Double-Effect Absorption Refrigeration

Overview

Double-effect absorption chillers represent a significant advancement over single-effect systems, utilizing two generator stages in a cascaded configuration to achieve substantially higher thermal efficiency. By extracting useful heat from the high-pressure generator vapor to drive a second low-pressure generator, these systems achieve COP values of 1.0 to 1.2, representing approximately 40-50% improvement in efficiency compared to single-effect designs (COP 0.7-0.8).

The fundamental advantage stems from the double use of input thermal energy. Heat supplied to the high-pressure generator produces refrigerant vapor that subsequently serves as the heat source for the low-pressure generator, effectively doubling the refrigerant production per unit of input energy. This cascaded energy utilization justifies the increased system complexity and higher initial cost for large-scale commercial and industrial applications.

Thermodynamic Principles

Cascaded Generator Operation

The double-effect cycle operates two generator stages at different pressure and temperature levels:

High-Pressure Generator (HPG):

• Operating pressure: 8-12 bar (116-174 psia)
• Solution temperature: 150-180°C (302-356°F)
• Requires high-temperature heat source
• Produces high-pressure refrigerant vapor
• Concentration change: 50-60% to 63-67% LiBr solution

Low-Pressure Generator (LPG):

• Operating pressure: 0.8-1.2 bar (11.6-17.4 psia)
• Solution temperature: 85-95°C (185-203°F)
• Heat source: condensing vapor from HPG
• Produces low-pressure refrigerant vapor
• Concentration change: 55-60% to 62-65% LiBr solution

The refrigerant vapor from the HPG condenses on tube surfaces within the LPG, releasing latent heat that drives further refrigerant generation from the dilute solution. This internal heat recovery represents the core efficiency advantage of double-effect systems.

Heat Source Requirements

Double-effect chillers demand significantly higher temperature heat sources than single-effect units:

The high-temperature requirement limits heat source options but enables superior performance when appropriate thermal energy is available. Direct-fired configurations represent the most common application, utilizing natural gas combustion to generate the necessary driving temperature.

System Configurations

Series Flow Cycle

The series flow arrangement passes solution sequentially through both generators:

1. Concentrated solution from absorber enters HPG
2. Partial dilution occurs in HPG as refrigerant boils off
3. Intermediate-concentration solution flows to LPG
4. Additional refrigerant generation produces fully diluted solution
5. Dilute solution returns to absorber via heat exchangers

Characteristics:

• Simpler piping configuration
• Lower solution flow rates through heat exchangers
• Pressure drop considerations between generators
• Solution temperature stratification benefits

Performance:

• COP: 1.0-1.15 typical
• Part-load efficiency: good down to 40% capacity
• Turndown ratio: 10-100% with hot gas bypass

Parallel Flow Cycle

Parallel flow splits dilute solution flow to both generators simultaneously:

1. Dilute solution from absorber divides into two streams
2. One stream enters HPG, another enters LPG
3. Both generators produce concentrated solution
4. Concentrated streams recombine before returning to absorber
5. Independent generator operation enables optimization

Characteristics:

• More complex piping and control
• Higher solution circulation rates
• Independent generator temperature control
• Requires careful flow balancing

Performance:

• COP: 1.05-1.2 typical (highest efficiency)
• Superior part-load performance
• Better tolerance to varying heat source conditions
• Enhanced capacity modulation capability

Reverse Parallel Flow

This advanced configuration combines elements of both arrangements:

1. Concentrated solution enters LPG first
2. Partial dilution occurs in LPG
3. Solution then flows to HPG for additional dilution
4. Refrigerant vapor from HPG heats LPG
5. Fully diluted solution returns to absorber

Advantages:

• Optimized heat exchanger effectiveness
• Reduced crystallization risk in HPG
• Better control of solution concentrations
• Improved heat recovery efficiency

Applications:

• Variable heat source temperature conditions
• Systems requiring maximum crystallization protection
• Applications with fluctuating cooling loads

Coefficient of Performance (COP)

Theoretical Performance

The Carnot COP limit for double-effect absorption follows:

COP_Carnot = (T_evap / T_absorber) × [(T_generator_high - T_absorber) / T_generator_high]

For typical operating conditions:

• Evaporator: 5°C (278 K)
• Absorber: 35°C (308 K)
• High-pressure generator: 170°C (443 K)

COP_Carnot = (278/308) × [(443-308)/443] = 0.90 × 0.305 = 2.75

Actual double-effect chillers achieve 35-45% of theoretical Carnot efficiency.

Practical COP Values

Real-world performance varies with operating conditions:

Efficiency Comparison

Relative performance versus other absorption technologies:

• Single-effect absorption: COP 0.70-0.75 (baseline)
• Double-effect absorption: COP 1.10-1.20 (+47-60% improvement)
• Triple-effect absorption: COP 1.40-1.50 (+100-114% improvement)
• Vapor compression (electric): COP 5.0-6.5 (different energy form)

The 40-60% COP improvement over single-effect systems translates directly to fuel cost savings, typically achieving payback in 3-7 years for direct-fired installations replacing electric chillers in high-energy-cost regions.

Direct-Fired Configurations

Burner Integration

Direct-fired double-effect chillers incorporate gas or oil burners directly into the high-pressure generator:

Burner Types:

• Atmospheric burners: simple, reliable, lower cost
• Power burners: precise control, lower emissions, higher efficiency
• Modulating burners: continuous capacity control (10-100%)
• Staged burners: step control (typically 25-50-75-100%)

Combustion System Components:

• Burner assembly with flame retention
• Ignition system (direct spark or pilot)
• Flame safeguard controls (UV or IR detection)
• Combustion air fan (power burner types)
• Flue gas venting system

Thermal Efficiency

Direct-fired systems achieve high seasonal efficiency:

• Combustion efficiency: 96-98% (condensing heat recovery)
• Heat transfer to solution: 92-95%
• Overall thermal efficiency: 88-93%
• Flue gas heat recovery via economizer: +3-5% efficiency

Modern direct-fired units incorporate flue gas economizers that preheat incoming dilute solution, recovering sensible heat from combustion products before venting. This feature improves overall efficiency by 3-5 percentage points.

Emissions Control

Environmental performance meets stringent air quality regulations:

Ultra-low NOx burners achieve 9-12 ppm through:

• Staged combustion (fuel or air staging)
• Flue gas recirculation (FGR)
• Lean premix technology
• Selective catalytic reduction (SCR) for <9 ppm

Fuel Flexibility

Direct-fired chillers accommodate various fuel sources:

Natural Gas:

• Primary fuel choice (>90% of installations)
• Clean combustion, low emissions
• Continuous supply via utility connection
• Variable pricing (time-of-use considerations)

Propane (LPG):

• Alternative where natural gas unavailable
• Higher fuel cost typically
• On-site storage required
• Similar combustion characteristics

Fuel Oil (No. 2):

• Backup fuel option
• Dual-fuel capability for energy security
• Higher emissions, more maintenance
• On-site storage and handling requirements

Steam-Driven Configurations

Steam Supply Requirements

Steam-driven double-effect chillers require high-pressure steam:

Steam Parameters:

• Pressure: 8-12 bar (115-175 psig)
• Temperature: 170-185°C (338-365°F)
• Quality: >95% (minimal moisture content)
• Flow rate: 1.8-2.2 kg/kW-cooling (3.0-3.7 lb/ton-hr)

Steam enters the HPG tube bundle, condenses on tube surfaces, and returns as condensate to the steam system. Proper steam trap selection and condensate return design ensure efficient operation and prevent waterlogging.

Integration Considerations

Successful steam-driven installations address:

Steam System Interface:

• Pressure reducing valve (PRV) if higher pressure available
• Steam separator for dry steam delivery
• Isolation valves for chiller shutdown
• Strainer upstream of steam control valve

Condensate Management:

• Appropriate steam trap sizing (2-3× condensate flow)
• Condensate return pump if needed
• Flash steam recovery opportunities
• Water treatment for corrosion control

Control Integration:

• Modulating steam valve (equal percentage characteristic)
• Condensate level control in generator
• Steam pressure monitoring and control
• Coordination with plant steam header

Performance Characteristics

Steam-driven advantages and considerations:

Benefits:

• No combustion equipment or emissions
• Quieter operation (no burner noise)
• Simpler maintenance (fewer moving parts)
• Excellent part-load efficiency
• Faster response to load changes

Limitations:

• Requires existing steam infrastructure
• Steam cost considerations (boiler efficiency)
• System complexity for small loads
• Seasonal steam availability in some facilities

Efficiency Improvements Over Single-Effect

Energy Utilization Comparison

The efficiency advantage of double-effect systems becomes apparent through direct comparison:

Single-Effect System:

• Heat input to generator: 100 units
• Refrigerant generated: 100 units
• Cooling effect: 70 units (allowing for losses)
• COP: 0.70

Double-Effect System:

• Heat input to HPG: 100 units
• Refrigerant from HPG: 50 units
• Heat released from HPG vapor: 90 units (latent heat)
• Heat input to LPG: 90 units (from HPG vapor condensation)
• Refrigerant from LPG: 45 units
• Total refrigerant generated: 95 units
• Cooling effect: 110 units (improved effectiveness)
• COP: 1.10

The cascaded energy use in the double-effect design produces 57% more cooling from the same heat input.

Economic Analysis

Operational cost comparison for 1,000 kW (284 ton) cooling load:

Energy Consumption:

• Single-effect: 1,429 kW thermal input (COP 0.70)
• Double-effect: 909 kW thermal input (COP 1.10)
• Energy savings: 520 kW (36% reduction)

Annual Operating Cost (8,000 hours, $0.30/therm gas):

• Single-effect: $146,700/year
• Double-effect: $93,300/year
• Annual savings: $53,400/year

Capital Cost Premium:

• Double-effect premium: $180,000-$250,000
• Simple payback: 3.4-4.7 years
• Life-cycle savings (20-year): >$800,000

Application Selection Criteria

Double-effect systems prove economical when:

1. Cooling load exceeds 350 kW (100 tons)

   • Capital cost justified by energy savings
   • Economies of scale favor larger capacities

2. Annual operating hours exceed 2,000 hours

   • Sufficient runtime to recover capital premium
   • Continuous or high-duty-cycle applications optimal

3. Energy costs exceed $0.20/therm (gas) or $0.08/kWh (electric)

   • Higher energy costs improve payback
   • Peak demand charges favor thermal chillers

4. Appropriate heat source available

   • Natural gas service for direct-fired
   • High-pressure steam (8-12 bar) for steam-driven
   • High-temperature waste heat (160-180°C)

5. Electric demand reduction valued

   • Utility demand charge savings
   • Peak load shaving benefits
   • Grid independence requirements

Performance Monitoring and Optimization

Key Performance Indicators

Monitor these parameters for optimal efficiency:

Energy Efficiency:

• COP calculation: cooling output ÷ thermal input
• Target: maintain >1.08 at design conditions
• Trend analysis for degradation detection

Solution Concentrations:

• HPG outlet: 63-67% LiBr (refractometer measurement)
• LPG outlet: 62-65% LiBr
• Monitor for crystallization risk (>65% dangerous)

Temperature Differentials:

• HPG approach: 3-5°C between flue gas/steam and solution
• LPG approach: 2-4°C between HPG vapor and solution
• Condenser approach: 2-3°C between refrigerant and cooling water
• Evaporator approach: 1-2°C between chilled water and refrigerant

Pressure Relationships:

• HPG pressure: 8-12 bar absolute
• LPG pressure: 0.8-1.2 bar absolute
• Condenser pressure: matches LPG pressure
• Evaporator pressure: 0.8-1.0 kPa absolute (6-8 mmHg)

These parameters enable early detection of fouling, air ingress, crystallization risk, and other performance-degrading conditions.