Refrigeration unit operation options: operation with normal overheating; with insufficient overheating; severe overheating.
Operation with normal overheating.
Refrigeration unit diagram
For example, the refrigerant is supplied at a pressure of 18 bar, and the suction pressure is 3 bar. The temperature at which the refrigerant boils in the evaporator is t 0 = −10 °C, at the outlet of the evaporator the temperature of the pipe with the refrigerant is t t = −3 °C.
Useful superheat ∆t = t t − t 0 = −3− (−10)= 7. This is the normal operation of a refrigeration unit with air heat exchanger. IN evaporator Freon boils away completely in about 1/10 of the evaporator (closer to the end of the evaporator), turning into gas. The gas will then be heated by the room temperature.
Overheating is insufficient.
The outlet temperature will no longer be, for example, −3, but −6 °C. Then the overheating is only 4 °C. The point where the liquid refrigerant stops boiling moves closer to the evaporator outlet. Thus, most of the evaporator is filled with liquid refrigerant. This can happen if the thermostatic expansion valve (TEV) supplies more freon to the evaporator.
The more freon there is in the evaporator, the more vapors will be formed, the higher the suction pressure will be and the boiling point of freon will increase (let’s say it’s no longer −10, but −5 °C). The compressor will begin to fill with liquid freon because the pressure has increased, the refrigerant flow rate has increased and the compressor does not have time to pump out all the vapors (if the compressor does not have additional capacity). With this type of operation, the cooling capacity will increase, but the compressor may fail.
Severe overheating.
If the performance of the expansion valve is lower, then less freon will enter the evaporator and it will boil off earlier (the boiling point will shift closer to the evaporator inlet). The entire expansion valve and the tubes after it will freeze and become covered with ice, but 70 percent of the evaporator will not freeze at all. The freon vapors in the evaporator will heat up, and their temperature can reach the room temperature, hence ∆t ˃ 7. In this case, the cooling capacity of the system will decrease, the suction pressure will decrease, and the heated freon vapors can damage the compressor stator.
Undercooling of condensate is understood as a decrease in the temperature of the condensate against the temperature saturated steam, entering the capacitor. It was noted above that the amount of condensate supercooling is determined by the temperature difference t n -t To .
Subcooling of the condensate leads to a noticeable decrease in the efficiency of the installation, since with subcooling of the condensate, the amount of heat transferred in the condenser to the cooling water increases. An increase in condensate subcooling by 1°C causes excess fuel consumption in installations without regenerative heating of feedwater by 0.5%. With regenerative heating of feedwater, the excess fuel consumption in the installation is somewhat less. IN modern installations in the presence of regenerative type condensers, condensate subcooling under normal operating conditions condensing unit does not exceed 0.5-1°C. Subcooling of condensate is caused by the following reasons:
a) violation of the air density of the vacuum system and increased air suction;
b) high level condensate in the condenser;
c) excessive flow of cooling water through the condenser;
d) design flaws of the capacitor.
Increasing the air content in the steam-air
mixture leads to an increase in the partial pressure of air and, accordingly, to a decrease in the partial pressure of water vapor relative to the total pressure of the mixture. As a result, the temperature of saturated water vapor, and therefore the temperature of the condensate, will be lower than it was before the increase in air content. Thus, one of the important measures aimed at reducing condensate subcooling is to ensure good air density of the vacuum system of the turbine unit.
With a significant increase in the level of condensate in the condenser, a phenomenon may occur that the lower rows of cooling tubes will be washed by condensate, as a result of which the condensate will be supercooled. Therefore, it is necessary to ensure that the condensate level is always below the lower row of cooling tubes. The best remedy preventing an unacceptable increase in the level of condensate is a device for automatically regulating it in the condenser.
Excessive water flow through the condenser, especially at low temperatures, will lead to an increase in the vacuum in the condenser due to a decrease in the partial pressure of water vapor. Therefore, the flow of cooling water through the condenser must be adjusted depending on steam load on the condenser and on the temperature of the cooling water. At correct adjustment cooling water flow rate in the condenser, an economic vacuum will be maintained and the subcooling of the condensate will not go beyond the minimum value for a given condenser.
Overcooling of condensate can occur due to design flaws of the condenser. In some condenser designs, as a result of the close arrangement of the cooling tubes and their unsuccessful distribution along the tube sheets, a large vapor resistance is created, reaching in some cases 15-18 mm Hg. Art. The high vapor resistance of the condenser leads to a significant decrease in pressure above the condensate level. A decrease in the pressure of the mixture above the condensate level occurs due to a decrease in the partial pressure of water vapor. Thus, the condensate temperature is significantly lower than the temperature of the saturated steam entering the condenser. In such cases, in order to reduce the supercooling of the condensate, it is necessary to make structural modifications, namely, to remove some of the cooling tubes in order to install corridors in the tube bundle and reduce the vapor resistance of the condenser.
It should be borne in mind that the removal of part of the cooling tubes and the resulting reduction in the cooling surface of the condenser leads to an increase in the specific load of the condenser. However, increasing the specific steam load is usually quite acceptable since older condenser designs have a relatively low specific steam load.
We reviewed the main issues of operating condensing unit equipment steam turbine. From the above it follows that the main attention when operating a condensing unit should be paid to maintaining an economic vacuum in the condenser and ensuring minimal subcooling of the condensate. These two parameters significantly affect the efficiency of the turbine unit. For this purpose, it is necessary to maintain good air density vacuum system turbine units, ensure the normal operation of air removal devices, circulation and condensate pumps, keep the condenser tubes clean, monitor the water density of the condenser, prevent an increase in raw water suction, ensure the normal operation of cooling devices. The instrumentation, automatic regulators, signaling and control devices available at the installation allow maintenance personnel to monitor the condition of the equipment and the operating mode of the installation and maintain such operating modes that ensure highly economical and reliable operation of the installation.
In the condenser, the gaseous refrigerant compressed by the compressor turns into a liquid state (condenses). Depending on the operating conditions of the refrigeration circuit, the refrigerant vapor may condense completely or partially. For proper functioning of the refrigeration circuit, complete condensation of the refrigerant vapor in the condenser is necessary. The condensation process occurs at constant temperature, called the condensation temperature.
Refrigerant subcooling is the difference between the condensing temperature and the refrigerant temperature leaving the condenser. As long as there is at least one gas molecule in the mixture of gaseous and liquid refrigerant, the temperature of the mixture will be equal to the condensation temperature. Therefore, if the temperature of the mixture at the condenser outlet is equal to the condensation temperature, then the refrigerant mixture contains vapor, and if the temperature of the refrigerant at the condenser outlet is lower than the condensation temperature, then this clearly indicates that the refrigerant has completely turned into a liquid state.
Refrigerant overheating is the difference between the temperature of the refrigerant leaving the evaporator and the boiling point of the refrigerant in the evaporator.
Why do you need to overheat the vapors of already boiled-off refrigerant? The point of this is to make sure that all the refrigerant is guaranteed to change into a gaseous state. The presence of a liquid phase in the refrigerant entering the compressor can lead to water hammer and damage the compressor. And since the boiling of the refrigerant occurs at a constant temperature, we cannot say that all the refrigerant has boiled away until its temperature exceeds its boiling point.
In internal combustion engines we have to deal with the phenomenon torsional vibrations shafts If these vibrations threaten the strength of the crankshaft in the operating range of shaft rotation speed, then anti-vibrators and dampers are used. They are placed at the free end of the crankshaft, i.e. where the greatest torsional forces occur
fluctuations.
external forces force the diesel crankshaft to undergo torsional vibrations
These forces are gas pressure and inertia forces of the connecting rod and crank mechanism, under the variable action of which a continuously changing torque is created. Under the influence of uneven torque, sections of the crankshaft are deformed: they twist and unwind. In other words, torsional vibrations occur in the crankshaft. The complex dependence of torque on the angle of rotation of the crankshaft can be represented as a sum of sinusoidal (harmonic) curves with different amplitudes and frequencies. At a certain crankshaft rotation frequency, the frequency of the disturbing force, in in this case any component of the torque may coincide with the natural frequency of the shaft, i.e., a resonance phenomenon will occur, in which the amplitudes of the torsional vibrations of the shaft can become so large that the shaft can collapse.
To eliminate the phenomenon of resonance in modern diesel engines, special devices are used - anti-vibrators. One type of such device, the pendulum antivibrator, has become widespread. At the moment when the movement of the flywheel accelerates during each of its oscillations, the load of the antivibrator, according to the law of inertia, will tend to maintain its movement at the same speed, i.e., it will begin to lag at a certain angle from the section of the shaft to which the antivibrator is attached (position II) . The load (or rather, its inertial force) will, as it were, “slow down” the shaft. When the angular velocity of the flywheel (shaft) begins to decrease during the same oscillation, the load, obeying the law of inertia, will tend to “pull” the shaft along with it (position III),
Thus, the inertial forces of the suspended load during each oscillation will periodically act on the shaft in the direction opposite to the acceleration or deceleration of the shaft, and thereby change the frequency of its own oscillations.
Silicone Dampers. The damper consists of a sealed housing, inside of which a flywheel (mass) is located. The flywheel can rotate freely relative to the housing mounted at the end of the crankshaft. The space between the housing and the flywheel is filled with silicone liquid, which has a high viscosity. When the crankshaft rotates uniformly, the flywheel, due to friction forces in the fluid, acquires the same frequency (speed) of rotation as the shaft. What if torsional vibrations of the crankshaft occur? Then their energy is transferred to the body and will be absorbed by the forces of viscous friction arising between the body and the inertial mass of the flywheel.
Low speed and load modes. The transition of the main engines to low speed modes, as well as the transition of auxiliary engines to low load modes, is associated with a significant reduction in the fuel supply to the cylinders and an increase in excess air. At the same time, the air parameters at the end of compression decrease. The change in PC and Tc is especially noticeable in engines with gas turbine supercharging, since the gas turbine compressor practically does not work at low loads and the engine automatically switches to the naturally aspirated operating mode. Small portions of burning fuel and a large excess of air reduce the temperature in the combustion chamber.
Due to the low temperatures of the cycle, the fuel combustion process is sluggish and slow; part of the fuel does not have time to burn and flows down the cylinder walls into the crankcase or is carried away with the exhaust gases into the exhaust system.
Poor mixture formation of fuel with air, caused by a decrease in fuel injection pressure when the load drops and the rotation speed decreases, also contributes to the deterioration of fuel combustion. Uneven and unstable fuel injection, as well as low temperatures in the cylinders, cause unstable engine operation, often accompanied by misfiring and increased smoking.
Carbon formation is especially intense when heavy fuels are used in engines. When operating at low loads, due to poor atomization and relatively low temperatures in the cylinder, drops of heavy fuel do not burn out completely. When a drop is heated, the light fractions gradually evaporate and burn, and only heavy, high-boiling fractions remain in its core, the basis of which is aromatic hydrocarbons, which have the strongest bonds between atoms. Therefore, their oxidation leads to the formation of intermediate products - asphaltenes and resins, which have high stickiness and can be firmly adhered to metal surfaces.
Due to the above circumstances, when engines operate for a long time at low speeds and loads, intensive contamination of the cylinders and especially the exhaust tract occurs with products of incomplete combustion of fuel and oil. The exhaust channels of the working cylinder covers and exhaust pipes are covered with a dense layer of asphalt-resinous substances and coke, often reducing their flow area by 50-70%. In the exhaust pipe, the thickness of the carbon layer reaches 10-20mm. These deposits periodically ignite as the engine load increases, causing a fire in the exhaust system. All oily deposits burn out, and the dry carbon dioxide substances formed during combustion are blown into the atmosphere.
Formulations of the second law of thermodynamics.
For existence heat engine 2 sources are needed - a hot spring and a cold spring (environment). If a heat engine operates from only one source, then it is called a perpetual motion machine of the 2nd kind.
1 formulation (Ostwald):
"A perpetual motion machine of the 2nd kind is impossible."
A perpetual motion machine of the 1st kind is a heat engine for which L>Q1, where Q1 is the supplied heat. The first law of thermodynamics “allows” the possibility of creating a heat engine that completely converts the supplied heat Q1 into work L, i.e. L = Q1. The second law imposes more stringent restrictions and states that the work must be less than the heat supplied (L
“Heat cannot spontaneously transfer from a colder body to a warmer one.”
To operate a heat engine, two sources are needed - hot and cold. 3rd formulation (Carnot):
“Where there is a temperature difference, work can be done.”
All these formulations are interconnected; from one formulation you can get another.
Indicator efficiency depends on: compression ratio, excess air ratio, combustion chamber design, advance angle, rotation speed, fuel injection duration, atomization quality and mixture formation.
Increasing indicator efficiency(by improving the combustion process and reducing fuel heat losses during compression and expansion processes)
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Modern engines are characterized by a high level of thermal stress of the cylinder-piston group, due to the acceleration of their working process. This requires technically competent maintenance of the cooling system. The necessary heat removal from the heated surfaces of the engine can be achieved either by increasing the difference in water temperature T = T in.out - T in.in, or by increasing its flow rate. Most diesel manufacturing companies recommend T = 5 – 7 degrees C for MOD, and t = 10 – 20 degrees C for SOD and VOD. The limitation of the water temperature difference is caused by the desire to maintain minimum temperature stresses of the cylinders and bushings along their height. Intensification of heat transfer is carried out due to high speeds of water movement.
When cooling with sea water, the maximum temperature is 50 degrees C. Only closed cooling systems can take advantage of high temperature cooling. When the coolant temperature rises. water, friction losses in the piston group decrease and the eff. increases slightly. power and efficiency of the engine, with an increase in TV, the temperature gradient across the thickness of the bushing decreases, and thermal stresses also decrease. When the cooling temperature decreases. water, chemical corrosion increases due to condensation of sulfuric acid on the cylinder, especially when burning sulfur fuels. However, there is a limitation of the water temperature due to the limitation of the temperature of the cylinder mirror (180 degrees C) and its further increase can lead to a violation of the strength of the oil film, its disappearance and the appearance of dry friction. Therefore, most companies limit the temperature to 50 -60 g. C and only when burning high-sulfur fuels 70 -75 g is allowed. WITH.
Heat transfer coefficient- a unit that denotes the passage of a heat flux of 1 W through a building structure element with an area of 1 m2 at a difference in outside and inside air temperatures of 1 Kelvin W/(m2K).
The definition of heat transfer coefficient is as follows: the loss of energy per square meter of surface with a difference in external and internal temperatures. This definition entails the relationship between watts, square meters and Kelvin W/(m2·K).
To calculate heat exchangers, a kinetic equation is widely used, which expresses the relationship between the heat flow Q and the heat transfer surface F, called basic heat transfer equation: Q = KF∆tсрτ, where K is the kinetic coefficient (heat transfer coefficient characterizing the rate of heat transfer; ∆tср is the average driving force or the average temperature difference between coolants (average temperature difference) along the heat transfer surface; τ is time.
The greatest difficulty is the calculation heat transfer coefficient K, which characterizes the rate of the heat transfer process involving all three types of heat transfer. The physical meaning of the heat transfer coefficient follows from the equation (); its dimension:
In Fig. 244 OB = R - crank radius and AB=L - connecting rod length. Let's denote the ratio L0 = L/ R - is called the relative length of the connecting rod, for marine diesel engines it is in the range of 3.5-4.5.
however, in the KSM theory, THE REVERSE QUANTITY λ= R / L IS USED
The distance between the piston pin axis and the shaft axis when it is rotated through an angle a
AO = AD + DO = LcosB + Rcosa
When the piston is in. m.t., then this distance is equal to L+R.
Consequently, the path traveled by the piston when turning the crank through an angle a will be equal to x=L+R-AO.
By mathematical calculations we obtain the formula for the piston path
X = R ( 1-cosa +1/ λ(1-cosB) ) (1)
average speed piston Vm, along with the rotation speed, is an indicator of the engine speed. It is determined by the formula Vm = Sn/30, where S is the piston stroke, m; n - rotation speed, min-1. It is believed that for MOD vm = 4-6 m/s, for SOD vm = 6s-9 m/s and for VOD vm > 9 m/s. The higher vm, the greater the dynamic stresses in engine parts and the greater the likelihood of their wear - primarily the cylinder-piston group (CPG). Currently, the vm parameter has reached a certain limit (15-18.5 m/s), due to the strength of materials used in engine construction, especially since the dynamic tension of the cylinder head is proportional to the square of the vm value. Thus, with an increase in vm by a factor of 3, the stresses in the parts will increase by a factor of 9, which will require a corresponding increase in the strength characteristics of the materials used for the manufacture of CPG parts.
The average piston speed is always indicated in the engine manufacturer's passport (certificate).
The true speed of the piston, i.e. its speed at a given moment (in m/sec), is defined as the first derivative of the path with respect to time. Let's substitute a= ω t into formula (2), where ω is the shaft rotation frequency in rad/sec, t is the time in sec. After mathematical transformations we obtain the formula for piston speed:
C=Rω(sina+0.5λsin2a) (3)
where R is the radius of the crank vm\
ω - angular frequency of crankshaft rotation in rad/sec;
a - angle of rotation of the crankshaft in degrees;
λ= R/L-ratio of crank radius to connecting rod length;
Co is the peripheral speed of the center of the crank pin, vm/sec;
L - connecting rod length inm.
With an infinite connecting rod length (L=∞ and λ =0), the piston speed is equal to
Differentiating formula (1) in a similar way, we obtain
С= Rω sin (a +B) / cosB (4)
The values of the function sin(a+B) are taken from the tables given in reference books and manuals depending on a and λ.
Obviously, the maximum value of the piston speed at L=∞ will be at а=90° and а=270°:
Cmax= Rω sin a.. Since Co= πRn/30 and Cm=Sn/30=2Rn/30=Rn/15 then
Co/Cm= πRn15/Rn30=π/2=1.57 whence Co=1.57 Cm
Consequently, the maximum speed of the piston will be equal. Cmax = 1.57 St.
Let us represent the velocity equation in the form
С = Rωsin a +1/2λ Rωsin2a.
Graphically, both terms on the right side of this equation will be depicted as sinusoids. The first term Rωsin a, representing the piston speed for an infinite length of the connecting rod, will be represented by a first-order sinusoid, and the second term 1/2λ Rωsin2a-correction for the influence of the finite length of the connecting rod - by a second-order sinusoid.
By constructing the indicated sinusoids and adding them algebraically, we obtain a speed graph taking into account the indirect influence of the connecting rod.
In Fig. 247 are shown: 1 - curve Rωsin a,
2 - curve1/2λ Rωsin2a
3 - curveC.
Operational properties are understood as objective characteristics of the fuel that manifest themselves during its use in an engine or unit. The combustion process is the most important and determines its operational properties. The process of fuel combustion is, of course, preceded by the processes of its evaporation, ignition and many others. The nature of the behavior of the fuel in each of these processes is the essence of the main operational properties of fuels. The following performance properties of fuels are currently being assessed.
Volatility characterizes the ability of a fuel to change from a liquid to a vapor state. This property is formed from such fuel quality indicators as fractional composition, saturated vapor pressure at different temperatures, surface tension and others. Volatility is important when selecting fuel and largely determines the technical, economic and performance characteristics engines.
Flammability characterizes the features of the ignition process of mixtures of fuel vapors and air. The assessment of this property is based on quality indicators such as temperature and concentration limits ignition, flash point and self-ignition, etc. The flammability index of a fuel has the same meaning as its flammability; in what follows, these two properties are considered together.
Flammability determines the efficiency of the combustion process of fuel-air mixtures in engine combustion chambers and combustion devices.
Pumpability characterizes the behavior of fuel when pumping it through pipelines and fuel systems, as well as when filtering it. This property determines the uninterrupted supply of fuel to the engine at different operating temperatures. The pumpability of fuels is assessed by viscosity-temperature properties, cloud point and pour point, filterability limit temperature, water content, mechanical impurities, etc.
Deposit proneness is the ability of a fuel to form various types of deposits in the combustion chambers, fuel systems, intake and exhaust valves. The assessment of this property is based on such indicators as ash content, coking capacity, content of resinous substances, unsaturated hydrocarbons etc.
Corrosivity and compatibility with non-metallic materials characterizes the ability of a fuel to cause corrosion of metals, swelling, destruction or change in the properties of rubber seals, sealants and other materials. This performance property provides for a quantitative assessment of the content of corrosive substances in the fuel, testing the resistance of various metals, rubbers and sealants in contact with fuel.
Protective ability is the ability of the fuel to protect the materials of engines and units from corrosion when they come into contact with an aggressive environment in the presence of fuel and, first of all, the ability of the fuel to protect metals from electrochemical corrosion when water enters. This property is assessed special methods, involving the impact of ordinary, sea and rain water on metals in the presence of fuel.
Anti-wear properties characterize the reduction in wear of rubbing surfaces in the presence of fuel. These properties are important for engines in which fuel pumps and fuel control equipment are lubricated only by the fuel itself without the use of lubricant (for example, in a plunger fuel pump high pressure). The property is assessed by viscosity and lubricity.
Cooling capacity determines the ability of the fuel to absorb and remove heat from heated surfaces when using the fuel as a coolant. The assessment of properties is based on quality indicators such as heat capacity and thermal conductivity.
Stability characterizes the preservation of fuel quality indicators during storage and transportation. This property evaluates the physical and chemical stability of the fuel and its susceptibility to biological attack by bacteria, fungi and mold. The level of this property makes it possible to establish the guaranteed shelf life of fuel in various climatic conditions.
Environmental properties characterize the impact of fuel and its combustion products on humans and environment. The assessment of this property is based on the toxicity of fuel and its combustion products and fire and explosion hazard.
The vast expanses of the sea are plowed by large vessels obedient to the hands and will of man, driven by powerful engines that use various types of marine fuel. Transport vessels can use different engines, however, most of these floating structures are equipped with diesel engines. Marine engine fuel used in marine diesel engines is divided into two classes - distillate and heavy. Distillate fuel includes diesel summer fuel, as well as foreign fuels Marine Diesel Oil, Gas Oil and others. It has a low viscosity, so it does not
requires preheating when starting the engine. It is used in high-speed and medium-speed diesel engines, and in some cases, in low-speed diesel engines in start-up mode. It is sometimes used as an additive to heavy fuel in cases where it is necessary to reduce its viscosity. Heavy varieties fuels differ from distillate fuels by increased viscosity, more high temperature solidification, presence more heavy fractions, high content of ash, sulfur, mechanical impurities and water. Prices for marine fuel of this type are significantly lower.
Most ships use the cheapest heavy diesel fuel for ship engines, or fuel oil. The use of fuel oil is dictated primarily for economic reasons, because the prices for marine fuel, as well as the overall costs of transporting goods by sea, are significantly reduced when using fuel oil. As an example, it can be noted that the difference in the cost of fuel oil and other types of fuel used for marine engines is about two hundred euros per ton.
However, the Rules of Maritime Shipping prescribe in certain operating modes, for example, when maneuvering, the use of more expensive low-viscosity marine fuel, or diesel fuel. In some marine areas, for example, the English Channel, due to the complexity of navigation and the need to comply with environmental requirements, the use of fuel oil as the main fuel is generally prohibited.
Fuel selection depends largely on the temperature at which it will be used. Normal starting and scheduled operation of the diesel engine is ensured in summer period with a cetane number of 40-45, in winter period it is necessary to increase it to 50-55. For motor fuels and fuel oils, the cetane number is in the range of 30-35, for diesel fuels – 40-52.
Ts diagrams are used primarily for illustrative purposes because in a Pv diagram the area under the curve expresses the work done by a pure substance in a reversible process, while in a Ts diagram the area under the curve represents the heat received for the same conditions.
Toxic components are: carbon monoxide CO, hydrocarbons CH, nitrogen oxides NOx, particulate matter, benzene, toluene, polycyclic aromatic hydrocarbons PAHs, benzopyrene, soot and particulate matter, lead and sulfur.
Currently emission standards harmful substances marine diesels established by IMO, the international maritime organization. All currently produced marine diesel engines must meet these standards.
The main components dangerous to humans in exhaust gases are: NOx, CO, CnHm.
A number of methods, for example, direct water injection, can only be implemented at the design and manufacturing stage of the engine and its systems. For an existing model range engines, these methods are unacceptable or require significant costs for upgrading the engine, replacing its components and systems. In a situation where a significant reduction in nitrogen oxides is necessary without re-equipping serial diesel engines - and here is exactly such a case, the most effective way is the use of a three-way catalytic converter. The use of a neutralizer is justified in areas where there are high requirements for NOx emissions, for example in large cities.
Thus, the main directions for reducing harmful exhaust emissions from diesel engines can be divided into two groups:
1)-improvement of engine design and systems;
2) - methods that do not require engine modernization: the use of catalytic converters and other means of exhaust gas purification, improvement of fuel composition, use of alternative fuels.
Air conditioner
Filling an air conditioner with freon can be done in several ways, each of them has its own advantages, disadvantages and accuracy.
The choice of method for refilling air conditioners depends on the level of professionalism of the technician, the required precision and the tools used.
It is also necessary to remember that not all refrigerants can be refilled, but only single-component (R22) or conditionally isotropic (R410a).
Multicomponent freons consist of a mixture of gases with different physical properties, which, when leaked, evaporate unevenly and even with a small leak, their composition changes, so systems using such refrigerants must be completely recharged.
Each air conditioner is charged at the factory with a certain amount of refrigerant, the mass of which is indicated in the documentation for the air conditioner (also indicated on the nameplate), information about the amount of freon that must be added additionally per meter is also indicated there. freon route(usually 5-15 gr.)
When refueling using this method, it is necessary to completely empty the refrigeration circuit of the remaining freon (into a cylinder or vent it into the atmosphere, this does not harm the environment at all - read about this in the article on the influence of freon on the climate) and evacuate it. Then fill the system with the specified amount of refrigerant using a scale or using a filling cylinder.
The advantages of this method are high precision and the fairly simple process of refilling the air conditioner. The disadvantages include the need to evacuate freon and evacuate the circuit, and the filling cylinder also has a limited volume of 2 or 4 kilograms and large dimensions, which allows it to be used mainly in stationary conditions.
Subcooling temperature is the difference between the freon condensation temperature determined from a table or pressure gauge scale (determined by the pressure read from a pressure gauge connected to the high-pressure line directly on the scale or table) and the temperature at the outlet of the condenser. The supercooling temperature should usually be in the range of 10-12 0 C ( exact value manufacturers indicate)
A hypothermia value below these values indicates a lack of freon - it does not have time to cool sufficiently. In this case, it must be refueled
If the subcooling is higher than the specified range, then there is an excess of freon in the system and it must be drained until the optimal subcooling values are reached.
You can refill this method using special devices, which immediately determine the amount of subcooling and condensation pressure, or can be done using separate instruments - a manometric manifold and a thermometer.
The advantages of this method include sufficient accuracy of filling. But the accuracy of this method is affected by contamination of the heat exchanger, so before refueling with this method, it is necessary to clean (rinse) the condenser of the outdoor unit.
Superheat is the difference between the evaporation temperature of the refrigerant determined by the saturation pressure in the refrigeration circuit and the temperature after the evaporator. It is practically determined by measuring the pressure at the air conditioner suction valve and the temperature of the suction tube at a distance of 15-20 cm from the compressor.
Superheat is usually within 5-7 0 C (the exact value is indicated by the manufacturer)
A decrease in overheating indicates an excess of freon - it must be drained.
Subcooling above normal indicates a lack of refrigerant; the system must be charged until the required superheat value is reached.
This method is quite accurate and can be significantly simplified if special devices are used.
If the system has an inspection window, then the presence of bubbles can indicate a lack of freon. In this case, fill the refrigeration circuit until the flow of bubbles disappears; this must be done in portions, after each portion wait for the pressure to stabilize and the absence of bubbles.
You can also fill by pressure, achieving the condensation and evaporation temperatures specified by the manufacturer. The accuracy of this method depends on the cleanliness of the condenser and evaporator.
-> 03/13/2012 - Hypothermia in refrigeration units
Subcooling the liquid refrigerant after the condenser is a significant way to increase the cooling capacity of a refrigeration unit. A decrease in the temperature of the subcooled refrigerant by one degree corresponds to an increase in the performance of a normally functioning refrigeration unit by approximately 1% at the same level of energy consumption. The effect is achieved by reducing, during supercooling, the proportion of steam in the vapor-liquid mixture, which is the condensed refrigerant supplied to the evaporator expansion valve even from the receiver.
In low-temperature refrigeration units, the use of subcooling is especially effective. They supercool the condensed refrigerant to significant negative temperatures allows you to increase the cooling capacity of the installation by more than 1.5 times.
Depending on the size and design of the refrigeration units, this factor can be realized in an additional heat exchanger installed on the liquid line between the receiver and the evaporator expansion valve in various ways.
subcooling systems using external sources cold is still quite rarely used in practice. Subcooling from cold water sources is used, as a rule, in heat pumps - water heating installations, as well as in medium- and high-temperature installations, where in the immediate vicinity there is a source of cool water - used artesian wells, natural reservoirs for ship installations, etc. . Hypothermia from external additional refrigeration machines is implemented extremely rarely and only in very large installations industrial cold.
Subcooling in air heat exchangers is also used very infrequently, since this option of refrigeration units is still poorly understood and unusual for Russian refrigeration manufacturers. In addition, designers are confused by seasonal fluctuations in the increase in cooling capacity of installations from the use of air subcoolers.
Subcooling systems that use internal resources are widely used in modern refrigeration units, with almost all types of compressors. In installations with screw and two-stage piston compressors, the use of subcooling confidently dominates, since the ability to provide suction of vapors with intermediate pressure is implemented directly in the design of these types of compressors.
The main challenge currently facing manufacturers of refrigeration and air conditioning units for various purposes, is to increase the productivity and efficiency of the compressors included in them and heat exchange equipment. This idea has not lost its relevance throughout the development of refrigeration equipment from the inception of this industry to the present day. Today, when the cost of energy resources, as well as the size of the fleet of operated and commissioned refrigeration equipment has reached such impressive heights, increasing the efficiency of systems that produce and consume cold has become an urgent global problem. Taking into account the fact that this problem is complex, the current legislation of the majority European countries stimulate developers refrigeration systems to improve their efficiency and productivity.
