Executive Summary

Heat exchangers are an ubiquitous part of chemical engineering process design, and plate heat exchangers are among the most widely used and effective of the prevailing heat exchanger designs. For this reason it is important to understand the variables that impact the performance of the heat exchanger and the degree to which they affect the heat transfer. While it is possible to change the total heat transfer by adjusting the configuration of the plates within a plate heat exchanger, this can be a difficult and time consuming process. Therefore, it is often better to change the flow configuration, one or both of the flow rates, or both the configuration and rates to achieve the desired overall heat transfer.To explore the relationship between heat transfer and flow rates, a plate heat exchanger was connected to both hot water and cold water feeds. Thermocouples and pressure gauges were connected to the outflow and inflow lines for both the cold and hot streams. Upon determining that the system had reached equilibrium, a computer was used to collect data for inflow and outflow temperatures and pressures for each stream over a period of approximately 30 seconds. This was done for a total of seven different hot and cold flow rate combinations; for each combination, three sets of data were recorded in an attempt to prevent erroneous data analysis due to random fluctuation and anomalous readings. This entire procedure was completed twice: once for cocurrent flow and once for countercurrent flow. To analyze the collected data it was necessary to calculate the heat exchange for both the cold and hot flows, as well as the effectiveness of the hot stream, which is defined as the ratio between the actual heat transfer and the theoretical maximum heat transfer. Graphical analysis of the relationship between mass flow rate and heat transfer confirmed the theoretical assertion that heat transfer increases linearly with an increase in mass flow rate. More significantly, the analysis also revealed that countercurrent flow resulted in significantly higher heat transfer than concurrent flow, with a hot stream heat transfer of approximately 6.1 kJ/s for countercurrent compared to approximately 1.4 kJ/s for coccurent. Analysis of the effectiveness of the hot stream also indicated that countercurrent flow resulted in greater heat transfer than cocurrent flow, suggesting that countercurrent flow might reach an effectiveness of 1 at a mass flow rate of only 0.37 kg/s, while cocurrent would require a mass flow rate of 0.82 kg/s. Analysis of the effects of changing flow rates on the difference between the inflow and outflow of the hot stream revealed that the changing flow rates had a more dramatic impact on counter current flow than concurrent flow; the slope of the linear relationship for cocurrent flow was -0.6737 whereas the slope for countercurrent flow was -3.3587.In order to conduct a secondary analysis of the collected data, it was necessary to calculate the dimensionless Nusselt Number, which in turn is a function of the dimensionless Reynolds and Prandtl Numbers. The Nusselt Number represents the ratio between convective and conductive heat transfer in a heat exchanger, which means that as the Nusselt Number increases, convective heat transfer becomes increasingly significant, where any Nusselt Number over one is indicative of heat transfer driven by convection. A graph of the Nusselt Number as a function of mass flow rate yields a linear trend that is identical within the bounds of uncertainty for both cocurrent and countercurrent. It is worth noting that the trend line intercepts the vertical axis at approximately 6.3, indicating that there is no flow rate at which conduction plays a more significant role than convection in the overall heat transfer.