Reconditioning of a shell and tube heat exchanger testing rig
Kitonyi, Thomas M
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ABSTRACT This project was carried out to improve the operation of a heat exchanger testing rig and assess its performance. The rig would be used by mechanical engineering students for experiments to test heat transfer theories and develop understanding of the parameters applicable in determination of the performance of heat exchangers and cooling towers. The rig consisted of an electrode boiler for generation of steam, a super heater to increase the enthalpy of the steam before entry to the heat exchanger, a heat exchanger (condenser), a counter flow induced mechanical draft cooling tower (for evaporative cooling of the heat exchanger cooling water), and two cooling water pumps. The boiler was operated at a pressure of about 6.2 bar. The heat exchanger was a shell and tube type comprising of two tube pass (with 85 tubes) and one shell pass, an arrangement otherwise known as "1:2 heat exchanger". In operation, cold water flowed through the tubes while steam flowed through the shell. The condenser cooling water then flowed through the cooling tower. In the reconditioning of the rig, a metric X series rota meter, tube size 35X with float type S was installed in the flow line between the heat exchanger and the cooling tower, for measurement of the cooling water flow rate. A measuring tank of internal diameter 306mm was installed to measure the flow rate of water leaving the tower. The wood fill material in the cooling tower was replaced with perspex in order to reduce maintenance and prolong service life. Wood requires periodical treatment to prevent rotting. Metal ring plates were also mounted round the inside and outside wall surface of the cooling tower tank to minimise water loss through the air intake holes. The heating element in the super heater was substituted with one of a higher power rating for improved heating capacity, and a thermostat installed to limit the element temperature to a maximum of 285°C. In the cooling water and steam pipe lines, faulty valves were replaced and leakages sealed. Pressure gauges at inlet and outlet of the heat exchanger were calibrated and faulty ones replaced. Glass mercury thermometers were replaced with 8mm bayonet spring loaded thermocouples for the measurement of temperatures at seven of the nine temperature measurement stations in the testing rig Experimental tests were performed after reconditioning and commissioning. From the analysis of the results obtained, various heat exchanger and cooling tower performance characteristics were determined. It was found that the heat exchanger performance varied with cooling water flow rate. For optimal heat duty, cooling water flow rate through the heat exchanger under study was found to be 0.27kg/s. At this cooling water flow rate and a steam flow rate of 0.02398kg/s, the following values were obtained for the heat exchanger; heat duty, 42.5kW; steam pressure drop, 0.22 bar; cooling water pressure drop, 0.21 bar; effectiveness by applying mean temperature values, 35%; effectiveness by applying £-NTU method, 35%; corrected LMTD, 57.4°C; overall heat transfer coefficient, 90.8W/m2K. At the same cooling water flow rate and a steam flow rate ofO.02415kg/s; heat duty was found to be 44.9 kW; steam pressure drop, 0.21 bar; cooling water pressure drop, 0.19 bar; effectiveness by applying mean temperature values, 43%; effectiveness by applying £-NTU method, 40%; corrected LMTD, 45.8°C; and overall heat transfer coefficient, 119.9 W/m2K. It was found that the cooling tower performance varied with water flow rate through it. At entry water flow rate of 0.27kg/s and heat exchanger steam flow rate of 0.02398kg/s, the cooling tower range was found to be 14.7°C; cooling capacity 16.39 kW; measured evaporation loss 0.0302m3/hr. At the same entry water flow rate and a heat exchanger steam flow rate of 0.02415kg/s, range was found to be 19.1°C; approach 17.0°C; effectiveness 52.9%; cooling capacity 21.29 kW; measured evaporation loss 0.0302m3/hr. Comparing values calculated at steam flow rate of 0.02398kg/s with those calculated at steam flow rate ofO.02415kg/s, it was found that heat exchanger and cooling tower performance increased with increase in steam flow rate.