Subcooling conditions in a freon condenser. Effect of overheating on the refrigeration capacity of the refrigeration system
Rice. 1.21. Dendrite seme
Thus, the mechanism of crystallization of metal melts at high cooling rates is fundamentally different in that in small volumes of the melt, high degree hypothermia. The consequence of this is the development of bulk crystallization, which can be homogeneous for pure metals. Crystallization centers larger than the critical size are capable of further growth.
For metals and alloys, the most typical dendritic form of growth was first described back in 1868 by D.K. Chernov. In fig. 1.21 shows a sketch by D.K. Chernov, explaining the diagram of the structure of the dendrite. Usually, a dendrite consists of a trunk (first-order axis), from which branches go - axes of the second and subsequent orders. Dendritic growth proceeds in certain crystallographic directions with branches at regular intervals. In structures with lattices of face-centered and body-centered cubes, dendritic growth occurs in three mutually perpendicular directions. It was found experimentally that dendritic growth is observed only in a supercooled melt. The growth rate is determined by the degree of hypothermia. The problem of theoretically determining the growth rate as a function of the degree of hypothermia has not yet received a substantiated solution. Based on the experimental data, it is believed that this dependence can be approximately considered in the form V ~ (D T) 2.
Many researchers believe that at a certain critical degree of hypothermia, an avalanche-like increase in the number of crystallization centers capable of further growth is observed. The nucleation of more and more new crystals can interrupt dendritic growth.
Rice. 1.22. Transformation of structures
According to the latest foreign data, with an increase in the degree of supercooling and the temperature gradient ahead of the crystallization front, the structure of the rapidly solidifying alloy transforms from dendritic to equiaxial, microcrystalline, nanocrystalline and further to amorphous state (Fig. 1.22).
1.11.5. Melt amorphization
In fig. 1.23 illustrates an idealized TTT diagram (Time-Temperature-Transaction), which explains the features of solidification of alloyed metal melts depending on the cooling rate.
Rice. 1.23. TTT diagram: 1 - moderate cooling rate:
2 - very high cooling rate;
3 - intermediate cooling rate
Temperature is plotted along the vertical axis, and time is plotted along the horizontal axis. Above a certain melting point - Т P, the liquid phase (melt) is stable. Below this temperature, the liquid is supercooled and becomes unstable, since the possibility of nucleation and growth of crystallization centers appears. However, with sharp cooling, the motion of atoms in a strongly supercooled liquid may cease, and an amorphous solid phase will form at a temperature below T3. For many alloys, the temperature of the onset of amorphization, ТЗ, lies in the range from 400 to 500 ºC. Most conventional ingots and castings are cooled slowly according to curve 1 in Fig. 1.23. During cooling, crystallization centers appear and grow, forming the crystalline structure of the alloy in the solid state. At a very high cooling rate (curve 2), an amorphous solid phase is formed. The intermediate cooling rate (curve 3) is also of interest. For this case, a mixed version of solidification is possible with the presence of both crystalline and amorphous structure... This option takes place in the case when the crystallization process that has begun does not have time to complete during the cooling time to the temperature TZ.The mixed version of solidification with the formation of small amorphous particles is illustrated by a simplified diagram shown in Fig. 1.24.
Rice. 1.24. Formation of small amorphous particles
On the left in this figure, a large melt droplet is shown containing 7 crystallization centers in the volume that are capable of subsequent growth. In the middle, the same drop is divided into 4 parts, one of which does not contain crystallization centers. This particle will solidify amorphous. On the right in the figure, the original particle is divided into 16 parts, 9 of which will become amorphous. In fig. 1.25. the actual dependence of the number of amorphous particles of a highly alloyed nickel alloy on the particle size and the intensity of cooling in a gaseous medium (argon, helium) is presented.
Rice. 1.25. Dependence of the number of amorphous particles of a nickel alloy on
particle size and intensity of cooling in a gaseous environment
The transition of a metallic melt to an amorphous, or as it is also called, glassy state is a complex process and depends on many factors. In principle, all substances can be obtained in an amorphous state, but pure metals require such high cooling rates that cannot yet be provided by modern technical means... At the same time, highly alloyed alloys, including eutectic alloys of metals with metalloids (B, C, Si, P) solidify in an amorphous state at lower cooling rates. Table 1.9 shows the critical cooling rates during the amorphization of nickel melts and some alloys.
Table 1.9
Air conditioner
Filling the air conditioner with freon can be done in several ways, each of them has its own advantages, disadvantages and accuracy.
The choice of the method of filling the air conditioners depends on the level of professionalism of the master, the required accuracy and the tools used.
It is also necessary to remember that not all refrigerants can be topped up, but only one-component (R22) or conditionally isotropic (R410a).
Multicomponent freons consist of a mixture of gases with different physical properties, which in the event of a leak, volatilize unevenly and even with a small leak, their composition changes, therefore, systems using such refrigerants must be completely refilled.
Filling the air conditioner with freon by weight
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), there is also information about the amount of freon that must be added additionally for each meter of the freon route (usually 5-15 gr.)
When refueling using this method, it is necessary to completely release the refrigeration circuit from the remaining freon (into a cylinder or vent into the atmosphere, this does not harm the environment at all - read about this in the article on the effect of freon on the climate) and evacuate. Then fill the system with the specified amount of refrigerant by weight or using the filling cylinder.
The advantages of this method in high precision and sufficient simplicity of the process of filling the air conditioner. The disadvantages include the need to evacuate freon and evacuate the circuit, and the filling cylinder also has limited volume 2 or 4 kilograms and large dimensions, which allows it to be used mainly in stationary conditions.
Filling the air conditioner with freon for hypothermia
The subcooling temperature is the difference between the condensation temperature of freon determined according to the table or manometer scale (determined by the pressure read from the manometer connected to the line high pressure directly on the scale or according to the table) and the temperature at the outlet of the condenser. The hypothermia temperature should usually be in the range of 10-12 0 C ( exact value indicated by manufacturers)
A subcooling value below these values indicates a lack of freon - it does not have time to cool down enough. In this case, you need to refuel it.
If the subcooling is above 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.
Refueling in this way can be done with the help of special devices that immediately determine the amount of subcooling and condensation pressure, or it is possible with the help of separate instruments - a manometric manifold and a thermometer.
The advantages of this method include sufficient filling accuracy. But for accuracy this method the contamination of the heat exchanger affects, therefore, before refueling with this method, it is necessary to clean (rinse) the condenser of the outdoor unit.
Charging the air conditioner with overheating refrigerant
Overheating is the difference between the evaporating temperature of the refrigerant determined from the saturation pressure in the refrigeration circuit and the temperature after the evaporator. Practically determined by measuring the pressure at the suction valve of the air conditioner and the temperature of the suction pipe at a distance of 15-20 cm from the compressor.
Overheating is usually in the range of 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 the norm indicates a lack of refrigerant - the system must be charged until the required superheat is reached.
This method is quite accurate and can be significantly simplified if you use special devices.
Other methods of charging refrigeration systems
If the system has a viewing window, then by the presence of bubbles, one can judge about the lack of freon. In this case, the refrigeration circuit is charged until the flow of bubbles disappears, this must be done in portions, after each wait for the pressure to stabilize and the absence of bubbles.
It can also be charged by pressure, while achieving the condensing and evaporating temperatures specified by the manufacturer. The accuracy of this method depends on the cleanliness of the condenser and evaporator.
19.10.2015
The degree of subcooling of the liquid obtained at the outlet of the condenser is an important indicator that characterizes stable operation. refrigeration circuit... Subcooling is the temperature difference between liquid and condensation at a given pressure.
Under normal atmospheric pressure, condensation of water has a temperature of 100 degrees Celsius. According to the laws of physics, water that is 20 degrees is considered 80 degrees Celsius supercooled.
Subcooling at the outlet of the heat exchanger varies as the difference between the temperature of the liquid and the condensation. Based on figure 2.5, hypothermia will be 6 K or 38-32.
In capacitors with air cooled the hypothermia indicator should be from 4 to 7 K. If it has a different value, then this indicates unstable work.
Interaction of condenser and fan: air temperature difference.
The air blown by the fan has an indicator of 25 degrees Celsius (Figure 2.3). It takes heat from freon, due to which its temperature changes up to 31 degrees.
Figure 2.4 shows a more detailed change:
Tae is the temperature mark of the air supplied to the condenser;
Tas - air with new condenser temperature after cooling;
Tk - condensation temperature reading from the pressure gauge;
Δθ is the difference in temperature indicators.
The calculation of the temperature difference in an air-cooled condenser is carried out using the formula:
Δθ = (tas - tae), where K has a range of 5–10 K. On the graph, this value is 6 K.
The difference in temperature difference at point D, that is, at the exit from the capacitor, in this case is 7 K, since it is in the same limit. The temperature head is 10-20 K, in the figure it is (tk-tae). Most often the value this indicator stops at 15 K, but in this example, 13 K.
In this article we will talk about the most exact way refueling air conditioners.
You can refuel any freons. Refuel - only one-component freons (eg: R-22) or isotropic (conditionally isotropic, eg: R-410) mixtures
When carrying out diagnostics of refrigeration and air conditioning systems, the processes taking place inside the condenser are hidden from service engineer, and often it is from them that one can understand why the efficiency of the system as a whole has fallen.
Let's take a quick look at them:
- Superheated refrigerant vapor flows from the compressor to the condenser
- Under the influence of the air flow, the freon temperature is reduced to the condensation temperature
- Until the last freon molecule enters the liquid phase, the temperature remains the same throughout the entire section of the pipeline where the condensation process takes place.
- Under the action of the cooling air flow, the temperature of the refrigerant decreases from the condensation temperature to the temperature of the cooled liquid freon
Knowing the pressure, according to the special tables of the freon manufacturer, you can determine the condensation temperature under current conditions. The difference between the condensation temperature and the temperature of the cooled freon at the outlet of the condenser - the subcooling temperature - is a usually known value (to be specified with the system manufacturer) and the range of these values for this system is fixed (for example: 10-12 ° C).
If the subcooling value is below the range specified by the manufacturer, then the freon does not have time to cool in the condenser - it is not enough and refueling is required. The lack of freon reduces the efficiency of the system and increases the load on it.
If the subcooling value is higher than the range - there is too much freon, it is required to drain a part until the optimum value is reached. An excess of freon increases the load on the system and reduces its service life.
Subcooling refueling without use:
- We connect the gauge manifold and the freon cylinder to the system.
- Install a thermometer / temperature sensor on the high pressure line.
- We start the system.
- Using the pressure gauge on the high pressure line (liquid line), we measure the pressure, calculate the condensation temperature for a given freon.
- Using a thermometer, we control the temperature of the supercooled freon at the outlet from the condenser (it must be in the range of values of the sum of the condensation temperature and the subcooling temperature).
- If the freon temperature exceeds the permissible one (the hypothermia temperature is below the required range) - freon is not enough, slowly add it to the system until the desired temperature is reached
- If the freon temperature is below the permissible one (the hypothermia temperature is above the range) - freon is in excess, some must be slowly bleed off until the desired temperature is reached.
- We reset the device to zero, put it into hypothermia mode, set the type of freon.
- We connect the gauge manifold and the freon cylinder to the system, and the high pressure (liquid) hose is connected through the T-shaped tee supplied with the device.
- We install the SH-36N temperature sensor on the high pressure line.
- We turn on the system, the hypothermia value will be displayed on the screen, we compare it with the required range and, depending on whether the displayed value is higher or lower, we gradually bleed or add freon.
Alexey Matveev,
technical specialist of the company "Rashodka"
In the condenser, the gaseous refrigerant compressed by the compressor turns into a liquid state (condenses). Depending on the operating conditions of the refrigerant circuit, refrigerant vapors may condense completely or partially. For the refrigerant circuit to function properly, complete condensation of the refrigerant vapor in the condenser is required. The condensation process takes place at a constant temperature called the condensation temperature.
Refrigerant subcooling is the difference between the condensing temperature and the temperature of the refrigerant leaving the condenser. As long as there is at least one gas molecule in the mixture of gaseous and liquid refrigerants, the temperature of the mixture will be equal to the condensation temperature. Therefore, if the temperature of the mixture leaving the condenser is equal to the condensing temperature, it means that the refrigerant mixture contains steam, and if the temperature of the refrigerant leaving the condenser is lower than the condensing temperature, then this clearly indicates that the refrigerant has completely passed into the 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 vapors of already boiled refrigerant? The idea behind this is to make sure that all the refrigerant is guaranteed to be gaseous. The presence of a liquid phase in the refrigerant entering the compressor can cause 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, one has to deal with the phenomenon torsional vibrations shafts. If these vibrations threaten the strength of the crankshaft in the operating range of the shaft speed, then anti-vibration and dampers are used. They are placed at the free end of the crankshaft, i.e., where the greatest torsional
fluctuations.
external forces force the diesel crankshaft to perform torsional vibrations
These forces are the pressure of gases and the forces of inertia of the connecting rod-crank mechanism, under the variable action of which a continuously changing torque is created. Under the influence of uneven torque, the sections of the crankshaft are deformed: they twist and unwind. In other words, torsional vibrations occur in the crankshaft. The complex dependence of the 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 frequency of rotation of the crankshaft, the frequency of the perturbing force, in this case, some component of the torque, can coincide with the frequency of natural vibrations of the shaft, i.e., the resonance phenomenon occurs, in which the amplitudes of 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. Wide use received one of the types of such a device - a pendulum anti-vibration device. At the moment when the movement of the flywheel during each of its oscillations will accelerate, the load of the anti-vibration device, according to the law of inertia, will tend to maintain its motion at the same speed, i.e., it will begin to lag behind the shaft section to which the anti-vibration device 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) during the same oscillation begins to decrease, the load, obeying the law of inertia, will tend to "pull" the shaft along (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 natural oscillations.
Silicone Dampers... The damper consists of a sealed housing with a flywheel (mass) inside. The flywheel can rotate freely relative to the housing mounted on the end of the crankshaft. The space between the housing and the flywheel is filled with a highly viscous silicone fluid. When the crankshaft rotates uniformly, the flywheel, due to the frictional forces in the fluid, acquires the same frequency (speed) of rotation as the shaft. And if torsional vibrations of the crankshaft occur? Then their energy is transferred to the body and will be absorbed by the viscous friction forces that arise 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 fuel supply to the cylinders and an increase in excess air. At the same time, the parameters of the air 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 operation mode. Small portions of combustion fuel and a large excess of air reduce the temperature in the combustion chamber.
Because of low temperatures cycle, the fuel combustion process proceeds sluggishly, slowly, part of the fuel does not have time to burn and flows down the walls of the cylinder into the crankcase or is carried away with the exhaust gases into the exhaust system.
Poor fuel-air mixing also contributes to the deterioration of fuel combustion, due to a decrease in the fuel injection pressure when the load drops and the speed decreases. Uneven and unstable fuel injection, as well as low cylinder temperatures, cause erratic engine operation, often accompanied by misfiring and increased smoke.
Carbon formation occurs especially intensively when heavy fuels are used in engines. When operating at low loads, due to poor atomization and relatively low temperatures in the cylinder, droplets of heavy fuel do not completely burn out. When the droplet is heated, the light fractions gradually evaporate and burn, and extremely heavy high-boiling fractions remain in its core, which are based on aromatic hydrocarbons, which have the strongest bond between atoms. Therefore, their oxidation leads to the formation of intermediate products - asphaltenes and resins, which are highly tacky and can be firmly adhered to metal surfaces.
Due to the above circumstances, when long work engines at low speed and load modes, intensive contamination of the cylinders and especially the exhaust tract with products of incomplete combustion of fuel and oil occurs. Outlet channels working cylinder covers and outlet 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 will periodically ignite as the engine load increases, causing a fire in the exhaust system. All oily deposits are burned out, and dry carbon dioxide formed during combustion is blown out into the atmosphere.
Formulation of the second law of thermodynamics.
For the existence of a heat engine, 2 sources are required - a hot source and a cold source (environment). If a heat engine works from only one source, then it is called a perpetual motion machine of the 2nd kind.
1 formulation (Ostwald):
"Perpetual motion machine of the 2nd kind is impossible."
A perpetual motion machine of the 1st kind is a heat engine with L> Q1, where Q1 is the supplied heat. The first law of thermodynamics "allows" the ability to create 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 asserts that the work should be less than the supplied heat (L
"Heat cannot spontaneously pass from a colder body to a warmer one."
For the operation of a heat engine, 2 sources are required - hot and cold. 3rd formulation (Carnot):
"Where there is temperature difference, work is possible."
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, speed, fuel injection duration, atomization and mixture formation quality.
Increase in indicator efficiency(by improving the combustion process and reducing fuel heat losses in the compression and expansion processes)
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Modern engines are characterized by a high level of thermal stress in the CPG, due to the forcing of their working process. This requires technically competent maintenance of the cooling system. The required 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 consumption. Most of the diesel-building companies recommend T = 5 - 7 degrees C for MOD, t = 10 - 20 degrees C for SOD and VOD. The limitation of the water temperature drop is caused by the desire to maintain the minimum temperature stresses of the cylinders and bushings along their height. Heat transfer is intensified due to the high velocities of water movement.
When cooled by seawater, the maximum temperature is 50 ° C. Only closed-loop cooling systems can take advantage of high temperature cooling. When the temperature rises, cool. water, friction losses in the piston group decrease and eff. power and efficiency of the engine, with an increase in T, the temperature gradient along the thickness of the sleeve decreases, and thermal stresses also decrease. With a decrease in temperature, cool. water increases chemical corrosion due to condensation on the cylinder of sulfuric acid, especially when burning sulfurous 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 firms limit the temperature to the limits of 50-60 gr. With 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 with a power of 1 W through an element of a building structure with an area of 1 m2 at a temperature difference between the outside and inside air of 1 Kelvin W / (m2K).
The definition of the heat transfer coefficient is as follows: the loss of energy per square meter of the surface with a temperature difference between the external and internal. This definition entails the relationship of watts, square meters and Kelvin W / (m2 K).
To calculate heat exchangers, the kinetic equation is widely used, which expresses the relationship between the heat flux Q and the surface F of heat transfer, called the basic heat transfer equation: Q = KF∆tсрτ, where K is the kinetic coefficient (heat transfer coefficient, which characterizes the rate of heat transfer; ∆tav - the average driving force or the average temperature difference between the heat carriers (average temperature head) over the heat transfer surface; τ - 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 is the radius of the crank and AB = L is the length of the connecting rod. Let us designate 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 theory of CSM, they use the INVERSE VALUE λ = R / L
The distance between the axis of the piston pin and the axis of the shaft when turning it through an angle a
AO = AD + DO = LcosB + Rcosa
When the piston is in. m., then this distance is equal to L + R.
Therefore, the path traveled by the piston when the crank is turned through an angle a will be equal to x = L + R-AO.
Using mathematical calculations, we obtain the formula for the piston path
X = R (1- cosa + 1 / λ (1-cosB)) (1)
average speed The piston Vm, along with the speed, is an indicator of the speed of the engine. It is determined by the formula Vm = Sn / 30, where S is the piston stroke, m; n - rotation frequency, min-1. It is considered that vm = 4-6 m / s for the MOD, vm = 6s-9 m / s for the SOD, and vm> 9 m / s for the FOS. The higher vm, the greater the dynamic stresses in the engine parts and the greater the likelihood of their wear - primarily of the cylinder-piston group (CPG). At present, the vm parameter has reached a certain limit (15-18.5 m / s) due to the strength of the materials used in engine building, especially since the dynamic tension of the CPG is proportional to the square of the vm value. So, with an increase in vm by 3 times, the stresses in the parts will increase by 9 times, 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 manufacturer's passport (certificate) of the engine.
The true speed of the piston, i.e. its speed in this moment(in m / s), is defined as the first time derivative of the path. Let's substitute in the formula (2) a = ω t, where ω is the shaft rotation frequency in rad / sec, t is the time in sec. After mathematical transformations, we get the formula for the piston speed:
C = Rω (sina + 0.5λsin2a) (3)
where R is the radius of the crank vm \
ω - angular frequency of rotation of the crankshaft in rad / sec;
a - the angle of rotation of the crankshaft in the city;
λ = R / L-ratio of the radius of the crank to the length of the connecting rod;
Co - the peripheral speed of the center, crank neck vm / sec;
L is the length of the connecting rod, vm.
With an infinite length of the connecting rod (L = ∞ and λ = 0), the piston speed is
Differentiating formula (1) in a similar way, we obtain
С = Rω sin (a + B) / cosB (4)
The values of the sin (a + B) function are taken from tables given in reference books and manuals depending on a and λ.
It's obvious that maximum value piston speed at L = ∞ will be at a = 90 ° and a = 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 piston speed will be equal. Cmax = 1.57 Art.
We represent the equation of speed in the form
С = Rωsin a + 1 / 2λ Rωsin2a.
Graphically, both terms on the right side of this equation will be represented by sinusoids. The first term Rωsin a, representing the speed of the piston with an infinite length of the connecting rod, is represented by a first-order sinusoid, and the second term 1 / 2λ Rωsin2a, a correction for the effect of the finite length of the connecting rod, is represented by a second-order sinusoid.
Having built the indicated sinusoids and adding them algebraically, we get a speed graph taking into account the indirect influence of the connecting rod.
In fig. 247 depicts: 1 - curve Rωsin a,
2 - curve 1 / 2λ Rωsin2a
3 - curve C.
The operational properties are understood as the objective features of the fuel, which are manifested in the process of using it in an engine or unit. The combustion process is the most important and determines its operational properties. The process of fuel combustion, of course, is preceded by the processes of its evaporation, ignition and many others. The nature of the fuel behavior in each of these processes is the essence of the main operational properties of fuels. The following performance properties of fuels are currently being evaluated.
Volatility characterizes the ability of the fuel to pass from liquid state into vapor. This property is formed from such indicators of fuel quality as fractional composition, pressure saturated vapors at different temperatures, surface tension other. Evaporation is important in the selection of fuel and largely determines the technical, economic and performance characteristics engines.
Flammability characterizes the features of the process of ignition of mixtures of fuel vapors with air. Evaluation of this property is based on such quality indicators as temperature and concentration limits of ignition, flash point and self-ignition, etc. The flammability index of a fuel has the same value 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 combustion chambers of engines and combustion devices.
The pumpability characterizes the behavior of the fuel when it is pumped through pipelines and fuel systems, as well as when it is filtered. 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, limiting filterability temperature, water content, mechanical impurities, etc.
Sediment tendency is the ability of a fuel to form various kinds of deposits in combustion chambers, fuel systems, intake and exhaust valves. The assessment of this property is based on indicators such as ash content, coking capacity, resinous substances, unsaturated hydrocarbons, etc.
Corrosion activity and compatibility with non-metallic materials characterizes the ability of a fuel to cause corrosive damage to 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, a test of resistance various metals, rubbers and sealants in contact with fuel.
The 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 if water gets in. This property is rated special methods, involving the effect of normal, sea and rainwater on metals in the presence of fuels.
Antiwear properties characterize the reduction of wear of rubbing surfaces in the presence of fuel. These properties are important for engines in which fuel pumps and fuel-regulating equipment are lubricated only by the fuel itself without the use of lubricant (for example, in a high-pressure plunger fuel pump). The property is evaluated by indicators of viscosity and lubricity.
The cooling capacity determines the ability of the fuel to absorb and remove heat from heated surfaces when using the fuel as a heat carrier. Assessment of properties is based on such quality indicators as heat capacity and thermal conductivity.
Stability characterizes the persistence of fuel quality indicators during storage and transportation. This property evaluates the physical and chemical stability of the fuel and its tendency to biological damage by bacteria, fungi and mold. The level of this property makes it possible to establish a guaranteed storage period for 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 indicators of the toxicity of the fuel and its combustion products and the fire and explosion hazard.
The endless expanses of the sea are plowed by large ships obedient to the hands and will of man, propelled by powerful engines who use marine fuel of various types. Transport ships can use different engines, however most of these floating structures are equipped with diesel engines. Marine fuel used in marine diesel engines is divided into two classes - distillate and heavy... Distillate fuel includes summer diesel 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 the starting mode. It is sometimes used as an additive to heavy fuel oil when it is necessary to lower its viscosity. Heavy varieties fuels differ from distillate ones with increased viscosity, more high temperature solidification, presence more heavy fractions, high content of ash, sulfur, mechanical impurities and water. The prices for this type of marine fuel are much lower.
Most ships use the cheapest heavy diesel fuel for ship engines, or fuel oil. The use of fuel oil is dictated, first of all, for economic reasons, because the prices for marine fuel, as well as the total cost of transporting goods by sea, when using fuel oil, are significantly reduced. 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 Maritime Navigation Rules prescribe in certain operating modes, for example, when maneuvering, to use more expensive low-viscosity marine fuel, or diesel fuel. In some sea areas, for example, the English Channel, due to the difficulty in navigation and the need to comply with environmental requirements, the use of fuel oil as the main fuel is generally prohibited.
Fuel selection largely depends on the temperature at which it will be used. Normal start-up and scheduled operation of the diesel engine are 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 - 40-52.
Ts charts are used primarily for illustrative purposes, because in Pv charts, the area under the curve expresses the work being done. pure substance in a reversible process, and in the Ts diagram, the area under the curve represents the heat obtained for the same conditions.
Toxic components are: carbon monoxide CO, CH hydrocarbons, nitrogen oxides NOx, particulate matter, benzene, toluene, polycyclic aromatic hydrocarbons PAH, benzopyrene, soot and particulate matter, lead and sulfur.
Current emission standards harmful substances marine diesel engines are installed by IMO, the international maritime organization. All currently produced marine diesel engines must meet these standards.
The main components hazardous to humans in exhaust gases are: NOx, CO, CnHm.
A number of methods, for example, direct water injection, can only be implemented during the design and manufacture of the engine and its systems. For already existing lineup engines, these methods are unacceptable or require significant costs for the modernization of the engine, replacement of its units and systems. In a situation where it is necessary to significantly reduce nitrogen oxides without re-equipment of serial diesel engines - and here it is just such a case, the most effective way is the use of a three-way catalytic converter. The use of a neutralizer is justified in those areas where there are high requirements for NOx emissions, for example, in large cities.
Thus, the main directions for reducing harmful emissions Diesel exhaust gases 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 the fuel composition, the use of alternative fuels.
