TRAINING
Using low-GWP refrigerants to heat dwellings
F
By Martin Cook of Business Edge.
rom the Victorian age to the 1960’s CO2 refrigeration /
heat pump systems were widely used. One important industry that employed CO2
was refrigerated storage
aboard ships transporting fresh food halfway around the world. Fluorinated refrigerants entered use in the 1940’s, although more expensive to manufacture they had lower operating pressures and this perceived lower risk meant they were becoming the preferred option. Working knowledge of CO2
was almost lost, but, in the
1990’s interest was re-kindled as the search for alternatives to fluorinated refrigerants grew due to their environmental toxicity. Today we see that a small but growing percentage of systems from refrigeration to heat pumps are using CO2 Training for CO2
. not just the operating pressures of CO2
systems is essential for technicians. It is that differentiates it
from other refrigerants, systems can include extra circuits and
components and there are certain properties that make CO2 behave in a different way to familiar Fluorinated refrigerants. CO2
is given the ASHRAE (American Society for Heating,
Refrigeration and Airconditioning Engineers) notation R744. The most efficient use of R744 within a transcritical cycle is as a refrigerant in a heat pump circuit, and to understand why this is it will be useful to explain what a critical point is. A refrigerant in a vapour compression refrigeration system
will change from a liquid to a vapour in the evaporator and from a vapour to a liquid in the condenser. In the evaporator the change of state describes a process in which the refrigerant collects energy and when in the condenser loses energy. If a refrigerant is at a pressure in which it is above the critical point, when it passes through the condenser, it will not condense. R744 has a critical point at approximately 31°C at a pressure close to 75 bar(g). If the design of the refrigeration system requires heat to be rejected into temperatures that are above this critical point, the refrigerant will not condense. Heat can be rejected but the R744 will not change state (it is in a special condition, supercritical high-density vapour), this is described as gas cooling.
When a R744 heat pump is used to heat a water circuit, the dense supercritical vapour at high temperature allows for higher flow temperatures in the water circuit, when compared to a Fluorinated refrigerant. The higher flow temperature is more compatible for retrofitting and can be used as a more efficient way for water microbial prevention (most systems use an electrical heater to bring the water temperature to a high temperature (above 60°C)). If the difference in the water flow and return temperatures are high, approximately 15- 20K or greater, the R744 transcritical system performs more efficiently.
34 February 2023 •
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