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Calculation of refrigeration equipment in display cases (2)
Calculation of refrigeration equipment in display cases (2)
5. Selection of screw refrigeration compressors Because of the structural characteristics of screw refrigeration compressors, its internal volume ratio is dependent on the outside temperature There are 2.6, 3.6 and 5.0 regulations in my country. Three different spool valves can be selected to meet the needs of different working conditions. The new display cases can move the slide valve type screw compressor, which can carry out stepless adjustment of the internal volume ratio. The screw refrigeration compressor has a large single-stage compression ratio and wide operating conditions. The single-stage screw refrigeration compressor with economizer can get higher operating efficiency. However, under low temperature conditions, because to is very low, a two-stage screw refrigeration compressor should be selected.
7. Selection of compressor under non-nominal working conditions. When the refrigeration compressor leaves the factory, the cooling capacity indicated on the nameplate of the machine by the manufacturer generally refers to the cooling capacity under nominal working conditions. However, in actual work, general working conditions The conditions will not be under nominal conditions. Therefore, in order to accurately select the compressor, one method is to check the cooling capacity under actual working conditions according to the compressor performance curve provided by the product sample, and the other method is to convert the compressor cooling capacity.
8. Two-stage compressor selection
1) Selection calculation of two-stage refrigeration compressor with group The selection calculation of two-stage refrigeration compressor with group is actually based on the intermediate cooling temperature as the condensation temperature of the low-pressure stage. The refrigeration compressor is selected and calculated. After the low-pressure compressor is determined, the high-pressure compressor is determined according to the ratio of the theoretical gas output of the high and low pressure compressors. The steps are as follows.
(1) determines the ratio of the high and low pressure mechanism wheels. At present, the ratio usually used in our country is 0.33-0.5 (that is, l/3~1/2).
(2) Determine the intermediate cooling temperature tZ according to t0.tk and special calculations.
(3) Determine the supercooling temperature according to the intermediate cooling temperature, which is generally 3 to 5°C higher than the intermediate cooling temperature.
(4) Determine the air delivery coefficient of the low-pressure stage compressor according to the evaporation temperature and the intermediate cooling temperature, or according to the product sample.
(5) Determine the unit volume refrigeration capacity of the low-pressure stage compressor according to the evaporating temperature and the subcooling temperature.
(6) Calculate the theoretical air volume required by the low-pressure stage compressor of the display cases.
(7) Determine the model and number of low-pressure stage compressors from the single-stage compressor product sample based on the calculated theoretical gas delivery volume of the low-pressure stage compressor and the compressor selection principle.
(8) According to the selected high-pressure and low-pressure compressor theoretical gas transmission ratio and the determined low-pressure compressor theoretical gas transmission capacity, the theoretical gas transmission capacity of the high-pressure compressor is determined, and then from the refrigeration compressor product The model and number of high-pressure stage compressors are determined in the sample. The selection of the compressor of the Freon two-stage compression refrigeration system can also refer to the selection method of the ammonia compressor.
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