CO2/NH3 Cascade Industrial Refrigeration Systems


While striving to support cost effective( both initial & operating) and energy efficient ecofriendly green refrigeration systems, it is time we adopt the attitude of Can-Do credo by switching from traditional two-stage compound ammonia refrigeration system to hyper cascade system. We may all want to keep track and embrace this system which has already attained popularity in North America and widely accepted in EU countries. Such systems are considered the natural refrigerant system for large scale (high product tonnage) refrigeration projects and process refrigeration applications in the industrial refrigeration community. Cascade refrigeration systems are recommended for use where storage temp requirements are perhaps -4F (-20C) where many chilling/freezing rooms as well as blast freezing applications are incorporated.

A. Safeguarding Perishable Foodstuff

Problem of storage of food items are not simple, for a whole series of biological factors prior to application of chosen techniques, which may influence occurring losses during storage procedures and distribution.

Besides, the fundamental problem of feeding the growing population of the world (presently stands at 7 Billion) cannot be solved unless the efforts to increase production of food items with improved logistics accompanied by efforts tending to reduce, if not eliminate, the losses which otherwise are considerable during logistics and/ or subsequent processing.

B. Natural Refrigerants (R744 & R717)

Compared with other refrigerants commonly used in refrigeration industry, the non-corrosive carbon dioxide (CO2) has a very low critical temperature of 31.05 C (73.8 ba), so that heat discharge into the ambient atmosphere above this temperature is impossible through condensation as is the case in unusual vapor compression cycle; CO2 can only be used in this cycle when the heat discharge temperature is lower than the critical temperature. For heat rejection at supercritical pressure, the refrigerant can only be cooled in a gaseous state without being condensed. This cycle is known as “Transcritcal cycle”. In reality CO2 was used as a refrigerant in the mid-19th Century, reaching its peak in the 1920s, declining when CFC refrigerant were introduced in the market.

Key advantages of CO2:

  • Low risk hazard for our planet earth
  • Ozone depletion Potential (ODP) is Zero
  • Global Warming Potential (GWP) is 1
  • Adhere to Protocol of Montreal and Kyoto
  • Superior thermodynamics properties
  • High volumetric refrigerating capacity
  • Approx. 30% cheaper than ammonia
  • Maintains texture and nutritious value of product to be frozen by minimize the freezing time. Also, it is nonflammable, nontoxic, non-corrosive, odorless, hence can be used in direct contact with food products.

Key advantage of NH3:

  • Adhere to Protocol of Montreal and Kyoto
  • Unique eco status at ODP= 0 and GWP=0
  • Adhere to Protocol of Montreal and Kyoto
  • Superior thermodynamics properties
  • Toxic but high alarm warning effect
  • Available worldwide and reasonably priced
  • Commonly applied in industrial refrigeration for over 100 years

C. CO2 Cascade vs. 2-Stage (traditional) NH3 Compound System

The cascade system operate with CO2 on the low temperature side, and ammonia on the high temperature side. In this manner ammonia charge is confined to the plant room. A heat exchanger where the condensation heat of CO2 is removed with evaporation NH3 connects both stages of the cascade. This CO2/NH3 cascade refrigerating system was first introduced by the giant Nestle’ food conglomerate who, per their business policy of promoting natural refrigerants on their refrigeration systems, during 2001 built the largest system then (the present largest one built in 2017 by Yosemite Meat Co) incorporating industrial refrigeration cascade systems. Before proceeding to further analyze the cascade system, basic features of the two types of Refrigerating Systems are herein discussed:

D. Types of Refrigerating Systems:

A Refrigerating system of more than one stage of compression is defined as a multi stage system. The first stage compressor often termed as “Booster”, the two specific types of which are compound and cascade. The Compound system may further be divided into:

1A) The Booster System: Consisting of separate boosting (low stage) and high stage compressors.

1B) Internally compounded system: When both stage of compression are handled by a two stage internally compounded (partitioned) Compressor.

2) The Cascade System: Where one refrigerant is used as cooling media to condense the other refrigerant.

Typical simple system with flash type inter-cooled P-H (Pressure Enthalpy) Diagram as shown in Figure 1.

Figure 1: P-H diagram for 2-Stage NH3 systems with flash

In a cascade refrigeration system, there can be more than one refrigerant depending on the application or requirement of the plant. In such a cascade system, each refrigerant circuit is separate. For every application CO2 used as a refrigerant for low temperature circuit and ammonia used for high temperature circuit.

Figure 2: Typical Traditional  Hi/Low Stage NH3 Flow Diagram

For a traditional two stage ammonia refrigeration system with overfeed recirculation feed, refer to Figure 2.

The Condenser of CO2 circuit act as the evaporator of NH3 circuit (generally known as cascade condenser). Thus, there will be inter-stage cooler. For better understanding refer to figure 3 which indicates a schematic arrangement (flow diagram) for CO2/NH3 cascade system:

Figure 3: Schematic arrangement for CO2/NH3 cascade system

Now for an analysis comparison of the refrigeration system, as a Case study already performed in California for a modelled pork processing facility, where after 3 months of monitored operation, its basic design data were conducted as follows:

  • Process Temperature: – 40 ‘C
  • Ambient Temperature: +39C
  • Evaporating Temperature: – 45 ‘C (for Low-stage of low temperature cycle)
  • Condensing Temperature: + 40 ‘C
  • Plant Cooling demand required: 200 TR (704 kWR
Figure 4; P-H Diagram for CO2/NH3 cascade system

To efficiently operate a system such as this case study , key elements of consideration were use of electronic expansion valves for each evaporator, distributed controls allowing plant operators visual indication of system operation features (refrigeration, defrost, alarm, cleaning etc.) and distributed smart controls which enabled zone like control strategies for temperature monitoring, variable speed evaporator fan controls, leak detection, hot gas defrost and refrigeration control, as well as electronic valve control. Remote defrost panels were located near the evaporator loads, minimizing wiring requirements for high and low voltage needs.

For the CO2/NH3 cascade system, the low temperature CO2 compressor operated at -45 ‘C suction and -5 ‘C saturated condensing temperature, while the high temperature ammonia compressor operated at -10 ‘C evaporation and +4 ‘C saturated condensing temperature. However for the 2 stage traditional compound NH3 refrigeration system, the low stage compressor operated at -45 ‘C SST and -10 ‘C saturated condensing temperature, while the high stage compressor operates at -10 ’C evaporating temperature and +35 ‘C saturated condensing temperature.

E. Advantages & Disadvantages :

Based on the above operating temperature, the following benefits were achieved by the CO2/NH3 Cascade system as compared to the traditional 2-Stage Ammonia Compound system:

1) Compressor Size (or compressor swept volume) required on the CO2 low stage circuit was approximately 97% less at a saturated vapor of -45 ‘C, since at low temperature, CO2 has far lower vapor volume as compared to ammonia.

2) On similar operating conditions, the COP for CO2 low stage is far higher as compared to that of ammonia.

3) The compression ratio (CR) is lower on CO2 low stage circuit, approximately 45% less as compared to that of ammonia low stage.

4) A purge unit to remove non-condensable are not required on the low stage CO2 systems, because its compressor suction is higher than atmospheric pressure.

5) The liquid separator vessels on CO2 systems are much smaller, because of its lower vapor volume flow rate as compared to that of ammonia when operating at similar capacity and temperature conditions.

6) The suction vapor volume on CO2, are far lower than that on ammonia circuit, hence substantial initial cost saving achieved both on pipe sizes and its relevant insulation thickness.

7) With smaller size suction and discharge line and with smaller size liquid separator, the overall refrigerant charge on CO2 cascade system are approximately 65% less, as compared to that of traditional 2-stage compound ammonia system. Hence often lower overall investment on the plant is achieved with CO2/NH3 cascade industrial refrigeration system.

8) For similar capacity and operating conditions, CO2 compressors are smaller and less quantity of compressors required, as compared to NH3 compressors, therefore in spite of approx.80% more expensive food grade synthetic lube oil

called for by CO2 compressors, yet no price impact is encountered.

9)While the CO2/NH3 cascade system minimizes ammonia charge, integrated with modern technology compressors and efficient heat exchangers, it also provides favorable price performance ratio with respect to both capital cost and operating cost.

Furthermore the CO2/NH3 cascade refrigerating systems as compared to the traditional 2-stage ammonia systems has few limitations such as:

1) For carbon dioxide the saturated pressure is much higher (more than 75 bar) when liquid refrigerant is warmed to ambient temperature (say 40 ‘C). This condition would require that all components in low temperature circuit be suitable for such high pressure, which is not viable economically.

2) In case of liquid on overfeed refrigeration systems, the CO2 liquid pump capacity requires 2.5 to 3.5 times higher NH3 pump for similar operating parameters. Thus liquid lined sizes for such a pump suction and discharge will be higher compared to ammonia.

3)The CO2 pressure vessel and heat exchanger design pressure is higher compared to the booster ammonia pressure vessel and heat exchangers.

4)The CO2 refrigerant should not be charged upon completion of evacuation when plant is under vacuum. This is because dry ice will form internally at the charging port when the system pressure is less than -5.5 bar. Therefore, it is necessary to increase the pressure level to over 5.5 bar before charging liquid CO2.

Conclusion: Inspite of above limitations we observe that the use of CO2/NH3 cascade systems are preferred over the traditional 2 stage compound ammonia systems. From point of view of industry safety due to large amount of ammonia charge , ammonia toxicity and when concentrations of ammonia vapor in the air exceeds the explosion limits, safety requirements of OSHA’s PSM & EPA’s RMP regulations should strictly be enforced.

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