Absorption chillers and heat pumps: questions and answers

How does an absorption chiller work?

It makes cooling from heat, not electricity. The refrigerant is plain water and the absorbent is a lithium bromide solution. The shell holds a vacuum of about 6 mm Hg, so water boils at 3–5 °C and chills the customer's water, typically to 7/12 °C. Strong solution absorbs the water vapor in the absorber, and in the generator steam, hot water or a burner boils the water back out of it. There is no compressor and no refrigerant gas: if the shell leaks, air gets in and nothing gets out. More in the article «What is an absorption chiller»

When is an absorption chiller cheaper to run than an electric chiller?

When heat is cheap or wasted and grid capacity is expensive or unavailable. We check four conditions:

1. Waste heat from 80 °C: summer boiler-house water, gas engine jacket water and exhaust, process steam.
2. Grid capacity is short, and a new connection takes years and its own budget.
3. Gas is available with a supply limit, and electricity is expensive.
4. Cooling is needed for 3,000–4,000 hours a year or more.

Often the answer is "both": absorption for the base load, an electric chiller for peaks and backup. More in the guide «Absorption or electric chiller»

From what capacity does an absorption chiller make sense, and what is the payback?

It is worth considering from 300 kW of cooling and 3,000–4,000 operating hours a year; then payback is usually 3–5 years. Below 100 kW the machine is expensive per kilowatt and few models exist, and below 2,000 hours a year it will not pay back. That is why a small gas engine often does not justify one: 300 kW of hot water gives a single-effect machine about 200 kW of cooling. On waste steam we have seen payback within a few months. More in the article «How to cut industrial cooling costs»

Is it true that an absorption chiller needs no electricity?

No, but it needs little. Power goes to the solution and refrigerant pumps and the controls, plus the burner on a gas-fired unit. With no compressor, this is a small fraction of what a compressor chiller of the same capacity draws. Pumps of the three water circuits and tower fans are counted separately, and the burner gets its own line in the power brief. Without power the machine cannot shut down properly, so it needs backup for the dilution cycle. The exact figure is on the model's data sheet.

What heat can an absorption chiller run on: steam, hot water, gas, exhaust?

Any of them, but each needs its own machine type. Single-effect units run on hot water at 70–110 °C (economically from 80 °C) or low-pressure steam, with a thermal COP of about 0.7. Double-effect units take steam at 4–9 bar or burn gas or diesel and reach 1.30–1.45. Exhaust-fired units (275–600 °C) come in both versions. Only direct-fired machines have a burner; hot water machines do not. With steam at 5 bar or more, compare double- and single-effect over a 10-year horizon. More in the article «What is an absorption chiller»

Can a plant's waste heat be turned into cooling?

Yes, if the heat is hot enough: process steam, hot water from 80 °C, engine exhaust, furnace heat. Warm air at 45 °C or server return water at 30–45 °C will not drive an absorption chiller, which needs 80 °C or more. For selection we need the source data: pressure and dryness of the steam, or supply and return temperatures and flow of the water, plus an hourly profile. Low-grade heat at 25–50 °C can also be put to work, but as hot water from an absorption heat pump. More in the article «Absorption heat pump: recovering waste heat»

Why does an absorption chiller need a bigger cooling tower than an electric chiller?

Because the tower takes both the customer's heat and all the driving heat. Per 1 kW of cooling, a single-effect machine rejects 2.45 kW, a double-effect one 1.72 kW, and an electric chiller with COP 5 only 1.20 kW. Per 1,000 kW of cooling, tower evaporation is about 3.6 m³/h for a single-effect machine against 1.8 m³/h for the chiller. A tower, pumps and pipes sized "as for a chiller" will not carry an absorption machine, and it shows in the first hot summer. More in the article «What is an absorption chiller»

What is crystallization in an absorption chiller and how do you prevent it?

Salt drops out of an overcooled strong lithium bromide solution and the machine stops. Main causes: cooling water too cold in spring and autumn, and a long power cut without a dilution cycle. Protection:

1. A three-way cooling tower bypass that discharges into the tower basin, valve stroke no longer than 120 s.
2. Minimum cooling water temperature per model data sheet: 10 °C for Thermax double-effect machines, 20 °C for the LT and 5G hot water series.
3. A UPS or backup feeder for the dilution cycle.

More in the resource «7 design guides for engineers»

How much cooling can 1 MW of heat or a tonne of steam give?

The thermal COP gives the estimate. A single-effect machine (about 0.7) makes roughly 700 kW of cooling from 1 MW of heat, a double-effect one (1.30–1.45) makes 1.3–1.45 MW. For steam: a double-effect machine needs about 1.075 kg/h of steam at 8 kgf/cm² gauge per 1 kW of cooling, a single-effect one about 2.36 kg/h. So one tonne of steam per hour gives roughly 930 or 420 kW of cooling. Use these figures to check a calculation; the working figure comes from the manufacturer's selection for your conditions. More in the resource «7 design guides for engineers»

Which absorption chiller suits a gas engine in a trigeneration plant?

It depends on which engine heat you use. For jacket water at 90–95 °C, a single-effect hot water machine with a COP of 0.7–0.85. If the exhaust at 275–600 °C is free, an exhaust-fired machine gives almost twice the cooling from the same heat. The price is a back-pressure check: the machine itself adds 200–250 mm H₂O, plus ducts, diverter damper and silencer, and the total must stay within the engine's allowable limit. Select on the engine data at 100, 75 and 50 % load. More in the article «Trigeneration: when it pays off»

What happens to an absorption chiller during a power cut?

A dip of up to 3 minutes is fine. A longer outage needs a dilution cycle, otherwise the solution may crystallize, so the design includes a UPS or a backup feeder for the machine. The steam control valve must close on power failure. A second interlock matters too: if chilled water flow stops, cooling water must stop within a minute, or the water in the evaporator can freeze. More in the article «What is an absorption chiller»

Can an absorption chiller deliver water below 5 °C or sub-zero temperatures?

Down to −5 °C, yes; below that, no. The LT and 5G hot water series reach 3.5 °C, the gas-fired 2V reaches 1 °C on water and −2 °C on glycol, and special designs go to −5 °C. That is the limit of lithium bromide absorption. A cold store at −18 °C needs a compressor machine on glycol. More in the article «What is an absorption chiller»

What happens to cooling if the gas engine stops or heat is unavailable?

Cooling is lost unless backup is designed in. In trigeneration the site switches to the grid and the absorption chiller is fed from a boiler house, so both stay in the scheme. If heat is missing part of the year, an electric chiller in the same plant is the backup. After a stop the absorption chiller needs about 30 minutes to reach full output, so continuous processes and data centers need a buffer or chiller beside it. Tier IV needs a second independent heat source or full-load chillers. More in the article «Absorption chillers for data centers short of power»

How long does an absorption chiller last, and is it hard to maintain?

Service life is 25 years or more, and real installations run 25–30 years without a major overhaul. Every machine is helium leak tested before shipment, and a corrosion inhibitor is added to the solution. There is no compressor, only pumps move, the controls run without a permanent operator, and staff training takes 2–3 days. The main operating issue is water: 1 mm of deposits in the absorber and condenser costs about 14 % of capacity. Solution concentration and vacuum are checked too, since air leaks sharply cut output. More in the resource «Summer cooling readiness checklist»

What is an absorption heat pump and where does it pay off?

An absorption machine that makes 60–105 °C hot water from 25–50 °C waste water by adding steam, gas or hot water. A single-effect unit gives 1.65–1.75 kW of heat per 1 kW of driving heat, so about 40 % of the heat is free; against a gas boiler that saves about a third of the gas. It makes sense when the site has a waste stream, a hot water user and driving heat at the same time, most of the year. It goes in a heated room, never outdoors. More in the article «Absorption heat pump: recovering waste heat»

How does a Type I heat pump differ from a Type II heat transformer?

Type I adds heat; Type II raises the temperature of part of the waste heat with no purchased fuel. A heat pump needs high-grade heat, waste heat and a user for all of it; a single-effect unit has a COP of 1.65–1.75. A heat transformer uses only 40–60 °C waste heat, rejects about half to 15–20 °C water, and at a COP of about 0.5 lifts temperature by 15–30 °C. Do not compare 0.5 and 1.7 directly: Type I divides by purchased fuel, Type II by heat already being wasted. More in the article «Absorption heat pump: recovering waste heat»

How much can an absorption heat pump lift temperature, and how is the lift counted?

A single-effect unit lifts up to 40 °C with confidence; the Thermax working limit is 55 °C. Count the lift from the waste source inlet temperature to the hot water outlet, not "outlet minus return": that halves the lift and produces a COP the machine will never reach. A bigger lift means a lower COP and less capacity: the same SS 80D frame gives 13,886 kW at a 90 °C outlet and 7,504 kW at 105 °C. A lift above 80 °C needs a cascade of two machines. More in the article «Absorption heat pump: recovering waste heat»

Where does the heat pump's heat go in summer, when space heating is off?

Into a backup sink: a cooling tower or dry cooler for the remaining capacity. Without it the machine hits its upper temperature limit, goes into dilution and stops, wasting steam. Better still, find a year-round user such as process heat or domestic hot water. If the site needs cooling in summer, there is a chiller-heater version: one machine instead of a chiller and a hot water generator saves up to 32 % of steam. Modes switch twice a year by calendar: every switch is a stop with a 20-minute dilution. More in the article «Absorption heat pump: recovering waste heat»

Which heat pump COP should go into the savings calculation?

For money, use the COP from the gas meter. 1.72 is heating capacity divided by heat input; it compares machines and in theory equals 1 plus the cooling COP of 0.70–0.75. 1.46 comes from gas use, including the burner and the gap between higher and lower heating value, and goes into the economics; steam units have no such split. Annual savings follow the smallest of the source, user and driving heat hours: with space heating at 4,600 hours, counting by a source running 8,400 hours overstates savings almost twofold. More in the resource «7 design guides for engineers»

How heavy is an absorption chiller, and what room does it need?

Size the floor for the flooded weight, not the operating weight: the machine completely filled with water. Example for a 400 kW unit: 4.8 t dry, 5.8 t operating, 9.4 t flooded. Thermax issues exact weights and foundation drawings 2–4 weeks after the order; the preliminary design stage uses the catalog. The machine room must be heated: we require above +5 °C. The machine arrives assembled, so the opening and rigging route follow its overall size, and it must be level within 1 mm per 1,000 mm. More in the resource «7 design guides for engineers»

What is the minimum hot water temperature for an absorption chiller?

The LT series runs from 70 °C, but capacity there is well below rated; economically, from 80 °C. If the water is cooler or there is less of it than designed, the machine gives less cooling: a different duty means a different capacity, recalculated by the manufacturer from supply, return and flow. Sharp swings in hot water flow and temperature hit capacity immediately. If the source gives only about 75 °C in summer, compare such a machine with a direct gas-fired absorption chiller. More in the resource «7 design guides for engineers»

Can an absorption chiller work with a dry cooler instead of a wet cooling tower?

Only with a special Thermax selection for warmer water, at reduced capacity. The rated cooling water inlet is 29.4 °C, while a dry cooler gives 33–38 °C in hot weather. The working option is an evaporative cooling tower. A closed tower keeps the absorber water clean but delivers water 2–3 °C warmer than an open one, which the selection takes into account. More in the article «Cooling tower vs dry cooler»

Can an absorption chiller run in winter and all year round?

Yes, if the cooling water circuit allows it. Tower water must stay above the model minimum: 10 °C for Thermax double-effect units, 20 °C for LT and 5G. So the design has a tower bypass to the basin, the fan stays off at start-up until the machine inlet reaches 20 °C, the tower is rated for winter and the room is heated. Often it pays to stop the machine in winter: at our 4.6 MW plant towers send 12 → 7 °C water straight to production through heat exchangers. More in the article «What is an absorption chiller»

What should a gas-fired absorption chiller design cover: gas, flue, shared stack?

Put the gas demand into the utility request from day one: a 1,000 kW unit with a COP of 1.4 needs about 77 m³/h. Burner inlet pressure is 60–100 mbar. Flue pressure at the outlet stays below 5 mm H₂O, so keep the duct short with 45° bends. Thermax does not recommend a stack shared with a boiler or another machine. If unavoidable: own damper and draft regulator on each, connections at least 1 m apart vertically, no common horizontal header. Room and gas piping follow local gas-safety rules. More in the resource «7 design guides for engineers»

Two-way or three-way valve on the absorption chiller's hot water side?

Three-way. Thermax requires a modulating three-way valve at the hot water inlet, controlled by the chilled water outlet temperature. Two layouts are correct. Mixing: the machine has its own pump and constant flow through the generator. Diverting: surplus water bypasses the machine to the return; this suits a gas engine that needs constant flow through its jacket. A valve on the generator outlet is a mistake. Also check the scope: the three-way valve ships only if it is listed in the supply. More in the resource «7 design guides for engineers»