Absorption Chillers Explained: Cooling Without a Compressor

Almost all the cooling in the world runs on one machine. An electric compressor squeezes a refrigerant, the refrigerant expands, and the expansion absorbs heat. It works, it is thoroughly understood, and it consumes a great deal of electricity at exactly the hours when electricity costs most.

There is an alternative that most engineers have heard of and relatively few have examined closely. It has no compressor, draws almost no power, and runs on heat that many plants are already throwing away.

What Is an Absorption Chiller?

An absorption chiller produces chilled water using a chemical process in place of mechanical compression. Water evaporates under deep vacuum, drawing heat from the circuit being cooled, the same physics that makes perspiration cooling. The resulting vapour is then absorbed into a concentrated salt solution, typically lithium bromide or lithium chloride, rather than being compressed by a machine.

The solution dilutes as it takes up water. To regenerate it, heat is applied in a generator, driving the water back out as vapour, which is condensed and returned to the evaporator. The reconcentrated solution flows back to the absorber and the cycle repeats continuously.

Electricity is required only for solution pumps and controls. Against a compressor of equivalent capacity, that is a rounding error.

Why the salt solution matters

The whole cycle turns on vapour pressure. A concentrated lithium halide solution has a vapour pressure far below that of pure water at the same temperature, and that difference is the force pulling vapour across from the evaporator. The stronger the solution, the harder it pulls.

This is why concentration control sits at the centre of operating one of these machines well, and why the specification of the salt is not a trivial purchasing detail. Everything the machine does depends on the solution behaving as designed.

Where the Economics Work

The heat source decides everything

The case for absorption cooling rests almost entirely on where the driving heat comes from. If it must be generated specifically for the purpose, the arithmetic rarely works: burning fuel to make heat to make cooling is less efficient than burning the same fuel to make electricity for a compressor.

Most industrial sites, however, are not short of waste heat. Exhaust from gas turbines, steam that has already done its useful work, hot process streams on their way to a cooling tower, all of it is routinely dumped to atmosphere. Routing it into an absorption chiller converts a disposal problem into cooling capacity, and the marginal running cost approaches zero.

Demand charges

There is a second economic argument that often decides the project. Many industrial tariffs penalise peak electrical load heavily, and air conditioning in a hot climate peaks at the same hour as everything else on site. Moving that load off the electrical system entirely can reduce a demand charge enough to justify the plant on its own, independent of energy savings.

Where it is installed

Refineries and chemical works with surplus process heat. District energy schemes. Large hospitals and hotels running combined heat and power, where the engine jacket water and exhaust would otherwise be wasted. Increasingly, solar thermal installations, where collector output peaks at precisely the hour cooling demand peaks, a coincidence that makes the technology unusually well suited to hot climates.

The Practical Constraints

Size and siting

Absorption plant is not a drop-in replacement for a compressor chiller. The machines are physically larger for the same capacity, considerably heavier, and need a reliable heat supply at the right temperature. Too cool a source and the generator cannot drive water out of the solution fast enough to sustain the cycle.

Vacuum integrity

The machine operates under deep vacuum, so air ingress degrades performance immediately and allows corrosion to begin. Leak detection and purge systems are not optional extras; they are what keeps the machine working.

Corrosion and the inhibitor

Lithium halide solutions are corrosive to carbon steel. A corrosion inhibitor is dosed into the solution and must be monitored through the machine life. Allow it to deplete and the shell begins to be attacked, generating hydrogen that further spoils the vacuum, a failure that compounds rather than stabilises.

Crystallisation

If solution concentration rises too far, the salt comes out of solution and blocks the heat exchanger. This typically happens because cooling water to the absorber is too cold, or because the machine has been pushed beyond its design duty. Recovery is possible but disruptive and expensive, and avoiding it is the principal reason concentration is monitored so closely.

Lithium Chloride and Liquid Desiccant Systems

The same property that makes lithium chloride useful in absorption cooling, an intense affinity for water vapour, puts it at the centre of liquid desiccant dehumidification. A concentrated solution pulls moisture directly out of an air stream without first cooling the air below its dew point, which is how conventional systems do it.

That matters where latent load dominates: textile plants, pharmaceutical facilities, cold stores, swimming pool halls, anywhere humidity rather than temperature is the problem. Removing moisture without over-cooling and reheating saves a great deal of energy.

Purity carries more weight here than buyers often expect. Sulphate and heavy metal contamination accelerates corrosion and shortens equipment life, and the cost of replacing a fouled absorber dwarfs any saving on the salt. Suppliers such as Bee Chems manufacture the lithium salts these systems depend on, supplying against a batch certificate of analysis so the specification is documented rather than assumed.

Frequently Asked Questions

Q1. How does an absorption chiller work without a compressor?

Ans. Water evaporates under vacuum and absorbs heat from the chilled water circuit. A concentrated lithium salt solution then absorbs the resulting vapour, and applied heat drives that water back out so the solution can be reused. The salt solution performs the role the compressor plays in a conventional machine.

Q2. Is an absorption chiller more efficient than an electric chiller?

Ans. Not in thermodynamic terms. Its coefficient of performance is lower. It wins when the driving heat is free or already being wasted, because the comparison that matters then is operating cost and electrical demand, not efficiency.

Q3. What is the difference between lithium bromide and lithium chloride in these systems?

Ans. Both are strongly hygroscopic lithium halides used as absorbents. Lithium bromide is the more common choice in large absorption chillers; lithium chloride is widely used in liquid desiccant dehumidification and in smaller or specialised systems. The selection depends on the operating range, materials of construction and the machine manufacturer design.

Q4. What causes crystallisation in an absorption chiller?

Ans. Solution concentration rising beyond the solubility limit, usually because absorber cooling water is too cold or the machine is operating outside its design envelope. The salt precipitates and blocks the heat exchanger, stopping the machine.

Q5. Why does the corrosion inhibitor need monitoring?

Ans. The lithium halide solution attacks carbon steel. The inhibitor protects the shell, and as it depletes, corrosion begins and generates hydrogen that degrades the vacuum. Both effects compound, so inhibitor level is checked as part of routine solution analysis.

In Summary

For plants with heat to spare, absorption cooling remains one of the few genuinely free lunches in energy engineering. The technology is old, which occasionally counts against it in procurement discussions, but the physics has not changed, the waste heat is still being discarded, and the electricity it displaces has only grown more expensive.

What determines whether a machine runs for twenty years or needs its shell replaced in eight is rarely the make. It is the solution chemistry, the inhibitor package and the purity of the salt, which makes it worth specifying properly at the outset.