Discussion on refrigerant in medium and high temperature air compressor

In order to build a resource-saving and environment-friendly society, the state encourages the use of renewable energy and heat recovery. As a kind of energy-saving, environmentally-friendly and sustainable development strategy, energy-saving and energy-saving technologies are widely used in geothermal tail water, oil extraction and transportation, and chemical, food, steel, pharmaceutical and other industries. The medium and high temperature heat pump can use 25~60 waste heat source to provide hot water with temperature above 60, direct heating or process heating, which greatly reduces energy waste, and avoids excessive temperature of the discharged wastewater and adverse effects on the environment. One of the key technologies of medium and high temperature heat pumps is to select suitable refrigerants.

Commonly used working fluids are difficult to produce more than 70 hot water due to their nature and compressor-related technology. Therefore, it is necessary to select from the new refrigerant. In the early days, CFC11 and CFC114 were used as research labors. However, due to its high environmental pollution, it has been banned. The selection of medium and high temperature heat pump working fluids generally follows the following principles.

(1) The condensing pressure is below 2.4 MPa; (2) The evaporation pressure is above 0.1 MPa; (3) The thermodynamic index is good, the critical temperature is high; (4) The oil solubility is good, the chemical property is stable; (5) The environmental hazard is small, Non-toxic, non-polluting; (6) has a high COP. According to the above principle, Kobe Steel in Japan uses a non-azeotropic mixture of R22/R142b as a working fluid test, and the water supply temperature can reach 85 or more. Gianfranco compared several working fluids and found that R-236fa has a higher critical temperature and is suitable for use in high temperature heat pumps, but it is currently in the experimental phase. In China, Tsinghua University has studied medium and high temperature heat pumps with refrigerants named HTR01 and HTR02. The high temperature heat pump research group of Tianjin University has studied the medium and high temperature heat pump working fluid of ZHR series. From the thermal performance calculations, it is concluded that the ZMR series of working fluids are suitable for medium and high temperature heat pumps. Among them, ZHR02 can meet the requirements of producing about 70 hot water, non-toxic, non-flammable, and has good comprehensive performance. It can be directly filled in the R22 compressor. The working fluid has applied for a national patent and has been published. In this paper, a comprehensive analysis of the experimental study of the medium and high temperature heat pump system with ZHR02 as the refrigerant is carried out.

The basic properties of 2ZHR02 and the basic properties of the medium and high temperature heat pump design parameters 2.1ZHR02.

The medium-high temperature environmental protection work ZHR02 is a kind of non-azeotropic mixed working fluid with better comprehensive performance. Table 1 lists the basic physical properties of ZHR02. The GWP and ODP of ZHR02 are relatively low, and the comprehensive environmental performance is good. Further material compatibility test, oil solubility test and flammability test on ZHR02 show that ZHR02 working fluid can be directly filled in the R22 compressor, and the oil solubility meets the requirements, which is safe and reliable.

2.2 Medium and high temperature heat pump design parameters.

ZHR02 is selected as the medium and high temperature heat pump working medium. The design conditions are: evaporation temperature is 25, condensation temperature is 75, superheat degree and supercooling degree are 5, and compressor indicating efficiency is 0.65. Due to the slip temperature of non-azeotropic working medium In the calculation, the evaporation temperature is selected as the dew point temperature of the working medium under the evaporation pressure, and the condensation temperature is selected as the bubble point temperature of the working medium under the condensation pressure. The theoretical calculation data are shown in Table 2. Pe: evaporation pressure, MPa; Pc: condensation pressure, MPa; te: temperature slip of the working medium in the evaporator; tc: temperature slip of the working medium in the condenser; n: compression Ratio; qh: unit mass of heat, kJ / kg; qv: unit volume of heat: kJ / kg 3; COP: coefficient of performance.

Table 2 compares the theoretical cycle performance of three working fluids of ZHR02, R134a and R114. From the pressure point of view, R134a exhaust pressure is the largest, ZHR02 working fluid pressure is slightly larger than R114, but they are less than the upper limit of the pressure of conventional heat pump system equipment; from the aspect of performance coefficient, the three working COPs are not much different, energy saving The ZHR02 unit volume heating capacity is 1.22 times that of R134a and 3.15 times that of R114, which is beneficial to the miniaturization of the compressor.

ZHR02 pressure is relatively small, the direct filling tank has good performance in R22 compressor, and the exhaust gas temperature is less than 120. ZHR02 is a non-azeotropic mixing medium, and the phase change slip temperature in the evaporator and condenser is 8.95, 7.09, respectively. This is matched with the temperature change of the water source in the two devices, which can effectively reduce the irreversible loss of temperature difference heat transfer between the evaporator and the condenser, and improve the available energy efficiency of the system.

The whole thing.

The inspection platform consists of four systems, namely the working fluid circulation system, the water circulation system, the automatic control system and the measuring device system. The working fluid circulation system consists of a shell and tube evaporator, a shell and tube condenser, and a semi-closed integral screw compressor. , dry filter cartridge, electronic expansion valve and other components; water circulation system consists of water pump, expansion tank, mixing tank, electric valve, globe valve, etc.; temperature test using Pt100 platinum resistance thermometer, measurement error is! 0.5, respectively arranged at the inlet and outlet of the refrigerant and water side of each device; the electromagnetic flowmeter is used to test the flow of the water side of the heat exchanger. All measuring instruments are automatically acquired with the Agilent data acquisition system. The automatic control system combined with the water system allows the high temperature heat pump unit to be operated under set conditions.

When the experiment is carried out, first open the deep water well pump to make the system fill all the water system of the experimental platform until the overflow water tank overflows, and exhaust the air collected in the system, adjust the relevant valve to the test state; respectively turn on the heat source side, use the side two The system circulates the pump and exhausts the air again, so that the flow is stabilized on the coarse flow value, the fluctuation range! 2%; After checking the circuit system is completely correct, turn on the heat pump unit, turn on the mixed water pump matched with the tested heat pump unit, and adjust the two electric valves to make the temperature of the two systems reach the set working condition value. The maximum deviation of each temperature reading from the rated condition is! 0.5, when the temperature of each temperature measurement point reaches the set value, the system will automatically collect data and record.

4 experimental conditions and results analysis.

4.1 Experimental conditions.

In this experiment, the automatic control system is used to adjust the temperature of the evaporator and the inlet and outlet water of the condenser, and simulate the various working conditions of the medium and high temperature heat pump unit specified in the draft of the GB/T# water source high temperature heat pump unit: nominal working condition, maximum working condition , minimum working conditions and variable working conditions.

The experimental conditions are shown in Table 3. The working conditions of the variable working conditions are as follows: (1) Keep the temperature and flow of the inlet and outlet of the condenser under the nominal working condition, and the inlet water temperature T3 of the evaporator changes from 28 to 53 at intervals of 8; Keep the temperature and flow of the inlet and outlet of the evaporator under the nominal working condition value, and the inlet water temperature T1 of the condenser changes at intervals of 8-10.

Combine a variety of operating conditions, and test the measured values ​​of the unit system under various working conditions, calculate the performance coefficient, heating capacity and other parameters under various working conditions. .

The coefficient of performance of the heat pump is calculated as .

COPh=Qc/P(1)

Qc=Cmc(T2-T1)(2)

Where, COPh: heat pump test unit performance coefficient; Qc: heating capacity, kW; P: compression power consumption, kW; mc: condenser side cycle 45 phase 7 refrigeration technology Refrigeration ring water flow, kg / s; me: evaporation Circulating water flow on the side of the device, kg/s; C: specific heat capacity of water, J/(kg.); T1, T2: circulating water in the condenser side, water temperature.

4.2 Results and analysis.

When the condenser inlet water temperature is 62, the evaporator water flow rate is 48.3 m 3 /h, and the condenser water flow rate is 64.3 m 3 /h, the heating capacity, COP, condenser outlet water temperature as a function of the evaporator inlet temperature.

It can be seen from the figure that when the ZHR02 working fluid is operated under variable working conditions, the heating capacity increases greatly with the increase of the inlet water temperature of the evaporator. When the evaporator inlet water temperature is 28,53, the heating capacity is 487.38kW and 786.70kW, respectively, and the heating capacity is increased by 61%; the COP is increased from 3.7 to 5.21, an increase of 40.8%. The condenser outlet water temperature is raised from 68 to 68. 72, under the nominal conditions of heat, COP, condenser outlet water temperature were 612. 85kW, 4.35, 70. Stable operation under nominal conditions, fully meet the national standard requirements.

When the evaporator inlet water temperature is 38, the evaporator water flow rate is 48.3m 3 / h, the condenser water flow rate is 64.3m 3 / h, the heating capacity, COP, high and low pressure pressure and the condenser outlet water temperature with the condenser inlet temperature Curve.

It can be seen from the figure that when the ZHR02 working fluid is operated under variable working conditions, the heating capacity is greatly reduced as the inlet water temperature of the condensing temperature increases. When the condenser inlet water temperature is 38,67, the heating capacity is 660.57 and 605.52 kW, respectively, down 8.3%, COP is reduced from 6.9 to 3.93, which is reduced by 43.0%. The high pressure is less than 2.5 MPa, and the traditional equipment pressure can fully meet the safety. Claim. The heating capacity under nominal conditions is 620.62 kW and 4.41. The stable operation under nominal conditions is completely full of national standards.

The highest water temperature of ZHR02 is 75. 5 conclusion.

In this paper, a test bench is built with ZHR02 as the medium-high temperature heat pump unit. The influence of the inlet water temperature of the evaporator and the inlet water temperature parameter of the condenser (the temperature range of the inlet and outlet water specified by the national standard) on the system performance is studied. The main conclusions are as follows. : (1) With ZHR02 as the working medium, the theoretical calculation of calorific value is slightly larger than the experimental data under nominal conditions. The reasons are as follows: (a) There is an error in the reading of the measuring instrument. (b) Heat loss. The evaporator, condenser and system piping are not sufficiently insulated and there is a loss of heat transfer from the environment.

(c) ZHR02 is directly filled in the R22 compressor, and the heat is deviated.

(2) Through the intermediate high temperature heat pump working substance ZHR02, the experimental test proves that the working medium ZHR02 is suitable for medium and high temperature heat pump refrigerant. The unit operates stably under nominal conditions, the COP reaches 4.3, the maximum pressure is less than 2.4MPa, and the high temperature heat source can be utilized. The hot water of more than 65 is obtained, and the highest outlet temperature is 75. (3) There are still some shortcomings in the designed test unit. Through the optimized design of the system, the cycle performance of the unit will be better.