Compressor Performance Parameters

Compressor performance parameters define the operational characteristics and efficiency of refrigeration system compressors. Understanding these metrics enables proper equipment selection, accurate capacity predictions, and optimal system design.

Volumetric Efficiency

Volumetric efficiency represents the ratio of actual refrigerant mass flow to theoretical displacement capacity:

η_v = (ṁ_actual × v_suction) / (V_swept × n × ρ_suction)

Where:

• ṁ_actual = Actual mass flow rate
• v_suction = Specific volume at suction conditions
• V_swept = Swept volume per revolution
• n = Compressor speed (revolutions per unit time)
• ρ_suction = Refrigerant density at suction

Volumetric efficiency degrades due to:

Re-expansion losses: Clearance volume gas expands during the intake stroke, reducing effective intake volume. Clearance ratio typically ranges from 2-6% for reciprocating compressors.

Pressure drop losses: Flow resistance through suction and discharge valves, ports, and internal passages reduces effective pressure differential and gas density.

Heat transfer effects: Heat transfer from discharge gas and motor windings to suction gas reduces suction density by 5-15°F equivalent superheat increase.

Leakage losses: Internal leakage past valves, pistons, or rotating seals returns compressed gas to suction side, reducing net flow.

Typical volumetric efficiency values:

Volumetric efficiency decreases as compression ratio increases, following approximately:

η_v = η_v,ref - C(CR - CR_ref)

Where C is an empirically determined coefficient specific to compressor design.

Isentropic Efficiency

Isentropic efficiency compares actual compression work to ideal isentropic compression work:

η_s = (h_discharge,isentropic - h_suction) / (h_discharge,actual - h_suction)

This efficiency accounts for irreversibilities during compression including:

• Friction between moving parts
• Gas friction and turbulence
• Heat transfer during compression
• Throttling losses through valves

Isentropic efficiency typically ranges from 60-75% for reciprocating compressors, 65-75% for scrolls, and 70-80% for screw compressors at design conditions.

The actual discharge temperature exceeds isentropic discharge temperature by:

ΔT_excess = (T_discharge,isentropic - T_suction) × ((1/η_s) - 1)

This temperature rise affects oil viscosity, material stress, and discharge gas superheat.

Mechanical Efficiency

Mechanical efficiency relates indicated power (power transferred to gas) to shaft power:

η_m = W_indicated / W_shaft

Mechanical losses include:

• Bearing friction
• Seal friction
• Windage losses
• Oil pump work

Mechanical efficiency typically exceeds 90% for well-designed compressors.

Overall Efficiency

Overall isentropic efficiency combines volumetric, isentropic, and mechanical efficiencies:

η_overall = η_v × η_s × η_m

This parameter determines actual compressor power consumption:

W_actual = ṁ × (h_discharge,isentropic - h_suction) / η_overall

AHRI Capacity Rating Conditions

The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) establishes standardized rating conditions for compressor capacity and efficiency comparison.

AHRI Standard 540 conditions for refrigeration compressors:

AHRI Standard 210/240 conditions for air-conditioning compressors:

• Cooling capacity: 95°F outdoor, 80°F DB / 67°F WB indoor
• Heating capacity: 47°F outdoor DB / 43°F WB, 70°F indoor
• High-temperature heating: 17°F outdoor

Compressor capacity at AHRI conditions serves as baseline for performance correction to actual operating conditions using manufacturer’s correction factors.

Compression Ratio Effects

Compression ratio profoundly impacts compressor performance:

CR = P_discharge / P_suction

Higher compression ratios produce:

Reduced volumetric efficiency: Re-expansion losses increase proportionally with pressure ratio. A 10:1 compression ratio may reduce volumetric efficiency by 20-30% compared to 3:1 operation.

Increased discharge temperature: Following the isentropic relation:

T_discharge = T_suction × (CR)^((k-1)/k) / η_s

Where k = specific heat ratio (typically 1.15-1.20 for refrigerants).

Reduced reliability: Discharge temperatures exceeding 250-275°F cause oil breakdown, valve damage, and motor winding degradation.

Higher power consumption: Work input increases faster than capacity, reducing coefficient of performance (COP).

Increased leakage: Greater pressure differential across internal sealing surfaces increases blow-by losses.

Most compressors limit maximum compression ratio to 8:1-10:1 for reciprocating types, 15:1-20:1 for scrolls, and 20:1-25:1 for screws.

Power Consumption

Compressor power consumption depends on refrigerant properties, operating conditions, and efficiency:

W = ṁ × (h_discharge - h_suction) / η_overall

For semi-hermetic and hermetic compressors, motor heat rejection increases refrigeration effect:

Q_evap = ṁ × (h_suction - h_liquid) + W_motor × f_motor_heat

Where f_motor_heat = fraction of motor heat rejected to suction gas (typically 0.85-1.0).

Power consumption increases with:

• Higher condensing temperature
• Lower evaporating temperature
• Increased superheat (reduces mass flow)
• Reduced efficiency
• Liquid refrigerant carryover (liquid slugging)

Superheat Impact

Suction superheat affects compressor performance through multiple mechanisms:

Density reduction: Each 10°F superheat increase reduces suction density by approximately 3-4%, reducing mass flow proportionally at constant volumetric efficiency.

Capacity reduction: Lower mass flow directly reduces cooling capacity:

Q_reduced = Q_nominal × (ρ_superheated / ρ_saturated)

Discharge temperature increase: Additional superheat increases discharge temperature approximately 1:1, potentially approaching thermal limits.

Motor cooling impact: For hermetic compressors, insufficient superheat (approaching saturated conditions) risks liquid floodback, while excessive superheat reduces motor cooling effectiveness.

Optimal suction superheat typically ranges from 10-20°F for refrigeration applications and 5-15°F for air-conditioning applications, balancing capacity, efficiency, and reliability.

Performance Mapping

Compressor performance maps graphically represent capacity and power as functions of operating conditions. Manufacturers provide performance data as:

Capacity maps: Plot cooling capacity versus evaporating temperature with condensing temperature as parameter, typically normalized to AHRI rating point.

Power maps: Plot power consumption versus operating conditions on same axes as capacity maps.

Efficiency maps: Display EER or COP across operating envelope, identifying optimal efficiency regions.

Envelope limits: Define maximum compression ratio, discharge temperature, discharge pressure, and motor current boundaries.

Performance correction factors adjust AHRI rated capacity to actual conditions:

Q_actual = Q_AHRI × CF_evap × CF_cond × CF_superheat × CF_subcool

Where correction factors typically follow polynomial relationships:

CF_evap = a₀ + a₁T_evap + a₂T_evap²

Accurate performance mapping requires consideration of:

• Refrigerant-specific thermodynamic properties
• Return gas temperature effects on motor cooling
• Liquid subcooling impact on expansion device operation
• Oil circulation rate variations
• Transient operation during start-up and cycling

Modern performance analysis employs curve-fitting algorithms to generate continuous performance functions from discrete test data, enabling accurate capacity and power predictions at any operating condition within the compressor envelope.

Practical Application

Compressor selection requires:

1. Calculate required capacity at actual evaporating and condensing temperatures
2. Verify compression ratio remains within manufacturer limits
3. Confirm discharge temperature below 250-275°F
4. Calculate expected power consumption and verify electrical supply adequacy
5. Check capacity turndown requirements against minimum stable operation limits
6. Verify motor service factor accommodates expected overload conditions
7. Evaluate part-load efficiency for variable-load applications

Performance monitoring during operation tracks:

• Suction and discharge pressures (compression ratio)
• Suction and discharge temperatures
• Power consumption and current draw
• Operating efficiency (EER or COP)
• Capacity deviation from design

Degraded performance indicators include reduced capacity, increased power, elevated discharge temperature, or decreased efficiency, signaling maintenance requirements such as valve replacement, refrigerant charge adjustment, or system cleaning.