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Mass produced thermostatic valve for water pasteurization
- 1 ABSTRACT
- 2 Introduction
- 3 Project Requirements
- 4 Design
- 5 Discussion
- 6 Progress and Further Considerations
- 7 Testing Procedure
- 8 References
The purpose of this project is to create a thermostatic valve that can be used in a solar water pasteurization unit. Water pasteurization is a vital part of maintaining health standards in less fortunate parts of the world. The development of this project came from the investigation into building a health care centre in Peru. Research on this issue returned the fact one of the largest health problems in Peru is the lack of safe and clean drinking water . Bacteria from unsafe drinking water can lead to serious health problems which cannot always be properly dealt with in countries such as Peru where the health care system may not have all the resources needed. To fix this problem it is proposed that people are given or are provided with the knowledge and resources to build their own household scale water pasteurization devices.
It is currently common practice to boil water in order to get rid of all unwanted contaminants. In reality the water only needs to be heated to a temperature of 65°C in order for all disease causing bacteria to be killed off . A solar device can easily heat water to this temperature and uses much less energy than if the water was being brought to a boil. A water pasteurization unit makes use of a thermostatic valve that responds to changes in temperature and can therefore open and close at threshold temperatures.
In order to achieve such a task, two thermostatic valve designs have been created with the above intentions. Many factors were taken into account when coming up with the designs. The main task at hand is to create a valve that is easily manufactured through mass production, so simplicity governs the designs. However other factors such as reliability, functionality and cost are also considered in the design. The valves must be fully functional and not let any dirty water through. Most thermostatic valves currently on the market are fairly expensive and range from $50 US to as much as $300 US , therefore one of the main criteria is to minimize this cost since the valve will be used in impoverished areas.
The objective of this project is to design and implement a thermostatic valve that can be easily mass produced for a solar water pasteurizing system. A thermostatic valve is an automatic device for regulating temperature that controls the supply of water to or from a heating apparatus . This valve will need to open at a specified threshold temperature of 70°C to allow pasteurized water to flow into a clean water reservoir. The valve must also close at a temperature of 65°C to insure no contaminated water reaches the clean water reservoir. The valve must be perfectly sealed as it is absolutely imperative that not a single drop of infected water makes it through. Furthermore, the valve must contain as few moving parts as possible and must be able to work for years opening and closing many times in a day. The valve should be as small as possible and ideally made for a 0.250” I.D. x 0.375” O.D. polycarbonate tube.
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When trying to come up with a design for a thermostatic valve, many factors were taken into account. First and foremost it must be functional. Another important factor in the design is its suitability for mass production. This means simple and as few parts as possible and inexpensive. Simplicity plays its part in what mechanism is used, as in how the valve actually functions. The more complicated the mechanism is, the more expensive it will be to create. Fewer parts means easier to produce and therefore can sell for less, which is beneficial for people in developing countries. In order to keep costs low, both materials and processes need to be considered. The following two designs were created with the intention of addressing the factors listed above. These thermostatic valves if ever put into mass production must be very cheap and manufactured with materials that ensure durability.
The first design involves a piston movement valve mechanism that opens and closes based on the expansion of some medium. In this design, wax is considered the most likely candidate for the medium. The heated water enters the piston casing and encircles the expansion chamber. A circular water heating chamber was considered to be more effective than a small inlet, for the circular design allows for even heat distribution across the expansion chamber wall and also within the expansion chamber itself. A spring wrapped around the piston rod keeps the valve closed at temperatures below 70°C. The valve opens when the force of the expanding medium overcomes the force of the spring holding the valve closed. However, once 70°C has been reached, there should be sufficient expansion as to allow for water passage.
One of the possible issues with this design is that the piston may become stuck open after continual use. There is also the issue of seals, for if the parts are not properly sealed, contaminated water can leak through the valve. This design also has many complexities and therefore material usage must be reduced to lower costs. One way to reduce the amount of material used is to omit the whole portion of the design that rests on top of the pipe and just have the mechanism entirely inside the pipe. This eliminates all the extra materials that would be needed in order to construct this extra part.
While the actual mechanism of the second design is simpler, the medium which will provide the expansion is more difficult to modify. This design consists of a metal coil wound around a central rod which is attached to a swing valve inside the water pipe. When the water heats the metal, it will expand, causing the rod to twist and open the valve, thus allowing water to flow through. The main issue with this design is that the swing valve may open prematurely due to the linear expansion of the metal, allowing contaminated water to get through. Another issue is that after continual usage, the metal coil may become weakened to the point of failure or may not close properly resulting once again in the allowance of contaminated water flow
One of our biggest problems in the design process is choosing suitable materials. The chosen material needs to be able to do the desired task and also be cheap and readily available. When determining which material will be the best to use in our prototype, there are a number of aspects that need to be considered. Much of the primary research was focused on the expanding and contracting material that will inevitably dictate how well the thermostatic valve works. The following is the order in which we ranked these factors:
- Material Properties vs. Temperature
- Thermal Expansion Coefficient
The properties and expansion coefficient of a material are important deciding factors because it is these qualities that will determine how functional the overall device will be. In this particular project, the material must expand at a temperature slightly higher than 65⁰C, and must have a relatively high thermal expansion coefficient. A higher thermal expansion coefficient means that the material will expand by a larger factor.
The price of the material is also very important because one of the main objectives is to produce a valve suitable for mass production. Cost is very important when thinking about mass production because so many of the same products are going to be made and therefore a lot of materials are going to be required. Reliability is the next important aspect to consider, because the valve must be completely functional with no faults or flaws. The material must expand and contract at the correct temperature and also expand enough to actually open the valve. It is also good to be aware of the availability of all the materials being considered. If a material is chosen that is not available in great quantities, it will be impossible to acquire enough materials in order to mass produce the valve. Lastly, the purity of our expandable material must be judged. We cannot have any harmful toxins leaking into our water supply. The water running through the pasteurizer can be contaminated with the slightest irregularity of the expandable material.
When looking for a material that has all the necessary thermal expansion properties and is also inexpensive, some sort of rubber or wax is the clearest option. There are many different kinds of waxes, all with different melting and expansion properties. Generally, waxes are solid at 20⁰C and melt at a temperature somewhere above 40⁰C . For the purpose of this project a wax is needed that expands at 70°C and contracts at 65°C. Waxes have a low viscosity at temperatures just slightly above their melting point, and have a plastic nature that allows it to deform under pressure .
There are two main types of wax, natural waxes and synthetic waxes. Natural waxes include animal waxes like beeswax and lanolin, vegetable waxes like soy and carnauba, and mineral waxes such as ceresin and paraffin . Then there are synthetic waxes such as ethlenic polymers, chlorinated napthalenes and hydrocarbon type. For the purpose of this project three waxes in particular were researched with more detail. The first is beeswax, which is a natural animal wax. It burns slowly and cleanly with a melting point of 65.55°C, which is very close to the desired temperature of 70°C. If beeswax were to be used it would have to be modified to melt at a higher temperature to leave room for error. The greatest advantage of using beeswax is that it is very cheap and can be found in wide availability . The second wax is paraffin, which is a natural mineral wax. Paraffin has varying melting points so it may be useful when trying to tweak material properties to arrive at the right temperature expansion. While paraffin is also cheap, it is made from refined oil. A fully refined paraffin wax typically meets FDA criteria for use in food packaging products and is also biodegradable. It is available in a variety of melt points and is a hydrophobic material. The third and final wax researched is a distilled wax called Astorstat®. This is a wax that is currently used in thermostatic valves of different purposes. It has customized characteristics, durability and a narrow range of melting temperatures . So far it has been difficult to find an existing wax that fits all the specifications. For this reason it will probably become necessary to experiment with mixing various waxes in order to create a medium that will have the desired characteristics
This aspect of the project focuses on making a valve that is most suitable for mass production. This leads to a study of what methods are best used for mass production and which ones can be used in the production of a thermostatic valve. On the manufacturing side of mass production, the most commonly used method is that of an assembly line . The use of an assembly line allows for maximum productivity as the production is split into several different steps. This process is both cost and time efficient. In both of the proposed designs for the thermostatic valve, some sort of casing is involved. One of the materials being considered for this casing, as plastic is both durable and inexpensive. There are two main methods of manufacturing plastics that are cheap and easy. The first is injection moulding and the second is blow moulding. Both of these methods make use of moulds, which is good for mass production because it creates consistency . The only problem is that the moulds themselves can be rather expensive, but depending on how successful the resulting product is, this cost can easily be compensated .
Various methods of metal manufacturing were also researched for the possibility that metal gets incorporated into the final design. Design 2 involves the use of a metal coil and there is even a possibility of using metals in design one for the casing around the expansion chamber. This is because metal is a better conductor than plastic and will more easily transfer the heat from the water to the expanding material.
Some common forms of metal manufacturing involve casting, forging, rolling and machining. The downside to these methods is they can be a little more expensive than the plastic manufacturing methods mentioned. A solution to this problem could be a process known as Metal Injection Moulding (MIM) which is a combination of injection moulding and powder-metal sintering . Another possible method is a metal nanoparticle method. This method involves forming a mixture through dissociation of a metal precursor in a fatty acid and then adding a catalyst . The pros of these two methods are that they are cheaper than other more common forms of metal manufacturing. MIM costs about the same as its counterpart plastic injection moulding and still retains the strength of the metal. The nanoparticle method is environmentally friendly because the process does not use a reducing agent. The cons of metal manufacturing are that even with reduced costs of newer methods it can still be a very costly and complex process.
Progress and Further Considerations
So far, most of the focus has been on researching thermostatic valves currently on the market. This allows for the formation of a deeper understanding of the specific function of the device that needs to be designed. Two designs have been proposed and are currently undergoing a compare and contrast stage of planning. By looking at how one design could possibly be better than the other, possible flaws or problems can be brought to the surface. Then using this analysis the best design for the desired specifications can be chosen. The main difference between the two is the expandable material that is used. Different expanding materials have been researched in an effort to decide which design would work best. A ranking scheme for both the material and the overall design was created to determine the factors that are most important developing a final product. Thought and research have also been put into possible manufacturing processes that could be used for mass production.
The next step needed for further development of the design is to research different prices of materials. More research also needs to done in regards to what material will be used for the outer casing of the device. From here, the ranking system will be used to decide exactly what materials will be both most cost efficient and most functional. A tentative plan for the coming weeks includes having all this research done by week 7. During this process, evaluation of the design also needs to take place in order to identify any areas that could be improved. This includes looking at the situation from all angles and brainstorming anything and everything that could potentially go wrong and how these problems can be fixed or avoided altogether. At this time different methods of testing both the materials and the whole design will be considered. A plan to have a prototype constructed by week 8 is in place. The testing and modifying will happen throughout weeks 9 and 10. During week 11 final evaluation of the design will take place and final results will be recorded in the final report.
Recently we have obtained a pump and some parrafin wax that we will use to begin conducted testing on how much the wax expands. Then taking this information we can start thinking about how we are going to amplify the expansion so that we reach the final desired result.
The purpose of these experiments is to determine a suitable expansion material, observe the conditions in which the materials expands, and view the way the piston moves based on this expansion.
It is believed that the materials that will be used will expand at the desired temperature (70°C). However there may be unforeseen obstacles that have yet to be addressed in the design, this is why this preliminary testing is taking place. It is not clear by how much the piston will move due to the expansion of the material, but it should move if the expansion chamber is completely sealed.
Materials Required for Testing:
1) Ball Pump: this is the piston mechanism in which the material will be inserted and heated. 2) Para film Wax: this is the expansion medium in question, melting temperature is generally around 71°C. 3) Candle Wax: this is also another expansion medium that can be tested, unlike the Para film there are many various types of candles with different compositions, therefore it is somewhat doubtful that candle wax will be used in the final product. 4) Heating Plate: apparatus used to heat the water surrounding the ball pump. 5) Beaker: holds the water and the ball pump. 6) Thermometer: records various temperatures of the water in the beaker. 7) Stopwatch: records time with respect to temperature, this will allow for the observers to see at what time the ball pump moves if at all. 8) Ruler: used to record the linear movement of the piston with respect to temperature and time.
• Water may leak into expansion chamber and interfere with piston movement • Heat transfer between water and expansion chamber may be inadequate • Linear movement may be so small that tweaking design may be extremely difficult • Expansion may occur at higher or lower temperature than required resulting in contamination or waste of energy • The friction of the piston may be to great resulting in the valve not opening or not closing Procedure:
1) Acquire the necessary materials and set up the following apparatus as illustrated in the diagram. 2) Add the necessary amount of water to the beaker as to completely immerse the ball pump, place thermometer in beaker, turn on heating plate and start stop watch. There probably won’t be any movement for awhile, but still have the ruler ready to record movement. 3) Record the following sets of information (the time interval used will be 30 seconds until a temperature of around 50°C is reached, this interval is shortened to 10 seconds above this temperature); time, temperature, and piston movement. Continue the 10 second time interval until a temperature of 100°C. The following temperatures are not based on any theoretical testing, there are simply arbitrary valves that can be changed but for ease of testing, these valves have been chosen because the desired temperature is nearly half way in between the lower and upper test temperatures. 4) Once testing is complete, allow to cool, remove pump, inspect the ball pump for leaking and also remove the Para film wax. Take observations and record the condition of the wax. Has anything changed? Consistency? 5) Change expansion medium and redo procedure if necessary.
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