In the evaporator, the process of transition of the refrigerant from the liquid phase state to the gaseous state occurs with the same pressure; the pressure inside the evaporator is the same everywhere. During the process of transition of a substance from liquid to gaseous (its boiling away) in the evaporator, the evaporator absorbs heat, unlike the condenser, which releases heat to the environment. That. through two heat exchangers, the process of heat exchange occurs between two substances: the cooled substance, which is located around the evaporator and the outside air, which is located around the condenser.
Liquid freon flow diagram
Solenoid valve - shuts off or opens the flow of refrigerant to the evaporator, is always either completely open or completely closed (may not be present in the system)
Thermostatic expansion valve (TEV) is a precise device that regulates the flow of refrigerant into the evaporator depending on the intensity of the refrigerant boiling in the evaporator. It prevents liquid refrigerant from entering the compressor.
Liquid freon enters the expansion valve, the refrigerant is throttled through the membrane in the expansion valve (freon is sprayed) and begins to boil due to the pressure drop, the droplets gradually turn into gas throughout the entire section of the evaporator pipeline. Starting from the throttling device of the expansion valve, the pressure remains constant. Freon continues to boil and in a certain section of the evaporator it 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 gas in the evaporator, begins to heat up and at the exit from the evaporator its temperature should be equal to -3, -4 °C, thus Δt ( the difference between the boiling point of the refrigerant and the gas temperature at the evaporator outlet) should be = 7-8, this is the normal operation of the system. For a given Δt, we will know that at the exit from the evaporator there will be no particles of unboiled freon (there should not be any); if boiling occurs in the pipe, then not all the power is used to cool the substance. The pipe is thermally insulated so that the freon does not heat up to ambient temperature, because The refrigerant gas cools the compressor stator. If liquid freon still gets into the pipe, it means that the dose supplied 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 away and the system does not work correctly, the compressor is also filled with liquid freon and fails. Overheating on a larger side is not as dangerous as overheating on a smaller side; at Δt ˃ 7, overheating of the compressor stator may occur, but a slight excess of overheating may not be felt by the compressor and is preferable during operation.
With the help of fans located in the air cooler, cold is removed from the evaporator. If this did not happen, then the tubes would become covered with ice and at the same time the refrigerant would reach its saturation temperature, at which it stops boiling, and then, even regardless of the pressure drop, liquid freon would enter the evaporator without evaporating, flooding the compressor.
→ Installation of refrigeration units
Installation of main apparatus and auxiliary equipment
The main devices of a refrigeration unit include devices directly involved in mass and heat transfer processes: condensers, evaporators, subcoolers, air coolers, etc. Receivers, oil separators, dirt traps, air separators, pumps, fans and other equipment included in the refrigeration unit include to auxiliary equipment.
The installation technology is determined by the degree of factory readiness and design features of the devices, their weight and installation design. First, the main equipment is installed, which allows you to begin laying pipelines. To prevent moisture insulation on the supporting surface of devices operating at low temperatures, apply a layer of waterproofing, lay a thermal insulation layer, and then again a layer of waterproofing. To create conditions that prevent the formation of thermal bridges, all metal parts (fastening belts) are placed on the devices through wooden antiseptic bars or gaskets 100-250 mm thick.
Heat exchangers. Most heat exchangers are supplied by factories ready for installation. Thus, shell-and-tube condensers, evaporators, subcoolers are supplied assembled, elemental, spray, evaporative condensers and panel, submersible evaporators are supplied as assembly units. Finned tube evaporators, direct coils and brine evaporators can be manufactured installation organization in place from sections of finned pipes.
Shell-and-tube devices (as well as capacitive equipment) are mounted in a combined flow method. When laying welded apparatus on supports, make sure that all welds are accessible for inspection, tapping with a hammer during inspection, and also for repair.
The horizontality and verticality of the devices are checked by level and plumb line or using surveying instruments. The permissible deviations of the devices from the vertical are 0.2 mm, horizontally - 0.5 mm per 1 m. If the device has a collection or settling tank, a slope only in their direction is permissible. The verticality of shell-and-tube vertical condensers is especially carefully verified, since it is necessary to ensure film flow of water along the walls of the pipes.
Elemental capacitors (due to their high metal consumption they are used in rare cases in industrial installations) are installed on metal frame, above the receiver, element by element from bottom to top, checking the horizontality of the elements, the uniform plane of the fitting flanges and the verticality of each section.
Installation of irrigation and evaporative condensers consists of sequential installation of a pan, heat exchange pipes or coils, fans, oil separator, pump and fittings.
Devices with air cooled, used as condensers for refrigeration units, are mounted on a pedestal. 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 centered to the gearbox.
Panel brine evaporators are placed on an insulating layer, on a concrete pad. The metal evaporator tank is installed on wooden beams, install the stirrer and brine valves, connect the drain pipe and test the tank for density by filling it with water. The water level should not fall 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 sections are mounted in the tank one by one, manifolds, fittings, and 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 capacitors and receivers using the combined flow method:
a, b - in a building under construction; c - on supports; g - on overpasses; I - position of the capacitor before slinging; II, III - positions when moving the crane boom; IV - installation on support structures
Rice. 2. Installation of capacitors:
0 - elemental: 1 - supporting metal structures; 2 - receiver; 3 - capacitor element; 4 - plumb line for checking the verticality of the section; 5 - level for checking the horizontality of the element; 6 - ruler for checking the location of the flanges in the same plane; b - irrigation: 1 - draining water; 2 - pallet; 3 - receiver; 4 - sections of coils; 5 - supporting metal structures; 6 - water distribution trays; 7 - water supply; 8 - overflow funnel; c - evaporative: 1 - water collector; 2 - receiver; 3, 4 - level indicator; 5 - nozzles; 6 - drop eliminator; 7 - oil separator; 8 - safety valves; 9 - fans; 10 - precondenser; 11 - float water level regulator; 12 - overflow funnel; 13 - pump; g - air: 1 - supporting metal structures; 2 - drive frame; 3 - guide vane; 4 - section of finned heat exchange pipes; 5 - flanges for connecting sections to collectors
Submersible evaporators are mounted in a similar way and are tested at an inert gas pressure of 1.0 MPa for systems with R12 and 1.6 MPa for systems with R22.
Rice. 2. Installation of panel brine evaporator:
a - testing the tank with water; b - testing panel sections with air; c - installation of panel sections; d - test of the evaporator assembly with water and air; 1 - wooden beams; 2 - tank; 3 - stirrer; 4 - panel section; 5 - goats; 6 - air supply ramp for testing; 7 - water drain; 8 - oil collector; 9-liquid separator; 10 - thermal insulation
Capacitive equipment and auxiliary devices. Linear ammonia receivers are mounted on the side high pressure below the condenser (sometimes under it) on the same foundation, and the steam zones of the devices are connected by an equalizing line, which creates conditions for draining the liquid from the condenser by gravity. During installation, maintain a difference in heights 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 oil separator overflow cup I of at least 1500 mm (Fig. 25). Depending on the brands of the oil separator and linear receiver, the differences in elevations of the condenser, receiver and oil separator are maintained: Yar, Yar, Nm and Ni, specified in the reference literature.
On the side low pressure install drainage receivers to drain ammonia from cooling devices when the snow coat is thawed by hot ammonia vapors and protective receivers in the absence of pumping schemes for receiving liquid in the event of its release from the batteries when the thermal load increases, as well as circulation receivers. Horizontal circulation receivers are mounted together with liquid separators placed above them. In vertical circulation receivers, steam is separated from the liquid in the receiver.
Rice. 3. Installation diagram of a condenser, linear receiver, oil separator and air cooler in an ammonia refrigeration unit: KD - condenser; LR - linear receiver; HERE - air separator; SP - overflow glass; MO - oil separator
In aggregated freon installations, linear receivers are installed above the condenser (without an equalizing line), and the freon enters the receiver in a pulsating flow as the condenser is filled.
All receivers are equipped with safety valves, pressure gauges, 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 cooling freon batteries are supplied by manufacturers ready for installation. Brine and ammonia batteries are manufactured at the installation site. Brine batteries are made from electric-welded steel pipes. For the manufacture of ammonia batteries, seamless hot-rolled steel pipes (usually with a diameter of 38X3 mm) are used from steel 20 for operation at temperatures down to -40 °C and from steel 10G2 for operation at temperatures up to -70 °C.
For cross-spiral finning of battery tubes, cold-rolled steel strip made of low-carbon steel is used. The pipes are finned using semi-automatic equipment in the conditions of procurement workshops with a random check with a probe for the tightness of the fins to the pipe and the specified fin pitch (usually 20 or 30 mm). Finished pipe sections are hot-dip galvanized. In the manufacture of batteries, semi-automatic welding in a carbon dioxide environment or manual electric arc is used. Finned tubes connect batteries with collectors or coils. Collector, rack and coil batteries are assembled from standardized 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 electroplating gun.
Brine batteries are tested with water after installation to a pressure equal to 1.25 working.
The batteries are attached to embedded parts or metal structures on ceilings (ceiling batteries) or on walls (wall batteries). Ceiling batteries are mounted at a distance of 200-300 mm from the axis of the pipes to the ceiling, wall batteries - at a distance of 130-150 mm from the axis of the pipes 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: height ± 10 mm, deviation from verticality of wall-mounted batteries is 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 refrigerant vapor. Wall batteries are installed using cranes before installing floor slabs or using boom loaders. Ceiling batteries are mounted using winches through blocks attached to the ceilings.
Air coolers. They are installed on a pedestal (on-pedestal air coolers) or attached to embedded parts on the ceilings (mounted air coolers).
Pedestal air coolers are installed using a flow-combined method using a jib crane. Before installation, insulation is laid on the 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 into the sewer network. Mounted air coolers are installed in the same way as ceiling radiators.
Rice. 4. Battery installation:
a - batteries for an electric forklift; b - ceiling battery with winches; 1 - overlap; 2- embedded parts; 3 - block; 4 - slings; 5 - battery; 6 - winch; 7 - electric forklift
Cooling batteries and air coolers made of glass pipes. Glass pipes are used to make coil-type brine batteries. Pipes are attached to racks only in straight sections (rolls are not secured). 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 batteries to a height of 1.5 m are protected with mesh fences. Glass pipes of air coolers are also installed in a similar way.
For the manufacture of batteries and air coolers, pipes with smooth ends are taken, connecting them with flanges. After installation, the batteries are tested with water at a pressure equal to 1.25 working.
Pumps. Centrifugal pumps are used to pump ammonia and other liquid refrigerants, coolants and chilled water, condensate, as well as to empty drainage wells and circulate cooling water. To supply liquid refrigerants, only sealed, sealless pumps of the CG 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 the same shaft with the impellers. The shaft bearings are cooled and lubricated by liquid refrigerant taken from the discharge pipe and then transferred to the suction side. Sealed pumps are installed below the liquid intake point at a liquid temperature below -20 ° C (to avoid disruption of the pump, the suction head is 3.5 m).
Rice. 5. Installation and alignment of pumps and fans:
a - installation centrifugal pump along the joists using a winch; b - installation of the fan with a winch using guy ropes
Before installing stuffing box pumps, check their completeness and, if necessary, carry out an inspection.
Centrifugal pumps are installed on the foundation by a crane, a hoist, or along joists 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 mass, wooden beams are placed near the bolts so as not to jam the threads (Fig. 5, a). Check the elevation, horizontalness, alignment, presence of oil in the system, smooth rotation of the rotor and packing of the stuffing box (oil seal). Stuffing box
The gland should be carefully stuffed and bent evenly without distortion. Excessive tightening of the gland leads to its overheating and increased energy 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 ready-to-install unit. After installing the fan with a crane or winch with guy ropes (Fig. 5, b) on the foundation, pedestal or metal structures (through vibration-isolating elements), the elevation and horizontal position of the installation are verified (Fig. 5, c). Then remove the rotor-locking device, inspect the rotor and housing, make sure there are no dents or other damage, manually check the smooth rotation of the rotor and the reliability of fastening of all parts. Check the gap between the outer surface of the rotor and the housing (no more than 0.01 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. The gaps between the rotating and stationary parts of the fan are also measured, which should be within 1 mm (Fig. 5, d).
During a test run, the noise and vibration levels are checked within 10 minutes, and after stopping, the reliability of fastening of all connections, heating of the bearings and the condition of the oil system. The duration of the load tests is 4 hours, during which the stability of the fan operation is checked under operating conditions.
Installation of cooling towers. Small film-type cooling towers (I PV) are supplied for installation with a high degree of factory readiness. 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 uniformity of irrigation of the nozzles made of miplast or polyvinyl chloride plates is adjusted by changing the position of the water spray nozzles.
When installing larger cooling towers after the construction of a swimming pool and building structures install the fan, check its alignment with the cooling tower diffuser, adjust the position of the water distribution gutters or collectors and nozzles for uniform distribution of water over the irrigation surface.
Rice. 6. Alignment of the impeller of the axial fan of the cooling tower with the guide vane:
a - by moving the frame relative to the supporting metal structures; b - cable tension: 1 - impeller hub; 2 - blades; 3 - guide vane; 4 - cooling tower casing; 5 - supporting metal structures; 6 - gearbox; 7 - electric motor; 8 - centering cables
Alignment is adjusted by moving the frame and electric motor in the grooves for the fastening bolts (Fig. 6, a), and in the largest fans, coaxiality 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, runout and vibration level at operating shaft rotation speeds.
One of the most important elements For vapor compression machine is . He performs main process refrigeration cycle – selection from the cooled environment. Other elements of the refrigeration circuit, such as a condenser, expansion device, compressor, etc., only provide reliable operation evaporator, therefore it is the choice of the latter that must be given due attention.
It follows from this that when selecting equipment for a refrigeration unit, it is necessary to start with the evaporator. Many novice repairmen often make the mistake typical mistake and start completing the installation with a compressor.
In Fig. Figure 1 shows a diagram of the most common vapor compression refrigeration machine. Its cycle, specified 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 the same in Fig. 1a (functions of state parameters).
Rice. 1 – Diagram and in coordinates of a conventional vapor compression machine: RU expansion device, Pk– condensation pressure, Ro– boiling pressure.
Graphic representation fig. 1b shows the state and functions of the refrigerant, which vary depending on pressure and enthalpy. Line segment AB on the curve in Fig. 1b characterizes the refrigerant in the state of saturated vapor. Its temperature corresponds to the starting point of boiling. The refrigerant vapor fraction is 100%, and superheat is close to zero. To the right of the curve AB the refrigerant has a state (the temperature of the refrigerant is greater than the boiling point).
Dot IN 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 segment BC, the refrigerant has the state of a saturated liquid, and on the left side - 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 occurring 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 share of steam depends on the specific refrigeration cycle and is 10-30%.
At the exit from the evaporator, the boiling process may not be completed, period 1 may not coincide with the point 7 . If the temperature of the refrigerant at the outlet of the evaporator is higher than the boiling point, then we get an overheated evaporator. Its size ΔToverheat represents the difference between the temperature of the refrigerant at the outlet of the evaporator (point 1) and its temperature at the saturation line AB (point 7):
ΔToverheat=T1 – T7
If points 1 and 7 coincide, then the refrigerant temperature is equal to the boiling point, and the superheat ΔToverheat will be equal to zero. Thus, we get a flooded evaporator. Therefore, when choosing an evaporator, you first need to make a choice between a flooded evaporator and an overheated evaporator.
Note that, under equal conditions, a flooded evaporator is more advantageous in terms of the intensity of the heat extraction process than with overheating. But it should be taken into account 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 probability of water hammer occurring, 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 give preference to an evaporator with overheating, then you do not need to worry about protecting the compressor and getting saturated steam into it. The likelihood of water hammer occurring will only occur if the superheat value deviates from the required value. IN normal conditions operation of the refrigeration unit, superheat value ΔToverheat should be within 4-7 K.
When the superheat indicator decreases ΔToverheat, the intensity of heat extraction from the environment increases. But at extremely low values ΔToverheat(less than 3K) there is a possibility of wet steam entering the compressor, which can cause water hammer and, consequently, damage to the mechanical components of the compressor.
Otherwise, with a high reading ΔToverheat(more than 10 K), this indicates that insufficient refrigerant is entering the evaporator. The intensity of heat extraction from the cooled medium sharply decreases and the thermal conditions of the compressor worsen.
When choosing an evaporator, another question arises related to the boiling point of the refrigerant in the evaporator. To solve this, it is first necessary to determine what temperature of the cooled medium should be ensured for normal operation of the refrigeration unit. If air is used as the cooled medium, then in addition to the temperature at the outlet of the evaporator, it is also necessary to take into account the humidity at the outlet of the evaporator. Now let us consider the behavior of the temperatures of the cooled medium around the evaporator during operation of a conventional refrigeration unit (Fig. 1a).
In order not to delve into this topic, we will neglect the pressure losses on the evaporator. We will also assume that the heat exchange occurring between the refrigerant and environment carried out according to a direct-flow scheme.
In practice, such a scheme is not often used, since in terms of heat transfer efficiency it is inferior to a counterflow scheme. But if one of the coolants has a constant temperature, and the overheating readings are small, then forward flow and counter flow will be equivalent. It is known that the average temperature difference does not depend on the flow pattern. Consideration of the direct-flow circuit will provide us with a more clear idea of the heat exchange that occurs between the refrigerant and the cooled medium.
First, let's introduce the virtual quantity L, equal to the length of the heat exchange device (condenser or evaporator). Its value can be determined from the following expression: L=W/S, Where W– corresponds to the internal volume of the heat exchange device in which the refrigerant circulates, m3; S– heat exchange surface area m2.
If we're talking about about a refrigeration machine, then the equivalent length of the evaporator is almost equal to the length of the tube in which the process takes place 6-1 . Therefore, its outer surface is washed by a cooled medium.
First, let's pay attention to the evaporator, which acts as an air cooler. In it, the process of removing 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 units 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, which provides forced air flow to the evaporator and is a tubular-fin heat exchanger (Fig. 2). Its schematic representation is shown in Fig. 2b. Let's consider the main quantities that characterize the blowing process.
Temperature difference
The temperature difference across the evaporator is calculated as follows:ΔT=Ta1-Ta2,
Where ΔTa is in the range from 2 to 8 K (for tubular-fin evaporators with forced air flow).
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;
- FF– refrigerant temperature;
- L– equivalent length of the evaporator;
- That– boiling point of the refrigerant in the evaporator.
Maximum temperature difference
The maximum temperature pressure of air at the evaporator inlet is determined as follows:DTmax=Ta1 – To
This indicator is used when selecting air coolers, since foreign manufacturers refrigeration technology provide evaporator cooling capacities Qsp depending on size DTmax. Let's consider the method for selecting an air cooler for a refrigeration unit and determine the calculated values DTmax. To do this, let us give as an example generally accepted recommendations for selecting the value DTmax:
- for freezers DTmax is within 4-6 K;
- for storage rooms for unpackaged products – 7-9 K;
- for storage rooms for hermetically packaged products – 10-14 K;
- for air conditioning units – 18-22 K.
Degree of steam superheat at the evaporator outlet
To determine the degree of steam superheat at the outlet of the evaporator, use the following form: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 foreign catalogs stipulate that the readings of the cooling 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. Let us assume that F=0, Then ΔТoverload=0, and the refrigerant leaving the evaporator will be in the state of saturated vapor. For this air cooler model, the actual cooling capacity will be 10-15% greater than the figure given in the catalog.
If F>0.65, then the cooling capacity for a given model of air cooler must be less than the value given in the catalog. Let's assume that F>0.8, then the actual performance for this model will be 25-30% greater than the value given in the catalog.
If F->1, then the evaporator cooling capacity Quse->0(Fig. 3).
Fig. 3 – dependence of the evaporator cooling capacity Qsp from overheating F
The process depicted in Fig. 2b is also characterized by other parameters:
- arithmetic mean temperature difference DTsr=Tasr-T0;
- average temperature of the air that passes through the evaporator Tasp=(Ta1+Ta2)/2;
- minimum temperature difference DTmin=Ta2-To.
Rice. 4 – Diagram and temperature parameters showing the process of water cooling on the evaporator:
Where Te1 And Te2 water temperature at the evaporator inlets and outlets;
- FF – coolant temperature;
- L – equivalent length of the evaporator;
- T is the boiling point of the refrigerant in the evaporator.
If the temperature difference across the 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 indicator ΔTe will be within 5±1.5 K.
Unlike air coolers, in liquid coolers it is necessary to maintain not a maximum, but a minimum temperature pressure DTmin=Te2-To– 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.
For shell-and-tube evaporators, the minimum temperature difference is DTmin=Te2-To should be maintained within 4-6 K, and for plate evaporators - 3-5 K.
The specified 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: as the difference increases, the cooling intensity begins to decrease, and as it decreases, the risk of freezing of the cooled liquid in the evaporator increases, which can cause its mechanical failure. destruction.
Evaporator design solutions
Regardless of the method of using various refrigerants, the heat exchange processes occurring in the evaporator are subject to the main technological cycle of refrigeration consuming production, according to which refrigeration units and heat exchangers are created. Thus, in order to solve the problem of optimizing the heat exchange process, it is necessary to take into account the conditions for the rational organization of the technological cycle of refrigeration-consuming production.As is known, cooling of a certain environment is possible using a heat exchanger. His constructive 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 thermal treatment of the environment, which is possible under the following conditions:
- maintaining a given temperature of the working process and control (regulation) over temperature conditions;
- selection of device material according to the chemical properties of the environment;
- control over the length of time the medium remains in the device;
- correspondence of operating speeds and pressure.
- ensuring the necessary speed of working media to implement turbulent conditions;
- creating the most suitable conditions to remove condensate, scale, frost, etc.;
- Creation favorable conditions for the movement of working media;
- preventing possible contamination of the device.
The ease of use and reliability of the device is influenced by factors such as the strength and tightness of detachable connections, compensation for temperature deformations, and ease of maintenance and repair of the device. These requirements form the basis for the design and selection of a heat exchange unit. Main role this involves ensuring the required technological process in refrigeration production.
In order to choose the correct design solution for the evaporator, you must be guided by the following rules. 1) cooling of liquids is best done using a rigid tubular heat exchanger or a compact plate heat exchanger; 2) the use of tubular-fin devices is due to the following conditions: The heat transfer between the working media and the wall on both sides of the heating surface differs significantly. In this case, the fins must be installed on the side with the lowest heat transfer coefficient.
To increase the intensity of heat exchange in heat exchangers, it is necessary to adhere to the following rules:
- ensuring proper conditions for condensate removal in air coolers;
- reducing the thickness of the hydrodynamic boundary layer by increasing the speed of movement of the working fluids (installation of inter-tube partitions and dividing the tube bundle into passages);
- improving the flow of working fluids around the heat exchange surface (the entire surface should actively participate in the heat exchange process);
- compliance with basic temperature indicators, thermal resistances, etc.
Improving heat exchange 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 exchange surfaces are being developed, which will make heat exchangers more compact.
Choosing a rational approach for calculating the evaporator
When designing an evaporator, structural, hydraulic, strength, thermal and technical and economic calculations should be carried out. They are performed in several versions, the choice of which depends on performance indicators: technical and economic indicators, efficiency, etc.To make a thermal calculation of a surface heat exchanger, it is necessary to solve the heat balance equation, taking into account certain operating conditions of the device ( design dimensions heat transfer surfaces, temperature change limits and patterns regarding 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 obtain results from the original data. But due to numerous factors, find common decision not possible for different heat exchangers. At the same time, there are many methods for approximate calculations that are easy to perform manually or by machine.
Modern technologies allow you to select an evaporator using special programs. They are mainly provided by manufacturers of heat exchange equipment and allow you to quickly select the required model. When using such programs, it is necessary to take into account 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 the evaporator design you have chosen, relative to its actual operating conditions.
In the case when the consumption of the vapor phase of liquefied gas exceeds the rate of natural evaporation in the container, it is necessary to use evaporators, which, due to electrical heating, accelerate the process of evaporation of the liquid phase into the vapor phase and guarantee the supply of gas to the consumer in the calculated volume.
The purpose of the LPG evaporator is the transformation of the liquid phase of liquefied hydrocarbon gases (LPG) into a vapor phase, which occurs through the use of electrically heated evaporators. Evaporation units can be equipped with one, two, three or more electric evaporators.
Installation of evaporators allows the operation of one evaporator or several in parallel. Thus, the productivity of the installation may vary depending on the number of evaporators operating simultaneously.
Operating principle of the evaporation unit:
When the evaporation unit is turned on, the automation heats the evaporation unit to 55C. The solenoid valve at the liquid phase inlet to the evaporation unit will be closed until the temperature reaches these parameters. The level control sensor in the shut-off valve (if there is a level gauge in the shut-off valve) monitors the level and closes the inlet valve when overfilled.
The evaporator begins to heat up. When 55°C is reached, the inlet magnetic valve will open. The liquefied gas enters the heated pipe register and evaporates. At this time, the evaporator continues to heat up, and when the core temperature reaches 70-75°C, the heating coil will be turned off.
The evaporation process continues. The evaporator core gradually cools down, and when the temperature drops to 65°C, the heating coil will be turned on again. The cycle repeats.
Evaporation unit complete set:
The evaporation unit can be equipped with one or two regulatory groups to duplicate the reduction system, as well as the vapor phase bypass line, bypassing the evaporation unit for using the steam phase of natural evaporation in gas holders.
Pressure regulators are used to set the desired pressure at the outlet of the evaporation unit to the consumer.
- 1st stage - medium pressure adjustment (from 16 to 1.5 bar).
- 2nd stage - low pressure adjustment from 1.5 bar to the pressure required when supplying to the consumer (for example, to a gas boiler or gas piston power plant).
Advantages of PP-TEC evaporation units “Innovative Fluessiggas Technik” (Germany)
1. Compact design, light weight;
2. Economical and safe operation;
3. Big thermal power;
4. Long service life;
5. Stable operation at low temperatures;
6. Duplicated control system for the exit of the liquid phase from the evaporator (mechanical and electronic);
7. Anti-icing of filter and solenoid valve (PP-TEC only)
Package Included:
Double thermostat for gas temperature control,
- liquid level control sensors,
- solenoid valves at the liquid phase inlet
- set of safety fittings,
- thermometers,
- ball valves for emptying and deaeration,
- built-in liquid phase gas separator,
- inlet/outlet fittings,
- terminal boxes for connecting power supply,
- electrical control panel.
Advantages of PP-TEC evaporators
When designing an evaporation plant, three elements must always be taken into account:
1. Ensure the specified performance,
2. Create the necessary protection against hypothermia and overheating of the evaporator core.
3. Correctly calculate the geometry of the location of the coolant to the gas conductor in the evaporator
The performance of the evaporator depends not only on the amount of power supply voltage consumed from the network. An important factor is the geometry of the location.
A correctly calculated arrangement ensures efficient use of the heat transfer mirror and, as a result, increases the efficiency of the evaporator.
In the evaporators “PP-TEC “Innovative Fluessiggas Technik” (Germany), through correct calculations, the company’s engineers achieved an increase in this coefficient to 98%.
Evaporative installations of the company “PP-TEC “Innovative Fluessiggas Technik” (Germany) lose only two percent of heat. The remaining amount is used to evaporate the gas.
Almost all European and American manufacturers of evaporation equipment completely erroneously interpret the concept of “redundant protection” (a condition for the implementation of duplication of protection functions against overheating and overcooling).
The concept of “redundant protection” implies the implementation of “safety net” of individual working units and units or entire equipment, through the use of duplicated elements from different manufacturers and with different principles of operation. Only in this case can the possibility of equipment failure be minimized.
Many manufacturers try to implement this function (while protecting against hypothermia and the ingress of the liquid fraction of LPG to the consumer) by installing two magnetic valves connected in series from the same manufacturer on the input supply line. Or they use two temperature sensors for switching on/opening valves connected in series.
Imagine the situation. One solenoid valve is stuck open. How can you determine that the valve has failed? NO WAY! The installation will continue to operate, having lost the opportunity to ensure safe operation in time during overcooling in the event of failure of the second valve.
In PP-TEC evaporators this function was implemented in a completely different way.
In evaporation installations, the company “PP-TEC “Innovative Fluessiggas Technik” (Germany) uses an algorithm for the combined operation of three elements of protection against hypothermia:
1. Electronic device
2. Magnetic valve
3. Mechanical shut-off valve in the shut-off valve.
All three elements have absolutely different principle actions, which allows us to speak with confidence about the impossibility of a situation in which non-evaporated gas in liquid form enters the consumer pipeline.
In the evaporation installations of the company “PP-TEC “Innovative Fluessiggas Technik” (Germany), the same thing was implemented when protecting the evaporator from overheating. The elements involve both electronics and mechanics.
The company “PP-TEC “Innovative Fluessiggas Technik” (Germany) was the first in the world to implement the function of integrating a liquid cut-off valve into the cavity of the evaporator itself with the possibility of constant heating of the cut-off valve.
No evaporation technology manufacturer uses this proprietary function. Using a heated cutter, evaporation units “PP-TEC “Innovative Fluessiggas Technik” (Germany) were able to evaporate heavy components of LPG.
Many manufacturers, copying from each other, install a cut-off valve at the outlet in front of the regulators. The mercaptans, sulfur and heavy gases contained in the gas, which have a very high density When entering a cold pipeline, they condense and are deposited on the walls of pipes, cut-off valves and regulators, which significantly reduces the service life of the equipment.
In PP-TEC “Innovative Fluessiggas Technik” (Germany) evaporators, heavy sediments in a molten state are kept in a separator until they are removed through a discharge ball valve in the evaporation unit.
By cutting off mercaptans, the company “PP-TEC “Innovative Fluessiggas Technik” (Germany) was able to achieve a significant increase in the service life of installations and regulatory groups. This means taking care of operating costs that do not require constant replacement of regulator membranes, or their complete expensive replacement, leading to downtime of the evaporation unit.
And the implemented function of heating the solenoid valve and filter at the inlet to the evaporation unit prevents water from accumulating in them even when freezing in solenoid valves disable when triggered. Or limit the entry of the liquid phase into the evaporation unit.
Evaporation plants German company“PP-TEC “Innovative Fluessiggas Technik” (Germany) is reliable and stable operation for for long years operation.
In order to increase the safety of operation of the refrigeration unit, it is recommended that condensers, linear receivers and oil separators (high pressure devices) with big amount refrigerant should be placed outside the engine room.
This equipment, as well as receivers for storing refrigerant reserves, must be surrounded by a metal barrier with a lockable entrance. Receivers must be protected from sunlight and precipitation by a canopy. Apparatuses and vessels installed indoors can be located in a compressor shop or a special equipment room if it has a separate exit to the outside. The passage between the smooth wall and the device must be at least 0.8 m, but installation of devices against walls without passages is allowed. The distance between the protruding parts of the devices 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. It is prohibited to punch holes in columns to secure equipment.
For installation of devices and further maintenance of condensers and circulation receivers, metal platforms with fencing and stairs are installed. If the length of the platform is more than 6 m, there should be two stairs.
Platforms and stairs must have handrails and edges. The height of the handrails is 1 m, the edge is at least 0.15 m. The distance between the handrail posts is no more than 2 m.
Tests of apparatus, vessels and pipeline systems for strength and density are carried out upon completion installation work and within the time limits provided for by the “Rules for the design and safe operation of ammonia refrigeration units.”
Horizontal cylindrical devices. 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 linear length towards the oil sump.
The devices rest on antiseptic wooden beams at least 200 mm wide 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 no less than the thickness of the thermal insulation, and under
Wooden blocks with a length of 50-100 mm and a height equal to the thickness of the insulation are placed in belts at a distance of 250-300 mm from each other around the circumference (Fig. 11).
To clean condenser and evaporator pipes from contamination, the distance between their end caps and walls should be 0.8 m on one side and 1.5-2.0 m on the other. When installing devices in a room to replace pipes of condensers and evaporators, a “false window” is installed (in the wall opposite the cover of the device). To do this, an opening is left in the building's masonry, which is filled thermal insulation material, sewed up with boards and plastered. When repairing devices, the “false window” is opened and restored upon completion of the repair. Upon completion of work on placing the devices, automation and control devices, shut-off valves, and safety valves are installed on them.
The cavity of the apparatus for the refrigerant is purged compressed air, testing for strength and density is carried out with the covers removed. When installing a condenser-receiver unit, a horizontal shell-and-tube condenser is installed on the platform above the linear receiver. The size of the site must ensure all-round maintenance of the device.
Vertical shell and tube condensers. The devices are installed outdoors on a massive foundation with a pit for draining water. When making the foundation, the bolts for securing the lower flange of the apparatus are placed in concrete. The capacitor is installed crane for packs of linings and wedges. By tamping wedges, the apparatus is positioned strictly vertically using plumb lines located in two mutually perpendicular planes. In order to prevent the plumb lines from swinging 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 device, water will not normally wash the surface of the pipes. Upon completion of the alignment of the apparatus, the linings and wedges are welded into bags and the foundation is poured.
Evaporative condensers. They are supplied assembled for installation and installed on a platform whose dimensions allow for all-round maintenance of these devices. ‘The height of the platform is taken into account the placement of linear receivers under it. For ease of maintenance, the platform is equipped with a ladder, and if the fans are located at the top, it is additionally installed between the platform and the upper plane of the device.
After installing the evaporative condenser, connect it to circulation pump and pipelines.
The most widely used are evaporative condensers of the TVKA and Evako types produced by VNR. The drop-repellent layer of these devices is made of plastic, so welding and other work with open flames should be prohibited in the area where the devices are installed. Fan motors are grounded. When installing the device on a hill (for example, on the roof of a building), lightning protection must be used.
Panel evaporators. They are supplied as separate units and are assembled during installation work.
The evaporator tank is tested for leaks by pouring water and installed on a 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 laid between the foundation and the tank. Panel sections are installed in the tank strictly horizontally, level. Side surfaces The tank is insulated and plastered, and the mixer is adjusted.
Chamber devices. Wall and ceiling batteries are assembled from standardized sections (Fig. 13) at the installation site.
For ammonia batteries, sections of pipes with a diameter of 38X2.5 mm are used, for coolant - with a diameter of 38X3 mm. The pipes are finned with spirally wound fins made of 1X45 mm steel tape with fin spacing of 20 and 30 mm. The characteristics of the sections are presented in table. 6.
The total length of 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).
Battery hoses are placed strictly horizontally and level.
Ceiling air coolers are supplied assembled for installation. Bearing structures devices (channels) are connected to the channels of embedded parts. The horizontal installation of the devices is checked using 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 linear length.
To remove melt water during defrosting, they are installed drain pipes, on which heating elements of the ENGL-180 type are fixed. The heating element is a glass fiber tape, which is based on metal heating cores made of an alloy with high resistivity. Heating elements they are wound onto the pipeline spirally or laid linearly, secured to the pipeline with glass tape (for example, tape LES-0.2X20). On the vertical section of the drain pipeline, heaters are installed only in a spiral manner. When laying linearly, the heaters are secured to the pipeline with glass tape in increments of no more than 0.5 m. After the heaters are secured, the pipeline is insulated with non-flammable insulation and sheathed with a protective metal sheath. In places where the heater bends significantly (for example, on flanges), it is necessary to place aluminum tape 0.2-1.0 mm thick and 40-80 mm wide to avoid local overheating.
Upon completion of installation, all devices are tested for strength and density.
