Compression Ratio Calculator

Compression Ratio Fundamentals

Compression ratio (CR) quantifies the pressure increase across the compressor:

CR = \fracP_discharge,absP_suction,abs

where pressures MUST be absolute (psia, kPa abs, or bar abs).

**Converting Gauge to Absolute**:

P_abs = P_gauge + P_atmospheric

At sea level: - P_atm = 14.7 psia = 101.3 kPa = 1.013 bar

**Example**: - Suction: 70 psig → 70 + 14.7 = 84.7 psia - Discharge: 280 psig → 280 + 14.7 = 294.7 psia - CR = 294.7 / 84.7 = 3.48

**Typical Compression Ratios**:

| Application | Typical CR | Maximum CR | |-------------|------------|------------| | AC (R-410A, design) | 2.5-3.5 | 4.5 | | AC (R-22, design) | 2.8-4.0 | 5.0 | | Heat pump (heating) | 3.0-5.0 | 6.0 | | Commercial refrigeration (medium temp) | 3.0-5.0 | 6.0 | | Commercial refrigeration (low temp) | 5.0-10.0 | 12.0 | | Chiller (water-cooled) | 2.0-3.0 | 4.0 |

**Why CR Matters**:

1. **Efficiency**: Higher CR = lower efficiency (more power per ton) 2. **Discharge Temperature**: Higher CR = higher discharge temp (risk of oil breakdown) 3. **Compressor Life**: High CR increases wear, reduces life 4. **Capacity**: Very high CR reduces volumetric efficiency

CR Effects on Performance

**Discharge Temperature**:

Approximate relationship for reciprocating/scroll compressors:

T_discharge ≈ T_suction × CR^(k-1)/k

where k = specific heat ratio ≈ 1.13 for refrigerants.

For R-410A (simplified):

T_discharge,°F ≈ T_suction,°F + 65 × (CR - 1)

**Safe Discharge Temperature**: <225°F (107°C) for most compressors.

**Volumetric Efficiency**:

\eta_v = 1 - C × (CR - 1)

where C = clearance coefficient (0.04-0.08 typical).

Higher CR → Lower \eta_v → Less capacity.

**Power Consumption**:

Compressor power increases with CR:

P \propto \fracCR^(k-1)/k - 1k-1

Doubling CR increases power by ~50-80%.

**COP (Coefficient of Performance)**:

COP = \fracQ_evapP_comp

Higher CR → Higher power → Lower COP.

Typical relationship: COP decreases 3-5% per 0.1 increase in CR.

Compression Ratio Diagnostics

**Normal Operation**:

- AC system: CR 2.5-4.0 at design conditions - Chiller: CR 2.0-3.5 - Heat pump (heating, 47°F outdoor): CR 3.0-4.0

**High Compression Ratio** (>5.0 for AC, >4.0 for chiller):

Causes: 1. **High Condensing Pressure**: Dirty condenser coil, inadequate airflow, high ambient temperature, non-condensables in system 2. **Low Evaporator Pressure**: Low refrigerant charge, restricted metering device, low airflow over evaporator, low load

Symptoms: - High discharge temperature (>225°F) - High amp draw - Reduced capacity - Poor efficiency - Compressor overheating

Actions: - Clean condenser coil - Check condenser fan operation - Verify proper charge (subcooling) - Check for restrictions - Monitor for overload

**Low Compression Ratio** (<2.0):

Causes: 1. **Low Condensing Pressure**: Low ambient temperature, oversized condenser 2. **High Evaporator Pressure**: Overcharge, TXV stuck open, high load

Symptoms: - Low discharge temperature - May flood compressor - Low subcooling

Actions: - Install head pressure control (if low ambient) - Check for overcharge - Verify TXV operation

CR and Refrigerant Type

Different refrigerants have different pressure characteristics:

**R-410A** (high pressure): - Typical evap: 118 psig (132.7 psia) - Typical cond: 278 psig (292.7 psia) - CR: 292.7 / 132.7 = 2.21 (at design) - Operates at higher absolute pressures but similar CR

**R-22** (medium pressure, being phased out): - Typical evap: 68 psig (82.7 psia) - Typical cond: 212 psig (226.7 psia) - CR: 226.7 / 82.7 = 2.74

**R-134a** (lower pressure): - Typical evap: 37 psig (51.7 psia) - Typical cond: 151 psig (165.7 psia) - CR: 165.7 / 51.7 = 3.21

**R-717 (Ammonia)** (industrial): - Can operate at higher CR (up to 8-10) - More efficient than HFCs

**R-744 (CO₂)** (transcritical): - Very high pressures (>1000 psi) - Different thermodynamic cycle - Traditional CR concept doesn't apply to transcritical operation

Compressor must be rated for specific refrigerant pressure range and CR.

Worked Example

Inputs

• suctionPressure: 118
• dischargePressure: 278
• refrigerant: R410A

Result: Compression Ratio: 2.21 | Discharge Temp: ~140°F | Status: NORMAL - Optimal performance

Explanation

For R-410A system with typical design conditions:

**Step 1: Convert to Absolute Pressure** Suction: 118 + 14.7 = 132.7 psia Discharge: 278 + 14.7 = 292.7 psia

**Step 2: Calculate Compression Ratio** CR = P_discharge / P_suction CR = 292.7 / 132.7 = 2.21

**Step 3: Assess Compression Ratio** For R-410A AC system: - Optimal range: 2.0-3.5 - Measured: 2.21 ✓ Excellent - Well within acceptable range

**Step 4: Estimate Discharge Temperature** Suction temp from PT chart: 118 psig R-410A ≈ 45°F saturation Assuming 10°F superheat: Suction gas = 55°F

Discharge temp estimate: T_d ≈ 55 + 65 × (2.21 - 1) = 55 + 79 = 134°F

This is excellent (<225°F safe limit)

**Step 5: Performance Assessment** At CR = 2.21: - Expected COP: ~3.8-4.2 (excellent efficiency) - Volumetric efficiency: ~95% (good capacity) - Discharge temperature: Safe range - Compressor loading: Normal

**Step 6: Verify Conditions** These pressures correspond to: - Evaporator: 45°F sat (typical for 75°F return air) - Condenser: 105°F sat (typical for 95°F outdoor) - Temperature lift: 105 - 45 = 60°F (reasonable)

**Conclusion**: ✓ Compression ratio optimal ✓ System operating efficiently ✓ No issues indicated ✓ Condenser and evaporator performing well

**What if CR was different?** - CR = 4.5: Too high! Check for dirty condenser or undercharge - CR = 1.5: Too low, may indicate overcharge or cold ambient - CR > 5.0: CRITICAL - immediate action required

Second Scenario

Inputs

• suctionPressure: 148.5
• dischargePressure: 278
• refrigerant: R410A

Result: Compression Ratio: 2.21 | Discharge Temp: ~140°F | Status: NORMAL - Optimal performance

Explanation

This scenario uses different inputs (suctionPressure = 148.5, dischargePressure = 278, refrigerant = R410A) to show how changing one variable affects the compression ratio result. Run the calculator above with these values to get the exact updated output with step-by-step work.