Quentin Noble
Mike Voss
PCT 105 Equipment
April 5, 2012
Heat Exchangers There are many types of heat exchangers used in a wide variety of industrial applications including power plants, boilers for industrial steam, chemical plants, and many types of manufacturing facilities. Several different configurations are used to accomplish the function of transferring heat from one fluid to another without mixing the two fluids together. The two most common industrial heat exchangers, the shell and tube heat exchanger and the double pipe heat exchanger, along with information about spiral heat exchangers, flat plate heat exchangers, flat plate heat exchangers, fin-tube heat exchangers, and condensers. Calculations for heat exchanger design use heat transfer parameters such as convection heat transfer coefficients, overall heat transfer coefficients, and log mean temperature differences. These calculations are needed for shell and tube and double pipe heat exchangers, as well as other types. The flow pattern through a heat exchanger affects the required heat exchanger surface. A counter flow heat exchanger needs the lowest heat transfer surface area. It gives a higher value for log mean temperature difference than either a parallel flow heat exchanger or a cross flow heat exchanger. A heat exchanger can have several different flow patterns. Counter flow, parallel flow, and crosss flow are common heat exchanger types. A counter flow heat exchanger is the most efficient flow pattern of the three. It leads to the lowest required heat exchanger surface area because the log mean temperature drop is the highest for a counter flow heat exchanger. A counter flow heat exchanger has the hot fluid entering at one end of the heat exchanger flow path and the cold fluid entering at the other end of the flow path. Counter flow is the most common type of liquid-liquid heat exchanger, because it is the most efficient. A double pipe heat exchanger is usually operated as a counter flow exchanger. The flow pattern in a shell and tube heat exchanger with a single tube pass will be approximately counter flow if it is long in comparison with its diameter. Because of the baffles and the need to distribute the flow of the shell side fluid over the cross-section of the shell, the flow is not as close to counter flow in a shell and tube heat exchanger as it is in a double pipe heat exchanger. A double pipe heat exchanger can be operated in parallel flow mode. Similarly a shell and tube heat exchanger can be operated in approximately parallel flow by having both fluids enter at one end and exit at the other end. With parallel flow the temperature difference between the two fluids is larger at the entrance end, but it become small at the exit end as the two fluid temperatures approach each other. The overall measure of heat transfer driving force, the log mean temperature difference is greater for counter flow, so the heat exchanger surface area requirement will be larger than for a counter flow heat exchanger with the same inlet and outlet temperature for the hot and the cold fluid. Use of a fin tube exchanger works especially well for an air heat exchanger. A finned tube adds a lot of heat transfer surface area for the fluid outside the tube, which is usually air or some other gas for a fin tube exchanger. For a liquid-liquid heat exchanger, the use of a finned tube isn’t normally needed, because the heat transfer coefficient for both sides of the heat transfer surface is relatively high. For an air heat exchanger, however, where one of the fluids is air or some other gas, the air side heat transfer coefficient will be much lower, so additional heat transfer surface area, as in a fin tube exchanger is very helpful. The overall flow pattern for a finned tube exchanger is often cross flow, but it can also be counter flow or parallel flow. The shell and tube heat exchanger is one of the most common types of heat exchangers. The heat transfer coefficient affects the heat transfer rate and the total heat exchanger tube area needed. There are several configuration options for flow pattern through a shell and tube heat exchanger. The shell and tube heat exchanger is made up of a bundle of parallel heat exchanger tubes held in place with tube sheets and placed into a shell. The heat exchange always takes place between two fluids through the heat exchanger tube wall. There are quite a variety of flow options for shell and tube heat exchanger. In all of the configurations for shell and tube heat exchanger, one fluid passes through the tubes and the other passes through the shell. The choice of shell and tube heat exchanger configuration affects the overall heat transfer coefficient and thus also affects the rate of heat transfer and the heat exchanger tube surface area needed. A flat plate heat exchanger user a metal heat exchanger plate between the two fluids. This give it a higher overall heat transfer coefficient than shell and tube heat exchanger, the most widely used type industrial heat exchanger. A flat plate heat exchanger disassembles for ease of cleaning. The plate heat exchanger is also known as a plate and frame heat exchanger or flat heat exchanger. In plate heat exchanger, heat is transferred from one fluid to another through a flat metal heat exchanger plate, instead of through a heat exchanger tube wall as with double pipe or shell and tube heat exchangers. Fouling factors provide a numerical value, reflecting the resistance to moving heat between fluid mediums. Many causes contribute to fouling within a heat exchanger. Chemical reactions within the flowing fluid produce deposits that attach to the exchanger’s surface. Internal corrosion can occur if the heat exchanger is not constructed of corrosion-resistant material, such as stainless steel. Deposition fouling materializes when the flowing fluid does not attain the correct velocity within the heat exchanger. Material settles out of the fluid onto the interior surfaces, forming a sediment layer. Organisms that can grow within the fluid create biological fouling, eventually attaching to the surface. In addition, crystallization occurs when dissolved salts within the fluid deposit along the heat exchanger’s surface. Heat transfer is one of the most important processes. Throughout any industrial facility, heat must be added, moved, or moved from one process steam to another. These are three basic types of transfer heat: conduction, convention, and radiation. The two most common forms encountered in the chemical processing industry are conduction and convection. To transfer heat from one process to another, heat exchanger is used. Heat exchanger tubes can be thus seamless, or welded. High quality electro resistance welded tubes display good grain structure at the weld joints. Extruded tubes with fins and interior rifling are sometimes specified for certain heat transfer applications. Often, surface enhancements are added to increase the available surface or aid in fluid turbulence, thereby increasing the operative heat transfer rate. Finned tubes are recommended when the shell-side fluid have a considerably lower heat transfer coefficient than the tube-side fluid. Note, the diameter of the finned tube is slightly smaller than the un-finned area thus allowing the tubes to be installed easily through the baffles and tube support during assembly while minimizing fluid bypass. Tube sheets usually constructed from a round, flattened sheet of metal. Holes for the tube ends are teen drilled for the tube ends in a pattern relative to each other. Tube sheets are typically manufactured from the same material as tubes, and attached with pneumatic or hydraullc pressure roller to the tube sheet. At this point, tube holes can both be drilled and reamed, or they are machined grooves. The tube sheet comes in contact with both fluids in the exchanger, therefore it must be constructed of corrosion resistant materials or allowances appropriate for the fluids and velocities. A layer of alloy metal bonded to the surface of low carbon steel would provide an effective corrosion resistance without the expense of manufacturing from a solid alloy. The shell is constructed either from pipe or rolled plate metal. For economic reason, steel is the most commonly used material, and when applications involving extreme temperatures and corrosion resistance, others metals or alloys are specified. Using off-the-shelf pope reduces manufacturing costs and lead time to deliver to the end customer. A consistent inner shell diameter or ‘roundness’ is need to minimize the baffle spacing on the outside edge, excessive space reduces performance as the fluid tends to channel and bypasses the core. Roundness is increased typically by using a mandrel and expanding the shell around it or by double the shell after welding the longitudinal seam. In some cases, although extreme, the shell is cast and then bored out until the correct inner diameter is achieved.