Single-Effect Absorption Refrigeration Systems

Single-effect absorption refrigeration systems represent the most common configuration for thermal-driven cooling applications utilizing low-grade heat sources. These systems accomplish the refrigeration cycle through a single generator stage, distinguishing them from multi-effect configurations that cascade heat input through multiple generator levels. The single-effect architecture provides optimal thermodynamic performance for heat sources in the 80-120°C temperature range, making these systems ideal for waste heat recovery, solar thermal cooling, and low-pressure steam applications.

Thermodynamic Operating Principles

The single-effect absorption cycle operates through four primary heat exchange processes occurring at two distinct pressure levels. In the low-pressure circuit (evaporator and absorber), the refrigerant evaporates at approximately 4-6°C while absorbing heat from the chilled water system, then the refrigerant vapor is absorbed into the concentrated absorbent solution, releasing the heat of absorption to the cooling water system. In the high-pressure circuit (generator and condenser), heat input drives refrigerant vapor from the dilute solution, then the refrigerant vapor condenses while rejecting heat to the cooling water.

The designation “single-effect” specifically indicates that the generator heat input is utilized once to produce refrigerant vapor. This contrasts with double-effect systems where the high-temperature generator vapor condenses in a second generator, effectively using the heat input twice. The single-stage configuration limits theoretical efficiency but enables operation with substantially lower temperature heat sources that represent the majority of available waste heat streams in commercial and industrial facilities.

Heat transfer in the generator establishes the fundamental limitation on single-effect performance. The temperature differential between the heat source and the solution boiling temperature must overcome both the temperature rise required to reduce solution refrigerant concentration and the finite heat transfer coefficients in the tube bundle. For hot water heat sources, a minimum approach of 5-8°C is required, meaning an 85°C hot water supply can support approximately 77-80°C generator solution temperature.

Coefficient of Performance Characteristics

Single-effect absorption chillers achieve coefficient of performance (COP) values ranging from 0.6 to 0.7 under design conditions, representing the ratio of refrigeration effect (evaporator cooling) to heat input (generator thermal energy). This performance level reflects fundamental thermodynamic constraints imposed by operating with a single generator stage and moderate heat source temperatures.

The COP calculation accounts for all heat inputs to the system:

COP = Q_evap / (Q_gen + W_pump)

Where Q_evap represents evaporator heat absorption, Q_gen represents generator heat input, and W_pump represents solution pump work. In practice, pump work typically constitutes less than 1-2% of generator heat input and is often neglected in simplified COP calculations, yielding the common expression COP = Q_evap / Q_gen.

Performance sensitivity to operating conditions follows predictable thermodynamic trends. Each 1°C increase in chilled water supply temperature improves COP by approximately 2-3% as the evaporator pressure rises, increasing the concentration difference between strong and weak solution and reducing required circulation ratio. Conversely, each 1°C increase in cooling water supply temperature reduces COP by 2-3% as higher condenser and absorber pressures compress the operating concentration range.

Heat source temperature exerts particularly strong influence on single-effect performance. Generator temperatures below 75°C typically cause severe capacity degradation and COP reduction below 0.5 as insufficient driving force fails to achieve adequate solution concentration differential. Temperatures above 100°C provide marginal additional benefit, with COP improvements limited to 5-10% over the 100-120°C range due to increased heat losses and parasitic loads that offset thermodynamic gains.

The theoretical maximum COP for a single-effect absorption system operating between three temperature reservoirs (evaporator at T_e, absorber/condenser at T_a, generator at T_g) derives from Carnot efficiency principles:

COP_Carnot = [(T_g - T_a) / T_g] × [T_e / (T_a - T_e)]

This expression demonstrates that maximum theoretical efficiency increases with higher generator temperature and lower absorber/condenser temperature while being relatively insensitive to evaporator temperature over the typical air conditioning range. Actual COP values achieve 40-50% of Carnot efficiency due to finite temperature differences in heat exchangers, pressure drops, and irreversibilities in absorption and desorption processes.

Heat Source Temperature Requirements

Single-effect absorption chillers operate with heat sources ranging from 80°C to 120°C, with optimal performance occurring in the 90-110°C range. This temperature requirement positions single-effect systems ideally for integration with low-pressure steam (0.5-1.5 bar gauge), engine jacket water cooling (80-95°C), solar thermal collectors (flat-plate or evacuated tube producing 80-100°C), and numerous industrial waste heat streams.

Low-pressure steam represents the most common heat source for single-effect chillers due to superior heat transfer characteristics compared to hot water. Steam condensation in the generator tubes provides isothermal heat transfer with coefficients exceeding 5000 W/m²·K, enabling compact generator design and stable operation across load ranges. Steam systems require proper condensate drainage and non-condensable venting to maintain performance, with air accumulation severely degrading heat transfer and reducing capacity.

Hot water heat sources require substantially larger generator heat transfer area due to single-phase convection coefficients typically ranging from 1500-3000 W/m²·K depending on velocity and temperature. The temperature glide during sensible cooling of the heat source fluid complicates design, as the logarithmic mean temperature difference (LMTD) calculation must account for both fluids changing temperature. Hot water systems typically require 10-15°C temperature drop (rise) through the generator to deliver rated heat input at acceptable flow rates.

Generator heat transfer design must accommodate fouling factors per ASHRAE Standard 90.1 requirements. For hot water systems, typical design fouling resistances range from 0.0002 m²·K/W for clean closed-loop water to 0.0004 m²·K/W for open cooling tower water or treated well water. Steam systems require lower fouling factors of 0.00009 m²·K/W on the steam side but must account for non-condensable gas resistance that increases effective fouling over time without proper venting.

Heat source availability and stability critically affect single-effect chiller economics and integration. Intermittent heat sources (solar thermal, batch process waste heat) require thermal storage or hybrid operation with auxiliary heating to maintain continuous cooling. Fluctuating heat source temperatures cause capacity modulation and efficiency variations, potentially requiring sophisticated control strategies to maintain chilled water supply temperature within acceptable tolerances.

Solution Circulation and Heat Exchange

The solution circuit constitutes the thermodynamic heart of single-effect absorption systems, transporting absorbent solution between the low-pressure absorber and high-pressure generator while exchanging sensible heat through the solution heat exchanger. This internal heat recovery mechanism represents the single most important efficiency enhancement in absorption refrigeration, typically recovering 70-90% of available sensible heat and improving COP by 40-60% compared to systems without heat exchange.

Strong solution (high refrigerant concentration) flows from the absorber through the solution pump, which raises pressure from evaporator level (approximately 0.7-1.2 kPa for LiBr-water systems) to generator level (approximately 7-10 kPa). The pump provides the only significant mechanical work input to the cycle, consuming 0.5-1.5 kW per 100 tons of refrigeration capacity. The small work input relative to vapor compression systems (approximately 1-2% of equivalent compressor power) explains the dramatic electrical demand reduction achievable with absorption technology.

After pressurization, the cold strong solution enters the solution heat exchanger where it receives sensible heat from the hot weak solution (low refrigerant concentration) returning from the generator. This heat exchange preheats the strong solution from absorber temperature (approximately 30-40°C) to within 5-10°C of generator temperature (approximately 75-90°C), substantially reducing the generator heat input required to reach boiling conditions. Heat exchanger effectiveness (actual heat transfer divided by maximum possible heat transfer) typically ranges from 0.70 to 0.85, with higher effectiveness improving COP but requiring larger, more expensive heat exchangers.

The preheated strong solution enters the generator where heat input drives refrigerant vapor from the solution, concentrating the absorbent and reducing refrigerant mass fraction. The boiling process occurs across a range of temperatures as solution concentration increases, creating temperature glide that affects heat transfer design. For LiBr-water systems, concentration typically changes from 55-58% LiBr (strong solution) to 62-65% LiBr (weak solution), with exact values depending on operating temperatures and crystallization margin.

Weak solution exits the generator at high concentration and temperature (approximately 70-95°C), flowing through the solution heat exchanger where it transfers sensible heat to the incoming strong solution. After cooling to within 5-10°C of absorber temperature, the weak solution passes through a pressure-reducing valve or orifice that drops pressure from generator level to absorber level. The pressure reduction may cause solution flashing if temperature exceeds saturation conditions at absorber pressure, with vapor separating from solution and flowing directly to the absorber.

Lithium Bromide-Water Systems

LiBr-water single-effect absorption chillers dominate commercial air conditioning applications due to superior thermodynamic properties, non-toxicity, and compatibility with water as refrigerant. Water’s high latent heat of vaporization (2450 kJ/kg at 5°C) provides excellent refrigeration effect per unit mass of circulated refrigerant, while lithium bromide’s negligible vapor pressure ensures essentially pure water vapor generation in the generator without rectification requirements.

The system operates entirely under vacuum, with absolute pressures ranging from 0.7 kPa in the evaporator (corresponding to 4°C saturation temperature) to approximately 7-10 kPa in the generator and condenser (corresponding to 35-45°C saturation temperature). Vacuum operation necessitates robust pressure vessel design to resist atmospheric pressure loading, hermetic sealing to prevent air infiltration, and continuous purge systems to remove non-condensable gases that inevitably enter through microscopic leaks and diffusion through seals.

Air infiltration represents a persistent challenge in LiBr-water systems because any system operating below atmospheric pressure inevitably experiences inward leakage. Oxygen from infiltrated air accelerates corrosion of carbon steel components, requiring corrosion inhibitor packages (typically chromates, molybdates, or nitrates) and pH control in the 10.5-11.0 range. Hydrogen gas generated by corrosion reactions accumulates in the evaporator and condenser, degrading heat transfer and requiring removal through automatic purge units that operate on 15-30 minute cycles.

Crystallization presents the primary operational risk in LiBr-water systems, occurring when solution concentration exceeds solubility limits or temperature drops below saturation temperature for a given concentration. Crystallization potential increases at high cooling water temperatures (concentrating the solution to maintain capacity) and low heat source temperatures (reducing the dilution range between strong and weak solution). Manufacturers provide crystallization margin guarantees, typically maintaining 5-10°C safety margin below the crystallization boundary under all rated operating conditions.

Anti-crystallization protection includes multiple strategies: dilution cycles that add water to reduce solution concentration when approaching crystallization limits, heating during shutdown to maintain solution temperature above crystallization point, and automatic temperature and concentration monitoring with shutdown interlocks. If crystallization occurs, the solidified salt blocks solution flow passages, requiring heat input to re-dissolve the crystals before normal operation can resume.

Ammonia-Water Systems

Ammonia-water single-effect absorption systems provide refrigeration at temperatures well below 0°C, enabling industrial process cooling, food freezing, and ice production applications impossible with LiBr-water systems limited by water’s freezing point. Ammonia serves as the refrigerant while water functions as the absorbent, reversed from the more common LiBr-water arrangement where water is the refrigerant.

The system operates at substantially higher pressures than LiBr-water configurations, with evaporator pressures ranging from 200-400 kPa absolute (2-4 bar) at -10°C to 0°C and condenser pressures ranging from 1200-1800 kPa absolute (12-18 bar) at 30-40°C condensing temperature. These elevated pressures eliminate vacuum operation concerns and air infiltration problems but require more robust pressure vessel construction designed for internal pressure loading rather than external atmospheric crushing.

Both ammonia and water exhibit significant volatility, creating a fundamental challenge absent in LiBr-water systems. When heat input to the generator drives ammonia vapor from the solution, water vapor co-evaporates due to its finite vapor pressure at generator temperatures. Without separation, water vapor would flow to the condenser and evaporator, accumulating in the refrigerant circuit and severely degrading performance through reduced evaporator pressure and latent heat.

Rectification columns installed above the generator separate water from ammonia vapor through partial condensation. Cooling provided by external water supply or internal reflux preferentially condenses water vapor (higher boiling point at system pressure), which returns to the generator as reflux. The overhead vapor stream achieves 99.5-99.9% ammonia purity through this distillation process, with residual water content controlled through reflux ratio and column design. Insufficient rectification manifests as capacity loss and efficiency degradation as water accumulation in the evaporator reduces refrigerant circulation rate and latent heat.

Ammonia’s ASHRAE Standard 34 classification as Group B2L (higher toxicity, lower flammability, mildly flammable) imposes safety requirements including refrigerant detection and alarm systems, mechanical ventilation interlocked with detection, emergency ventilation capability, and restricted access to machinery rooms. However, ammonia provides environmental advantages with zero ozone depletion potential (ODP = 0) and negligible global warming potential (GWP = 0), along with superior thermodynamic and heat transfer properties compared to synthetic refrigerants.

Application Scenarios and Integration

Single-effect absorption chillers excel in applications where low-grade thermal energy is available at minimal cost or represents waste heat requiring disposal. Combined heat and power (CHP) installations utilize engine jacket cooling water (80-95°C) or exhaust heat recovery (300-500°C stepped down through heat exchangers) to drive absorption cooling, achieving overall system efficiencies exceeding 75-80% through simultaneous electricity generation, heating provision, and cooling production (tri-generation).

Industrial facilities generate substantial waste heat streams at appropriate temperatures for single-effect absorption including process cooling water, heat treatment furnace exhaust, compressor aftercoolers, and condenser reject heat from higher-temperature processes. Direct integration of absorption chillers with these waste streams recovers otherwise-rejected energy for refrigeration duty, reducing both thermal pollution and electrical cooling equipment consumption. Economic viability depends on waste heat quantity, temperature stability, proximity to cooling loads, and avoided electrical cooling costs.

Solar thermal cooling systems pair evacuated tube or concentrating collectors with single-effect absorption chillers to provide renewable cooling with zero direct emissions. Evacuated tube collectors achieve 80-100°C fluid temperatures suitable for single-effect operation with collection efficiencies of 50-70% depending on ambient conditions and operating temperature. System design must address solar intermittency through thermal storage (12-24 hours of generator heat input), auxiliary heating for low insolation periods, or hybrid operation with electrical backup chillers.

District energy systems employ large single-effect absorption chillers (1000-5000 tons) to utilize central plant steam or hot water during cooling season, maintaining high annual load factors on heating-optimized cogeneration equipment. The absorption chillers convert summer thermal energy (otherwise unused when heating loads disappear) into cooling for distribution through chilled water networks, improving overall system economics and energy efficiency compared to separate heating and cooling systems.

Performance Monitoring and Optimization

Absorption chiller performance monitoring requires tracking multiple parameters beyond the simple entering/leaving temperatures and flow rates sufficient for vapor compression analysis. Generator heat input measurement (steam flow rate or hot water flow and temperature differential) provides the denominator for COP calculations and indicates heat source adequacy. Solution concentration measurement through density or specific gravity enables crystallization margin assessment and verification of proper solution inventory.

Cooling water temperature critically affects both capacity and efficiency, with each 1°C increase in cooling water supply temperature typically reducing capacity by 3-5% and proportionally increasing heat input requirement. Oversized cooling tower designs maintaining approach within 2-3°C of wet bulb temperature provide substantial performance benefits exceeding the tower cost premium through improved absorption chiller COP and reduced generator heat consumption.

ASHRAE Standard 90.1-2019 establishes minimum efficiency requirements for single-effect absorption chillers at Path A designation (for water-cooled LiBr absorption units). The standard specifies minimum COP of 0.60 at AHRI Standard 560 test conditions: 44°F (6.7°C) leaving chilled water temperature, 85°F (29.4°C) entering condenser water temperature, 12°F (6.7°C) chilled water temperature range, and 10°F (5.6°C) condenser water temperature range. High-efficiency single-effect models achieve COP values of 0.68-0.72 under these conditions through enhanced heat transfer surfaces, optimized solution circulation, and advanced control strategies.

Part-load performance characteristics differ substantially from vapor compression equipment. Single-effect absorption chillers maintain relatively constant COP from 100% down to approximately 50% load through proportional reduction of heat input to the generator. Below 50% load, efficiency degrades as fixed parasitic loads (solution pump power, purge system operation, standing thermal losses) represent increasing fractions of total input. Minimum stable load typically ranges from 10-15% of rated capacity, substantially higher than vapor compression chillers capable of 5% minimum load with variable-speed compressors.

Control optimization focuses on minimizing cooling water supply temperature through aggressive cooling tower operation (running all cells, maximum fan speed) during moderate ambient conditions. Each 1°C reduction in cooling water temperature enables approximately 3-5% generator heat input reduction at constant capacity. For installations with variable-cost heat sources, control logic should maximize cooling water system electrical consumption (running additional tower fans) when the heat input cost savings exceed fan power cost.

Long-term performance degradation typically results from tube fouling in the generator, condenser, evaporator, and absorber heat exchangers. Annual inspection and cleaning of tube bundles per manufacturer recommendations maintains design heat transfer coefficients and prevents progressive capacity loss. Solution sampling and analysis verifies corrosion inhibitor concentrations, pH control, and absence of contamination that degrades absorption characteristics or accelerates corrosion rates.

Understanding these operating principles, performance characteristics, and application requirements enables effective single-effect absorption chiller selection, integration within broader HVAC systems, and ongoing optimization to maintain design efficiency throughout the equipment service life of 25-30 years typical for absorption technology.