Absorption Chiller Maintenance: Vacuum, LiBr Chemistry & Crystallisation

Absorption chillers are famously long-lived — 25 to 30 years is routine — but only when three disciplines are respected: vacuum integrity, lithium bromide chemistry, and crystallisation prevention. Neglect any one and the same machine can be scrap in a decade. This guide covers what actually matters, drawn from field service across the UK and Europe.

An absorption chiller has very few moving parts — two or three hermetic pumps and some valves — so it does not fail the way a compressor-based machine fails. It fails slowly and quietly, through neglected chemistry and a creeping loss of vacuum, until one summer day the capacity simply is not there. The good news is that the failure modes are well understood and entirely preventable. This is the maintenance guide we wish every operator had.

Discipline 1: vacuum integrity

The machine's entire function depends on absolute pressures of around 1 kilopascal — roughly one-hundredth of atmosphere. Because everything is below atmospheric pressure, any leak draws air in, never refrigerant out. And even small amounts of non-condensable gas devastate performance by blanketing the absorber surfaces and raising evaporator pressure.

Symptoms of vacuum loss

• Chilled-water temperature creeping above setpoint at unchanged load.
• The purge unit running more frequently, or continuously.
• Rising solution concentrations as the controls fight to hold capacity.
• Gradual capacity fade over weeks — the classic slow-leak signature.

What good practice looks like

• A 24-hour vacuum decay test at least annually: isolate the machine, log the absolute pressure, and compare the pressure rise against the manufacturer's pass criterion.
• Purge-system maintenance. The purge is the machine's immune system — its pump, valves and pick-up lines must be serviced, not merely observed.
• Palladium cell and hydrogen management where fitted, since internal corrosion generates hydrogen even in otherwise tight machines.
• Leak detection — helium or pressure methods — the moment decay tests begin trending the wrong way.

Discipline 2: lithium bromide chemistry

The solution is the machine's bloodstream, and it degrades invisibly. Annual laboratory analysis should verify four things:

• Corrosion-inhibitor level (typically lithium chromate or molybdate systems). Depleted inhibitor means the steel shell is being consumed from the inside — the single most expensive failure to let develop.
• Alkalinity. A drifting pH accelerates corrosion and copper transport around the machine.
• Performance additive (octyl alcohol) content. This dramatically improves absorption; it also degrades over time and needs replenishment to maintain capacity.
• Contaminants. Iron and copper in the solution are the corrosion products of a machine consuming itself. Rising trends demand action, not just observation.

Discipline 3: crystallisation prevention

Concentrated lithium bromide solution sits near a solubility cliff. If it becomes too concentrated while its temperature falls too low, the salt crystallises — turning solid in the solution heat exchanger and stopping the machine. The common triggers we see in the field are:

• Cooling water too cold — a tower running unrestrained in cool weather chills the concentrated solution below its crystallisation limit.
• Power loss during operation, which skips the dilution cycle that safely mixes the solution before standstill.
• Air leaks forcing the controls toward ever-higher concentrations to hold duty — pushing the solution toward the cliff edge.
• Faulty temperature or level sensors steering the concentration control wrongly.

Prevention is procedural: correct shutdown (always allow the dilution cycle to complete), cooling-water temperature control, prompt attention to vacuum, and sensor calibration. Recovery from crystallisation — controlled heating of the affected heat exchanger — is a specialist job, and done wrongly it damages the machine further.

The supporting cast

• Tube cleaning. Fouled absorber and condenser tubes show up as widening approach temperatures. Clean on condition — typically every one to three years depending on water quality.
• Hermetic pumps. Monitor amps and noise; rebuild on interval, not on failure.
• Controls and sensors. Annual calibration of the handful of sensors on which the safety of the whole machine rests.
• Trend logging. The machines announce their problems weeks in advance to anyone recording concentrations, pressures and approach temperatures. Remote monitoring turns that early whisper into an email before it becomes a breakdown.

A sensible annual service, summarised

The payoff for discipline

Do these things every year and an absorption chiller becomes one of the most dependable assets on site — quiet, low-carbon, and reliable for a quarter-century or more. Skip them, and the machine will keep its complaints to itself until the summer day it cannot hold setpoint. The difference between a twelve-year machine and a thirty-year machine is almost never the make or the model. It is whether someone kept the vacuum tight, the chemistry healthy, and the solution safely away from the crystallisation cliff.

Catch Energy provides planned maintenance, LiBr analysis, vacuum testing and emergency support for absorption chillers across the UK and Europe. Whether you want a one-off health audit or a multi-year contract, our chiller service is built entirely around these disciplines — and our engineers are happy to answer maintenance questions even if you never become a customer.