【文章內(nèi)容簡介】 ng head, multipass。 or a fixed head heat exchanger (Figure ), tube sheets are attached to the shell. Fixed head heat exchangers are designed to handle temperature differentials up to 200176。F (176。C). Thermal expansion prevents a fixed head heat exchanger from exceeding this differential temperature. It is best suited for condenser or heater head heat exchangers are designed for high temperature differentia is above 200176。F (176。C).During operation, one tube sheet is fixed and the other “floats” inside the floating end is not attached to the shell and is free to expand.Figure Fixed Head Heat ExchangerReboilers are heat exchangers that are used to add heat to a liquid that was once boiling until the liquid boils again. Types monly used in industry are kettle reboilers and thermosyphon reboilers.Plateandframe heat exchangers are posed of thin, alternating metal plates that are designed for hot and cold service. Each plate has an outer gasket that seals each partment. Plateandframe heat exchangers have a cold and hot fluid inlet and outlet. Cold and hot fluid headers are formed inside the plate pack, allowing access from every other plate on the hot and cold sides. This device is best suited for viscous or corrosive fluid slurries. It provides excellent high heat transfer. Plateandframe heat exchangers are pact and easy to clean. Operating limits of 350 to 500176。F (176。C to 260176。C) are designed to protect the internal gasket. Because of the design specification, plateandframe heat exchangers are not suited for boiling and condensing. Most industrial processes use this design in liquidliquid service.Aircooled heat exchangers do not require the use of a shell in operation. Process tubes are connected to an inlet and a return header box. The tubes can be finned or plain. A fan is used to push or pull outside air over the exposed tubes. Aircooled heat exchangers are primarily used in condensing operations where a high level of heat transfer is required.Spiral heat exchangers are characterized by a pact concentric design that generates high fluid turbulence in the process medium. As do other exchangers, the spiral heat exchanger has coldmedium inlet and outlet and a hotmedium inlet and outlet. Internal surface area provides the conductive transfer element. Spiral heat exchangers have two internal chambers.The Tubular Exchanger Manufacturers Association (TEMA) classifies heat exchangers by a variety of design specifications including American Society of Mechanical Engineers (ASME) construction code, tolerances, and mechanical design:l Class B, Designed for generalpurpose operation (economy and pact design)l Class C. Designed for moderate service and generalpurpose operation (economy and pact design)l Class R. Designed for severe conditions (safety and durability)Heat Transfer and Fluid FlowThe methods of heat transfer are conduction, convection, and radiant heat transfer (Figure ). In the petrochemical, refinery, and laboratory environments, these methods need to be understood well. A bination of conduction and convection heat transfer processes can be found in all heat exchangers. The best conditions for heat transfer are large temperature differences between the products being heated and cooled (the higher the temperature difference, the greater the heat transfer), high heating or coolant flow rates, and a large crosssectional area of the exchanger.ConductionHeat energy is transferred through solid objects such as tubes, heads, baffles, plates, fins, and shell, by conduction. This process occurs when the molecules that make up the solid matrix begin to absorb heat energy from a hotter source. Since the molecules are in a fixed matrix and cannot move, they begin to vibrate and, in so doing, transfer the energy from the hot side to the cooler side.ConvectionConvection occurs in fluids when warmer molecules move toward cooler molecules. The movement of the molecules sets up currents in the fluid that redistribute heat energy. This process will continue until the energy is distributed equally. In a heat exchanger, this process occurs in the moving fluid media as they pass by each other in the exchanger. Baffle arrangements and flow direction will determine how this convective process will occur in the various sections of the exchanger.Radiant Heat TransferThe best example of radiant heat is the sun’s warming of the earth. The sun’s heat is conveyed by electromagnetic waves. Radiant heat transfer is a lineofsight process, so the position of the source and that of the receiver are important. Radiant heat transfer is not used in a heat exchanger.Laminar and Turbulent FlowTwo major classifications of fluid flow are laminar and turbulent (Figure ). Laminar—or streamline—flow moves through a system in thin cylindrical layers of liquid flowing in parallel fashion. This type of flow will have little if any turbulence (swirling or eddying) in it. Laminar flow usually exists atlow flow rates. As flow rates increase, the laminar flow pattern changes into a turbulent flow pattern. Turbulent flow is the random movement or mixing of fluids. Once the turbulent flow is initiated, molecular activity speeds up until the fluid is uniformly turbulent.Turbulent flow allows molecules of fluid to mix and absorb heat more readily than does laminar flow. Laminar flow promotes the development of static film, which acts as an insulator. Turbulent flow decreases the thickness of static film, increasing the rate of heat transfer.Parallel and Series FlowHeat exchangers can be connected in a variety of ways. The two most mon are series and parallel (Figure ). In series flow (Figure ), the tubeside flow in a multipass heat exchanger is discharged into the tube