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Author:
Anna Frank, Equipment Selection Expert
Anna Frank, Equipment Selection Expert
Heat Rejection in Absorption Chillers
An absorption chiller cooling tower carries a heavier duty than the evaporative cell paired with an electric machine of the same tonnage. The reason sits inside the cycle: a single-effect LiBr unit runs at a COP near 0.7, so every refrigeration ton pushes roughly 29,000 Btu/h to the condenser loop. A centrifugal compressor at COP 5.5 rejects about 14,200 Btu/h per ton. Engineers often reduce the cooling tower and absorption relationship to a single figure, the heat rejection factor: load multiplied by (1 + 1/COP). For single-effect machines that factor lands between 2.3 and 2.5; double-effect designs bring it down to 1.7–1.9; vapor-compression units sit near 1.2.
That factor defines the cooling tower requirement for an absorption chiller: the cell must absorb the evaporator load plus the full generator heat input. In tower tons (15,000 Btu/h each) a 500-ton single-effect machine looks like a 950–970-ton evaporative plant. Ignore this at the bid stage and the project inherits an undersized cell, elevated condenser temperatures, and a machine that never reaches nameplate capacity.
That factor defines the cooling tower requirement for an absorption chiller: the cell must absorb the evaporator load plus the full generator heat input. In tower tons (15,000 Btu/h each) a 500-ton single-effect machine looks like a 950–970-ton evaporative plant. Ignore this at the bid stage and the project inherits an undersized cell, elevated condenser temperatures, and a machine that never reaches nameplate capacity.
Cooling Tower by Torch-Air
One more feature separates this duty from a compressor plant: inside the machine the condenser loop usually flows in series, through the absorber bundle first and the condenser bundle second, because dilution in the absorber releases more heat than refrigerant condensation does. The loop therefore sees a higher total rise across the machine, and the firing source matters too — steam, hot exhaust, or 190°F process liquid each shape the rejection profile differently at part load.
Tower Types for Absorption Duty
Selecting a cooling tower for absorption chiller service starts with the draft and flow arrangement. Counterflow induced-draft cells dominate this application: air travels against the falling film, the coldest droplets meet the driest air, and approach temperatures of 5–7°F stay achievable at 4+ gpm/ton loading. Crossflow units accept the higher flow with lower pumping head, at the cost of a wider footprint. Field-erected counterflow structures serve plants above roughly 1,000 tons; factory-assembled cells cover the rest. Closed-circuit (fluid cooler) designs isolate the condenser loop from airborne debris and suit machines that cycle seasonally, though the extra approach of the internal coil, typically 5–8°F, must be bought back with more fill surface or airflow.
Towers by Torch-Air
Materials deserve early attention because absorption plants often run year-round on heat recovery. FRP casings and stainless hardware outlast galvanized steel where the circulating liquid carries chlorides; basin heaters, equalizer lines, and remote sumps keep winter operation safe when the machine tracks a data-center or process load through freezing weather.
Process sites add one more layer. Facilities running absorption distillation and cooling towers on a shared circuit — refineries, gas plants, solvent recovery units — must budget the loop for the dirtiest user, because column condensers shed hydrocarbons and fines that end up on the fill. CTI STD-201 thermal certification is worth specifying in either case; an uncertified cell that under-delivers by 10% erases the entire safety margin built into the heat rejection factor. For loops too aggressive for standard equipment — plating rinses, scrubber blowdown, brine — we take on custom OEM design of corrosion-proof heat-rejection cells in polypropylene and FRP, drawing on the same fan, drive, and drift-control packages we build for gas cleaning.
Process sites add one more layer. Facilities running absorption distillation and cooling towers on a shared circuit — refineries, gas plants, solvent recovery units — must budget the loop for the dirtiest user, because column condensers shed hydrocarbons and fines that end up on the fill. CTI STD-201 thermal certification is worth specifying in either case; an uncertified cell that under-delivers by 10% erases the entire safety margin built into the heat rejection factor. For loops too aggressive for standard equipment — plating rinses, scrubber blowdown, brine — we take on custom OEM design of corrosion-proof heat-rejection cells in polypropylene and FRP, drawing on the same fan, drive, and drift-control packages we build for gas cleaning.
Packed Bed and Trayed Internals
Terminology trips up even experienced HVAC engineers here, because two industries use the same words for different hardware. In evaporative service, a packed bed cooling tower means a cell filled with film media: cross-corrugated PVC sheets with 45–70 ft²/ft³ of specific surface, or splash bars and grid decks where the circulating flow carries solids. In mass-transfer service, a packed column holds random rings or structured sheets and strips a gas into a liquid. The contact physics is identical — maximize interfacial area, minimize pressure drop — which is why packed bed cooling tower equipment and gas-cleaning columns share media suppliers, support grids, and distributor designs. We build on the same principle: our TORNADO RP vertical wet scrubber carries a random-packing bed rated for 600–24,000 CFM, and the Sirocco line of adsorbers applies a fixed granular bed where the media itself does the filtration work.
Trays follow the same logic. Queries about trayed tower absorption refrigeration almost always point at the machine internals, at columns inside ammonia–water units where bubble-cap or sieve plates rectify the vapor leaving the generator. A trayed tower in gas absorption chiller service handles two jobs at once: it strips refrigerant vapor from the weak solution and knocks entrained liquid back down. LiBr machines skip trays entirely and rely on falling-film tube bundles. The evaporative cell outside the machine never contains plates; if a spec sheet mentions them, it describes the generator column. Plate contactors remain current in gas cleaning — our TYPHOON tray tower scrubber covers 100–175,000 CFM on exactly this principle, with the perforated plates acting as the filtration stage for soluble gases and droplets.
Trays follow the same logic. Queries about trayed tower absorption refrigeration almost always point at the machine internals, at columns inside ammonia–water units where bubble-cap or sieve plates rectify the vapor leaving the generator. A trayed tower in gas absorption chiller service handles two jobs at once: it strips refrigerant vapor from the weak solution and knocks entrained liquid back down. LiBr machines skip trays entirely and rely on falling-film tube bundles. The evaporative cell outside the machine never contains plates; if a spec sheet mentions them, it describes the generator column. Plate contactors remain current in gas cleaning — our TYPHOON tray tower scrubber covers 100–175,000 CFM on exactly this principle, with the perforated plates acting as the filtration stage for soluble gases and droplets.
Packed-bed foam column
Sizing the Cooling Tower
Here is how to size a cooling tower for an absorption chiller in five steps, using a 300-ton single-effect unit as the example. First, compute rejection: 300 × 29,000 = 8.7 MMBtu/h. Second, set the condenser loop flow; per AHRI Standard 560 rating conditions the machine expects 85°F entering liquid, and at 4.5 gpm/ton the loop runs 1,350 gpm. Third, derive the range: 8,700,000 ÷ (1,350 × 500) = 12.9°F, so the return comes back near 98°F. Fourth, fix the design wet bulb for the site — 78°F across much of the Gulf states — and check that 85°F entering equals a 7°F approach, which is realistic for a counterflow cell at this loading. Fifth, select the cell for 8.7 MMBtu/h at 12.9°F range and 7°F approach, then verify fan power and drift data against the vendor curve.
Sizing cooling towers for absorption chillers by rule of thumb — taking the nameplate tons and adding 60% — works only until the wet bulb, the loop flow, or the effect count changes. The five-step sequence above catches all three. A parallel habit from the design of packed towers for absorption chillers transfers well: always check the media at maximum liquid loading, because film fill floods and loses efficiency just as random packing does. Two verification items close the exercise. Confirm the vendor curve at 105% of design flow, since commissioning teams routinely trim pump impellers late; and compare fan power between candidate cells, because a larger, slower unit frequently pays back its footprint within two summers of operation.
Sizing cooling towers for absorption chillers by rule of thumb — taking the nameplate tons and adding 60% — works only until the wet bulb, the loop flow, or the effect count changes. The five-step sequence above catches all three. A parallel habit from the design of packed towers for absorption chillers transfers well: always check the media at maximum liquid loading, because film fill floods and loses efficiency just as random packing does. Two verification items close the exercise. Confirm the vendor curve at 105% of design flow, since commissioning teams routinely trim pump impellers late; and compare fan power between candidate cells, because a larger, slower unit frequently pays back its footprint within two summers of operation.
TORNADO RP Vertical Wet Scrubber With Random Packing
Condenser Water Temperature Control
Absorption machines punish cold condenser flow. Below roughly 70°F entering, the concentrated LiBr solution in a single-effect unit cools faster than it dilutes, and crystallization becomes a real risk; the salt solidifies in the heat exchanger and the machine locks up until a service crew melts it out. Manufacturers publish a minimum entering temperature — commonly 68–75°F — and the tower controls must hold it through winter operation and morning starts. Standard practice stages the fans first, then modulates a bypass valve blending warm return into the supply. VFD-driven fans hold the setpoint tighter than on-off staging and cut parasitic power at part load.
Cold starts need their own sequence. A machine waking up against a 55°F basin should see the bypass fully open and the fans parked; as the loop warms past the manufacturer's floor, fans stage in and the valve trims toward the setpoint. Program the ramp over 20–30 minutes — snapping to full airflow drags the entering temperature back down and stalls dilution just as the cycle establishes itself.
The same discipline applies to trayed tower absorption chillers of the ammonia–water type: rectifier plates lose separation efficiency when reflux runs too cold, so the condenser loop setpoint protects process performance as much as hardware. Instrument the loop honestly — supply and return RTDs, a flow meter on the pump discharge — because a drifting sensor that reads 5°F high will quietly starve the machine for months.
Cold starts need their own sequence. A machine waking up against a 55°F basin should see the bypass fully open and the fans parked; as the loop warms past the manufacturer's floor, fans stage in and the valve trims toward the setpoint. Program the ramp over 20–30 minutes — snapping to full airflow drags the entering temperature back down and stalls dilution just as the cycle establishes itself.
The same discipline applies to trayed tower absorption chillers of the ammonia–water type: rectifier plates lose separation efficiency when reflux runs too cold, so the condenser loop setpoint protects process performance as much as hardware. Instrument the loop honestly — supply and return RTDs, a flow meter on the pump discharge — because a drifting sensor that reads 5°F high will quietly starve the machine for months.
Water Quality and Fouling Prevention
Evaporation concentrates everything the makeup brings in. At 3–6 cycles of concentration, blowdown equals evaporation divided by (cycles − 1), and the evaporation itself runs close to 1% of loop flow per 12.5°F of range. Absorption duty tolerates fouling worse than comfort duty because the machine's internal temperature differences are small; a 0.001 hr·ft²·°F/Btu fouling factor on the condenser tubes can shave several percent off capacity. Side-stream filtration on 3–5% of the flow keeps fines out of film media, and drift eliminators rated at 0.005% limit both liquid loss and the plume of dissolved solids leaving the cell. The vane packs doing that job are close kin to the mist eliminators we fabricate for scrubber service, sized by the same droplet-capture math. Track the Langelier index of the blended circulating stream, and hold it slightly negative if the media has narrow passages: scale that grows on 19-mm flutes closes them far sooner than it would a splash deck. Media selection should start from the measured makeup analysis rather than a clean-supply assumption — passage size, flute geometry, and material each set a ceiling on tolerable solids.
Biological control rounds out the program. Warm, aerated, nutrient-rich flow is an ideal incubator, and fouled fill loses thermal capability long before it plugs. Quarterly inspection of the media, the distribution basin, and the eliminators costs an afternoon; replacing collapsed film packs costs a season.
Biological control rounds out the program. Warm, aerated, nutrient-rich flow is an ideal incubator, and fouled fill loses thermal capability long before it plugs. Quarterly inspection of the media, the distribution basin, and the eliminators costs an afternoon; replacing collapsed film packs costs a season.
Mist Eliminator. High-Efficiency Droplet Separation for Clean Air Systems
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Vladimir Nikulin
Vladimir Nikulin
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