Refrigeration control system. Automation of refrigeration machines

Purpose

Propane refrigeration units natural gas are designed to simultaneously provide the required dew point parameters for water and hydrocarbons through condensation of the water and hydrocarbon fraction (HC) at low temperatures (down to minus 30 0 C). The source of cold is an external propane refrigeration cycle.

The main advantage of such installations is the low pressure loss of the feed stream (throttled natural gas flow is not required) and the ability to extract the C3+ product fraction.

To prevent hydrate formation, injection of an inhibitor is used: ethylene glycol (for temperatures not lower than minus 35 0 C) and methanol (for temperatures up to minus 60 0 C).

Main advantages

Reliability

• A continuous process based on the condensation of water and hydrocarbon fractions in the presence of a hydrate formation inhibitor.
• No cyclical fluctuations.
• Shell and tube gas-gas heat exchanger with low temperature pressure.
• Motor service factor refrigeration compressor 110%.
• Automatic system for maintaining pressure in the receiver during operation in cold climates.
• Electric heating of the inhibitor collector in a three-phase separator.

Efficiency

• Cold separator with efficient coalescing packings and long residence time.
• Gas-propane heat exchanger (chiller) with submerged tube bundle.

Possible options

• Refrigeration cycle economizer (standard for systems over 150 kW and evaporation temperature below minus 10 0 C).
• Input separator.
• Gas-liquid heat exchanger (allows you to reduce compressor power consumption).

Technology system

The moisture-saturated natural gas stream is supplied to the inlet separator (1), in which free water and hydrocarbon fractions are removed from the stream. The gas fraction is sent to the gas-gas heat exchanger (2) for pre-cooling with a flow of dry stripped gas from the cold separator. To prevent hydrate formation, the heat exchanger is equipped with nozzle devices for injecting an inhibitor (methanol or ethylene glycol).

Rice. 3 Schematic diagram of a propane refrigeration unit

After pre-cooling in the gas-gas heat exchanger, the flow is supplied to the gas-propane heat exchanger (chiller) (4), in which the flow temperature is reduced to a predetermined value through heat exchange with the boiling propane flow. The feed stream is located in a tube bundle, which in turn is immersed in the refrigerant volume.

The vapor-liquid mixture formed as a result of cooling is sent for separation to a low-temperature three-phase separator (5), where it is separated into streams of stripped gas, condensate and a water-saturated hydrate formation inhibitor.

Dry stripped gas (DSG) is supplied countercurrently to the gas-gas heat exchanger (2) and then discharged outside the installation.

Liquid fractions are discharged by independent automatic level controllers into the appropriate lines.

Articles on the topic

Gas processing made easy

One of our main tasks is to combat the myth that gas processing is difficult, time-consuming and expensive. Surprisingly, projects that are implemented in the USA in 10 months take up to three years in the CIS. Installations that occupy 5,000 m2 in the USA can hardly fit into 20,000 m2 in the CIS. Projects that pay off in the USA in 3-5 years, even with a significantly lower cost of product sales, never pay off in Russia and Kazakhstan.

The maintenance personnel of a non-automated refrigeration unit starts and stops the refrigeration machine, regulates the supply of liquid agent to the evaporator, and regulates the temperature in refrigeration chambers and cooling capacity of compressors, monitors the operation of devices, mechanisms, etc.

With automatic control of refrigeration machines, these manual operations are eliminated. Operating an automated installation is much cheaper than operating a manually adjusted installation (reducing operating personnel costs). Automated installation more economical in energy consumption, more accurately maintains the specified temperature conditions. Automation devices quickly react to any deviations from normal operating conditions, and if danger arises, they turn off the installation.

Various automatic devices are used - control, regulation, protection, alarm and monitoring.

Devices automatic control turn on or off machines and mechanisms in a certain sequence; turn on backup equipment when the system is overloaded; include auxiliary devices when thawing frost from the surface of cooling batteries, releasing oil, air, etc.

Automatic control devices maintain, within certain limits, the basic parameters (temperature, pressure, liquid level), on which the normal operation of the refrigeration unit depends, or regulate them in accordance with a given program.

Devices automatic protection if dangerous conditions arise (excessive increase in discharge pressure, overflow of separators with liquid ammonia, damage to the lubrication system), turn off the refrigeration unit or its parts.

Automatic alarm devices provide light or sound signals when the controlled value reaches the specified or maximum permissible values.

N. D. Kochetkov

322 Automation refrigeration units

Devices automatic control(recorders) record machine parameters (temperature in different points, pressure, amount of circulating agent, etc.).

Complex automation includes refrigeration equipment automatic devices management, regulation and protection. Control and signaling means are necessary only to monitor right action these devices.

Currently, a small and significant part of medium-capacity plants are fully automated; Large installations are in most cases partially automated (semi-automatic installations).

AUTOMATIC REFRIGERATION CONTROL

INSTALLATIONS

The automatic control devices used are distinguished by the variety of functions they perform and principles of operation.

Each automatic regulator consists of a sensitive element that senses changes in the controlled parameter; regulatory authority; intermediate connection connecting the sensing element and the regulatory body. Let's consider ways to regulate the main parameters and the most typical devices.

Regulating the temperature of refrigeration chambers. Refrigeration rooms must maintain constant temperatures, even if the thermal load for cooling batteries.

A constant temperature is maintained by regulating the cooling capacity of the batteries. A simple and common one is the two-position control system. With this system, an individual temperature relay is installed in each chamber, for example, type TDDA - two-position remote thermal relay (Fig. 193), or other types. A solenoid valve is installed on the liquid refrigerant or brine pipeline before entering the batteries (Fig. 194). When the air temperature rises to the upper specified limit, the temperature controller automatically closes the electrical circuit of the solenoid valve. The valve opens completely and coolant flows into the batteries; the chambers are cooled. When the air temperature drops to the lower specified limit, the temperature controller, on the contrary, opens the valve circuit, stopping the supply of cold liquid to the batteries.

Thermal balloon 1 (sensitive cartridge) of the TDDA temperature regulator (see Fig. 193), partially filled with liquid freon-12,

Automatic regulation refrigeration units 323

placed in a refrigeration chamber, the temperature of which must be regulated. The freon pressure in the thermal cylinder depends on its temperature, which is equal to the chamber air temperature. As this temperature increases, the pressure in the thermal cylinder increases. The increased pressure is transmitted through capillary tube 2 to chamber 3, in which bellows 4 is located, representing

which is a corrugated tube. The bellows compresses and moves the needle 5 in the axial direction, which rotates the angular lever 6 (see also the diagram on the right) around axis 7 counterclockwise, overcoming the resistance of the spring 22. Lever 6 carries a plate spring with a rod 8 attached to it, which, when the lever moves counterclockwise, moves to the left. A finger 10 is attached to the rod 8, moving in the slot of the contact plate 12. At some point, the finger comes into contact with the lever 9 and turns this lever, as well as the contact plate 12 (which is connected to the lever by a spring 11) around the axis 13 (in this case, counterclockwise ). In it

324 Automation of refrigeration units

time, the lower end of the contact plate approaches the permanent horseshoe magnet 18 and is quickly attracted by it. The main 17 and spark-extinguishing contacts 26 are closed. The control circuit of the solenoid valve installed on the liquid line closes, the valve opens, and liquid flows into the batteries.

As the air temperature decreases, the pressure in the thermal cylinder and in chamber 3, where the bellows is located, decreases and the angle lever 6 rotates clockwise under the action of spring 22. Finger 10 moves from lever 9 to the end of the slot in contact plate 12 (free play), presses on the plate and, overcoming the attraction of the magnet, sharply turns it clockwise. At this moment, the electrical contacts open, the solenoid valve closes and the supply of liquid to the batteries stops.

Automatic control of refrigeration units 325

The chamber temperature at which the electrical contacts open is set depending on the tension of spring 22. To set the device to a certain temperature opening moves the carriage 21 with the pointer 20 to the corresponding division of the temperature scale 19, which is achieved by rotating the screw 23 with the handle 24.

The device is regulated to a certain temperature difference between closing and opening electrical contacts. This difference depends on the amount of free play of the finger 10 in the slot of the contact plate. The free play changes when the upper end of the lever 9 moves along the slot, which is achieved by turning the cam 14 around the axis 13. The larger the radius of the cam at the point of contact of the lever 9, the greater the free play and the greater the temperature difference between closing and opening the contacts.

The TDDA temperature regulator ensures that the solenoid valve is turned off within the temperature scale from -25 to 0° C. Possible error is ±1° C. The minimum differential of the device is 2° C, the maximum is not less than 8° C. The weight of the device is 3.5 kg , capillary length 3 m.

For large refrigerators, a multi-point centralized system for automatic temperature control in the chambers has been developed - the Amur machine. Such machines are manufactured with 40, 60 and 80 control points. They can be used not only to regulate air temperature, but also the boiling point of the refrigerant, brine temperature, etc. The machine has devices for measuring temperature at control points.

Solenoid (electromagnetic) valves (see Fig. 194) operate as follows. When voltage is applied to the electromagnet coil, a electric field, which retracts the core; the associated unloader valve lifts to reveal a small diameter seat. After this, the liquid from the discharge side, i.e., from the cavity above the valve (in the SVA valve) or above the membrane (in the SVM valve) through the through holes and small seat, enters the cavity under the valve. The valve is relieved of the pressure that pressed it to the seat and opens to allow fluid to flow under pressure from the discharge pipeline. After turning off the solenoid coil, on the contrary, the core with the unloading valve drops down, covering the small diameter seat. The pressure from above on the main valve increases, and under the influence of its own weight and spring it lowers onto its seat, blocking the flow of liquid.

Solenoid valves are among the most common automation devices for ammonia and freon refrigeration units.

326 Automation of refrigeration units

new For liquid and gaseous freon and ammonia, brine and water, solenoid valves are produced with a nominal diameter of 6 to 70 mm. Previously, predominantly piston solenoid valves of the SBA type were used; V Lately Diaphragm valves of the SVM type with an improved design are used. The temperature of the working environment can range from -40 to +50° C. The solenoid valve (with a filter in front of it) is installed on a horizontal section of the pipeline in a vertical position.

Air temperature control is also possible by changing the temperature or flow rate of the refrigerant (with brine cooling of the coolant) in the batteries using proportional temperature controllers PRT. Such regulators are rarely used.

To automatically control the air temperature when using small freon installations with one cooled object, turn on and off the compressor. To turn it on and off, devices are used that respond to the temperature or boiling pressure in the evaporator, or directly to the air temperature of the chamber.

Regulating the cooling capacity of compressors. The heat load of refrigeration chambers can vary widely depending on the quantity and temperature of incoming products, temperature environment and other factors. The refrigerating capacity of the installed compressors is selected to maintain the required temperatures under the most difficult conditions.

In small direct evaporation freon installations, the performance of the compressors is regulated simultaneously with the regulation of the temperature of the cooled object using the start-and-stop method at the corresponding values ​​of one of the adjustable parameters.

In machines with brine cooling, the most convenient parameter for regulating compressor performance is the temperature of the brine at the outlet of the evaporator. If the heat load decreases, the temperature of the brine in the evaporator quickly drops to the lower set limit and the temperature controller (for example, type TDDA), opening the circuit of the magnetic starter coil, stops the compressor electric motor. When the temperature rises to the upper specified limit, the temperature controller turns the compressor back into operation. The greater the thermal load on the evaporator (cooling batteries), the longer the compressor operates. By changing the working time ratio, the required Automatic control of refrigeration units 327

average compressor performance.

In medium and large installations the system contains a large number of batteries designed to cool many rooms. When the set temperatures are reached separate rooms Some of the cooling batteries must be turned off and the cooling capacity of the compressors must be reduced accordingly.

The most acceptable in this case is multi-position (step) regulation by changing the working volume described by the compressor pistons. In installations with several compressors, multi-position control is carried out by turning on and off individual compressors controlled by temperature controllers with shifted setting limits. The presence of two identical compressors allows you to obtain three stages of cooling performance: 100-50-0%. Two compressors AB-100 and AU-200 provide four stages of cooling capacity: 100-67-33-0%. Step control of multi-cylinder indirect compressors is possible by shutting down individual cylinders by depressing the suction valves using a special mechanism controlled by a low pressure relay.

Much less often they use smooth control of compressor performance by throttling the intake steam, changing the value of the dead volume of the compressor, etc. These methods are energetically disadvantageous. A relatively promising method is the method of regulating cooling capacity by changing the speed of the compressor (use of multi-speed electric motors).

Regulating the supply of refrigerant to the evaporator. Regardless of the magnitude of the heat load, automatic control devices must ensure correct filling of the evaporator with refrigerant. Excess liquid in the evaporator must not be allowed, as this leads to a decrease in operating efficiency and to the occurrence of water hammer(“wet run”)

If there is a lack of liquid, some part of the surface is not used, which also worsens the operating mode due to a decrease in the evaporation temperature.

The devices that regulate the supply of liquid to the evaporator are thermostatic expansion valves (TRV) and float control valves (TRV). In the same devices, the process of throttling the liquid is carried out.

The main type of manufactured thermostatic valves is membrane, in a metal case. The connection diagram for the expansion valve is shown in Fig. 195. The operation of the device depends on the overheating of the pyre emerging from the evaporation

328 Automation of refrigeration units

body The absence of overheating indicates that there is excess liquid in the evaporator and the possibility of it entering the suction line and the compressor. In this case, the expansion valve automatically stops supplying liquid to the evaporator. Large overheating of refrigerant vapor during suction is, on the contrary, a sign of its deficiency in the evaporator. Under this condition, the expansion valve increases the flow of liquid.

In the TRVA ammonia valve, the thermal cylinder (the sensitive element of the device) is filled with freon-22, which is close in operating pressure to ammonia. The thermal cylinder is tightly attached to the suction pipeline; it has the temperature of the ammonia vapor leaving the evaporator.

Automatic control of refrigeration units 329

When the temperature changes, the pressure in the thermal cylinder changes. The valve valve is mechanically connected to the membrane, on which the steam pressure from the thermal cylinder, transmitted through the capillary tube, is applied from above, and the pressure from the evaporator is applied from the bottom through the equalizing tube (through fitting 7). The movement of the membrane, and at the same time the opening of the valve that regulates the supply of liquid to the evaporator, depends on the difference in the indicated pressures, proportional to the superheat of the steam at the outlet of the evaporator. Ammonia enters the TRVA through fitting 10. Throttling occurs both in the valve hole and partially in the throttle tube 8, which ensures a calmer and more uniform flow of the agent through the valve.

During operation of the machine, the TRVA maintains constant superheating of the steam; With the appropriate setting, the amount of overheating can be changed in the range from 2 to 10 ° C. The adjustment is carried out using screw 4 and the associated adjusting gears. When the screw rotates, the tension of spring 3, which counteracts the opening of the valve, changes.

TRVA allows you to reliably regulate the supply of ammonia to various types of evaporators at boiling temperatures from 0 to -30 ° C. The power supply of shell-and-tube evaporators for brine cooling is adjusted at low overheating (from 2 to 4 ° C). Various TRVA models are available, designed for cooling capacity from 6 to 230 kW (~5-200 Mcal/h).

TRV for 12-190 kW 10-160 Mcal/h) for freon installations are close in design to valves of the TRVA type. In small freon machines, membrane expansion valves without equalizing lines are used.

Regulation of the supply of ammonia to evaporators and vessels with a free liquid level is possible using low-pressure float control valves PRV (Fig. 196).

The PRV is set at the level that it is desirable to maintain in the evaporator (or other vessel). The device body is connected to the evaporator by equalizing lines (liquid and steam). A change in the liquid level in the evaporator leads to a change in the level in the valve body. At the same time, the position of the float inside the housing changes, which causes the valve to move and change the cross-sectional area for the flow of liquid from the condenser to the evaporator.

In non-through type float valves, the refrigerant, after throttling in the valve hole, enters directly into the evaporator, bypassing the float chamber. In straight-through valves, the refrigerant, after throttling, enters the float chamber, and from it is discharged to the evaporator.

330 Automation of refrigeration units

Automatic control of refrigeration units 331

monitoring the liquid level in evaporators and vessels. Unlike low pressure valves, PR-1 can be installed at different levels in relation to the evaporator and condenser.

A fitting is welded to the valve body, connecting the valve to the lower part of the condenser. Inside the body there is a float connected by a lever to a needle valve. Ammonia passes through the hole in the valve seat, channel and throttle tube to the outlet

the fitting and through it into the pipeline to the evaporator. There is a capillary tube inside the valve body. Its upper end is open, and the lower end is connected to the throttle tube using channels. The pressure in the valve is set slightly lower than in the condenser; the liquid from it enters the valve body. Under the influence of liquid, the float floats up. The more liquid enters the pop-shop body, the more the valve opens to allow it to pass into the evaporator. When using a PR-1 type valve, the condenser is free of liquid. Therefore, the amount of ammonia in the system must be such that when the ammonia completely flows into the evaporator, the liquid level in it is no higher than between the first and second rows of evaporator pipes from the top. With this filling

332 Automation of refrigeration units

eliminates the risk of liquid ammonia entering the suction line and creates favorable conditions for intensive heat exchange in the evaporator.

For positional control of the liquid level in refrigeration units, indirect level regulators are often used, consisting of a remote level indicator (for example,

DU-4, RU-4, PRU-2) and the solenoid valve controlled by it. These devices are included in the circuit (Fig. 198) so that in the event of an excessive increase in the liquid level in the device, the remote indicator opens the electrical control circuit of the solenoid valve and it closes, stopping the supply of refrigerant to the evaporator.

If the liquid level in the evaporator decreases compared to the optimal level, the remote indicator will again close the electrical circuit of the solenoid valve; the fluid supply will be restored.

Regulating the supply of cooling water to the condenser.

Water is supplied to the condenser through a water control valve

(Fig. 199), maintaining approximately constant pressure and condensation temperature at different loads. The condensation pressure is perceived by the valve membrane or bellows, which changes the position of the spindle and the cross-section for the passage of water. In installations with cooling towers, water control valves are not used.
Automatic protection and alarm 333

FROM DANGEROUS MODES

During the operation of refrigeration machines and installations, due to failures of individual components or assemblies, as well as due to disruptions in energy and water supply systems, dangerous conditions may arise: an increase in pressure and temperature, liquid level in individual devices or machine components, loss of lubrication of rubbing parts steam, lack of cooling water, etc. If prompt action is not taken, compressors, heat exchangers or other plant components may be damaged or destroyed. This poses a serious danger to the health and life of operating personnel.

Protection of refrigeration machines and installations includes a whole range of technical and organizational measures to ensure their safe operation. In this chapter, only those of them that are performed on the basis of automatic instruments and devices will be considered.

METHODS OF PROTECTION

Protection methods include stopping the machine or the entire installation, turning on emergency devices, releasing the working substance into the atmosphere or transferring it to other devices.

Stopping the machine or the entire plant. This method is carried out using an automatic protection system (APS), which consists of primary devices - sensors-protection relays (or simply protection relays) and an electrical circuit that converts signals from the protection relay into a stop signal. This signal is transmitted to the automatic control circuit.

Protection relays perceive controlled technological quantities and, when they reach the maximum permissible values, generate an alarm signal. These devices most often have relay on-off characteristics. The number of sensors-relays included in the control system is determined minimally required quantity controlled quantities.

The electrical circuit is carried out in one of three options, according to which the SAZ can be single-acting, repeated-acting and combined.

Single action SAZ stops the machine or installation when any protection relay is triggered and makes it impossible to automatically start until the intervention of maintenance personnel. This type of SAZ is common mainly on large and medium-sized machines. If the installation operates without continuous maintenance and the equipment does not have an automatically switched reserve, then the emergency control system is supplemented with a special alarm system for emergency call of personnel.

SAZ with restart stops the machine when the protection relay is activated and does not prevent it from automatically turning on when the relay returns to its normal state. It is used mainly in small commercial-type installations, where they strive to simplify the automation circuit.

In combined SAZ Some of the protection relays that control the most dangerous parameters are included in a single-acting electrical circuit, and some with less dangerous parameters are included in a repeated-action circuit. This allows you to automatically restart the machine without the help of personnel, if this does not involve the risk of an accident.

In practice, there is also a type of protection called blocking. Its difference is that the signal is received not from a protection relay, but from an element of the monitoring or control circuit of another unit or installation unit (for example, a pump, fan, etc.). The blocking prevents the start or operation of the machine if the specified order of start-up of the controlled units is not followed. Typically, blocking is performed using a re-closing scheme.

Activation of emergency devices. This method is also carried out by SAZ.

Emergency devices include:

Warning alarm about dangerous modes, which is used in particularly large installations with continuous maintenance, in order to avoid stopping the machine if possible;

An alarm that informs personnel about the protection being triggered, as well as deciphering the specific cause of the emergency;

Emergency ventilation, activated when the local or general concentration in the air of explosive and flammable, as well as toxic working substances (for example, ammonia) increases.

Release of the working substance into the atmosphere or transfer to other devices. This method is carried out using special safety devices (safety valves, safety plates, fusible plugs, etc.) that are not included in the CAZ. Their purpose is to prevent the destruction or explosion of vessels and apparatus when pressure increases as a result of an installation malfunction, as well as in the event of a fire. Choice safety devices and the rules for their use are determined regulatory documents in accordance with the safety and operation rules for pressure vessels.

BUILDING PROTECTION SYSTEMS

Protection systems vary depending on the type of refrigeration unit, its size, the adopted method of operation, etc. When constructing all safety systems, it is necessary to take into account the general principles that ensure the greatest degree of operational safety. As an example, we consider a schematic diagram of a SAZ compression refrigeration unit, consisting of a KM compressor with an electric motor D, heat exchangers TA and auxiliary devices VU - pumps, fans, etc. (Fig. 7.1). The diagram is presented in general form without indicating specific quantities and parameters subject to control.

Rice. 7.1. Schematic diagram of the SAZ

It should be agreed that the SAZ is designed to stop the compressor when one of the parameters reaches the maximum permissible value.

SAZ has ten protection channels. Channels 1-8 operate from the corresponding protection relays that sense process parameters. Channels 9 and 10 provide blocking of the compressor and auxiliary devices.

The system includes a key with which, if necessary (during testing and running-in), you can turn off part protective relays and blocking circuits (2, 3, 5, 6, 8, 9, 10). Those protections that must function in any operating mode of the installation cannot be turned off.

The electrical circuit of the SAZ consists of two parts. The first part, which includes channels 2, 5, 9 and 10, operates according to the restart method, and the second with the remaining channels provides protection operating on the single-action principle and controls the most critical parameters. When they reach the maximum permissible values, the SAZ stops the compressor. Its subsequent start-up is possible only after the intervention of personnel, who use a special button to put the protection into operation.

Signals from the electrical circuit of the automatic control system are supplied to the automatic control circuit of the control unit. These signals stop the compressor motor regardless of the operational control signals of the op-amp.

In addition to the main function of the SAZ - emergency stop of the compressor, it also performs auxiliary operations: turning on the necessary emergency devices, as well as light and sound alarms. The decoding protection alarm with re-enablement operates only until the monitored parameter has entered normal limits. The single-action protection alarm remains on after activation until the start button is pressed, regardless of the actual state of the monitored parameter. Such a scheme “remembers” the protection that has occurred and informs personnel for an unlimited time.

The presented diagram can only be considered as an example of constructing a control system. Specific systems may differ from it in the number of channels and methods of their inclusion.

The main requirement for the SAZ is high reliability, which is achieved by using highly reliable protection relays and electrical circuit elements, redundancy of relays and other protection elements in particularly critical cases, reducing the number of elements sequentially included in the SAZ, using the safest options for electrical circuits, organizing preventive checks and repairs during operation.

The use of highly reliable protection relays and electrical circuit elements is the simplest and most natural way, since, other things being equal, the use of more reliable elements allows you to create a more reliable system. It should only be borne in mind that during operation, relays and other elements of the safety protection system have a very small cyclic operating time (small number of operations). Therefore, when assessing reliability, one should take into account not cyclic durability and cyclic time between failures, but other indicators that characterize the ability of elements to remain ready to operate (for example, time between failures). In this case, any violation of the element’s ability to operate is taken as a failure.

Redundancy is the parallel inclusion of two or more homogeneous and jointly working elements that perform the same functions. The failure of one of them does not disrupt the performance of the system as a whole. Redundancy is used in especially dangerous cases, when a sudden failure of the control system can lead to serious consequences. Such cases include, for example, protection against liquid ammonia entering a piston compressor. To do this, main and backup level switches are installed on the vessels in front of the compressor.

The simplified diagram (Fig. 7.2) shows a coolant liquid ammonia separator installed between the evaporator and the Km compressor. During normal operation, there is no liquid ammonia in the liquid separator. When liquid is released from the evaporator, it accumulates in the liquid ammonia separator, and if its level reaches the permissible limit, protection relays РЗ 1 and РЗ 2 are activated (their primary converters are shown in the diagram). Both relays are constantly switched on and perform the same function. This redundancy significantly increases reliability, since the probability of simultaneous failure of both relays is extremely low.

Reducing the number of elements sequentially included in the BAS is one of the ways to increase the reliability of the electrical circuits of the BAS. Most reliable system, in which the protection relays are connected directly to the compressor motor starter without intermediate elements. However, this scheme is used only in the smallest installations. In larger installations, intermediate relays must be used, which reduces reliability. Therefore, the number of consecutive intermediate elements included in the circuit emergency shutdown compressor should be minimal.

Rice. 7.2. Simplified diagram of a liquid separator with redundant protection relays

from wet running of the compressor

When using the safest electrical circuits, the compressor stops when failures occur in the control system. The most typical failure of an electrical circuit is a break (disappearance of voltage or current), which can occur when wires are physically broken, contacts are burned, radio-electronic elements (diodes, transistors, resistors, etc.) fail, or power sources malfunction. In order for these failures to be signaled as emergencies, it is necessary that current circulates in the protection circuits under normal conditions, and the emergency stop signal corresponds to its termination. Therefore, the safest is an electrical protection circuit based on normally closed contacts or other elements.

Thus, in the circuit (Fig. 7.3), the contacts of the protection relays РЗ 1, РЗ 2 and РЗ 3 are closed if the controlled values ​​are within normal limits, and are open when the maximum permissible values ​​are reached. These contacts are connected in series to the winding circuit of the electromagnetic relay RA, which, when the protection is triggered, turns off the winding of the magnetic starter (not shown in the diagram) and stops the compressor.

Rice. 7.3. Electrical protection circuit for normally closed contacts

When all contacts of the protection relay are closed, the electromagnetic relay circuit can be put into operation by briefly pressing the KVZ button. In this case, current will flow through the winding of the electromagnetic relay, this relay will operate and close its RA contact. After releasing the button, the circuit remains energized. It is enough for one of the protection relays to open the contact, and the electromagnetic relay will release and its contact will open. Restarting will only be possible after pressing the button. This is a single action scheme. In a restart circuit, the PA contact and button are not required.

The organization of preventive inspections and repairs during operation plays a decisive role in ensuring the safe operation of installations. These measures, if carried out at the required intervals, virtually eliminate dangerous situations associated with sudden failures in the system.

To organize preventive checks, it is necessary that the safety protection systems be equipped with devices and devices that allow, if possible, to fully check the performance of the protections. In this case, it is desirable that the check does not cause the installation to go beyond the maximum permissible modes. Thus, in the diagram (see Fig. 7.2), you can check the operation of the protection relay without filling the liquid separator.

During normal operation, valves B 1 and B 2 are open, and valve B 3 is closed. The primary converters of the protection relays RZ 1 and RZ 2 are connected to the vessel.

To check, close valve B 2 and open valve B 3. From the pipeline, liquid is supplied directly to the float chambers of the level switch and fills them. If the relays are working properly, then when they are triggered, they give out the corresponding signals.

After this, valve B 3 is closed, and valve B 2 is opened. The liquid flows into the vessel, which indicates that the connecting pipe is not clogged.

During operation, a schedule of preventive checks must be in place, the frequency of which should be selected taking into account actual reliability indicators.

COMPOSITION OF SAZ

The number of parameters controlled using a safety protection system depends on the type of equipment, its size and performance, type of refrigerant, etc. Typically, the number of protections increases with the size of the equipment. More complex control systems are usually used in ammonia plants.

In table 7.1 provides a recommended list of monitored parameters for the most common types refrigeration equipment. For some types of equipment, several protection options are offered, which are selected based on specific conditions. So, for hermetic compressors, two options can be used. The option with built-in devices for protection against temperature rise of electric motor windings is preferable, since the same number of devices provides protection against more malfunctions.

In table 7.1 does not include compressors for household refrigerators and air conditioners.

Some of the protections included in the SAZ do not have to be included in a single-action circuit; if necessary, they can be included in a repeated circuit.

On particularly large installations with screw and centrifugal compressors, it is advisable to use a warning alarm. When the parameters reach the maximum permissible values, a warning alarm is activated. The compressor stops only if, after a specified period of time, the parameter does not fall within normal limits. Parameters that can be activated via a warning alarm are also noted in Table. 7.1. In this case, pay attention to the reliability of the time delay device and, if necessary, take appropriate measures, such as redundancy.

Table 7.1

End of table. 7.1

Note. An asterisk (*) means that protection is provided:

* Switching on according to the circuit with repeated switching is allowed.

** It is allowed to stop the compressor after the warning alarm is turned on.

*** Activation through a warning alarm is allowed.

SYSTEMS AUTOMATION

AIR CONDITIONING

Related information.

Cold is used in the technologies of many processes of processing agricultural products. Thanks to refrigerators, losses during storage of products are significantly reduced. Chilled products can be transported over long distances.

Milk intended for processing or sale is usually pre-cooled. Before being sent to a dairy industry enterprise, milk may be stored for no more than 20 hours at a temperature not exceeding 10 °C.

IN agriculture Meat is chilled mainly on farms and poultry farms. The following cooling methods are used: in air, cold water, in water with melting ice and cold water irrigation. Freezing of poultry meat is done either with cold air or by immersion in cold brine. Air freezing is carried out at an air temperature in refrigeration chambers from -23 to -25 ° C and an air speed of 3...4 m/s. For freezing by immersion in brine, solutions of calcium chloride or propylene glycol with a temperature of -10 ° C and below are used.

Meat intended for long-term storage is frozen using the same methods as freezing. Freezing

with air is carried out at a temperature of cooled air from -30 to -40 °C; when freezing in brine, the temperature of the solution is -25...-28 °C.

Eggs are stored in refrigerators at a temperature of -1...-2 °C and a relative humidity of 85...88%. After cooling to 2...3 °C, they are placed in a storage chamber.

Fruits and vegetables are cooled in stationary storage facilities. Fruit and vegetable products are stored in refrigerated chambers with cooling batteries in which a cold agent or brine circulates.

In air-cooled systems, air is first cooled, which is then forced into storage chambers by fans. In mixed systems, products are cooled with cold air and from a battery.

In agriculture, cold is obtained both machine-free (glaciers, ice-salted cooling) and using special refrigeration machines. In machine refrigeration, heat from the cooled medium is removed to the external environment using low-boiling refrigerants (freon or ammonia).

Steam compressors and absorption refrigeration machines are widely used in agriculture.

The simplest way to obtain a working fluid temperature below ambient temperature is that this working fluid (refrigerant) is compressed in a compressor, then cooled to ambient temperature and then subjected to adiabatic expansion. In this case, the working fluid does work due to its internal energy and its temperature decreases compared to the ambient temperature. Thus, the working fluid becomes a source of cold.

In principle, any steam or gas can be used as refrigerants. In the first mechanically driven refrigeration machines, air was used as a refrigerant, but already from the end of the 19th century. it was replaced by ammonia and carbon dioxide, since the air refrigeration machine is less economical and more cumbersome than the steam one, due to high flow rate air due to its low heat capacity.

In modern refrigeration units, the working fluid is a vapor of liquids that, at pressures close to atmospheric, boil at low temperatures. Examples of such refrigerants include ammonia NH3, sulfur dioxide SO2, carbon dioxide C0 2 and freons - chlorofluorocarbon derivatives of the type C m H x F y Cl2. Boiling point of ammonia at atmospheric pressure is 33.5 °C, “Freona-12” -30 °C, “Freona-22” -42 °C.

Freons are widely used as refrigerants - halogen derivatives of saturated hydrocarbons (C m H n), obtained by replacing hydrogen atoms with chlorine and fluorine atoms. In technology, due to the wide variety of freons and their relatively complex names, a conventional numerical designation system has been established, according to which each such compound, depending on chemical formula has its own number. The first digits in this number conventionally indicate the hydrocarbon of which this freon is a derivative: methane - 1, ethane - 11, propane - 21. If the compound contains unsubstituted hydrogen atoms, then their number is added to these numbers. Next, to the resulting amount or to the original number (if all the hydrogen atoms in the compound are replaced), a figure expressing the number of fluorine atoms is added in the form of the next sign. This is how the designations are obtained: R11 instead of monofluorotrichloromethane CFCI2, R12 instead of difluorodichloromethane CF 2 C1 2, etc.

In refrigeration units, R12 is usually used as a refrigerant, and in the future R22 and R142 will be widely used. The advantages of freons are relative harmlessness, chemical inertness, non-flammability and explosion safety; Disadvantages - low viscosity, which promotes leakage, and the ability to dissolve in oil.

Figure 8.15 shows the circuit diagram steam compressor refrigeration unit and her ideal cycle in the 75 diagram. In the compressor 1 the wet vapor of the refrigerant is compressed, resulting in (section a-b) the result is dry saturated or superheated steam. Usually the degree of overheating does not exceed

130... 140 “C, so as not to complicate the operation of the compressor due to increased mechanical stress and not to use oils

Rice. 8.15.

/ - compressor; 2 - refrigerated room; 3- throttle valve; 4 - special grade capacitor. Superheated steam from the compressor with parameters pi and 02 enters the cooler (condenser 2). In a condenser at constant pressure, superheated steam gives off superheating heat to the cooling water (process b-c) and its temperature becomes equal to the saturation temperature 0 n2. Subsequently releasing the heat of vaporization (the process c-d), saturated steam turns into boiling liquid (point d). This fluid flows to the throttle valve 3, after passing through which it turns into saturated steam with a slight degree of dryness (x 5 = 0.1...0.2).

It is known that the enthalpy of the working fluid before and after throttling is the same, and the pressure and temperature decrease. The 7s diagram shows a dashed line of constant enthalpy d-e, dot e which characterizes the state of steam after throttling.

Next, the wet steam enters a cooled container called a refrigerator 4. Here, at constant pressure and temperature, the steam expands (the process e-a), taking away a certain amount of heat. The degree of dryness of the steam increases (x| = 0.9...0.95). Pair with state parameters characterized by a point 1, is sucked into the compressor, and the operation of the installation is repeated.

In practice, the steam after the throttle valve does not enter the refrigerator, but into the evaporator, where it takes away heat from the brine, which, in turn, takes heat away from the refrigerator. This is explained by the fact that in most cases the refrigeration unit serves a number of cold consumers, and then the non-freezing brine serves as an intermediate coolant, continuously circulating between the evaporator, where it is cooled, and special air coolers in refrigerators. Used as brines aqueous solutions sodium chloride and calcium chloride, having fairly low freezing temperatures. The solutions are suitable for use only at temperatures above those at which they freeze as a homogeneous mixture, forming salt ice (the so-called cryohydrate point). Cryohydrate point for NaCl solution with mass concentration 22.4% corresponds to a temperature of -21.2 °C, and for a CaCl 2 solution with a concentration of 29.9 - a temperature of -55 °C.

An indicator of the energy efficiency of refrigeration units is the refrigeration coefficient e, which is the ratio of the specific refrigeration capacity to the energy consumed.

The actual cycle of a vapor compressor refrigeration unit differs from the theoretical one in that, due to the presence of internal friction losses, compression in the compressor occurs not along an adiabatic path, but along a polytrope. As a result, energy consumption in the compressor is reduced and the refrigeration coefficient is reduced.

To obtain low temperatures (-40...70 °C) required in some technological processes, single-stage steam compressor units turn out to be either uneconomical or completely unsuitable due to a decrease in compressor efficiency caused by high temperatures of the working fluid at the end of the compression process. In such cases, either special refrigeration cycles are used, or in most cases, two-stage or multi-stage compression. For example, two-stage compression of ammonia vapor produces temperatures down to -50 °C, and three-stage compression - up to -70 °C.

Main advantage absorption refrigeration units Compared to compressor engines, they use not electrical, but thermal energy of low and medium potentials to produce cold. The latter can be obtained from water vapor taken, for example, from a turbine in thermal power plants.

Absorption is the phenomenon of absorption of vapor by a liquid substance (absorbent). In this case, the temperature of the steam may be lower than the temperature of the absorbent that absorbs the steam. For the absorption process, it is necessary that the concentration of the absorbed vapor be equal to or greater than the equilibrium concentration of this vapor above the absorbent. Naturally, in absorption refrigeration units, liquid absorbents must absorb the refrigerant at a sufficient speed, and at the same pressures, their boiling point must be significantly higher than the boiling point of the refrigerant.

The most common are water-ammonia absorption plants, in which ammonia serves as a refrigerant and water as an absorbent. Ammonia is highly soluble in water. For example, at 0 °C, up to 1148 volumes of vaporous ammonia are dissolved in one volume of water, and heat of about 1220 kJ/kg is released.

The cold in the absorption unit is produced according to the scheme shown in Figure 8.16. This diagram shows approximate values ​​of the parameters of the working fluid in the installation without taking into account pressure losses in pipelines and losses in temperature pressure in the condenser.

In the generator 1 evaporation of a saturated ammonia solution occurs when it is heated with water steam. As a result, the low-boiling component - ammonia steam with a slight admixture of water vapor - is distilled off. If you maintain the solution temperature at about 20 °C, then the saturation pressure of ammonia vapor will be approximately 0.88 MPa. To prevent the NH 3 content in the solution from decreasing, use a transfer pump 10 from the absorber to the generator a strong concentrated

Rice. 8.16.

/-generator; 2- capacitor; 3 - throttle valve; 4- evaporator; 5-pump; b-bypass valve; 7- refrigerated container; absorber; 9-coil; 10- pump

bath ammonia solution. Saturated ammonia steam (x = 1), produced in the generator, is sent to the condenser 2, where ammonia turns into liquid (x = 0). After throttle 3 ammonia enters the evaporator 4, in this case, its pressure decreases to 0.3 MPa (/n = -10 °C) and the degree of dryness becomes approximately 0.2.„0.3. In the evaporator, the ammonia solution is evaporated due to the heat supplied by the brine from the cooled container 7. In this case, the temperature of the brine decreases from -5 to -8 °C. With pump 5 it is distilled back into container 7, where it is again heated to -5 °C, taking heat from the room and maintaining a constant temperature in it, approximately -2 °C. Ammonia evaporated in the evaporator with a degree of dryness x = 1 enters the absorber 8, where it is absorbed by a weak solution supplied through the bypass valve 6 from the generator. Since absorption is an exothermic reaction, to ensure continuity of the heat exchange process, the absorbent is removed with cooling water. The strong ammonia solution obtained in the absorber pump 10 pumped to the generator.

Thus, in the considered installation there are two devices (generator and evaporator), where heat is supplied to the working fluid from the outside, and two devices (condenser and absorber), in which heat is removed from the working fluid. Comparing the schematic diagrams of steam compressor and absorption plants, it can be noted that the generator in the absorption plant replaces the discharge part, and the absorber replaces the suction part of the piston compressor. Compression of the refrigerant occurs without the expenditure of mechanical energy, except for the small costs of pumping a strong solution from the absorber to the generator.

In practical calculations, the refrigeration coefficient e, which is the ratio of the amount of heat q 2 perceived by the working fluid in the evaporator to the amount of heat q u spent in the generator. The refrigeration coefficient calculated in this way is always less than the refrigeration coefficient of the steam compressor unit. However, a comparative assessment of the energy efficiency of the considered methods for producing cold as a result of a direct comparison of the methods of only the refrigeration coefficients of absorption and steam compressor units is incorrect, since it is determined not only by the quantity, but also by the type of energy expended. The two methods of obtaining cold should be compared based on the value of the reduced coefficient of performance, which is the ratio of the cooling capacity q 2 to fuel heat consumption q it i.e. ? pr = Yag Ya- It turns out that at evaporation temperatures from -15 to -20 °C (used by the majority of consumers), the e-efficiency of absorption units is higher than that of steam compressor units, as a result of which, in some cases, absorption units are more profitable not only when supplying them with steam taken from turbines, but also when supplied with steam directly from steam boilers.

Automation of refrigeration units involves equipping them with automatic devices (instruments and automation equipment), with the help of which they are ensured safe work and conducting production process or individual operations without the direct participation of service personnel or with their partial participation.

Automation objects together with automatic devices form automation systems with various functions: control, alarm, protection, regulation and management. Automation increases the economic efficiency of refrigeration units, as the number of operating personnel is reduced, the consumption of electricity, water and other materials is reduced, and the service life of the units increases due to maintenance by automatic devices. optimal mode their work. Automation requires capital costs, so it must be carried out based on the results of a technical and economic analysis.

The refrigeration unit can be automated partially, completely or comprehensively.

Partial automation provides for mandatory automatic protection for all refrigeration units, as well as monitoring, alarm and often control. Maintenance personnel regulate the basic parameters (temperature and humidity in the chambers, boiling point and condensation temperature of the refrigerant, etc.) when they deviate from the set values ​​and malfunction of the equipment, which is reported by control and alarm systems, and some auxiliary periodic processes ( thawing of frost from the surface of cooling devices, removal of oil from the system) are performed manually.

Full automation covers all processes related to maintaining the required parameters in refrigerated rooms and elements of the refrigeration unit. Maintenance personnel may only be present periodically. They fully automate small-scale refrigeration units, are trouble-free and durable.

For large industrial refrigeration units it is more typical complex automation automatic control, alarm, protection).

Automatic control ensures remote sensing, and sometimes recording parameters that determine the operating mode of the equipment.

Automatic alarm - notification using a sound and light signal about the achievement of specified values, certain parameters, turning on or off the elements of the refrigeration unit. Automatic alarm are divided into technological, preventive and emergency.

Process alarm - light, informs about the operation of compressors and the presence of voltage in electrical circuits.

A warning alarm on protective circulation receivers reports that the value of the monitored parameter is approaching the maximum permissible value.

The alarm system notifies with light and sound signals that the automatic protection has been activated.

Automatic protection ensuring the safety of operating personnel is mandatory for any production. It prevents the occurrence of emergency situations by turning off individual elements or the installation as a whole when the controlled parameter reaches the maximum permissible value.

Reliable protection in case of dangerous situation should be provided by an automatic protection system (APS). In the simplest version, the SAZ consists of a sensor-relay (protection relay), which controls the parameter value and generates a signal when its limit value is reached, and a device that converts the protection relay signal into a stop signal, which is sent to the control system.

In refrigeration units high power The SAZ is designed so that after the protection relay is triggered, automatic start-up of the failed element without eliminating the cause that caused the stop is impossible. In small refrigeration units, for example in retail establishments, where an accident cannot lead to serious consequences, there is no constant maintenance; the facility turns on automatically if the value of the monitored parameter returns to the acceptable range.

Compressors have the greatest number of types of protection, since according to operating experience, 75% of all accidents in refrigeration units occur with them.

The number of parameters controlled by the BAS depends on the type of compressor power and the type of refrigerant.

Type of compressor protection:

From an unacceptable increase in discharge pressure - prevents violation of the tightness of connections or destruction of elements;

An unacceptable decrease in suction pressure - prevents an increase in the load on the compressor seal, foaming of the oil in the crankcase, freezing of the coolant in the evaporator (high and low pressure switches are equipped on almost all compressors);

Reducing the pressure difference (before and after the pump) in the oil system - prevents emergency wear of rubbing parts and jamming of the compressor movement mechanism, the pressure difference relay controls the pressure difference on the discharge and suction side of the oil pump;

Unacceptable increase in discharge temperature - prevents disruption of the cylinder lubrication regime and emergency wear of rubbing parts;

Increasing the temperature of the windings of the built-in electric motor of sealed and sealless refrigerant compressors - prevents overheating of the windings, jamming of the rotor and operation in two phases;

Water hammer (entry of liquid refrigerant into the compression cavity) - prevents a serious failure of the piston compressor: loss of density, and sometimes destruction.

Types of protection for other elements of the refrigeration unit:

• - from freezing of the coolant - prevents rupture of evaporator pipes;
• - overflow of the linear receiver - protects against a decrease in the efficiency of the condenser as a result of filling part of its volume with liquid refrigerant;
• - emptying the linear receiver - prevents gas breakthrough high pressure into the evaporation system and the danger of water hammer.

Prevention of an emergency provides protection against unacceptable concentrations of ammonia in the room, which can cause a fire and explosion. The concentration of ammonia (maximum 1.5 g/m3, or 0.021% by volume) in the air is monitored by a gas analyzer.