The choice of a refrigerating machine cycle particularly depends on the required temperature of the flow to be cooled (the temperature maintenance in a cooling chamber) and on the ambient temperature. A type of compressor and heat exchangers, a working substance and a plant scheme itself have a substantial influence on the cycle.
The ammonia is applied in refrigerating machines (RMs) at a condensation temperature not higher than 55 0C and at a boiling temperature up to –30 0C in case of using one-stage cycles, and up to –60 0C for two-stage ones. The power of ammonia RMs used is within the range from several dozens of kW to some MW.
The RM main elements are a compressor, a condenser, an evaporator and a working substance expanding device. It is worthwhile noting the absorption RMs, which are widely used in various manufacturing sectors, and operate on the ammonia as a working medium. A mechanical compressor in these machines is replaced at once with several mechanisms: a generator, an absorber and a pump.
The one-stage vapourRM with an expansion valve is the simplest scheme and applied in low-powered units. The RM scheme and cycle are presented in fig. 1.
Fig. 1. The scheme and the cycle in i-p-diagram (enthalpy-pressure) of one-stage RM with an expansion valve: I – compressor, II – condenser, III – expansion valve, IV – evaporator.
To protect the compressor equipment from water ingress the steam driers (vessels, in which fluid drops are separated from vapour) are applied in these schemes. For improvement of the effectiveness of this scheme, working substance additional cooling downstream of the condenser should be arranged in the heat exchanger by using water. The RM scheme and cycle with supercooling are presented in fig. 2.
Fig. 2. The scheme and the cycle in i-p-diagram (enthalpy-pressure) of one-stage RM with working substance supercooling: I – compressor, II – condenser, III – heat exchanger, IV – expansion valve, V – evaporator.
The pressure ratio and difference of refrigerant boiling and condensing increase at boiling temperature lowering or condensation temperature rising of a working substance. It leads to the specific refrigeration capacity decrease of one-stage cycle, capital and operating expenditures for refrigerating. Also the pressure ratio increase in a compressor leads to the discharge temperature rise, which can cause the inadmissible temperature deformation and oil sticking in discharge valves. These factors are reasons, for which it is necessary to proceed to the multistage compression at pк/ p0 ≥ 8.
Two-stage RM schemes and cycles with a coiled intermediate vessel, with complete and incomplete intercooling, are presented in fig. 3 and fig. 4, respectively.
Fig. 3. The scheme and the cycle in i-p-diagram (enthalpy-pressure) of two-stage RM with a coiled intermediate vessel and complete intercooling: I – first-stage compressor, II – intercooler, III – second-stage compressor, IV – condenser, V, VII – expansion valve, VI – intermediate vessel, VIII – evaporator.
Fig. 4. The scheme and the cycle in i-p- diagram (enthalpy-pressure) of two-stage RM with a coiled intermediate vessel and incomplete intercooling.
A working substance upstream of the intermediate vessel falls into two flows in both schemes: the greater part is directed to the intermediate vessel coil, where the refrigerant is additionally supercooled in advance of main expansion, the minor part is expanded till reaching the interstage pressure and comes into the intermediate vessel. The distinction is that in the scheme of fig. 3 the whole gas flow coming to the compressor 2nd stage for suction is chilled, and in fig. 4 – only part of it.
The scheme and the cycle of two-stage RM with two-fold expansion and refrigerant complete intercooling are given in fig. 5. This scheme is characterized by expanding the whole working fluid flow until the interstage pressure is reached.
Fig. 5. The scheme and the cycle in i-p-diagram (enthalpy-pressure) of two-stage RM with two-fold expansion and refrigerant complete intercooling.
In recent decades the cascade RMs are applied for providing low temperatures in facilities to be cooled (fig. 6). The carbon dioxide (R744) is used as a refrigerant in the cascade RM lower circuit, and the ammonia – in the upper one. Such solution essentially enables to reduce the ammonia content in refrigeration systems.
Fig. 6. The cascade RM scheme.
The cascade schemes are considered as an alternative to two-stage RMs at a boiling temperature less than –40 0C in the cycle lower circuit. One of the main disadvantages of this scheme is that an autonomous thermostating refrigeration unit for pressure maintenance in R744 loop should be available during system shutdowns.
The application of absorption RMs is potential and commercially reasonable in facilities, where there are exhaust heat flows with sufficient capacity and the cold is required (fig. 7). The ammonia-water solution is used in such units.
Fig. 7. The scheme and the process in ξ–i-diagram (concentration-enthalpy) of absorption ammonia-water RM with a heat exchanger: I – absorber, II – pump, III – solution heat exchanger, IV – generator, V – condenser, VI – expansion valve, VII – evaporator, VIII – solution expansion valve.
The temperatures of a heating source, a refrigerating medium and an object to be cooled, have a great impact on absorption RM processes, its performance and energy efficiency.