PLANT ENGINEERING, January 11, 1979

By RICHARD S. LEVINE

LITHIUM BROMIDE, the working fluid of a steam absorption refrigeration machine, is a highly corrosive substance that attacks the copper and ferrous portions of the system. Consequently, absorption equipment must be constantly checked to prevent serious damage and ensure proper performance.

The relationships between lithium bromide and absorption chillers are discussed in this three-part series. The first article (PE 10/12/78, p 157, file #2510) explained the basic operation of absorption chillers, and how to test, analyze, and interpret the chemical nature of the lithium bromide solution. The second article (PE 11/23/78, p 101, file #2510) described the corrosive nature of lithium bromide and discussed lithium nitrate and lithium chromate inhibitors that are used to control corrosive attack. This article outlines procedures for cleaning accumulated corrosion debris from the machine.

Corrosion Cannot Be Completely Stopped—Regardless of inhibitor concentration and chemical supervision, absorbers corrode and eventually become loaded with corrosion debris. Most of the debris stays in the system and plugs narrow orifices such as spray headers, heat exchangers, and pump components with close tolerances. The absorption system, which depends upon precise evaporation, heat transfer, and fluid flow, will be shut down.

Corrosion is a chemical reaction, or a complex series of simultaneous reactions, governed by kinetics. Some reactions are rapid and spontaneous; others may continue for years before the effects of corrosion are seen.

However, the time required for a chemical reaction to occur can be altered by inhibitors. Adding lithium nitrate or lithium chromate to the absorption system will not eliminate corrosion, but it will slow down the process. There will always be dirty absorbers no matter what chemical adjustments are made.

The plant engineer must be as concerned with removing corrosive debris and with cleaning the internal sections of the steam absorber as he is with maintaining the correct chemical balance to minimize corrosive attack in the machine.

Effects of Corrosive Debris— The appearance of the lithium bromide solution initially added to an absorber is similar to that of water. However, after corrosion has developed and debris has been produced, this clear liquid looks muddy.

Dirt in an absorption machine can range from very fine, silt-like solids to large chunks. The fine debris will not necessarily clog internal portions since it will tend to flow along with the lithium bromide charge; however, it will have an abrasive effect and contribute significantly to wear of the system. The fine material can also accumulate and condense into larger, more troublesome waste.

Large chunks of debris create the major problems inside the absorber. Oxides forming on the mild-steel internal shell eventually become heavy enough that layers fall into the lithium bromide solution.

Spray header nozzles, which are designed to provide a uniform distribution of lithium bromide and water for flash evaporation, are easily clogged by the debris. In the heat exchanger, which warms the diluted lithium bromide that is to be concentrated and simultaneously cools the boiled brine that is returning for fresh absorption, debris collects and reduces heat transfer. The result is a decline in overall absorber performance.

Removing the dirty lithium bromide solution and replacing it with a new charge will not solve the problem. When the new liquid and accumulated dirt in low areas of the absorber or on the shell come into contact, the debris is loosened and the dense lithium bromide charge becomes loaded with harmful waste again.

Filtering the Debris—A filter can be used to reduce the debris; however, it must be installed so that the absorber’s operation is not affected.

Adding a filter ahead of the spray header prevents debris from clogging the nozzles. If the filter clogs, however, the system shuts down because the solution cannot reach the header. The best filter arrangement is a bypass loop, because the rest of the machine’s operation will be unaffected when the filter clogs.

Another problem with any filter is deciding what size filtration will be most useful. A filter that removes chunks of large debris is rather ineffective for fine material. And one for small particles clogs instantly from large debris. A graduated arrangement in which the internal element of the filter is changed to provide finer and finer filtration is the best choice.

Proper size is also important so that the solution can be filtered in a reasonable period, or so that the filter removes the debris almost as fast as it is generated. Placing a small filter on a 1200 ton machine will clean the charge, but at a very slow rate.

An external unit can be used for rapid filtration. This type of device permits filtration outside the absorber when equipment is not in use. A large percentage of the debris is eliminated, but once the portable filter is removed, the solution reverts to a dirty state since there is no other cleaning device.

A filter must be able to hold suitable vacuum to prevent air leakage into the machine and must be easily replaceable so that it can be changed without any loss of cooling capacity.

Chemical Cleaning—There are three chemical methods for cleaning the interior of absorption machines: acid cleaning, rinse techniques, and chromate conversion.

Acid cleaning is an excellent means to remove debris; however, it can also cause corrosion if not properly controlled. The problem is to dissolve the debris while preventing new attack on the fresh, exposed metallic surfaces.

Acids are strong reactive chemicals. When placed in contact with metals, acids are very corrosive and readily dissolve the metal and its oxides. It is impossible for an acid to differentiate between what is to be dissolved and what must be protected.

Oxides and corrosion products usually dissolve more readily than pure metal because they have already experienced a chemical reaction. This occurred when the pure metal was attacked by oxygen or hydroxide in solution to produce its oxide.

Acid is added to the machine after the lithium bromide charge has been removed. The acid produces rapid dissolution of a large amount of oxide waste, and then attacks the metal surface exposed when the debris is eliminated. Controlling contact time between acid and metal is the key to successful acid cleaning. Leaving the acid in contact with the interior for just the right amount of time will dissolve debris without subjecting the metal surface to prolonged corrosive attack.

After the internal surfaces have been rinsed with water to remove the acid, they will be clean and have a dull appearance. Waste will have been reduced, but some debris will remain.

Repeating the process several times, with proper rinsing between steps, achieves excellent cleaning. A detailed chemical study of the metal versus the acid in the dirt matrix is essential to successful acid cleaning.

At the end of the acid exposure, adding a large dose of corrosion inhibitor will provide temporary protection. The inhibitor should be flushed out before the lithium bromide charge is returned to the absorber.

Acid cleaning has a drawback besides its tendency to dissolve the metal. If there are latent corrosion failures in the machine, acid usually precipitates them and causes the worn component to fail sooner than it would otherwise.

However, the premature failure is not necessarily bad. For example, assume a machine is experiencing slow stress corrosion cracking of its copper tubes. As the cracks form, they are sealed by corrosion debris. The acid dissolves the debris and reveals the crack.

The failure will occur regardless of the acid, and a corroded part will certainly not improve. Discovering this condition under controlled circumstances is better than being surprised by a breakdown on a hot day.

In general, acid cleaning is reserved for exceptionally dirty machines, such as units that have experienced significant corrosion for years and equipment with large accumulations of internal debris.

Rinse agents do not dissolve oxide waste, but, rather facilitate its removal. If water alone is used to wash out the interior, a large percentage of debris will become suspended in the water as it is agitated. However, as the water solution is pumped from the machine, the oxides settle out and only the water is removed. Adding various chemicals improves the process.

The most common rinse agent used in steam absorption machines is lithium hydroxide, an alkaline material that is not particularly corrosive to mild steel but is somewhat corrosive to copper. The lithium hydroxide makes the water slippery and the debris becomes more fluid and easier to pump out of the machine.

Surfactants can also be used to rinse away corrosion products. Surfactants are a class of chemicals that interact with water to change its surface tension. They, too, make water slippery, and they function like lithium hydroxide.

In addition, surfactants can possess chelating ability. Dirt removal is improved because the surfactants chemically bond to the debris and tend to keep the waste in physical suspension longer. The suspended material is removed from the absorber with the water.

The disadvantage of using rinse techniques, and to some degree acid cleaning, is that there are stagnant areas inside the absorber. If the dirt cannot be stirred into solution so that the acid or rinse agent contacts the debris, it will not be removed. In some cases, special pipes must be installed to flush away debris in stagnant areas.

Converting an absorber from a lithium nitrate to a lithium chromate inhibitor produces some cleaning internally. However, the inhibitor is not changed to improve internal cleaning. Switching from nitrate to chromate is recommended to control corrosion in machines in which nitrate was insufficient to ameliorate the corrosion demand.

The table illustrates the cleaning action associated with chromate conversion. The 6/15 analysis shows levels of dissolved copper, dissolved iron, and suspended solids of a lithium bromide solution through a 0.45 micron filter. This nitrate-inhibited-solution analysis is for an absorber that was actively corroding.

The 7/23 analysis shows levels 2 days after chromate was first added to the solution. The level of active copper corrosion increased as a result of the chemical addition, and the debris was stirred up, causing a higher suspended solids concentration. In essence, a large portion of the waste deposited on the internal shell wall was knocked loose into suspension. The dissolved copper subsequently dropped to a low level, and the level of suspended solids was reduced below the original level.

These results are typical for the first 6 months of a chromate conversion program. In the later stages, the dissolved copper content was reduced much further and the solution, which was muddy at the beginning, clarified to the basic yellow chromate color. Some debris was still present at the end of the conversion.

Richard Levine is a consultant dealing with Lithium Bromide-related problems in absorption refrigeration. He has 30 years experience in the field of Lithium Bromide analysis, interpretation, internal cleaning, and corrosion control. He can be reached at:

Richard Levine
LBD ASSOCIATES, LLC.
Randolph, NJ USA
973-895-5207
mailto:rslevine@lbdassociates.com