Modeling of Flow and Temperature Field in an Economizer

Applied Computer Science, vol. 12, no. 2, pp. 63–73 Submitted: 2016-02-09 Revised: 2016-06-12 Accepted: 2016-06-20 Economizer, main part of boiler, modelling, CFD František BRUMERČÍK*, Dusan SOJCAK**, Michal LUKÁČ***, Aleksander NIEOCZYM***, Sławomir WIERZBICKI***** MODELING OF FLOW AND TEMPERATURE FIELD IN AN ECONOMIZER Abstract This article deals with the economizer as one of the main parts of a boiler. Economizers and air heaters perform a key function in providing high overall boiler thermal efficiency by recovering the low level energy from the flue gas before it is exhausted to the atmosphere. The most common and reliable economizer design is the bare-tube, in-line, cross-flow type. To reduce capital costs, most boiler manufacturers build economizers with a variety of designs to enhance the controlling gas-side heat transfer rate. From this point of view it creates a lack for an investigation and modeling of these parts. 1. INTRODUCTION Economizers are basically tubular heat transfer surfaces used to preheat boiler feedwater before it enters the steam drum (recirculating units) or furnace surfaces (once-through units). The term economizer comes from early use of such heat exchangers to reduce operating costs or economize on fuel by recovering extra energy from the flue gas [2]. Economizers also reduce the potential of thermal * University of Zilina, Faculty of Mechanical Engineering, Univerzitná 1, 010 26 Žilina, Slovakia, ** Enersense International Oy Gallen-Kallelankatu 7, 28100 Pori, Finland, *** University of Zilina, Faculty of Mechanical Engineering, Univerzitná 1, 010 26 Žilina, Slovakia, **** Lublin University of Technology, Faculty of Mechanical Engineering, Nadbystrzycka 36, 20-618 Lublin, Poland, ***** University of Warmia and Mazury in Olsztyn. Faculty of Technical Sciences, Słoneczna 6A, 10-710 Olsztyn, 63 shock and strong water temperature fluctuations as the feedwater enters the drum or waterwalls. Fig. 1 shows the location of an economizer in a coal-fired boiler. The economizer is typically the last water-cooled heat transfer surface up-stream of the air heater. Fig. 1. Economizer and air heater locations in a typical coal fired boiler [9] Modern heat exchangers (economizers) are characterized by high steam pressure and temperature values, and the tubes used in these devices have complex cross-sectional shapes [3, 4]. Tube geometry allows the construction of heat exchanger plates with smooth sidewalls, which can be placed in the upper part of the combustion chamber. This prevents erosion of superheater tubes and deposition of slag and ash in the spaces between adjacent tubes. The mechanisms of heat transfer in flow conditions are complex because they may comprise the phenomena of natural forced convection and convection associated with the turbulent flow of medium in the tubes. Computational Fluid Dynamics (CFD) software and the results of laboratory tests of thermal conductivity make it possible to model the real phenomena occurring in heat exchangers. Other test methods are dedicated to specific economizer designs. The authors of [7], for example, present a mathematical model of a shell-and-tube heat exchanger with helical baffles (STHXsHB) in which the rate of heat transfer and the total cost of energy are subject to multi-objective optimization. 64 Modeling results can also be used to validate the adequacy of mathematical models and the accuracy of equations used in those models. Simulations conducted using FLUENT 6.3 software have been used to solve the equations of continuity, momentum, and energy in [8]. Studies [4, 6] present the results of modeling of flow and heat processes in superheaters with complex flow systems. In those works, 3D models have been replaced by 2D models. The proposed computation method can be used in subcritical and supercritical boilers. The application of simplified models based on the finite element method (FEM) to calculate the steady-state temperature field and flow in tubes remains a basic study method that allows one to obtain results with an error not exceeding 5% [1]. The results one obtains are the basis for optimizing the geometry of economizers and selection of pumps which set the fluid in motion. In this article, FEM modeling was used to determine the stress fields and temperatures of economizer components. 2. ECONOMIZER SURFACE TYPES Bare Tube – the most common and reliable economizer design is the bare tube, inline, cross flow type shown in Fig. 2a. When coal is fired, the fly ash creates a high fouling and erosive environment. The bare tube, in line arrangement minimizes the likelihood of erosion and trapping of ash as compared to a staggered arrangement shown in Fig. 2b. It is also the easiest geometry to be kept clean by sootblowers. However, these benefits must be evaluated against the possible larger weight, volume and cost of this arrangement. Extended Surface – to reduce capital costs, most boiler manufacturers build economizers with a variety of fin types to enhance the controlling gas-side heat transfer rate. Fins are inexpensive parts which can reduce the overall size and cost of an economizer. However, successful application is very sensitive to the flue gas environment. Surface cleanability is a key concern. In selected boilers, such as PRB coal-fired units, extended surface economizers are not recommended because of their peculiar flyash characteristics. Stud fins: Stud fins work reasonably well in gas-fired boilers. However, stud finned economizers can have a higher gas-side pressure drop than a comparable unit with helically-finned tubes. Studded fins perform poorly in coal-fired boilers because of high erosion, loss of heat transfer, increased pressure loss and plugging resulting from flyash deposits. 65 Fig. 2. Bare tube economizer arrangements [source: own study] Longitudinal fins: Longitudinally-finned tubes in staggered crossflow arrangements (Fig. 3) also do not perform well over long operating periods. Excessive plugging and erosion in coal-fired boilers have resulted in the replacement of many of these economizers. In oil- and gas-fired boilers, cracks occur at the points where the fins terminate. These cracks propagate into the tube wall and cause tube failures in some applications. Plugging with flyash can also be a problem (tight spaces). 66 Fig. 3. Longitudinal fins, staggered tube arrangement [source: own study] Helical fins: Helically-finned tubes (Fig. 4a) have been successfully applied in some coal-, oil- and gas-fired units. The fins can be tightly spaced in the case of gas firing due to the absence of coal flyash or oil ash. Four fins per inch (1 fin per 6.4 ram), 1.5 to 1.9 mm thick and 19.1 mm high are typical. For 51 mm outside diameter tubes, these fins provide ten times the effective area of bare tubes per unit tube length. If heavy fuel oil or coal is fired, a wider fin spacing must be used and adequate measures taken to keep the heating (...truncated)