ASHRAE Journal - February 2009 - 35

ence year data are available5) to investigate the behavior of the system when varying the condensation conditions. Refrigeration System Suction Line The supermarket refrigeration system Compressors Room is divided into two independent circuits: low temperature (Tev= –34°C [–29°F]) and medium temperature (Tev= –13.5°C [–29°F]) with, respectively, 75 kW and 325 kW refrigerating capacity. The refrigLiquid Line erant is R-404A, and its condensation is provided by two rooftop condensers: three Figure 1: Refrigeration system: machine room layout. units for medium-temperature circuit and one unit for low-temperature circuit (476 kW, 21.4 m3/s [45,360 the possible results in other climates, the influence of the outside cfm] for each unit). Refrigerating and heat rejection capacities temperature on the possible performance of the two systems must have to be considered as nominal powers. The retrofit solution be analyzed. Because the display cabinet’s refrigerating load involved the creation, in each display cabinet, of two parallel cannot be exactly predicted due to several stochastic reasons (reexpansion lines: one with a TEV and the other with an EEV frigerating capacity is strongly influenced by customer frequency, , which are activated alternately by solenoid valves on a daily by thermal and physical characteristics of the packaged goods basis. This allowed comparative tests to be carried out between and by internal gain of the building), electrical consumption of the two systems in the same environmental and load conditions compressors as a function of outside air temperature is difficult (Figure 1). A remote machine room hosts the compressors (three to estimate on the basis of recorded data. Instead of developper circuit) that work in parallel with an inverter-based control ing a formula based on recorded data, it has been preferred to system. Suction pressure is set at a fixed value (0.35 MPa [3.5 calculate compressors’ power consumption on the basis of the bar] and 0.15 MPa [1.5 bar], respectively for medium-temperature thermodynamic cycle by means of a well-known refrigeration and low-temperature circuit, for both TEV and EEV), while the cycles’ modeling software, assuming, for the two circuits and for minimum head pressure is regulated at (nearly) 1.6 MPa (16 bar) the two kinds of valves, the hypotheses is summarized in Table for TEV and 1.2 MPa (12 bar) for EEV (condensation setpoint 1. The cycle’s behavior with EEV has been simulated by using a is followed by varying fans turning speed). lower superheating (Table 1) and a lower condensation pressure Desuperheating of the refrigerant allows heat recovery: than TEV The values used in the simulation were inferred by . two heat exchangers in low-temperature circuit (for the tap recorded experimental data. Condensation temperatures for EEV water circuit and for space heating) and one heat exchanger in and TEV have been considered about 10°C higher than outside air medium-temperature circuit (only for space heating). temperature. Previously reported annual consumption data and information about measurements as shown in Figure 1 are work well for checking the reliability of the predicted annual energy Energy Analysis A one-year experimental survey started in April 2004. Data was savings, depending on the climate. The results of the simulations are depicted in Figures 2 and logged and recorded (outdoor and indoor air dry-bulb temperatures, total electrical consumption, state of compressors, fans and drivers, 3, where Pel of compressors is the specific power per unit of thermophysical properties of refrigerant, heat recovery by means of refrigeration capacity. Note that for outside air temperature magnetic volumetric flow rate meters and PT100 thermoresistances higher than 37°C to 38°C (99°F to 100°F), the two curves (in placed on water circuits, etc.) with a small time interval (varying both circuits) intersect. Under these conditions, EEVs provide from 5 to 120 seconds depending on the variable).4 no energy advantages with respect to TEVs. This result agrees The annual experimental results reveal an important advan- with previous works,6,7 where a high energy savings was tage in using EEV in this application instead of TEV In fact, the claimed (due to the great modulation and adjustment capabil. same service was provided for the medium-temperature circuit, ity of EEV with respect to TEV) at low condensation pressure requiring 1 529 433 MJ (424,746 kWh) of electricity using EEV (that implies low outside air temperature), while energy saving and 2 589 989 MJ (719,278 kWh) with the TEV. These needs is decreasing with the increasing of condensation temperature. are the projection for the whole year, i.e., the measured energy This is directly connected to a refrigerating capacity increase is half the reported because each system operated every other of the cabinets with EEV because of: day. The possible saving with the low-temperature circuit is less • Higher refrigerant mass flow rate for lower condensation remarkable, but quite relevant. EEV required 1 063 142 MJ pressures; (295,250 kWh) versus 1 430 914 MJ (397,386 kWh) of TEV. • More steady operation; and This article discusses the superiority of EEV in this application • Lower superheating of the refrigerant at the evaporator for the climate where this experiment was performed. To study outlet. TEV TEV EEV February 2009 TEV EEV EEV ASHRAE Journal 35

ASHRAE Journal - February 2009

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