RAC Flashcards
is the process of removing heat from an area or a substance. It is usually done by an artificial means of lowering the temperature, such as by the use of ice or mechanical refrigeration, which is a mechanical system or apparatus, designed and constructed to transfer heat from one substance to another.
Refrigeration
is a form of energy contained to some extent in every substance on earth
Heat
are made up of very small particles
atoms
are in a constant state of motion
Molecules
a form of molecular energy that results from the motion of these molecules
Heat
dictates to a degree the molecular activity within a substance.
The temperature of the molecules
two most common thermometer scales
Fahrenheit and the Celsius.
The usual means of measuring temperature. It measures the degree or intensity of heat and usually consists of a glass tube with a bulb at the lower portion of the tube that contains mercury, colored alcohol, or a volatile liquid.
Thermometer
are used to calibrate the controls of refrigeration.
Thermometer
When two substances of different temperatures are brought in contact with each other, the heat will immediately flow from the ? substance to the ? substance.
Warmer to the colder susbstance
The greater the difference in temperature between the two substances, the ? the heat flow.
faster
is a method of heat transfer in which heat moves by passing from one molecule of a substance in another molecule of the same different substance. Heat flow by means of direct contact.
conduction
As the temperature of the substances tends to equalize, the flow of heat ? and ? completely when the temperatures are equalized.
slows and stops
is a method of heat transfer that occurs in fluids, where the molecules are free to move around
convection
The heat of the air, of the lining of the refrigerator, and of the food to be preserved is transferred to a colder substance
Refrigerant
is a method of heat transfer which flows through wave motion similar to light waves wherein the energy is transmitted from one body or another.
radiation
is the ratio between the quantity of heat required to change the temperature of 1 pound of any substance 1°F, as compared to the quantity of heat required to change 1 pound of water 1°F.
Specific Heat
also expresses the heat-holding capacity of a substance compared to that of water.
Specific Heat
is the term used for the heat absorbed or given off by a substance while it is changing its physical state. When this occurs, the heat given off or absorbed does NOT cause a temperature change in the substance.
Latent Heat
In other words, sensible heat is the term for heat that affects the temperature of things; latent heat is the term for heat that affects the physical state of things.
Heat that is added to, or subtracted from, a substance that changes its temperature but not its physical state is called ?. It is the heat that can be indicated on a thermometer.
Sensible Heat
The sum of sensible heat and latent heat
Total Heat
Data tables giving the heat content of the most commonly used refrigerants start at ?°F below zero as the assumed point of no heat; tables for water and steam start at ?°F above zero. Tables of data usually contain a notation showing the starting point for heat content measurement.
40 and 32
? is the amount of refrigeration produced by melting 1 ton of ice at a temperature of 32°F in 24 hours. A ? is often used to express the amount of cooling produced by a refrigerator or air conditioner.
Day-Ton of Refrigeration
is defined as a force per unit area. It is usually measured in pounds per square inch (psi).
Pressure
states that pressure applied to a confined liquid is transmitted undiminished in all directions and acts with equal force on all equal areas, at right angles to those areas.
Pascal’s law
The layer of air that surrounds the earth is several miles deep. The weight of the air above exerts pressure in all directions. This pressure is called,
Atmospheric Pressure
Atmospheric pressure at sea level is ?
14.7 psi.
Below sea level in excavations and depressions, atmospheric pressure?
increases
The exertion of pressure on a substance with a constant temperature decreases its volume in proportion to the increase of pressure.
Pressure – Volume Relationship
Pressure has a relationship to the boiling point of a substance. There is a definite temperature at which a liquid boils for every definite pressure exerted upon it.
Pressure – Temperature Relationship
shows the change of the thermodynamic properties of the refrigerant, the energy, and work transfer between the refrigerant and surroundings.
A refrigeration process
When a refrigerant undergoes a series of processes like evaporation, compression, condensation, throttling, and expansion, absorbing heat from a low-temperature source and rejecting it to a higher temperature sink, it is said to have undergone a ?
refrigeration cycle
If its final state is equal to its initial state, it is a ?
closed cycle
if the final state does not equal the initial state,
open cycle.
is often used to calculate the energy transfer and analyze the refrigeration cycle’s performance.
A pressure-enthalpy or p-h diagram
cycle consists of two regions: the evaporator pressure in the “low side” and the condenser pressure in the “high side”. These pressure areas are divided by the other two components; on one end, is the metering device (expansion valve) which controls the refrigerant flow and on the other end is the compressor.
A single-stage refrigeration
uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the low side and subsequently rejects that heat in the high side. The process involves changes of thermodynamic properties of the refrigerant and the energy transfer between the refrigerant and the surroundings.
The vapor-compression refrigeration system
In this process, the refrigerant evaporates (liquid to gas) at a lower temperature than that of its surroundings, absorbing its latent heat of vaporization (Qc).
This process of heat extraction at low temperature represents the useful part of the refrigerator. The low pressure necessary for the evaporation at the required temperature is maintained by the suction of the compressor.
Evaporation (process 4-1)
Refrigerant is compressed to a higher pressure and temperature for condensation.
The compression process takes the gas from low pressure and low temperature to high pressure and high temperature. Work energy (Qw) is required and is usually provided by an electric motor.
Compression (process 1 -2)
Gaseous refrigerant is condensed to liquid form by being desuperheated, then condensed, and finally sub-cooled, giving up its latent heat of condensation to some cooling medium.
Heat (Qc + Qw) is removed from high temperature, high pressure refrigerant vapor in the condenser, which is a heat exchanger cooled generally by water or air.
Condensation (process 2 -3)
The higher-pressure liquid refrigerant is throttled to the lower evaporating pressure and is ready for evaporation.
The temperature also drops considerably at constant enthalpy. Because of the reduction in temperature and pressure, the refrigerant exits as a two-phase liquid-vapor typically a mixture of 75% liquid, 25% vapor.
Throttling and expansion (process 3- 4)
is a complex piece of machinery that requires numerous parts and accessories to function properly
Refrigerator
Some of the essential parts of a refrigerator include the
compressor,condenser, evaporator, thermostat, etc.
power refrigerators. It initiates full cooling. The cooling agent, refrigerant, is circulated throughout the system, adding pressure to the warm area and heating it. The compressor raises the temperature and pressure of the vapor refrigerant as it circulates through the refrigerator.
Compressors
are located on the rear of a refrigerator which handles the heat exchange process. It is composed of serpentine-shaped copper tubes with aluminum fins.
is a heat exchanger that cools the refrigerant and transforms it into a liquid by transferring heat to the outside.
Condenser
It is placed in the middle of the evaporator and condenser coils. The ?creates a quick pressure drop in the refrigerant, converting it back to a vapor state.
Its purpose is to regulate the release of pressure as the state transitions from liquid to vapor.
Expansion valve
is where the refrigerant enters after exiting the condenser. Between the condenser and evaporator is a fixed-length tubular structure with a very tiny diameter.
usually 0.5 to 0.28mm
Capillary tube
It has copper or aluminum tubes inside the refrigerator. While turning refrigerant liquid into gas, it absorbs heat from the chamber’s air.
the refrigerant cools the area around it, creating the ideal food-preserving environment.
Evaporator
It initiates full cooling. The cooling agent, refrigerant, is circulated throughout the system, adding pressure to the warm area and heating it. The compressor raises the temperature and pressure of the vapor refrigerant as it circulates through the refrigerator.
compressor
It has two main duties: of compressor
Circulating refrigerant
Changing the pressure of the refrigerant
Compressors are divided into 3 groups according to their appearance:
Hermetic compressors
Semi hermetic compressors
Open type
Compressors are divided into 4 groups according to their function:
Reciprocating compressors
Rotary scroll compressors
Helical- Rotary Screw compressors
Centrifugal compressors
is typically locatedin the liquid line, between the condenser and the expansion valve.
Filter dryer
This key refrigerator component has two functions:
Absorb system contaminants such as water which can create acid and hurt the compressor.
Provide physical filtration
is a fluid circulating in the refrigeration compression cycle. It absorbs heat from the food with the help of the evaporator and sends it out by the condenser.
Refrigerant
is a simple refrigerator component which is used in single evaporator refrigerators as explained before. It adjusts the cold air flow from the freezer to the refrigerator chamber.
has a sensor which is controlled by the thermostat. If it senses the refrigerator temperature is too hot, opens the vent between the two chambers and leads the cold air from the freezer to the refrigerator
Diffuser or damper
This refrigerator component is in charge of defrosting the extra ice or frost in the freezer as its name indicates. The job can be done manually by pressing thermostat button or automatically by a system consisting of a heater and a timer.
Defrost system
Since food has some moisture and this moisture can be absorbed by the evaporator surface and turns into frost. This frost disturbs the heat exchange of the evaporator. So, the refrigerator temperature gradually goes upper and upper. Evaporator should have a mechanism to melt this frost and this is done by a small refrigerator component called heater element.
The frost melted by the heater goes out of the refrigerator through a duct and normally evaporates on the compressor or condenser.
Heater element
This refrigerator component is in fact the refrigerant tank. It saves the refrigerant and releases it when necessary. It has another responsibility which is preventing water from entering the compressor in order to protect it.
Accumulator
The defrost water is accumulated in this ?. The water is then evaporated by the compressor heat.
Drain compartment
This refrigerator component is installed in some units to make it easier for you to drink cold water. It occupies a big part of the door but is a useful component.
Water dispenser
