Intro to Chillers : Absorption Chillers

 Overview:

• Difference from Typical Chillers:
  • No mechanical compression; instead, uses a chemical reaction.
  • Compressor is replaced by an absorber, using heat rather than compression.
  • Liquid absorbs vapor from the evaporator.

Major Components:

• Evaporator
• Absorber
• Heat Exchanger
• Generator
• Condenser

Refrigerants Used:

• Ammonia (R717):
  • Common in industrial applications and RV refrigeration systems.
  • Caustic and hazardous, requires strict safety measures.
• Lithium Bromide:
  • Safer than ammonia, with a high affinity for water.
  • Distilled water is the refrigerant; vaporizing water absorbs 970 BTUs per pound.

Operation of the Evaporator:

• Vacuum Operation:
  • Typically operates at about ½ inch of mercury or 13,000 microns.
  • Water boils at 40°F due to low pressure.

Absorption Chiller Process:

• Evaporator:
  • Contains distilled water refrigerant boiling at 40°F due to vacuum.
  • Vaporized water travels to the absorber.
• Absorber:
  • Water vapor is absorbed into the lithium bromide solution.
  • Weak solution is pumped to the generator.
• Generator:
  • Uses a heat source (e.g., hot water, steam, direct-fired gas burners) to drive water vapor out of the lithium bromide, creating a concentrated solution that returns to the absorber.
• Condenser:
  • Water vapor condenses back into distilled water after releasing heat to cooling tower water.
  • Condensed water returns to the evaporator, continuing the cycle.

Absorption Process with Temperature Example:

• Evaporator:
  • Water boils at 40°F, absorbing heat from chilled water (inlet at 45°F, outlet at 55°F).
• Generator:
  • Heat source drives water vapor out of the solution.
  • Concentrated solution returns to the absorber at around 155°F.
• Condenser:
  • Water vapor condenses after heat rejection to cooling tower water.
  • Condensed water returns to the evaporator, passing through a metering device to lower pressure and boil at 40°F, continuing the absorption process.