Experimental Investigation on Scaling in Gasketed Plate Heat Exchangers

Abstract

Gasketed Plate Heat Exchangers (GPHEs) are a sub-class of heat exchangers that enable heat transfer between 2 fluids via metal plates. A common problem encountered in all heat exchangers is fouling. Fouling can be described as the deposition and accumulation of unwanted materials such as scale, algae, suspended solids and insoluble salts on a heat transfer surface. Despite the widespread use of GPHEs and the common occurrence of fouling in heat exchangers, fouling is not clearly understood in GPHEs. Scale, one of the several types of fouling, has the most harmful effect on heat exchangers out of all fouling mechanisms and is the most common problem encountered in cooling water systems. When formed on heat-exchanging surfaces, scale retards heat exchange, accelerates fouling, promotes certain types of corrosion and microbial growth, and increases pumping back pressure. These, in turn, result in decreased plant efficiency, reduced productivity, schedule delays, more downtime for maintenance, and increased costs for equipment repair and replacement. Since Alfa Laval is an active part of this project, and a large portion of the GPHEs they sell are used in cooling water applications, the focus of this project is to understand the effect of flowrate, inlet temperature and GPHE duty on scaling in GPHEs. The most common scale deposited on heat exchangers is calcium carbonate (CaCO3), and therefore this study will only focus on CaCO3 scaling.

In order to quantify the effect of changes in a parameter on scaling tendency and GPHE performance, an experimental setup is built. The experimental design ensures that when altering one parameter, all other operating conditions of the GPHE remain constant, thereby attributing any changes in scaling tendency solely to the test parameter. Throughout each experiment, media temperatures are monitored, and the overall heat transfer coefficient and fouling resistance are plotted against time to observe changes in GPHE performance. Additionally, since the deposited scale initially increases the performance of the GPHE, a time for ’onset of scale’ is also determined for each experiment, where ’onset of scale’ is defined as the moment when the deposited scale is detrimental to GPHE performance. Moreover, following each experiment, the GPHE is disassembled, and the scaled plates are weighed to determine the quantity of scale deposited on the plates. Finally, images of the scaled plates are compared with those of a clean plate to observe the distribution of scale on the plate surface.

After performing and analysing the results from all experiments, it can be concluded that all 3 parameters have a significant influence on GPHE performance. It was observed that higher flowrates led to a smaller amount of scale deposits on the plate and result in a greater duration to achieve the onset of scale condition. Conversely, higher inlet temperatures and GPHE duty led to a greater amount of scale deposits and result in a shorter time to achieve the onset of scale condition. The results also indicated that a change in GPHE duty has the greatest influence on scaling from all 3 parameters, provided the flowrate is greater than 250 kg/hr. However, for flowrates below 250 kg/hr, the flowrate of the test solution has the biggest impact on scaling tendency and GPHE performance. Additionally, an optimal performance region which would result in minimal scale deposition on the plates is determined for the current system. Finally, to predict the amount of scale deposited on the plates as a function of the test parameters, regression analysis is performed on the data and a 5th-degree polynomial function is established.