Energy Efficiency Reference/Refrigeration/Walkthrough Checklist
Refrigeration: Walkthrough Checklist
Following are "hints and rules of thumb" that can help increase familiarity with refrigeration equipment and common terminology.
1) Suction Pressure = low side pressure or evaporator pressure 2) Compressor power drops 2% - 3% for each degree F of suction temperature increase. 3) Discharge pressure = compressor discharge pressure, high side pressure, condensing pressure, or (cylinder) head pressure. 4) Compressor power drops 1%-1.5% for each degree F of condensing temperature drop. 5) Reduce "lift" (maximize suction pressure and minimize condensing pressure) to reduce power 6) Condensers are hot (or wet) - outside the refrigerator 7) Evaporators are cold - "inside the refrigerator"
Operation: Following are typical opportunities for improving the operation of compressors
- Reduce Discharge Pressure
Discharge pressure/temperature should approach ambient by 10 to 15 degrees F with adequately sized condensers. Decrease discharge pressure to reduc compressor power.
The minimum condensing pressure for 60 degree F condensing temperature varies with refrigerant.
1) Ammoniz: 93 psig 2) R12: 72 psig 3) R134a: 72 psig 4) R22: 116 psig
- Reduce discharge pressure (halocarbon systems): Flash gas caused by system losses at low discharge pressure can be avoided with a liquid pressure amplifier (LPA) pump. LPA is small pump with magnetically driven impeller used after high-pressure receiver to increase pressure and sub-cool liquid before an expansion valve.
- Add Condensing capacity to reduce condenser approach temperature: Compressor savings with lower discharge pressure will generally pay for the cost installing and operating additional condensers if the approach temperature exceeds 20 degrees F.
- Raise Suction Pressure: Suction temperature should be no more than 10 to 15 degrees F below the space of product temperature with adequately sized and maintained evaporators.
- Add Evaporator Capacity: The additional evaporator fan energy required to reduce evaporator approach temperature is often less than the compressor savings from operating with higher suction pressure.
- Reset Suction Pressure: Raise suction temperature to the highest possible for particular loads at any time. Drop suction (low-side) pressure/temperature to maintain colder loads. Raising suction pressure decreases compressor work.
- Minimize Back Pressure Regulators (BPR): BPR's control higher temperature loads by increasing pressure to selected system branches above the suction pressure. One result may be lower suction pressure, and therefore system efficiency than necessary. Raise suction pressur to the highes pressure that will meet the requirements of the lowest temperature load. This will eliminate some BPR's and reduce pressure drop through through others, resulting in higher system efficiency.
- Optimize Interstage Pressure: A two-stage system has three separate pressures maintained by high and low-stage compressors. Choose intermediate pressure so that each compressor has approximately the same pressure ratio to minimize compressor energy use.
- Eliminate Back Pressure Regulators: BPR's control various refrigeration loads on a single system. Loads requiring lower suction pressure might operate on a system separate from loads requiring higher suction pressure.
- Cycle Evaporator Fans: Evaporator fans move air across refrigerant coils. Fans can turn off when not needed for temperature control or to destratify air in the cooler. Cycling fans saves fan and compressor energy. A 50% duty cycle is often possible. ASD or 2 speed fans save even more. A timer can cycle fan after the space reaches the set temperature.
- Cycle Condenser Fans: Condenser fans move air across external coils. Cycling fans saves fan energy. Fans can turn off when not needed for temperature control. ASD or 2 speed fans save even more.
- System: Following are typical design features which when implemented can improve compressor performance.
- Install a Cooling Tower: A cooling tower in series with chillers on a chilled water or glycol loop will reduce the water loop temperature before contact with refrigerant. A cooling tower will decrease chiller load when the wet-bulb temperature is more than 1 degrees F below the water loop temperature.
- Install Thermosyphon Oil Cooling: Install a thermosyphon oil cooler to replace liquid injection oil cooling. Oil seals, cools, and lubricates screw compressors. liquid-refrigerant injection cooling uses 5-15% of compressor power to recompress refrigerant. Thermosyphon system cools oil with a heat exchanger that evaporates refrigerant. The refrigerant vapor rises to a condenser where it returns to a liquid state. A separate condenser may use additional energy for a fan and water pump, or the system condenser may be used.
- Increase System Piping Diameter: Small piping diameter requires higher pressure to overcome friction losses.
- Separate Loads by Temperature: Consolidate and re-pipe loads with similar suction requirement. Consider dedicating selected compressor(s) to those loads rather than operate the entire system at lower suction pressure than necessary.
- Economized Single-Stage System: An economized single stage system may allow operation at slightly lower suction temperatures while avoiding the cost of a two-stage system. however, for low(<15 degrees F) temperature operation, 2-stage system efficiency may repay the higher system cost.
- Consider Two-Stage Operation: Single-stage above -15 degrees F. Use two-stage system for lower temperatures.
- Replace with more Efficient System: Are you using old air cooled refrigeration equipment? Refrigerants based on chlorofluorocarbons are being phased out. Take the opportunity to replace your older equipment with more efficient newer equipment using ammonia or the newer refrigerants. Replace air-cooled condensers with evaporative condensers on larger systems.
- Use Condenser Waste Heat: Use waste heat for defrost or preheating cleanup or process water.
For systems with Multiple Compressors
- Sequence Compressors: Overlap suction pressure switches on compressors connected to a common header to sequence operation with the goal of turning off one or more compressors.
Install Electronic Control System
1) Compressor Sequence 2) Condenser 3) Evaporator 4) Pressure Optimization 5) Defrost 6) Trend Logs
- Maintenance: Opportunities to improve compressor performance through maintenance activities.
- Maintain Evaporative Condensers: Water evaporates as it absorbs heat from the condensing refrigerant. The heat exchange surface must be clean and free of corrosion. Air must pass through freely. Treat water to reduce scale, corrosion, and biological growth.
- Purge Non-Condensable Gases: Non-Condensable gases, such as air or CO2, reduce the effective surface area of the condenser that could condense refrigerant vapor, tereby decreasing heat exchanger efficiency. In general, non condensable gases enter the system when the low stage suction pressure is less than atmospheric pressure. Automative purging controls are available. The system can also be purged manually when an operator notices an increase in the discharge pressure. A refrigeration log helps identify operating condition changes.
- Defrost: Evaporator coils must be free of ice to maximize heat transfer. Use hot gas defrost instead of electric defrost. High-pressure refrigerant uses less energy than electric heaters.
- Reduce Defrost Times: Warm fluid (refrigerant or water) commonly is used to defrost evaporators. Heat will warm the space after the ice has melted. Airflow sensors and thermocouples can stop the defrost system as soon as the ice has melted.
- Use Water Defrost Instead of Hot gas Defrost: Warm process water can also defrost the evaporator. No extra compressor electric energy use. This can be more efficient and faster in a blast freezer, for example, where the defrost can occur with water without the product being present.