MODULAR FIREBOX DESIGN AND ANALISYS. Miklós Gábor Várkuli Mechanical Engineer student University of Miskolc, Institute of Machine and Product Design

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1 MultiScience - XXXI. microcad International Multidisciplinary Scientific Conference University of Miskolc, Hungary, April 2017 ISBN MODULAR FIREBOX DESIGN AND ANALISYS Miklós Gábor Várkuli Mechanical Engineer student University of Miskolc, Institute of Machine and Product Design 1. INTRODUCTION In modern heat generation there are a lot of challenges, that makes designing an efficient firebox hard. Modern emission standards make fireplaces and industrial firebox designs that use solid fuel a high cost investment. The main reasons are the following: - high durability heat resistant materials, - active controlled combustion with fans, - CPU controlled flue-gas, temperature sensors, - primary, secondary and tertiary combustion air control. The above mentioned systems make the designs more complex than it should be with a lot of error possibility and high production costs. Further problems that production methods require high energy and the end product has a bad recycling value. From the user side, there is the improper use of devices, e.g., heating with not seasoned, wet firewood that drastically reduces efficiency. In this paper I show a possible solution, that can eliminate most if not all the above mentioned drawbacks and the same time makes it possible to build eco friendly products for both industrial and private use. 2. DESIGNING THE FIREBOX 2.1 Main goals Here we introduce the main goals from designing and construction perspective: - simple yet durable eco friendly materials - simple and efficient firebox geometry - simple construction with minimal tool requirements - end product can be repaired or replaced without special tools or materials - repairs can be initiated safely without any specialised knowledge Moreover, the goals form user side are the following: - safe system with long life-span - can be built as a DIY kit project - safely maintainable and repairable by the owner without any specialised repairman - non toxic materials used in the product (no unstable polymers, heavy metals, or any toxic painting) DOI: /musci

2 2.2 Possible materials As mentioned above, only reusable and natural materials will be applied in this design. In Table 1 we review some of the possible material group choices and their usability. Material Heat resistance Heat conductivity Reusability Required energy for production Heat shock resistance cast iron high medium High high medium Bad steel high high High high high Bad clay medium lowmedium vermiculite high Usability complete medium-high low-medium badaverage complete low-medium medium-high Excellent fireclay high lowmedium lowmedium mediumhigh low-medium Table 1. Material requirements medium-high Good As we see the clay type and vermiculite materials are far superior in firebox building and have a low energy requirement for production purposes. In this design fireclay will be used. 2.3 Firebox types The inspiration for my model was an existing wood fired design, the rocket stove and rocket mass heater, originated from Dakota fire pit. Figure 1. Rocket mass heater [2]

3 Rocket-mass heaters are hand built heaters mostly made of natural clay and dirt. A more durable design can be made with the use of firebricks. Building a mass heater is a very time consuming DIY project and not without difficulties. One of these difficulty is that there is no adequate blueprint for every size, and model type so it can be built only by experimenting with measurement. Home used and some industrial heaters are using the batch type firebox. In these types of heaters all of the fuel is put into the firebox at once and the entire batch will burn. This kind of burning has no adequate air and flue gas mixing. A more reliable solution is the continuous burning variant like above in Fig. 1. In this case only a portion of the wood is burning and there is room around the burning zone for the air to mix with the flue gases. In this way a much cleaner combustion can be achieved. The other fact is that the small chimney-like fire tube, reaches a much higher initial temperature, than a big batch type firebox. The high temperature makes cleaner burning possible. In my design I used these principles and combined it with modular design. 2.4 Firebox part design The main design guideline was to keep modules as simple as possible. The designed modules are shown in Fig. 2. The 3D models were made in FreeCAD 3D designing and analysis software. Figure 2. T, L, I-shape firebox building blocks

4 From these 3 types of block any simple and complex self loading firebox can be built in a few minutes or in a few hours depending on the complexity. The first prototype provided valuable information on the strength and the weaknesses of the design. The advantages can be summarized as follows: - the combustion was clean, smokeless and odorless as predicted, with minimal warm up time; 1,5-5 minutes depending on the external temperature. - the blocks have high mechanical stress tolerance. (One of the blocks was accidentally fallen 1 m without any structural error, only with small surface scratches.) - the prototype is easy to repair only water and a small amount of the original material is needed to make the necessary maintenance. The drawbacks are the following: - L-shaped main fire chamber is hard to start at lower external temperatures. - T- shaped main fire chamber is easy to start but does not have adequate ash storing capacity The first prototype in action (no smoke) is exhibited in Fig. 3. Figure 3. Prototype These blocks were made with simple impregnated paper mold. Industrial scale production can be made with wooden or steel molds. No heating or any high energy procedure is required. After the first tests a new fire block design was made.

5 Figure 4. Advanced fire chamber design The advanced T-shape fire chamber eliminates the ash storage problem and adds a supply air pre heating function to the original blocks; so, the prior combustion chamber is improved. The above mentioned blocks are scalable for any power range. The prototype has the potential to heat an average sized room despite of the small size 10x10x20 cm. The prototype has a potential heating power of 3kW if it is combined with a heat exchanger and adequate radiation surfaces. Possible usage of blocks can be listed: - it can be used to build a small or large mass heater, - the heat exchanger can be attached to use as a central heating system, - more mass can be added to make it a combined heat generation and storage unit (no external fluid based heat storage is required) - it can be used as a waste burning unit, - an automatic refueling mechanism can be added for any type of bio fuel - the possible fuels are: wood, grass or any natural biomass, oils, biogas, - Peltier modules can be mounted to the surface as auxiliary power supply. 3. FINITE ELEMENT ANALYSIS The goal of the analysis was to get preliminary data on heat exchange and heat distribution in the material. A slow but steady heat exchange is necessary for good comfort in a room while the heat distribution affects the smooth heating of the living space.

6 3.1 Boundary conditions and starting values In the numerical simulations performed by commercial software ANSYS R17.2 the following settings were applied [1], [2], [3], [4], [5], [6], [7]: Firebox sizes: block outer measurement: 300x300x300 mm inner tube: 100x100x300 mm square cross-section Firebox material properties: material: fireclay density : ρ=2000 kg/m 3 specific heat capacity: c=0,88 kj/kg K thermal conductivity: λ=0,84 W/m K Boundary conditions: - firebox inner temperature : 900 C o - block maximal outer surface temperature : 100 C o - external temperature : 20 C o - base plate is fixed and there is no heat exchange in this direction 3.2 Heat distribution Fig. 5 exhibits the heat distribution in the blocks at time t=0s Figure 5. Heat distribution at t=0s

7 3.3 Heat exchange Fig. 6 exhibits the heat exchange in the cooling block at time t=6s 4. Conclusions Figure 6. The heat flux and heat exchange in the cooling period at t=6s The simulations show that the system has very steady heat transfer and heat distribution ability. These simulations are in accordance with the test run of the prototype. Modular building makes the design versatile and eco friendly, thus it is far superior in comparison with single designs. This technique can be used in any possible heating situation without compromise. Acknowledgement: Special thanks for my mentors Gabriella, Vadászné Bognár and Ferenc, Szabó J. for their valuable help in my project. REFERENCES [1] [2] [3] Homonnay Györgyné, Molnár Zoltán: Fűtéstechnika, Műszaki Könyvkiadó Budapest 1979

8 [4] Épületgépészet I. Alapismeretek, Épületgépészet Kiadó Kft Budapest, 2000 [5] Épületgépészet II. Fűtéstechnika, Épületgépészet Kiadó Kft Budapest, 2000 [6] Épületgépészet a gyakorlatban I kötet, VERLAG DASHÖFFER Szakkiadó Kft Budapes 2001 [7] Épületgépészet a gyakorlatban II kötet, VERLAG DASHÖFFER Szakkiadó Kft Budapes 2001