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 anefficient firebox hard. Modern emission standards make fireplaces and industrial fireboxdesigns 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 lotof error possibility and high production costs. Further problems that production methodsrequire 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 notseasoned, wet firewood that drastically reduces efficiency.

In this paper I show a possible solution, that can eliminate most if not all the abovementioned drawbacks and the same time makes it possible to build eco friendly products forboth 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 toxicpainting)

MultiScience - XXXI. microCAD International Multidisciplinary Scientific ConferenceUniversity of Miskolc, Hungary, 20-21 April 2017

ISBN 978-963-358-132-2

DOI: 10.26649/musci.2017.088

2.2 Possible materials

As mentioned above, only reusable and natural materials will be applied in this design. InTable 1 we review some of the possible material group choices and their usability.

Material Heat resistance

Heat conductivity

Reusability Required energy forproduction

Heat shock resistance

Usability

cast iron high medium High high medium Bad

steel high high High high high Bad

clay medium low-medium

complete medium-high low-medium bad-average

fireclay high low-medium

complete low-medium medium-high Excellent

vermiculite high low-medium

medium-high

low-medium medium-high Good

Table 1. Material requirements

As we see the clay type and vermiculite materials are far superior in firebox building andhave 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 rocketmass heater, originated from Dakota fire pit.

Figure 1. Rocket mass heater [2]

Rocket-mass heaters are hand built heaters mostly made of natural clay and dirt. Amore durable design can be made with the use of firebricks. Building a mass heater is a verytime consuming DIY project and not without difficulties. One of these difficulty is that thereis no adequate blueprint for every size, and model type so it can be built only byexperimenting with measurement.

Home used and some industrial heaters are using the batch type firebox. In thesetypes 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 thiscase only a portion of the wood is burning and there is room around the burning zone for theair to mix with the flue gases. In this way a much cleaner combustion can be achieved. Theother 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

From these 3 types of block any simple and complex self loading firebox can be built in afew minutes or in a few hours depending on the complexity. The first prototype providedvaluable 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 capacityThe 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 productioncan be made with wooden or steel molds. No heating or any high energy procedure isrequired.After the first tests a new fire block design was made.

Figure 4. Advanced fire chamber design

The advanced T-shape fire chamber eliminates the ash storage problem and adds a supplyair pre heating function to the original blocks; so, the prior combustion chamber isimproved.

The above mentioned blocks are scalable for any power range. The prototype has thepotential to heat an average sized room despite of the small size 10x10x20 cm. Theprototype has a potential heating power of 3kW if it is combined with a heat exchanger andadequate 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 distributionin the material. A slow but steady heat exchange is necessary for good comfort in a roomwhile the heat distribution affects the smooth heating of the living space.

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/m3

– specific heat capacity: c=0,88 kJ/kg K⋅– thermal conductivity: λ=0,84 W/m K⋅

Boundary conditions:- firebox inner temperature : 900 Co

- block maximal outer surface temperature : 100 Co

- external temperature : 20 Co

- 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

3.3 Heat exchange

Fig. 6 exhibits the heat exchange in the cooling block at time t=6s

Figure 6. The heat flux and heat exchange in the cooling period at t=6s

4. Conclusions

The simulations show that the system has very steady heat transfer and heat distributionability. 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 incomparison with single designs. This technique can be used in any possible heating situationwithout compromise.

Acknowledgement: Special thanks for my mentors Gabriella, Vadászné Bognár and Ferenc,Szabó J. for their valuable help in my project.

REFERENCES

[1] http://www.kamintechkandallo.hu/cikk/2011augusztus.htm 2017.02.15.

[2] https://s-media-cache-ak0.pinimg.com/originals/fe/d0/f5/fed0f5b10ec7119f1db0007977cee92d.jpg 2017.02.15.

[3] Homonnay Györgyné, Molnár Zoltán: Fűtéstechnika, Műszaki Könyvkiadó Budapest 1979

[4] Épületgépészet 2000 - I. Alapismeretek, Épületgépészet Kiadó Kft Budapest, 2000

[5] Épületgépészet 2000 - 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