how to desing a oil separator and what factors impacting its performance

[Xin]

hi,all

i have some trouble with the system oil concentration in evaporator(falling film), and the oil separator(OS) is highly suspected as the root cause. our separator is an helical(centrifugal) type. i don't have any book directly studying the OS,so i have to search the related info on the website. there are generally four type of OS,which are Reduction of velocity, impingement,coalescing and centrifugal. now i have some questions about these OS.

1, is there any reference or book that is specially discuss the problems concerning the OS?
2, how are these factors such as discharge superheat, condenser pressure and load conditions (full or part) influencing the efficiency of OS? From the website, I get a very rough conclusion: high discharge superheat, high condenser pressure and part (lower) load conditions are beneficial to the performance of OS. But I don’t know the exact reason for these conclusions. Just excerpt the paragraphs below:

Discharge superheat:

Excess oil mixed with the refrigerant in the evaporator will cause severe foaming when the chiller is started. This foaming carries liquid refrigerant into the compressor, lowering the compressor discharge temperature. As the compressor discharge temperature approaches the condenser saturation temperature (low discharge superheat), the refrigerant gas has a greater affinity to absorb more oil. When the discharge super heat is reduced, oil cannot be separated from the refrigerant gas in the oil separator.

Condenser pressure:
oil starvation to the screw compressor, the reason is the pressure differential is not enough. Velocity will increase with volume flow, for a given pipe size. If you reduce the discharge pressure the volume flow will increase (the compressor pumps more gas due to the lower thermal lift) .A lower discharge temperature will in turn results in a higher oil carry over because of dilution.
Part load:

Although these newer types of separators claim efficiencies of close to 100%, it must be remembered that this is only at part-load or low-load conditions. The efficiency falls off substantially with any separator as the load is increased. One possible reason for this is that there is not as much benefit from the reduction of velocity inside the separator at full-load as at low-load conditions. A number of years ago a major supermarket rack OEM did a laboratory test comparing different brands and styles of oil separators. This included coalescing and centrifugal types as well. The test was done in accordance with ASHRAE Standard 69-1990. It was done using a large five-compressor rack with the same instrumentation, operators, and the testing was done in the same time frame. It was probably the most accurate and comprehensive comparison test ever attempted to determine oil separator performance. At low-load conditions, all the separators tested had efficiencies above 95%. The testing also showed that the efficiency of all types and styles of oil separators tested fell off substantially as catalog capacity ratings were approached.

The above is data taken from website, including this forum as well.

I wonder if these conclusion are correct, if correct, why and what are the detailed explanations? If not, what are other factors and what are conclusions, why?

In fact, I want to know how to really design an OS instead of a qualitative analysis or results.

For the centrifugal OS, in my opinion, the more velocity, the more centrifugal force, the more performance of OS, I am not sure if I am correct? But from the part load perspective, it seems I am wrong? What are your ideas?

i really appreciate you help in advance, you can also contact me with mail wlxzju@163.com.

Xin

[wilsoncheung]

No one answer this? Hope you guys spare time to answer this, I 'm also very interested in this topic too.
Thanks!

[Brian_UK]

Perhaps no answer because there is no real question within his post. It is unclear what he wants.

He quotes sections from unstated websites and has not given any facts and figures regarding his problem.

Also very very few members are involved with the design of machine components, we just use them.

Also, he has not returned to the forum since his post last November.

[jose regueiro]

The OIL SEPARATOR NEVER CACTH 100% of oil, the pipe return the oil. with R22 suction velocity vertical up, 1500 feet per minute