Refrigeration and Air Conditioning Flashcards
What are refrigerants primarily classified into ?
Primary and Secondary Refrigerants
What is a primary refrigerant ?
It directly takes part in the refrigeration system
What is a secondary refrigerant ?
It is first cooled and then used
What is used to cool a secondary refrigerant ?
A primary refrigerant
What are primary refrigerants classified into ?
Halocarbon
Azeotrope
Inorganic
Hydrocarbon
What are the three types of properties refrigerants are checked for before use ?
Physical, Chemical and Thermodynamic
Why should a refrigerant have a low boiling point at atmospheric pressure ?
It provides for lower refrigeration temperature
How low should it be, generally ?
Lower than the required refrigeration temperature
What should be the condensing pressure of a refrigerant at normal temperature ?
Low
Why should a refrigerant have high critical temperature ?
Easy to provide refrigeration, even at room temperature
What is critical temperature ?
the temperature at and above which vapor of the substance cannot be liquefied, no matter how much pressure is applied.
Why should refrigerants have low freezing point ?
Easy to circulate even at room temperature
What kind of a coefficient of performance (COP), is desired (high or low) ?
High
What is coefficient of performance (COP) ?
Ratio of refrigeration effect to work input
What is a desirable horsepower/ ton of refrigeration ?
Low
What is ton of refrigeration ?
Amount of ice melted in 24 hours from 0 degree centigrade water
What is the required flammability of a refrigerant ?
Low flammability
Why should refrigerants have low water dissolvity ?
Reduces moisture chocking of expansion devices with narrow passages
What affect should refrigerants have on metals ?
They should be non corrosive to metals
How should refrigerants mix with lubricating oil ?
Should mix well
What kind of a dielectric strength in required for a refrigerant ?
Strong dielectric strength, especially for hermetically sealed units
What is the property of a hermetic seal ?
It is air tight
Why should refrigerants have high thermal conductivity ?
It increases heat transfer rates
What is the desired viscosity of a refrigerant ?
Low viscosity
What should be the thermal capacity of a refrigerant at evaporating temperature ?
High thermal capacity at evaporating temperature
What should be the thermal capacity of a refrigerant at condensing temperature ?
Low thermal capacity at condensing temperature
What is thermal capacity ?
Measurable physical quantity equal to the ratio of the heat added to (or removed from) an object to the resulting temperature change
What are some common refrigeration systems ?
Vapour Compression
Vapour Absorption
Air Refrigeration
What are some special refrigeration systems ?
Thermoelectric Refrigeration
Steam Jet Refrigeration
Vortex tube Refrigeration
Passive Refrigeration
What are the four main parts of a vapour compression refrigeration system ?
Compressor
Condensor
Expansion Device
Evaporator
What is the main function of a compressor ?
To suck vapours
What does a compressor do after sucking in vapours ?
It increases the pressure
Where do the vapours come to the compressor from ?
Evaporator
What happens to the vapours after the pressure is increased ?
Easily condensed at higher temperatures
What happens when the vapours are compressed ?
A large amount of heat is extracted
What is the main function of a condensor ?
To provide a heat transfer area
What does a condensor provide a heat transfer area to ?
Hot vapours
How do the vapours become liquid ?
Due to their high pressure
On what principle do they lose heat ?
Second law of thermodynamics
What is the second law of thermodynamics ?
Every natural thermodynamic process proceeds in the sense in which the sum of the entropies of all bodies taking part in the process is increased.
What is used as the heat removing medium ?
Generally, air
What is the function of the expansion device ?
Control the amount of flow of refrigerant
What is another function of the expansion device ?
Reduces pressure of condensed refrigerants
What does this facilitate for the refrigerant ?
Lower temperature at constant enthalphy
What is enthalpy ?
Enthalpy is the amount of heat content used or released in a system at constant pressure
What does the evaporator do ?
Provides heat transfer surface for refrigerant to increase temperature
What are some of the other components of vapour compression system ?
Receiver and Refrigeration lines
What does the receiver do ?
Stores the condensed liquid
Why does the receiver store the condensed liquid ?
To maintain sufficient supply of liquid to evaporator
What do the refrigeration lines do ?
Transfers refrigerant from one component to another
Why does the discharge line do ?
Deliver high P, high T from compressor to condensor
What do low temperature lines do ?
Deliver low P, low T vapours from evaporator to condensor
What comes out of the compressor ?
High P and High T vapours
What comes out of the condensor ?
Liquid, which is at low T, but high T
What comes out of the expansion valve ?
Pressure reduces at constant pressure
What kind of energy does vapour absorption use ?
Heat
Which part does vapour Absorption eliminate ?
Compressor
How does vapour absorption handle vapours ?
Absorbed in a liquid, pumped to a generator which is heated for vapours to form
What are two common absorbent refrigerant pairs in vapour absorption cycle ?
Ammonia - Water
Water - Lithium Bromide
What is the replacement for compressor in vapour absorption cycle ?
Absorber, generator, throttled valve and pump
Where is the Ammonia- Water mixture kept ?
Generator
Where does the heat energy come from ?
An external source
Where do the Ammonia vapours go ?
Flow to the condensor through pump
What happens to the vapour in the condensor ?
Reject heat externally
What happens to the vapours in the throttle valve ?
Both temperature and pressure fall
What happens to the liquid ammonia in the refrigeration ?
They evaporate
How do they evaporate ?
By absorbing the latent heat of vaporisation
What does this achieve ?
Produces refrigeration effect
What happens to the newly formed vapours now ?
It enters the absorber
What happens in the absorber ?
Weak solution of Ammonia absorbs ammonia to form a strong solution
Where does the weak solution of Ammonia come from ?
The throttle valve
What happens to the strong solution of Ammonia ?
It is pumped to the generator
Does it take a lot of energy to pump ?
No, it is negligible compared to the heat produced and refrigeration effect
What is the COP of Vapour Absorption in comparison to Vapour Compression in the same temperature ?
It is 20-30% of the COP at the same temperature
What are room air conditioners also known as ?
Window air conditioners
What is the basic function of a window air conditioner ?
To provide filtered cool air, and maintain desired temperature
Where is the refrigerant kept in room air conditioners ?
Compressor
Where is it sent after being compressed ?
It is cooled in the condensor
Why is the refrigerant passed through a filter drier ?
Any moisture is absorbed
How is the liquid refrigerant reduced to low pressure ?
By passing it through the capillary tube
What is a capillary tube ?
A narrow tube (hair-like thickness)
Where does the capillary tube take the liquid refrigerant ?
Into the evaporator
What does the evaporator fan do ?
It flows the air over the evaporator
What happens to the refrigerator when the air flows over the evaporator ?
The refrigerator evaporates
Are the evaporator and condenser fan run by the same motor ?
Yes, but in the opposite direction
Where is the final filter kept ?
In front of the evaporator, to filter cooled air and allow it to enter the room
Why is air recirculated ?
Because the occupants of the room heat the air
How much of air is exhausted ?
About 10%
Why does exhaust ventilator have damper ?
To control the amount of air exhausted into atmosphere
Why is there a decorative grill in front of the filter ?
To change the direction of air flow
Why is there a tray below the evaporator ?
To collect the condensate over evaporator and drain it outside
Why is there a thermostat in front to f the filter ?
To measure and control temperature of air entering room
Why are some safety devices incorporated in compressor ?
To protect against high pressure delivery, high motor temperature, etc.
How is the compressor sealed in room air conditioner ?
Hermetically (Air tight)
What is refrigeration ?
Process of moving heat from one place to another
What is refrigeration system ?
Equipment employed in maintaining object at lower temperature
What is refrigeration effect ?
Amount of heat absorbed by the refrigeration system from the space or object to be cooled
What is the unit of refrigeration ?
Ton, And in SI units, kW
What is ton of refrigeration ?
It is the amount of ice formed or melted in 24 hours from 0 degree centigrade water
What is ice making capacity ?
Amount of ice produced in one hour from the water a 0 degree celcius to 0 degree ice
What is coefficient of performance (COP) ?
Ratio of refrigeration effect to work input
What is relative COP ?
Ratio of actual COP to theoretical COP
