Many owners and service personnel who deal with steam absorption refrigeration equipment are not aware of the "other side" of this particular type of chiller. The mechanical aspects are appreciated, but the chemical nature of the Lithium Bromide solution, the chiller’s blood, is not too readily understood.

Lithium bromide is used in the steam absorption cycle to absorb water vapor. Water is the chiller’s refrigerant and the evaporation of water provides the cooling effect desired. However, with the Lithium Bromide solution, the evaporation of water would lose its efficiency as the water vapor saturated the enclosure. Hence, the Lithium Bromide serves as a vehicle to perpetuate a refrigeration cycle based upon water.

This whole process occurs in a vacuum and the precise absorption and release of water is critical to the system’s operation.

The process is simple and the mechanics of the steam absorption chiller are composed of pumps to circulate the various fluids, heat exchangers to economize energy, and tubes to transfer energy in and out of the unit.

Nonetheless, the Lithium Bromide system does have its own unique problems, such as plugged spray nozzles, clogged heat exchangers, worn pump impellers, and gas generation. These arise from the chemical aspects of the absorber. For this reason, it is important to regularly, about once each year, chemically check the Lithium Bromide solution and make any necessary chemical adjustments.

The parameters, usually analyzed are specific gravity, pH, alkalinity, dissolved copper and iron, and inhibitor strength. For some machines, Lithium Nitrate is used as the inhibitor while for others, Lithium Chromate is chosen. When nitrate is applied, the solution must be checked for nitrate and Ammonia. When chromate is sued, the chromate level must, obviously, be observed.

All of these parameters can be viewed in the overall to give a composite analysis and interpretation of the chemistry of each particular lithium bromide solution and can depict quite accurately, in many cases, potential air leaks and major corrosion failures, such as copper-stress corrosion cracking.

Consider the solution alkalinity, for example. Alkalinity, loosely defined, represents the capacity of the solution to absorb acid. It is directly related to the amount of oxygen in the solution.

If the solution alkalinity is too high, based upon manufacturers’ specifications, this condition could be indicative of excess oxygen, or air, in solution. Hence, the probability of an air leak is considered.

In conjunction with alkalinity are the dissolved copper and iron contents. High alkalinities usually give rise to high copper corrosion rates in Lithium Bromide.

If the alkalinity were high, a greater dissolved copper content would be anticipated. On the other hand, if the alkalinity were low and the copper corrosion rate high, this could indicate a leak even though it was not directly observed from the alkalinity.

This raises an interesting point. If, for example, the solution analysis shows 200 ppm (parts per million) copper in solution, does this represent a major air leak or a major corrosion failure? The answer must be made based upon prior history of the steam absorption unit.

If 200 ppm copper were generated by a corrosive attack by the Lithium Bromide on the copper tubing in a period of three weeks, this would be characteristic of a more severe problem than were the 200 ppm copper to have developed over a year. Thus, the need for a repetitive analysis schedule is very important to the maintenance of any steam absorber.

The monitoring of nitrate or chromate is essential for corrosion protection. Without proper inhibitor balance, the system becomes very corrosive and non-condensibles are generated. These destroy the vacuum, or at the very least, make the vacuum more difficult to maintain. Correspondingly, the efficiency of the absorber suffers.

The Ammonia content is very important as it relates to a specific corrosion problem sometimes found in steam absorption equipment – copper-stress corrosion cracking of the tubes.

Stress corrosion has the visual appearance of cracks, usually along the axis of the tube. These cracks appear to have been caused by the tube "opening up" rather than from bursting from excessive mechanical stress. It is an expensive failure and its detection depends, in part, upon being able to accurately monitor the amount of Ammonia in the Lithium Bromide solution.

Industrial Corrosion Management, Inc. at our Randolph Twp., New Jersey, office has been involved with the analysis and interpretation of Lithium Bromide solutions from all models of all manufacturers. Our observation has been that a routine checking of this solution is vital to the performance and operation of an absorption machine.

Furthermore, by analyzing the Lithium Bromide solution yearly under the same set of standards, trends can be observed which quite accurately depict tendencies or pending problems long before they actually develop. This information is most important for the service mechanic.

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