Refrigerant hypothermation system. Analysis of VRF systems. Refreshing system of refrigerant Supplement in air-cooled condensers
Carrier.
Installation, commissioning and maintenance instructions
Calculation of supercooling and overheating
Supercooling
1. Definition
condensation of a saturated pair of refrigerant (TC)
and temperature in the liquid line (TZh):
PO \u003d TK TJ.
Collector
temperatures)
3. Stages of measurement
electronic on the liquid line next to the filter
desiccant. Make sure the surface of the pipe is clean,
and the thermometer touches it tightly. Cover flask or
sensor foam to heat the thermometer
from ambient air.
low pressure).
pressure in the injection line.
Measurements should be made when the unit
works in optimal design conditions and develops
maximum performance.
4. According to the table recalculation table for the temperature for R 22
find the condensation temperature of a saturated pair
refrigerant (TC).
5. Record the temperature measured by the thermometer
on the liquid line (TZH) and deduct it from the temperature
condensation. The resulting difference and will be the value
supercooling.
6. With the right refueling system of the refrigerant
the supercooling ranges from 8 to 11 ° C.
If the supercooling turned out to be less than 8 ° C, you need
add refrigerant, and if more than 11 ° С Remove
surplus Freon.
Pressure in the injection line (on the sensor):
Condensation temperature (from table):
Temperature in the liquid line (by thermometer): 45 ° С
Supercooling (by calculation)
Add the refrigerant according to the calculation results.
Overheat
1. Definition
Precooling is the difference between the temperature
suspension (TV) and saturated evaporation temperature
(TI):
GH \u003d TV TI.
2. Equipment for measuring
Collector
Normal or electronic thermometer (with sensor
temperatures)
Filter or heat insulating foam
Table of recalculation of pressure in the temperature for R 22.
3. Stages of measurement
1. Place the flask of the liquid thermometer or the sensor
electronic suction line next to
compressor (10 20 cm). Make sure the surface
pipes clean, and the thermometer tights its top
parts, otherwise, the thermometer readings will be incorrect.
Cover the flask or sensor foam to heat
tell the thermometer from the ambient air.
2. Insert the collector in the injection line (sensor
high Pressure) and Suction Line (Sensor
low pressure).
3. After the conditions stabilize, write down
pressure in the injection line. On the conversion table
pressure in temperature for R 22 Find the temperature
rich evaporation of refrigerant (TI).
4. Record the temperature measured by the thermometer
on the suction line (TV) at 10 20 cm from the compressor.
Spend a few measurements and calculate
the average temperature of the suction line.
5. Delete the temperature of evaporation from temperature
suction. The resulting difference and will be the value
overheating of the refrigerant.
6. When setting up the expansion valve
overheating is from 4 to 6 ° C. With little
overheating to the evaporator too much
refrigerant, and you need to cover the valve (turn screw
clockwise). With greater overheating in
the evaporator gets too little refrigerant, and
need to open the valve (turn the screw against
clockwise).
4. Example of calculating supercooling
Pressure in the suction line (on the sensor):
Evaporation temperature (from the table):
Temperature in the suction line (by thermometer): 15 ° С
Overheating (by calculation)
Open the expansion valve according to
the results of the calculation (too much overheating).
ATTENTION
COMMENT
After adjusting the expansion valve, do not forget
return its lid to place. Change overheating only
after adjusting the hypothermia.
19.10.2015
The degree of hypothermia of the liquid obtained at the output of the condenser is an important indicator that characterizes the stable operation of the refrigeration circuit. The supercooling is called the temperature difference between the liquid and condensation at this pressure.
With normal atmospheric pressure, water condensation has a temperature indicator of 100 degrees Celsius. According to the laws of physics, water, which is 20 degrees, is considered to be overcoiled by 80 degrees Celsius.
The overcooling at the outlet of the heat exchanger is changed as the difference between the temperature fluid and condensation. Based on Figure 2.5, the supercooling will be equal to 6 to or 38-32.
In air-cooled capacitors, the hypothermation indicator should be from 4 to 7 K. In case it has a different value, this indicates unstable work.
Condenser and fan interaction: air temperature difference.
The injected air fan has an indicator of 25 degrees Celsius (Figure 2.3). He takes the heat from Freon, due to which its temperature changes up to 31 degrees.
Figure 2.4 shows a more detailed change:
TAE is a temperature mark of air supplied to the condenser;
TAS - air with a new condenser temperature after cooling;
TK -C pressure gauge testimony about condensation temperature;
Δθ - the difference in temperature indicators.
The calculation of the temperature difference in the air-cooled condenser occurs by the formula:
Δθ \u003d (TAS - TAE), where K has a limit 5-10 K. On the chart, this value is 6 K.
The difference between the temperature difference at the point D, that is, at the outlet of the condenser, in this case the 7 K is equal to, as it is in the same limit. Temperature pressure is 10-20 k, in the figure it is (TK-TAE). Most often, the value of this indicator stops at the mark of 15 K, but in this example - 13 K.
Recall that VRF systems (Variable refrigerant flow - systems with variable refrigerant flow) are today the most dynamically developing class of air conditioning systems. The global growth in sales of class VRF systems annually increases by 20-25%, leaving the market competing air conditioning options from the market. Due to this growth?
Firstly, thanks to the wide range of systems of Variable Refrigerant Flow: a large selection of external blocks - from mini-VRF to large combinatorial systems. Huge selection of internal blocks. Pipeline lengths - up to 1000 m (Fig. 1).
Secondly, due to the high energy efficiency of systems. The inverter drive of the compressor, the absence of intermediate heat exchangers (unlike water systems), individual refrigerant consumption - all this provides minimal power consumption.
Thirdly, a positive role is played by the modular structure. The desired system performance is recruited from individual modules, which is no doubt very convenient and increases overall reliability as a whole.
That is why today VRF-systems occupy at least 40% of the global central air conditioning systems and this share is growing every year.
Refrigerant supercooling system
What is the maximum length of freon pipelines can be a split-system of air conditioning? For household systems with a capacity of up to 7 kW cold, it is 30 m. For semi-industrial equipment, this figure can reach 75 m (inverter outdoor unit). For split systems, the maximum value, but for the VRF class systems, the maximum pipeline length (equivalent) can be significantly greater - up to 190 m (total - up to 1000 m).
Obviously, the VRF systems differ fundamentally from split systems from the point of view of the freon contour, and this allows them to work at large pipelines. This difference lies in the presence of a special device in the outer block, which is called the refrigerant superchain or Subcooler (Fig. 2).
Before considering the features of the work of VRF systems, let's pay attention to the freon contour circuit of split-systems and understand what happens with the refrigerant at large freon pipelines.
Refrigerated Split Systems Cycle
In fig. 3 shows the classic freon cycle in the air conditioner circuit in the "Pressure Enhaulpia". And this is a cycle for any split systems on the R410A freon, that is, from the performance of the air conditioner or mark, the view of this diagram does not depend.
Let's start from point d, with the initial parameters in which (temperature 75 ° C, pressure 27.2 bar) freon enters the outer block condenser. Freon is currently superheated gas, which first cools up to a saturation temperature (about 45 ° C), then begins to condense and completely passes from the state of the gas into the liquid. Next, the fluid is undercooling to point A (temperature 40 ° C). It is believed that the optimal amount of hypothermia is 5 ° C.
After the heat exchanger of the outdoor unit, the refrigerant enters the throttling device in the outer block - the thermostatic valve or the capillary tube, and its parameters change to the point B (temperature 5 ° C, pressure 9.3 bar). We note that the point in is in the zone of the mixture of liquid and gas (Fig. 3). Consequently, after throttling, a mixture of liquid and gas flows into the liquid pipeline. The greater the magnitude of freon hypothermia in the condenser, the greater the proportion of liquid freon enters the internal unit, the higher the efficiency of the air conditioner.
In fig. 3 The following processes are indicated: B-C is the boiling process of freon in the inner unit with a constant temperature of about 5 ° C; C-C - Overheating of Freon to +10 ° C; C -L - the process of suction of the refrigerant to the compressor (pressure losses in the gas pipeline and the elements of the freon contour from the heat exchanger of the inner block to the compressor); L-M is the process of compression of gaseous freon in the compressor with an increase in pressure and temperature; M-D is the process of injection of gaseous refrigerant from the compressor to the condenser.
The pressure loss in the system depends on the rate of freon V and the hydraulic characteristics of the network:
What will happen to air-conditioned with an increase in the hydraulic characteristics of the network (due to increased length or large number of local resistances)? Increased pressure losses in the gas pipeline will lead to a pressure drop at the entrance to the compressor. The compressor will begin to capture a smaller pressure refrigerant and, it means less density. Refrigerant flow will fall. At the outlet, the compressor will produce less pressure and, accordingly, the condensation temperature will fall. The reduced condensation temperature will lead to a reduced evaporation temperature and gas pipeline frost.
If the increased pressure loss will occur on a liquid pipeline, the process is even more interesting: since we found out that the freon is in a saturated state in the liquid pipeline, or rather, in the form of a mixture of liquid and gas bubbles, then any pressure loss will lead to a small boosting refrigerant and increase in the share of gas.
The latter will result in a sharp increase in the volume of the vapor-gas mixture and an increase in the speed of movement along the liquid pipeline. The increased speed will again cause an additional pressure loss, the process will become "avalanche".
In fig. 4 shows a conditional schedule of specific pressure losses depending on the speed of the refrigerant movement in the pipeline.
If, for example, pressure loss with a length of pipelines 15 m is 400 pa, then with an increase in the length of the pipelines twice (up to 30 m), the losses are increasing not twice (up to 800 pa), and seven times - up to 2800 pa.
Therefore, a simple increase in pipeline length is doubled relative to standard lengths for split-system with ON-OFF-compressor fatal. The refrigerant flow fell several times, the compressor will overheat and very soon fail.
Refrigerated VRF-Systems Cycle with Freon Procesor
In fig. 5 schematically depicts the principle of operation of the refrigerant superchain. In fig. 6 shows the same refrigeration cycle in the "Pressure Enhaulpia" diagram. Consider in detail what we have with a refrigerant during the work of the Variable Refrigerant Flow system.
1-2: The liquid refrigerant after the capacitor at point 1 is divided into two streams. Most of the part passes through the countercurrent heat exchanger. It takes cooling the main part of the refrigerant to + 15 ... + 25 ° C (depending on its effectiveness), which further enters the liquid pipeline (point 2).
1-5: The second part of the flow of the liquid refrigerant from point 1 passes through the TRV, its temperature decreases to +5 ° C (point 5), comes to the same countercurrent heat exchanger. In the latter, its boiling and cooling of the main part of the refrigerant occurs. After boiling, the freon gas immediately enters the compressor absorption (point 7).
2-3: At the outdoor unit (point 2), the liquid refrigerant passes through the pipelines to the internal blocks. At the same time, the heat exchange with the environment is practically not occurring, but a portion of the pressure is lost (point 3). In some manufacturers, the throttling is made partially in the outer block of the VRF system, so the pressure at 2 is less than on our schedule.
3-4: The pressure loss of refrigerant in the electronic adjusting valve (ERV), which is located in front of each inner block.
4-6: Evaporation of refrigerant in the inner block.
6-7: The pressure loss of the refrigerant when it is returned to the outer block along the gas pipeline.
7-8: Compression of the gaseous refrigerant in the compressor.
8-1: Cooling the refrigerant in the heat exchanger of the outdoor unit and its condensation.
Consider a detail from point 1 to point 5. In VRF systems without a refrigerant system, the process from point 1 immediately goes to point 5 (according to the blue line. 6). The specific amount of refrigerant performance (incoming to the internal blocks) is proportional to the length of the line 5-6. In systems where the supercooler is present, the beneficial productivity of the refrigerant is proportional to the line 4-6. Comparing the lengths of the line 5-6 and 4-6, the work of the Freon's superchain becomes clear. Improving the cooling efficiency of the circulating refrigerant occurs at least 25%. But this does not mean that the performance of the entire system has become more than 25%. The fact is that part of the refrigerant did not admit to the internal blocks, and immediately went to the absorption of the compressor (line 1-5-6).
It is in this that the balance is: what magnitude the productivity of freon, which goes to the internal blocks, has increased, the performance of the system as a whole decreased to the same amount.
So then what is the meaning of the use of the refrigerant superchain, if the overall performance of the VRF system does it increase? To answer this question, back to fig. 1. The meaning of the application of the monitoring agent is to reduce losses on long tracks of the Variable Refrigerant Flow systems.
The fact is that all the characteristics of the VRFsystem are given with the standard length of the pipelines of 7.5 m. That is, it is not entirely correct to compare the VRF systems of different manufacturers according to the directory data, since the real length of pipelines will be much more - as a rule, from 40 to 150 m. The greater the length of the pipeline from the standard, the greater the pressure loss in the system, the greater the refrigerant boosts in the liquid pipelines. The performance loss of the outdoor unit in length is given in special charts in service manuals (Fig. 7). It is according to these graphs that it is necessary to compare the efficiency of the operation of systems in the presence of a refrigerant superchain and in its absence. Performance loss of VRF systems without a superchargeder on long tracks is up to 30%.
conclusions
1. The refrigerant superchild is an essential element for VRF systems. Its functions are, firstly, an increase in the energetic capacity of the refrigerant entering the internal blocks, secondly, a decrease in pressure losses in the system on long tracks.
2. Not all manufacturers of VRF systems provide their systems with a refrigerant superchild. Especially often exclude the supercharger of the OEM brands to reduce the cost of construction.
In the condenser, the gaseous refrigerant, compressed by the compressor, goes into a liquid state (condensed). Depending on the working conditions of the refrigeration circuit, the refrigerant pair can be condensed completely or partially. For the proper functioning of the refrigeration circuit, complete condensation of the refrigerant vapor in the condenser is necessary. The condensation process takes place at a constant temperature called condensation temperature.
The hyposhee of the refrigerant is the difference between the temperature of the condensation and the refrigerant temperature at the outlet of the condenser. While there is at least one gas molecule in a mixture of gaseous and liquid refrigerant, the mixture temperature will be equal to the condensation temperature. Therefore, if the temperature of the mixture at the outlet of the capacitor is equal to the condensation temperature, it means that there are pairs in the refrigerant mixture, and if the refrigerant temperature at the output of the capacitor is below the condensation temperature, this clearly indicates that the refrigerant has completely passed into a liquid state.
Overheating of refrigerant - This is the difference between the refrigerant temperature at the exit of the evaporator and the boiling point of the refrigerant in the evaporator.
Why do you need to overheat a couple of already swollen refrigerant? The meaning of this is to be confident that the entire refrigerant is guaranteed to go into a gaseous state. The presence of a liquid phase in the refrigerant entering the compressor can lead to a hydraulic impact and output compressor. And since the boiling of the refrigerant occurs at a constant temperature, we cannot argue that the entire refrigerant has flown up until its temperature exceeds its boiling point.
In the internal combustion engines have to face phenomenon cutyl oscillations shafts. If these oscillations threaten the strength of the crankshaft in the operating range of the rotational frequency of the shaft, anti-vibrators and dampers are used. They are placed on the free end of the crankshaft, i.e., where the greatest twears arise
oscillations.
external forces make the crankshaft of a diesel engine to perform twist fluctuations
These forces are the pressure of gases and the inertia forces of the rocker-crank mechanism, under the variables of which a continuously changing torque is created. Under the influence of uneven torque, the segment of the crankshaft is deformed: twisted and spinned. In other words, in the crankshaft of the shaft there are twisting oscillations. The complex dependence of the torque from the corner of the rotation of the crankshaft can be represented as the sum of sinusoidal (harmonic) curves with different amplitudes and frequencies. At a certain rotational speed of the crankshaft, the frequency of the perturbation force, in this case of any component of the torque, may coincide with the frequency of the shaft's own oscillations, i.e. the phenomenon of the resonance will come, in which the amplitudes of the rolling oscillations of the shaft can become so great that the shaft may collapse.
To eliminate The phenomenon of resonance in modern diesel engines, special devices are applied. Wide distribution received one of the types of such a device - the pendulum anti-vibrator. At that moment, when the movement of the flywheel during each of his oscillations will accelerate, the cargo of the anti-vibrator according to the law of inertia will strive to preserve its movement at the same speed, i.e. it will begin to lag at some angle from the section of the shaft, to which the anti-virus is attached (position II) . The load (or rather, its inertial force) will be as it were to "slow down" the shaft. When the angular velocity of the flywheel (shaft) during the same oscillation will begin to decrease, the cargo, obeying the law of inertia, will strive to "pull" the shaft (position III),
Thus, the inertial forces of the suspended cargo during each oscillation will periodically affect 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 hermetic case, inside which the flywheel (mass) is located. The flywheel can freely rotate relative to the hull reinforced at the end of the crankshaft. The space between the case and the flywheel is filled with silicone fluid having a greater viscosity. When the crankshaft rotates uniformly, the flywheel at the expense of friction forces in the fluid acquires the same with the shaft the frequency (speed) of rotation. And if there are twisted oscillations of the crankshaft? Then their energy is transferred to the body and will be absorbed by viscous friction by the body and inertial weight of the flywheel.
Modes of small revolutions and loads. The transition of the main engines to the modes of small revolutions, as well as the transition of auxiliary on the modes of small loads, is associated with a significant reduction in fuel supply to cylinders and an increase in excess air. At the same time, air parameters are reduced at the end of the compression. It is especially noticeable to change the RS and TC in the engines with gas turbine supervision, since the gas turbocompressor on small loads practically does not work and the engine automatically goes to the mode of operation without chance. Small portions of burning fuel and high air excess reduce the temperature in the combustion chamber.
Due to the low temperature of the cycle, the combustion process of fuel flows sluggishly, slowly, part of the fuel does not have time to burn and flows through the cylinder walls in the crankcase or carrying out the exhaust gases into the exhaust system.
The worsening of the combustion of the fuel also contributes to the poor mixing of fuel with air, due to a decrease in the fuel injection pressure when the load is dropped and reduce the speed of rotation. The uneven and unstable fuel injection, as well as low temperatures in the cylinders, cause unstable operation of the engine, often accompanied by flash passes and increased smoking.
Nagara formation proceeds especially intensively when used in heavy fuel engines. When working on low loads due to poor spraying and relatively low temperatures in a heavy fuel drop cylinder, do not completely fade away. When the droplets are heated, the light fractions gradually evaporate and burned, and in its kernel there are extremely heavy high-boiling fractions, the basis of which is aromatic hydrocarbons with the most durable bonding between atoms. Therefore, the oxidation of them leads to the formation of intermediate products - asphaltenes and a resin with high stickiness and capable of firmly kept on metal surfaces.
By virtue of the circumstances, the prolonged operation of the engines on the modes of small revolutions and loads occurs intensive contamination of cylinders and especially the exhaust path of the products of incomplete combustion of fuel and oil. The outlet channels of the operating cylinder covers and the outlet nozzles are covered with a dense layer of asphalt-resinous substances and coke, often by 50-70% of the flowing section that reduce their passage section. In the outlet pipe, the thickness of the Nagar layer reaches 10-20 mm. These deposits when improving the load on the engine are periodically flammorated, causing fire in the exhaust system. All oily deposits burn out, and dry carbon dioxide generated during combustion blows into the atmosphere.
The wording of the Second Law of Thermodynamics.
For the existence of a thermal engine, 2 sources are needed - a hot source and a cold source (environment). If the thermal motor works only from one source, it is called the 2nd birth eternal engine.
1 Formulation (OSVALDA):
"The Eternal Engine of the 2nd kind is impossible."
The perpetual engine of the 1st genus is a thermal motor, in which L\u003e Q1, where Q1 is the suspended heat. The first law of thermodynamics "allows" the ability to create a heat engine fully turning the expended heat of the Q1V operation L, i.e. L \u003d Q1. The second law imposes more stringent restrictions and argues that the work should be less than the heated heat (L
"The heat cannot spontaneously moves from a colder body to the more heated."
For the operation of the thermal engine, 2 sources are needed - hot and cold. 3rd wording (carno):
"Where there is a difference in temperatures, perhaps doing work."
All these formulations are interrelated, from one formulation you can get another.
Indicator efficiency It depends on: compression degree, excess air coefficient, combustion chamber design, advance angle, speed, fuel injection duration, spraying and mixing quality.
Increase indicator efficiency (by improving the combustion process and reduce fuel heat losses in compression and expansion processes)
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For modern engines, a high level of thermal tension of CPGs is characterized due to the forcing of their workflow. This requires technically competent care of the cooling system. The necessary heat sink from the heated surfaces of the engine can be achieved either by an increase in the difference in the temperature of the water T \u003d T.V.V. - T.VX, or an increase in its consumption. Most of the diesel firms are recommended for modes T \u003d 5 - 7 gr.c, for soda and water T \u003d 10 - 20 gr. The limitation of the temperature difference is caused by the desire to preserve the minimum temperature stresses of cylinders and the sleeves at their height. The intensification of heat transfer is carried out due to the high speed of water movement.
When cooled by intricate water, the maximum temperature-ra 50 gr. Only closed cooling systems allow you to use the advantages of high-temperature cooling. With increasing the temperature of ox. Waters reduce friction losses in the piston group and slightly increases eff. The power and efficiency of the engine, with an increase in TV, the temperature gradient in the thickness of the sleeve is reduced, thermal stresses are reduced. With a decrease in the temperature ox. Water increases chemical corrosion due to condensation on a sulfuric acid cylinder, especially when burning sulfur fuels. However, there is a restriction of water temperature due to limitations of the in-cylinder mirror (180 gr. 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 are limited to the volume of 50 -60 gr. C and only when burning high-continuous fuels is allowed 70-75 gr. FROM.
Heat transfer coefficient - a unit that denotes the passage of heat flux with a capacity of 1 W through an element of a construction structure with an area of \u200b\u200b1 m2 when the outer air temperature difference and the internal temperature in 1 Kelvin W / (M2K).
The definition of the heat transfer coefficient sounds as follows: energy loss by a square meter of the surface with the difference in temperature of the outer and internal. This definition entails the relationship of Watt, square meters and Kelvin W / (m2 · k).
To calculate heat exchangers, a kinetic equation is widely used, which expresses the relationship between the heat flux Q and the surface F of heat transfer, called the main equation of heat transfer: Q \u003d KFΔTCRτ, where K is a kinetic coefficient (heat transfer coefficient characterizing the heat transfer rate; Δtcs - the average driving force or the average temperature difference between the coolants (average temperature pressure) on the heat transfer surface; τ - time.
The greatest difficulty causes the calculation coefficient of heat transfer K.characterizing the speed of heat transfer process with the participation of all three types of heat transfer. The physical meaning of the heat transfer coefficient flows from the equation (); Its dimension:
In fig. 244 OB \u003d R - radius of crank and AB \u003d L - the length of the connecting rod. Denote by the ratio L0 \u003d L / R- is called the relative length of the connecting rod, for ship diesel engines is within 3.5-4.5.
however, in the theory of KSM, the inverse value λ \u003d r / l is used
The distance between the piston finger axis and the shaft axis when it turns it to the angle
AO \u003d AD + DO \u003d LCOSB + RCOSA
When the piston is in c. m. t., This distance is equal to L + R.
Therefore, the path passed by the piston when the crank is rotated at the angle A, will be equal \u003d L + R-AO.
By mathematical calculations we get the formula of the piston path
X \u003d R (1- Cosa + 1 / λ (1-Cosb)) (1)
The average speed of the VM piston along with the speed of the rotation is the indicator of the speed mode of the engine. It is determined by the formula Vm \u003d Sn / 30, where S is the stroke of the piston, m; P - rotational speed, min-1. It is believed that for modes Vm \u003d 4-6 m / s, for software Vm \u003d 6S-9 m / s and for waters Vm\u003e 9 m / s. The higher the VM, the greater the dynamic stresses in the parts of the engine and the greater the likelihood of their wear - primarily the cylindrophone group (CPG). Currently, the VM parameter has reached a certain limit (15-18.5 m / s), due to the strength of materials used in the engine, especially since the dynamic tension of the CPG is proportional to the square VM value. Thus, with an increase in VM, 3 times voltage in details will increase by 9 times, which will require an appropriate enhancement of the strength characteristics of materials used for the manufacture of parts of the CPG.
The average piston rate is always indicated in the factory passport (certificate) of the engine.
True piston rate, i.e. the speed of it at the moment (in m / s) is defined as the first derivative of the time in time. We substitute in formula (2) a \u003d ω T, where ω is the rotation frequency of the shaft in rad / s, T- time in sec. After mathematical transformations, we get a piston speed formula:
C \u003d RΩ (SINA + 0.5λSIN2A) (3)
where r - radius crank VM \\
ω - the angular frequency of rotation of the crankshaft in rad / s;
a - the angle of rotation of the crankshaft vigrarad;
λ \u003d r / L-ratio of the radius of the crank to the length of the connecting rod;
CO - the district speed of the center, the crank cervical cerial / s;
L is the length of the ride rod.
With the infinite length of the connecting rod (L \u003d ∞ and λ \u003d 0) the speed of the piston is equal
Differentizing the formula (1) in the same way
C \u003d Rω SIN (A + B) / COSB (4)
The values \u200b\u200bof the SIN function (A + B) are taken from tables of reference books and benefits depending on the directories.
Obviously, the maximum value of the piston velocity at L \u003d ∞ will be \u003d 90 ° and a \u003d 270 °:
Camax \u003d RΩ sin a .. Since CO \u003d πRN / 30 ICM \u003d SN / 30 \u003d 2RN / 30 \u003d RN / 15
CO / CM \u003d πRN15 / RN30 \u003d π / 2 \u003d 1.57 from where co \u003d 1.57 cm
Consequently, the maximum piston rate will be equal. Smaks \u003d 1.57 tbsp.
Imagine the speed equation in the form
C \u003d RωSIN A + 1 / 2λ RωSIN2A.
Graphically both members of the right part of this equation will be depicted with sinusoids. The first term of RωSIN A, representing the piston rate with an infinite length of the connecting rod, is depicted by a first-order sinusoid, and the second member1 / 2λ RωSIN2A-correction on the effect of the final length of the rod-sinusoid of the second order.
by building the specified sinusoids and folding them algebraically, we obtain a speed chart with regard to the indirect influence of the connecting rod.
In fig. 247 depicted: 1 - curveRωSIN A,
2 - curve1 / 2λ RΩSin2a
3 - Crowd.
Under the operational properties understand the objective features of fuel, which are manifested in the process of using it in the engine or unit. The combustion process is the most important and determining its operational properties. The process of combustion of fuel is definitely preceded by the processes of its evaporation, ignition and many others. The nature of the behavior of fuel in each of these processes is the essence of the main operational properties of fuels. Currently, the following operational properties of fuels are evaluated.
Evaporability characterizes the ability of fuel to move from a liquid state in vapor. This property is formed from such fuel quality indicators, as a fractional composition, the pressure of saturated vapors at different temperatures, surface tension and others. Evaporability is essential in the selection of fuel and largely determines the technical and economic and operational characteristics of the engines.
The flammability characterizes the characteristics of the process of ignition of mixtures of fuel vapor with air. The evaluation of this property is based on such quality indicators, as temperature and concentration limits of ignition, flare and self-ignition temperature, etc. The fuel flammability indicator has the same value as its flammability; In the future, these two properties are considered jointly.
The combustibility determines the effectiveness of the process of burning fuel-air mixtures in the combustion chambers of the engines and the furnace devices.
Pouring characterizes the behavior of fuel when pumping it through pipelines and fuel systems, as well as when filtering it. This property determines the smoothness of the supply of fuel into the engine at different temperatures of operation. Pulmonary fuels is evaluated by viscous-temperature properties, turbidity and frozen temperatures, limiting filtral temperature, water content, mechanical impurities, etc.
The addiction to the formation of deposits is the ability of fuel to form deposits of various kinds in combustion chambers, in fuel systems, in inlet and exhaust valves. Evaluation of this property is based on such indicators as ash content, coking, the content of resinous substances, unsaturated hydrocarbons, etc.
Corrosion activity and compatibility with non-metallic materials characterizes the ability of the fuel to cause corrosion lesions of metals, swelling, destruction, or changing the properties of rubber seals, sealants and other materials. This operating property provides for a quantitative assessment of the content of corrosion-active substances in fuel, the test of the resistance of various metals, rubber and sealants during contact with the fuel.
Protective ability is the ability of fuel to protect against corrosion materials of engines and aggregates when they contact them with an aggressive medium in the presence of fuel and primarily the ability of fuel to protect metals from electrochemical corrosion when water enters. This property is assessed by special methods involving the impact of ordinary, marine and rainwater on metals in the presence of fuel.
Anti-wear properties characterize the decrease in the 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 the high pressure plunger fuel pump). The property is estimated by viscosity and lubricity.
Cooling capacity determines the possibility of fuel to penetrate and remove heat from heated surfaces when using fuel as a coolant. Evaluation of properties is based on such quality indicators as heat capacity and thermal conductivity.
Stability characterizes the retainability of fuel quality indicators during storage and transportation. This property evaluates the physical and chemical stability of fuel and its tendency to biological accuracy with bacteria, fungi and mold. The level of this property allows you to establish a warranty life of fuel in various climatic conditions.
Environmental properties characterize the effects of fuel and its combustion products on humans and the environment. The assessment of this property is based on the fuel toxicity indicators and its combustion and fire and explosion products.
Beavenly maritime expanses furrowed obedient hands and the will of man big vessels given in motion using powerful engines that use ship fuel of various types. Transport ships can use different engines, but most of these floating structures are equipped with diesel engines. Fuel for ship engines used in ship diesels, divide into two classes - distillate and heavy. Diesel summer fuel refers to distillate fuel, as well as foreign fuels "Marin Diesel Oil", "Gas Oil" and others. It has a slight viscosity, so not
Requires at the start of the preheating engine. It is used in high-speed and medium-round diesel engines, and in some cases, and in low-state diesel engines in start-up mode. Sometimes it is used as an additive to severe fuel in cases where it is necessary to lower its viscosity. Heavy varieties Fuels are distinguished from distillate increased viscosity, higher frosted temperature, the presence of a larger number of heavy fractions, greater ash, sulfur, mechanical impurities and water. Prices for ship fuel of this species are significantly lower..
Most of the 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 considerations, because prices for ship fuel, as well as, the total costs of transportation of goods by marine transport 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 ship engines is about two hundred euros per ton.
However, maritime shipping rules are prescribed in certain modes of operation, for example, when maneuvering, use more expensive low-viscous ship fuel, or, solarium. In some marine waters, for example, the La Mans Strait, due to the complexity in the favings and the need to comply with the requirements of the ecology, 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 launch and planned operation of a diesel engine is provided in the summer period with a cetane number of 40-45, in winter it is necessary to increase it to 50-55. In motor fuels and fuel oil, the cetane number is within 30-35, in diesel - 40-52.
TS-diagrams are used primarily for the purposes of illustration, since in the PV diagram area under the curve expresses the work produced by a pure substance in the reversible process, and in the TS diagram area under the curve is depicted for the same conditions obtained heat.
Toxic components are: carbon oxide CO, CH hydrocarbons, nitrogen oxides NOX, solid particles, benzene, toluene, polycyclic aromatic hydrocarbons PAU, benzapine, soot and solid particles, lead and sulfur.
Currently, the norms for emissions of harmful substances by ship dieselks establishes IMO, an international maritime organization. These standards should satisfy all current ship diesel engines.
The main components dangerous for a person in exhaust gases are: NOX, CO, CNHM.
A number of ways, for example, a direct injection of water can only be implemented at the design and manufacture of engine and its systems. For an already existing model range of engines, these methods are unacceptable or require substantial costs for engine upgrading, replacing its aggregates and systems. In a situation where a significant reduction in nitrogen oxides without re-equipment of serial diesel engines is necessary, and here this case, the most effective way is the use of a three-component catalytic neutralizer. The use of the neutralizer is justified in those areas where there are high requirements for NOX emissions, for example in large cities.
Thus, the main directions to reduce harmful emissions of diesel engines can be divided into two groups:
1)-improving the design and engine systems;
2) Promotions that do not require engine modernization: the use of catalytic neutralizers and other means of cleaning OG, improving the composition of the fuel, the use of alternative fuels.
2.1. Normal work
Consider the scheme in fig. 2.1, representing air cooling capacitor at normal operation in the context. Suppose that the refrigerant R22 is received into the condenser.
Point A. R22 pairs, superheated to a temperature of about 70 ° C, leave the pumping nozzle of the compressor and fall into the capacitor at a pressure of about 14 bar.
Line A-B. Steating vapors is reduced at constant pressure.
Point V.The first drops of fluid R22 appear. The temperature is 38 ° C, the pressure is still about 14 bar.
Line B-s. Gas molecules continue to condense. More and more liquid appears, it remains less and less vapor.
Pressure and temperature remain constant (14 bar and 38 ° C) in accordance with the "pressure temperature" ratio for R22.
Point S. The last gas molecules are condensed at a temperature of 38 ° C, except for fluid in the circuit there is nothing. Temperature and pressure remain constant, accounted for about 38 ° C and 14 bar, respectively.
C-D Line. The entire refrigerant was condensed, the liquid under the action of air, the cooling capacitor using a fan, continues to cool.
Point D. R22 at the outlet of the condenser only in the liquid phase. The pressure is still about 14 bar, but the temperature of the liquid dropped to about 32 ° C.
The behavior of the mixing refrigerants of the type of hydrochlorofluorocker (HCFCs) with a large temperature globe, see paragraph B partition 58.
The behavior of the refrigerants of the type of hydrofluorocarbons (HFCs), for example, R407C and R410A, see section 102.
The change in the phase state R22 in the condenser can be represented as follows (see Fig. 2.2).
From A to B. Reduced overheating of the vapor R22 from 70 to 38 ° C (Zone A-B is the overheating zone in the condenser).
At the point in the first drops of the liquid R22 appear.
From to C. Condensation R22 at 38 ° C and 14 bars (Zone B - C is a condensation zone in the condenser).
At the point, the last steam molecule was condensed.
From from to D. The supercooling of liquid R22 from 38 to 32 ° C (zone C-D is the zone of hypothermation of liquid R22 in the condenser).
Throughout this process, the pressure remains constant equal to the testimony of the VD pressure gauge (in our case 14 bar).
Consider now how cooling air behaves (see Fig. 2.3).
The outer air, which cools the condenser and enters the inlet with a temperature of 25 ° C, heats up to 31 ° C, selecting the heat released by the refrigerant.
We can present changes in the temperature of the cooling air when it passes through the condenser and the temperature of the condenser in the form of a graph (see Fig. 2.4) where:
tae. - Air temperature at the entrance to the condenser.
tas. - Temperaturewood at the exit of the condenser.
tK - The condensation temperature is read from the VD pressure gauge.
A6. (Read: Delta Teta) difference (drop) temperature.
In general, in air-cooled capacitors, the temperature difference in air A0. = (tas - Tae.) It has values \u200b\u200bfrom 5 to 10 K (in our example 6 K).
The difference value between the condensation temperature and the air temperature at the outlet of the condenser also has order from 5 to 10 K (in our example 7 K).
Thus, the full temperature pressure ( tK - Tae.) It can be from 10 to 20 K (as a rule, its value is located near 15 K, and in our example it is 13 K).
The concept of complete temperature pressure is very important, since for this condenser this value remains almost constant.
Using the values \u200b\u200bgiven in the above example, it can be said that for the outer air temperature at the inlet into the condenser, equal to 30 ° C (i.e., Tae \u003d 30 ° C), the condensation temperature TK should be equal to:
tae + DBPPs \u003d 30 + 13 \u003d 43 ° C,
what will correspond to the testimony of the DV pressure gauge about 15.5 bar for R22; 10.1 bar for R134A and 18.5 bar for R404a.
| 2.2. Supercooling in air-cooled capacitors |
One of the most important characteristics during the operation of the refrigeration circuit, no doubt, is the degree of hypothermation of fluid at the outlet of the condenser.
We will call the solution of fluid to the difference between the temperature of the fluid condensation at a given pressure and the temperature of the fluid itself at the same time.
We know that the temperature of the condensation of water at atmospheric pressure is 100 ° C. Therefore, when you drink a glass of water having a temperature of 20 ° C, from the position of the thermal physics, you drink water superched on 80 k!
In the condenser, the supercooling is defined as the difference between the condensation temperature (read from the VD pressure gauge) and the fluid temperature measured at the output from the condenser (or in the receiver).
In the example shown in Fig. 2.5, supercooling P / O \u003d 38 - 32 \u003d 6 K.
The normal amount of refrigerant supercooling in air-cooled capacitors is usually in the range from 4 to 7 K.
When the magnitude of the hypothermia goes beyond the normal temperature range, this often indicates an anomalous current of the workflow.
Therefore, below we analyze various cases of abnormal hypothermia.
| 2.3. Analysis of abnormal hypothermia cases. |
One of the greatest difficulties in the work of the repairman is that it cannot see the processes occurring inside the pipelines and in the refrigeration circuit. However, the measurement of the magnitude of the supercooling may allow us to obtain a relatively accurate picture of the behavior of the refrigerant inside the contour.
Note that most of the constructors choose the size of air-cooled capacitors in such a way as to provide overcooling at the output from the condenser in the range from 4 to 7 K. Consider what happens in the condenser if the magnitude of the hypothermia goes beyond this range.
A) reduced supercooling (as a rule, less than 4 K).
In fig. 2.6 shows the difference in the refrigerant state inside the condenser under normal and abnormal overcooling.
The temperature at the points TB \u003d Tc \u003d TE \u003d 38 ° C \u003d condensation temperature TK. The temperature at point D gives the value TD \u003d 35 ° C, the hypothermia of 3 K.
Explanation. When the refrigeration circuit works fine, the last pair molecules are condensed at the point C. Next, the liquid continues to cool and the pipeline along the entire length (C-D zone) is filled with a liquid phase, which allows to achieve a normal overcooling magnitude (for example, 6 K).
In the event of a lack of refrigerant in the condenser, the C-D zone is not completely flooded with a liquid, there is only a small portion of this zone, fully occupied by the liquid (E-D zone), and its length is not enough to ensure normal overcooling.
As a result, when measuring the hypothermia at point D, you will definitely get its value below normal (in the example in Fig. 2.6 - 3 K).
And the smaller the refrigerant will be in the installation, the smaller it will be its liquid phase at the outlet of the condenser and the less its degree of hypothermia will be.
In the limit, with a significant lack of refrigerant in the refrigeration circuit, the release of the condenser will be a pop-sick mixture, the temperature of which will be equal to the condensation temperature, that is, the movement will be equal to (see Fig. 2.7).
Thus, the insufficient refueling of the refrigerant always leads to a decrease in supercooling.
It follows that the competent repairman will not add the refrigerant to the installation without regardless of the absence of leaks and without making sure that the supercooling is abnormally low!
Note that as refrigerant refers to the contour, the fluid level at the bottom of the capacitor will increase by causing an increase in hypothermia.
We now turn to the consideration of the opposite phenomenon, that is, too much supercooling.
B) increased overcooling (usually more than 7 K).
Explanation. Above, we were convinced that the lack of refrigerant in the circuit leads to a decrease in supercooling. On the other hand, an excessive amount of refrigerant will accumulate at the bottom of the condenser.
In this case, the length of the condenser zone, completely flooded with liquid, increases and can occupy the entire section E-D. The amount of fluid in contact with the cooling air increases and the magnitude of the supercooling, therefore, also becomes greater (in the example in Fig. 2.8 P / O \u003d 9 K).
In conclusion, we indicate that measurements of the magnitude of the hypothermia are ideal for diagnosing the process of functioning of a classic refrigeration unit.
In the course of a detailed analysis of typical faults, we will see as in each particular case to accurately interpret the data of these measurements.
Too small supercooling (less than 4 K) indicates a lack of refrigerant in the condenser. Incooling (more than 7 K) indicates an excess refrigerant in the condenser.
Under the action of gravity, the liquid is accumulated at the bottom of the capacitor, so the input of vapors into the capacitor should always be located on top. Consequently, options 2 and 4 at least represent a strange solution that will not be operational.
The difference between variants 1 and 3 is mainly in the air temperature, which blows the hypothermia zone. In the 1st embodiment, the air that provides supercooling is entered into the hyposion zone already heated, since it passed through the condenser. The design of the 3rd option should be considered the most successful, since it has a heat exchange between the refrigerant and air on the principle of countercurrent.
This option has the best characteristics of heat exchange and installation design as a whole.
Think about it if you have not yet decided what the direction of coolant (or water) through the condenser you choose.
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