With more than 14,000 Hillphoenix systems installed worldwide as of 2023, the case for CO₂ as a refrigerant is compelling.

Using CO₂ can lower costs during installation and operation of the systems, and in meeting increasingly stringent safety and regulatory compliance.

CO₂ as refrigerant (R-744) has over the past decade become more and more popular in industrial applications such as cold storage and distribution, food processing and production, pharmaceutical and other types of facilities. To many current and potential users, it may seem to be an alternative to the almost 100-years of reliance upon ammonia, synthetic CFC, HFC and HCFC refrigerants. But before these refrigerants became as widely used as they have been, CO₂ was already in use.

Refrigerant List

CO₂ has long been known to have superior thermo-dynamic properties as an approved A1 refrigerant, and unlike most synthetics such as A3 refrigerants that include propane, or B2L refrigerants that include ammonia, it is non-flammable and non-toxic. But while the higher pressures required for its operation were long seen as a potential barrier to its wider spread acceptance, in some ways that factor actually adds up to an advantage since it allows for higher volumetric capacity which in turn allows for more compact component and system designs. CO₂ refrigeration racks (the metal frames and various components such as compressors, receivers and heat exchangers that make up the typically single largest pieces of equipment apart from condensers) compared to similar capacity HFC racks are generally smaller. Likewise, comparatively less refrigerant is needed for CO₂ systems.

CO₂ (R744) Refrigeration Rack

CO₂ within the past dozen years began being used as a secondary coolant for low-temperature applications. Eventually, new developments in the technology further led to its use as a low-temperature direct expansion (DX) refrigerant in cascade systems. The difference between the two is that whereas CO₂ used in secondary systems, circulates as a chilled fluid that absorbs heat without changing state, in a cascade system it is used as a conventional refrigerant undergoing changes in state as it moves from one heat exchanger to another. What makes the latter approach a cascade system is that a heat exchanger connecting the upper and lower cascade is the condenser for the lower while at the same time also working as an evaporator for the upper. In other words, the upper section cascades its cooling to the lower section. Both of these approaches however, continued to rely on the use of HFCs in the primary side of secondary systems and in the upper section of cascade systems, albeit in far reduced volumes compared to conventional systems.

An All-CO₂ Refrigeration System

Building upon the success of these advances, industrial companies finally began adopting the use of CO₂ as a sole refrigerant in a variety of industrial applications. This approach, using what’s known as a booster system, leverages the advantages of CO₂ to deliver a more efficient sustainable and ultimately less costly approach to refrigeration.

A booster system instead of using an HFC for the primary side of the system and CO₂ for the lower side uses only CO₂ for the entire system, both for medium-temperature and low-temperature loads. It is able to do this by virtue of some significant advances in technology that take advantage of the thermo-dynamic properties of CO₂.

Prior to the development of booster systems, major challenges to the use of CO₂ as a DX refrigerant stood in the way of its acceptance. These obstacles were mostly a consequence of the relatively high-pressures that such a system would be required to operate at. These difficulties were overcome by the development of certain key components now commonly used in booster systems – among them a special type of high-pressure control valve, condenser (gas cooler) and use of high-pressure copper piping.

Apart from these two parts of the system, most other aspects of CO₂ booster systems are familiar to anyone who knows how traditional DX systems work. Like those, booster systems also have the four main components that include compressors, evaporators, expansion valves & condenser (gas cooler). The difference is that these components are rated for higher pressure and that in place of mechanical expansion valves, electronic versions, and case controllers for evaporator temperature control are used.

Another key difference from other types of DX systems is that booster systems work with the same refrigerant moving between the low- and medium-temperature compressors. The low-temperature compressors are subcritical as they discharge to the suction of the medium-temperature compressors which are transcritical as they discharge to the ambient. In other words, the low-temperature compressors serve as a booster to the medium-temperature compressors. This may seem like a similar approach as the cascade system described above but instead of one system with two different refrigerants, a booster system only has one. This point is worth emphasizing: a CO₂ booster system uses only CO₂; there is no other type of refrigerant in the system.

A booster system is also referred to as a transcritical system meaning that its operation transfers from a subcritical to a supercritical state depending on the ambient conditions such as when the outside temperature reaches 88°F, the refrigerant exceeds its critical point. From here on, the ambient air does not allow for all of the heat in the refrigerant.

to be removed until the pressure and temperature drops below that point. The only way to reduce the heat load of the refrigerant is to release as much of it as possible to the outside air, and this is also why under these conditions the condenser is referred to as a gas cooler since it is cooling the gas as opposed to fully condensing it.

Enthalpy – P-H Diagram

In the booster system, low-temperature subcritical compressors operate at around 200 psig, well below the critical point for CO₂ receiving suction gas from the low-temperature display case and freezer evaporators. The low-temperature compressors then discharge that gas at about 425 psig combining it with the medium-temperature suction gas from the medium-temperature display cases and walk-in cooler evaporators before entering the medium-temperature transcritical compressors. Medium-temperature discharge gas leaves the compressors, depending on ambient conditions, anywhere from 560 psig to as much as 1450 psig, which is above the critical point of CO₂ at 1070 psig (at 88°F). Under warmer conditions in which the pressure rises above 1055 psig, the system operates in the transcritical range.

Under any conditions, however, the discharge gas from the medium-temperature compressors feeds to the gas cooler, where the heat it carries is rejected to the outside environment. This heat rejection process is standard in any refrigeration system.

The CO₂ leaving the condenser feeds to a high-pressure control valve that controls the pressure in the gas cooler, similar to a traditional hold back valve that allows a pressure drop to assure some liquid CO2 refrigerant flows into an intermediate pressure receiver called a flash tank (a high-pressure tank similar to a traditional receiver). The gas enters the valve from 560 to 1450 psig, depending on ambient conditions, and regardless of those conditions exits the valve at 540 psig. The valve is designed to work somewhat like a holdback valve in order to maintain optimum pressure through the condenser when it is working as gas cooler for the most efficient operational performance of the system.

The flash tank works the same way, in principle, as a component used on most types of refrigeration systems that’s called a receiver. This is usually a vessel that contains all of the refrigerant charge of the system when needed such as during maintenance or if the system ever has to shut down. The pressure in the tank is held at a constant level that is sufficient to maintain differential pressure throughout the system.

From the flash tank, where both liquid and gas can be present, the liquid refrigerant is supplied to the medium- and low-temperature evaporators controlled by conventional electronic expansion valves. Vapor from the flash tank is fed through a flash gas bypass valve (another specialized component) back to the medium-temperature compressors. The flash gas bypass valve maintains a constant pressure in the flash tank and removes the gas buildup in the flash tank.

Apart from some of the special components just described, the system works similarly to other types of DX systems. The main differences are related to the two-stage design of the system and the fact that all evaporators in the system are supplied with liquid from the same source. For most experienced technicians, the system will not seem overly complicated.