34
Cover Story
Specific Energy Consumption of Industrial Natural Refrigerant Systems Type of System
SEC (kWh/m3*a)
Refrigerated Space (m3)
NewTon NH3/CO2 secondary
351
44,000
Low-Charge NH3 DX
20-242
44,000
2-stage pumped NH3
24-872
27,000-215,000
Transcritical CO2
13.73
135,213
every 1.2 part of liquid ammonia, one part evaporates), compared to the 3-1 ratio at a traditional Henningsen facility.
Source: 1. Mayekawa 2. Scantec Refrigeration Technologies 3. Henningsen Cold Storage, which measures the SEC of its entire facility, not just the refrigerated space
Base
Another technique Henningsen employs to cut ammonia charge is to use what Lepschat calls a “pump-down system” in lieu of a high-pressure receiver. The system features a small vessel that maintains a low-level of ammonia and is continuously feeding the gas to a low-pressure receiver via a subcooler. In its second low-charge warehouse in Salem, built in 2017, Henningsen was able to reduce SEC to 0.288kWh/ ft³*a (10.3kWh/m³*a), the lowest in the company, and one of the lowest in the cold-storage industry. This became the standard for all future facilities, including the first CO 2 warehouse in Grandview. (H e n nin gse n c o nve r te d anoth e r warehouse, in Scranton, Pennsylvania (U.S.), to transcritical CO 2 in 2018.)
Accelerate Magazine // March 2020
The installed cost of the Grandview CO 2 system was $534,000 less than the Salem ammonia system; between $200,000 and $400,000 was saved by not building a machine room, which also cut 5-6 weeks in installation time. The reliability of the CO² system and low-charge-ammonia standard has been the same. But on an energy basis, the Grandview CO 2 system, which uses an adiabatic gas cooler, had much higher energy costs than the standard facility’s system, though that was before several energy-efficiency measures were added. Those included: dock dehumidification, evaporator/condenser fan control, hot-gas defrost, gas cooler optimization, compressor VFDs and a glycol pump VFD. In the subsequent commissioning of the system, the low-temperature suction setpoint was changed from -25°F (-32°C) to -16°F (-27°C). In addition, evaporator and gas cooler fan speeds were changed from fixed to modulating.