Screw refrigerators all adopt oil injection lubrication. According to the different connection methods between the compressor and the motor, they are classified into three types: open type, semi-hermetic type and hermetic type.
Open-type unit: The compressor is connected to the motor through a coupling. It is required to install a reliable shaft seal on the extended shaft of the compressor to prevent the leakage of refrigerant and lubricating oil.
Semi-hermetic units: The motor and the compressor are integrated and connected by a flange in the middle, which can effectively prevent the leakage of refrigerant and lubricating oil. The motor is cooled by refrigerant, eliminating the noise of the motor cooling fan in open units.
Fully enclosed unit: The motor and compressor are enclosed in one container, completely eliminating the leakage of refrigerant and lubricating oil, and the noise is lower. Under standard operating conditions (evaporation temperature: -15℃, condensation temperature: 30℃), the cooling capacity range of this refrigeration compressor has exceeded 3500KW.
Hermetic screw compressor units are mainly suitable for independent air conditioning or central air conditioning systems in places that are particularly sensitive to noise, such as office buildings, transportation vehicles, libraries, shopping malls, hospitals, residential buildings, and hotels. Open-type and semi-hermetic units are widely used in industrial and mining enterprises as well as civil air defense projects. The following is an analysis of the comparison between two types of compressor units:
Open-type unit
Advantages
The compressor and the motor are relatively separated, and there are no special requirements for the motor. Therefore, the application range of the compressor is relatively wide. Motors of different capacities can be matched according to different refrigerants and operating conditions. The same compressor can adapt to different refrigerants. Besides using halogenated hydrocarbon refrigerants, ammonia can also be used as the refrigerant by changing the material of some parts. The cooling capacity of a single-unit head unit can still reach over 2 million kcal, and its cost is relatively low, making it relatively inexpensive.
Disadvantage
The shaft seal is needed to seal the channels through which the refrigerant and lubricating oil leak, which is also a key point that users often maintain. According to the JB/T6906-93 standard for single-stage screw-type refrigeration compressors, the oil leakage at the shaft seal during the operation of open-type units should not exceed 3ml/h. Since Freon and refrigeration oil are miscible, the synchronous leakage of Freon and refrigeration oil during use is inevitable. Especially after 1000 hours of operation, due to the wear of the shaft seal, the leakage of Freon and refrigeration oil will be exacerbated, increasing maintenance and operation costs and affecting normal use.
It is necessary to configure separate oil separators, oil coolers and other complex oil system components, which results in a large volume of the unit, inconvenient use and maintenance, and a large weight and floor space. The accompanying motor rotates at high speed, the airflow noise formed by the cooling fan is large, and the noise of the compressor itself is also relatively high. The noise of open-type units is generally above 90dB(A), resulting in noise pollution to the environment. The efficiency is low. Due to the need for an external motor to drive the oil pump and the use of a common low-efficiency motor, the nominal working condition energy efficiency ratio of the air conditioner generally does not exceed 4.0.
2. Semi-enclosed unit
Advantages
Due to the adoption of A semi-hermetic method, the motor and the compressor are integrated, and an internal oil distribution muffler is added, which greatly reduces the operating noise. The noise difference between the open type and the semi-hermetic type with the same cooling capacity is approximately 15dB(A). The motor and compressor are designed as an integrated unit, resulting in low noise and minimal vibration. There is no leakage of Freon and lubricating oil as in open-type units, reducing the maintenance and management costs for users and preventing leakage from affecting their normal operation. For occasions with higher requirements, a dual-head or triple-head type can be adopted. Each refrigeration system is relatively independent. Once one system malfunctions, the others can operate normally without causing significant impact on production and the environment. As the built-in oil separator adopts the internal pressure difference oil supply mode, there is no need to be equipped with an external motor to drive the oil pump, which improves the energy efficiency ratio during operation. Under the nominal operating conditions of the air conditioner, the energy efficiency ratio is generally greater than 4.5.
Disadvantage
Due to the use of special motors that are resistant to Freon, lubricating oil and high temperatures, the material cost of the compressor has increased. Therefore, the price of semi-hermetic compressors of the same grade is higher than that of open compressors. The single-unit capacity is relatively small, and the capacity of a single-unit head generally does not exceed 1.5 million kcal. Most of the semi-hermetic screw compressors used in China are imported from abroad in their original form and are relatively expensive.
