ICR - Advances in vapour compression cycles and equipment design

R744.com reports on research presented at the International Congress of Refrigeration (ICR) on CO₂ vapour compression and equipment design. New research on oil-free systems could improve R744 system performance, whilst presented studies advised on CO₂ heat exchanger design and reviewed CO₂ compressor development.

The keynote presentation at the ICR vapour compression and equipment design technical session held on Tuesday afternoon focused on the key advantages of oil-free CO₂ systems. Two papers were then presented on heat exchanger design, which due to a working pressure 4 times higher (120 bar) and a temperature (150°C) double that of systems using synthetic refrigerants, require considerably different design. The final presentation in the session reviewed the development of compressors designed specially for CO₂.

Oil-free R744 systems for industrial/commercial applications, Hafner A., Neskå P., Ladam Y., Eikevik T.M., SINTEF Energy Research

R744 vapour compression systems rely on having oil in the compressors to provide lubrication of the mechanical moving parts and keep the system running smoothly. The addition of oil requires periodic drainage and replacement, in addition to which it migrates to other parts of the system, affecting the system performance.

This paper looked at the benefits of an oil-free system in: commercial refrigeration, transport refrigeration, onboard-refrigerated sea water (RSW) production, and industrial heat pumping systems, and finds the following benefits:

Heat transfer: oil-free compression systems could improve the heat transfer properties of the refrigerant, in particular for natural refrigerants like CO₂, where the reduction in heat transfer for evaporation and supercritical cooling is well known
System complexity: oil-free systems avoid the need for oil management (recovery) systems, reducing costs and system complexity, which in turn can lead to fewer systems “down” periods
Discharge temperature: the presence of lubricant oil limits compressor discharge temperature. Oil-free systems can work with higher pressure ratios, increasing the operation range of compressor stages, or reducing the number of necessary stages
Evaporating temperature: oil-free R744 systems allow for low temperature refrigeration, below 40°C, and introduce the possibility of production and storage of solid CO₂

Design of heat exchangers for heat recovery in transcritical CO₂ systems, Christensen R., Alfa Laval AB

Heat exchangers with different characteristics are required for transcritical and subcritical operation. The paper presented looked at the different operating conditions and system layouts and their influence on heat exchanger design, and in particular heat exchanger thermal length.

Based on the studies conducted, the author recommends the use of segmented models in the design of heat exchangers, which can be configured differently using the same plate to obtain the desired characteristics. Also recommended is the design of indirect systems for the desuperheaters or gas coolers, which can help avoid scaling problems.

Air cooled heat exchangers for CO₂ refrigeration cycles, Filipini S., Merlo U., LU-VE Group

As mentioned above, CO₂ systems require a significantly different design from the heat exchanger point of view. The research presented focused on having a coil with small diameter tubes, low refrigerant charge and circuiting which can lower the approach temperature. Results showed that reductions in airflow and of the coil front area brought about lower fan consumption, smaller size and some production cost savings. Also, the use of a water spray system was shown to help improve general efficiency at high ambient temperatures.

Development of a transcritical CO₂ compressor series for medium to large refrigerating capacities, Fröschle M, Gea Bock GmbH

The final presentation of the session provided an overview of the development of a new range of transcritical compressors designed for CO₂, for maximum pressures of up to 150 bar. Key factors in the design of the compressors were as follows:

Casing: cast iron graphite casting (GIS) casing was used
Drive: slim, drop-forged, split steel connecting rods with bearing steel and bearing brush were developed for the 4-6 cylinder compressors
Oil management: oils C55E and C85E, with anti-wear additives were especially developed for the use of CO₂
Limits of application: the designed compressors allow for a maximum overpressure of 150 bar at the high pressure side (HP) and 100 bar on the low pressure side (LP)

The compressors, designed by Bock, are available in 2, 4 and 6 cylinder versions for medium to large refrigeration capacities, with a displacement volume of 6,2 – 30,2 m³/h.