all help determine the complexity of a specific heat exchanger. One method that combines the characteristics of two or more heat exchangers and improves the performance of a heat exchanger is to have the two fluids pass each other several times within a single heat exchanger. When a heat exchanger’s fluids pass each other more than once, a heat exchanger is called a multi-pass heat exchanger. If the fluids pass each other only once, the heat exchanger is called a single-pass heat exchanger. Commonly, the multi- pass heat exchanger reverses the flow in the tubes by use of one or more sets of “U” bends in the tubes. The “U” bends allow the fluid to flow back and forth across the length of the heat exchanger. A second method to achieve multiple passes is to insert baffles on the shell side of the heat exchanger. These direct the shell side fluid back and forth across the tubes to achieve the multi-pass effect.
HEAT AND MASS TRANSFER LABTypes of H.E w.r.t ConstructionShell and tube heat exchanger:Shell and tube heat exchangers consist of a series of tubes. One set of these tubes containsthe fluid that must be either heated or cooled. The second fluid runs over the tubes that arebeing heated or cooled so that it can either provide the heat or absorb the heat required. A setof tubes is called the tube bundle and can be made up of several types of tubes: plain,longitudinally finned, etc. Shell and tube heat exchangers are typically used for high-pressureapplications (with pressures greater than 30 bar and temperatures greater than 260°C).This isbecause the shell and tube heat exchangers are robust due to their shape.Plate heat exchanger:Another type of heat exchanger is the plate heat exchanger. One is composed of multiple,thin, slightly-separated plates that have very large surface areas and fluid flow passages forheat transfer. This stacked-plate arrangement can be more effective, in a given space, thanthe shell and tube heat exchanger. Advances in gasket and brazing technology have madethe plate-type heat exchanger increasingly practical. InHVAC applications, large heat exchangers of this typeare called plate-and-frame; when used in open loops,these heat exchangers are normally of the gasket typeto allow periodic disassembly, cleaning, and inspection.There are many types of permanently-bonded plate heatexchangers, such as dip-brazed and vacuum-brazedplate varieties, and they are often specified for closed-
HEAT AND MASS TRANSFER LABloop applications such as refrigeration. Plate heat exchangers also differ in the types of platesthat are used, and in the configurations of those plates. Some plates may be stamped with"chevron" or other patterns, where others may have machined fins and/or grooves.Materials of construction:The materials of construction used in heat exchangers depend on the fluids or vapor beinghandled, process conditions, such as pressures, temperatures, etc., and a balance of initialcost against expected life and maintenance requirements. Any component or the entire unitcan be made of materials such as carbon steel, stainless steel, nickel, nickel alloys or otherspecial alloys. Selection of materials involves careful evaluation of factors other than the basiccost of possible metals compatible with the application.Main Components1- Channel Cover 2- Channel 3- Channel Flange4- Pass Partition 5- Stationary Tube 6- Shell Flang7- Tube 8- Shell 9- Baffles10- Floating Head backing Devi 11- Floating Tubesheet12- Floating Head 13- Floating Head Flange 14 –Shell CoverTEMA STANDARDS:The Tubular Exchanger Manufacturers Association (TEMA) produces the most widely knownstandard in the heat transfer business, which is on shell-and-tube heat exchangers. They
HEAT AND MASS TRANSFER LABcurrently update the standards every 10 years with "TEMA 98" having been published slightlylate, in 1999. This latest version includes the following main additions and changesDesign of floating head split ringsDesign of double tube sheetsModifications to the design of flexible shell elements (expansion joints)Two-phase flow added to vibration sectionInformation on the design of supports, lifting lugs and the reaction on foundationsTEMA(Tubular Exchanger Manufactures Association) Heat ExchangerFront Head TypeShell Types
HEAT AND MASS TRANSFER LABRear End Head Types Floating Head FloatingHead Pull-Through Floating HeadTEMA DESIGNATIONSThe TEMA designations for shell and tube heat exchangers are a series of threeletters. The first letter represents the front head of the tube side of the exchanger. Thesecond letter represents the shell type, and the third letter indicates the rear head typeof the tube side of the exchanger. Knowledge of the TEMA designations can providean engineer with a quick glimpse to the design and configuration of the shell and tubeheat exchanger.FRONT AND REAR HEAD TYPEDifferent front and rear heads offer varying advantages, depending on the application,process conditions, and ease of maintenance. Tubes are connected to the heads in atube sheet. The front and rear heads distribute process flow into and out of the tubes.Some head types are flat with removable covers that allow the tubes to be cleanedwithout disconnecting the exchanger from the system. Other head types have abonnet and single flange to lower cost.
HEAT AND MASS TRANSFER LABSHELL TYPES AND EXAMPLESShell types in TEMA shell and tube heat exchangers can offer a variety of flow andheat transfer options. Some shells have a one-pass fluid flow that only contacts thetubes one time before exiting. Others have multiple or divided flow paths that offerhigher heat transfer rates. Internal baffles are used to segregate the flows through theshell. A typical example of a TEMA heat exchanger configuration would be BEM. The"B" indicates a stationary bonnet with one flange. The "E" designates a standard one-pass shell, and the "M" indicates a fixed tube sheet and bonnet head similar to "B."Reference I. http://en.wikipedia.org/wiki/Heat_exchanger II. http://www.vesma.com/tutorial/hr_principles.htm III. http://www.thomasnet.com/articles/process- equipment/heat-exchanger-types IV. http://www.engineeringpage.com/heat_exchangers/tema. html V. http://www.hcheattransfer.com/shell_and_tube.html VI. http://en.wikipedia.org/wiki/NTU_method