| name | absorption-refrigeration |
| description | Absorption refrigeration — LiBr-water and NH3-water cycles, COP, generator/absorber/condenser/evaporator heat duties, waste heat driven cooling, double-effect cycles, ASHRAE standards. |
| metadata | {"priority":7,"promptSignals":{"phrases":["absorption refrigeration","absorption chiller","LiBr water cycle","ammonia water refrigeration","waste heat cooling","absorption cooling"],"minScore":3}} |
Absorption Refrigeration — Complete Skill
Cycle Overview
Absorption cycle replaces compressor with:
Generator (heat input) + Absorber (heat rejection) + Solution pump (low work)
Working pairs: LiBr-H₂O (water as refrigerant; LiBr as absorbent) or NH₃-H₂O (NH₃ as refrigerant)
Component functions:
- Generator: heat Q_gen desorbs refrigerant vapor from strong solution
- Condenser: refrigerant vapor condenses at high P (rejects Q_cond)
- Expansion valve: throttles refrigerant to evaporator pressure
- Evaporator: refrigerant evaporates, absorbing Q_evap (cooling effect)
- Absorber: weak solution absorbs refrigerant vapor; rejects Q_abs
- Solution pump: pumps weak solution from low P to high P (small W_pump)
- Solution HX: recovers heat between strong→absorber and weak→generator streams
Energy Balance
For cycle at steady state:
Q_gen + Q_evap + W_pump = Q_cond + Q_abs
Coefficient of Performance (COP):
COP = Q_evap / (Q_gen + W_pump) ≈ Q_evap / Q_gen [W_pump << Q_gen; neglected]
Theoretical maximum COP (Carnot-limited):
COP_max = (T_evap / (T_gen - T_evap)) × ((T_gen - T_cond) / T_cond)
T_evap = evaporator temperature [K]; T_gen = generator temperature [K]; T_cond = condenser/absorber T [K]
Typical single-effect COP:
LiBr-H₂O: COP = 0.65–0.80 (generator T = 80–100°C; condenser T = 35°C; chilled water 7°C)
NH₃-H₂O: COP = 0.50–0.65 (lower due to rectifier losses and refrigerant impurity)
LiBr-H₂O Cycle
Refrigerant: water (H₂O); excellent refrigerant properties; zero ODP/GWP
Absorbent: lithium bromide (LiBr); highly hygroscopic salt
Operating pressure: low side 0.8–1.0 kPa (0.006–0.007 bar) at 5°C evaporator; high side 7–8 kPa
Concentration:
Strong solution (leaving generator): 62–65% LiBr by mass
Weak solution (leaving absorber): 58–60% LiBr by mass
ΔX = 3–6% per cycle
Solution crystallization risk:
Crystallization occurs if solution concentration too high at low T → plugging
Crystallization line: X > 70% at 30°C; maintain safety margin
Purge/vacuum required: non-condensable gases (N₂, O₂) must be removed continuously
Heat duties:
Q_gen = m_r × (h_rv - h_sl) + m_ss × (h_ss - h_ws) [kW; m_r = refrigerant flow; h = enthalpy]
Q_evap = m_r × (h_rv_evap - h_cond_exit) [kW]
COP = Q_evap / Q_gen
Typical T ranges:
Generator supply: 70–95°C hot water (solar, waste heat, steam)
Condenser/absorber: 25–40°C cooling water
Evaporator: 5–12°C chilled water outlet
NH₃-H₂O Cycle
Advantage over LiBr: works below 0°C (refrigerant NH₃ freezes water above 0°C limit of LiBr systems); suitable for industrial refrigeration and heat pumps
Complication: H₂O vapor pressure non-negligible → requires rectifier to purify NH₃ vapor
Rectifier duty:
Q_rect = m_r × (h_vapor_out - h_rectified) [kW; reduces Q_evap effectiveness]
Typical generator T: 120–180°C (steam at 150–200 kPa or direct fired)
Evaporator T: -40°C to +10°C (industrial cold storage: -30°C)
COP: 0.45–0.65 (single effect); 0.8–1.1 (double effect)
Double-Effect LiBr Cycle
Configuration: two generators at high and low pressure; condenser heat from first effect drives second generator
COP improvement: ~1.2–1.4 (vs. 0.7 single effect)
Required generator T: 150–175°C (steam at 400–800 kPa)
First generator: T_gen1 = 140–160°C (high pressure)
Second generator: T_gen2 = 70–90°C (uses condenser heat from first generator)
COP (double-effect):
COP ≈ 2 × COP_single_effect × η_regeneration ≈ 1.2–1.4
Q_saved = extra refrigerant from second generator with same Q_gen input
Triple-Effect Cycle
Three generators; COP ≈ 1.6–1.8; generator T 200–230°C; commercially available in large sizes
Equipment Sizing
Chiller capacity:
Q_chiller = m_chw × c_p × (T_in - T_out) [kW; m_chw = chilled water mass flow; c_p = 4.18 kJ/(kg·K)]
Typical chilled water ΔT = 5–7°C
Generator heating coil:
Q_gen = Q_evap / COP [kW]
Steam consumption: m_steam = Q_gen / h_fg [kg/s; h_fg = 2200 kJ/kg at 100°C]
Cooling tower load:
Q_CT = Q_evap + Q_gen [kW] (total heat rejection)
Cooling water flow: m_cw = Q_CT / (c_p × ΔT_cw) [kg/s; ΔT_cw = 5–8°C]
Absorber sizing:
Q_abs ≈ Q_evap + Q_gen - Q_cond (from heat balance)
Heat transfer area: A = Q_abs / (U × LMTD) [m²; U ≈ 1.0–1.5 kW/(m²·K) for falling film]
Applications
| Application | Working pair | COP | Heat source |
|---|
| Building HVAC | LiBr-H₂O | 0.7 | Hot water 85°C |
| Solar cooling | LiBr-H₂O | 0.7 | Solar flat plate |
| Industrial cold | NH₃-H₂O | 0.5 | Steam 150°C |
| Cogeneration | LiBr-H₂O DE | 1.3 | Steam 175°C |
| Waste heat recovery | LiBr-H₂O | 0.7 | Flue gas HX |
Standards
| Standard | Scope |
|---|
| ASHRAE 147 | Absorption chillers; performance testing |
| ARI 560 | Absorption water chilling packages rating |
| ASHRAE 90.1 | Energy efficiency limits for chillers |
| ISO 5149 | Refrigerating systems and heat pumps |
Output
Provide: working pair (LiBr-H₂O/NH₃-H₂O), cycle type (single/double/triple effect), COP at operating conditions, heat duties Q_gen/Q_evap/Q_cond/Q_abs [kW], generator supply temperature [°C] and heat source type, cooling capacity [kW or TR], chilled water supply/return temperatures [°C], cooling water flow and temperature [°C], steam or hot water consumption rate [kg/s or kW], crystallization safety check (LiBr concentration vs. limit), applicable standard (ASHRAE 147, ARI 560).
