The major limiting factor for our project other than time was the limits of the equipment. The problem we were trying to solve requires deeper more accurate analysis than just testing the mass flow rate at low unrealistic pressures. Ideally, we would have created a system that
precisely can measure the mass flow rate and pressure at the inlet of the capillary tube and outlet of the capillary tube. In reality this is not possible as the Ramer connected are needed to connect the small diameter of the capillary tube to the larger diameter mass flow meters and pressure transducers.
FURTHER TESTING
Although we have conducted countless tests on capillary tubes in Friedrich’s database there is still more extensive testing that can be conducted. While we have been analyzing National copper and Minallum tubes, Friedrich has made it clear that Minallum tubes will no longer be used for AC units. We suggest that when Friedrich next gets a new supplier, they follow the testing methods outlined below.
1. Perform extensive testing on a large sample of the new capillary tubes at elevated pressures (greater than 70 psig) and compare the results of those tests to a large-scale test of national copper tubes at the same inlet pressure.
a. This data can be analyzed through calculating the friction factors of the capillary tube at the elevated pressure using the following equation.
𝑓 = ∆𝑝 2𝐷
𝐿𝜌𝑉
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The fraction factors from all the tubes can be compared and if it is found there is a substantial different, further performance testing can be conducted. When looking at the calculated friction factors of the capillary tubes, realizes this variable is not just of the tube but in reality, the friction factor of the entire system. The only variable that is changing however is the capillary tube. This is due to the limiting factor of the equipment and the technology of the testing system.
b. This data can also be analyzed through testing the tubes mass flow rates at increasing pressure intervals without the pressure drop transducer connected.
See Section 7.3 and Section 7.4 for an example of this analysis.
2. After a large quantity of tubes have been tested using the testing device, we
recommend selecting tubes for performance testing. We recommend selecting tubes from the new supplier that have high mass flow rates at high pressure or tubes with low friction factors when compared to seemingly identical national copper capillary tubes. Then select baseline tubes from national copper to compare the results with. See Section 7.5 and Section 7.6 for an example of this.
SURFACE ROUGHNESS OPTICAL TESTER
We also have another recommendation for Friedrich to consider. Purchasing an optical surface roughness testing device such as the one seen in Figure 81. With this device an engineer could cut open a capillary tube and directly test the surface roughness of the new suppliers’
capillary tubes. This device would be used in the early stages of receiving new capillary tubes as it would provide empirical evidence of differing manufacturing standards used in creating the capillary tubes. We still suspect Minallum and National copper have different manufacturing
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processes which result in different microscopic differences in the properties of the tubes. This difference is what causes different mass flow rates and different pressure drops.
Figure 81: Profilometer Surface Roughness Optics
LABVIEW PROGRAMMING
While we initially planned to develop the LabVIEW program during our time working on the project. However, it became clear that developing a new program was not vital to testing the pressure drop across the capillary tubes as the calibrated gauge provided the same useful
measurements required to do analysis. If in the future Fredrich develop our system more and find tolerances that the friction factors need to be withing for each specific capillary tube, then we would recommend developing the LabVIEW program we outlined in Section 5.5.
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CONSTRICTING CAPILLARY TUBE
One aspect of the design we did not have enough time to fully investigate was the coiled capillary tube at the outlet of the pressure drop component. When the coiled tube was connected it reduced the mass flow rate of the system which makes sense as it equivalent to testing two tubes at once. This again relates to how the equipment was limiting factor, but we would
recommend further development on this component. We recommend testing different diameters and lengths of coiled outlet tube with different inner diameter test capillary tubes. From what we found through our time testing capillary tubes having an outlet tube that had a larger inner diameter than that of the tube being testing. Something we did not investigate was reducing the length of the tube, but we would recommend this being the first place to start when trying to create a backpressure without effecting the mass flow rate.
MASS FLOW METER
The final recommendation we have is to investigate adding another mass flow meter after the capillary tube. Measuring the mass flow at the inlet and outlet may prove useful. While we are not confident this would help show differences in the capillary tube it could be a possible course of action in the future if the other recommendations we have provided don’t resolve the issue of the testing device falsely passing tubes that will eventually fail in AC units.
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