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Determination of the best charge of air conditioner refrigerant z*_maintenance

by:Arkool     2021-05-29

  Refrigeration network: The design of each air conditioner has the problem of how to determine the refrigerant charge, especially in the air conditioner that uses capillary tubes as throttling devices, because the capillary tubes have a higher regulating capacity The expansion valve is poor, and the change of the charge has a greater impact on its performance. At present, there are few researches in this area and lack of mature theoretical calculation methods. Manufacturers have to adopt experimental methods and carry out multiple tests based on empirical estimates, and finally determine the best charging amount of z* with z*. This repetitive work is not only costly, but also time-consuming and laborious. In order to make it simple and feasible to determine the z*best charge, this paper calculates the z*best charge of the split air conditioner based on the system steady-state performance simulation, and proposes to determine the z*best charge of the system The principle of quantity: When the air conditioner has a certain structural size and working conditions, and the cooling capacity meets the design requirements, the energy efficiency ratio of the system is greater than z*. At this time, the air conditioner and its components are in the best working condition. I have repeatedly tested the KFR-32GW/H split wall-mounted air conditioner, and the theoretical calculations are in good agreement with the test results.

Calculation of   1 charge volume

The state of    refrigerant in the refrigeration system can be divided into two types, single-phase and two-phase, and the refrigerant quality calculation of these two parts should be considered separately.

  1.1 Mass calculation of single-phase zone

   The calculation of refrigerant density in single-phase zone is relatively simple, and the refrigeration of each part in single-phase zone

The mass of agent can be integrated by Calculation.


   where m1 is the refrigerant mass, kg; ρ is the density, kg/m3; V is the volume, m3; Pv is the pressure, Pa; Tv is the refrigerant temperature, K.

  The refrigerant in the single-phase zone mainly exists in the superheating zone of the evaporator, the supercooling zone of the condenser, the connecting pipeline, the compressor shell, the filter and the lubricating oil, so the quality of the refrigerant in the single-phase zone is :


The subscript meaning of each parameter in    equation (2) is: filt filter, pipe pipeline, oil lubricating oil, com compressor, V single-phase zone volume.

   Considering that the temperature of the refrigerant in the compressor, filter, and nozzle does not change much, the average temperature is used to calculate the density in formula (2). The amount of refrigerant dissolved in the lubricating oil can be calculated based on the oil quality and the solubility of the refrigerant.

  1.2 Calculation of the mass in the two-phase zone

The difficulty in the calculation of the charge amount in the two-phase zone is the determination of the refrigerant quantity in the two-phase zone. The key is the calculation of the cavity coefficient in the two-phase zone. Many scholars have done a lot of work in the research and verification of the modified model of the cavitation coefficient in the two-phase region. On this basis, in combination with the actual working conditions of the air conditioner, under steady-state conditions, assuming a constant heat load per unit area in the two-phase area of u200bu200bthe heat exchanger, the Hughmark model is used to calculate the amount of refrigerant in the two-phase area. Its mathematical expression is:


   where α is the cavitation coefficient, x is the dryness, β and kH are the coefficients, where kHu003df(z) is shown in Table 1.


   where G is the mass flow rate, kg/(m2·s); μ is the viscosity, Pa·S; Di is the inner diameter of the tube, m.

  This model coefficient calculation includes α, so it must be iterated when calculating α, which requires a large amount of calculation.

The amount of refrigerant in the two-phase zone m2:


In the formula, ls is the length of the two-phase zone, m; l is the length of the refrigerant pipe, m.

  The total charge of refrigerant m is the sum of the charge of each part:


  2 charge Impact on air conditioner performance and test results

  The impact of different refrigerant charge on air conditioner performance is different. The author has calculated and tested the KFR-32GW/H split wall-mounted air conditioner under different charging volumes. Theoretical calculations and test results are shown in Table 2.

   Test conditions: outdoor ambient temperature: 35±0.5°C; wet bulb temperature: 19.5±0.3°C; indoor bulb temperature: 27±0.5°C; capillary tube size: length 450mm\inner diameter X1.4mm.

   In order to verify the model and study the changes of various parameters, the steady-state parameters of the air conditioner were measured. With the help of steady-state calculation results, 14 pairs of thermocouples and two pressure gauges were arranged along the direction of refrigerant flow, and the changes in the inlet and outlet parameters of each component were measured. Figure 1 shows the arrangement of measuring points in the cycle process.

   The charge of refrigerant is measured by an electronic scale, the range z* is 6kg, and the accuracy is ±2g. The filling port and the refrigerant cylinder are connected by a plastic hose, and the measurement is performed when there is gas in the pipe.

  Under different charging volume, the theoretical calculation and test results of the system parameters are shown in Figure 2-7.

  With the increase of the charge volume:

  ① Both the condensing pressure and the evaporation pressure increase (see Figure 2);

  ②The compressor suction temperature decreases , When the evaporator outlet is saturated or two-phase, the compressor suction temperature will no longer drop. It can be seen from Figure 3 that when the refrigerant charge is 690g, there is no superheat at the outlet of the evaporator, and the utilization efficiency of the evaporator at this time z* is high;

  ③Due to the suction temperature of the compressor Decrease, but the pressure ratio does not change much, the compressor discharge temperature drops, which is conducive to the stable operation of the compressor (see Figure 4);

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