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System Design of Water-cooled Screw Chiller for Air Conditioning

2014-09-25 10:32

Common air conditioning with water-cooled screw chillers according to the evaporator type is mainly divided into three categories: dry unit, full liquid unit, falling film unit.




The core component of the water-cooled screw chiller is the screw compressor. The market is dominated by twin screws, individual manufacturers use single screws, and semi-closed. Individual manufacturers also use open or fully closed type. In addition to large manufacturers such as the United States and Japan, most domestic manufacturers purchase compressors.

The system design of dry water-cooled screw chillers of various manufacturers is similar, mainly in the design of heat exchanger and throttle mechanism design. Condenser designs of mainstream manufacturers in the market are roughly the same (condenser tubes are mostly Φ 15.88 and Φ 19.05 pipe diameters); There are three main types of dry evaporator design: one is a dry evaporator with straight tubes and multiple processes (commonly used in units with large cooling capacity, with Φ φ12.7 and Φ 15.88 pipe diameters), and the second is a dry evaporator with U-shaped tubes (this type is more common, see Φ 9.52 pipe diameter), the third category is the use of small diameter straight pipe single process dry evaporator (this type is used by some manufacturers, more cost-effective, using Φ 7.94 pipe diameter). The key point in the design of dry evaporators is the distribution of liquid at the inlet, which directly determines the efficiency of the heat exchanger. Most of the throttling mechanisms of dry units use thermal expansion valves or electronic expansion valves; thermal expansion valves have a greater advantage in cost; as the cost of electronic expansion valves decreases, electronic expansion valves will be popularized.

The system design of each manufacturer's full-liquid unit has its own characteristics, mainly in the design of the evaporator, oil separator, oil return system and throttle mode. Most major manufacturers use φ19.05 full-liquid high-efficiency evaporation pipes with slightly different pipe types. Most full-liquid units need to return oil from the evaporator. The location and number of oil extraction points of the evaporator are different from manufacturer to manufacturer. Some manufacturers only take one point, others take 2 or 3 points. The heights of multiple oil extraction points are different, so as to facilitate the safety of oil return when the liquid level changes; but now most of the manufacturers of the full-liquid unit are used ester synthetic oil, its mutual solubility with the refrigerant is better, the evaporator oil and refrigerant will not appear stratification phenomenon, so as long as the location of the oil inlet can be set below the liquid level, liquid level control does not need special attention (most of these systems use exhaust superheat control). Generally, there will be an oil separator inside the screw compressor, but the liquid-filled unit will generally be equipped with a secondary oil separator. Generally, the oil return system of the full-liquid unit consists of two parts, one is the oil return of the secondary oil separator, and the other is the ejector oil return in the evaporator. The throttling modes of the full liquid type unit are: orifice plate throttling mode, electronic expansion valve throttling mode, etc.

Falling film unit is also called spray type unit, the technical difficulty of falling film evaporator is the design of the distributor in the evaporator, how to realize the uniform distribution of gas-liquid two-phase fluid after throttling is the key to the success of the evaporator design. The system design of falling film unit is similar to that of full-liquid unit except evaporator, and the design of throttling mechanism is mostly electronic expansion valve. Evaporator liquid level control or condenser liquid level control is adopted; The oil return of the oil separator is the same as that of full-liquid unit. The evaporator oil return can be equipped with oil collecting device for intermittent oil return or high-pressure gas ejection for continuous oil return. Due to the falling film unit than the full liquid unit refrigerant charge is greatly reduced, and the system efficiency can reach or even exceed the full liquid unit, so it is the future development direction of water-cooled screw chiller.

Lengde Company is a professional enterprise that designs and manufactures shell and tube heat exchangers for refrigeration. We have first-class heat exchanger design and system supporting service capabilities, and have industry-leading technical levels of dry, full liquid, falling film and other complete sets of heat exchanger products, and are widely used in the air-conditioning and refrigeration industry. We continue to pay attention to the forefront of industry technology development and continue to create the industry's most technologically advanced products.