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We have seen that the refrigerating machine with stepped compression can reduce evaporation temperatures used in the manufacture of cold. However, the lower the temperature, the more specific volume of steam suction on the low pressure stage, it follows that there must be more than the amount of compressor cylinders. This requirement is added limitation associated with the value of the triple point of the coolant. The corresponding temperature at the same time low evaporation temperature. For ammonia, for example, it is equal to ?77,9 DEGREES. ..
For this reason, when you want to get the temperature below ?70 C, as is often needed for industrial use, do not use stage of a refrigeration machine, and the cascade of the machine. Such machines contain two independent cooling circuit, with a vaporizer high pressure stage at the same time be a capacitor low pressure stage. However, each of these circuits can itself constitute a refrigerating machine with a few steps.
Cascade refrigerating machines are deprived of not only the above-mentioned shortcomings, but also allow you to avoid too big differences of temperatures of evaporation and condensation. These changes lead to problems with the viscosity and thermal resistance oils.
The refrigerants used at the lowest level, obviously, should have relatively low melting temperature, as, for example, in the case of R22 (-160 C), ethylene (-169,5 C), propane (-188 C) or R14 (-187 C). On the other hand, the requirements for environmental protection are forced to refuse from R12, R13, R502 and R503.
Among the mentioned refrigerants those that have a very small specific volume when used low temperatures, as a rule, such low values of the critical pressure, that their couples can not be turned into a liquid with the help of water, where there is a need to use a different refrigerant in cascade high pressure. |
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