Abstract: Automation of the refrigeration plant. Goals of automation of refrigeration plants Automation of refrigeration machines
Modern refrigeration machines and installations cannot be imagined without automation. They provide stable operation, protect against unacceptable operating conditions and extend the life of the entire system.
Refrigeration automation devices include thermostatic valves; regulators of productivity, pressure and oil level; pilot, safety and check valves; pressure and temperature relay; flow switch. This also includes various electrical and electronic devices: controllers, frequency converters, speed controllers, motor protection circuit breakers, timers, and so on. Unfortunately, quite often they try to save money on this critical piece of equipment. Often one has to deal with ignorance of the possibilities and specifics of the use of automation. In this article, we will try to give short review basic mechanical devices and tasks solved with their help.Automation devices
For smooth filling of the evaporator in order to maximize effective use thermostatic valves (TRV) are intended for its heat exchange surface. The filling indicator is the refrigerant superheat - the difference between its temperature at the inlet and outlet of the evaporator. It is on this parameter that the regulation takes place. There is an opinion that the expansion valve maintains the temperature of the cooled medium or the boiling pressure, but this is fundamentally impossible due to the design features of the expansion valve.
expansion valve(scheme 1) consists of a temperature-sensitive system (1) separated from the housing by a membrane; a capillary tube connecting the thermosensitive system with a thermoballoon (2); valve body with seat (3); adjusting spring (4).
The operation of the expansion valve depends on three main parameters: the pressure in the bulb acting on the upper surface of the membrane (P1), the evaporating pressure acting on the lower surface of the membrane (P2), and the pressure of the control spring also acting on the lower surface of the membrane (P3).The regulation is carried out by maintaining a balance between the pressure in the bulb and the sum of the boiling and spring pressures. The spring provides overheating regulation.
An expansion valve is installed in the liquid line between the condenser and evaporator. In it, the working substance is throttled from the condensation pressure to the boiling pressure. According to their design, expansion valves are divided into valves with external and internal pressure equalization; collapsible and non-collapsible. Expansion valves with internal equalization are typically used on small evaporators with low refrigerant pressure drops, such as commercial equipment.
Low-capacity expansion valves are non-separable (with replaceable or fixed throttling insert), and high-capacity expansion valves are dismountable, which makes it possible, if necessary, to replace individual elements, rather than the entire valve.
Condensing pressure regulators for condensers with air-cooled designed to maintain the minimum required operating condensing pressure when the temperature drops environment. They provide the so-called "winter regulation". Figure 2 shows a variant of such a solution for a condenser and receiver installed outdoors.For water-cooled condensers, valves are used that change the water flow depending on the pressure of the refrigerant. These valves allow you to maintain the condensing pressure with high accuracy.
Evaporating pressure regulators are installed in the suction line downstream of the evaporator to maintain the set evaporating pressure at refrigeration systems Oh. In systems with multiple evaporators, the regulator is installed downstream of the evaporator with the highest evaporating pressure.
Crankcase pressure regulators avoid starting and operating the compressor at too high a suction pressure, on the line of which they are installed directly before the compressor.
Such regulators are often used in refrigeration applications with hermetic or semi-hermetic compressors designed to operate at low temperatures.
Capacity regulators that compensate for the decrease in thermal load are used in systems with one compressor that is not equipped with other means of regulation (valve squeezing, frequency converter). They are installed on the bypass line between compressor suction and discharge, avoiding suction pressure drop and frequent compressor start-stops. The advantages of such regulators include simplicity and low cost, but there are a number of restrictions on their use. So, due to a decrease in the speed of the refrigerant in the system, leading to problems with the return of oil to the compressor, it is possible to compensate for the load drop by no more than 50%. Bypassing hot gas into the suction line of a hermetic or semi-hermetic compressor can lead to overheating of the motor windings. In addition, the discharge temperature also increases. Liquid injection from the discharge side may be required to lower the suction temperature, which requires careful selection and tuning of the system to avoid water hammer in the compressor.
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| Demountable TPB Danfoss TE12 |
The switch pressure is normally adjustable. For some models, the differential operation is also configured. Compact relays without the possibility of setting (cartridge pressure switches) are mainly used by large manufacturers of compressor, condensing units and monoblocks.
Differential pressure switches are widely used to protect compressors against a drop in oil pressure in the crankcase. These devices often include a timer that turns off the compressor if the oil pressure is kept below the minimum required for a given time - to properly lubricate the moving parts of the compressor.
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| Cutaway TPB |
In refrigeration, two types of filling of the sensitive element of the thermostat are used - steam and adsorption. Steam-filled thermostats are used in systems where the temperature change is slow (for example, in large-capacity cold stores). In such thermostats, the relay housing must be in a warmer room than the sensing element. Relays with adsorption charging can be used to control where the temperature changes rapidly.
Application of automation
Consider the use of automation devices on the example of a refrigeration system for a small refrigerator, made by the specialists of the Thermocool company using Danfoss automation.
The filling of the evaporator with refrigerant is regulated by means of a TEX 5-3 collapsible expansion valve with external pressure equalization. An electronic controller (not shown in the diagram) is responsible for the temperature in the chamber. solenoid valve EUR 10.
Maintaining the condensing pressure in winter period by means of the KVR condensing pressure regulator, NRD differential valve and NRV non-return valve. A characteristic feature of this technical solution is the installation of a KVR regulator before the condenser. This leads to a certain increase in the cost of the system, since a larger regulator is required compared to the regulator in the liquid line after the condenser. At the same time, it avoids problems with starting the system after a long stop when the condenser and receiver are installed outdoors or in an unheated room. To regulate the condensing pressure during operation of the unit, stepwise control of the condenser fans is used using two high-pressure switches KR 5 with automatic reset.The compressor is controlled by a two-block relay KP 17 W: the low pressure switch turns the compressor on and off in operating mode, the high pressure switch stops it if the operating value is exceeded. As an additional protection against shutdown due to high pressure, the unit is equipped with a manual reset relay KP 5.
This configuration of automation allows, at a relatively low cost of components, to obtain a simple and reliable refrigeration control system that ensures stable maintenance of the specified parameters.
The article was prepared by Sergey Smagin and Sergey Buchin. We thank the company "Thermocool" (www.thermocool.ru) for information support
Automation of refrigeration plants facilitates work, makes it safe, improves and simplifies technological processes. This essential condition technical progress. Automation is carried out to reduce the share of manual labor, maintain stable temperature, humidity, pressure parameters, as well as prevent accidents and increase service life. Since fewer operating personnel are required, the operation of automated units is cheaper.
Automation of refrigeration plants affects the management of individual operations - alarm, control, start-up and shutdown of certain mechanisms. In general, integrated management is carried out - regulation and protection. You can automate almost any process, but it is not always advisable. Steam jet and absorption units are the easiest to automate, since apart from pumps they have no extra moving mechanisms. With large compression models, everything is more complicated. They require constant monitoring and maintenance by qualified personnel, so only partial automation is used. The main elements of the system are a measuring sensor, a control element and a transmission device. All of them are interconnected.
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Types of automation devices There are several ways to automate, greatly simplifying production processes. Both single options and their complex are used.- Control. Special technical automation solutions are responsible for the independent switching on and off of compressors, pumps in accordance with the designated mode or during load fluctuations. Temperature and time relays are installed that respond to changes or track a specific schedule. Regulation. They help to maintain at the right level the main operating parameters - temperature, pressure, humidity. Smooth regulation of productivity allows to maintain a specific temperature of the coolant with a decrease in heat load. The regulation of the refrigerant supply to the evaporator is also applied. This is necessary to ensure the safety of the compressor, increase or decrease productivity. Alarm. Notifies about dangerous changes in performance, modes, malfunctions in the functioning of the system. Protection. Helps to eliminate the possibility of malfunctions, dangerous situations as a result of an unacceptable increase in pressure, temperature, malfunction of some devices. All kinds of sensors, thermometers, pressure gauges and much more are used here.
The main condition for the technical development of any industry is the automation of production processes, i.e. a set of technical measures that completely or partially exclude human participation in a certain stage of the production process.
main goals refrigeration automation are:
- mechanization of the production process;
- precise maintenance of the specified parameters of equipment operation;
- equipment breakdown prevention;
- service life increase refrigeration equipment;
- reducing staff and reducing labor costs;
- ensuring the safety of personnel.
Any operation performed by the machinist of modern refrigeration machines can be automated, but this does not mean that it is necessary to automate all processes. Automation for refrigeration equipment is necessary only in cases where the performance of operations does not require the qualification of the performer at all, or when the performer cannot achieve the necessary accuracy of regulation. It is also necessary to automate all processes that take place in explosive and harmful to human health conditions without fail.
According to the degree of automation, refrigeration equipment can be divided into three groups:
- 1. Refrigeration equipment with manual control– all control functions and refrigeration system control performed by the staff.
- 2. In partially automated refrigeration equipment, some processes are automated, but the equipment must operate with the constant presence of personnel; in such machines, most often the start-up occurs manually, and the stop is automated.
- 3. Fully automated refrigeration equipment does not require the constant presence of maintenance personnel, but does not eliminate the need for periodic inspections and Maintenance according to the established regulations. Basically, steam jet and absorption refrigeration units are fully automated due to the absence of moving mechanisms in them.
Varieties of refrigeration automation systems
An automation system is a combination of an automation object and automatic devices, through which it is possible to manage the work refrigeration systems without the participation of service personnel.
Types of automation systems:
Open-loop systems - rarely used, are divided into types:
- an open-loop automation system with a direct connection, in which tracking is based on an indirect parameter (for example, in ventilation systems based on the outdoor temperature);
- an open-loop automation system with feedback that performs only informational functions (measurement, signaling).
Closed systems, the principle of operation of which is to determine the deviation of the actual value of the regulating parameter from the given one. These automation systems are used forcontrol work refrigeration unit. Types of closed automation systems:
- automatic control systems, i.e. those that support parameters at a given level;
- automatic protection systems, i.e. those that automatically turn off the equipment when its normal operation is disturbed.
The main parts and devices of the refrigeration automation system
Main parts of the system refrigeration plant automation:
- measuring (sensitive) element equipped with a device for refrigeration control settings parameters to a given value;
- a sensor that registers a change in the controlled variable;
- refrigeration control panel, i.e. a regulating body, which, on the signal of the measuring element, changes the supply of a signal or energy to the regulated object;
- a transmission device that connects the sensor to the transmission mechanism.
Control panel of the refrigeration unit and automation devices of the refrigeration unit
The main element that controls the devices of refrigeration automation systems is refrigeration control panel. The control panel contains automatic control, regulation and protection devices, as well as signaling means, which ensure the normal functioning of the refrigeration system.
Automatic control devices located on refrigeration unit control panel, regulate the operation of pumps and compressors when the load changes. When the temperature of the refrigerant drops, as well as when the pressure in the evaporators drops below the limit value, the compressors automatically stop; when the temperature in the evaporator rises, the compressors automatically turn on. Sometimes a time relay is used for automatic control of compressors, which is programmed for a certain time when the units are turned on.
With the help of automatic control devices on the control panel, the key parameters of the refrigeration unit operation - temperature and pressure - are maintained at an optimal level. When the heat load decreases, the temperature of the coolant is maintained at a given level due to the smooth automatic control of the cooling capacity of the unit, which can be carried out in the following ways:
- 1) throttling of refrigerant vapors in front of the compressor, as a result of which the pressure decreases;
- 2) bypassing part of the vapor from the discharge line to the suction line;
- 3) an increase in dead space in the reciprocating compressor, as a result of which the suction of refrigerant vapor from the evaporator is reduced.
Automatic control devices that change the flow of refrigerant to the evaporator also ensure the safe operation of the compressor and its protection against water hammer.
Automatic alarm is used to notify the operator of the refrigeration plant about a change in the operating mode of the equipment, which can cause the operation of automatic protection. Also, automatic signaling with a sound signal notifies the operator about turning on and off the equipment, fittings and devices.
Automatic protection of refrigeration equipment allows you to avoid the dangerous consequences of a violation of the normal parameters of the operation of refrigeration machines. In case of sudden changes in the functioning parameters (strong increase in discharge pressure, decrease in pressure and evaporation temperature, non-compliance with the operating mode of the lubrication system, refrigeration system check and other situations) specially designed devices turn off refrigeration units, preventing their breakdown.
FROM DANGEROUS MODES
In the process of operation of refrigeration machines and installations, due to failures of individual components or assemblies, as well as due to violations in power and water supply systems, dangerous modes may occur: an increase in pressure and temperature, a liquid level in individual devices or machine components, the termination of lubrication of rubbing steam, lack of cooling water, etc. If timely measures are not taken, compressors, heat exchangers or other elements of the installation may be damaged or destroyed. In this case, there is a serious danger to the health and life of the operating personnel.
The protection of refrigeration machines and installations includes a whole range of technical and organizational measures that ensure their safe operation. This chapter will consider only those that are performed on the basis of automatic instruments and devices.
WAYS OF PROTECTION
The methods of protection include stopping the machine or the entire installation, turning on emergency devices, releasing the working substance into the atmosphere or bypassing it to other devices.
Stopping the machine or the entire plant. This method is carried out using an automatic protection system (SAZ), 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 passed to the automatic control circuit.
The protection relays perceive the controlled technological values 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 SAS is determined by the minimum required number of controlled values.
Wiring diagram It is carried out in one of three options, according to which the SAZ are single-acting, with repeated switching on and combined.
SAZ single action stops the machine or installation when any protection relay is activated and makes it impossible to start automatically until the intervention of the operating personnel. This type of SAZ is distributed mainly on large and medium-sized machines. If the installation is operating without continuous maintenance and the equipment does not have an automatically switched on reserve, then the SAS is supplemented with a special alarm system for emergency personnel calls.
SAZ with reclosing stops the machine when the protection relay is activated and does not prevent it from automatically turning on when the relay returns to normal. It is mainly used in small installations. commercial type, where they strive to simplify the automation scheme.
In combined SAZ part of the protection relays that control the most dangerous parameters is included in the single-action electrical circuit, and the part with less dangerous parameters is included in the re-closing circuit. This allows, without resorting to the help of personnel, to automatically start the machine again, if this is not associated with a risk of an accident.
In practice, there is also a kind of protection called blocking. Its difference lies in the fact that the signal is received not from the protection relay, but from an element of the control circuit or control of another unit or unit of the installation (for example, a pump, a fan, etc.). The blocking excludes the start or operation of the machine if the specified start order of the controlled units is not followed. Typically, blocking is performed according to a reclosing scheme.
Switching on emergency devices. This method is also carried out by SAZ.
Emergency devices include:
Warning alarm about dangerous modes, which is used in especially large installations with continuous service to avoid stopping the machine as much as possible;
Emergency signaling informing the personnel about the protection operation, as well as deciphering the specific cause of the emergency operation;
Emergency ventilation, which is switched on when the local or general concentration of explosive and flammable, as well as toxic working substances (for example, ammonia) in the air increases.
The release of the working substance into the atmosphere or bypass to other devices. This method is carried out by special safety devices (safety valves, safety plates, fusible plugs, etc.) that are not included in the SAS. Their purpose is to prevent the destruction or explosion of vessels and apparatus when the pressure rises as a result of a malfunction of the installation, as well as in the event of a fire. The choice of safety devices and the rules for their use are determined normative documents in accordance with the rules for the safety and operation of pressure vessels.
BUILDING PROTECTION SYSTEMS
Protection systems differ depending on the type of refrigeration unit, its size, the accepted method of operation, etc. When building all SAS, it is necessary to take into account the general principles that ensure the greatest possible safety of work. As an example, a schematic diagram of the CAS of a compression refrigeration unit is considered, consisting of a compressor Km with an electric motor D, heat exchangers TA and auxiliary devices VU - pumps, fans, etc. (Fig. 7.1). The scheme is presented in general view without indications of specific quantities and parameters subjected 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 allowable value.
SAZ has ten protection channels. Channels 1-8 operate from the corresponding protection relays that perceive technological parameters. Channels 9 and 10 provide blocking of the compressor and auxiliaries.
The system includes a key with which, if necessary (during trials and running-in), you can turn off part of the protective relays and blocking circuits (2, 3, 5, 6, 8, 9, 10). Those protections that must function in any operating mode of the installation are not subject to deactivation.
The electrical circuit of the SAZ consists of two parts. The first part, which includes channels 2, 5, 9 and 10, works according to the re-closing method, and the second, with the rest of the channels, provides protection that works on the principle of a single action 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 for putting the protection into operation.
Signals from the electrical circuit of the SAZ are fed into the automatic control circuit of the AC. These signals stop the compressor motor irrespective of the OS operational control signals.
In addition to the main function of the SAS - an emergency stop of the compressor, it also performs auxiliary operations: switching on the necessary emergency devices, as well as light and sound alarms. The decoding signaling of the reclosing protections is only active until the controlled parameter has entered the normal limits. The one-shot protection alarm remains on after a trip until the commissioning button is pressed, regardless of the actual state of the controlled parameter. Such a scheme, as it were, “remembers” the protection operation that has occurred and informs the personnel for an unlimited time.
The presented scheme can be considered only as an example of the construction of the SAZ. Specific systems may differ from it in the number of channels and how they are turned on.
The main requirement for SAZ is high reliability, which is achieved by using highly reliable protection relays and electrical circuit elements, redundant relays and other protection elements in especially critical cases, reducing the number of elements connected in series in the SAS, using the safest electrical circuit options, organizing preventive checks and repairs during operation.
The use of highly reliable protection relays and elements of electrical circuits 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, the relay and other elements of the SAS have a very small cyclic operating time (a small number of operations). Therefore, when evaluating reliability, one should take into account not cyclic durability and cyclic time between failures, but other indicators characterizing the ability of elements to maintain readiness for operation (for example, time between failures). In this case, any violation of the ability of the element to operate is taken as a failure.
Redundancy is a parallel connection of two or more homogeneous and jointly working elements that perform the same functions. The failure of one of them does not affect the performance of the system as a whole. Redundancy is used in especially dangerous cases, when a sudden failure of the ACS can lead to serious consequences. Such cases include, for example, protection against the ingress of liquid ammonia into a reciprocating compressor. To do this, the main and backup level switches are installed on the vessels in front of the compressor.
The simplified diagram (Fig. 7.2) shows the 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 ejected from the evaporator, it accumulates in the liquid ammonia separator, and if its level reaches allowable limit, the protection relays RZ 1 and RZ 2 are activated (the diagram shows their primary converters). Both relays are constantly on and perform the same function. This redundancy greatly improves reliability, since the probability of both relays failing at the same time is extremely low.
Reducing the number of elements connected in series in the ACS is one of the ways to improve the reliability of the electrical circuits of the ACS. The most reliable system is in which the protection relays are connected directly to the compressor motor starter without intermediate elements. However, this scheme is used only on the smallest installations. In larger installations, intermediate relays must be used, which reduces reliability. Therefore, the number of intermediate elements in series included in the compressor emergency shutdown circuit should be kept to a minimum.
Rice. 7.2. Simplified diagram of a liquid separator with protection relay redundancy
from the wet running of the compressor
When using the safest electrical circuits, it is ensured that the compressor stops in the event of failures in the SAS. The most characteristic failure of an electrical circuit is a break (disappearance of voltage or current), which can occur when there is a physical break in wires, burning of contacts, failure of radio-electronic elements (diodes, transistors, resistors, etc.), disturbances in the operation of power supplies. In order for these failures to be signaled as emergency, it is necessary that current circulate in the protection circuits in the normal state, and the emergency stop signal should correspond to its termination. Therefore, the most secure is the electrical protection circuit on normally closed contacts or other elements.
So, in the circuit (Fig. 7.3), the contacts of the protection relay RZ 1, RZ 2 and RZ 3 are closed if the controlled values are within normal limits, and open when the maximum allowable values are reached. These contacts are connected in series to the winding circuit of the RA electromagnetic relay, 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 on 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, a current will flow through the winding of the electromagnetic relay, this relay will work and close its contact RA. When the button is released, the circuit remains energized. It is enough for one of the protection relays to open the contact, as the electromagnetic relay will release and its contact will open. Reactivation will be possible only after pressing the button. This is a one-shot scheme. In the reclosing circuit, the PA contact and button are not required.
The organization of preventive checks and repairs during operation plays a decisive role in ensuring the safe operation of installations. These measures, if carried out at the necessary intervals, practically eliminate dangerous situations associated with sudden failures in saz.
For the organization of preventive checks, it is necessary that the SAZ be equipped with devices and devices that allow, if possible, to fully check the performance of the protections. At the same time, it is desirable that the check does not cause the output of the installation beyond the maximum permissible modes. So, in the circuit (see Fig. 7.2), you can check the operation of the protection relay without filling the liquid separator.
During normal operation, valves B1 and B2 are open and valve B3 is closed. The primary converters of the protection relay 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, then they, when triggered, give the appropriate signals.
After that, 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 should be in place, the frequency of which should be selected taking into account actual reliability indicators.
COMPOSITION OF SAZ
The number of parameters controlled by the SAS depends on the type of equipment, its dimensions and performance, the type of refrigerant, etc. Usually, the number of protections increases with the size of the equipment. More complex ACS are usually used in ammonia plants.
In table. 7.1 shows the recommended list of controlled parameters for the most common types of refrigeration equipment. For some types of equipment, several options for a set of protections are offered, which are selected based on specific conditions. So, for hermetic compressors, two options can be used. The variant with built-in motor winding temperature protection devices is preferred, as the same number of devices provides protection against more faults.
In table. 7.1 does not include compressors for household refrigerators and air conditioners.
Some of the protections that are part of the SAS do not have to be included in the single action circuit; if necessary, it is allowed to include them in the reactivation circuit.
On especially 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 the parameter does not fall within the normal limits after a predetermined period of time. Parameters that allow activation through a warning signal are also noted in Table. 7.1. At the same time, attention should be paid to the reliability of the time delay device and, if necessary, take appropriate measures, such as redundancy.
Table 7.1
| Equipment | Pressure | Temperature | Liquid level | Axial shaft shift | Application area | |||||||||
| boiling point (temperature) | suction | injection | injection | oils | gear oil | motor windings | bearings | outgoing coolant | ||||||
| Hermetic piston compressor | +* +* | +* +* +* +* | + | Freon compressors for small refrigeration units (commercial equipment, air conditioners, etc.) The same » | ||||||||||
| Sealless piston compressor | + + + + + +* | + + + + + +* | + + + + | + | + + | Medium capacity refrigerant compressors The same Large capacity refrigerant compressors The same Refrigerant compressors for small refrigeration units | ||||||||
| Piston open compressor | + + | + + | + + | + | Freon and ammonia compressors of medium capacity The same, large capacity | |||||||||
The end of the table. 7.1
| Equipment | Pressure | Pressure drop in the oil system | Temperature | Liquid level | Axial shaft shift | Application area | |||||||
| boiling point (temperature) | suction | injection | injection | oils | gear oil | motor windings | bearings | outgoing coolant | |||||
| Screw compressor unit | +** | + | + | +** | |||||||||
| Centrifugal compressor unit | +** | + | + | +** | +** | +** | +** | + | Ammonia and freon units | ||||
| Ammonia shell and tube evaporator | +*** | No limit | |||||||||||
| Freon evaporator with shell-to-tube boiling | +*** | Also | |||||||||||
| Freon evaporator with intratube boiling | +*** | » | |||||||||||
| Liquid separator, circulation receiver | + | » |
Note. An asterisk (*) means that protection is provided:
* Switching on according to the scheme with repeated switching on is allowed.
** It is allowed to stop the compressor after the warning alarm is activated.
*** Activation via warning signaling is allowed.
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