The Process of Heat Transfer - Essay Example

HEAT TRANSFER Due HEAT TRANSFER Introduction A distillation column is a cylinder made of metal, normally talland fitted internally with horizontal plates, which have perforations. The perforated plates promote miscible liquids separation as they ascend in the cylinder in form of a vapor. They also separate air into oxygen and nitrogen; its principal components. In the distillation column, separation of air happens in its liquid rather than its gaseous state. Distillation, which is the separation of liquid or vapor mixture into its component fractions of desired purity by heat application or removal, takes place in a distillation column (Kuppan, 2000). Distillation columns are in two categories; continuous and batch-process. The batch-process distills one batch of mixtures at a time; introduction of fresh batches only happens once the first batch is over. A continuous column feeds constantly on fresh mixture. The units deal with a continuous feed streams uninterruptedly (Nisenfeld & Seemann 1981). Distillation columns are vital in the industrial arena e.g. in distillation of crude oil. Crude oil heats to 350-400oC, and, the vapor and the liquid channeled into the distilling column. The vapor rises as it passes through the perforated plates and the liquid falls to the bottom. The hydrocarbons which are heavier condense faster and settle on the lower trays. The lighter hydrocarbons remain longer in their vapor state and later condense on the higher trays. A heat exchanger cools distillates. Distillation occurs at tremendously high temperatures. The columns use an air cooled heat exchanger for cooling. A heat exchanger is equipment built for transfer of heat from one medium to another, efficiently (Nellis & Klein, 2009). Heat exchangers find their uses in refrigeration, power plants, sewage treatment, air conditioning, and natural gas processing and petroleum refineries. The air cooled heat exchanger is a pressure vessel whose function is to cool the circulating fluid in finned tubes by forcing ambient air, by directing the heat to water or air. The radiator of a car is an example of an air cooler. The objective of using an air cooled heat exchanger is to increase the efficiency of a plant, and its environmentally friendly effect of not requiring an auxiliary supply of water and no water treatment chemicals required, compared to cooling tower counterparts. The problems of chemical and thermal pollution from cooling fluids do not exist while using the air cooled heat exchanger (Kroger, 2004). Controlling resistance across the air cooled heat exchanger Industrial applications, which do not have a reliable water source as a medium for cooling or out of choice due to its advantages, use air cooled heat exchanger. An optimized air cooled heat exchanger should enable reliable production. The air cooled heat exchanger faces the challenges of changing climatic conditions due to its exposure, problems of controlling the air coolant is thus, relevant. An air cooled heat exchange normally has a coil or more, with heat transfer surface, a fan, a driver and speed reduction device, a plenum (between the fan and the coil to direct air across the surface area), supporting structure, guards for rotation and protection of the coil area and louvers for outlet temperature control. The functioning of the heat exchanger depends on the soundness of each of these parts (American Society of Mechanical Engineers, & American National Standards Institute, 1991). Corrosion is a factor which brings about degradation of the heat exchangers. Due to their operations in the harsh environment and exposure to unfavorable climatic conditions, a mixture of rainwater and contaminants from the air cause corrosion on the fin materials and the tube, through seeping into the tube-fin bond. The tubes fail, and, the life of the exchanger cut off. The fins for the exchanger should be carefully selected to ensure that it’s appropriate for the prevailing conditions and last long. High thermal resistance which is synonymous to a low cooling capacity, high consumption of electrical power, degradation of performance due to foul and the fan noise are long-standing issues of the air cooled heat exchanger. After some period of being in operation, the surface of air cooled heat exchanger may not be in the superb condition as it was during installation. The surfaces get coated with particulates and deposits or microbiological films. Whatever the cause, it creates resistance to heat transfer and it tampers with the heat exchanger’s efficiency. The resistance is normally called the “fouling resistance” or “the fouling factor”, Rf. The overall coefficient of heat transfer rate is related to the rate of heat transfer and temperature difference by:     Or     U, the coefficient of overall heat transfer. A, the area, QTotal is the heat transfer rate and ΔT , the temperature difference. "LMTD" is the log means temperature difference using the hot and cold ends temperatures. "Mean", the temperature mean difference across the tube. The fouling factor is defined as                                             The fouled exchanger has a smaller heat transfer coefficient than that of the exchanger which is clean, thus, causing Rf to be positive. Through calculations and overall evaluation, resistance reduces the efficiency of the air cooled heat exchanger (Serth, 2007). One can rectify reduced performance due to reduced airflow by servicing the fan and or the drive or cleaning the fins externally. External fin cleaning reduces aerodynamic resistance and external fouling resistance (American Institute of Chemical Engineers, 1978). If the degradation of performance is as a result of increased tube-fin bond resistance, the solution will be tube replacement. As the heat flows from the fluid to the atmosphere, it encounters a series of thermal resistance. What causes thermal resistance at the tube-fin bond is loss of contact or bond pressure or corrosion at the fin base or root. Apparently, there is no repair for root-corroded or loses fins, the remedy is simply replacement or re-tubing. As long as the heat exchanger will be in use, its performance will remain upbeat with regular maintenance (Mukherjee, 2007). In the petrochemical and refining industry, the air cooled heat exchanger’s long-term reliability is vital for return on investment. Loss of/ reduction of production due to equipment degradation should be avoided through good maintenance of the equipment. References American Institute of Chemical Engineers. (1978). Air-cooled heat exchangers a guide to performance evaluation. New York, American Institute of Chemical Engineers. American Society of Mechanical Engineers, & American National Standards Institute. (1991). Air cooled heat exchangers. New York, American Society of Mechanical Engineers. Kroger, D. G. (2004). Air-cooled heat exchangers and cooling towers. Tulsa, Okl, Penwell Corp. Kuppan, T. (2000). Heat exchanger design handbook. New York, Marcel Dekker. Mukherjee, R. (2007). Practical thermal design of air-cooled heat exchangers. Redding, Conn, Begell House, Inc. (50 Cross Highway, Redding, CT 06896). (Accessed Nov, 24 2012). Nellis, G., & Klein, S. A. (2009). Heat transfer. Cambridge, Cambridge University Press. (Accessed Nov, 24 2012). Nisenfeld, A. E., & Seemann, R. C. (1981). Distillation columns. Research Triangle Park, NC, Instrument Society of America. Serth, R. W. (2007). Process heat transfer principles and applications. Amsterdam, Elsevier Academic Press. (Accessed Nov, 24 2012). Read More