Cold Energy Management: Refrigerate and reduce
Carbon dioxide refrigeration technology can reduce cost, energy in the right applications.
Want to reduce refrigeration system costs and energy at the same time? Cold food plant and warehouse operators should consider cascade carbon dioxide and ammonia (cascade CO2/NH3) refrigerating systems, which are becoming more popular in North America.
Is your facility the right candidate for this new type of technology? Before you can answer that question, you have to address a few more.
Here’s a starting place:
• What temperatures do you need to maintain?
• Are there product freezing requirements?
• Is it critical for your facility to maintain a reduced ammonia system charge?
Of course, cost always is an important factor in any project. If your project does not require storage temperatures below 0°F to -4°F (-17.8°C to -20°C) and there is no blast freezing and very little room freezing requirement, the most cost effective and efficient refrigerating option would be a single-stage economized ammonia refrigerating system. However, if your storage temperature requirements are below -4°F and there is considerable room and or blast freezing requirements, you should carefully consider a cascade CO2/NH3 refrigerating system’s benefits.
Depending upon the size of the refrigerated facility, the installed cost of a cascade CO2/NH3 refrigerating system often is less than an equivalent two-stage ammonia system. The primary factors contributing to the lower cost are: smaller low-stage compressors; smaller low-temperature suction piping, valves; liquid separating vessels; less piping and vessel insulation and lower refrigerant (CO2) costs.
When lower temperatures than mentioned above are required, the operating cost of a cascade CO2/NH3 refrigerating system becomes very attractive.
A third-party energy company recently studied results of a cascade CO2/NH3 system modeled against an equivalent two-stage ammonia system. The project included field installed instrumentation and data tracking equipment to monitor and evaluate the real-time performance of the refrigeration plant. The study included refrigerated storage spaces and ultra-low temperature blast freezers with ability to operate at temperatures lower than those commonly obtained with conventional ammonia systems.
The cascade CO2/NH3 system operates at the following nominal process temperatures (i.e., saturated suction temperatures): -58°F CO2 blast freezing; -20°F CO2 freezer storage; +20°F CO2 coolers and docks; and, +20°F NH3 high stage.
The conventional ammonia system was modeled for the following temperatures: -58°F NH3 blast freezing; -20°F NH3 freezer storage; and, +20°F NH3 high stage, coolers and docks.
The efficiency of the cascade CO2/NH3 refrigerating system was measured in kilowatts per ton of refrigeration load (kW/TR). After five months of monitored operation, these were the results of the cascade CO2/NH3 refrigerating system efficiency – compared to the efficiency calculated for the conventional two-stage ammonia system.
• The cascade CO2/NH3 combined (-58°F and -20°F suction groups) efficiency shows a 28.7 percent improvement compared to the conventional ammonia system.
• The cascade CO2/NH3 overall system efficiency is reduced somewhat as a result of a lower high-stage suction temperature compared with the conventional ammonia system; resulting from the 9°F temperature difference required by the cascade heat exchangers between the CO2 compressors +20°F saturated discharge temperature and the +11°F ammonia high stage saturated suction temperature.
• Moreover, the cascade CO2/NH3 system showed a 5.8 percent overall efficiency improvement over the conventional ammonia system.
Other costs, concerns
I’ve spoken with many refrigerating engineers about cascade CO2/NH3 system operations and maintenance. Most agree that there is little difference between maintaining a CO2 system and an ammonia system.
When it comes to reciprocating compressors, some engineers believe maintenance costs are higher than costs associated with a rotary screw compressor. Furthermore, when comparing the performance of a high pressure reciprocating compressor versus the high-pressure rotary screw compressor, the efficiency (BHP/TR) of the reciprocating compressor is considerably better. Consequently, if increased reciprocating compressor costs do occur the savings achieved by the lower BHP/TR would be offset, resulting in an overall lower operating cost.
Municipal and government agencies often require refrigerated facility to reduce the ammonia refrigerant charge. A cascade CO2/NH3 system usually will reduce the required amount of ammonia by a factor of 10. Since all of the ammonia is contained within the engine room and the condenser on the roof, there is no contaminating refrigerant in the storage or processing areas. This eliminates the risk of damaged product and possible injury to employees in the unlikely event of a refrigerant leak.
Last but not least, it’s important to remember that ammonia and carbon dioxide are both natural refrigerants. They do not harm the ozone layer and do not contribute to global warming.
NEW PROJECT PROFILE: ENERGY SAVINGS AT TESTA PRODUCE INC.
A broadline distributor of fresh, frozen and canned fruits and vegetables, Testa Produce Inc., is pursuing LEED Platinum certification for its newly constructed 91,300-square-foot distribution center in Chicago.
The facility includes separate temperature-controlled refrigeration zones ranging from 0