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Refrigerant CO2 in the ready meal-production

For the production and storage of ready meals for air passenger catering, the Frankenberg GmbH, domiciled in the German city of Würselen near Aachen, needed a new building for production, storage and administration. The refrigeration and air conditioning equipment for the new facilities was selected in close cooperation with the company York Deutschland GmbH. Together, they developed a refrigeration concept in which CO2 is used as refrigerant for production frosting and deep-freeze storage and R507 is used in the area of production cooling.

In order to develop an economically reasonable and at the same time environmentally friendly equipment for the new facilities of the Frankenberg GmbH, the company firstly conducted conversations with different refrigeration plant engineers and discussed different solutions based on the following consumption scenarios: To find a long-term reliable solution, the company developed a concept comparison between the different refrigeration concepts in cooperation with YORK Deutschland GmbH. Apart from the technical requirements, there were other conditions to fulfill in addition to the target of an exceptionally cost-effective investment. Thus, the company wanted:

  • maximum waste heat utilisation / heat recovery and good overall COP-value
  • production frosters should have pump circuits, refrigerant NH3 or CO2
  • no NH3 coolers in processing rooms / workplace regulation
  • no refrigerating system that is subject to authorization in accordance with BImSchG (federal law for the protection against emissions)
  • cost-effective investment

Comparison of system concepts
During a joint project cooperation of three to four months, the overall concept was examined from different angles. This approach was mandatory, as the refrigerating system plays an important role in the scope of the necessary investment (approx. 15 % of total investment) as well as in the current operating costs; besides, it was intended for a period of use of 20 to 30 years. Three different refrigeration concepts were compared in detail:

  • NH3 system, two-stage, with brine system for processing
  • NH3/CO2 cascade refrigerating system with brine system for processing
  • R507/CO2 cascade refrigerating system with R507 direct evaporation in processing

Moreover, in the scope of these concepts, the use of reciprocating compressor aggregates and screw compressor aggregates respectively were examined and compared with regard to investment requirements and maintenance costs as well as sound emissions.

Decision pro cascade refrigerating system
Finally, it was decided to use an R507/CO2 refrigerating system, as it is an economically as well as ecologically wise solution. CO2 is a common and established refrigerant and can be used in refrigerating systems in many areas of food refrigeration and deep freeze storage. The cascade refrigerating system with SABROE reciprocating compressor aggregates includes the following main components:

  • 3 CO2 reciprocating compressor aggregates
  • 2 CO2 condensers
  • 1 CO2 receiver
  • 1 CO2 pump separator
  • 3 R507 reciprocating compressor aggregates
  • 1 R507 separator
  • 1 R507 HD receiver
  • 1 evaporative condenser
  • 1 R507 plate heat exchanger for condensation
  • 1 horizontal R507 pressure gas desuperheater
  • 1 air-cooled R507 condenser
  • 1 R507 heat exchanger for brine cooling of continuous pass-through cooking machine
  • 3 froster valve stations for CO2 pump operation
  • 13 deep freeze storage coolers CO2-DX-operation
  • 22 room coolers R507-DX-operation•1 air conditioning water chiller plate heat exchanger

Equipment with many advantages
The decision to use this equipment was based on numerous advantages:

  • good partial load behaviour due to use of reciprocating compressor
  • high total COP-value: 1.42 (incl. pumps/fans and auxiliary drives)
  • in comparison to NH3 two-stage or NH3/CO2 cascade system, more favorable investment by about 100 - 150 thousand
  • no NH3 components in processing areas / production
  • high froster efficiency due to CO2 pump operation
  • reduced piping due to DX-R507 operation
  • BImSchG approval not necessary
  • good sound situation

In addition, this solution made it possible to reconstruct without problems an existing Samifi production froster for CO2 pump operation.

Successful operation
After a construction period of 5 months, the new Frankenberg plant has been in continuous operation since the beginning of March, and during the summer, it has already proven itself at exterior temperatures of +35 °C.

  • cold water for air conditioning: +12 °C/+6°C        80 kW
  • brine cooling forcooking facilities: ±0 °C/-5 °C    280 kW

freezing facilities (CO2)

  • spiral freezer I: -40 °C   410 kW
  • spiral freezer II: -40 °C  200 kW
  • spiral freezer III: -40 °C 190 kW
  • frozen foodstore I room temperature: -25 °C       90 kW
  • frozen foodstore II room temperature: -25 °C    128 kW
  • frozen foodstore III room temperature: -18 °C       5 kW
  • temporarystorage I room temperature: -18 °C     17 kW
  • temporarystorage II room temperature: -18 °C    40 kW
  • food freezer (CO2):                                                35 kW

room cooling (R507):

  • loadingramp Iroom temperature: +12 °C  36 kW
  • loadingramp IIroom temperature: +5 °C   24 kW
  • cold room I room temperature: +5 °C         7 kW
  • cold room II room temperature: +5 °C      13 kW
  • temporary storage I-VII room temperature: +5 °C  je 15 kW
  • garbage room temperature: +5 °C    15 kW
  • hot meal filling room temperature: +18 °C  90 kW
  • blast chiller room temperature: +2 °C   2 x 40 kW


  • Installierte Verdichterleistung HD (to = -8 °C) 1.500 kW
  • Installierte Verdichterleistung ND (to =- 40 °C) 1.000 kW

Wärmerückgewinnung aus

  • Druckgas HD 100 kW/+48 °C Warmwasser
  • Frischlufterwärmung/Kondensation 550 kW/+25 °C Zuluft
  • Frischwassererwärmung/Kondensation 245 kW/+22 °C Frischwasser

Frankenberg: "Warm meals for specific needs"
The Frankenberg GmbH, domiciled in the German city of Würselen near Aachen, specialises in developing, producing and distributing ready meals for air passenger catering. These "hot meal" and "ready meal" products depend on supreme quality and availability. As a business partner of many international airlines, particularly the smooth and flexible production is an important cornerstone of the company's success.

Refrigerant CO2
Carbon dioxide has been used as a refrigerant since 1850 and has a long tradition in refrigeration engineering. Considering the process to abandon the production and use of CFCs and the associated mounting relevance of so-called natural refrigerants, the economic advantages of using CO2 in refrigerating plants are being rediscovered, too. Some important characteristics are:

  • low greenhouse warming potential (GWP = 1)
  • nontoxic at low concentrations – ambient air = 330 ppm

    • limit of comfort = 1.000 - 1.500 ppm
    • MAC = 5.000 ppm (0,5 %, equals 9.000 mg/m3)
    • exhalation = 3-4 % vol.
    • IDHL = 40.000 ppm (4,0 % vol.)
    • benumbing effect at concentrations above 10 % vol. in the breathing air
    • instantaneously lethal at concentrations above 30 % vol.

  • non-flammable (used as fire extinguishing agent)
  • heavier than air

Refrigerating system

  • high volume flow-related refrigerating capacity (5 to 7 times higher than ammonia)

    • low compressor cubic capacity + piping cross sections

  • high refrigeration figure of merit at low temperatures
  • low viscosity

    • low pressure losses

  • pressure losses entail only small drops in temperature
  • high operating pressures (e.g. 40 bar bei +5 °C)
  • low pressure relations

    • high process efficiency of the compressors

  • high overall heat transfer rates during evaporation and condensation (up to 60 % above that of HFC)
  • good compatibility with common materials and machine oils for refrigerating systems
  • triple point at 5.18 bar and -56.6 °C
  • low critical temperature 31.05 °C
  • max. condensation temperature 10 - 15 °C in 50 bar-systems

History of CO2 as refrigerant

  • 1866/69 application by LOWE (dry ice)
  • 1877 condensation by RAYDT (begin of use as technical gas)
  • 1881/90 Design and construction of refrigerating systems by LINDE and WINDHAUSEN respectively
  • 1887-90 strong development in refrigerating systems, especially in Great Britain and in the U.S.A.
  • 1895 First vapor chart: MOLLIER
  • 1904 first pressure-enthalpy-diagram by MOLLIER (TH Dresden)
  • 1929 detailed p,h-diagram with solid phase: PLANK, KUPRIANOFF
  • 1932 NH3-CO2 cascade systems for temperatures of down to -50 °C
  • 1950 60 % of all refrigerating systems used on vessels and 10 % of all fractional and multiple horsepower refrigerating plants
  • 1960-65  CO2 is no longer used (‡ 0 %)
  • 1989-93  rediscovery and first suggestions regarding plant design (LORENTZEN et al.)

CO2 – a bonus for the environment
Carbon dioxide is primarily used in the beverage and food industry, for cooling during transportation, in water management, in metal processing and for fire fighting.



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