ANALYSIS OF THE INFLUENCE OF THE LAYER OF SCALE ON HEAT TRANSFER COEFFICIENT IN THE HORIZONTAL SHELL-AND-TUBE HEAT EXCHANGER

Abstract

Over the years, shell and tube heat exchangers have become the most widely used type of apparatus. This is primarily due to the reliability of the design, a large set of options for different operating conditions, in particular: single-phase flows, boiling and condensation on the hot and cold sides of the heat exchanger with vertical or horizontal execution; pressure range from vacuum to high values; sizes from small to very large (5000 m2) the possibility of using different materials in accordance with cost requirements, corrosion, temperature regime and pressure, etc. With poor water treatment for heat exchange equipment, over time, layers of scale, sediment will form, leading in turn to a negative impact on the heat transfer process in heat exchangers and corrosion on the surfaces of the equipment. The article investigated the effect of scale thickness in a horizontal shell-and-tube heat exchanger on the heat transfer coefficient from hot to cold water. The issues of creating a layer of scale on the surfaces of heat exchange equipment, types of scale and modern methods of dealing with it for heat exchangers, such as: self-cleaning, the use of chemical softeners, ultrasound, magnetic water treatment are considered. Identified by analysis of the study, currently the most effective method of dealing with scale in precisely shell-tube apparatus is magnetic water treatment due to safety, reliability of operation with stable water parameters, low operating costs and environmental friendliness. The article describes the traditional scheme of horizontal shell-and-tube equipment and features of its design. According to the results of calculations of linear heat transfer coefficients of a horizontal shell-and-tube heat exchanger with a counterflow of hot water in tubes without a layer of scale and taking into account the variable thickness of scale with the creation time, an analysis of the efficiency of heat transfer along the length of the apparatus was carried out. The obtained hyperbolic dependences of the heat transfer coefficient and heat transfer efficiency on the scale thickness for horizontal shell-and-tube heat exchangers. It was calculated and obtained that with an increase in the thickness of scale on the inner surface of pipes of a shell-and-tube heat exchanger from 0.0001 m to 0.002 m, the linear heat transfer coefficient Kln decreases from 126.35 W / (m×°С) to 31.5 W / (m×°С), which indicates reducing the efficiency of the heat transfer process from hot water to cold water in a horizontal shell-and-tube heat exchanger from 77.4% to 20.59% as a result of an increase in the thickness of scale with a time of 1 to 7 years of operation of the apparatus, respectively. It was revealed as a result of calculating linear heat transfer coefficients of scale and with a variable layer of scale on the inner surface of hot water tubes in a horizontal shell-and-tube heat exchanger with countercurrent a significant effect of the thickness of the layer of scale on the heat transfer efficiency even at a thickness of the scale of the heating surface 0.0005 m above which it is impossible to efficiently to ensure the heat transfer process in the apparatus, since the heat transfer coefficient decreases by 49% and, accordingly, the heat transfer efficiency decreases by 50% compared to the surface of the tubes without scale (153, 02 W / (m×°С) > Klн = 75,57 W / (m×°С)).