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72 Ion Zabet and Graţiela-Maria Ţârlea 2002). In the same time we will try to define the point where injection process take place on the: isentropic process, isochoric process or between them. The work is conducted in two phases: the first phase of the rig consists in the set-up of a test rig, its instrumentation, the calibration of the measurements devices, the set-up of the data acquisition system and a few verification tests; the second phase consists in the achievement of the 100 tests. On this test rig we mount a pressure sensor on the evaporator supply and a new system of expansion valves. This new system of the expansion valves allow us to extend the tests matrix to a wide range of capacities and a higher control over the system. The expansion valves are mounted three on the evaporator and two on the injection evaporator. 2. Description of Experimental Model The schematic representation of the test rig is given in Fig. 1. The refrigerant circuit (Zabet, 2011) comprises a main evaporator, an injection evaporator, the compressor, a condenser and different expansion valves. The refrigerant is R407C. The main evaporator and injection evaporator are fed by a glycol water closed loop. Glycol water is an aqueous solution 50% of propylene glycol. The glycol water loop is composed of a pump, an electrical boiler, an expansion tank, a bypass loop and different manual control valves (at the inlet and outlet of the main evaporator and the injection evaporator). The condenser is cooled by tap water. Two type of test was performed: with injection and without injection. The compressor characteristics are: type scroll; injection mode vapour; swept volume 166 cm 3 ; N= 2900 rpm. The main evaporator is a helical heat exchanger. The second evaporator is a helical heat exchanger which is used for heat the refrigerant liquid to be injected in the compressor. The boiler is built by using two concentric cylinders. The internal cylinder is completely closed and filled with gaseous nitrogen. Glycol water circulates upwards between the external and the internal cylinders. Glycol water is heated by 10 electrical resistances (6 resistances of 9 kW and 4 resistances of 6 kW). A flow sensor is installed at the boiler exhaust, to protect the boiler if no water movement is detected. The nominal flow rate of the pump is 40m 3 /h. The boiler is equipped with a temperature regulation system, which allows maintaining a stable temperature set point at the boiler exhaust (by switching on and off some resistances). On the second evaporator it was mounted two expansion valves and two different orifice types in order to varying the injected refrigerant mass flow. On the main evaporator it was mounted three expansion valves with different orifice size. The expansion valves are mounted in parallel for both types of evaporator. The condenser is shell and tubes heat exchanger and use tap water for refrigerant condensation.
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 73 Fig. 1 – Schematic representation of the test rig. 3. 3. Experimental Results Analyses Table 1 presents the results of the experimental test carried out without vapour injection. Each test point is characterized by the supply pressure (Psu), exhausted pressure (Pex), superheating temperature (ΔTSI) and subcooling temperature (ΔTsub). Table 2 presents the results of the experimental test carried out with vapour injection. Each test point is characterized by the supply pressure (Psu), exhausted pressure (Pex), superheating temperature (ΔTSI) and subcooling temperature (ΔTsub). Let be other types of situations. Figs. 2–8 (Zabet, 2011) present the cooling capacity (Φ0), heating capacity (Φcd), compressor power ( W ), coefficient of performance for heating & cp (COPh) and cooling (COPc), compressor exhausted temperature (tex;cp) versus pressure ratio (π) in the cases with/without injection. As can be seen from the figures, vapour injection method is better than the case without injection for pressure ratio between 5 and 7.5. For the vapour injection tests the injection mass flow rate ( inj M& ) varied from 0 to 0.027 kg/s, which corresponds to an injection ratio INJR from 0 to 9.6%. The injection ratio is defined by injection mass flow rate divided by supply mass flow ratio.
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BULETINUL INSTITUTULUI POLITEHNIC D
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