18-08-2012, 03:50 PM
Oil Effect on Performance in an Ammonia Refrigeration System
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INTRODUCTION
Ammonia refrigeration systems have become predominant throughout the food processing industries due to
their efficiency, and low operating costs. To sustain this efficiency and, low operating costs the compressors
at the heart of these refrigeration systems must be properly lubricated.
Like any piece of industrial machinery, the lubricant in an ammonia compressor must provide protection
against friction and wear and also serve as a coolant. If these were the only required properties, virtually any
type of lubricant could be used. However it has been found that a specialized compressor fluid must be used
in order to maintain a balanced system.
In an ammonia compressor the lubricant and the gaseous ammonia are compressed together. The
lubricant/ammonia gas mixture then passes through a separator/coalescer, which filters out the lubricant. The
filtered lubricant falls to the bottom of the separator, where it is collected and returned back to the
compressor. The gaseous ammonia passes through the separator where it then enters the evaporative
condenser and other parts of the ammonia refrigeration system (see figure 1).
Effects of Improper Lubricant/Ammonia Separation on Compressor
One area that is typically overlooked by most lubricating engineers and plant managers are the
problems related to the condensation of the ammonia gas within separator/coalescer. If the
ammonia refrigerant condenses within the separator, it will most likely return to the compressor
instead of continuing on to the evaporative condenser. Once within compressor the ammonia
refrigerant returns to a gas phase due to the high temperatures within the shaft seal. The
developing gas pressures forces out the lubricating film reducing or eliminating the lubricating film
(i.e. dry running). This dry running dramatically increases the chances of blistering (corona effect).
The temperature within separator typically governs the condensation of the ammonia gas within
the separator. Condensation is more likely to occur if the temperature within the separator drops
to low (for instance as a result of an exposure to a cold external air flow).
Effect of Base Fluid on Solubility with Ammonia
A lubricants affinity with ammonia is controlled by the aromatic content of base fluids. Base fluids
like alkyl benzenes and napthenic have a high aromatic content which increases the lubricants
solubility. This is beneficial in Freon refrigeration systems, which require refrigerant/lubricant
solubility, but is drawback in ammonia systems as it leads increased carryover rates. As shown in
figure #3 alkyl benzene base fluids, which have the highest aromatic content, also have highest
percent solubility with ammonia, napthenic base fluids closely follow. 2-stage Hydrocracked base
fluids have the lowest aromatic concentration and thus have the lowest % solubility.
Although PAO should have a very low solubility with ammonia, the addition of the alkyl benzene
base fluids to many formulations increases solubility. This is most likely due to the high aromatic
content of alkyl benzene base fluids. Field-testing has shown that many PAO/AB based
formulations have a higher rate of lubricant carryover then 2-stage Hydrocracked based
formulations (see figure #2).
Lubricants Effect on Seals
The high aromatic content of napthenic and alkyl benzene based fluids has been shown to swell
seals. This seal swelling has little effect on the performance of the seals but presents a problem
for customers switching from a napthenic based or alkyl benzene based formulation to either a 2-
stage Hydrocracked or PAO based fluid. PAO and 2-stage Hydrocracked base fluids have the
tendency to shrink these swollen seals. This presented a problem for early adopters of both 2-
stage Hydrocracked and PAO base fluids as these shrunken seals were more likely to leak.
Formulators of 2-stage Hydrocracked based formulations have compensated for this problem by
adding seal swelling additives. Formulators of PAO based formulations have attempted to correct
this problem by blending in alkyl benzene base fluids into the formulation. The high aromatic
content of the alkyl benzene base fluids has been shown to reduce seal shrinkage.
Fluid Life
Since ammonia refrigeration systems are sealed, oxidation of the fluid is typically negligible.
However, fluid breakdown still does occur, typically as a result of hydrolysis (water absorption).
Excessive water contamination is typically the condemning factor for most ammonia compressor
lubricants. The demulsibiity (the ability of the oil to shed water) is decreased by the presence of
aromatic and other polar molecules. Base fluids like napthenics and alkyl benzenes; with a high
concentration of aromatic molecules, are more susceptible to water contamination (see figure #8).
High water content is a problem in ammonia refrigeration systems as it can increase component
wear, increase rusting, and degrade the lubricant through the process of hydrolysis. As the water
content of the ammonia gas can vary greatly from application to application, the ammonia
compressor fluids must be stable in the presence of water and resist excessive water
contamination.