Problems related to refrigerant charging

　　【】Computer room air-conditioning units are generally charged with refrigerant at the installation site. There are two main methods for refrigerant charging: weighing method and observation pressure method. The two methods have their own characteristics, and in many cases, the combination of the two will have better results.

　　The charge amount of the computer room air conditioning unit can be accurately measured in the laboratory, and then appropriate corrections are made according to the on-site situation, and the charge amount of each computer room air conditioning unit installed on site can be basically determined , And then use the weighing method to charge the refrigerant into the unit. However, this method has two problems in practical application. Sometimes it is not easy to determine the exact value of the unit's charge, and sometimes it is not easy to find accurate weighing equipment on site. At this time, only the observation pressure method can be used to charge the refrigerant. .

　　 The essentials of observing the pressure filling method are mainly the evaporation pressure and the condensing pressure as a supplement. Because most of the condensing pressure of computer room air-conditioning units can be steplessly adjusted, a part of the refrigerant can be reduced in advance, and the condensing pressure can be stabilized at about 1.4～1.6 MPa (14～16 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 JH F32 units, the target evaporation pressure is about 0.56 MPa (5.6 kg/cm2), while for some computer room air conditioning units, the target pressure may be lower. For example, most computer room air conditioning units have evaporation pressure settings. The fixed value is 0.48 MPa (4.8 kg/cm2), which is related to the configuration of the evaporator and blower.

　　 But the observation pressure method is more 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 evaporating pressure and condensing pressure of the refrigeration system are already near the set value, if the refrigerant is continued to be charged at this time, the condensing fan can be steplessly adjusted according to the change of the condensing pressure. The rotation speed can still stabilize the condensing pressure at about 1.4～1.6 MPa (14～16 kg/cm2), so it is difficult to determine whether the refrigerant is overcharged at this time. In the summer, when the temperature is high, the overcharged unit will be squeezed by the excess liquid refrigerant in the condenser space, which is prone to high pressure alarm. At this time, the refrigerant needs to be discharged. Even if the discharge is too much, the condensing pressure can still be stabilized at 11 4～ 11 6 MPa (14～16 kg/cm2), therefore, it is not easy to find the refrigerant shortage of the unit even if the discharge is too much, and the unit with insufficient refrigerant may give a low-pressure alarm when the temperature is low, and so on.

　　 It can be seen that although the observation pressure method seems simple and easy to implement, it requires a high level of on-site debugging experience for maintenance personnel.

　　 In fact, the improper filling volume is not discovered until the unit high and low pressure alarms. Problems can also be found by observing changes in related components. If frosting is found at the outlet of the expansion valve, it can be preliminarily judged that the evaporation temperature is lower than 0 ℃ due to insufficient charging; frosting or condensation on the compressor casing is generally caused by overcharging. The specific reasons are as follows.

　　Excess refrigerant generally accumulates in the lower part of the condenser in liquid form, and is continuously cooled in the forced convection air, the degree of subcooling increases, and the amount of cold release per unit mass of refrigerant increases; Moreover, due to the increase of the condensing pressure, the flow capacity of the expansion valve is increased, and the refrigerant flow rate is also increased. When the heat load of the evaporator remains unchanged, the refrigerant is not easy to evaporate completely in the evaporator, resulting in the return of the evaporator with liquid. Under the suction of the compressor, the liquid refrigerant in the return air flashes at the suction port of the compressor, resulting in a low pressure at the suction port. At the same time, the flashing refrigerant absorbs heat and causes the water vapor in the air to be at the suction port of the compressor. Condensation nearby causes a lot of condensation or frost on the compressor housing.

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