Absorption Refrigeration System Components

Generator (Desorber)

The generator separates refrigerant vapor from the refrigerant-absorbent solution through thermal energy input. In lithium bromide-water systems, the generator receives dilute solution containing approximately 55-60% lithium bromide by weight and produces strong solution at 62-65% concentration while driving off pure water vapor.

Generator Types

Single-effect generators operate at 190-250°F (88-121°C) and require heat input of approximately 18,000-19,000 BTU/hr per ton of refrigeration. Direct-fired generators achieve temperatures up to 350°F (177°C) using natural gas burners with thermal efficiency ratings of 70-80%.

Double-effect generators employ a high-temperature generator at 300-360°F (149-182°C) and a low-temperature generator at 190-230°F (88-110°C). The refrigerant vapor from the high-temperature generator condenses in the low-temperature generator, providing cascade heat recovery that improves COP from 0.7 to 1.2.

Heat Transfer Design

Generator heat transfer surfaces use tube bundles with refrigerant-absorbent solution on the shell side and heating medium (steam, hot water, or combustion gases) on the tube side. Heat flux ranges from 8,000-12,000 BTU/hr-ft² for steam-fired units to 15,000-20,000 BTU/hr-ft² for direct-fired generators.

Tube materials include copper-nickel alloys (90/10 or 70/30) for corrosion resistance in lithium bromide service. Minimum tube wall thickness is 0.049 inches (18 BWG) to withstand thermal stress and crystallization effects.

Absorber Design

The absorber combines refrigerant vapor from the evaporator with strong solution from the generator, releasing heat of absorption that must be rejected to cooling water. This exothermic reaction generates 15,000-17,000 BTU/hr per ton of refrigeration capacity.

Absorption Process

Strong lithium bromide solution enters the absorber at 110-120°F (43-49°C) and absorbs water vapor at pressures of 0.15-0.30 psia. The dilute solution exits at 95-105°F (35-41°C) after rejecting heat to cooling water flowing through internal tube bundles.

Absorption efficiency depends on maintaining intimate contact between solution and vapor. Spray nozzles distribute solution in droplet sizes of 0.5-2.0 mm diameter, creating surface area of 50-100 ft²/ft³ of absorber volume.

Heat Rejection Requirements

Cooling water flow rates of 3.0-4.0 gpm per ton remove both sensible heat from the solution and latent heat of absorption. Water temperature rise is typically 10-15°F (5.6-8.3°C) with inlet temperatures maintained below 85°F (29°C) for optimal absorption rates.

Tube bundles use enhanced surfaces with internal rifling or external fins to achieve overall heat transfer coefficients of 150-250 BTU/hr-ft²-°F. Horizontal tube arrangements provide superior vapor distribution compared to vertical configurations.

Solution Heat Exchanger

The solution heat exchanger recovers thermal energy by transferring heat from hot strong solution leaving the generator to cold dilute solution returning from the absorber. This heat recovery improves cycle COP by 10-15% while reducing both generator heat input and absorber cooling requirements.

Thermal Performance

Effectiveness values of 0.60-0.75 are typical, with approach temperatures of 5-10°F (2.8-5.6°C). Strong solution temperature decreases from 200°F (93°C) to 130°F (54°C) while dilute solution temperature increases from 95°F (35°C) to 165°F (74°C) in a well-designed exchanger.

Heat transfer area requirements are approximately 0.5-1.0 ft² per ton of refrigeration capacity. Plate-and-frame heat exchangers provide compact design with overall U-values of 200-300 BTU/hr-ft²-°F, while shell-and-tube configurations offer 100-150 BTU/hr-ft²-°F.

Material Selection

Plate heat exchangers employ 316 stainless steel plates with thickness of 0.024-0.032 inches. Shell-and-tube designs use copper-nickel tubes (90/10 alloy minimum) with steel or stainless steel shells rated for 150 psig design pressure.

Evaporator Configuration

The evaporator produces chilled water by evaporating refrigerant (water in lithium bromide systems, ammonia in ammonia-water systems) at low pressure and temperature. Water evaporates at 40-45°F (4.4-7.2°C) corresponding to absolute pressures of 0.12-0.15 psia.

Flooded Tube Design

Chilled water flows through horizontal tube bundles at velocities of 3-8 ft/sec while refrigerant boils on the external tube surfaces. Enhanced tubes with porous coatings or machined nucleation sites increase boiling heat transfer coefficients from 800 BTU/hr-ft²-°F for plain tubes to 1,500-2,500 BTU/hr-ft²-°F.

Refrigerant liquid levels maintain submergence of 50-70% of tube bundle height. Distribution systems ensure uniform wetting of tube surfaces to prevent dry patches that reduce heat transfer effectiveness.

Capacity Control

Evaporator capacity modulates through solution flow control to the generator, adjusting refrigerant circulation rates. Response time is 2-5 minutes due to thermal mass of solution inventory and heat exchanger surfaces.

Chilled water temperature range is typically 42-48°F (5.6-8.9°C) leaving water temperature with 10-14°F (5.6-7.8°C) temperature differential. Lower evaporator temperatures require reduced operating pressures below 0.10 psia, which increases crystallization risk in lithium bromide systems.

Condenser Design

The condenser receives high-temperature refrigerant vapor from the generator and condenses it to liquid by rejecting heat to cooling water. Condensing occurs at 95-110°F (35-43°C) at absolute pressures of 0.9-1.5 psia for water refrigerant.

Heat Transfer Performance

Cooling water flows through horizontal tube bundles at 4-8 ft/sec, providing film coefficients of 800-1,200 BTU/hr-ft²-°F. Refrigerant condenses on external tube surfaces with coefficients of 1,500-3,000 BTU/hr-ft²-°F, yielding overall U-values of 400-600 BTU/hr-ft²-°F.

Heat rejection is approximately 12,500-13,500 BTU/hr per ton of refrigeration. Cooling water flow requirements are 2.5-3.0 gpm per ton with temperature rise of 8-12°F (4.4-6.7°C).

Non-Condensable Gas Impact

Air and other non-condensables accumulate in the condenser, creating thermal resistance that reduces condensing coefficients by 20-40%. A 1% air concentration by volume reduces capacity by approximately 15% and increases condensing temperature by 10-15°F (5.6-8.3°C).

Solution Pump

The solution pump circulates dilute solution from the absorber to the generator, overcoming the pressure difference between low-pressure absorber (0.15-0.30 psia) and higher-pressure generator (0.9-1.5 psia). Hermetically sealed centrifugal pumps handle flow rates of 8-15 gpm per ton of capacity.

Pump Design Requirements

Head requirements range from 15-40 feet of solution depending on system configuration and piping losses. Motor power is typically 0.010-0.015 kW per ton, representing less than 2% of total system power consumption.

Materials must resist lithium bromide corrosion, with 316 stainless steel impellers and casings. Mechanical seals employ carbon-ceramic or silicon carbide faces with secondary containment to prevent solution leakage into the vacuum environment.

Control Integration

Variable frequency drives modulate pump speed from 30-100% to match capacity requirements. Flow variation adjusts solution circulation ratio (mass flow of solution per mass flow of refrigerant) from 8:1 at full load to 12:1 at part load conditions.

Minimum flow protection prevents operation below 20% capacity where crystallization risk increases due to high solution concentration and low circulation rates.

Purge System for Non-Condensables

Non-condensable gases enter absorption systems through air leaks in sub-atmospheric components and desorption from solution. Purge systems continuously remove these gases to maintain vacuum integrity and heat transfer performance.

Purge Methods

Vacuum pump purge systems extract gas samples from the condenser high point, passing them through a small evaporator-absorber cycle that recovers refrigerant vapor before venting non-condensables. Recovery efficiency exceeds 95%, limiting refrigerant losses to less than 5 lb/year.

Palladium cell purgers employ hydrogen-selective membranes that separate hydrogen (the primary non-condensable in lithium bromide systems) from refrigerant vapor. Operating temperatures of 700-800°F (371-427°C) provide hydrogen permeation rates sufficient for systems up to 1,500 tons capacity.

Performance Monitoring

Non-condensable concentration is inferred from condenser temperature-pressure relationships. A 5°F (2.8°C) deviation between measured condensing temperature and saturation temperature at measured pressure indicates approximately 0.5-1.0% air by volume.

Continuous purge operation maintains non-condensable levels below 0.3% by volume. Intermittent purge cycles activate when temperature deviation exceeds setpoint, typically 3-5°F (1.7-2.8°C), operating for 15-30 minute intervals every 2-4 hours.