Evaporator Performance Analysis

Performance Fundamentals

Evaporator performance determines refrigeration system capacity, efficiency, and operating economics. Performance analysis requires understanding heat transfer mechanisms, refrigerant-side and load-side characteristics, and degradation factors affecting long-term operation.

The evaporator transfers heat from the cooling medium (air, water, or process fluid) to the evaporating refrigerant. Performance depends on:

• Heat transfer surface area and geometry
• Temperature difference between load and refrigerant
• Flow characteristics on both sides
• Surface conditions and cleanliness
• Refrigerant distribution quality

Heat Transfer Analysis Methods

Log Mean Temperature Difference (LMTD) Method

The LMTD method applies to evaporators with known inlet and outlet temperatures. For constant evaporating temperature (typical for direct expansion systems):

Heat Transfer Equation:

Q = U × A × LMTD

Where:

• Q = Heat transfer rate (Btu/hr or kW)
• U = Overall heat transfer coefficient (Btu/hr·ft²·°F or W/m²·K)
• A = Heat transfer surface area (ft² or m²)
• LMTD = Log mean temperature difference (°F or K)

LMTD Calculation for Constant Evaporating Temperature:

LMTD = (ΔT₁ - ΔT₂) / ln(ΔT₁/ΔT₂)

Where:

• ΔT₁ = Load inlet temperature - Evaporating temperature
• ΔT₂ = Load outlet temperature - Evaporating temperature

For air-cooling evaporators with significant temperature change, account for sensible and latent heat components separately.

Effectiveness-NTU Method

The effectiveness-NTU method applies when outlet temperatures are unknown, particularly useful for design calculations and performance prediction.

Heat Transfer Effectiveness:

ε = Q_actual / Q_maximum

Q_maximum = C_min × (T_load,in - T_evap)

Where:

• ε = Heat exchanger effectiveness (dimensionless)
• C_min = Minimum heat capacity rate (Btu/hr·°F or W/K)
• T_load,in = Load fluid inlet temperature
• T_evap = Evaporating temperature

Number of Transfer Units (NTU):

NTU = U × A / C_min

For evaporators with constant evaporating temperature (C_refrigerant approaches infinity):

ε = 1 - exp(-NTU)

This relationship simplifies performance prediction when overall heat transfer coefficient and surface area are known.

Comparison of Methods

Overall Heat Transfer Coefficient

The overall heat transfer coefficient combines thermal resistances in series:

1/U = 1/h_outside + t_wall/k_wall + R_fouling,outside + R_fouling,inside + 1/h_inside

Where:

• h_outside = Load-side convective coefficient
• h_inside = Refrigerant-side convective coefficient
• t_wall = Tube wall thickness
• k_wall = Tube material thermal conductivity
• R_fouling = Fouling resistance factors

Typical Heat Transfer Coefficients

Values represent clean conditions. Fouling reduces U significantly.

Approach Temperature Analysis

Approach temperature represents the temperature difference between load fluid outlet and refrigerant evaporating temperature. It indicates heat transfer effectiveness.

Approach Temperature:

TD_approach = T_load,out - T_evap

Lower approach temperatures indicate:

• Larger heat transfer surface area
• Better heat transfer characteristics
• Higher evaporating temperature for given load
• Improved system efficiency

Approach Temperature Optimization

Optimum approach temperature balances capital cost against operating cost:

Economic Analysis:

• Smaller approach → Larger evaporator → Higher first cost
• Smaller approach → Higher evaporating temperature → Lower compressor power
• Optimal design minimizes life cycle cost

Impact on System Performance

Each 1°F (0.56K) reduction in approach temperature:

• Increases evaporating temperature by approximately 1°F
• Reduces compressor power by 2-4% (varies with refrigerant and conditions)
• Requires 10-15% more evaporator surface area
• Reduces refrigerant-side pressure drop concerns

Fouling Effects on Performance

Fouling adds thermal resistance, reducing heat transfer and increasing approach temperature. Fouling occurs on load side and refrigerant side.

Load-Side Fouling

Air-Side Fouling:

• Dust and particulate accumulation
• Biological growth (mold, bacteria)
• Corrosion products
• Increases air-side resistance dramatically

Water-Side Fouling:

• Scale formation (calcium carbonate, magnesium silicate)
• Biological fouling (algae, biofilm)
• Corrosion products (iron oxide, copper oxide)
• Particulate deposition

Fouling Resistance Values

Convert to SI: 1 hr·ft²·°F/Btu = 0.1761 m²·K/W

Refrigerant-Side Fouling

Less common but critical when present:

• Oil accumulation (reduces heat transfer coefficient by 10-30%)
• Non-condensable gases (reduce effective surface area)
• Contaminants from system degradation
• Refrigerant decomposition products

Oil Effect on Heat Transfer:

For 5% oil concentration in evaporator:

• Heat transfer coefficient reduction: 15-25%
• Greater impact at low evaporating temperatures
• Flooded evaporators more susceptible than DX

Performance Degradation Factors

Frost Formation Effects

Frost accumulation on air-cooling evaporators creates multiple performance penalties:

Thermal Resistance:

• Frost adds insulating layer
• Thermal conductivity of frost: 0.12-0.25 Btu/hr·ft·°F (0.21-0.43 W/m·K)
• Reduces effective U-value by 20-60%

Airflow Restriction:

• Frost blocks airflow passages
• Increases air-side pressure drop exponentially
• Reduces airflow by 30-70% before defrost
• Non-uniform frost distribution causes air bypass

Frost Density Impact:

Frost density increases with time as consolidation occurs.

Refrigerant Maldistribution

Poor refrigerant distribution reduces effective surface area utilization:

Impact Factors:

• Overfed circuits: liquid carryover, reduced superheat control
• Underfed circuits: reduced capacity, excessive superheat
• Combined effect: 10-40% capacity loss possible

Distribution Quality Metric:

Circuit efficiency = (Actual capacity) / (Capacity if perfectly distributed) × 100%

Well-designed systems achieve 85-95% distribution efficiency.

Air or Water Flow Variation

Flow rate changes affect heat transfer coefficients non-linearly:

Air-Side Relationship (Turbulent Flow):

h ∝ V^0.8

Where V = air velocity

Reducing airflow by 20% reduces heat transfer coefficient by approximately 17%.

Water-Side Relationship (Turbulent Flow):

h ∝ V^0.8

Similar relationship, but water-side typically has lower thermal resistance.

Defrost System Performance

Defrost Methods and Efficiency

Hot Gas Defrost

Most common method for commercial refrigeration:

Process Sequence:

1. Liquid line solenoid closes
2. Hot gas valve opens
3. Evaporator fans stop (typically)
4. Hot refrigerant vapor enters evaporator
5. Frost melts from inside out
6. Termination by time or temperature

Performance Considerations:

• Requires 15-25% of system capacity during defrost
• Heat input: 12,000-18,000 Btu/hr per ton of evaporator capacity
• Refrigerant charge increases during defrost (receiver sizing critical)
• Multiple evaporators require sequenced defrost

Electric Defrost

Heater Sizing:

kW = (Evaporator capacity in tons × 12,000 × 1.5) / 3413

Factor 1.5 accounts for:

• Melting ice (144 Btu/lb at 32°F)
• Heating coil mass
• Heat losses
• Safety margin

Energy Impact:

Electric defrost typically adds 5-15% to total system energy consumption depending on:

• Defrost frequency (cycles per day)
• Duration per cycle
• Operating temperature
• Humidity conditions

Performance Monitoring and Maintenance

Key Performance Indicators

Maintenance Impact on Performance

Regular maintenance preserves design performance:

Air-Side Cleaning:

• Coil cleaning restores 80-95% of original capacity
• Frequency: 2-4 times per year (varies by environment)
• Methods: vacuum, compressed air, chemical wash, steam

Water-Side Cleaning:

• Chemical cleaning restores 85-100% of original capacity
• Frequency: annually or per water analysis
• Methods: acid cleaning, mechanical brushing (tube bundles)

Refrigerant Charge Optimization:

• Proper charge within ±5% of design
• Undercharge reduces capacity proportionally
• Overcharge in DX systems causes hunting, liquid slugging

Performance Testing and Verification

Field Performance Test Procedure:

1. Stabilize system at design conditions (minimum 30 minutes)
2. Measure refrigerant saturation temperature (evaporating temperature)
3. Measure load-side inlet and outlet temperatures
4. Measure load-side flow rate
5. Calculate heat transfer rate: Q = m × cp × ΔT
6. Calculate approach temperature
7. Compare to design values
8. Determine U-value and compare to clean design condition

Acceptance Criteria:

• Capacity within ±5% of design
• Approach temperature within ±2°F of design
• U-value within 10% of clean design (new installation)
• U-value within 25% of clean design (existing system)

Advanced Performance Considerations

Variable Load Operation

Part-load performance differs from design conditions:

• Reduced load → lower temperature difference → reduced LMTD
• May require capacity control (VFD fans, water flow modulation)
• Flooded systems maintain better part-load performance than DX
• Minimum loading: typically 25-40% of design capacity

Low-Temperature Applications

Below 0°F evaporating temperature:

• Frost formation accelerates dramatically
• Defrost frequency increases (4-12 cycles per day)
• Approach temperature typically larger (6-10°F)
• Material selection critical (brittle fracture concerns)
• Oil return becomes challenging

High-Efficiency Design Strategies

Maximize performance through:

1. Oversized surface area (reduce approach by 30-50%)
2. Enhanced surfaces (microfin tubes, louvered fins)
3. Optimized circuitry (minimize refrigerant-side pressure drop)
4. Superior distribution (multiple feedpoints, distributors)
5. Variable-speed fans (maintain optimum air velocity)
6. Water treatment programs (minimize fouling)
7. Advanced defrost controls (demand-based, not time-based)

ROI Analysis:

High-efficiency evaporator design typically adds 15-25% to evaporator cost but:

• Reduces annual energy cost by 8-15%
• Simple payback: 2-5 years (varies with utility rates)
• Extended equipment life through reduced compressor stress
• Improved process control and product quality

Performance Optimization Summary

Optimal evaporator performance requires:

• Proper initial sizing with appropriate approach temperature
• High-quality refrigerant distribution system
• Regular maintenance program (cleaning, refrigerant charge)
• Advanced controls for demand-based defrost
• Performance monitoring and trending
• Water treatment for water-cooled applications
• Adequate air filtration for air-cooled applications

Performance degradation is gradual and often unnoticed. Establish baseline performance at commissioning and monitor quarterly. A 20% capacity loss typically occurs over 2-3 years without proper maintenance, directly impacting system efficiency and operating cost.