【正文】 the refrigeration plant ponents [19–21].Moreover, a fuzzy controller with respect to PID controlgenerally might allow obtaining performances parableor sometimes better in terms of precision of the set pointrequired. Besides referring to a fuzzy controller theovershoot of the variables is small and the settling timefast as regards the dynamic response during the suddenvariations in the cooling load。 all this generally results in arobust control [22–24]. So, experimental tests have beenconducted to pare the plant performances obtainableusing as pressor refrigeration capacity control systems,both the fuzzy algorithm and the classical thermostat thatdetermines onoff cycles of the pressor that works at afrequency of 50 Hz. The working fluids tested, the R407C(R32/R125/R134a 23/25/52% in mass) and the R507 (R125/R143A 50/50% in mass), are among the most diffusesubstitutes of R22.2. Experimental plantThe vapor pression experimental plant, subjected toa mercially available cold store and shown in Fig. 1,ismade up of a semihermetic reciprocating pressor, an aircondenser followed by a liquid receiver, a manifold withtwo expansion valves, a thermostatic one and a manual onemounted in parallel, to feed an air cooling evaporator insidethe cold store. The pressor, as declared by themanufacturer, can work with the fluids R22, R507 andR407C。 it is lubricated with polyester oil and its speed isregulated by means of a PWM inverter. It is formed by arectifier that converts the threephase main voltage, .380 V, 50 Hz to DC voltage and by an inverter that invertsNomenclatureCOP coefficient of performanceE_x exergy (W)ex specific exergy (J/kg)f pressor electric motor supply frequency(Hz)_Lcppressor power input (W)_m mass flow rate (kg/s)_Q thermal power (W)T temperature (8C)T0environmental temperature (8C, K)t time (s)Tair。cscold store air temperature (8C)Tsetsetpoint temperature (8C)Greek symbolsd efficiency defecth efficiencyDh enthalpy variation (J/kg)DT temperature difference (8C)t dimensionless exergetic temperatureSubscriptsco condensercp pressordes destroyedev evaporatorex exergeticin inletis isentropicmt mean thermodynamicout outletref refrigerantC. Aprea et al. / International Journal of Refrigeration 27 (2020) 639–648640the DC voltage to a three phase AC supplyvoltage to thepressor motor。 at the output of the inverter the voltage isadjustable in frequency and magnitude. The manifold withboth valves has been mounted to solve possible troubles,because the expansion valves behavior, when the pressor speed varies, is unknown [13]. The expansion valvesused are specifically designed for the R407C and R507. Inthe evaporation temperature range 220 to 10 8Cata308Ccondensing temperature, working with the R407C at thenominal frequency of 50 Hz, the pressor refrigerationcapacity varies in the range – kW. To fix the airtemperature on the condenser and to simulate the externalconditions, the air flows under the influence of a blower in athermally insulated channel, where some electrical resistances are located. To exactly obtain the same temperaturefixed for the air, a regulator is used to control the electricalresistances supply. In some experimental tests, the coolingload in the cold store is simulated by means of some electricheaters linked to a regulator and the electric power ismeasured by means of a Wattmeter. Table 1 lists thespecifications of the transducers used. The test apparatus isequipped with 32 bit A/D acquisition cards linked to apersonal puter allowing a high sampling rate and amonitoring of all the measures carriedout by means of thetransducers. The data acquisition software has been realizedin a Labview environment and the R407C and R507thermodynamic properties have been evaluated using adedicated software that has also been used to determine theenergy and exergy balances.3. Experimental procedure descriptionTo evaluate the plant performances when an inverteris used, it is necessary to pare the plant energyconsumption when the refrigeration capacity is regulatedby on/off cycles of the pressor that works at a supplycurrent frequency of 50 Hz, and when the refrigerationcapacity is controlled by the fuzzy algorithm. In theexperimental tests, different types of cooling loads havebeen considered. First of all some experimental tests whenthe cooling load is due both to the periodic opening of thecold store door and to the inevitable heat exchanges withoutdoor air, even when the cold store door is closed, havebeen realized. These tests have been performed at varioustemperature levels for the air in the coldstore and, precisely,at 5, 0 and 25 8C, opening the cold store door every 20 minfor about 5 min with an outdoor air temperature of about18 8C. Moreover, in some tests the cooling load has beenobtained by means of controllable electrical heaters locatedin the cold store, while in other tests a real cooling load hasbeen considered represented by 200 kg of fruit andvegetables for whose preservation the temperature hasbeen fixed at 5 8C in the cold store. In these last twoFig. 1. Sketch of the experimental plant.C. Aprea et al. / International Journal of Refrigeration 27 (2020) 639–648 641situations, the cold store door has been opened every 10 minto simulate a real working condition。 moreover, the testshave been performed both in the winter and in the summerseason. As for the summer tests the outdoor air temperatureat the condenser has been kept at about 32 8C thanks to achannel where the air is heated by means of an electricheater, while in winter the outdoor air temperature has beenkept at 10 8C. The experimental results are mostly presentedin terms of electrical energy consumption, measured bymeans of an opportune electric energy meter, evaluating theenerg