Carbon Dioxide (CO2) DX Shell and Tube Heat Exchangers for Ice Rink Applications
Carbon Dioxide (CO2) has 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.
Due to its promising transport properties, particularly lower ratio of vapor to liquid specific volume and steep Pressure-Temperature (P-T) curve as shown in Table 1 and Figure 1, respectively, it stands out as a viable choice for DX applications.
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
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. The steep P-T curve indicates that 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 between the inlet and the outlet. For example, looking at Figure 1, a large change in pressure drop would not result in a large change in temperature. This quality helps in dampening the effect on the Log-Mean-Temperature-Difference (LMTD). With this said it is a leading candidate for the Ice Rink application. With proper circuiting the evaporator can be designed to work between 10-100% capacity. This attribute helps the owners to operate various size rinks at the same site without having multiple dedicated evaporators.