ASHRAE Journal - February 2009 - (Page 20) indicated previously, the field data indicated performance of the low-temperature CO2 secondary system that was somewhat better than expected. It was thought that the differences could be due to the changes in the distribution piping network, transitioning from a circuited HFC direct expansion system to a loop-type secondary coolant system. The impact of the distribution piping network on system operation in commercial refrigeration systems has traditionally not been quantified. The majority of the piping networks consist of insulated copper tubing operating at temperatures below indoor ambient and, for the most part, experience unwanted or parasitic heat gain resulting in temperature rise of the working fluid and an increase in the required refrigeration load of the system. Although this heat load is not generally calculated, it is common practice to add anywhere from 5% to 15% additional compressor capacity over the required refrigeration load to compensate for this, and in most situations this is sufficient to cover variations of this heat gain into distribution piping. When energy consumption is modeled for these systems, however, and decisions on technology strategy are made based on differences of 5%, this heat gain can become quite significant in the efficiency of the overall system. Of particular interest is the effect of the significantly smaller line sizes associated with using CO2, both as a secondary coolant and in the future, as a direct expansion refrigerant. Laboratory testing has also indicated that distribution piping effects can vary significantly, depending on system type, operating conditions, pipe sizing, and configuration of the piping network. To estimate heat gain into distribution piping, a simple heat transfer model of an insulated pipe can be used as shown in Figure 1. Insulation properties are available from the insulation manufacturers and internal and external heat transfer coefficients can be easily calculated for different system types using well-known correlations. Equation 1 can be used to calculate total heat transfer once the pipe’s UA value is calculated as shown in Equation 2. If the temperature change of the fluid in the piping is relatively small, an average fluid temperature can be used to calculate the heat transfer from the fluid to ambient and is shown in Equation 3, with the final outlet temperature T2 calculated using an iterative approach. The external heat transfer coefficient from the insulation surface to the surrounding low-velocity air was calculated from Equation 4,which is a simplified approximation shown separately in SI and I-P units, and the convective heat exchange inside the copper tube was calculated from Equations 5 and 6. (1) Nomenclature Q UA TAMB TFLUID L do di da hinside houtside ktube kinsulation · mFLUID h1 h2 T1 T2 V D Nuinside Re Pr kFLUID Re = Overall heat transfer in W (Btu/h) = Overall heat transfer coefficient in W/K (Btu/h · °F) = Ambient temperature surrounding pipe in °C (°F) = Average fluid temperature in pipe section in °C (°F) = Length of pipe section in m (ft) = Outside diameter of pipe in m (ft) = Inside diameter of pipe in m (ft) = Outside diameter if insulation in m (ft) = Convective heat transfer coefficient inside pipe in between fluid and internal pipe wall in W/m2 · K (Btu/h · ft2 · °F) = Convective heat transfer coefficient between ambient and surface of outside insulation in W/m2 · K (Btu/h · ft2 · °F) = Thermal conductivity of tube wall material in W/m · K (Btu/h · ft · °F) = Thermal conductivity of insulation material in W/m · K (Btu/h · ft · °F) = Mass flow rate of refrigerant inside pipe in kg/s (lb/h) = Fluid enthalpy at pipe inlet in kJ/kg (Btu/lb) = Fluid enthalpy at pipe outlet in kJ/kg (Btu/lb) = Fluid temperature at pipe inlet in °C (°F) = Fluid temperature at pipe outlet in °C (°F) = Ambient air velocity in m/s (ft/s) = Outside diameter of insulation in m (ft) = Nusselt number of internal pipe flow (nondimensional) = Reynolds number of internal pipe flow (nondimensional) = Prandtl number of internal pipe flow (nondimensional) = Thermal conductivity of fluid in W/m · K (Btu/h · ft · °F) = Reynolds number of internal pipe flow (nondimensional) (4)3 (2) (5) (3) (6) ashrae.org February 2009 20 ASHRAE Journal http://www.ashrae.org
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