Energy Efficiency Reference/Refrigeration/Recommendations/Increase Suction Pressure
Recommendation: Increase Suction Pressure
Summary: Increase refrigeration suction pressure to reduce compressor power and save energy. Typically, compressor power decreases 2% to 3% for every degree Fahrenheit that suction temperature increases.
When to apply: When suction pressures are lower than needed to meet refrigeration loads, or back-pressure regulators (BPR) are used. BPRs are used to hold evaporator pressures above the suction pressure, resulting in higher evaporating temperatures.
When the evaporator approach temperature difference is excessively low due to non-product related refrigeration loads.
Key Engineering Concepts:
- Compressor power is directly proportional to the compression ratio [verify]
- Increasing suction pressure will decrease compression ratio
- Suction pressure is directly controlled by compressor settings
- Refrigeration data sheets
- Existing Suction Pressure - (Pressure gauges or Display)
- Temperature of cooled product - (Measure)
- Desired temperature of cooled product - (thermostat or control settings)
- BPR settings (if any)
- Compressor Performance Data - (manufacturer's specifications)
Special Considerations: Suction pressure may be lower than needed due to high refrigeration loads. Refer to Recommendation No 6, Minimize Excess Refrigeration Loads for more information.
Analysis process: Choose Target Suction Pressure A 10 degrees F approach temperature difference is recommended across the evaporators.
If the minimum evaporator approach is larger than this, consider purchasing more evaporators. Otherwise, select an appropriate suction temperature based on the desired temperature of the cooled medium and the minimum approach temperature difference with all fans operating. Back pressure regulators may have to be removed to allow an increase in suction pressure, or excess loads.
Calculate Coefficient of Performance (COP) to be reduced.
Get Compressor Performance Specifications Call the manufacturer of the compressor and request performance specifications for the compressor model you are analyzing. BHP and tons of refrigeration are generally available at various suction and discharge pressures.
Compressor specifications contain tables of compressor power and refrigeration capacity (usually in tons) for various suction and discharge pressures. Use compressor specifications at the existing discharge pressure to calculate and graph COPs at various section pressures. Use the curves to find existing and proposed COP's at various suction pressures or temperatures.
3) Calculate COP: Coefficient of Performance is a measure of efficiency, calculated as the ratio of refrigeration effect to compressor power. COP is unitless, so convert power and refrigeration capacity to common units using the following conversion factors:
- COP = Refrigeration Effect / Compressor Power
Where 1 ton refrigeration = 12,000 Btu / hr
4)Calculate Energy Savings The proposed COP will be larger than the existing. Calculate the percent energy savings based on the change in COP.
- energy savings = 1 - (existing COP / proposed COP)
Calculate the current compressor energy use (CE) based on the operating schedule and the measured power, or volts and current. The total energy savings is the compressor energy multiplied be the percent energy savings.
- CES = CE x ES%
5) Calculate Cost Savings Calculate cost savings by multiplying the energy cost from the utility bills.
6) Estimate Implementation Cost The implementation cost depends on the action necessary. Cost can be for maintenance or for new equipment.
7)Determine payback Determine simple payback period by dividing the implementation cost by the annual cost savings.