After the evaluation of the different possibilities to deal with the problem of the water in the air, the decision of constructing an absorption filter for high pressure was made.
This was the cheapest solution and would allow to eliminate the water from the air cycle. The main challenges designing this filter were the following:
• High pressure. Because the filter would stand after the compressor, it would have to withstand up to 100 bar. Furthermore, the team had to find a way of preventing any leaks.
• Desiccant material. The conditions of high temperature limit the materials that can be used, because most desiccants start releasing water at a certain temperature.
The main idea was to build a stainless steel vessel that could easily be filled and emptied with the desiccant. To do so, the team first had to find the appropriate material. Knowing its properties, the amount of desiccant needed to remove the water could be determined. This quantity would impose the size of the air filter.
To start with, the only type of desiccant material that can work at high temperature are molecular sieves. As shown in figure 65, other common desiccants as silica gel have a poor performance at high temperatures (212ºF = 100ºC).
Figure 64 sensor placement
64 They would absorb almost no water, while the molecular sieves remain having a high absorption capacity. One of the most popular molecular sieves is zeolite, which is a microporous mineral used in plenty of industrial applications. This was the material chosen by the team.
With this kind of material and taking into consideration that about 15 grams of water must be removed every three complete cycles of the demo, the size of the filter can be established. According to figure 65 19, at a temperature of 200ºF (93ºC), with 100 grams of material, 15 grams of water can be absorbed before reaching the equilibrium point, when the material would start releasing the water.
Then, to know the volume of the vessel the density of the material is needed. This density varies between 550 and 800 g/l. Taking the lower value for the density, the minimum needed of 100 grams would occupy about 0,19 liters. Being this volume the minimum, the team aimed to build a filter about 2 to 3 times bigger to ensure a sufficient water absorption.
After this research, the team focused on the design of the air vessel. Different possibilities were evaluated to build it, but the easiest way was to use a stainless steel pipe and connect it to the air cycle. The selected pipe was manufactured and delivered by the company ASCHL. It was made of V2A stainless steel, was 40 cm long, had an outer diameter of 42,4mm and a thickness of 3,25mm. It also had both threaded ends of G1-1/2". This would result in a volume of 0,41 liters, which is 2,16 times bigger than the minimum previously mentioned. The working pressure was 158 bar and the maximum pressure was 501 bar, so it was safe to use it in the system.
Afterwards, there were various forms to perform the connection from G1-1/2" to 1/4" (the diameter of the rest of the cycle). Some of them included multiple couplings that would progressively decrease the diameter, but this would involve several expensive components and a bigger size of the filter, which is restrained by the table in which the demo is built. Therefore, the team decided to use threaded steel
19 (SorbentSystems. 2006)
Figure 65 Relation temperature and absorption capacity
65 caps to close the vessel, and then drill the necessary hole in the cap to insert a direct connection of 1/4".
Then, the main idea to correctly seal the air vessel was to install rubber O-rings in both ends of the pipe. The rings would come into contact with the steel cap and applying the appropriate force, they seal the container. This is a standard procedure in many applications, and a similar principle was used in the connection of other pipes in the demo.
However, the threaded length of the caps was longer than the one of the pipe. With this initial dimensions, much rubber would be needed, and the sealing would be more difficult to perform.
Therefore, the team had to cut 1,15 centimeters from the threaded caps using the lathe from the metal workshop at university.
Figure 66 Process of drilling the hole in the iron caps
Figure 67 Final result of the air vessel and the connectors
66 Lastly, to prevent the desiccant material to go throw the system, the team had to install a grid or a web at the ends of the vessel. This would allow the air to flow but would block the solid particles of the desiccant. To do so, first the size of the material should be known.
The chosen material was zeolite Z4-01 from the company Zeochem. This material forms spheres of between 2,5 and 5mm of diameter, so the holes in the grid had to be smaller than that. The team manually cut two circles from a steel sheet, so that it could fit in the cap. After that, multiple holes of 1,5 mm diameter were drilled and then it was sticked to the cap with special glue. (figure 68)
This way, the desiccant is easy to remove while also ensuring that it could not flow through the air cycle.