UNDERSTANDING AND ANALYSIS OF FOULING BEHAVIOR OF BARE-WIRE HEATING ELEMENTS IN ELECTRIC WATER HEATING

The aim of the present work is to understand the effects of crystallization fouling on bare-wire heating elements. To carry out the investigations, a unique experimental set-up was developed. It was used to determine the fouling resistance and investigate the local fouling growth over time using optical measuring methods. The fouling resistance was measured for various flowrates. The optical investigations were used to understand the local and temporal fouling growth. Furthermore, they aided in determining the locally deposited fouling thickness over the heating length of the bare-wire heating elements. The morphology of the deposited crystal layers on the bare-wire heating surface was also analyzed to determine their influence on the heat transfer. The findings of this work can be applied for developing enhanced bare-wire heating systems with improved durability to crystallization fouling. The experimental results were used to derive correlations between the fouling mass weight and the flowrate and between the lifetime and the flowrate. INTRODUCTION Electric water heating The electric water-heating industry includes a wide range of applications, e.g., tankless water heaters, hot-water storage tanks, and drinking-water heat pump applications. All these domestic appliances comprise electric heating elements. Many conventional heating elements for electric water heating are based on either a tubular or a barewire heating system. However, knowledge of the fouling behavior of electric heating elements is scarce. Such knowledge is necessary for the successful development of design principles for novel heating-element generations. Bare-wire heating elements are placed directly in the fluid without any electrical insulation between the heating surface and fluid. Therefore, they require a unique type of electrical insulation. The heating elements are placed into a pressure-tested insulated block. The insulation resistance is realized using long and narrow channels in front of and behind the electric heating elements. The resistance depends on the properties of the fluid flowing through the channels and on the channel geometry. Bare-wire heating elements are typically fabricated using a nickel-chromium alloy (NiCr). Fig. 1 shows a barewire heating system. Fig. 1. Schematic of a bare-wire heating system Bare-wire heating elements are especially suitable for hard-water applications, because they have lower weight and lower surface temperature than conventional tubular heating elements [1]. Owing to the properties of electric bare-wire heating elements, their fouling behavior is characterized by different Pressure-tested insulated block Outlet Inlet Bare-wire heating element Fluid Long and narrow channels in front of electric heating element Electrical connection Long and narrow channels behind electric heating element Heat Exchanger Fouling and Cleaning – 2019 ISBN: 978-0-9984188-1-0; Published online www.heatexchanger-fouling.com stages, which justifies the requirement of a closer investigation. Crystallization fouling Crystallization fouling is the deposition of undesirable precipitates on heat-transfer surfaces. According to several investigations [3-7], fouling can cause severe damage to the heat exchanger. In the application field of electric water heating, it is necessary to change the entire heating system when the bare-wire heating elements fail in order to make the device operational again. The process of crystallization fouling is complex; it begins with nucleation and is followed by crystal growth on the heating surface. The growth of crystals results in a decrease in the overall heat transfer coefficient. If fouling occurs, and if all the operating parameters of the heat exchanging fluid are maintained as constants, the fouling resistance Rf can be determined by