The term refrigeration is used here to include:
Both the cooling process for preserving food and comfort cooling.
What is Refrigeration?
The process of removing heat from a place where it is not wanted and transferring that heat to a place where it makes little or no difference.
Ice requires ____ Btu/lb to melt.
144
Melting 1 ton of ice requires ________ Btu.
288,000
_____ is the amount of heat required to melt 1 ton of ice in a 24-hour period.
One ton of refrigeration.
How can the boiling point of water be changed and controlled?
By controlling the vapor pressure above the water.
What is temperature/pressure relationship?
This refers to the temperature/pressure relationship of a liquid and vapor in a closed container. If the temperature increases, the pressure will also increase. If the temperature is lowered, the pressure will decrease.
What is a refrigerant?
A substance that can be changed readily to a vapor by boiling it and then changed to a liquid by condensing it.
The pressure and temperature of a refrigerant will correspond when both liquid and vapor are present under two conditions:
When the change of state is occurring and when the refrigerant is at equilibrium.
When both of these conditions occur the refrigerant is said to be: When the change of state is occurring and when the refrigerant is at equilibrium.
Saturated.
Saturation pressure:
The saturation temperature depends on the pressure of the liquid/vapor mixture.
Vapor pressure:
The vapor that was generated because of the increase in temperature.
When is vapor pressure generated?
Any time saturated vapor and liquid are together.
List the four major components that make up mechanical refrigeration systems:
Evaporator, Compressor, Condenser, Metering device.
What does the evaporator do?
Absorbs heat into the system.
What happens as refrigerant passes through the evaporator coil?
The refrigerant enters the coil form the bottom as a mixture of about 75% liquid and 25% vapor.
What is the purpose of the evaporator?
Is to boil all of the liquid into a vapor just before the end of the coil. This occurs approximately 90% of the way through the coil, leaving pure vapor.
What is Superheat considered for in relation to the compressor and why?
Insurance, because it ensures that no liquid leaves the evaporator and enters the compressor.
Summarize what the three main functions the evaporator are:
Absorb heat form the medium being cooled. Allow the heat to boil off the liquid refrigerant to a vapor in its tubing bundle. Allow heat to superheat the refrigerant vapor in its tubing bundle.
Describe what the suction line is and does:
Connects the evaporator to the compressor and provides a path for refrigerant vapor to travel.
What does a well insulated suction line do?
Helps the system operate as efficiently as possible and the insulation prevents the suction line form reaching the dew point temperature and sweating.
What is the compressor consider as in the refrigeration system and why?
The heart. Because it pumps heat through the system in the form of heat-laden refrigerant.
A compressor can be considered a ______, why?
Vapor pump. It reduces the pressure on the low-pressure of the system, which includes the evaporator, and increases the pressure on the high-pressure side of the system.
What are the most common compressors used in residential and light commercial A/C and refrigeration?
Reciprocating, the rotary, and the scroll.
Describe the rotary compressor.
A compressor that uses rotary motion to pump fluids. It is a positive displacement pump. Extremely efficient and have few moving parts. It uses a drum like piston that squeezes the vapor out the discharge line.
Describe the scroll compressor.
A compressor that uses two scroll-like components, one stationary and one orbiting, to compress vapor.
The scroll compressor is a positive displacement compressor until what happens?
Too much pressure differential builds up.
What type of compressor is used in large commercial systems and why?
Centrifugal compressors. Because it is much like a large fan and is not positive displacement. They must move more refrigerant vapor through the system.
The centrifugal compressor is referred to as a ________.
Kinetic displacement compressor.
Describe the screw compressor.
A positive displacement and is used in larger A/C and refrigeration applications. It is popular for use in low-temperature refrigeration applications.
Describe the condenser.
The component in a refrigeration system that transfers heat from the system by condensing refrigerant.
The condenser rejects:
Both sensible "measurable" and latent "hidden" heat from the refrigeration system.
What is the discharge line?
The hot gas line, between the compressor and the condenser.
In the HVAC/R industry, the vapor pressure read by the high-side gauge is often referred to as:
Head pressure, high-side pressure, discharge pressure, or condensing pressure.
What happens to refrigerant in the condenser?
When the condensing refrigerant gets about 90% of the way through the condenser, it becomes pure saturated liquid.
Describe subcooled liquid.
The temperature of a liquid when it is cooled below its condensing temperature.
Three important things happen to the refrigerant in the condenser:
The hot gas from the compressor is desuperheated. The refrigerant is condensed from a vapor to a liquid. The liquid refrigerant is subcooled.
Define a condensing unit.
A complete unit that includes the compressor and the condensing coil.
Describe the characteristics of refrigerant as it travels in a operating system.
The suction line carries low-pressure, low-temperature, superheated vapor to the condensing unit, and the liquid line carries high-pressure, high-temperature subcooled liquid from the condensing unit to the metering device.
What is one type of metering device that is a simple fixed-size known as?
An Orifice.
What is an orifice?
A small restriction of a fixed sized in the liquid line.
When the high-pressure subcooled liquid passes through the orifice what happens to the liquid?
Its pressure changes and some of the refrigerant flashes to a vapor called flash gas.
Describe flash gas, and tell how it affects system capacity.
The pressure drop in an expansion device when some of the liquid passing through the valve changes quickly to a gas and cools the remaining liquid to the corresponding temperature.
The superheat that is picked up only in the evaporator is referred to as:
Evaporator superheat.
The total superheat that is picked up in the evaporator and the suction line is referred to as:
System superheat.
The conditions for condensing are determined by:
The efficiency of the condenser. When the condensing temperature is about 30�F higher than the surrounding air used to absorb heat from the condenser.
What temperature will high efficiency condenser condense?
25�F above the surrounding air temperature.
Describe the temperature and pressures that corresponded to the condenser heat load.
The higher the heat load, the higher will be the condensing temperature and the corresponding pressure.
Why is proper ventilation important when working with refrigerants?
Because refrigerants are heavier than air, and the oxygen could be displaced by the refrigerant and a person could be overcome.
Many refrigerant leaks can be found by listening, what type of device is on the market that detects leaks by sound?
Ultrasonic.
What type of refrigerant leak detector is practical and yet simple to use?
Soap bubbles.
List the common methods used for refrigerant leak detection.
Ultrasonic, soap bubbles, halide torch, electronic, and ultraviolet.
What are the pumping characteristics of refrigerants?
System capacity is directly related to the number of pounds of refrigerant that are circulated through the system per unit if time.
ANSI is:
The American National Standards Institute.
ASHRE is:
The American Society of Heating, Refrigerating and Air-Conditioning Engineers.
____ and ___ are responsible for naming refrigerants and identifying their characteristics.
ANSI, ASHRE.
What does enthalpy describe?
How much heat a substance contains with respect to an accepted reference point.
What is net refrigeration effect, NRE?
It is expressed in Btu/lb. and is the quantity of heat that each pound of refrigerant absorbs from the refrigerated space to produce useful cooling.
What is THOR?
Total heat of rejection. The system must reject all of the heat introduced to the system.
What is heat of rejection?
The condenser must reject all of the heat that is absorbed in the evaporator and suction line as well as the heat generated during the compression process.
How does temperature glide occur?
When the refrigerant blend has many temperatures as it evaporates or condenses at a given pressure.