The ammonia plant is decompressed with liquid ammonia, and then evaporated under isothermal conditions to obtain a cold amount; then, by compressing the ammonia vapor, cooling condensation is performed at a higher pressure, and the ammonia vapor is exothermic under isostatic conditions to be condensed into a liquid. This liquid can be evaporated by throttling to a low pressure. Thereby forming a circulation process of the heat plexus low temperature substance to the high temperature substance.
A typical compressed steam refrigeration cycle, the cycle of which can be represented on a temperature entropy map, ie, a T-S diagram, as shown.
The cycle process is as follows: Point 1 represents the steam coming out of the evaporator, and enters the isentropic compression of the compressor to point 2. After the compression, the steam is cooled in the condenser to release the heat, reaching the saturated steam temperature Ta (point 3) corresponding to P2, and waiting for At the isothermal temperature, the heat is further condensed and condensed into a liquid to reach the point on the saturated liquid curve. 4. The liquid is throttled by the pressure reducing valve, the temperature drops to T1 below the ambient temperature, the pressure drops to P1, and part of the liquid is vaporized. At the state point 5 of the wet steam, the heat is absorbed in the evaporator for the purpose of freezing, and the liquid becomes the steam state point 1 under the conditions of T1 and P1, completing one cycle.
However, the actual refrigeration cycle is different from the above. Since the low temperature gas line entering the compressor has a loss of cooling capacity, the compressor enters the 1' point of superheated steam, and the compression process is not an isentropic process, which is actually an entropy increase process (1'-2'). Actually consume more shaft work. In addition, if the temperature of the cooling water is high, the ammonia gas cannot be completely condensed, which lowers the freezing coefficient.
In the refrigeration cycle, the evaporation pressure and compression ratio of ammonia are determined by the evaporation temperature and condensation temperature of ammonia, and the evaporation temperature of ammonia is again restricted by the heat transfer temperature difference between the ammonia cooler and the frozen material.
Compared with the refrigeration cycle process, if the lower temperature is to be obtained, the ammonia evaporation pressure is lower, so that the ammonia gas compression ratio is increased. Considering the different requirements of the user for the cold source, multi-stage compression is generally used in actual production. . In a large ammonia production plant, the requirements for the cold source in each process can be basically divided into two categories. Therefore, a two-stage ammonia refrigeration cycle is adopted, and a simple flow chart thereof is shown.
The T-S diagram of the above refrigeration cycle is as shown.
The pressure generated by the low-pressure ammonia evaporator and the low-temperature ammonia gas (as indicated by 1 point) are sucked in by the low-pressure compressor gas tank, and the ammonia gas is compressed to a higher pressure (as indicated by 2 points), entering the middle. The mixed heat exchange tank is cooled and evaporated by the lower temperature ammonia ammonia from the high pressure ammonia evaporator and the liquid ammonia from the high pressure condenser, and all the ammonia and ammonia are brought together (as indicated by 3 points) to enter the high pressure compression. After being compressed to a certain pressure (as shown at 4 o'clock), the machine enters the high-pressure condenser and is condensed into a liquid (as shown at points 5 and 6). The liquid ammonia from the high-pressure condenser is divided into two ways, after one-way throttling ( As shown at 7 o'clock, it enters the high-pressure ammonia evaporator, and the evaporated ammonia enters the intermediate mixed heat exchange tank; the other cross-flow directly enters the intermediate mixed heat exchange tank. The liquid ammonia from the intermediate mixing heat exchanger (shown at 8 o'clock) enters the low pressure ammonia evaporator after throttling (as indicated at 9 o'clock), and the vaporized ammonia returns to the inlet of the low pressure compressor to complete a cycle.
The simple process shown is a high-energy process. Because the ammonia temperature from the outlet of the low-pressure compressor is higher, the liquid ammonia decompressed at the outlet of the high-pressure compressor can be decompressed to achieve the purpose of warming. However, it consumes a large amount of liquid ammonia, and its ratio of gas ammonia to liquid ammonia is ~1:1.221. The total efficiency of the compressor unit is low, which is not conducive to energy saving and consumption reduction of the ice machine system. A utility circulating water system should be introduced to reduce the ammonia temperature of the low pressure compressor to around 40 °C. For the 1000t/day ammonia plant, this alone can save energy by 2745KW/h.
The centrifugal ammonia compressor is driven by a steam turbine, and a circulating water cooling system is provided at each outlet. The process is as shown in this process. The low pressure cold source is taken from the original heat exchanger to the liquid phase of the high pressure evaporator. After the throttle valve is depressurized, it enters the low-pressure ammonia evaporator, and the saturated ammonia vapor from the low-pressure evaporator is compressed by the low-pressure compressor, cooled in the low-pressure cooler, and mixed with the saturated steam from the high-pressure evaporator to enter the high-pressure compressor. . The high-pressure ammonia from the high-pressure compressor is condensed into a liquid phase in a high-pressure condenser, and then all goes to the high-pressure ammonia evaporator after the throttle valve. After the decompressed liquid ammonia absorbs the heat, the high-pressure ammonia evaporator is in a gaseous state, and part of The liquid phase of the high pressure evaporator is supplied to a low pressure ammonia evaporator to complete the entire cycle.
After switching to a centrifugal turbine driven by a steam turbine, the operating power consumption of the ice machine system can be reduced from the original 4100 KWH to 1840 KWH, saving energy consumption of 2260 KWh.
Data comparison before and after the transformation was carried out by introducing a circulating water cooling system to the low-pressure ice machine system and a motor-driven oil-filled screw-type refrigeration compressor to a centrifugal turbine-driven centrifugal ammonia compressor unit. The energy consumption of the ice machine system producing the same amount of cooling is very obvious. The energy consumption before and after the transformation is as follows: if the electricity cost is 0.50 yuan/kwh, the circulating water is 0.15 yuan/t, the steam is 120 yuan/t, and the steam turbine driven centrifugal is used. The ammonia compressor can save operating costs of 4 million yuan / year, and the economic benefits are very significant.
The author compares the data. If the ammonia plant produces both liquid ammonia and ammonia, the product plan changes and the impact on the entire refrigeration system is very obvious. Because of the ammonia plant that produces both liquid ammonia and ammonia, its advantage is that it requires less external cooling and a smaller total ammonia consumption per ton of ammonia.
Because this type of ammonia plant is designed in the ice machine system, it is basically impossible to consider the configuration of the ice machine system when the gas ammonia or liquid ammonia product has no sales. Because the difference between the two is very large, if this situation is considered, then the ice The machine system is operated under abnormal conditions for a long time. Therefore, once the ammonia or liquid ammonia product has no sales, the freezing capacity of the ice machine system is very limited, and the operation requirements of the device cannot be guaranteed.
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