11-10-2012, 05:23 PM
Heat exchangers
HEAT+EXCHANGER+KUMAR.doc (Size: 167 KB / Downloads: 23)
Introduction.
Heat exchangers are devices where two moving fluid streams exchange heat without mixing. Heat exchangers are widely used in various industries, and they come in various designs.
The simplest form of a heat exchanger is a double tube (also called tube and shell)
heat exchanger, shown in figure 1. It is composed of two concentric pipes of different diameters. One fluid flows in the inner pipe, and the other in the annular space between the two pipes. Heat is transferred from the hot fluid to the cold one through the wall separating them. Sometimes the inner tube makes a couple of turns inside the shell to increase the heat transfer area and thus the rate of heat transfer. The heat transfer rate associated with heat exchangers depends on how the control volume is selected. Heat exchangers are intended for heat transfer between two fluids within the device, and the outer shell is usually well insulated to prevent any heat loss to the surrounding medium.
Equipment/ Description of Experimental Apparatus.
In the case of Shell and Tube Water Heat Exchanger, a water stream flows through a pipe and exchange heat with a second water stream that flows through an annulus surrounding the pipe.
For the Water Heat Exchanger, the hot water flowing through the inner tube bundle will be cooled and the cold water flowing through the outer shell will be heated. There are 7 hot water tubes with inner and outer diameter of 5.15 mm and 6.35 mm respectively. All hot water tubes are 0.144 m in length.
Discussion.
The result obtained are not as expected because the several reasons. One of the reasons is the heat are not completely transfer from the hot water tube to the cold water tube. The heat, which is not transferred to the water tube, may be transferred to the environment. Other reasons are may be the controller or the valves are not is the good condition. This will lead to the unstable condition of the temperature indicator and flow rate indicators. The bubbles existing in the tube may also the cause of the experimental error. The efficiency of the temperature difference for the both streams increases as the mass flow rate of cold-water increases. The efficiency of the heat exchanger increases as the mass flow rate of cold-water increases. The outlet temperature of hot water decreases in consequence of the increase in the mass flow rate of cold water.
Conclusions.
A heat exchanger involves heat transfer from a hot medium to a cold medium without both being in contact. The parallel-flow heat exchanger is more efficient and provides better heat transfer. Performance of the heat exchanger increases as the flow rate increases. From this application also shown that the LMTD for the parallel-flow operation is more than the LMTD for the counter-flow operation. In the parallel-flow operation, the LMTD is increasing if the water flow rate is increased.