George Briley talks about lubricants and lubricant additives for ammonia refrigeration systems.

This month, I want to talk about additives. Now, before I get started, I want to point out there are additives, and then there are additives. What's the difference, you might say? Well, additives are what a lubricant manufacturer might add to the product to enhance its performance. By contrast, an additive is sold separately from the lubricant. Now, this is an important difference. I'll explain why.

Lubricants for ammonia refrigeration systems should be properly formulated and balanced in order to provide lubrication for the compression system. The lube should have a high film strength with stable inhibitors of rust, oxidation and foam. In selecting the rust inhibitors, careful consideration should be given to the metals in the system. Keep in mind the likelihood that besides ammonia, the system may contain a slight amount of water and air. In addition, viscosity extenders often are added if the system is to be operated at very low temperatures. Low volatility/vapor pressure also is important. Lubricant suppliers normally include additives that address these factors to enhance the quality of their product.

The manufacturers of lubricants used in ammonia systems typically test (or should test) their products in an ammonia atmosphere over an extended period of time. They want to make sure that these additives do not react with ammonia. This is done at temperatures approximating that of the compressor and that of the evaporator. The tests generally include adding small quantities of water and air plus representative metals that are used in the systems. As a result of these tests, the industry has learned that some chemicals react with ammonia, sometimes producing solids or a wax-like substance. Sometimes, there have been environmental problems; other times, in the presence of ammonia, the additives simply lose solubility in the lubricant in the presence of ammonia.

There are numerous other tests that lubricant manufacturers use to ensure product quality. The wrong combination of additives, or the additive itself, can influence the amount of foaming (a real concern in rotary screw compressors), pour point, low temperature fluidity, stability and lubricity. Therefore, lubricant manufacturers generally are careful in selecting additives and the amount of additives to be used.

Now let's talk about additives. Companies that do not market or manufacture lubricants usually supply these products. These companies usually tout one major advantage of their product. And, they often make claims of 10 to 30 percent improvements in system efficiency.

Now, before I go any further, I want to make it clear that I don't think that all additives are bad; nor do I think they are all good. I'm just saying that you ought to think about what you are doing and be sure to research the product -- particularly its compatibility with your system -- before you take the cap off and pour it in. Here are a few reasons why.

First of all, many previous manufacturers of additives are no longer in business. Second, some products are banned by the Environmental Protection Agency. Third, from my experience, additives don't always seem to make positive contributions. I know of a couple of cases where an additive was used that was not recommended by the lubricant manufacturer and it appears the additive may have caused some major problems. Both screw compressors and recips in the system required costly repairs.

There are many potential reasons for the problem I witnessed. Often, additives are added to systems that contain only pure ammonia and lubricants thought to be additive-free, when in fact, the lubricant did contain additives already included by the manufacturer to inhibit rust, suppress foam, extend viscosity and/or inhibit oxidation. It's not always clear whether a lubricant already contains additives. Lubricant manufacturers may not list the ingredients in the formula. Also, each ammonia system usually contains a bit of water and some air. Some older systems have accumulated wax leftover from lubricants used years ago.

Additives may vary widely in chemical composition and manufacturers may not publish information about lubricant composition. In my opinion, the trace amounts of additives employed by the lubricant manufacturers may, and probably will, react with the additives. This can result in a sludge that clogs lube filters and coalescers in the system, in turn rendering the lubricant useless. I have seen lube filters and coalescers coated with green goo. It may have resulted from chlorine in the system -- a component of an additive.

So, what's my best advice? Owners of refrigeration systems employing reciprocating or screw compressors should follow the lubricant supplier's and compressor manufacturer's recommendations regarding the type of lubricant and additives they should use in their systems. If you believe that an additive will enhance the efficiency of your system, check it out thoroughly before you put it in your system.