Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten. We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in industry is not in place.
Cayla asks: Grant could you please look into the use of R744 and how do these systems work. It is quite difficult to understand the operation of these systems. Also a particular concern is the very high pressures. Thank you.
Hi Cayla, yes I can help. In the last RACA issue we looked into CO2 as a refrigerant as well as the critical and triple point. We began looking at subcritical and transcritical systems and what is meant by these terms. Let us continue on this subject by looking at types of systems.
There are many configurations but I will try to look at the more common systems
Subcritical systems – Cascade
A subcritical cascade is a two-stage system with two refrigerants. The low-pressure or first stage is filled with R744 and the high-pressure second stage is filled with another second refrigerant, e.g. propane, ammonia, HFC or HFO.
Both stages are coupled with a heat exchanger. On the respective systems we find on the first stage subcritical CO2 condenser and on the second stage an evaporator
The subcritical CO2 cycle is similar to other refrigerant cycles, except a situation which occurs when the system cycles. The CO2 must be continuously subcooled and to achieve this a small plant with a secondary refrigerant is used.
If there is no off cycle, cooling the low-pressure stage requires a pressure resistance up to 90 bar (9,000 kPa).
In a cascade the first stage with the lowest evaporation pressure the vapour of the refrigerant condenses in a heat exchanger, which is the evaporator of the next stage. With the refrigerant R744, normally only two stages are built. In the second stage, the refrigerant evaporates 3 or 4K below the condensing temperature of the first stage, For example if the condensing temperature of R744 equals 20°C then the evaporating temperature of R290 equals 16 °C.
This system is used in supermarkets and cold rooms. Today, the second stage is often charged with a HFC, R717 (NH3) or hydrocarbons (e.g. R290).
Flooded systems
In flooded systems the refrigerant does not evaporate completely. Generally, the vapour quantity is between 25% and 50%. This mixture can flow in the piping. There are two kinds of flow generation: liquid pump or gravity. Both systems need a receiver, which is at the same time a separator of the vapour. There is no superheating in the evaporator.
Transcritical Systems
The transcritical system operates at higher pressure and some designs are as follows
Simple single stage
This is a simple system. There is only one regulation valve -Q2, the high-pressure valve. This regulation valve ensures the optimal high pressure in the gas cooler -E2. Before this valve only supercritical gas exists in the high-pressure side of the system.
The CO2 is expanded in this valve, therefore, behind this valve subcooled liquid and flash gas exists. The liquid absorbs the heat input of the evaporator -E1 and evaporates. The gas is suctioned and compressed from the transcritical compressor -T.
The low-pressure receiver is placed in the suction line.
This system is suitable for small plants and for training units.
Single stage with internal heat exchanger
This is another simple system There is only the thermostatic expansion valve -Q1 to regulate the mass flow. Before the expansion valve only supercritical gas exists in the high-pressure side of the system.
For higher efficiency the supercritical gas is subcooled in an internal heat exchanger (IHX) -E3. The suction gas absorbs the heat from the supercritical gas and is more superheated.
The subcooled CO2 is expanded in the thermostatic expansion valve, therefore behind this valve subcooled liquid and flash gas exists. The liquid absorbs the heat input of the evaporator -E1 and evaporates. The gas is suctioned and compressed from the transcritical compressor -T.
The low-pressure receiver is placed again in the suction line.
Single stage with gas bypass
This system is a one stage compression system but a two-stage expansion system. The suction flow coming from the evaporator is compressed and gives off the heat in the gas cooler. The subcooled transcritical gas expands in the high-pressure regulation valve. The gas is transferred in a two-phase flow. The liquid is collected in the receiver. The flash gas expands in the flash gas regulation valve into the suction line. The flash gas valve ensures the right intermediate pressure in the receiver.
The saturated liquid expands in the thermostatic expansion valve.
Vaporising liquid absorbs heat in the evaporator
Single stage with parallel compression
In the CO2 two stage systems both – the compression and the expansion – works in two stages. The first stage of expansion of the supercritical gas takes place in the high-pressure valve. The supercritical gas liquifies and gives off heat to the flash gas. The liquid collects in the receiver. The pressurised receiver has intermediate pressure. The second stage of expansion takes place in the thermostatic expansion valve. The evaporated refrigerant is compressed in the low-pressure (LP) stage to medium pressure (MP) and in the high-pressure stage to the supercritical pressure of the gas cooler (HP).
This system is similar to the gas bypass system. This system is applied also in supermarkets and other applications with continuous cooling capacity.
Two stage booster system
In the CO2 two stage systems both the compression and the expansion works in two steps. The first stage of expansion of the supercritical gas takes place in the high-pressure valve. The supercritical gas liquifies and gives off heat to the flash gas. The liquid collects in the receiver. The pressurised receiver has intermediate pressure. The second stage of expansion takes place in the thermostatic expansion valve. The evaporated refrigerant is compressed in the low-pressure (LP) stage to medium pressure (MP). The discharge flow of the low-pressure compressor is mixed with the suction flow of the intermediate pressure evaporators. The total mass flow is compressed in the high-pressure stage to the supercritical pressure of the gas cooler (HP).
Small ejector system
The intermediate pressure receiver works as separator of the liquid and saturated vapour. The compressor suctions the saturated vapour out of the receiver/separator. On the way to the compressor the cold vapour is used for subcooling the supercritical gas in an internal heat exchanger. The compressed transcritical gas rejects heat in the gas cooler.
The supercritical gas expands when it enters the nozzle of the ejector. While crossing the saturated liquid line the fluid is transferred into two-phase flow. The suction flow coming from the evaporator is mixed with the two-phase flow. The kinetic energy is transferred back in pressure energy. Thus, the two-phase flow is pre-compressed before entering the separator.
Small ejector system
For very high efficiency at warmer ambient temperatures, the low-pressure compressor is replaced by an ejector. The ejector uses the compression work of the compressor. This compression work is shown in the very high pressure in the gas cooler. The supercritical fluid expands in the motive nozzle of the ejector. The vapour of the low temperature evaporator flows into the suction chamber. The total flow mixes in the mixing chamber. In the diffusor the velocity decreases and the pressure increases up to the intermediate pressure. The flash gas valve ensures the maximum intermediate pressure for operation.
Efficiency of CO2 refrigeration
Compared to traditional HFC direct expansion systems, the CO2 transcritical system is more efficient at low ambient temperatures, but not yet at high ambient temperatures. The map visualises this “border” with higher efficiencies for CO2 systems and indicates the possible savings.
However as technology improves the equator is disappearing
Cayla hopefully this helps with your understanding of CO2 systems, components and operation. We will look at servicing procedures in the next issue.
Grant Laidlaw
REFERENCES:
ACRA
The Deutsche Gesellschaft für Internationale Zusammenarbeit GmbH,
ASHRAE
Carrier
A-Gas