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Classification of Compressors

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Classification of Compressors 

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ELEMENTS OF THE REFRIGERATING PLANTS
Compressors, classification and performance review.
Classification of Compressors:
Compressors are the devices in charge of passing the refrigerating fluid from the evaporation pressure, corresponding to the conditions of the cold focus, to the condensing pressure of the hot bulb, so it is necessary to make an external contribution of work.
The type of compressor depends on the type of refrigerant used. In the positive displacement machines an external force is applied to force a certain volume of gas or steam to move from the enclosure at low pressure to the enclosure
at high pressure.
In turbochargers, the movement of a rotor within the gas or vapor communicates an increase in kinetic energy, which subsequently results in an increase in pressure as the fluid passes through a duct of variable cross-section acting as a diffuser.
Compressors Types
?????????? Alternative Compressors.

The compression device consists primarily of a cylinder (fixed part) and a piston (movable part) which slides internally by the first, is moved by a motor through a connecting rod, which produces an alternative movement on the piston. The plunger absorbs, compresses, and expels the gas through the inlet and exhaust valves.
The segments placed on the piston provide the seal between the piston and the cylinder, separating the high pressure prevailing inside the cylinder, from the low pressure part prevailing in the crankcase.
In the case of small compressors instead of segments, pistons with grooves are used, which ensure the sealing by the important losses of load that the gas suffers when crossing them. This seal also contributes the film created by the lubricating oil.
The piston is the component of the compressor that reduces the volume of gas contained in the cylinder; and the movement described by the piston is reciprocal, name that receives this type of compressors.
The motor that drives the connecting rod is an electric induction motor (single-phase or three-phase), with the rotor in a squirrel cage, and damped its movement through springs. The lower housing is the engine crankcase, allowing the oil to be lubricated. The entire assembly is enclosed in a metal housing formed by two pieces of steel embedded and welded by the closing zone, so they are called hermetic type compressors.
When the engine shaft rotates a reservoir located in the lower part of the same, it picks up the oil during the rotation, and by effect of the centrifugal force, it makes it rise upwards through a conduction existing inside that axis; this oil exits through the top of the shaft bathing the different parts of the compressor.
Position 1 = A, 2 = B ... ..
Position 1 corresponds to the moment when the piston reaches its PMS. When the piston begins its downward stroke, the space inside the cylinder is still filled with gas at a slightly higher pressure than the discharge and in the course of the downstroke the pressure inside the cylinder decreases until the voltage of the corresponding spring closes the exhaust valve, whereby, during the first part of the expansion stroke both valves are closed. When the piston reaches point 2, the inlet valve opens, continues its downward stroke, whereupon steam is sucked in an essentially isobaric process until the PMI, position 3, is reached.
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When the piston starts its upstroke, the intake valve is closed due to a slight increase in the pressure inside the cylinder over the value of the suction pressure. From this moment, with the two valves closed,
the compression of the gas 3 is started according to an essentially adiabatic process, until the position 4 is reached.
At this time, position 4, the pressure inside the cylinder is slightly higher than the value of the pressure in the discharge and when the corresponding spring is overcome, the exhaust valve is opened. As the piston ascends, the gas contained inside the cylinder is driven outwardly, according to an essentially isobaric process, until it reaches the PMS again, at which time the described sequence is re-started.
The compressor body is generally cast and is divided into two parts, which are the cylinder block and the crankcase.
The walls of the cylinders are rectified and polished in mirror with rigorous tolerances, or they are equipped with carefully machined shirts. The outside of the compressor body is provided with fins, to facilitate cooling
of the cylinder block.
In the compressor body, the crank or eccentric friction bearings are disposed; the friction surfaces of the bearings are often grooved to facilitate lubrication.
The compressors with crankshaft on the side of the steering wheel have a side screwed bolt, where is housed the tow gland that allows the placement of the crankshaft; this cap is not necessary in eccentric compressors, a detail that allows to distinguish at a glance the type of compressor.
compressor-alternative
When weight is a factor to be taken into account, it is used to aluminum bodies, as in vehicles for transporting frozen or refrigerated products.
The intake and exhaust valves are different; in the case of the intake valve, it opens when the difference between the pressure in the suction line and the pressure inside the cylinder balances the tension of a spring, while the exhaust valve opens when the pressure difference between the inside of the cylinder and the drive line balances the tension of the corresponding spring.
The maximum displacement of the piston, equal to twice the length of the crank, is the stroke, which must be somewhat less than the length of the cylinder, in order to avoid inter alia that, due to thermal expansion, the piston can strike against the
valve plate during the upward stroke.
The highest position of the plunger is called the upper dead center PMS and the lowest dead center
PMI.
fig4-56
Rotary Compressors Rotary
compressors are particularly suitable for applications where high volumetric displacement is required at moderate operating pressures.
They are constituted by one or several rotating movement elements forming the rotor, and located inside a cylinder, stator.
Between the stator and the rotor there is a chamber in which the aspirated fluid is compressed, so that the motion of the rotor confines the fluid in said chamber eliminating communication with the suction line; subsequently the displacement of the rotor communicates the chamber with the line of impulse and when there exists a greater pressure in it, there is the entrance of fluid coming from the impulse in the chamber that compresses the steam, producing a compression by reflux; finally,
the movement of the rotor expels all the fluid from the chamber, forcing it to pass to the line of impulsion. There are many types of rotary compressors, among which we can mention pallet compressors, radial pistons,
axial pistons , Scroll, etc.
The fundamental difference between rotary and reciprocating compressors is, as far as operational qualities are concerned:
 On the one hand, the absence of alternative displacements reduces the presence of vibrations.
 On the other hand, gas mass consumption is much less pulsating.
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????? Scroll Compressors:
It can be considered as the last generation by its pallets, in which the latter have been replaced by a rotor in the form of a spiral, eccentric with respect to the drive shaft, that rolls on the surface of the stator, that instead of being circular has form of spiral concentric with the drive shaft.
The contact surface between both spirals is established in the stator (in all its generators) and in the rotor also in all its generatrices. As can be seen, there is another fundamental difference with respect to rotary vane compressors, and is that the movable motor spiral does not rotate together with the latter, but only moves with it parallel to itself.
The Scroll compressor has only one movable scroll that follows the path defined by the machining in a fixed spiral, unlike what happens with the piston technology, which requires a large number of moving parts to achieve the effect of the compression sought. The fixed coil is rigidly coupled to the compressor body. The movable scroll orbits, coupled to the axis of the compressor.
The orbital movement creates a series of gas compartments that move between both spirals. These compartments are formed at the periphery of the spirals and move towards the center, where the discharge occurs. As these compartments move, their volume decreases and the temperature and pressure of the gas inside them decreases, generating the desired compression effect.
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? Its operation can be defined in 3 phases:
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- Aspiration: In the first orbit (360 °), two cells are formed and completely filled with vapor at the pressure P1 on the outside of the coils.
- Compression: In the second orbit (360 °), compression occurs as these cells decrease in volume and approach towards the center of the fixed spiral, reaching the end of the second orbit, when its volume is V2 and the pressure exhaust pipe P2.
- Discharge: In the third and last orbit, both cells are placed in communication with the exhaust port, discharge (exhaust) through it.
Screw Compressor (single rotor)
The single-rotor helical compressor consists of a conductive rotor with six helical working chambers with a balloon profile, which drives two satellite toothed wheels having eleven teeth each, of the same profile as the working chambers, and located on both flanks of the driving rotor, the speed of the sprockets is (6/11) that of the main rotor.
The compression power is transferred directly from the main rotor to the steam; the sprockets do not have any energy except friction.
It is necessary that the movement clearances are small, (the optimum for each type of machine), in order to avoid leakage or steam leakage from the exhaust cavity to regions where the pressures are lower.
  Process of operation. 
- Aspiration: With the rotor confined in a cylindrical jacket, all the working chambers are in communication with the suction cavity at one of its ends. A tooth of one of the gears meshes with each chamber, suctioning the steam as it is moved into the chamber.
Compression: As the rotation continues, the chambers once completely filled, are closed and separated from the suction cavity by a tooth belonging to the other toothed wheel, progressively reducing its volume, thus compressing the steam.
- Exhaust: At a given instant, the working chambers with decreasing volumes and increasing pressures are brought into communication with the exhaust port, ceasing the compression and producing the expulsion of the vapor until the volume V2 is reduced to zero.
The velocity of the fluid at the inlet and outlet of the helical compressor increases according to the square of its dimensions, while the flow of refrigerant fluid increases with the cube of its dimensions. If for example the dimensions of the compressor are doubled, the input and output areas would multiply by a factor of 4 and the flow by 8; as the load losses are proportional to the square of the speed, doubling the size of a helical compressor supposes to multiply by 4 the losses of load.
It is customary to mount a check valve to this type of compressors, thus avoiding the reverse rotation of the rotors when the power supply to the motor is cut off. This check valve is operated by the internal suction pressure of the compressor, and thus during operation is kept open without causing an unnecessary pressure drop in the suction of the gas.
Volume control:
- By manually adjusting the position of the slide by turning the stem under the motor shaft, the manufacturer's instructions are followed.
- By means of the automatic control, by means of the oil pressure and the two solenoid valves controlled by a position transmitter between the two parts (remote controlled by remote control).
Advantages
• Low mechanical friction losses.
• Despicable wear on moving parts
Disadvantages
• Excessive lubricating oil.
• Increased price for using carbon fiber and Teflon materials to avoid wear.
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Turbochargers
A turbocharger TC is essentially constituted by a rotor rotating within a housing; the rotor is formed by a set of blades or blades and the steam is forced to circulate through the free space between each two blades.
The movement imposed on the rotor increases the absolute velocity of the vapor and subsequently the kinetic energy of the stream is converted into pressure energy by means of a diffuser. There are two types of CT: axial flow and centrifugal or radial flow.
In axial flow compressors, the fluid flows in a direction parallel to the axis of the rotor, while in the radial flow the movement of the fluid relative to the blade is normal to the axis of the rotor; the compression ratio depends on the number of stages of
compression (rotor and stator forming a pressure step), the shape of the blades, in particular of the outlet angle β2 and the peripheral speed of each impeller; however, the compression ratio of each step εc ranges from 1.25 to 1.30 and for the complete compressor between 15 and 20.
In the radial flow compressors, a compression ratio of up to 3 The axial flow compressor is used only when the mass of gas to be compressed is extremely high and therefore not usually used in the field of refrigeration.
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