Numerical and CFD Design of Heat Exchanger Used to Biomass Combustion

The article describes the design of a heat exchanger used for biomass combustion. The design takes into account simple maintenance of the exchanger, low input costs of construction and the highest possible efficiency. In the design is used the tubular type of heat exchanger. The construction consists of a tubular part flue gas part, inter-tube space heat transfer medium space. The output of the numerical model, CFD model is the heat transfer coefficient, heat exchanger power and final comparison of CFD and numerical model outputs. 1 Numerical design of heat exchanger We know various divisions of types of exchangers in terms of construction, type of media used, according to the purpose of use, the meaning of the flow of heat transfer media. When designing the exchanger, emphasis is placed on the smallest possible dimensional requirements, low costs, high reliability and easy maintenance. During process of burning biomass, it is necessary to take into account in particular the simplicity of construction, but also the easiest possible maintenance of equipment. The combustion of biomass, especially straw, produces emissions of solid pollutants, which gradually pollute the heat exchanger. Therefore, in our case, we decided for a tubular type exchanger. This type of exchanger enables the introduction of continuous cleaning, but also simple maintenance, when even after equipment failures, it is possible to replace individual parts (pipes) of the equipment that are exposed to the adverse corrosive effects of biomass flue gases [1, 2]. The exchanger consists of two identical parts, which are interchangeable. In the event of a fault, the device can also work with one part with reduced power [3]. In this way, it is possible to maintain the functionality of the device maintaining the operation of the heating system, repairing a non-functional part of the exchanger and quick re-commissioning of the device. The proposed exchanger uses flue gas and water heat media without segment baffles. * Corresponding author: marian.pafcuga@fstroj.uniza.sk © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). MATEC Web of Conferences 328, 02005 (2020) https://doi.org/10.1051/matecconf/202032802005 XXII. AEaNMiFMaE-2020


Numerical design of heat exchanger
We know various divisions of types of exchangers in terms of construction, type of media used, according to the purpose of use, the meaning of the flow of heat transfer media. When designing the exchanger, emphasis is placed on the smallest possible dimensional requirements, low costs, high reliability and easy maintenance. During process of burning biomass, it is necessary to take into account in particular the simplicity of construction, but also the easiest possible maintenance of equipment. The combustion of biomass, especially straw, produces emissions of solid pollutants, which gradually pollute the heat exchanger. Therefore, in our case, we decided for a tubular type exchanger. This type of exchanger enables the introduction of continuous cleaning, but also simple maintenance, when even after equipment failures, it is possible to replace individual parts (pipes) of the equipment that are exposed to the adverse corrosive effects of biomass flue gases [1,2].
The exchanger consists of two identical parts, which are interchangeable. In the event of a fault, the device can also work with one part with reduced power [3]. In this way, it is possible to maintain the functionality of the device -maintaining the operation of the heating system, repairing a non-functional part of the exchanger and quick re-commissioning of the device. The proposed exchanger uses flue gas and water heat media without segment baffles.

Calculation and design of heat exchanger
Two basic equations are used to calculate the heat exchanger parameters.
Heat balance equation: To calculate heat exchanger length, we need to know overall heat transfer coefficient used to calculate necessary heat transfer area of heat exchanger. Input parameters are listed in Table 1. Dimensions of heat exchanger are shown in Figure 2. Heat transfer coefficient consist of convection coefficient in pipe side (flue gas) and of convection coefficient of inner -tube space (water). To calculate the coefficients, it is necessary to know the properties of individual heat transfer media at the points of their mean temperatures [4]. Table 1. Heat exchanger design inputs [5].

Inner pipe space -water
Water will flow in inner pipe space. Inner diameter of pipe is 32 millimetres. The water properties are calculated for an average temperature of 65 °C, flow of 0.59 m 3 h -1 .
To calculation are used criterial equations. Scheme of tube bundle is shown in Figure 1.
Water properties are shown in Table 3.

Final design of heat exchanger
Results of calculation are shown in Table 4. Final length of heat exchanger is 1800 millimetres, and overall heat transfer coefficient is 7.42 W.m -1 K -1 .

Length of heat exchanger mm 1800
Width of heat exchanger mm 400 Overall heat transfer coefficient W.m -1 K -1 7.42

CFD design of heat exchanger
We designed CFD model for pipe area -flue gas in Ansys 20.1 -Workbench to comparison and verification our numerical model. CFD model was implemented in Fluid Flow (Fluent) flow analysis system. For the calculation was used simplified 3D model with flue-gas phase.
In model is used tetrahedral mesh shown inf Figure 3 with 2 508 553 elements. To calculation was used K-ε model with scalable wall function. Input parameters are listed in Table 5.
Setup converged [6,7].  As result of calculation was power of the exchanger, the resulting temperature Figure 4. and velocity field Figure 5. Heat power was calculated to 19.311 kW. From temperature field is obvious that outlet temperature of outlet flow of flue gas is 150 °C. Velocity field shown that mean velocity is around 2 m.s -1 .

Comparison of results from numerical model
Designed models allows comparison of numerical and CFD models. From final temperature field Figure 4. we can see, that outlet temperature is similar to input of numerical model. From velocity field Figure 5 we can see that that mean velocity of flow flue gas is similar to calculated velocity of numerical model. From Figure 6 we can see main difference between power curve calculated by numerical model and CFD model. This difference is due to different way of calculation overall heat transfer coefficient. To final calculation is necessary verify results with measurement of real model, design final equation, which can be used to calculate properties of same type heat exchanger as is shown in Figure 1 in future designs. Model is used to simulate heat transform from flue-gas to water.