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refrigeng
13-12-2014, 10:15 PM
Hi
1.which types of oil are suitable for ammonia refrigeration? 1)mineral 2)Synthetic alkylbenzene 3)Synthetic polyolester

2. What is the optimum viscosity of Ammonia oil?

2.Is there any difference between oil used in recip and screw compressors?

sterl
17-12-2014, 05:14 PM
For industrial scale equipment: any of the synthetic lubricants are going to lead to containment, compatability and or transportation considerations on a normally constructed ammonia circuit. Mineral oil the normal fluid.

Optimum viscosity is a consequence of the machine's characteristics and the operating temperature anywhere liquid refrigerant could be present on the system low side: but the generic answer is 68 Centistokes. As indicator, if the pour point of the oil is not a few degrees lower than the operating temperature low side, you will have to design return risers on the suction to apply a stronger shear to the oil than the force of gravity, which is usually not tolerable for low temperature refrigeration; the velocities and pressure drop required are too high for decent system performance....

A whole lot of both recips and screws use the 68 Cst oil. Some of the older, low speed recips need a higher viscosity oil. As a generality, the VW style recips using roller bearings on the crankshaft have considerable tolerance for operating viscosity but have a very low tolerance for refrigerant carryover...

Many of the Semi-Synthetics (which are highly refined mineral oils....) will have both pour points and viscosities acceptable for low temp duty. These are reflected in the manufacturer's oil selections. See second page link below.

www.gartner-refrig.com/pdf/resources/tips_tools/frick_compressor.pdf

Josip
18-12-2014, 07:55 AM
Hi, sterl :)


For industrial scale equipment: any of the synthetic lubricants are going to lead to containment, compatability and or transportation considerations on a normally constructed ammonia circuit. Mineral oil the normal fluid.

Optimum viscosity is a consequence of the machine's characteristics and the operating temperature anywhere liquid refrigerant could be present on the system low side: but the generic answer is 68 Centistokes. As indicator, if the pour point of the oil is not a few degrees lower than the operating temperature low side, you will have to design return risers on the suction to apply a stronger shear to the oil than the force of gravity, which is usually not tolerable for low temperature refrigeration; the velocities and pressure drop required are too high for decent system performance....

A whole lot of both recips and screws use the 68 Cst oil. Some of the older, low speed recips need a higher viscosity oil. As a generality, the VW style recips using roller bearings on the crankshaft have considerable tolerance for operating viscosity but have a very low tolerance for refrigerant carryover...

Many of the Semi-Synthetics (which are highly refined mineral oils....) will have both pour points and viscosities acceptable for low temp duty. These are reflected in the manufacturer's oil selections. See second page link below.

www.gartner-refrig.com/pdf/resources/tips_tools/ (http://www.gartner-refrig.com/pdf/resources/tips_tools/)frick_compressor.pdf


for me link is not valid :confused:

Best regards, Josip :)

sandybapat
22-12-2014, 05:23 AM
Thanks Sterl for your post.

The link provided by you is not working.

For temperatures around minus 40 C, mineral oils give some difficulty. Incase moisture is present in Ammonia, Mineral oils form a greasy mass and bloc the HT surface of evaporator. Pour point of these oils is around minus 35 C. he flash point is around +160 C. While using two stage reciprocating compressors the oil tends to carbonize and loose lubrication properties. In case of semi synthetic the pout point is near minus 40 C and flash point is above +225 C. Therefore the oil can be easily removed from the evaporator and oil does not get carbonized. The oil can be recovered and rectified easily.

It is observed that use of semi-synthetic oil for minus 40 C has resulted in less wear and tear and compressor maintenance cost has reduced.

sterl
28-01-2015, 09:30 PM
Sorry about the broken link: its leads to the Frick descriptions of their oil offerings which includes Low Speed Recips, VW recips, screws and rotary vanes so largely it covers all styles of machines.

But even without it: we need to ensure we are talking about the same things here....

http://www.eng-tips.com/viewthread.cfm?qid=355474


See Gallup's little history for the "catch phrase" Semi Synthetic. In the US at least, semi-synthetic has commercially evolved to mean highly hydrotreated mineral oil. We also have some marketing types that prefer the term "synthesized". They can't explain that one to me either.....


http://www.rm-support.com/index.php/en/expert-advice/item/pao-vs-2-stage-hydrocracked-base-oil.html

The difference between Highly Hydrocracked and 2-stage Hydrocracked is subtle but they both tend to lead to the same place: mineral oil free of contaminants that is not as broad a mixture in terms of light and heavy components as conventionally cracked oils. Hydrocracking sharpens the distribution of the mixture to where effectively, everything contained is virtually the same. For the most part: thats what we get in a barrel marked semi synthetic.

This permits it to stay consistent in content throughout the system, and minimizes volatiles at any given viscosity. It also means there is lower tramp chemicals in the oil so contaminants aren't there to cause reactive issues with gaskets, seal materials and the like. There are versions that incorporate additives for soft material preservation, particularly seal materials....

With this processing, for a mineral base stock, finish viscosity of 54 centistokes: the pour point can be as low as -58 C. The flash point for that oil is 232 deg. C.

The biggest outstanding issue we encounter with these oils is moisture content, as you indicate. We see a lot of oil extraction points that simply stop working once the moisture content on a low temperature vessel exceeds about 3%. This is a vintage system effect....Our own experience indicates the deleterious effects are more pronounced with water contamination of the PAB synthetics; the PAO's seem to be have greater tolerance for at least minor moisture content.

Are the elastomeric additives worthwhile? Sure, in a modern circuit a few years old. We don't think of it as money well spent for a screw machine with less than about 15K hours but if you are putting it in one machine you might just as well be putting it in all, it saves confusion.....

camelCase
16-04-2015, 09:43 PM
Most ammonai systems are either using a type of mineral oil, or a mineral oil blended with either an AB or a PAO.

Napthenic mineral oils are your lowest quality - per say. As said above, they are a hydrotreated or acid treated compound. They have a pretty broad range of hydrocarbon chain lengths. That means some chains will begin to volatize when others won't. This is why they usually have a pretty low flash point. When heated, they will create more vapors per kg of oil that a higher refined product. these vapors will directly lead to carryover since a separator can't separate them (only separates aerosol droplets).

AB fluids will sometimes either be used as straight AB or blended with a naphthenic MO. The AB helps get the pour point down, but since it's a cyclic compound - it does have a bit of volatility issues.

Most semi-synthetic ammonia oils will be a combination of these two.

The hydrocracked oils - which a lot of the OEMs are going towards now - are probably the best bet in the systems these days when it comes to cost per performance. The new pour point agents can help get to pour points of about -56C. As well, since they are more refined, a lot of the volatile components are pulled out and help reduce vapor carryover. You can also expect a bit more life out of these fluids.

when it comes to ultra low temperature, you'll see guys either dropping the viscosity down or blending in some PAO. A straight PAO isn't really ideal, but can work. Price per performance just really isn't there.

Most of the systems I see run an ISO-68, regardless of compressor type. Some of the higher pressure or older systems will go to an ISO-100. Some low temp systems will start going into the ISO-46 range.

The seal conditioner additives you'll see are usually only used when transitioning systems from a naphthenic/AB based oil over to a paraffinic hydrocracked oil. This is due to how the different oils interact with the seals. If you're already using a paraffinic oil, you can pour most other oils right on top (except for PAGs, Silicones, and Esters).

Not sure why someone would run a POE in an ammonia systems. Wayy too costly for what they offer. And the acids that form when they get in contact with water don't play too nicely with system internals usually. If you're running an HFC, that's one thing. With ammonia, your best bang for buck is going to come from a hydrocracked fluid. This is why some of the OEMS like Frick, FES, etc.etc. are switching their OEM recommendations to them. Some aftermarkets that immediately come to mind are NXT-717 and CP1009.