One of the most important elements for a vapor compression machine is. It runs the main process refrigeration cycle- selection from the medium to be cooled. Other elements of the refrigeration circuit, such as a condenser, an expansion device, a compressor, etc., only ensure reliable operation of the evaporator, therefore, proper attention must be paid to the choice of the latter.
It follows from this that, when choosing equipment for a refrigeration unit, it is necessary to start with the evaporator. Many novice repairmen often assume a typical mistake and begin to complete the installation with a compressor.
In fig. 1 shows a diagram of the most common vapor compression refrigeration machine. Its cycle, given in coordinates: pressure R and i... In fig. 1b points 1-7 of the refrigeration cycle is an indicator of the state of the refrigerant (pressure, temperature, specific volume) and coincides with that in Fig. 1a (functions of state parameters).
Rice. 1 - Scheme and coordinates of a conventional vapor compression machine: RU expansion device, Pk- condensation pressure, Ro- boiling pressure.
Graphical representation of fig. 1b displays the state and functions of the refrigerant, which change depending on pressure and enthalpy. Section AB on the curve in Fig. 1b characterizes the refrigerant in the saturated vapor state. Its temperature corresponds to the initial boiling point. The proportion of refrigerant vapor in is 100%, and the superheat is close to zero. To the right of the curve AB the refrigerant is in a state (the refrigerant temperature is higher than the evaporating temperature).
Dot V is critical for a given refrigerant, since it corresponds to the temperature at which the substance cannot go into a liquid state, no matter how high the pressure is. On the BC segment, the refrigerant has a saturated liquid state, and on the left side it is a supercooled liquid (the refrigerant temperature is less than the boiling point).
Inside the curve ABC the refrigerant is in the state of a vapor-liquid mixture (the proportion of vapor per unit volume is variable). The process taking place in the evaporator (Fig.1b) corresponds to the segment 6-1 ... The refrigerant enters the evaporator (point 6) in the state of a boiling vapor-liquid mixture. In this case, the proportion of steam depends on a certain refrigeration cycle and is 10-30%.
At the exit from the evaporator, the boiling process may not be completed and the point 1 may not match point 7 ... If the temperature of the refrigerant leaving the evaporator is higher than the boiling point, then the evaporator is overheated. Its magnitude ΔT overheating is the difference between the temperature of the refrigerant at the outlet of the evaporator (point 1) and its temperature on the AB saturation line (point 7):
ΔT overheating = T1 - T7
If point 1 and 7 coincide, then the temperature of the refrigerant is equal to the boiling point, and the superheat ΔT overheating will be zero. Thus, we get a flooded evaporator. Therefore, when choosing an evaporator, you must first make a choice between a flooded evaporator and a superheated evaporator.
Note that, under equal conditions, the flooded evaporator is more advantageous in terms of the intensity of the heat extraction process than with overheating. But it should be borne in mind that at the outlet of the flooded evaporator, the refrigerant is in a state of saturated vapor, and it is impossible to supply a humid environment to the compressor. Otherwise, there is a high likelihood of water hammering, which will be accompanied by mechanical destruction of compressor parts. It turns out that if you choose a flooded evaporator, then it is necessary to provide additional protection compressor from saturated steam entering it.
If you prefer an overheated evaporator, you do not need to worry about protecting the compressor and the ingress of saturated steam into it. The likelihood of water hammer will arise only in the event of a deviation from the required value of the overheating value. Under normal operating conditions of the refrigeration unit, the overheating value ΔT overheating should be in the range of 4-7 K.
With a decrease in the overheating indicator ΔT overheating, intensity of heat extraction environment rising. But at extremely low values ΔT overheating(less than 3K) there is a possibility of wet steam entering the compressor, which can cause a water hammer and, consequently, damage the mechanical components of the compressor.
Otherwise, with a high reading ΔT overheating(more than 10 K), this indicates that an insufficient amount of refrigerant enters the evaporator. The intensity of heat extraction from the medium to be cooled sharply decreases and the thermal regime of the compressor worsens.
When choosing an evaporator, another question arises related to the value of the boiling point of the refrigerant in the evaporator. To solve it, you first need to determine what temperature of the medium to be cooled should be provided for the normal operation of the refrigeration unit. If air is used as the medium to be cooled, then in addition to the temperature at the outlet from the evaporator, it is necessary to take into account the humidity at the outlet of the evaporator. Let us now consider the behavior of the temperatures of the medium to be cooled around the evaporator during the operation of a conventional refrigeration unit (Fig. 1a).
In order not to delve into this topic, the pressure loss across the evaporator will be neglected. We will also assume that the heat exchange occurring between the refrigerant and the environment is carried out according to the direct flow scheme.
In practice, such a scheme is not often used, since in terms of heat transfer efficiency it is inferior to the counterflow scheme. But if one of the coolants has a constant temperature, and the overheating readings are small, then the forward flow and counterflow will be equivalent. It is known that the average value of the temperature head does not depend on the flow pattern. Consideration of the once-through scheme will provide us with a more visual representation of the heat exchange that occurs between the refrigerant and the medium to be cooled.
First, we introduce the virtual value L equal to the length of the heat exchanger (condenser or evaporator). Its meaning can be determined from the following expression: L = W / S, where W- corresponds to the internal volume of the heat exchanger in which the refrigerant is circulated, m3; S- heat exchange surface area m2.
If we are talking about a refrigerating machine, then the equivalent length of the evaporator is practically equal to the length of the tube in which the process takes place 6-1 ... Therefore, its outer surface is washed by the medium to be cooled.
First, let's pay attention to the evaporator, which acts as an air cooler. In it, the process of extracting heat from the air occurs as a result of natural convection or with the help of forced blowing of the evaporator. Note that in modern refrigeration plants the first method is practically not used, since air cooling by natural convection is ineffective.
Thus, we will assume that the air cooler is equipped with a fan that provides forced air blowing of the evaporator and is a tubular-finned heat exchanger (Fig. 2). Its schematic representation is shown in Fig. 2b. Consider the main values that characterize the blowing process.
ΔТ = Ta1-Ta2,
where ΔTa is in the range from 2 to 8 K (for tubular-finned evaporators with forced blowing).
In other words, during normal operation of the refrigeration unit, the air passing through the evaporator must be cooled not lower than 2 K and not higher than 8 K.
Rice. 2 - Scheme and temperature parameters of air cooling on the air cooler:
Ta1 and Ta2- air temperature at the inlet and outlet of the air cooler;
DTmax = Ta1 - To
This indicator is used in the selection of air coolers, since foreign manufacturers of refrigeration equipment provide values for the cooling capacity of evaporators Qsp depending on the value DTmax... Consider the method of selecting an air cooler for a refrigeration unit and determine the calculated values DTmax... To do this, we will give as an example the generally accepted recommendations for the selection of the value DTmax:
F = ΔToverload / DTmax = (T1-T0) / (Ta1-T0),
where T1- temperature of the refrigerant vapor at the outlet of the evaporator.
This indicator is practically not used in our country, but in foreign catalogs it is stipulated that the indications of the refrigerating capacity of air coolers Qsp corresponds to the value F = 0.65.
During operation, the value F it is customary to take from 0 to 1. Suppose that F = 0, then ΔТoverload = 0 and the refrigerant leaving the evaporator will be in a saturated vapor state. For this model of air cooler, the actual refrigerating capacity will be 10-15% more than the indicator given in the catalog.
If F> 0.65, then the refrigerating capacity index for this model of the air cooler must be less than the value given in the catalog. Let us assume that F> 0.8, then the actual performance for this model will be 25-30% higher than the value given in the catalog.
If F-> 1, then the refrigerating capacity of the evaporator Qtest-> 0(fig. 3).
Fig. 3 - dependence of the evaporator cooling capacity Qsp from overheating F
The process shown in Fig. 2b is also characterized by other parameters:
Rice. 4 - Diagram and temperature parameters showing the process of cooling water on the evaporator:
where Te1 and Te2 water temperature at the inlet and outlet of the evaporator;
If the temperature difference in water ΔTe = Te1-Te2, then for shell-and-tube evaporators ΔTe should be maintained in the range of 5 ± 1 K, and for plate evaporators the value ΔTe will be in the range of 5 ± 1.5 K.
Unlike air coolers in liquid coolers, it is necessary to maintain not the maximum, but the minimum temperature head DTmin = Te2-To- the difference between the temperature of the medium to be cooled at the outlet of the evaporator and the boiling point of the refrigerant in the evaporator.
For shell and tube evaporators, the minimum temperature head is DTmin = Te2-To should be maintained within 4-6 K, and for plate evaporators - 3-5 K.
The set range (the difference between the temperature of the cooled medium at the outlet of the evaporator and the boiling point of the refrigerant in the evaporator) must be maintained for the following reasons: with an increase in the difference, the cooling intensity begins to decrease, and with a decrease, the risk of freezing of the cooled liquid in the evaporator increases, which can cause its mechanical destruction.
As you know, the cooling of a certain medium is possible with the help of a heat exchanger. Its design solution should be selected according to the technological requirements that apply to these devices. Especially important point is the compliance of the device with the technological process of heat treatment of the medium, which is possible under the following conditions:
The ease of use and reliability of the device is influenced by such factors as the strength and tightness of detachable joints, compensation of temperature deformations, convenience for maintenance and repair of the device. These requirements form the basis for the design and selection of a heat exchange unit. The main role in this is played by the provision of the required technological process in cold-consuming production.
In order to choose the correct design solution for the evaporator, the following rules must be followed. 1) the cooling of liquids is best done using tubular heat exchanger rigid or compact plate heat exchanger; 2) the use of tubular-finned devices is due to the following conditions: the heat transfer between the working media and the wall on both sides of the heating surface is significantly different. In this case, the fins must be installed from the side of the lowest heat transfer coefficient.
To increase the intensity of heat exchange in heat exchangers, it is necessary to adhere to the following rules:
Improving heat transfer processes is one of the main processes for improving heat exchange equipment refrigeration machines... In this regard, research is being carried out in the fields of energy and chemical engineering. This is the study of the regime characteristics of the flow, turbulization of the flow by creating artificial roughness. In addition, new heat transfer surfaces are being developed to make the heat exchangers more compact.
To make a thermal calculation of a surface heat exchanger, it is necessary to solve the equation and heat balance, taking into account certain operating conditions of the device (structural dimensions of heat transfer surfaces, limits of temperature change and circuits, relative to the movement of the cooling and cooled medium). To find a solution to this problem, you need to apply rules that will allow you to get results from the original data. But due to numerous factors, it is impossible to find a common solution for different heat exchangers. Along with this, there are many methods of approximate calculation that are easy to perform in a manual or machine version.
Modern technologies make it possible to select an evaporator using special programs. Basically, they are provided by manufacturers of heat exchange equipment and allow you to quickly select the required model. When using these programs, it must be borne in mind that they assume the operation of the evaporator under standard conditions. If actual conditions differ from standard conditions, the evaporator performance will be different. Thus, it is advisable to always carry out verification calculations of your chosen design of the evaporator, in relation to the actual conditions of its operation.
In the evaporator, the process of transition of the refrigerant from the liquid phase state to the gaseous one with the same pressure takes place, the pressure inside the evaporator is the same everywhere. In the process of transition of a substance from liquid to gaseous (its boiling off) in the evaporator - the evaporator absorbs heat, in contrast to the condenser, which releases heat into the environment. then. by means of two heat exchangers, the process of heat exchange takes place between two substances: the cooled substance, which is located around the evaporator and the outside air, which is around the condenser.
The scheme of movement of liquid freon
Solenoid valve - shuts off or opens the refrigerant supply to the evaporator, always either completely open or completely closed (may not be present in the system)
The thermostatic expansion valve (TRV) is a precision instrument that regulates the supply of refrigerant to the evaporator according to the boiling rate of the refrigerant in the evaporator. It prevents liquid refrigerant from entering the compressor.
Liquid freon enters the expansion valve, the refrigerant throttles through the membrane in the expansion valve (freon is sprayed) and begins to boil due to the pressure drop, gradually the drops turn into gas throughout the entire section of the evaporator pipeline. Starting from the expansion valve, the pressure remains constant. Freon continues to boil and in a certain area of the evaporator completely turns into gas and then, passing through the evaporator, the gas begins to be heated by the air that is in the chamber.
If, for example, the boiling point of freon is -10 ° C, the temperature in the chamber is +2 ° C, freon, having turned into a gas in the evaporator, begins to heat up and at the outlet of the evaporator its temperature should be equal to -3, -4 ° C, thus Δt ( the difference between the boiling point of the refrigerant and the temperature of the gas at the outlet of the evaporator) should be = 7-8, this is the normal operation of the system. With a given Δt, we will know that there will be no particles of non-boiled freon at the exit from the evaporator (they should not be), if boiling occurs in the pipe, then not all the power is used to cool the substance. The pipe is insulated so that the freon does not heat up to ambient temperature, because The refrigerant gas cools the compressor stator. If, nevertheless, liquid freon gets into the pipe, it means that the dose of its supply to the system is too large, or the evaporator is weak (short).
If Δt is less than 7, then the evaporator is filled with freon, it does not have time to boil off and the system does not work correctly, the compressor is also filled with liquid freon and fails. Overheating upward is not so dangerous than overheating downward; at Δt ˃ 7, overheating of the compressor stator may occur, but a slight excess of overheating may not be felt by the compressor in any way and it is preferable during operation.
With the help of fans located in the air cooler, the cold is removed from the evaporator. If this did not happen, then the tubes would be covered with ice and at the same time the refrigerant would reach its saturation temperature, at which it ceases to boil, and then, even regardless of the pressure drop, liquid freon would enter the evaporator without evaporating, filling the compressor.
In order to increase the safety of operation of the refrigeration unit, condensers, linear receivers and oil separators (devices high pressure) with a large amount of refrigerant, place outside the engine room.
This equipment, as well as receivers for storing the refrigerant stock, must be fenced off with a metal barrier with a lockable entrance. Receivers should be protected from sunlight and precipitation by a canopy. Apparatus and vessels installed in the room can be located in the compressor shop or in a special control room, if it has a separate exit to the outside. The passage between the smooth wall and the apparatus must be at least 0.8 m, but it is allowed to install the apparatus near the walls without passages. The distance between the protruding parts of the apparatus must be at least 1.0 m, and if this passage is the main one - 1.5 m.
When mounting vessels and apparatus on brackets or cantilever beams, the latter must be embedded in the main wall to a depth of at least 250 mm.
Installation of devices on columns using clamps is allowed. Do not punch holes in columns to support equipment.
For installation of devices and further maintenance of condensers and circulation receivers, metal platforms with a fence and a ladder are arranged. If the length of the site is more than 6 m, there should be two ladders.
Platforms and stairs should have handrails and edges. The height of the handrails is 1 m, the edges are not less than 0.15 m. The distance between the posts of the handrails is not more than 2 m.
Tests of apparatus, vessels and pipeline systems for strength and density are carried out at the end of the installation works and within the timeframes stipulated by the "Rules for the device and safe operation ammonia refrigeration units ".
Horizontal cylindrical apparatuses. Shell and tube evaporators, horizontal shell and tube condensers and horizontal receivers are installed on concrete foundations in the form of separate pedestals strictly horizontally with a permissible slope of 0.5 mm per 1 m of running length towards the oil sump.
The devices rest on wooden antiseptic beams with a width of at least 200 mm with a recess in the shape of the body (Fig. 10 and 11) and are attached to the foundation with steel belts with rubber gaskets.
Low-temperature devices are installed on beams with a thickness not less than the thickness of the thermal insulation, and under
belts place wooden blocks with a length of 50-100 mm and a height equal to the thickness of the insulation, at a distance of 250-300 mm from each other along the circumference (Fig. 11).
To clean the condenser and evaporator pipes from contamination, the distance between their end caps and the walls should be 0.8 m on one side and 1.5-2.0 m on the other. When installing devices in a room for replacing condenser and evaporator pipes, a "false window" is arranged (in the wall opposite the cover of the device). To do this, an opening is left in the masonry of the building, which is filled thermal insulation material, sewn up with boards and plaster. When repairing devices, the "false window" is opened, and after the repair is completed, it is restored. At the end of the work on the placement of devices, automation and control devices, shut-off valves, safety valves are mounted on them.
The cavity of the apparatus for the refrigerant is blown out with compressed air, the strength and tightness test is performed with the covers removed. When installing the condenser-receiver unit, the horizontal shell-and-tube condenser is installed on the site above the linear receiver. The size of the platform should provide circular service of the apparatus.
Vertical shell-and-tube condensers. The devices are installed outdoors on a massive foundation with a pit for draining water. In the manufacture of the foundation, the bolts of the lower flange of the apparatus are laid in the concrete. The condenser is installed with a crane on the packs of shims and wedges. By tamping wedges, the apparatus is set strictly vertically using plumb lines located in two mutually perpendicular planes. In order to exclude the swaying of the plumb lines by the wind, their weights are lowered into a container with water or oil. The vertical position of the apparatus is caused by the helical flow of water through its tubes. Even with a slight tilt of the apparatus, water will not normally wash over the surface of the pipes. At the end of the alignment of the apparatus, the linings and wedges are welded into packages and the foundation is poured.
Evaporative condensers. They are delivered assembled for installation and installed on a site, the dimensions of which allow for circular maintenance of these devices. ‘The height of the platform is taken taking into account the placement of linear receivers under it. For ease of maintenance, the platform is equipped with a ladder, and with the upper arrangement of the fans, it is additionally installed between the platform and the upper plane of the apparatus.
After installing the evaporative condenser, connect to it circulation pump and pipelines.
The most widespread are evaporative condensers of the TVKA and Evaco type, manufactured by BHR. The drop-baffle layer of these devices is made of plastic, therefore, welding and other work with an open flame should be prohibited in the area of installation of the devices. The fan motors are grounded. When installing the device on an elevated position (for example, on the roof of a building), it is necessary to use lightning protection.
Panel evaporators. They are supplied as separate units and are assembled during assembly work.
The evaporator tank is tested for tightness by bulk water and is installed on concrete slab 300-400 mm thick (Fig. 12), the height of the underground part of which is 100-150 mm. Antiseptic wooden beams or railway sleepers and thermal insulation are placed between the foundation and the tank. Panel sections are installed in the tank strictly horizontally, in accordance with the level. The side surfaces of the tank are insulated and plastered, and the mixer is adjusted.
Chamber instruments. Wall and ceiling batteries are assembled from unified sections (Fig. 13) at the installation site.
For ammonia batteries, sections of pipes with a diameter of 38X2.5 mm are used, for a coolant - with a diameter of 38X3 mm. Pipes are ribbed with spirally wound ribs made of 1X45 mm steel tape with rib spacing of 20 and 30 mm. The characteristics of the sections are presented in table. 6.
The total length of the battery hoses in pumping circuits should not exceed 100-200 m. The battery is installed in the chamber using embedded parts fixed in the ceiling during the construction of the building (Fig. 14).
The battery hoses are placed horizontally in level.
Ceiling unit coolers are supplied complete for installation. Bearing structures devices (channels) are connected to channels of embedded parts. The horizontal position of the apparatus is checked by the hydrostatic level.
Batteries and air coolers are lifted to the installation site by forklifts or other lifting devices. The permissible slope of the hoses should not exceed 0.5 mm per 1 m of running length.
To remove melt water during thawing, drain pipes are installed on which heating elements of the ENGL-180 type are fixed. The heating element is a glass strand tape based on metal heating conductors made of a high resistivity alloy. Heating elements are wound onto the pipeline spirally or laid linearly, fixing on the pipeline with glass tape (for example, tape LES-0.2X20). In the vertical section of the drain pipe, the heaters are installed only in a spiral manner. With linear laying, the heaters are fixed on the pipeline with glass tape with a step of not more than 0.5 m. After fixing the heaters, the pipeline is insulated with non-combustible insulation and sheathed with a protective metal sheath. In places of significant bends of the heater (for example, on flanges), an aluminum tape 0.2-1.0 mm thick and 40-80 mm wide must be placed under it to avoid local overheating.
At the end of the installation, all devices are tested for strength and tightness.
→ Installation of refrigeration units
Installation of main apparatus and auxiliary equipment
The installation technology is determined by the degree of factory readiness and design features of the apparatus, their weight and the installation design. First, the main devices are installed, which allows you to start laying pipelines. To prevent wetting of the thermal insulation on the supporting surface of the devices operating under low temperatures, apply a layer of waterproofing, lay a heat-insulating layer, and then again a layer of waterproofing. To create conditions that exclude the formation of thermal bridges, all metal parts (fastening belts) are placed on the apparatus through antiseptic wooden bars or gaskets 100-250 mm thick.
Heat exchangers. Most of the heat exchangers are supplied by factories ready for installation. So, shell-and-tube condensers, evaporators, subcoolers are supplied assembled, element, irrigation, evaporative condensers and panel, submersible evaporators are supplied as assembly units. Finned tube evaporators, direct expansion coils and brine can be fabricated on site by the installer from finned tube sections.
Shell-and-tube devices (as well as tank equipment) are mounted in a flow-combined manner. When laying welded machines on supports, make sure that all welded seams are accessible for inspection, tapping with a hammer during inspection, as well as for repair.
The horizontality and verticality of the apparatus are checked by level and plumb line or with the help of geodetic instruments. The permissible deviations of the apparatus from the vertical are 0.2 mm, horizontally - 0.5 mm per 1 m. The verticality of shell-and-tube vertical condensers is especially carefully checked, since it is necessary to ensure a film flow of water along the walls of the pipes.
Elemental capacitors (due to their high metal content, they are rarely used in industrial plants) are set to metal frame, above the receiver, along the elements from bottom to top, adjusting the horizontality of the elements, one-plane flanges of the fittings and the verticality of each section.
The installation of irrigation and evaporative condensers consists in the sequential installation of the sump, heat exchange pipes or coils, fans, oil separator, pump and fittings.
Apparatus with air cooled used as condensers in refrigeration units are mounted on a plinth. To center the axial fan relative to the guide vane, there are slots in the plate, which allow the gear plate to be moved in two directions. The fan motor is aligned to the gearbox.
Panel brine evaporators are placed on insulating layer, on a concrete cushion. The metal tank of the evaporator is installed on wooden beams, mount the mixer and brine valves, connect the drain pipe and test the tank for density by pouring water. The water level should not drop during the day. Then the water is drained, the bars are removed and the tank is lowered onto the base. Before installation, panel sections are tested with air at a pressure of 1.2 MPa. Then, one by one, the sections in the tank are mounted, collectors, fittings, a liquid separator are installed, the tank is filled with water and the evaporator assembly is again tested with air at a pressure of 1.2 MPa.
Rice. 1. Installation of horizontal condensers and receivers by the flow-combined method:
a, b - in a building under construction; c - on supports; d - on overpasses; I - position of the capacitor before slinging; II, III - positions when moving the crane boom; IV - installation on supporting structures
Rice. 2. Installation of capacitors:
0 - elemental: 1 - supporting metal structures; 2 - receiver; 3 - capacitor element; 4 - plumb line for verifying the verticality of the section; 5 - level for checking the horizontal element; 6 - a ruler for checking the location of the flanges in one plane; b - irrigation: 1 - water drain; 2 - pallet; 3 - receiver; 4 - sections of coils; 5 - supporting metal structures; 6 - water distribution trays; 7 - water supply; 8 - overflow funnel; in - evaporative: 1 - catchment; 2 - receiver; 3, 4 - level indicator; 5 - nozzles; 6 - droplet separator; 7 - oil separator; 8 - safety valves; 9 - fans; 10 - pre-capacitor; 11 - float water level regulator; 12 - overflow funnel; 13 - pump; d - air: 1 - supporting metal structures; 2 - drive frame; 3 - guiding device; 4 - section of finned heat exchange tubes; 5 - flanges for connecting sections to collectors
Immersion evaporators are mounted in a similar manner and tested with an inert gas pressure of 1.0 MPa for systems with R12 and 1.6 MPa for systems with R22.
Rice. 2. Installation of the panel brine evaporator:
a - testing the tank with water; b - air testing of panel sections; c - installation of panel sections; d - test of the evaporator with water and air as an assembly; 1 - wooden beams; 2 - tank; 3 - stirrer; 4 - panel section; 5 - goats; 6 - air supply ramp for testing; 7 - water drain; 8 - oil sump; 9-liquid separator; 10 - thermal insulation
Capacitive equipment and auxiliary apparatus... Linear ammonia receivers are mounted on the high pressure side below the condenser (sometimes under it) on the same foundation, and the vapor zones of the apparatus are connected by an equalizing line, which creates conditions for the liquid to drain from the condenser by gravity. During installation, the difference in elevation from the liquid level in the condenser (the level of the outlet pipe from the vertical condenser) to the level of the liquid pipe from the overflow cup of the oil separator And is not less than 1500 mm (Fig. 25). Depending on the brands of the oil separator and the linear receiver, the differences in elevation marks of the condenser, receiver and oil separator Yar, Yar, Nm and Ni, set in the reference literature, are maintained.
On the side low pressure install drainage receivers for draining ammonia from cooling devices during thawing of a snow coat with hot ammonia vapors and protective receivers in non-pumping circuits for receiving liquid in case of its release from the batteries when the heat load increases, as well as circulation receivers. Horizontal circulation receivers are mounted together with liquid separators located above them. In vertical circulating receivers, the vapor from the liquid is separated in the receiver.
Rice. 3. Installation diagram of a condenser, a linear receiver, an oil separator and an air cooler in an ammonia refrigeration unit: КД - condenser; LR - linear receiver; VOT - air separator; SP - overflow glass; MO - oil separator
In aggregated freon units, linear receivers are installed above the condenser (without equalizing line), and freon enters the receiver in a pulsating flow as the condenser fills.
All receivers are equipped with safety valves, manometers, level indicators and shut-off valves.
Intermediate vessels are installed on supporting structures on wooden beams, taking into account the thickness of the thermal insulation.
Cooling batteries. Direct refrigeration refrigeration batteries are supplied by manufacturers in a ready-to-install form. Brine and ammonia batteries are manufactured at the installation site. Brine batteries are made of steel welded pipes. For the manufacture of ammonia batteries, seamless hot-rolled steel pipes (usually with a diameter of 38X3 mm) of steel 20 are used for operation at temperatures up to -40 ° C and of steel 10G2 for operation at temperatures up to -70 ° C.
Cold rolled steel strip made of low carbon steel is used for the transverse spiral finning of the tubes of the batteries. Pipes are ribbed on a semi-automatic tooling in the conditions of procurement workshops with a spot check with a probe of the tightness of the fit of the ribbing to the pipe and the specified pitch of the ribbing (usually 20 or 30 mm). Finished pipe sections are hot-dip galvanized. In the manufacture of batteries, semi-automatic welding in carbon dioxide or manual arc welding is used. Finned pipes connect the batteries with collectors or rolls. Collector, rack and coil batteries are assembled from unified sections.
After testing ammonia batteries with air for 5 minutes for strength (1.6 MPa) and for 15 minutes for density (1 MPa), the welded joints are galvanized with an electrometallization gun.
Brine batteries are tested with water after installation at a pressure equal to 1.25 working pressure.
The batteries are attached to embedded parts or metal structures on ceilings (ceiling panels) or on walls (wall panels). Ceiling batteries are fixed at a distance of 200-300 mm from the pipe axis to the ceiling, wall-mounted - at a distance of 130-150 mm from the pipe axis to the wall and at least 250 mm from the floor to the bottom of the pipe. When installing ammonia batteries, the following tolerances are maintained: in height ± 10 mm, deviation from verticality of wall batteries - no more than 1 mm per 1 m of height. When installing batteries, a slope of no more than 0.002 is allowed, and in the direction opposite to the movement of the refrigerant vapor. Wall-mounted batteries are installed by cranes before installing the floor slabs or using loaders with an arrow. Ceiling batteries are mounted using winches through blocks attached to the ceilings.
Air coolers. They are installed on a pedestal (per-staing air coolers) or attached to embedded parts on ceilings (hinged air coolers).
Pedestal air coolers are mounted using the flow-combined method using a jib crane. Before installation, the insulation is laid on a pedestal and a hole is made to connect the drainage pipeline, which is laid with a slope of at least 0.01 towards the drain in sewer network... Suspended air coolers are mounted in the same way as ceiling radiators.
Rice. 4. Battery installation:
a - batteries by an electric forklift; b - ceiling battery with winches; 1 - overlap; 2- embedded parts; 3 - block; 4 - slings; 5 - battery; 6 - winch; 7 - electric forklift
Glass tube cooling batteries and air coolers. For the manufacture of coil-type brine batteries, glass pipes are used. The pipes are attached to the racks only in straight sections (the rolls are not fixed). The supporting metal structures of the batteries are attached to the walls or suspended from the ceilings. The distance between the posts should not exceed 2500 mm. Wall-mounted batteries to a height of 1.5 m are protected with mesh fences. Glass tubes of air coolers are mounted in a similar way.
For the manufacture of batteries and air coolers, pipes with smooth ends are taken, connecting them with flanges. After the installation is completed, the batteries are tested with water at a pressure equal to 1.25 working pressure.
Pumps. Centrifugal pumps are used to pump ammonia and other liquid refrigerants, refrigerants and chilled water, condensate, and also to empty drainage wells and circulate cooling water. For the supply of liquid refrigerants, only sealed, non-sealed pumps of the KhG type with an electric motor built into the pump housing are used. The stator of the electric motor is sealed, and the rotor is mounted on one shaft with impellers. The shaft bearings are cooled and lubricated with liquid refrigerant taken from the discharge pipe and then bypassed to the suction side. Sealed pumps are installed below the liquid intake point at a liquid temperature below -20 ° C (in order to avoid disruption of the pump, the suction head is 3.5 m).
Rice. 5. Installation and alignment of pumps and fans:
a - installation of a centrifugal pump along the logs using a winch; b - installation of the fan with a winch using guy wires
Before installing the stuffing box pumps, check their completeness and, if necessary, carry out an audit.
Centrifugal pumps are installed on the foundation with a crane, hoist, or along logs on rollers or a sheet of metal using a winch or levers. When installing the pump on a foundation with blind bolts embedded in its array, wooden beams are laid near the bolts so as not to jam the threads (Fig. 5, a). Check the elevation, horizontal position, centering, the presence of oil in the system, the smoothness of rotation of the rotor and the stuffing box packing (stuffing box). Stuffing box
The wives must be carefully stuffed and bent evenly without distortion. Excessive tightening of the stuffing box leads to its overheating and an increase in power consumption. When installing the pump above the receiving tank, a check valve is installed on the suction pipe.
Fans. Most fans are supplied as a unit ready for installation. After installing the fan with a crane or a winch with guy wires (Fig. 5, b) on the foundation, pedestal or metal structures (through vibration-insulating elements), the height mark and the horizontal position of the installation are verified (Fig. 5, c). Then they remove the rotor locking device, inspect the rotor and the housing, make sure that there are no dents or other damage, check manually the smoothness of the rotor rotation and the reliability of fastening of all parts. Check the gap between the outer surface of the rotor and the casing (no more than 0.01 of the wheel diameter). The radial and axial runout of the rotor is measured. Depending on the size of the fan (its number), the maximum radial runout is 1.5-3 mm, axial 2-5 mm. If the measurement shows that the tolerance is exceeded, static balancing is carried out. Also measure the gaps between the rotating and stationary parts of the fan, which should be within 1 mm (Fig. 5, d).
During a test run within 10 minutes, the level of noise and vibration is checked, and after a shutdown, the reliability of fastening of all connections, heating of the bearings and the state of the oil system. Duration of tests under load - 4 hours, while checking the stability of the fan under operating conditions.
Installation of cooling towers. Small film-type cooling towers (I PV) are delivered for installation with a high degree of prefabrication. The horizontal installation of the cooling tower is verified, connected to the pipeline system, and after filling the water circulation system with softened water, the irrigation uniformity of the nozzle made of miplastic or PVC plates is adjusted by changing the position of the water spray nozzles.
When installing larger cooling towers after the construction of the pool and building structures install the fan, check its alignment with the cooling tower diffuser, adjust the position of the water distribution troughs or headers and nozzles to evenly distribute water over the irrigation surface.
Rice. 6. Alignment of the coaxiality of the impeller of the cooling tower axial fan with the guide vanes:
a - by moving the frame relative to the supporting metal structures; b - by the tension of the cables: 1 - impeller hub; 2 - blades; 3 - guiding device; 4 - cooling tower cladding; 5 - supporting metal structures; 6 - reducer; 7 - electric motor; 8 - centering cables
The alignment is regulated by moving the frame and the electric motor in the slots for the fastening bolts (Fig. 6, a), and in the largest fans, alignment is achieved by adjusting the tension of the cables attached to the guide vane and supporting metal structures (Fig. 6, b). Then check the direction of rotation of the electric motor, smoothness, beating and vibration level at operating speeds of rotation of the shaft.
Evaporators
In the evaporator, the liquid refrigerant boils and turns into a vapor state, removing heat from the medium to be cooled.
Evaporators are subdivided:
by the type of medium to be cooled - for cooling gaseous media (air or other gas mixtures), for cooling liquid heat carriers (coolants), for cooling solids (products, technological substances), evaporators-condensers (in cascade refrigeration machines);
depending on the conditions of movement of the medium to be cooled - with natural circulation of the medium to be cooled, with forced circulation of the medium to be cooled, for cooling stationary media (contact cooling or freezing of products);
by filling method - flooded and non-flooded types;
according to the method of organizing the movement of the refrigerant in the apparatus - with natural circulation of the refrigerant (circulation of the refrigerant under the influence of the pressure difference); with forced circulation of the coolant (with a circulation pump);
depending on the way of organizing the circulation of the liquid to be cooled - with a closed system of the cooled liquid (shell-and-tube, shell-and-shell), with open system cooled liquid (panel).
Most often, the medium for cooling is air - a universal heat carrier that is always available. Evaporators differ in the type of channels in which the refrigerant flows and boils, the profile of the heat exchange surface and the organization of air movement.
Types of evaporators
Sheet-tube evaporators are used in household refrigerators. Made from two sheets with stamped channels. After aligning the channels, the sheets are connected by roller welding. The assembled evaporator can be given the appearance of a U- or O-shaped structure (in the form of a low-temperature chamber). The heat transfer coefficient of sheet-tube evaporators is from 4 to 8 V / (m-square * K) at a temperature head of 10 K.
a, b - O-shaped; в - panel (evaporator shelf)
Smooth tube evaporators are coiled tubes that are braced or brazed to the racks. For ease of installation, smooth-tube evaporators are made in the form of wall-mounted batteries. A battery of this type (wall-mounted smooth-tube evaporating batteries of the BN and BNI types) is used on ships to equip chambers for storing food products. For cooling the provision chambers, smooth-tube wall batteries designed by VNIIkholodmash (ON26-03) are used
Finned tube evaporators are most widely used in commercial refrigeration equipment. Evaporators are made of copper pipes with a diameter of 12, 16, 18 and 20 mm with a wall thickness of 1 mm or brass tape L62-T-0.4 with a thickness of 0.4 mm. To protect the surface of the pipes from contact corrosion, they are coated with a layer of zinc or chromium-plated.
To equip refrigerating machines with a capacity of 3.5 to 10.5 kW, IRSN evaporators are used (wall-mounted finned tube evaporator). Evaporators are made from copper pipe with a diameter of 18 x 1 mm, the ribbing is made of a brass tape 0.4 mm thick with a rib pitch of 12.5 mm.
A finned tube evaporator equipped with a fan for forced air circulation is called an air cooler. The heat transfer coefficient of such a heat exchanger is higher than that of a finned evaporator, and therefore the dimensions and weight of the apparatus are smaller.
evaporator failure technical heat transfer
Shell and tube evaporators are evaporators with a closed circulation of the cooled liquid (heat carrier or liquid process medium). The liquid to be cooled flows through the evaporator under the pressure generated by the circulation pump.
In shell and tube flooded type evaporators, the refrigerant boils on the outside of the tubes and the liquid to be cooled flows inside the tubes. Closed system circulation allows you to reduce the cooling system due to the reduction of contact with air.
For cooling water, shell-and-tube evaporators are often used with boiling of the refrigerant inside the pipes. The heat exchange surface is made in the form of pipes with internal ribbing and the coolant boils inside the pipes, and the cooled liquid flows in the annular space.
Operation of evaporators
· When using evaporators, it is necessary to comply with the instructions of the manufacturers, these Rules and production instructions.
· When the pressure on the discharge lines of the evaporators is higher than that provided for by the project, the electric motors and coolants of the evaporators must be automatically turned off.
· It is not allowed to operate evaporators with faulty or switched off ventilation, with faulty instrumentation or their absence, in the presence of a gas concentration in the room exceeding 20% of the lower concentration limit of flame propagation.
· Information on the operating mode, the number of hours worked by the compressors, pumps and evaporators, as well as malfunctions in the work should be reflected in the operating log.
· The withdrawal of the evaporators from the operating mode to the reserve must be carried out in accordance with the production instructions.
· After shutting down the evaporator, the shut-off valves on the suction and discharge lines must be closed.
· The air temperature in the evaporating compartments during working hours must not be lower than 10 ° С. When the air temperature is below 10 ° C, it is necessary to drain the water from the water supply system, as well as from the cooling system of the compressors and the heating system of the evaporators.
The evaporation compartments must be technological schemes equipment, pipelines and instrumentation, operating instructions for installations and operational logs.
· Maintenance Evaporators are carried out by operating personnel under the guidance of a specialist.
· Maintenance evaporative equipment includes maintenance and inspection operations, partial disassembly of equipment with repair and replacement of wearing parts and parts.
· When operating the evaporators, the requirements for the safe operation of pressure vessels must be met.
Maintenance and repair of evaporators should be carried out in the amount and terms specified in the manufacturer's passport. Maintenance and repair of gas pipelines, fittings, safety automation devices and instrumentation of evaporators should be carried out within the time limits established for this equipment.
Operation of evaporators is not allowed in the following cases:
1) increase or decrease in pressure of the liquid and vapor phases above or below the established norms ;
2) malfunctions of safety valves, instrumentation and automation equipment;
3) failure to verify control and measuring devices;
4) faulty fasteners;
5) detection of gas leakage or sweating in welds, bolted joints, as well as violation of the integrity of the evaporator structure;
6) ingress of the liquid phase into the gas pipeline of the vapor phase;
7) stopping the supply of the coolant to the evaporator.
Evaporator repair
Evaporator too weak ... Generalization of symptoms
In this section, we will use the term “evaporator too weak” to mean any malfunction that results in an abnormal decrease in cooling capacity due to the fault of the evaporator itself.
Diagnostic algorithm
A “too weak evaporator” fault and, as a consequence, an abnormal drop in evaporating pressure is most easily identified, since this is the only malfunction in which normal or slightly reduced superheating occurs simultaneously with an abnormal drop in evaporating pressure.
Practical aspects
3 tubes and heat exchange fins of the evaporator are dirty
The risk of this defect arises mainly in installations that are poorly maintained. A typical example of such an installation is an air conditioner that lacks air filter at the inlet to the evaporator.
When cleaning the evaporator, it is sometimes sufficient to blow the fins with a jet of compressed air or nitrogen in the direction opposite to the air movement during the operation of the unit, but in order to completely cope with the dirt, it is often necessary to use special cleaning and detergents... In some particularly severe cases, it may even be necessary to replace the evaporator.
Dirty air filter
In air conditioners, contamination of the air filters installed at the inlet to the evaporator leads to an increase in the resistance to air flow and, as a result, to a decrease in the air flow through the evaporator, which leads to an increase in the temperature difference. Then the repairman must clean or change the air filters (for filters of similar quality), not forgetting to provide free access to outside air when installing new filters.
It is helpful to remember that the air filters must be in perfect condition. Especially at the outlet facing the evaporator. The filter material should not be torn or lost thickness during repeated washes.
If the air filter is in poor condition or not suitable for this evaporator, dust particles will not be well captured and over time will cause fouling of the evaporator tubes and fins.
Evaporator fan belt slips or torn
If the fan belt (or belts) slips, the fan speed decreases, which leads to a decrease in air flow through the evaporator and an increase in the air temperature difference (in the limit, if the belt is torn, there is no air flow at all).
Before tightening the belt, the repairer should check its wear and, if necessary, replace it. Of course, the repairer should also check the alignment of the belts and completely inspect the drive (cleanliness, mechanical clearances, grease, tension) and the condition of the drive motor with the same care as the fan itself. Each repairer, of course, cannot have all existing models of drive belts in stock in his car, so you first need to check with the client and choose the right kit.
Poorly adjusted pulley with variable chute width
Most modern air conditioners are equipped with fan drive motors, on the axis of which a pulley of variable diameter (variable chute width) is installed.
At the end of the adjustment, it is necessary to fix the movable cheek on the threaded part of the hub using a locking screw, while the screw should be tightened as tightly as possible, carefully making sure that the screw leg rests on a special flat on the threaded part of the hub and prevents damage to the thread. Otherwise, if the thread is crumpled by the locking screw, further adjustment of the depth of the groove will be difficult, if not impossible. After adjusting the pulley, in any case, check the amperage consumed by the electric motor (see the description of the next malfunction).
High pressure losses in the evaporator air path
If the variable-diameter pulley is adjusted to the maximum fan speed, and the air flow remains insufficient, which means that the losses in the air path are too large in relation to the maximum fan speed.
After you are firmly convinced that there are no other malfunctions (a shutter or valve is closed, for example), it should be considered advisable to replace the pulley in such a way as to increase the fan speed. Unfortunately, increasing the fan speed not only requires replacing the pulley, but also entails other consequences.
The evaporator fan rotates in the opposite direction
The risk of such a malfunction always exists during commissioning. new installation when the evaporator fan is equipped with a three-phase drive motor (in this case, it may be sufficient to swap two phases to restore the correct direction of rotation).
The fan motor, being designed for power supply from a 60 Hz mains, is connected to a 50 Hz mains
This problem, fortunately quite rare, can mainly affect motors manufactured in the USA and intended to be connected to a 60 Hz AC mains. Note that some motors manufactured in Europe for export may also require a 60 Hz supply frequency. To quickly understand the cause of this malfunction, you can very simply just read the repairman specifications motor on a special plate attached to it.
3 contamination of a large number of evaporator fins
If many fins of the evaporator are covered with dirt, resistance to air movement through it increased, which leads to a decrease in air flow through the evaporator and an increase in the air temperature difference.
And then the repairman will have no choice but to thoroughly clean the contaminated parts of the evaporator fins on both sides using a special comb with a tooth pitch that exactly matches the distance between the fins.
Evaporator maintenance
It consists in providing heat removal from the heat transfer surface. For this purpose, the supply of liquid refrigerant to the evaporators and air coolers is regulated to create the required level of flooded systems or in the amount required to ensure optimal superheating of the exhaust steam in non-flooded systems.
The safety of the evaporator systems largely depends on the regulation of the refrigerant supply and the order of switching on and off the evaporators. The regulation of the refrigerant supply is carried out in such a way as to prevent the breakthrough of vapors from the high pressure side. This is achieved by smooth control operations, maintaining the required level in the linear receiver. When disconnected evaporators are connected to the operating system, it is necessary to prevent the compressor from running wet, which can occur due to the release of steam from the heated evaporator together with drops of liquid refrigerant during its sharp boiling up after careless or thoughtless opening of the shut-off valves.
The procedure for connecting the evaporator, regardless of the duration of the shutdown, must always be as follows. Cut off the supply of refrigerant to the running evaporator. Close the suction valve on the compressor and gradually open the shut-off valve on the evaporator. Thereafter, the suction valve of the compressor is also gradually opened. Then the supply of refrigerant to the evaporators is controlled.
To ensure an efficient heat transfer process, the evaporators of refrigeration units with brine systems ensure that the entire heat transfer surface is immersed in the brine. In evaporators open type the brine level should be 100-150 mm above the evaporator section. When operating shell-and-tube evaporators, the timely release of air through the air taps is monitored.
When servicing evaporative systems, they monitor the timeliness of thawing (warming up) the frost layer on the batteries and air coolers, check if the melt water drainage pipeline is frozen, monitor the operation of the fans, the tightness of closing hatches and doors in order to avoid losses of cooled air.
During defrosting, the uniformity of the heating vapor supply is monitored, avoiding uneven heating of individual parts of the apparatus and not exceeding the heating rate of 30 S h.
The supply of liquid refrigerant to air coolers in non-pumping units is controlled by the circuit according to the level in the air cooler.
In installations with a pumping circuit, the uniformity of the flow of refrigerant into all air coolers is controlled depending on the freezing rate.
Bibliography
Installation, operation and repair refrigeration equipment... Textbook (Ignatiev V.G., Samoilov A.I.)
