Objective 2

The overall heat transfer coefficients and the effectiveness of a heat exchanger are very important in many engineering applications because they help predict the outlet temperatures which would be useful in designing a plant. The overall heat transfer coefficient is the reciprocal of the total resistance within the heat exchanger unit. It tells about how heat is easily transferred within the unit. On the other hand, effectiveness is defined as the ratio between the actual amount of heat transferred and the maximum amount of heat, which is the heat that could be transferred if the area were infinite. The table below summarizes the experimental and theoretical values of the overall heat transfer coefficients and the effectiveness of the heat exchanger.

TABLE 1

There are three main types of heat transfer mechanisms- conduction, convection and radiation. For this experiment, the effect of the last one is considered negligible because of the low temperatures involved during the experiment. As mentioned before, the driving force of the changes in temperature is the differences of the temperatures of the two fluids. Heat is transferred from the hot fluid to the cooler fluid. First, heat is transferred through convection from the fluid to the interface of the hot fluid and the inner wall of the inner pipe. Then, heat is then transferred through conduction through the inner wall and finally, heat is once again transferred through convection from the interface of the outer wall of the inner pipe and the cooler fluid. Every type of material that the heat goes through offers a certain amount of resistance which in turn, dictate the amount heat will be allowed to pass through. The overall heat transfer coefficient measures these resistances. If there was an infinite area for exchanging of heat, a maximum of the amount heat can be transferred. The effectiveness of a heat exchanger measures how close is the actual amount of heat transferred to the maximum allowable heat that can be transferred.

Based from the results, the theoretical and the actual values for the overall heat transfer coefficients greatly differ. This is due to the certain assumptions that were done in computing the theoretical values for the heat transfer coefficients. First, for the theoretical calculations, it was assumed that there were only two fluids responsible for the convection mechanism, the hot fluid and the cold fluid. However, during the experiment, several air bubbles were observed inside the pipes. This greatly affects because the amount of convective heat transfer is different for liquids and gases. Second, the heat losses due to friction in the tubes and due to the fittings were not taken into account. Next, the outer pipe was not properly insulated. Heat losses through the outer walls were also neglected. Finally, another possible source of error is due to the deteriorating performance of the equipment, due to its age, in transferring heat and measuring the temperature.

Based from the results, the effectiveness of the countercurrent regime is higher than the effectivene ss of the cocurrent regime. The value for the effectiveness for the countercurrent flow should be higher than the effectiveness of the concurrent regime. As mentioned before, this is because in the cocurrent configuration, the temperatures of both fluids just approach a temperature between the inlet

temperatures of the two fluids while on the other hand, for the countercurrent configuration, the temperatures of the two fluids approach the inlet temperatures of its opposite fluid.