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Issues Related To Refrigerant Charging

Jun 22, 2020

Issues related to refrigerant charging


There are two main methods of refrigerant charging: weighing method and observation pressure method. Both methods have their own characteristics. In many occasions, the combined use of the two methods will be better.


The charging amount of the computer room air conditioning unit can be accurately measured in the laboratory, and then appropriately modified according to the on-site conditions, the charging amount of each on-site installed computer room air conditioning unit can be basically determined, and then the refrigerant can be weighed according to this method Charge the unit. However, this method has two problems in practical application. Sometimes the exact value of the charging amount of the unit is not easy to determine, and sometimes it is not easy to find the accurate weighing equipment on site. At this time, the refrigerant can only be charged by the observation pressure method. .


  The main method of observing the pressure filling method is to evaporate pressure as the main, and condensing pressure as the auxiliary. Because most of the condensing pressure of the computer room air conditioning unit can be adjusted steplessly, a part of the refrigerant can be pre-charged, and the condensing pressure should be stabilized at about 1.4 to 1.6 MPa (14 to 16 kg/ cm2 ). Then adjust the evaporation pressure. When the indoor temperature is 24 ℃, ensure that the evaporation pressure is stable near the target value. For the JH F32 unit, the target evaporating pressure is about 0.56 MPa (5.6 kg/cm2), while for some computer room air conditioning units, the target pressure may be lower, such as the evaporating pressure of most computer room air conditioning units. The fixed value is 0.48 MPa (4.8 kg/cm2), which is related to the configuration of evaporator and blower.


  However, the observation pressure method is greatly affected by the outdoor temperature. It is not easy to charge accurately when the outdoor temperature is relatively low. For example, when the outdoor temperature is very low in winter, when the liquid is filled, when the evaporation pressure and condensation pressure of the refrigeration system are already near the set value, if the refrigerant continues to be charged at this time, the condensation fan can be adjusted steplessly according to the change in condensation pressure The rotation speed can still stabilize the condensing pressure at about 1.4 to 1.6 MPa (14 to 16 kg/cm2), so it is difficult to determine whether the refrigerant is overcharged at this time. However, when the temperature is high in summer, the excessively charged units are easily occupied by excess liquid refrigerant due to the condenser space, which is susceptible to high-pressure alarms. At this time, the refrigerant needs to be discharged. Even if it is discharged too much, the condensation pressure can be stabilized at 11 4 ~ 11 6 MPa (14~16 kg/ cm2 ), therefore, even if the volume is too large, the refrigerant shortage of the unit is not easy to find, and the unit with insufficient refrigerant may give a low pressure alarm when the temperature is low, and so cycle.


  It can be seen that although the observation pressure method looks simple and easy to implement, it requires a high level of maintenance personnel's field debugging experience.


  In fact, the improper charging volume is not found until the unit's high and low pressure alarm. Problems can also be discovered by observing changes in related parts. If frost at the outlet of the expansion valve is found, it can be preliminarily determined that the insufficient charging causes the evaporation temperature to be lower than 0 ℃; and the frost or dew of the compressor casing is generally caused by excessive charging, the specific reasons are as follows.


Excess refrigerant generally accumulates in the lower part of the condenser in liquid form and continues to be cooled in the forced convection air. The degree of supercooling increases and the amount of cold that can be released per unit mass of refrigerant increases; and due to the increase in condensation pressure, As a result, the flow capacity of the expansion valve is enhanced, and the refrigerant flow rate is also increased. When the heat load of the evaporator is unchanged, the refrigerant is not easily evaporated in the evaporator, causing the evaporator to return liquid with liquid, which is sucked in the compressor. Under the action, the liquid refrigerant in the return air flashes at the suction port of the compressor, resulting in low pressure at the suction port. At the same time, the flash refrigerant absorbs heat and causes water vapor in the air to condense near the suction port of the compressor, resulting in the compressor casing There is a lot of condensation or frost.