13 Components used in carbon dioxide refrigeration systems

Carbon dioxide as a refrigerant

Among all the fluids currently used or proposed for use as refrigerants carbon dioxide (CO2) is something of an oddity. Unlike any other substance in the refrigeration and air-conditioning world it is applicable to a range of systems from low temperature freezer plants at -50°C to hot water heating systems at 80°C in sizes from small domestic units up to huge industrial facilities. Also unlike most other refrigerant fluids CO2 combines the twin advantages of being non-flammable and non-toxic with the environmental benefits of having no ozone depletion potential and a global warming potential of 1.

There is no single reason for this universal applicability of CO2; rather it is the result of
several system properties which mark it out from the majority of working fluids. The most
obvious of these is the higher operating pressure of CO2 systems, but to this we can add extremely low viscosity, even at the lowest operating temperatures, and the low critical temperature of CO2.

In the 1950s and 60s CO2 was displaced in new plants by synthetic refrigerants, touted by the chemical industry as so-called safety refrigerants. It wasn’t rediscovered until the early 1990s, when researchers and practitioners realized that CO2 can be an excellent refrigerant for today’s energy and environmental situation. Now it is possible to source a wide range of components for use in refrigerating systems up to 120 bar operating pressure, as demonstrated in this paper. It is also apparent that the high pressure of the CO2 delivers many of the inherent benefits of these systems. The compressors are relatively small and so is the pipework. The gas is very dense, and as a result gives good transport and heat transfer performance. The system efficiency is relatively unaffected by pressure drop, because the saturation temperature profile is so flat. It is fair to say that the high operating pressure of CO2 offers opportunities to create significantly more efficient systems than the current best practice, provided the whole system is designed to capitalise on these advantages.

The viscosity of liquid CO2 is about one-tenth of that of water at 5°C. Unlike brine and glycol solutions this figure remains almost constant at lower temperatures, even down to -50°C. This gives CO2 a significant advantage as a low temperature heat transfer fluid, and if the liquid is allowed to boil in the cooling heat exchanger the benefit in performance is even greater. This property has been exploited in a wide range of applications including food processing equipment, cold storage and IT cooling systems.

The critical temperature of 31°C is also often said to be a disadvantage, but like operating pressure it offers opportunities to do things differently, and in many cases to generate significant advantage over conventional technology. High grade heat recovery is the most common advantage. This is available whether the operator chooses to exploit it or not, and the range of domestic, commercial and light industrial heat pumps already on the market show that this advantage is sufficiently large to justify system installation in its own right, rather than as a supplement to a cooling system. The benefits of this property are even greater when the cooling and heating systems are integrated.

Although CO2 was first used as a refrigerant nearly one hundred and fifty years ago, it has only been intensively studied in the last fifteen years. There is still significant scope for further product and system development. With increased attention paid to the environmental implications of refrigeration system efficiency the possible advantages to be gained from the use of CO2 are as important today as they have ever been.

Components for carbon dioxide refrigeration systems

Compressors (open type reciprocating and screw compressors)

Sub-critical applications
Grasso 5 HP (50 bar design pressure) reciprocating range, 4 different models, 100 to 200
m3/h swept volume, 152 up to 303 kW at -40°C/-5°C
Grasso screw compressors in 22 different types in their HP version (52 bar design pressure),
231 to 8,570 m3/h, 328 up to > 10,000 kW at -40°C/-5°C, all screw compressors are suitable for hot gas defrost in due to its unique design pressure of 52 bar.

Trans-critical applications
Grasso developed an open type screw compressor particularly for the use on higher
pressures which occur in trans-critical CO2 applications and NH3 heat pumps.
Speed range: 2,500 to 6,000 rpm
Swept volume: 60 to 154 m³/h
Capacity: 300 to 770 kW
at t0/tGC-out = 5°C / 45°C / 120 bar
Shaft power: 110 to 360 kW

Compressors (semi-hermetic reciprocating compressors)

Sub-critical applications
With the introduction of the further developed Bitzer Octagon® CO2 compressor, there are
now 17 models available in the range from 2.7 to 81 kW refrigerating capacity (-35°C/-5°C) for sub-critical CO2 cascade and booster applications. This new series of compressors replaces the K-series which has been used successfully all over the world in CO2 projects already since the 1990s. A particularly robust drive version and valve plate design adapted specially to the demands of CO2 permit further increases in efficiency and operating reliability. As well as expanding the product range with closer graduations in capacity, the operating limitations of the compressors have been broadened and harmonized.

Trans-critical applications
Based on the Octagon® compressor series, an extra series has been developed for the
specific requirements of trans-critical CO2 applications. This means that a series is now
available for such systems with the attributes of the proven Octagon® compressors. The
wide operating range of the compressors permits their use in a large number of system
variants. This particular series addresses interested companies involved in corresponding developments with trans-critical CO2 systems. The warranty for such machines is subject to a special agreement, because of the specific requirements applying to the installation and operation of these systems.

Condensers/gas coolers (air-cooled)

Tube and fin condenser, made of stainless steel tubes and aluminium fins, that is used as condenser in subcritical operation mode and as gas cooler in trans-critical operating mode

Condensers/gas coolers (liquid-cooled), inner heat exchangers

Plate heat exchanger made of stainless steel, copper soldered in vacuum. For use as
condenser in sub-critical operating mode, as gas cooler, suction gas heater, economizer and oil cooler in trans-critical operating mode.

Evaporators

Tube and fin evaporators for direct expansion, made of copper or stainless steel tubes
(pump systems with hot gas defrost)

Heat exchangers

High side float regulator

NH3/CO2 cascade systems

Pressure vessels

Pumping stations

Witt refrigeration pumping stations comprise the combination low pressure vessel (surge
drum) with refrigerant pumps interconnecting pipework and liquid level controls. All mounted
on a steel base support frame.

Refrigeration pumps suitable for NH3 and CO2

Witt hermetic refrigerant pumps with radial impellers are suitable for pumping liquid with
entrained vapour. This is of importance for boiling liquids, such as CO2, because during
reduction of the pressure gas bubbles will form. The stator housing is designed to withstand
the high nominal pressure of the pump, giving unrivalled safety. Developments of the
hermetic pump include 40 bar models for CO2, with a further development of 90 bar designs
for high pressure CO2 applications.

Temper-40 heat transfer fluid (used as defrost media in low temperature CO2 refrigeration
installations)

Valves (shut off refrigeration valves, filters, check valves, hand regulating valves and stop
check valves)
The range of alternative CO2 valves is complete from DN4 to DN450 which incorporate drop
forged low temperature steel bodies up to DN65. From DN80 the valves are made from cast
low temperature steel.

Ball valves
The RFF refrigeration ball valves are constructed from low temperature or stainless steel bar
material. They can be supplied with reduced bore DN20 to DN100 or full bore DN10 to
DN80.

Refrigeration purge/drain/gauge valve
These small components are all suitable for use with CO2 and incorporate low temperature
and simple steel. A hot forged one piece design. Stainless steel spindle with serviceable ‘O’
ring seals. The single design pressure rating is 65 bar (PN65) for all small valves DN4 and
DN8.