The Compressor: A Fundamental Part of Refrigeration SystemsIt is known that refrigeration equipment, air conditioning, dehumidifiers, among others use the compression refrigeration cycle to produce cold, and practically all of them have an airtight motor compressor that in addition to being the most expensive part of the equipment is the one that performs if not everything, much of the energy consumption.
Both reciprocating and centrifugal compressors are on the market forming hermetic equipment that includes the engine. These closed engines are of a different type than conventional because they are cooled by the same liquid or coolant vapor, at much lower temperatures than the air used to cool the open engines. These engines can work with higher speed temperatures but without exceeding the maximum temperature allowed to the design conditions.
As the work of these closed motors (frequent stops and starts) is different from normal, they are not usually classified by their permanent speed power, but by the starting and full load intensities. The meaning of this classification is clearly observed at the time of selecting the teams.
There is no general rule for designating the size of airtight compressors. Previously, compressor sizes were indicated in HP (CV), but this unit of measurement did not represent a clear definition of cooling characteristics.
Hermetic compressors are designed to be used in steam compression refrigeration cycles and are classified according to the pressure corresponding to the evaporation temperature range in which the compressor operates, within the application category of high, medium and low pressure.
"The condenser and evaporator are heat exchangers, and can be associated with a fan to force the passage of air through them"
Figure 1 Compressor body
Compressor definition
It consists of mechanically forcing the circulation of a fluid in a closed circuit creating zones of high and low pressure in order for the fluid to absorb heat in one place and dissipate it in the other.
The cooling process involves a closed circuit, and the refrigerant is not allowed to expand outdoors.
When the refrigerant goes to the evaporator, it is fed by a tank. The pressure in the tank will be high, until its pressure is equal to that of the evaporator. Therefore, the circulation of the refrigerant will cease and the temperature in both the tank and the evaporator will gradually rise until it reaches room temperature.
To maintain a lower pressure and with this a lower temperature, it is necessary to remove the steam from the evaporator. This is done by the compressor which vacuums it. In simple terms, the compressor can be compared to a pump that transports steam in the refrigerant circuit.
In a closed circuit in the long run, a condition of equilibrium prevails. To further expand this concept we have to see if the compressor sucks steam more quickly, than that which can be formed in the evaporator, the pressure will drop and with this the temperature in the evaporator. On the contrary, if the load in the evaporator rises, the refrigerant will evaporate more quickly which will produce a higher pressure and therefore a higher temperature in the evaporator. The refrigerant leaves the evaporator, either as saturated or slightly overheated steam and enters the compressor where it is compressed. See Figures 2 and 3.
Figure 2 Gas circulation Figure 3 Discharge system
Compression refrigeration cycle
The method of cold production in condensable fluid machines is based on changes in state (liquid-gas and gas-liquid) of a substance (refrigerant fluid) in a closed circuit. To do this, the quality of the fluids is used (Thermodynamic process). The temperature necessary to produce the change of state of the Refrigerant fluid will depend on the pressure at which the fluids are within the operating conditions; that is, at low pressure the temperature is low, and if the pressure rises, the temperature increases.
The reason for taking advantage of state changes is because latent heats (change of state) are greater than sensitive heats (temperature change), with the consequent decrease in the amount of refrigerant fluid and the capacity of refrigeration equipment.
The way to obtain cold with this system describes a theoretical cycle that we can summarize as follows:
• The refrigerant is compressed in a gaseous state by means of a compressor, so that the pressure and temperature of the gas is raised.
• The fluid (gas) is circulated by a condenser, in it the refrigerant condenses at constant pressure (liquid), giving heat to the outside environment, usually air or water.
• The coolant is passed through an expansion stage where it loses pressure and temperature evaporating a small fraction of the liquid.
• The refrigerant with low temperatures and pressure is passed through an evaporator in which the refrigerant evaporates (gas), absorbing heat from the outside environment and thus achieving the desired refrigeration effect.
• Finally the gas is recompressed, restarting the cycle.
The compressor is usually driven by an electric motor, and in equipment of small power almost exclusively are hermetic motor compressors driven by single-phase asynchronous motors.
The condenser and evaporator are heat exchangers, and can be associated with a fan to force the passage of air through them, to complete our cooling cycle. See Figure 4.
Figure 4. Compression system
"Thermodynamics is the tree and sun of physics that studies energy, the transformation between its different manifestations, such as heat, and its ability to produce a work."
Electrical Attachments
Figure 5. Electrical connection
Manufacturers of engines intended for hermetic cooling equipment mount the rotor and stator assemblies inside a common housing for the compressor, and adapt the appropriate bearings to it. The windings are perfectly insulated and guaranteed, especially in large motors. Small motors are usually single-phase and as contacts capable of producing sparks cannot be used, they are usually a type of split phase with the capacitors and switches located on the outside of the compressor (Junction Box). See Figure 5.
The motors of these units are usually single-phase induction motors (SPIM), and due to their zero starting torque, they need an auxiliary system for their start-up, which can be achieved with different devices, such as relays or centrifugal switches. Each of them gives the engine different operating characteristics.
A problem with monophonic or biphasic compressors is that they generate a pulsating magnetic field, so they have zero starting torque, which is why they are not able to start up on their own. To cause the start in these motors they are equipped with an auxiliary starter winding (start, S) with a lag with respect to the main (run, R), so that between this auxiliary winding and the main one the necessary torque is provided at the start, and once this is produced, the auxiliary winding can be disconnected.
These single-phase motors over three-phase motors have the advantage of being able to connect to the basic electricity distribution network, which makes it suitable for domestic applications.
The fastest way to select or replace a compressor is through the tables and curves of operating characteristics provided by manufacturers, which not only present the capabilities and conditions of evaluation, but give the capacity and power for a variety of evaporation and condensation temperature.
But it must be said that these constitute an average of laboratory evaluation with equipment designed to mediate the operating conditions commonly called calorimeter. Recent research shows how the cooling capacity given by the catalog of some manufacturers is higher than the real one.
Figure 6. Useful overheating and overheating in the suction line
The evidence was demonstrated through information from catalogs, calculation and experimentation and the concepts between useful overheating and total overheating. The total overheating is formed by the product in the suction line and that produced in the evaporator (called useful reheating), the latter being the only one that contributes to increase the useful cooling capacity. See Figure 6.
Some Recommendations for good use of the compressor
To ensure a long compressor life, out-of-design operating conditions that lead to thermal decomposition of the materials used in the compressor must be avoided.
Some materials used that affect the life of the compressor are the following: Refrigerant Gas, type of oil and materials for engine insulation.
Examples:
When selecting a compressor we have to have well defined the application and use that will be given to our equipment, since with that we can define the type of refrigerant gas that we will use, as well as the type of appropriate oil, if we do not take into account this recommendation we are guaranteeing a short-term problem in our compressor.
The insulation of the motor is formed by the enamel for the copper winding, which supports an internal temperature inside the compressor (factory defined values), if it exceeds the design temperature, it will cause electric arcs to be generated, and therefore severe damage to the compressor.
Machines that apply compression cooling
• Air conditioning or air conditioner
• Domestic refrigerator, refrigerator or refrigerator
• Water cooler
• Ice factory
• Cooling chamber
• Milk tank
There are countless criteria and parameters regarding the subject of compressors, specific information, technical specification sheets, performance tables, selection recommendations, installation and maintenance, the manufacturer will be responsible for offering all the necessary information that will be very useful when selecting the compressors that best suit your requirements.
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Glossary
• Centrifugal Switch: is an electrical switch designed to activate or deactivate an electric motor depending on the rotational speed of the shaft. square (psi).
• Gas: one of the three basic states of matter. While air is a gas, in the field of pneumatics the term gas is generally applied to any gas other than air.
• Displacement: the volume swept by the piston or rotor(s) per unit of time, usually expressed in cubic feet per minute.
• Speed: The term speed in the case of a compressor refers to the number of revolutions per minute (rpm) of the transmission shaft or rotor shaft of the compressor.
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