PLC control transformation of the hottest refriger

  • Detail

PLC control transformation of refrigeration compressor introduction ammonia plant of Juhua Co., Ltd. mainly produces carbon ammonia, urea, methanol, liquid ammonia and other products. In the process flow of each product, it is required to provide a large amount of refrigeration capacity. The synthetic ammonia plant uses gas ammonia and liquid ammonia for energy conversion, and supplies a large amount of refrigeration capacity through the ice freezer. Considering the overall plan for energy-saving transformation of synthetic ammonia plant and the demand for refrigeration capacity, combined with the requirements for increasing production and energy saving and updating and transformation of refrigeration posts, the technical transformation work is carried out for the problems of the original old process piston compressor, such as high loss, small volume, low operation efficiency, too cumbersome electrical equipment and so on. The original btd-icc piston ice machine adopts relay control, which has the problems of complicated control circuit wiring, high damage rate, many mechanical transmission parts, troublesome operation, frequent faults, inconvenient maintenance and so on. Therefore, the ammonia plant decided to replace the original piston ice machine with a screw ice machine with a refrigerating capacity of 1million kcal/h and a power consumption of 450KW

PLC control is adopted in the electrical control circuit, because PLC has high reliability and strong anti-interference ability; The control program is variable and has good flexibility; Programming is simple and easy to use; Perfect function; Convenient expansion and flexible combination; Small size, light weight and other advantages, this design has achieved the expected effect in practice

2 introduction to process flow

the working process of ice freezer is based on physical conversion: (pressure × Volume)/temperature = constant (i.e. p1v1/t1= p2v2/t2) the physical process of converting gas ammonia into liquid ammonia

therefore, the pressure and temperature of gas ammonia are important parameters for process control

in production, the gas ammonia with the pressure lower than 2kg/m3 enters the ammonia separator at the inlet through the gas ammonia main pipe of the system to separate the liquid ammonia droplets. The gas ammonia that filters the liquid ammonia droplets flows through the system pipe and enters the suction filter of the compressor unit, and then filters the small debris carried by the gas ammonia through the filter (the suction filter is equipped with a thermometer to indicate the suction temperature, and a stop valve is connected with the suction pressure gauge to indicate the suction pressure). The clean gas ammonia enters the screw compressor for compression and pressure rise (i.e. the air pressure rises from 0.3MPa to 1.57mpa), and the compressed gas is discharged to the exhaust port. During the operation of the compressor, the oil pump injects about 0.5 ~ 1 volume flow into the compressor to avoid conflict; For force measurement, there is% lubricating oil, which plays the role of cooling, sealing and lubrication. At this time, it is required that the injection pressure of 6000kn high-precision load sensor must be greater than the pressure of ammonia in the compressor to ensure the smooth injection of lubricating oil. The detection point of oil pressure difference here is an important parameter. These lubricating oils are discharged into the oil separator along with the exhaust gas for oil separation. A safety valve is installed in the oil separator to vent through the safety valve when the pressure in the separator is too high. After that, the system is divided into gas circuit process and oil circuit process

from the gas path process, the ammonia separated by oil enters the condenser at a temperature of 60 ~ 70 ℃ and a pressure of 1.35 ~ 1.40MPa to condense into liquid ammonia, which enters the liquid ammonia collector; From the oil circuit process, the oil separated from the oil separator passes through the oil cooler, and the cooled oil enters the coarse oil filter through the check valve (which can only flow in one direction). After filtering out large particle impurities such as iron filings, it enters the inlet of the fuel injection pump. After being boosted by the oil pump, it is further filtered by the fine oil filter, and then flows back to the fuel injection main pipe and enters the compressor. The oil pump is connected with an attached line valve to regulate the pressure of the oil pump, and the fine oil filter is connected with a pressure gauge (the pressure value should be small ≤ 0.07MPa under normal conditions, and when the pressure value is large, it means that the filter in the filter is blocked and needs to be cleaned). The basic process flow diagram is shown in Figure 1:

Figure 1 basic process flow diagram

because the compression regenerative plastic granulator has a broad development space in China, the front and rear bearing shells of the main engine are heated due to long-term operation, Add oil for cooling and lubrication. To this end, two oil pumps in the thin oil station are added to suck oil from the oil tank and inject oil into the bearing bush through the oil filter and oil cooler. Generally, when the pressure is sufficient, one oil pump supplies oil and the other is used as a standby machine; When the oil pump pressure is insufficient, start two pumps to supply oil at the same time, and the oil pressure injected into the bearing bush is generally 0.15Mpa

3 PLC control

programmable controller is an industrial control product based on microprocessor, which integrates computer technology and automatic control technology. It is developed on the basis of hard wired logic control technology and computer technology. Generally, PLC is regarded as a device composed of equivalent relays, timers, counters and other elements. Unlike relay control, PLC is connected to many real hardware relays. It is composed of some "soft relays", which avoids the maintenance due to component wear and a series of complicated wiring work

(1) main features

high reliability and strong anti-interference ability

the control program is variable and has good flexibility

simple programming and convenient use

perfect function

convenient expansion and flexible combination

the workload of control system design and construction is reduced

small in size and light in weight, it is a unique product of "mechatronics"

from the perspective of electrical instruments, the centralized control interface can be used to flexibly use PLC control, fieldbus control system (FCS) or distributed system (DCS) to realize the display and control of process parameters. In terms of the scale, investment price and process control points of this transformation, we use programmable controller to realize electrical index display, tripping and alarm

(2) PLC selection

plc selection is mainly based on the required function and capacity, and considering the convenience of maintenance, the versatility of spare parts, whether it is easy to expand, whether there are special functional requirements, etc. Through comparison, we choose the fxon series of Mitsubishi micro programmable controller. Fxon series is a micro programmable controller that condenses many functions in a subminiature housing

compared with f1/f2 series, the installation area of fxon series is only 41% of f1/f2 series, and the volume is only 37%. The controller is equipped with convenient functions such as analog potentiometer and run/stop switch. Through the connection between the expansion unit, expansion module and the basic, you can freely choose to use input and output points. Fx

on series inherits the fixed collocation and flexibility of the original series

(3) design of PLC control system

according to the conditions and control requirements proposed by the process, the specific design idea is as follows: the screw ice machine has a circulating oil pump, and the oil pressure is adjusted through the auxiliary line valve during operation. Two thin oil pumps. When the oil pressure is normal, one is in operation and one is standby, which can be switched automatically. When the oil pressure is low, two thin oil pumps are started at the same time; When the oil pressure difference is too low, the trip will be delayed for 6S. In addition, high exhaust temperature, high oil temperature, high North bearing temperature, high South bearing temperature, high exhaust pressure and high differential pressure of oil fine filter will all cause tripping. However, in the thin oil station, the oil pressure is low, the oil pressure difference is low, the DC power supply loses pressure, the circulating oil pump is overloaded, and the 1 # and 2 # thin oil pumps do not trip when overloaded, but only send alarm signals. To realize the above functions, dozens of intermediate relays are needed, and the wiring is very complex, the maintenance is extremely difficult, and the reliability is poor. However, the wiring after using PLC is quite simple, and the reliability is greatly improved. Its ladder diagram is shown in Figure 2

Figure 2 PLC control ladder diagram

4 conclusion

using screw ice machine to replace piston ice machine can save 11.5 tons of refrigerator lubricating oil per year Set. The traditional fixed program relay contactor is replaced by a small PLC industrial control computer. (end)

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