1. Overview & Applications
Mechanical refrigeration systems use the vapor-compression cycle to transfer heat from a low-temperature process stream to ambient conditions. Essential for cryogenic gas processing, LNG production, and petrochemical operations.
NGL recovery
Gas plant chilling
Propane refrigeration cools inlet gas to -20°F to -40°F for ethane+ recovery.
LNG production
Multi-stage cascade
Propane, ethylene, methane cascade to liquefy natural gas at -260°F.
Gas dew pointing
Pipeline spec conditioning
Mechanical refrigeration to meet -20°F hydrocarbon dew point at 800 psia.
Ethylene plants
Cryogenic separation
Multi-level refrigeration for ethylene/ethane splitter at -150°F to -100°F.
Refrigeration vs Expander Cooling
| Method | Temperature Range | Efficiency | Applications |
|---|---|---|---|
| Mechanical refrigeration | -150°F to +40°F | COP = 2-4 (1 HP removes 2-4 HP of heat) | Reliable, flexible, good for variable loads |
| Turboexpander (JT valve) | -200°F to -50°F | η = 80-88% isentropic efficiency | Gas processing, no external power, recovers work |
| Hybrid (expander + refrigeration) | -150°F to -40°F | Combined benefits | Deep NGL recovery, optimize expander + propane refrigeration |
Refrigeration Load Sources
Total refrigeration duty is the sum of all heat loads that must be removed:
- Process cooling load: Cool gas from 100°F to -40°F (sensible heat + latent heat if condensing)
- Demethanizer reboiler: Evaporate bottom product using refrigerant evaporator
- Subcooling reflux: Subcool overhead liquid reflux before returning to column
- Heat leak: Heat ingress through insulation, pipe flanges, valves (1-5% of total duty typical)
- Pump work: Liquid pumps add heat to process (W_pump / η_pump)
Why refrigeration matters: Refrigeration is typically 20-40% of total operating cost in cryogenic gas plants and LNG facilities. A 1% improvement in COP (coefficient of performance) can save $100,000-500,000/year in power costs for a 100 MMscfd gas plant. Proper design optimizes equipment size, refrigerant selection, and operating conditions.
2. Vapor-Compression Refrigeration Cycle
The basic refrigeration cycle consists of four processes: compression, condensation, expansion, and evaporation.
Ideal Cycle (Carnot Efficiency)
Carnot COP (Theoretical Maximum):
COP_Carnot = T_cold / (T_hot - T_cold)
Where:
T_cold = Evaporator temperature (absolute, °R or K)
T_hot = Condenser temperature (absolute, °R or K)
Example:
Evaporator at -40°F = 420°R
Condenser at 100°F = 560°R
COP_Carnot = 420 / (560 - 420) = 420 / 140 = 3.0
Carnot COP is theoretical maximum (reversible cycle).
Real cycles achieve 40-70% of Carnot efficiency due to:
- Compressor inefficiency
- Pressure drops in piping/heat exchangers
- Temperature differences in evaporator/condenser
- Superheat and subcooling
Standard Vapor-Compression Cycle
The practical cycle consists of four components operating in a closed loop:
Cycle States (clockwise on P-h diagram):
State 1 → 2: Compression (isentropic ideal, polytropic real)
- Inlet: Saturated or superheated vapor at evaporator pressure
- Outlet: Superheated vapor at condenser pressure
- Work input: W_comp = m × (h₂ - h₁) / η_comp
State 2 → 3: Condensation (isobaric)
- Cool superheated vapor to saturated vapor (desuperheat)
- Condense vapor to saturated liquid
- Subcool liquid below saturation (optional)
- Heat rejection: Q_cond = m × (h₂ - h₃)
State 3 → 4: Expansion (isenthalpic, h₃ = h₄)
- Throttle high-pressure liquid to low pressure
- Flash evaporation creates two-phase mixture
- No work recovered (in basic cycle)
State 4 → 1: Evaporation (isobaric)
- Two-phase mixture evaporates to saturated vapor
- Absorbs heat from process stream
- Refrigeration effect: Q_evap = m × (h₁ - h₄)
Performance Metrics:
COP = Q_evap / W_comp = (h₁ - h₄) / (h₂ - h₁)
Refrigeration tons = Q_evap / 12,000 Btu/hr (1 ton = 3.517 kW)
Compressor power (HP) = W_comp / 2545 Btu/hr
Example: Propane Refrigeration Cycle
Operating Conditions:
Evaporator temperature: T_evap = -40°F (P = 14.2 psia)
Condenser temperature: T_cond = 100°F (P = 190 psia)
Refrigerant: Propane (C3H8)
Refrigeration load: 1,000,000 Btu/hr
State Point Properties (from propane tables or EOS):
State 1 (evaporator outlet, saturated vapor):
P₁ = 14.2 psia, T₁ = -40°F
h₁ = 276 Btu/lb, s₁ = 1.125 Btu/lb·°R
State 2 (compressor discharge):
P₂ = 190 psia, s₂ = s₁ (isentropic compression)
From isentropic compression: h₂s = 315 Btu/lb
Assume compressor efficiency η_comp = 0.80
h₂ = h₁ + (h₂s - h₁)/η_comp = 276 + (315-276)/0.80 = 324.8 Btu/lb
T₂ ≈ 180°F (superheated)
State 3 (condenser outlet, subcooled liquid):
P₃ = 190 psia, T₃ = 90°F (10°F subcooling)
h₃ = 145 Btu/lb
State 4 (evaporator inlet, two-phase):
P₄ = 14.2 psia, h₄ = h₃ = 145 Btu/lb (isenthalpic expansion)
Quality χ₄ = (h₄ - h_f) / h_fg ≈ 0.35 (35% vapor, 65% liquid)
Performance Calculations:
Refrigeration effect per lb:
q_evap = h₁ - h₄ = 276 - 145 = 131 Btu/lb
Compressor work per lb:
w_comp = h₂ - h₁ = 324.8 - 276 = 48.8 Btu/lb
COP = q_evap / w_comp = 131 / 48.8 = 2.68
Mass flow rate:
m = Q_evap / q_evap = 1,000,000 / 131 = 7,634 lb/hr
Compressor power:
W_comp = m × w_comp = 7,634 × 48.8 = 372,500 Btu/hr = 146 HP
Condenser duty:
Q_cond = m × (h₂ - h₃) = 7,634 × (324.8 - 145) = 1,373,000 Btu/hr
Check energy balance: Q_cond = Q_evap + W_comp
1,373,000 ≈ 1,000,000 + 372,500 = 1,372,500 ✓
Cycle Enhancements
1. Economizer (Flash Gas Removal)
Add intermediate pressure flash drum to remove vapor before final expansion:
- Benefit: Reduces flash gas entering evaporator → higher refrigeration effect → 5-15% COP improvement
- Cost: Additional flash drum, piping, compressor suction port at intermediate pressure
- Application: Large systems (> 500 HP), high compression ratios (> 5:1)
2. Subcooling
Cool condenser liquid below saturation temperature:
- Benefit: Reduces flash gas, increases net refrigeration effect by 2-5%
- Method: Use cold evaporator vapor to subcool liquid (internal heat exchange)
- Tradeoff: Superheat compressor suction → slightly higher compression work
3. Multi-Stage Compression
For high compression ratios (> 8:1), use two-stage compression with intercooling:
- Benefit: Lower discharge temperature, better efficiency, reduced risk of oil breakdown
- Optimal intermediate pressure: P_int = √(P_evap × P_cond)
- Application: Cascade systems, very low evaporator temperatures (< -80°F)
COP optimization: COP increases with higher evaporator temperature and lower condenser temperature. Every 10°F reduction in lift (T_cond - T_evap) improves COP by 10-15%. Minimize approach temperatures in heat exchangers (5-10°F typical), use economizers for large systems, and optimize condenser cooling (water vs air).
3. Compressor, Condenser, and Evaporator Design
Compressor Selection
| Compressor Type | Capacity Range | Efficiency | Applications |
|---|---|---|---|
| Reciprocating (piston) | 10-2000 HP | η_isen = 0.70-0.80 | Small/medium plants, high compression ratios, multiple stages |
| Screw (rotary) | 50-3000 HP | η_isen = 0.65-0.75 | Medium plants, smooth operation, oil-flooded or oil-free |
| Centrifugal | 500-40,000 HP | η_isen = 0.75-0.85 | Large LNG plants, high flow rates, low compression ratios |
| Scroll | 1-50 HP | η_isen = 0.65-0.72 | Small HVAC, skid-mounted packages |
Compressor Sizing
Volumetric Flow Rate (Inlet):
ICFM = (m / ρ₁) × 60 (inlet cubic feet per minute)
Where:
m = Mass flow rate (lb/s)
ρ₁ = Suction density (lb/ft³)
Compression Ratio:
r = P_discharge / P_suction
Design limits:
Single-stage reciprocating: r < 8-10
Single-stage screw: r < 10-15
Single-stage centrifugal: r < 3-4
For higher ratios, use multi-stage compression.
Discharge Temperature:
T₂ / T₁ = (P₂ / P₁)^((k-1)/k) (isentropic)
Where k = Cp/Cv ≈ 1.13 for propane, 1.22 for ethylene
For real compression:
T₂ = T₁ × [1 + (1/η_isen) × ((P₂/P₁)^((k-1)/k) - 1)]
Limit discharge temperature:
T₂ < 250-300°F to prevent oil breakdown (reciprocating/screw)
T₂ < 300-350°F for ethylene (no oil concerns in centrifugal)
Condenser Design
Condensers reject heat from hot refrigerant vapor to cooling medium (air or water):
Air-Cooled Condensers
Heat Transfer Area:
A = Q / (U × LMTD)
Where:
Q = Condenser duty (Btu/hr)
U = Overall heat transfer coefficient (Btu/hr·ft²·°F)
Typical U = 15-25 for air-cooled finned tube
LMTD = Log mean temperature difference
LMTD Calculation:
For condensation (isothermal on refrigerant side):
T_ref = Condensing temperature (constant)
T_air,in = Ambient air temperature
T_air,out = Air outlet temperature
ΔT₁ = T_ref - T_air,in
ΔT₂ = T_ref - T_air,out
LMTD = (ΔT₁ - ΔT₂) / ln(ΔT₁/ΔT₂)
Design Approach:
Typical approach: T_ref - T_air,in = 15-25°F
Example:
Ambient air = 95°F → Condensing temp = 110-120°F → P_cond = 210-250 psia (propane)
Fan power:
P_fan = (CFM × ΔP) / (6356 × η_fan) (HP)
Typical CFM = 1500-2500 cfm per ton of refrigeration
Water-Cooled Condensers
Shell-and-Tube Condenser:
Refrigerant condenses on shell side (horizontal or vertical)
Cooling water flows through tubes
U = 100-200 Btu/hr·ft²·°F (much higher than air-cooled)
Design approach: T_ref - T_water,out = 5-10°F
Cooling Water Flow Rate:
m_water = Q / (Cp × ΔT)
Where:
Q = Condenser duty (Btu/hr)
Cp = 1.0 Btu/lb·°F (water)
ΔT = T_out - T_in (typically 10-20°F rise)
Example:
Q = 10 MMBtu/hr, ΔT = 15°F
m_water = 10,000,000 / (1.0 × 15) = 667,000 lb/hr = 1340 gpm
Fouling factor: Add 0.001-0.002 (hr·ft²·°F/Btu) for cooling tower water
Evaporator Design
Evaporators absorb heat from process stream to cold refrigerant:
| Type | Configuration | U (Btu/hr·ft²·°F) | Applications |
|---|---|---|---|
| Flooded shell-and-tube | Refrigerant on shell, process in tubes | 50-150 | Liquid cooling (brine, glycol), chilled water |
| Kettle reboiler | Boiling refrigerant on tube bundle | 100-200 | Column reboiler (demethanizer, deethanizer) |
| Plate-fin (BAHX) | Brazed aluminum, multi-stream | 200-400 | Gas cooling, cryogenic service, compact |
| Direct expansion coil | Refrigerant in tubes, air/gas over fins | 10-30 | Air cooling, small systems |
Evaporator Sizing:
A = Q / (U × LMTD)
LMTD for evaporation (constant T_ref on refrigerant side):
ΔT₁ = T_process,in - T_ref
ΔT₂ = T_process,out - T_ref
LMTD = (ΔT₁ - ΔT₂) / ln(ΔT₁/ΔT₂)
Minimum Approach Temperature:
ΔT_min = T_process,out - T_ref
Typical values:
Gas-gas (plate-fin): ΔT_min = 5-10°F
Liquid-refrigerant (shell-tube): ΔT_min = 5-8°F
Reboiler (nucleate boiling): ΔT_min = 10-20°F
Smaller ΔT_min → larger heat exchanger → higher capital cost
Larger ΔT_min → lower refrigeration efficiency → higher operating cost
Optimize for total cost (capital + operating).
Equipment sizing tradeoffs: Larger heat exchangers (more area, smaller ΔT_min) reduce required refrigerant lift → higher COP → lower compressor power. But larger exchangers cost more capital. Typical economic optimum: ΔT_min = 5-10°F for process gas cooling, 10-15°F for reboilers. For LNG plants with 20+ year life, invest in low ΔT_min (3-5°F) to minimize power costs.
4. Refrigerant Selection
Refrigerant choice depends on required temperature range, thermophysical properties, safety, environmental impact, and cost.
Common Industrial Refrigerants
| Refrigerant | Temp Range (°F) | Pressure (psia @ 100°F) | Applications |
|---|---|---|---|
| Ammonia (R-717, NH₃) | -50 to +50 | 247 | Industrial refrigeration, cold storage, ice plants (toxic, flammable) |
| Propane (R-290, C₃H₈) | -50 to +50 | 190 | NGL recovery, gas dew pointing, LNG (flammable, excellent properties) |
| Ethylene (R-1150, C₂H₄) | -150 to -40 | 870 @ 0°F | Deep NGL recovery, ethylene plants, LNG cascade (flammable) |
| Methane (R-50, CH₄) | -260 to -150 | Very high | LNG final stage, cryogenic applications (flammable, high pressure) |
| R-134a (HFC) | -20 to +60 | 136 | HVAC, small industrial (non-flammable, low toxicity, GWP=1430) |
| R-404A (HFC blend) | -40 to +50 | 215 | Commercial refrigeration, cold storage (non-flammable, high GWP=3922) |
Refrigerant Properties Comparison
Key Properties (at typical evaporator conditions):
Propane at -40°F evaporator, 100°F condenser:
P_evap = 14.2 psia, P_cond = 190 psia
Latent heat λ = 131 Btu/lb
Compression ratio r = 13.4
Ideal COP ≈ 2.8-3.2
Ethylene at -100°F evaporator, 80°F condenser:
P_evap = 37 psia, P_cond = 630 psia
Latent heat λ = 183 Btu/lb
Compression ratio r = 17
Ideal COP ≈ 2.2-2.8 (lower due to larger lift)
Ammonia at -40°F evaporator, 100°F condenser:
P_evap = 10.4 psia, P_cond = 247 psia
Latent heat λ = 565 Btu/lb (highest of common refrigerants)
Compression ratio r = 23.7
Ideal COP ≈ 2.8-3.3
R-134a at -20°F evaporator, 100°F condenser:
P_evap = 12.2 psia, P_cond = 136 psia
Latent heat λ = 84 Btu/lb
Compression ratio r = 11.1
Ideal COP ≈ 2.4-3.0
Hydrocarbon Refrigerants in Oil & Gas
Propane and ethylene dominate oil/gas applications despite flammability:
Advantages of Hydrocarbon Refrigerants
- Excellent thermodynamic properties: High latent heat, low compression ratio, good COP
- Already present in facility: Propane/ethylene available from process streams, no import needed
- Low cost: Propane $0.50-1.50/gal vs R-404A $5-15/lb
- Environmental: GWP ≈ 3-5 (vs 1000-4000 for HFCs), zero ODP
- Compatibility: Compatible with mineral oils, metals, elastomers
Safety Considerations
- Flammability: LEL 2.1% for propane, design per API RP 521 for flammable fluid handling
- Ventilation: Equipment in well-ventilated areas, gas detection, isolation valves
- Electrical classification: Class I, Division 2 hazardous area (or Division 1 near potential leak sources)
- Fire protection: Water deluge system for compressor area, emergency shutdown systems
- Inventory minimization: Use compact heat exchangers (plate-fin) to reduce refrigerant inventory
Refrigerant selection for gas processing: Propane is standard for -50°F to 0°F applications (NGL recovery, dew pointing). Ethylene for -150°F to -50°F (deep NGL, ethane rejection). Ammonia for HVAC and cold storage (excellent properties but toxic). HFCs being phased out due to high GWP (Montreal Protocol, Kigali Amendment). Future: Low-GWP alternatives (R-1234yf, CO₂, natural refrigerants).
5. Cascade Refrigeration Systems
Cascade systems use two or more refrigeration cycles in series, with the condenser of the low-temperature cycle rejecting heat to the evaporator of the high-temperature cycle. Essential for cryogenic applications.
Why Cascade?
Single-stage compression becomes impractical for large temperature lifts due to:
- High compression ratio: For propane, -150°F evaporator and 100°F condenser → r = 50+ (excessive)
- High discharge temperature: T_discharge > 400°F → oil breakdown, compressor damage
- Low volumetric efficiency: High compression ratio → large clearance volume losses
- Poor COP: Efficiency drops sharply with compression ratio > 10-15
Two-Stage Cascade Configuration
Typical LNG Cascade:
High-temperature stage (warm loop):
Refrigerant: Propane (C₃)
Evaporator: -40°F to 0°F
Condenser: 80-100°F (air or water cooled)
Process duty: Cool feed gas, condense NGLs
Low-temperature stage (cold loop):
Refrigerant: Ethylene (C₂)
Evaporator: -120°F to -100°F
Condenser: -50°F to -40°F (cooled by propane evaporator)
Process duty: Subcool LNG, demethanizer reboiler
Cascade heat exchanger:
Propane evaporator = Ethylene condenser
ΔT_approach = 5-10°F
Overall COP:
COP_cascade = Q_evap,cold / (W_comp,hot + W_comp,cold)
Typically COP_cascade = 1.8-2.5 (lower than single-stage due to two compression stages)
But this enables temperature ranges impossible for single-stage.
Three-Stage Cascade for LNG
Large LNG plants use propane-ethylene-methane cascade to reach -260°F:
| Stage | Refrigerant | Evaporator Temp | Process Duty |
|---|---|---|---|
| 1 (warm) | Propane (C₃) | -40°F to +10°F | Pre-cool feed gas, remove heavy HCs |
| 2 (intermediate) | Ethylene (C₂) | -140°F to -60°F | Liquefy methane, sub-cool LNG |
| 3 (cold) | Methane (C₁) | -260°F to -200°F | Final LNG sub-cooling, nitrogen rejection |
Mixed Refrigerant (MR) Alternative
Single mixed-refrigerant cycle uses blend of components (N₂, CH₄, C₂H₆, C₃H₈) instead of pure refrigerants:
Mixed Refrigerant Composition (typical):
Nitrogen (N₂): 5-10 mol%
Methane (CH₄): 40-50%
Ethane (C₂H₆): 20-30%
Propane (C₃H₈): 15-25%
How it works:
Mixture condenses over temperature range (not isothermal)
- Propane condenses first at -40°F
- Ethane condenses at -80°F
- Methane condenses at -150°F
- Nitrogen remains vapor
Temperature glide during condensation matches process cooling curve
→ Better thermodynamic efficiency than cascade (smaller ΔT_min)
After expansion, mixture evaporates in reverse order:
- Light components (N₂, CH₄) evaporate first at cold end (-260°F)
- Heavy components (C₃) evaporate last at warm end (-40°F)
Advantages vs Cascade:
- 10-20% higher COP (better temperature matching)
- Simpler: single compressor loop (but multi-stage compression)
- Lower capital cost for large LNG plants
Disadvantages:
- Complex composition control (makeup, bleed)
- Sensitive to composition changes
- Requires rigorous simulation (Aspen HYSYS, ProMax)
Cascade vs Mixed Refrigerant Selection
| Application | Preferred System | Reason |
|---|---|---|
| Small NGL plant (< 50 MMscfd) | Single-stage propane | Simple, reliable, -40°F adequate |
| Deep NGL recovery (90%+ C2) | Propane/ethylene cascade | -100°F to -120°F required, proven technology |
| Small LNG (< 1 MTPA) | Propane/ethylene cascade | Simpler operation, easier composition control |
| Large LNG (> 3 MTPA) | Mixed refrigerant (single or dual MR) | 10-15% efficiency gain justifies complexity at scale |
| Offshore LNG (FLNG) | Nitrogen-expander or single MR | Weight/space constraints favor compact systems |
LNG Plant Refrigeration Power
Specific Power Consumption:
Modern LNG plants: 0.25-0.35 kWh/kg LNG
For 5 MTPA LNG plant:
Production rate = 5,000,000 MT/yr / (8760 hr/yr) = 570 MT/hr = 570,000 kg/hr
Power consumption = 570,000 kg/hr × 0.30 kWh/kg = 171,000 kW = 171 MW
Compressor drivers:
- Gas turbines (aeroderivative or heavy-duty)
- Electric motors (if grid power available)
Fuel cost at $3/MMBtu, heat rate 10,000 Btu/kWh:
Fuel cost = 171 MW × 10,000 Btu/kWh × $3/MMBtu / 1000 = $5.1 million/hr
→ $45 million/year continuous operation
1% efficiency improvement saves $450,000/year in fuel.
Cascade system design: Use two-stage cascade (propane/ethylene) for gas processing down to -150°F. Use three-stage cascade or mixed refrigerant for LNG production (-260°F). Optimize inter-stage temperatures to balance compression ratios (geometric mean: T_mid = √(T_cold × T_hot) on absolute scale). Size cascade heat exchangers with 5-10°F approach to minimize exergy destruction. For large LNG, evaluate MR vs cascade based on capital cost, operating cost, and reliability.
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