Working Pairs in Absorption Refrigeration
Absorption refrigeration systems rely on specific refrigerant-absorbent working pairs with distinct thermodynamic properties that enable the absorption-desorption cycle. The selection of working pairs fundamentally determines system performance, operating range, and application suitability.
Thermodynamic Requirements for Working Pairs
Effective refrigerant-absorbent pairs must satisfy specific criteria:
Volatility Differential: The refrigerant must exhibit significantly higher vapor pressure than the absorbent at operating temperatures. This allows selective evaporation of the refrigerant during generation and condensation during absorption.
Mutual Solubility: High solubility of refrigerant in absorbent across the operating temperature range enables concentrated solution formation. The solubility must remain adequate at both low absorber temperatures and high generator temperatures.
Chemical Stability: Both substances must remain chemically stable and non-reactive with each other and system materials throughout the operating cycle. Decomposition, corrosion, or chemical reactions compromise system integrity and performance.
Favorable Transport Properties: Low solution viscosity enhances heat and mass transfer. High thermal conductivity improves heat exchanger effectiveness. These properties directly impact required heat transfer surface area and pumping power.
Environmental and Safety Compatibility: The working pair must meet safety standards for toxicity, flammability, and environmental impact based on application requirements.
Lithium Bromide-Water (LiBr-H₂O)
Lithium bromide-water represents the dominant working pair for comfort cooling applications, where water serves as the refrigerant and lithium bromide as the absorbent.
Properties and Performance Characteristics
Vapor Pressure Characteristics: Water exhibits significantly higher vapor pressure than lithium bromide solutions, providing excellent volatility separation. At 40°C, pure water vapor pressure reaches 7.38 kPa while LiBr solution vapor pressure remains negligible.
Absorption Capacity: LiBr demonstrates exceptional water absorption capacity. Solutions can achieve concentrations exceeding 65% LiBr by mass, providing strong solution concentrations for effective refrigerant removal.
Heat of Absorption: The heat released during water absorption into LiBr solution approaches 2400 kJ/kg at typical operating conditions, requiring effective heat rejection in the absorber.
Transport Properties: LiBr-water solutions exhibit favorable viscosity at operating temperatures, typically 1.5-3.0 mPa·s at absorber conditions, facilitating film formation and mass transfer.
Crystallization Limits and Operating Range
Crystallization Phenomenon: Lithium bromide crystallizes from solution when concentration exceeds solubility limits at a given temperature. Crystallization blocks flow passages and terminates system operation, requiring system shutdown and crystal dissolution through heating.
Solubility Boundaries: The crystallization boundary varies with temperature and concentration. At 35°C, the crystallization limit occurs at approximately 64% LiBr concentration. At 25°C, this limit decreases to approximately 60%.
Operating Safety Margin: Practical systems maintain 3-5% concentration margin below crystallization limits to prevent inadvertent crystal formation during transient conditions or control variations.
Temperature-Concentration Control: The solution concentration ratio between strong and weak solutions must remain within safe operating boundaries across the full load range. Typical weak solution concentrations range from 55-58% LiBr, while strong solutions operate at 60-64% LiBr.
Application Range and Limitations
Evaporator Temperature Limits: Water’s freezing point restricts minimum evaporator temperature to approximately 4-5°C in practical systems. This limitation confines LiBr-water systems to comfort cooling and process applications above water’s freezing point.
Vacuum Operation: Water’s low vapor pressure necessitates sub-atmospheric operation throughout the cycle. Evaporator pressures typically operate at 0.7-1.2 kPa (5-9 mmHg), requiring absolute sealing and vacuum maintenance.
Cooling Water Requirements: LiBr-water systems require cooling water temperatures below 32°C for effective operation. Higher cooling water temperatures reduce capacity and may cause crystallization risk.
Corrosion Considerations: Lithium bromide solutions exhibit corrosive properties toward carbon steel and other metals in the presence of oxygen. Corrosion inhibitors containing chromates, molybdates, or lithium hydroxide are essential. Oxygen ingress through leaks accelerates corrosion, requiring hermetic construction.
Ammonia-Water (NH₃-H₂O)
Ammonia-water pairs utilize ammonia as refrigerant and water as absorbent, suitable for low-temperature refrigeration and applications where LiBr-water proves inadequate.
Thermodynamic Properties
Volatility Relationship: While ammonia is significantly more volatile than water, the volatility ratio is less favorable than LiBr-water. At 30°C, ammonia vapor pressure reaches 1167 kPa while water remains at 4.25 kPa, but water evaporation remains significant.
Solution Concentration Range: Ammonia-water solutions operate at 30-50% ammonia concentration by mass, depending on system design and operating conditions. The narrower concentration differential compared to LiBr-water reduces circulation ratio efficiency.
Heat of Absorption: Ammonia absorption in water releases approximately 1370 kJ/kg at typical operating conditions, lower than LiBr-water but still requiring substantial heat rejection.
Rectification Requirements
Water Contamination Problem: Water vapor accompanies ammonia during generation due to insufficient volatility separation. Water carried to the evaporator freezes, blocking refrigerant flow and deteriorating heat transfer.
Rectification Process: A rectification column positioned above the generator separates water vapor from ammonia vapor. Cool weak solution or dedicated cooling water condenses water vapor while allowing ammonia to pass. The rectification column operates as a fractional distillation process.
Rectifier Design: Practical rectifiers incorporate packed beds or trayed columns to maximize vapor-liquid contact surface area. Reflux ratios of 3:1 to 5:1 are typical, meaning 3-5 kg of vapor must be condensed to purify 1 kg of ammonia vapor.
Purity Requirements: Commercial refrigeration applications require ammonia purity exceeding 99.5% to prevent evaporator freeze-up. Industrial applications with evaporator temperatures above 0°C may tolerate slightly lower purity levels.
Low-Temperature Capability
Extended Temperature Range: Ammonia’s low freezing point (-77°C) and favorable vapor pressure characteristics enable evaporator temperatures from -60°C to +10°C, suitable for cold storage, ice production, and industrial refrigeration.
Pressure-Temperature Relationship: Ammonia systems operate at positive pressures across most of the operating range. At -40°C evaporator temperature, ammonia pressure remains at 71 kPa (absolute), eliminating vacuum requirements.
Cascade Applications: For temperatures below -60°C, ammonia-water absorption systems serve as the high-stage in cascade arrangements with secondary refrigerants or alternative absorption pairs providing additional temperature reduction.
Safety Considerations
Toxicity: Ammonia is classified as a toxic refrigerant with TLV-TWA of 25 ppm and IDLH of 300 ppm. Systems require ventilation, detection, and emergency response provisions complying with IIAR and ASHRAE 15 standards.
Flammability: Ammonia exhibits flammability limits of 15-28% by volume in air. While less flammable than hydrocarbons, ignition sources must be controlled in machinery rooms and areas with potential release.
Pressure Levels: Higher operating pressures compared to LiBr-water systems (1500-2000 kPa condenser pressure) require pressure vessel construction and safety relief provisions per ASME Section VIII.
Material Compatibility: Ammonia is incompatible with copper and copper alloys, causing corrosion and contamination. Steel, stainless steel, aluminum, and cast iron are standard construction materials.
Alternative Working Pairs
Lithium Chloride-Water (LiCl-H₂O)
Lithium chloride functions similarly to lithium bromide with water as refrigerant. LiCl offers higher solubility limits, reducing crystallization risk, but exhibits higher solution viscosity and reduced availability. Applications include specialized systems where crystallization prevention is critical.
Water-Silica Gel and Other Solid Absorbents
Adsorption Systems: Silica gel, zeolites, and activated carbon physically adsorb water vapor rather than forming liquid solutions. These systems operate intermittently or with multiple beds cycling between adsorption and regeneration.
Operating Characteristics: Solid adsorbent systems achieve evaporator temperatures of 5-15°C, suitable for cooling applications. Regeneration requires temperatures of 70-95°C, compatible with solar thermal or waste heat sources.
Capacity Limitations: Adsorption capacity per unit mass of adsorbent is limited (0.2-0.4 kg water per kg adsorbent), requiring larger adsorbent quantities and bed volumes compared to liquid absorption systems.
Organic Working Pairs
TFE-E181: Trifluoroethanol (TFE) as absorbent with E181 (1,1,1,3,3,3-hexafluoropropane) as refrigerant offers non-corrosive operation and operates at positive pressures for air-conditioning applications. Limited commercial availability restricts applications.
Methanol-Based Pairs: Methanol with various organic refrigerants provides alternatives for specific applications but faces challenges with toxicity, flammability, and performance compared to established working pairs.
Working Pair Selection Criteria
Selection depends on multiple application-specific factors:
Operating Temperature Range: Evaporator and condenser temperatures determine refrigerant requirements. Water refrigerant limits minimum temperature to approximately 4°C. Ammonia enables temperatures to -60°C.
Heat Source Availability: Generator heat source temperature and type (steam, hot water, waste heat, direct-fired) influence working pair selection and system design.
Safety and Environmental Requirements: Occupied spaces favor non-toxic LiBr-water. Industrial facilities may accept ammonia with appropriate safety measures.
Cooling Water Temperature: Available cooling water or air temperature affects condenser and absorber performance, influencing crystallization risk in LiBr systems.
System Complexity: Ammonia-water requires rectification, increasing complexity. LiBr-water operates with simpler cycle configuration but requires vacuum maintenance and crystallization prevention.
Economic Considerations: Material costs, maintenance requirements, and replacement fluid expenses vary significantly between working pairs.