ASHRAE Journal - February 2009 - (Page 21) Pipe Insulation 2.0 1.8 1.6 Total Heat Gain, kW 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 –25 (–13) Experimental Versus Calculated Heat Gain Results 160 Total Heat Gain, kBtu/Day 140 120 100 80 TAMB, houtside TFLUID, hinside Total Heat Gain – Calculated Total Heat Gain – Measured 60 40 20 0 –13 (8.6) –23 (–9.4) –21 –(5.8) –19 (–2.2) –17 (1.4) –15 (5) di do da Liquid Supply Temperature, °C (°F) Figure 2: Heat gain calculation verification. Figure 1. Distribution piping heat transfer. To verify that the heat gain calculation method was appropriately applied, measurements were made on an existing 22 kW (75 kBtu/h) experimental low-temperature CO2 cascade system installed in the laboratory. System piping included several supply and return lines installed in a loop-style network in the same manner as installations in the field, and consisted of copper piping from 3/8 in. to 1 5/8 in. outside diameter (9 to 41 mm), insulated with 1 in. thick (25 mm) closed cell elastomeric foam and in lengths from 12 to 30 m (40 to 100 ft.). Several conditions were tested at various temperatures and measurements were made on the supply and return lines of the system. Pressure and temperature were measured at the entrance and exit of each supply and return line using calibrated Low-Temperature HFC DX Baseline Condenser thermocouples and pressure transducers, and fluid mass flow rate measurements were made using a coreolis meter. Figure 2 shows the calculated and measured total heat gain into the supply and return distribution piping based on the methodology outlined previously. The calculations show good agreement with the measured values with a maximum difference of 7%. With the methodology established, an analysis of a typical supermarket was carried out to quantify the differences in the heat gain into the distribution piping network for a variety of system types. A representative fixture plan was selected on which to base the analysis. Figure 4 shows the general layout of the 3600 m2 (39,000 ft2) store with the medium-temperature loads shown in green, and the low-temperature loads under investigation shown in blue. Low-temperature system types under consideration for the analysis included: • Low-temperature HFC direct expansion system with two suction groups (existing system); Low-Temperature CO2 DX Cascade Condenser Low-Temperature CO2 Secondary Condenser Receiver Receiver Receiver Medium-Temp. HFC Comp. SLHE EEV EEV Cascade HX Subcooler LowTemp. HFC Comp. Subcooler Low-Temp. HFC Comp. Condenser Evaporator SLHE Low-Temperature Loads Separator Low-Temp. Loads CO2 Pump SLHE EEV Low-Temp. Loads Figure 3: Basic piping configuration of three low-temperature systems under analysis. February 2009 ASHRAE Journal 21
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