Chemistry 101 for absorption chillers
Under vacuum, water vapor flash boils at temperatures less than 100Â°C. The boiling action provides a mechanism whereby heat can be removed from a chilled-water loop, which circulates through the chiller and building areas to be cooled.
But as the water vapor accumulates inside the chiller, the vacuum is soon lost. Hence, lithium bromide brine at 58% to 62% concentrations is mixed with the water vapor to absorb it, thereby maintaining the essential vacuum. This absorption process can continue until the lithium bromide becomes too diluted with absorbed water vapor and its absorption properties are lost.
At this point, the diluted lithium bromide brine needs to be reconcentrated by boiling out the excess absorbed water. The excess water is recycled to be flash boiled again; likewise, the lithium bromide is recycled at a strong concentration to absorb more water vapor.
This is the thermodynamic cycle of lithium bromide refrigeration.
Fouled and corrodedWhen absorption chillers were first introduced, steam from an external source was used to concentrate the lithium bromide. Over the years, direct boiling of the diluted bromide has become the method of choice, and this is what is used in today’s absorption chillers.
The entire absorption chiller seems very simple, mechanically. Pumps are needed to circulate the bromide and water. A heat source is required. Heat exchangers, spray trees, tubing bundles, electrical controls, etc., complete the system. The entire operation depends upon circulating the right mix of water and lithium bromide through unobstructed piping. But there is another aspect of the absorption chiller that literally fouls up the workings of the chiller — corrosion. Lithium bromide, a highly corrosive brine, readily attacks ferrous metals such as steel.
The corrosion process generates hydrogen gas that reduces the internal vacuum inside the chiller. With the reduction in vacuum, the unit operates poorly. In addition, the debris resulting from the corrosion fouls narrow openings in spray headers, heat exchangers, etc.
Proper operation of absorption chillers, therefore, does not merely depend upon servicing the mechanical aspects of the unit. An understanding of lithium bromide chemistry is also required.
Chemistry unpluggedFor any corrosive process to occur chemically, two separate electrochemical reactions are required.
The obvious first process is the one where the metal dissolves. The free metal gives up its electrons, becomes a positively charged ion, and goes into solution.
This can be summarized by the following equation:
Metal —> Positive Metal Ion + Free Electrons
Free electrons don’t last very long; they are readily consumed by other chemical species. In aqueous (water-based) systems, the corrosion mechanism usually involves oxygen that scavenges the free electrons according to the following equation:
Oxygen + Free Electrons + Water —> Hydroxide Ions
In the lithium bromide-based absorption chiller, the iron component of the steel dissolves when air (containing oxygen) gets into the system. The result is ferrous hydroxide, a rust-like substance. When heated, the ferrous hydroxide changes to the black iron oxide found in every absorber.
This is obviously an oversimplification of the corrosion inside the absorption chiller. The lithium ion from the lithium bromide brine is very aggressive and will readily associate with the free hydroxide ions in solution. This tends to force the oxygen into giving up its electrons. With all of these electrons available, the iron dissolves quite easily and rapidly.
In addition, the pumps provide a source of stray electricity (electrons) as well. Having such an abundant supply of electrons in a brine solution such as lithium bromide makes the corrosion of iron and copper metal very likely to occur.
This is, then, the chemical mechanism whereby lithium bromide attacks the internal metal components of absorption chillers. It cannot be ignored because the action creates havoc on the workings of the unit. Owners and operators of absorption chillers must deal with its chemistry as well as its mechanical systems.
Necessary testingIn order to monitor what’s going on chemically inside the chiller, a chemical analysis of the lithium bromide is required. This usually consists of tests for the following (summarized in Table 1):
- Specific gravity;
- Alkalinity and pH;
- Copper content;
- Suspended solids; and
- Inhibitor concentration.
The specific gravity will indicate the concentration of the lithium bromide, whether it is at working strength, or too dilute to function as an absorption medium.
The pH and alkalinity can provide an indication of any air leakage into the chiller. It can also give some insight into which metals are likely to be attacked. For example, at low alkalinity and pH values, the ferrous or steel components are susceptible to corrosion. Raising the alkalinity can protect these metals while promoting an attack on the copper portions, such as the tubes.
The copper content and suspended solids will show the magnitude of the corrosive attack and also indicate how aggressively the lithium bromide charge is dissolving the machine’s components. It will likewise indicate the degree of fouling inside the chiller, and whether or not the spray headers and heat exchangers are likely to be clogged with debris.
Finally, measuring the inhibitor will show how these additional chemicals are performing inside the solution. Inhibitors are added to the lithium bromide brine to protect the various metallic surfaces and slow the rate of corrosion to acceptable levels.
Corrosion can never be totally stopped; it can only be minimized. This is the function of the inhibitor.
Inhibiting corrosionSeveral different corrosion inhibitors have been used to reduce the brine’s attack on the internal metals. These include high-solution alkalinity, lithium nitrate, lithium chromate, lithium arsenite, and lithium molybdate.
Lithium bromide brine at 54% concentration has a pH close to 7.0 or neutral. But because it is highly aggressive on ferrous metals, raising the alkalinity will tend to calm this attack. However, in the copper-iron world of the absorption chiller, a higher alkalinity will promote copper corrosion. So it is a trade-off; if you protect the iron, you sacrifice the copper-based tubing.
At lower alkalinity levels from those beneficial to ferrous metals, the corrosion of the copper tubing is less, but the iron and steel will suffer.
Lithium nitrate was used along with a slightly lower alkalinity to protect the steel surface. It encouraged the development of the black iron oxide material. In theory, if the black oxide covered the exposed metal surface, then fresh corrosion could be blocked. But the oxide layer accumulated to the point where it became too thick and it fell off the metal surface. This fouled the tubes, etc., and exposed fresh surfaces to additional attack.
Further, when nitrate was added to the complex chemistry of the lithium bromide absorption chiller, it formed ammonia. This not only created more noncondensables in the system, reducing its internal vacuum, it added a new, specific corrosive agent.
Ammonia is highly corrosive on copper-based metals, and its attack is not limited to general dissolution of the metal. Ammonia can weaken the microscopic grain boundaries of the copper and cause copper materials to literally come apart or crack under stress. This phenomenon is known as stress corrosion.
Similar to nitrate, lithium arsenite was used for a short while in some absorbers. Most of these units contained stainless steel tubes, but under specific conditions, these likewise developed stress-related failures. Worse still, the ammonia equivalent in the arsenic-based system is arsine gas, which is toxic.
Lithium chromate is a true corrosion inhibitor used in the lithium bromide system. At lower alkalinity levels, the chromate is able to dramatically reduce the corrosive effect of the brine on mild steel. Unlike the nitrate, which promoted the iron oxide growth, the chromate merely passivated (made inactive or less reactive) the steel surface. This meant that the steel did not corrode, and noncondensable gas (mostly hydrogen) was not produced.
The chromate at lower alkalinity levels protected the copper portions of the units as well, and since it did not provide a source of ammonia production, the possibility of copper stress corrosion was gone.
However, chromates are pollutants and known carcinogens; therefore, they must be controlled. They cannot be discharged into the environment, and workers handling them must wear appropriate protection. Nonetheless, chromate remains highly effective in lithium bromide absorption chillers.
To moderate some of the environmental and health concerns associated with chromate, lithium molybdate is currently being used. Like chromates, molybdates are effective in reducing lithium bromide corrosion.
But, simply put, molybdates aren’t as effective as chromate. Steel corrosion rates associated with lithium bromide-molybdate systems are higher than those found where chromate is used.
This describes briefly the chemistry of lithium bromide absorption chillers.
An understanding of the chemical aspects of lithium bromide, as well as the various types of chemicals used as inhibitors, can help maintenance personnel and operators achieve better efficiency from the absorption chiller.