Phase SummaryDuring this phase we split our design into 2 unique sub-systems. A leaf drying system that will target the drying of tea leaves. And a water pasteurization system which will aim to heat water to 70 deg. C.
Although the analysis of these sub-systems was conducted individually there will be significant overlap. Risks and engineering requirements were taken into account, and edited where necessary. Overall, both sub-systems pass our feasibility analysis and we plan to move forward with our designs.
Team Vision for Subsystem-Level Design Phase
For a complete list of action items our team planned to accomplish during Phase II, please click here.
For a list of actual action items and their progress during this phase, please click here.
Link to phase 3 risk assessment
Link to phase 3 Engineering Requirements Spreadsheet
Feasibility of Leaf Dryer Sub-system
- Will the fire produce enough heat to dry the tea leaves?
- Will the dryer be within the cost constraints?
- Will our team have access to the tools and materials needed to build the dryer?
- Can the dryer be built with local materials found near Nepal?
- Will farmers be able to construct the dryer with local tools and manufacturing processes?
- Will the dryer compromise the existing Kon Tiki Kiln?
- Will the dryer alter the quality of the biochar?
- Clay dehydrator box
- 4 wood frame trays with wire mesh
- Metal sheet for bottom of box
- 1-D heat transfer
- Phase Change
- Standard atmospheric pressure
- Heat loss as moisture travels through leaf to its surface is negligible
- Neglect relative humidity of air
- Initial leaf moisture content is 82%
- Final leaf moisture content is 2%
Feasibility Analysis and Conclusions
1. Will the fire produce enough heat to dry the tea leaves?
The data shown above was then put into a spreadsheet so variables could easily be changed. The graph below shows the sensitivity of 'flux needed' to 'change in time'.
Currently the leaves are dried for 15 minutes. At 15 minutes, a flux of only 4 kW/m^2 is needed. A time between 10-20 minutes (flux between 3 and 6 kW/m^2) may be best as the slower drying time increases the quality of the tea flavor. However, there is a trade-off between flavor quality and storage quality.
From this analysis, there is definitely enough heat provided from the fire to dry the leaves. Actually, a concern is it might provide too much heat. This analysis was done assuming the leaf initially had a high moisture content. This may not always be the case, and a lower moisture content would shorten the time needed even more. However, this analysis is just a rough estimate and in actuality it may take a longer time at that flux than shown as this analysis didn't take into account the humidity of the air.
2. Will the dryer be within the cost constraints?
Most of the materials needed for our design are cost effective. However, the cost of clay may pose an issue.
3. Will our team have access to the tools and materials needed to build the dryer?
According to our BOM, our team will have access to the materials and tools needed to build the leaf dryer. However, some materials like sheet metal and clay are costly.
4. Can the dryer be built with local materials found near Nepal?
Material cost and accessibility differs between the U.S. and Nepal. Our design uses materials like clay and wood that are available in Nepal. Hinges and wire mesh may be more difficult to buy or make in Nepal.
5. Will farmers be able to construct the dryer with local tools and manufacturing processes?
Farmers in Nepal should be able to make the clay box and wooden trays with local tools. The more difficult manufacturing process is the wire mesh and access to a staple gun.
6. Will the dryer compromise the existing Kon Tiki Kiln? The leaf dryer is not in direct contact with the kiln. Therefore, it will not compromise the existing Kon Tiki kiln.
7. Will the dryer alter the quality of the biochar?
The design is using heat that the Kon Tiki is releasing into the atmosphere, so it does not interfere with the pyrolysis process needed to make biochar.
Feasibility of Water Pasteurization Sub-System
- Feed tank
- Flex tube inlet
- Heat exchanger
- Flex tube outlet
- Temperature regulator
- Output tank
- Tank rack
- Heat exchanger mount
- Pulley system
- Average temperature above kiln: TSource=200 deg. C
- Average heat flux from burn: Q’’=200 kW/m^2
- Min-max burn time: t= 1.5 - 4 hours
- Initial water temperature: T1= 27 deg. C
- Final water temperature: T1= 70 deg. C
- Volume of water needed by 1 person per day: Vperson/day= 1.65 L
- Number of people per family: #Family= 4 people
- Volume of water needed by 1 family for 1 week: 46.2 L
- Diameter of pipe: DPipe=2.54 cm
- Length of pipe: LPipe= 2.26 m (circumference of kiln ½ way up)
- Input/output tank sizes: Vtank= 512 L (0.8 m * 0.8 m * 0.8m)
General System Analysis
- No phase change of water
- Constant properties (k, U, Cp)
Heat Transfer Analysis
- Neglect radiation losses from kiln to environment
- Neglect radiation losses from heat exchanger to environment (all goes to water)
- Neglect heat transfer resistance
- Uniform heat flux across system
- Uniform temperature output across system
Fluid Dynamics Analysis
- Laminar flow of water
- Neglect head losses due to bend and friction
Numerical Analysis and Results
This is the list of equations used in the heat transfer and fluid dynamic analysis.
This is the data from the heat transfer and fluid dynamic analysis. Important values are highlighted with a yellow background.
Results show that accepted heat output is greater than the maximum calculated value. Accepted output is found from Source 2 in the Documentation section below.
- (Q=36,068 W) > (Qmax=31,199 W)
In minimum burn time (1.5 hours), we need to produce at least 46.2 L water to be useful.
- (46.2 L)*(1 kg / 1 L) / (5400 s) = (mdot)min= 0.0086 kg/s
Results from both heat transfer and fluid dynamics calculations show that minimum calculated mass flow rate is greater than required minimum mass flow rate.
- From heat transfer analysis: (0.0086 kg/s < 0.0087 kg/s)
- From fluid dynamics analysis: (.0086 kg/s < 0.7094 kg/s)
Results from coefficient analysis prove that the assumed heat transfer coefficient values in the heat transfer analysis are accurate and that they fall within our estimated range.
This is a preliminary model of the water sub-system. Water will be loaded into the top tank, flow through the heat exchanger, and then into the lower output tank. The heat exchanger will hang from the rack above the kiln and will have the ability to be raised and lowered (to get closer to the heat source as well as move when the user needs to add more feedstock).
Bill of Materials (BOM)
BOM of Leaf Dryer Sub-system
Excel link for BOM spreadsheet.
Flow Charts, Drawings, Schematics, etc.
Leaf System Functional Architecture
Water System Functional Architecture
Final Concept Functional Architecture
- “Household Pasteurization of Drinking-water: The Chulli Water-treatment System” Mohammad Fakhrul Islam, Richard B. Johnston <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3013256/>
- “Heat Transfer Principles in Thermal Calculations of Structures in Fire” Chao Zhang, Asif Usamani <http://www.sciencedirect.com/science/article/pii/S0379711215300138>
- “Heating Water in a Copper Pipe From a Wood Oven”. Matt Sawyer <http://www.scienceforums.net/topic/65916-heating-water-in-a-copper-pipe-from-a-wood-oven/>