problem in Auto Refrigeration Cascade System

Hi, everybody! I set up a three stage Auto Refrigeration Cascade system to achieve the -180 F box temperature, and the refrigerant was chosen as R404A\R23\R14 mixture, the lubricating oil was synthesize POE. When run this system, I found the evaporate temperature fluctuated in a wide range, after checked the system, it has been found there was a blockage in the gas line by the sticky oil. I think the oil is the big problem. So, does anyone know which type oil should be used in this ARC system? Any input is appreciated!

best Rgds

ernest.

try adding pentane or propane to system

Hi there,
I had the same problem with synthetic oil.
These oils are very hygroscopic (absorb moisture). When this happens then you will get sticky oil like chewing gum which will mess up the entire system:eek: .
To prevent this you have to work with the oil and the whole system for that matter, very very carefully, i.e., the vacuum process must be done very carefully and for a long time, then brake the vacuum with Nitrogen and then vacuum it again.
If this sticky oil is generated then YOU HAVE A HUGE PROBLEM.
I hope you can solve it somehow.
Good luck
Cheers:)

Have you checked the oils properties for the low temperature operation?

Here are some ideas on different things to consider...

Pour Point - is a widely used low-temperature flow indicator and is 5°F above the temperature to which a normally liquid petroleum product maintains fluidity. It is a significant factor in cold-weather start-up, but must be considered along with pumpability, the ease with which an oil pumps at low temperatures. Paraffinic oils contain wax which forms a honeycomb of crystals at low temperatures near the pour point. However, agitation by a pump breaks down this wax structure and allows paraffinic oils to be pumped at temperatures well below their pour point. Napthenic oils, on the other hand, contain little or no wax and reach their pour point through increase in viscosity: they cannot be pumped readily near the pour point. ASTM D 97 is used to determine pour point. See also pour depressant under additives. ASTM D 97 also provides for the determination of cloud point, the lowest temperature (°F) at which the sample becomes clouded by the formation of was crystals. Clouding is a characteristic only of paraffinic oils. It is a consideration in the evaluation of fuels whose filtration might be impaired by the plugging effect of wax crystals.

Viscosity - measure of a fluid’s resistance to flow. It is ordinarily expressed in terms of the time required for a standard quantity of the fluid at a certain temperature to flow through a standard orifice. The higher the value, the more viscous the fluid. Since viscosity varies inversely with temperature, its value is meaningless unless accompanied by the temperature at which it is determined. With petroleum oils, viscosity is commonly reported in Saybolt seconds, Universal-SSU or SUS-or, for very viscous oils, in Saybolt seconds, Furol-SSF. For a given oil, the Saybolt Universal viscosity will run about 10 times that of the Saybolt Furol viscosity measured at the same temperature. Both values are determined in accordance with the ASTM Method D88. Kinematic viscosity is now widely used in connection with petroleum oils, and the unit is the stoke or - more generally - the centistoke (cSt). Less common are the Engler and Redwood viscosity scales, whose principal applications are outside of North America.

Viscosity Index (V.I.) - the measure of the rate of change of viscosity with temperature. This change is common to all fluids - some more, some less. Heating tends to make them thinner - cooling, thicker. The higher the V.I., the less the tendency for the viscosity to change. V.I. is determined by formula from the viscosities at 100°F and 210°F in accordance with the ASTM Test Method D 567 or D 2270. The latter test is required for V.I.’s above 100. High -V.I. oils are often preferred for service in which a relatively constant viscosity is desired under conditions of varying temperature. Some hydraulic systems require this property. Paraffinic oils are inherently high in V.I., and the V.I. of any petroleum oil can be increased by the addition of a V.I. improver. Naphthenic oils are inherently low in V.I. and aromatics are still lower - often having negative numbers.

Above information taken from:
http://www.cpieng.com/Reference.htm

Add some R134a to flush the system, this R134a will remain liquid throughout the system and R134a has a good rinsing effect.

A -180 autocascade system is new to me. What is it that requires such low temperatures, and how large a refrigerator is this? Lana's advice, as usual, is spot on. Extremely low temperatures are unforgiving. Iceman's response teases me. What are aromatic oils? I was surprised to see that mineral oils are common in cascade systems. I would think synthetic oils would be more popular. What sort of expansion device are you using? I want to learn more about these systems, and the problems experienced with them.

Dan,
Autocascade always use capillary expansion.

In case you or others don't know the working principle of this: in the 1st stage, the complete liquid refrigerant mix (in this case R404a, R13 and R14) is fed through a HE, mostly cooled by water.
The 1st refrigerant with the highest boiling point is condensed here to its liquid phase.
The liquid (R404a)and remaining gas mix (R14 and R13) is then also separated.
This gaseous mix (in this case R13 and R14) is fed to a second HE and the liquid of the 1st stage is used to cool this down so that the second gas condens now (R13)

Again the two gasses are separated in a liquid and a gaseous phase.

The liquid of the 2nd stage is now used to cool further down the gas mix so that the last liquid is condensed out of the mix.
This last liquid (R14) is fed to the final HE (mostly a blown evaporator)

You can imagine that it takes some time to stabilize these systems, everything must become cold and very cold till its working temperature conditions. So all the HE exchangers are extremely good isolated.

The load must also be very stable, otherwise the systems will fail.

Adding some R134a in this system will make that the R134a will always be in its liquid form, so it will flow throughout the whole system as a liquid and flush and rinse it.

We have some autocascades running on metalising machines. These machines are used to evaporate and condens aluminum on a thin plastic foil, used in capacitors.
Another application is the shiny covers you see when you buy pralines which are mostly packed in a very shining plastic.

Or the first shoes from a baby are also metalized.

In these metalising machines is a Meissner coil which is cooled to -100°C (-148°F) to achieve fast (!) a very high vacuum. The metalizer works also without this coil but the vacuum pumping time without the Meissner coil is +/- 45 minutes compared with 15 minutes when the Meissner coil is working.
Each metalising cycle only lasts +/- 30 minutes to 1 hour, so the pumping down time is very important.
I made a basic schematic of it, at least the way I see it..

Hi Peter,

That was very good explanation of the operation. I especially liked your drawing.

Thanks US Iceman, so the system we're servicing has a Meissner coil which is in fact a bare copper tube of 15 m (+/- 45 ft) and is cooled by a 7.5 HP.

The working principle of this very cold coil is that the last water molecules which are 'flying' in the vacuum chamber are trapped as ice on this cold coil. They don't need to be removed by the vacuum pumps and so is teh vacuum pump time reduced.

In attachment some pictures of the Meissner coil we made.

There are - I think - 4 of these vacuum pumps for the 1st stage, the 2nd stage again 4 pumps of this size and 3th stage is a diffusion vacuum pump of +/- 2.5 m (7.5 ft) high and a diameter of +/- 1.5 m (7 ft)
http://en.wikipedia.org/wiki/Diffusion_pump
http://www.chemassociates.com/produc...usion_pump.pdf

Meissner coil www.sciaky.com/uploaded/files/cves.pdf The machine you see is a Polycold autocascade

If it is a small system there are few things that are rules of thumb. Try to make the evap down feed so that oil will drain back to the comp. Oil seperators, use more than one. The less oil in the eveaporators the better. Vacuum is your friend, let it run for days. Air and moisture is your enemy. The filter driers should be of the molecular seive type. Got to run Ill post more latter.

Further on this thread, a technician of this machine - Danny - tried something this morning for me and I'm very grateful for it he helped me.

These machines achieve very high vacuum levels (10-5 mbar), something we only can dream of with our small pumps.

I wondered how fast water evaporates at very high vacuum levels so that we could visualize a vacuum process in our refrigeration systems better.

So he tied a bottle of water with duct tape behind the window of his huge machine and pulled it vacuum.
It evaporates first and i finally freezes and the splashes freezes against the window.

I think this is a unique movie.

Danny mailed me that he will do this test once again with a better camera.
Someone have special requests?

I posted this movie (33MB) on http://rapidshare.com/files/190366409/MOV01357.MPG

5 millibar is 3,75 Torr which is 3750 microns
Modern two stage vacuum pump has normally 25 microns as end vacuum!
I don't see how is that pump, you are talking about , "something we only can dream of with our small pumps".

I see you never saw a vacuum chamber for metalizing purposes 0.00001 mbar (10 exp -5 mBar, is that better?)

Quote:

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Originally Posted by Peter_1

I see you never saw a vacuum chamber for metalizing purposes 0.00001 mbar (10 exp -5 mBar, is that better?)

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I taught that you wrote 10 to 5 milibars instead of 10 at -5 milibars (10^-5).

Just updated the link

Hi
I am trying to design a R23-R22 cascade system. I have decided to have a 6HP scroll Comp for low cycle and 10HP scroll comp for high cycle and a Plate cascade heat exchanger and air cooled condensor. My cooling load is at most 5KW and operating temperatures are -70C for evaporating and 35C for condensing.
I have to have a tight temperature control +/-1C.
I wanted to know if it is ok?!
I don`t have enough catalogues for selecting my compressor, Plate HE, TEV, Solenoid valves,...
whould you please help me?

about the arc system i worked on a -80c cascade and i used a little r290 for to assist oil return it seemed to work ok