Science, Technology, Engineering, Manufacturing, and Mathematical Principles Involved in the Design of the Household Desalination Unit

Introduction

Fig. 1: The Atacama Desert of Northern Chile

The Atacama Desert (Fig 1.) has long been known as the driest on Earth. The average rainfall there is one millimetre per year, and some weather stations have never reported precipitation. The small towns clustered around the edges and near oases have not prospered, but have managed to survive. However, since the mining boom of the 1990s, large corporations have begun to take over the mineral-rich area. Chile's free-market water rights system allowed the mining companies to buy the rights to nearby fresh rivers and lakes to use in their plants, poisoning the only water source that the towns have. The only water resources left are the salt lakes on the flats and the oceanic inlets.

Fig 2: The final product

If villages had freshwater, they could maintain their culture. This group has designed and constructed a compact solar desalination unit (Fig 2.) for use on a per-household basis. The product will supplement the water supply of an Atacaman family without requiring undue maintenance or power. This working solution could be manufactured and spread throughout the world, helping millions.

Engineering
System Classification
In engineering, concepts and devices are classified as inventions or innovations based on whether or not the idea is unique. For example, the wheel was an invention – its like has never been seen before. Innovations, however, evolve existing ideas and capitalize upon them. Typically, the most pervasive form of an idea is its innovation, not the original invention. This can be seen everywhere in modern society, from rollerblades (adding together two existing inventions) to iPods (making existing technology more user-friendly).
This device falls into the classification of innovation. Solar humidification devices already exist, but this particular device builds upon that concept by making the technology more accessible through the materials and the user interface facility. Innovations are often concerned with bringing high concepts to the consumer masses, and this innovation serves a similar goal – to bring long-known technology to those communities which need it most.
System Category
Any engineering system can be categorized as open or closed depending upon how it interacts with its environment. Closed systems are basically completely autonomous, whereas open systems have some sort of interface with the environment,, be it of energy, matter, or information. For example, a pocket watch is considered a closed system in that it maintains its purpose regardless of input from the surroundings, but a human being (a prime example of an open system) is completely dependent upon and intrinsically linked to the environment in which it lives.
Our device is also an open system – it takes place in both matter and energy exchanges with the ambient environment. Obviously, the water added to the desalination chamber and taken from the storage receptacle constitutes a matter exchange, but the very efficacy of the device is dependent on a significant heat variance on the interface between the exterior and interior of the primary desalination chamber.
System Type
A system is typified by what sort of purpose it serves. Information systems serve to educate the user as to a specific set of facts. Economic systems strive to structure the flow of goods and principal in a certain area. This desalination unit system, however, is known as a technology system. Technology systems encompass physical devices that are useful to the human population, as well as systems of use and organizational techniques related to this. The desalination unit is a material object that betters the lives of the people that use the device, which is the end goal of technological systems.
Engineering Type
The type of engineering most closely involved in designing and constructing the primary component of the desalination unit was hydrodynamics – the study of the movement of water in a system. To properly optimize the process, many different components of the unit were examined. Within the intake and outtake pipes, the water is in open channel flow – it never experiences enough volume or pressure to eradicate the free surface of the water (the interface between the water and Earth’s atmosphere). Seals were constructed accordingly and the angle that the intake and outtake pipes entered and left the barrel took this open-channel flow into account.
During testing, hydrodynamics will again come into play. To find how quickly the unit is producing, the group will have to conduct flow measurement. To keep costs low and methods streamlined, the testing will occur within the auspices of the bucket-and-stopwatch method. The desalination unit will be filled and left in an ideal location for a solar day, and the amount of water in the final reservoir at sunset will be measured. This volume can be divided by time to give a rate of productivity. More specificity on this test is found in the “Math” section of this report.

Manufacturing
Type
In an ideal situation, no single type of manufacturing would suffice for the entire process. A mix of different principles would help balance the process. Prefabrication in the US or in another industrial country would probably be best for making the raw materials – the barrel, PVC, and other plastic accoutrements. However, in an effort to further help the Atacama region, the group’s goal would be to set up small assembly “workshops” where certain parts (the cone, the intake and outtake pipes, etc) could be assembled by Atacaman residents. These small workshops would benefit the people by helping them feel more invested in the product, and it would also provide income for those people who cannot find work on mines or farms. Within the workshops, construction would follow the English system, wherein workers would be trained to make parts and assemble entire devices, rather than simply shaping a part and passing it down an assembly line. This would increase the investment that the Chilean people would have in their desalination units.
Principles
Manufacturing of these systems would also follow the principles of leagile manufacturing – a combination of lean and agile manufacturing theories.
Lean manufacturing focuses on getting rid of waste – of materials, of time, of space, and of potential talent (among other things). Lean manufacturing requires simplifying your system, working with a low inventory and always prioritizing “value adding activities”, or those activities which add value to the final product. OEE (overall equipment effectiveness) measurements would be used to optimize the production of any processes undertaken, and the top goals of the entire design/construction cycle would be to improve quality and reduce waste.
Agile manufacturing is similar to lean in that it prioritizes quality and deliverables. It also emphasizes the necessity of being able to respond quickly to changes in demand or customer needs. This is most useful if the COC (customer order cycle, or time between order placed and delivery of unit) is short.
Leagile manufacturing combines these two systems to form a highly efficient and quality-driven enterprise, which is precisely the kind of business that should be constructing and distributing these units throughout the world.
Category
Since this device is heavily reliant on PVC pipes and barrels, it would be best categorized under Plastics manufacturing. Any factory that typically works with synthetic or semi-organic moldable solids would be equipped to prefabricate the parts needed for this device. There are innumerable such factories in the US, so finding one that could serve the needs of this project would not be challenging. Parts could be formed, cut to size, and packaged in the US factory, then shipped to Chile. In this way, even the manufacturing of this device benefits the Atacaman communities.
Scientific Concepts
Evaporation
Origin

Fig 3: Water evaporating from tea

Evaporation, or the vaporization of liquid which occurs solely at the surface, is one of the fundamental concepts of the desalination unit design. In evaporation, when the free surface of a body of liquid is exposed to heat energy (Fig 3), the molecules are able to slowly convert into vapor. When they vaporize, they take some energy with them, lowering the net kinetic energy of the body of liquid. Thus, in order to enact sustained evaporation, the source of outside heat must also be sustained.
There is no single person who is credited with discovering evaporation, since people have been evaporating water to make salt since ancient times. Some say that Swedish scientist N. Wallerius definitively proved the origin of water vapor in his works trying to prove the existence of a shrinking ocean, but the fact remains that humans were aware of the properties of evaporation, particularly the tendency to separate a substrate during the process, for longer than we have records.
Examples
For an easy visualization of the concept of evaporation, think of things that happen in daily life. Frequently, one sees muddy puddles during a rainstorm, only to return during good weather to find them dried up, with mud the only remainder. It was not dried up through a magical celestial mop – the water evaporated, leaving behind the dirt particles which had been suspended in the water column. When we hang clothes on a line, the birds do not wring them out. The water evaporates out from between the fibers, leaving the fabric dry to the touch.
Condensation
Origin

Fig 4: Water condensing on a cold bottle

Condensation is also integral to the efficacy of this device. Why would it be useful to convert impure water into pure vapor if there was no way to convert the vapor back to water again? In the device, we aim to force the vapor to condense via adhesion onto the cooler surface of the desalination cone. Since the cone is clear rather than dark (like the walls of the barrel) it will be more conducive to adhesion due to the difference in temperature – condensation occurs most readily on a cool surface.
Again, there is no one person credited with discovering condensation – since the times of early humans, it has been noted that water collected on leaves in the morning or on cold surfaces – even animals, such as the Darkling Beetle or Thorny Devil recognize and take advantage of condensation.
Examples
Again, daily life holds many examples of condensation. Water beading on the outside of a cold container of water (Fig. 4), dew forming on the surfaces of leaves, windows fogging up during rainstorms – these all perfectly illustrate condensation.
Technology
Barrel

Fig 5: A barrel during a compression test

The type of barrel used for this project is more commonly knows as a 55-gallon drum. They are mainly used for bulk transport of cargo, which ranges from olives to cider to fertilizers. There are two common sub-types, the open-top and welded-top types. The drum used by the group was of the welded-top variety, leading to the need to separate the lid from the body with a jigsaw. The drum is typically strengthened at intervals along its height with rings of thicker plastic, to stabilize the drum when the product is rolled sideways or loaded in stacks. Before being put into widespread usage, they are subjected to many tests like the one in Fig. 5 to assess their durability and safety for transport. Thus, the use of this type of barrel ensures a sturdy base for the device and a long life for the component.


PVC Pipes

Fig 6: PVC pipes ready to ship

PVC pipes like the ones used in the solar humidification component of the desalination unit are typically used for plumbing, both residential and commercial. The pipes are petroleum products, and are typically extruded and sold in precut lengths (Fig. 6). PVC pipe first appeared in 1932, though the technology to make PVC had been around for almost a century. Though it took a long time before the manufacturing process was completely refined, PVC was perfectly suited to drinking water due to its chemical resistance, lack of taste or odor, and smooth interior surface. Today, the PVC pipe is almost ubiquitous, in first-world society at least, as an endlessly useful building material and transporter of fluid.


Funnels

Fig 7: A typical plastic funnel

The primary purpose of a funnel is to consolidate a mass of fluid into an easily controlled and directed flow. This serves to enhance the user interface and reduce waste by making spillage less likely. Funneling technology has been in use since ancient times, with references in Egyptian art and Greek texts. In this particular device, plastic funnels like the one seen in Figure 7 are used on both the intake and outflow pipes. The intake pipe funnel is the very beginning of the system, directing the saltwater, poured in by hand, to the solar humidification chamber. Inside the chamber, a slightly smaller funnel waits underneath the point of the desalination cone in order to collect the droplets of condensed fresh water and transport them to the storage receptacle. Without these funnels, waste and contamination would be a huge problem for the device.


Plexiglas

Fig 8: Sheets of Plexiglas

Plexiglas (Fig. 8), also known as poly(methyl methacrylate), is commonly known as a less shatter-prone alternative to glass. It was invented simultaneously in many laboratories in the year 1928, and was first marketed Rohm and Haas in 1933. It has also been called Lucite and Perspex. Due to its low cost and easy processing, it is widely used, although it is brittle when being handled and is more prone to scratching than traditional glass. In this device, it is used directly as an inorganic glass substitute, due to ease of construction (cutting circles in Plexiglas is simpler than in glass) and cost concerns. Due to the clarity of the Plexiglas, the lid of the device does not retain heat, therefore better facilitating condensation on the plastic cone below, due to the heat gradient differential.

Desalination Cone

Fig 9: The Watercycler, a large-scale Desal Cone

After the fresh water is vaporized and separated from all salt constituents, it is necessary to have a place for the vapor to condense and collect before running into the final receptacle. Using a cone shape allows the water droplets to coalesce to a single point, facilitating cohesion and speeding up entrance into the storage component. This can be seen large scale in Fig 9. Typically in solar humidification units, the cone is not inverted in this way, but after careful study of such technologies as conventional alcohol stills, the decision was made to invert.

Adhesives

Fig 10: PVC liquid weld

The many adhesives used thoughout the device – hot glue, epoxies, caulk – all have their roots in tree sap glues from 4,000 BC. The ancient Greeks used adhesives in their daily lives, as did many other sophisticated ancient cultures. The two main types of adhesives used in this device are insulating water-potable epoxy and PVC liquid welds. The epoxy is used to connect the funnels to the pipes and to the barrel. The epoxy is silicon based and therefore completely safe for drinking water – although the water never comes in direct contact with the epoxy, this was a case of better safe than sorry. The PVC liquid weld was a two-part process – first, a primer was applied to the ends of the PVC that were to be adhered together, as seen in Fig. 10. This primer acted as a chemical abrasive, breaking down the outer layer of PVC into its molecular components. After the addition of the second component, the cement, the two PVC pieces are seated into position and held for a period of 15-20 seconds, after which they are effectively one contiguous piece of PVC.

Mathematics
Volumetrics
In order to ensure that our unit could produce enough water to sustain a family, the team had to size the receptacles appropriately. Overflow of either receptacle would not be acceptable, particularly in the first desalination chamber. If saltwater was allowed to touch the desalination cone, the entire process would be compromised. Thus, the intake pipe was placed at a level that would only allow a certain level of water in the primary receptacle. This level was decided upon through simple volumetric calculations. The barrel had a diameter of 81.5cm (33.5”) and the team needed to desalinate at least 40L (10.5 gal) in one cycle. Since the volume of a cylinder is given by
Πr2h=V
with r being the radius of the barrel (40.75 cm) and V being the minimum volume (40,000 cm3), one finds that the minimum height of the free surface of the water in the desalination chamber would have to be 7.67 cm. To make sure that the target demographic would not have to refill their receptacles every day, we capped the maximum free surface height at 30 cm However, we designed the intake spout such that it discharged less than 5 cm above the bottom of the barrel, so that the water would not splash as it filled. A 30 cm high water body does not provide significant risk of splashing.
Evaporation Rate
In order to estimate the speed and efficiency of the device, the evaporation rate had to be known. Evaporation rate is influenced by several factors, including the temperature of the water at the air-water interface, the humidity of the air, the area of the air-water surface, and the temperature of the air. Airflow and currents are usually also taken into account, but since the system is closed from air and other water bodies, the team disregarded these. Since the evaporation is taking place inside a closed container, vapor pressure and saturation also had to be taken into account – the Clausius-Claypen relation demonstrates the saturation of an air mass as water evaporates:

where P1, P2 are the vapor pressures at temperatures T1, T2 respectively, ΔHvap is the enthalpy of vaporization, and R is the universal gas constant. The group does not expect that the air in the barrel will become saturated due to the condensation and removal og water vapor that is simultaneously taking place. Since water is constantly being physically removed from the solar humidification system, evaporation will retain a constant rate.
Heat Intensity
Heat intensity and the heat exchange between the sun and the water within the barrel were very important in calculating the rate of freshwater creation. Heat (radiation) intensity (shown as I) is a measure of the distribution of radiant heat flux per unit area and solid angle, in a particular direction, defined according to

Where dA is the infinitesimal source area, dq is the outgoing heat transfer from the area dA, dω is the solid angle subtended by the infinitesimal 'target' (or 'aperture') area dAa, θ is the angle between the source area normal vector and the line-of-sight between the source and the target areas. The plastic barrel acts as a conductor, attracting heat energy due to its dark coloration. The top of the barrel and the cone, by contrast, are clear and do not store as much heat. This allows for the heat differential to encourage evaporation in the lower part of the barrel and condensation near the top.
Conclusion
This desalination unit is an innovative, open-system device that will be constructed using hydrodynamics engineering and a combination of prefabrication and English-system manufacturing, all operating under the principles of leagile manufacturing. The solution uses solar humidification to remove salt and other contaminants from water in order to produce a potable hydration source for a specific demographic of people – the rural inhabitants of the Atacama Desert in northern Chile. Using the principles of evaporation and condensation, the team was able to successfully separate out impure constituents of water, transferring the drinkable freshwater into a separate receptacle. By incorporating technologies that have been in use for many years, the device conquers a problem that heretofore required much more costly infrastructure. This desalination unit is exactly what the Atacama region needs.

___________________________________________________________________

Works Cited
Mary Bellis. (2008). “What Sticks: History of Adhesives and Glue.” Ask.com. Retrieved from on 13 January 2012
O. Gurel. (11 October 2007). “Invention and Innovation: Similar Words, Different Concepts.” IP Frontline. Retrieved from on 3 January 2012
M.M. Helms. (2006). "Open and Closed Systems." Encyclopedia of Management. Vol. 1. Gale Cengage. eNotes.com. Retrieved from on 3 January 2012
Rajesh Krishnamurthy, Charlene A. Yauch, (2007) "Leagile manufacturing: a proposed corporate infrastructure", International Journal of Operations & Production Management, Vol. 27 Iss: 6, pp.588 – 604. Retrieved from on 22 December 2011
Robert Walker. (1990). “The Early History of PVC Pipe.” Uni-Bell PVC Pipe News. Retrieved from on 12 January 2012
Wikipedia. (19 November 2011). “English System of Manufacturing.” Wikipedia. Retrieved from on 21 December 2011
Wikipedia. (17 December 2011). “Open-channel Flow”. Wikipedia. Retrieved from on 4 January 2012
Wikipedia. (4 January 2012) “Poly(methyl methacrylate).” Wikipedia. Retrieved from on 13 January 2012
Wikipedia. (9 August 2011). “Technology Systems”. Wikipedia. Retrieved from on 22 December 2011