ASHRAE Journal - February 2009 - (Page 22) • Low-temperature carbon dioxide secondary coolant system with single suction group; and • Low-temperature carbon dioxide direct expansion cascade system with two suction groups. Basic piping diagrams for the three systems are shown in Figure 3. In the DX baseline system and the CO2 secondary system, the HFC component is a low-temperature system, which can be subcooled by another medium-temperature system installed nearby. Subcooling of low-temperature systems using a system operating at higher saturated suction temperature is an important energy saving feature that is considered in the following analysis. Circuiting and line sizing for the low-temperature HFC DX baseline system was based on the existing circuited system configuration that is currently manufactured. Line sizes for the various CO2 systems were selected specifically for this analysis based on current practice, which dictates a loop-type of installation for both systems as control valves are located at the individual evaporator coils rather than having common valving for multiple coils (evaporator pressure regulators) as is the case with most HFC systems. It was assumed that all lines would be copper piping, of the thickness required for the specific applications. In several instances, the CO2 piping required Type-K copper pipe. All lines were assumed to run inside the building envelope and in air-conditioned space with a constant ambient temperature of 24°C (75°F). With system configurations established, the analysis for each system type was carried out. The bar graph in Figure 5 shows the results of the analysis of distribution piping heat gain for the three different low-temperature systems. The analysis indicates that although the line temperatures are lower for the CO2 secondary coolant system, the HFC direct expansion system has significantly higher heat gain due to the larger pipe diameters and installed lengths of copper piping. The smaller lines and somewhat warmer pipe temperatures in the CO2 cascade system lead to further reductions in heat gain. In addition to heat gain, it is interesting to look at how the different system types compare with each other in terms of installed feet and weight of copper. This gives some indication of potential differences in installation cost between the various system types. Table 1 summarizes the installed copper length and weight of copper pipe for each of the system 22 ASHRAE Journal Low-Temperature Loads Medium-Temperature Loads Refrigeration System Figure 4: Representative store layout. Low-Temperature Heat Gain, % of Total Low-Temperature Load 16% 14% 12% 12.29% 10% 8% 6% 4% 2% 0% 6.06% Insulation: Low-Temperature HFC DX: 0.75 in. Supply, 1.0 in. Return Low-Temperature CO2 SC: 1.0 in. Supply, 1.0 in. Return Low-Temperature CO2 DX: 0.75 in. Supply, 1.0 in. Return 9.12% Low-Temperature HFC DX Baseline Low-Temperature CO2 Secondary Low-Temperature CO2 DX Cascade Figure 5: Low-temperature system heat gain into distribution piping. 2,400 2,200 2,000 1,800 1,600 1,400 1,200 1,000 800 600 400 200 0 –17.7 (0) –12.2 (10) –6.7 (20) –1.1 (30) 4.4 (40) 10 (50) 15.6 (60) 21.1 (70) Atlanta Los Angeles Boston Hours Per Year 26.7 (80) 32.2 (90) 37.8 (100) Ambient Temperature, °C (°F) Figure 6: Bin weather data for selected cities. ashrae.org February 2009 http://www.ashrae.org
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.