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Kingston Hot Press: Process Improvements
Check out the project on wasteforlife.org Hot Press Discussion Page
Currently, I am investigating the potential to design a plastic/paper composite extruder which would facilitate the production of feedstock for the Kingston Hot Press.
- 1 Introduction
- 2 Problem Definition and Scope
- 3 Constraints
- 4 Prior Art
- 5 Theory and Methodology
- 6 Final Design and Analysis
- 7 Cost Analysis
- 8 Conclusions and Recommendations
- 9 Future Work
- 10 References
The organization Waste for Life (WFL) defines itself as "a loosely joined network of scientists, engineers, educators, architects, artists, designers, and cooperatives who work together to develop poverty-reducing solutions to specific ecological problems." Through a collaboration with researchers and community members at Queen's University, the Centro Experimental de la Produccion (CEP) in Argentina, the Rhode Island School of Design (RISD), Smith College, and the University of Western Australia, the Kingston Hot Press has been designed and developed to provide the means of production to smaller cooperatives in communities in Argentina and Lesotho. The Hot Press allows the user to produce a value-added composite tile out of waste plastic and fiber (most commonly cardboard and paper). Currently three prototypes have been built, one at Queen's, one at RISD, and one at CEP. Detailed design drawings are available at the WFL website.
Problem Definition and Scope
The WFL team has identified several key areas of design development that they would like to pursue :
- Dimensions - The current Kingston Hot Press can produce a 24"x24"x1/4" sheet. A wider gap in the press could allow more width and perhaps allow for 3D molds. The size constraints could be circumvented if pieces could be produced in modules and then connected post-production.
- Heating Sources - Currently the plate heaters require electricity. CEP has expressed interest in a gas powered system.
- Temperature Consistency - Initial tests had shown inconsistencies between the temperature distribution on the top and bottom plates. To remedy this, researchers at Queen's replaced the 1/4" steel heating plates with 1/2" aluminum plates.
- Opening between top and bottom plates - Currently molds are slid between the heating plates. A "clam shell" lid design could allow for a more diverse range of geometries and facilitate the use of 3D molds.
- Safety and Environmental Issues - Fire safety, emissions, pollutants.
- Production Speed - The current system requires a time intensive production process to arrive at a single 1/4" composite tile. CEP has expressed interest in finding ways to improve throughput.
Currently the temperature consistency problem has been resolved with very expensive aluminum plates. A heat transfer model could assist in the evaluation of different solutions and hopefully provide an optimal solution that could use less costly and more widely available materials.
There are several key stakeholders that I have identified for this project. Here at Queen's I am working to assist Dr. Matovic with design improvements based on the Hot Press prototype in Kingston. More broadly, I hope my work can contribute to the Waste For Life team. Finally the end-user of the hot press is the cartoneros, the workers who partake in the informal economy of waste in Argentina.
I would like to develop a useful heat transfer model which could assist the WFL team in reducing the cost of the Hot Press. I would also like to take this opportunity to expand the reach of the WFL team and share their innovative design. Finally, I would like to provide a clear pictographic instruction manual for users of the Hot Press.
The hot press should provide a low-cost tool to access the means of production and add value to "waste" products. As such, materials should be as economical and accessible to the carteneros communities as possible.
Design work must be limited to theoretical analysis as significant empirical testing would require equipment not currently available for the budget and scope of the project. Future work could include an empirical evaluation of several plate designs using an array of thermal transducers (thermistors, or thermocouples) to determine realized temperature gradients in the hot press.
At Queen's University Dr. Matovic has produced CAD drawings fully detailing the design and dimensions of the Kingston Hot Press.
The original design used 1/4" steel plates to press and heat the tiles. To overcome temperature inconsistencies which were producing burnt profiles the prototype plates were replaced with 1/2" aluminum plates.
Theory and Methodology
Although the design of a hot press plate must ideally involve both a heat transfer model and a finite element static stress analysis it will be assumed for the following optimization that based on previous work by the WFL team, the 1/4" steel plate provides a minimum benchmark thickness for the plate and a lower limit for allowable bending and deformation of the plate. Therefore the following analysis will focus on better understanding the heat transfer mechanisms in the heat press, and how these effect the performance of the press.
(see also the Wikipedia article)
The Kingston Hot Press presents a particularly challenging heat transfer system to model. Six 750W Omega OT-2107 strip heaters are clamped to the two "press" plates which transfer heat and pressure to the mold and tile composite material. A cork rubber gasket provides insulation between the press plates and the steel weldments which provide the necessary structural support for the device. A simple on/off controller regulates the temperature of the center of the plate with a thermocouple transducer. Standard setpoints range between 150°C and 250°C. Since a tile (or film) can be pressed within 5-35 minutes depending on the thickness of the mold, the problem almost certainly falls within the transient time period. However, the plates are preheated to the setpoint temperature, and so a steady state model can assist in determining the ideal performance of the heating plates upon the initiation of the press cycle.
A preliminary 2D model of the plate was developed to allow for a general understanding of the behaviour of the plate which could be made accessible for the community through open office. Using a 2D finite element control volume approach, a rough estimate of the temperature distribution can be arrived at in a simple spreadsheet. Using each cell as a finite control volume the energy balance equation can be used to derive an equation for the control volume (spreadsheet cell) temperature.
The energy balance equation for a 2D control volume is given by:
This simplifies in a medium surrounded by a material with identical conductivity to the average of the neighbouring cell temperatures:
Assuming heat loss through the edges due to convective heat transfer we can derive the temperatures for edge cells and corner cells:
These equations when put into Excel or Calc can be solved iteratively (See this help for Excel). The screenshot below shows the three strip heaters (approximated as constant temperature surfaces) surrounded by the aluminum plate and the resulting temperature distribution. Although this is a rudimentary model, it provides an immediate sense of the geometry and gradient in a way that can be easily distributed and modified through OpenOffice.
Final Design and Analysis
|0||Step 0: Turn on the Kingston Hot Press to preheat. This will save you time later.|
|1||Choose dimensions (length, width, and height) and cut wood. The device requires 2 pieces of each the length and width, and 8 of the height.|
|2||Create 2 U shaped frames (U-1), with the thick part of the timber forming the thickness of the frame. Nail the base to the other two pieces as shown in the figure by the arrows.|
|3||Create 2 more U shaped frames (U-2). The thick part of the timber should still be the thickness, but this time attach together as shown by the arrows in the image.|
|4||Cut the jute cloth and chicken wire to fit the four frames that were created. This should correspond to pieces about 1ft x 1ft. 8 pieces of jute cloth and 9 pieces of chicken wire are required.|
|5||Fasten the jute cloth to one side of each frame using the finishing nails. Place one nail in each corner, and additional nails as required. A staple gun and staples could also be used for the fastening.|
|6||Fasten the chicken wire on top of the jute cloth of each frame. Fastening nails can be used, but must be bent over the wire mesh to hold it in place. Care should be taken when handling the mesh wire as edges are sharp.|
|7||On each of the U-1 frames, fasten both the jute and finishing wire to the other side.|
|Now there should be:||
|8||Nail the U-2 frame to the U-1 frame to form a three dimensional L-shape. Nail locations are indicated by the arrows.|
|9||Nail the other U-2 frame to the U-1 frame to form a three dimensional U shape.|
|10||Measure the board to fit the bottom of the cooler. Cut the board at the appropriate length. Nail the board onto the bottom.|
|11||Fasten the jute cloth and chicken wire to the remaining two sides on the outside of the cooler.|
|12||Attach three nails on each of the edges of the frame, pointing diagonally towards the middle of the cooler.|
|13||Using a piece of chicken wire, form a shelf in the middle of the box. This is done by weaving the mesh onto the protruding nails. Test the shelf by putting some pressure on it to see if it will be able to hold food. As a substitution, a board may be used to form the shelf, or woven reeds/bamboo, however a non solid material will be more effective.|
|14||Attach the hinges to the open face of the cooler.|
|15||Attach the remaining U-1 Frame to the hinges to form a door for the cooler. If the door does not close, a latch can be installed to hold it closed as necessary.|
|16||Fill the cavities formed by the jute cloth and chicken wire with charcoal. The charcoal should be evenly dispersed throughout the cavity. Charcoal should be in chunks about 0.5cm in diameter. The mesh wire should be strong enough to hold the charcoal in place and prevent the cavities from bulging.|
|17||Tie the end of the hose. Pour some water into the hose to ensure the tie is sufficient to block the end of the hose. If tie is not sufficient, a stopper must be used to prevent water from flowing through the hose.**|
|18||Starting at the opening of the door, lay the hose over the open sides of the box. Fasten the hose in place by using the ties to attach the hose to the mesh wire. Ensure that the holes are pointing down into the charcoal filled cavities.|
|19||Poke holes along approximately 4’ of the hose. The holes should be spaced about 0.5-1cm apart and can be made using a nail. The size and spacing of the holes takes a bit of experimentation and depends on the rate of evaporation for the given climate. The charcoal should be kept continually moist, but should not be so wet that it is dripping out the bottom of the cooler. The flow rate of water though the holes should therefore equal the rate of evaporation. If the holes made are too large, candle wax can be used to fill them, and new holes can be created through the wax with a pin.|
|The device should now look like this:|
|20||Place cloth or woven reids across the top of the box and fasten it in place.|
|21||Attach the free end of the hose to the base of an elevated bucket. As the bucket is filled with water, the water will trickle into the cavities, mositening the charcoal and cloth material.|
- ** If hose is unavailable and tins are used, the tins can be fastened to the top of the frame, with holes poked by nails on the charcoal cavities. If this method is used it is recommended that the tins have lids to prevent evaporation of the water from the surface of the tins.
Conclusions and Recommendations
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