Isotherm, Inc.

Why does Carbon Dioxide (CO2) stand out in a DX Heat Exchanger in low temperature systems?

Since the Kigali Amendment of Montreal Protocol in 2016, the world refrigeration community has been looking more into the natural refrigerant route. The Kigali Amendment calls for the phase out of Hydrofluorocarbons (HFC) as follows:

  • Developed countries: 10% reduction in 2019 and delivering an 85% cut by 2036 (compared to the 2011–2013 baseline)
  • Developing countries two groups
    • Group 1: China and African nations: 10% reduction starting 2029 and 85% cut by 2045.
    • Group 2: India, Iran, Iraq, Pakistan, and the Gulf countries: 10% reduction starting 2032 and 85% cut by 2047.

A prime candidate is Carbon Dioxide (CO2) with an Ozone Depletion Potential (ODP) of zero and Global Warming Potential (GWP) of one.  It is nontoxic and non-flammable. It is readily available at a low cost. The only major drawback is its low critical point and therefore high pressure.

The unique characteristic of this refrigerant is its high vapor density and steep Pressure-Temperature (P-T) curve as shown in Table 1 and Figure 1, respectively.

Table 1: Specific Volume Ratio Comparison [vg/vl]

Temp               R-744              R-717              R134a


-40°F               42.7                 1071.9             511.9

-20°F               27.8                 620.1               297.9

   0°F               18.4                 376.9               182.1


The ratio of specific volume of vapor to liquid is orders of magnitude lower across the temperature range compared to Ammonia and R-134a.

Now let us look at the P-T curves in Figure 1.  It shows that the CO2 curve stands out.  All other four refrigerants have almost a flat trend, especially at temperatures below 0°C.

Why do these two features alone act as positive attributes in a DX evaporator?  A smaller specific volume ratio helps in reducing the potential of flow mal distribution inside the tubes along the flow path in each pass, which means better heat transfer and fewer expansion valve issues. It also helps in pressure drop. With a steep P-T curve, there is less sensitivity between the saturation temperature and pressure. In other words, pressure drop does not impose a radical change in the saturation temperature.  For example, looking at Figure 1 at temperatures below -20°C, a minor change in pressure results in a large change in temperature. This directly affects the performance of the shell and tube heat exchanger or an evaporator coil in the form of drop in Log-Mean-Temperature-Difference (LMTD).

When both the above stated attributes are coupled together, it becomes a nightmare for the designer and the user.  This is one reason that one does not find DX evaporator coils in cold storage facilities operating at lower temperatures.       

Vapor pressure curves for different refrigerants

Fig. 1  Vapor pressure curves for different refrigerants