Principles/Sources of Refrigeration Flashcards
What did early forms of refrigeration depend on?
Ice (stored in insulated container)
How does ice provide cooling in ice-refrigerators?
melting ice (latent heat change) -> absorb heat NATURAL CONVECTION: stored above chamber so cool air flows down
Why is the ice stored above the chamber?
cold air is denser -> will sink down; warm air rises
What innovation in fridges was made in the early 1900s?
electric fridges
What is the primary source of refrigeration today? What is the principle?
mechanical
heat absorbed from cooling chamber -> released outside
What makes the heat transfer from cold to hot air possible in a refrigeration system?
use of a refrigerant and compression/decompression
Describe how the refrigerant is used for cooling:
DECOMPRESS -> cools down -> ABSORBS heat when inside cooling chamber
COMPRESS -> heat up -> RELEASE heat outside
How does compression/decompression affect state?
compress -> higher boil pt -> liquid form
decompress -> lower boil pt -> gas
What are common refrigerants?
freons, ammonia
Compression will decrease ____, and increase _____, ______, _____
volume
pressure, temperature, boil point
decompression will increase ____, and decrease ___, ____, and _____
volume
pressure, boil pt, temperature
the 5 parts of a vapor compression refrigeration system:
evaporator compressor condensor receiver expansion valve
The condensor is located _____.
The evaporating coil is located ____.
outside fridge
inside
The earliest refrigerant used is:
What are advantages/disadvantages?
ammonia
good: high latent heat (L) - can absorb a lot, non-corrosive (except copper)
bad: irritating to eyes, membranes; toxic above 0.5%; leak can damage produce
The smell of sulfur in a old fridge system indicates:
ammonia leak
True/False: ammonia is the most commonly used refrigerant
false; mostly replaced by FREONS
What is an alternative name for freons? What are some common types?
fluorocarbons
Refrigerant 12 (difluro-dichloro-methane)
R-22 (chloro-difluoro-methane)
R-30 (methylene-chloride)
What are characteristics of a good refrigerant?
high latent heat of vaporization
moderate condensing pressure
freezing pt «< evaporator temperature
critical temp»_space;> ambient temperature
low toxicity/corrosiveness, stable (SAFE)
NOT inflammable
cheap
leaks easily detected
What is the environmental concern about refrigerants?
CFCs:
- very stable (don’t break down)
- cleaved by UV, react with and damage ozone layer
What is an alternative to CFCs, and why are they better?
hydrofluorocarbons (HFC)
hydrochlorofluorocarbons (HClFC)
less stable -> more easily degraded on earth
The 2 types of modern refrigeration:
vapor-compression
vapor absorption
How does vapor absorption refrigeration work? How does it compare to vapor compression?
refrigerant with LOW BOIL PT (ex: ammonia)
- evaporate -> COOLS FOOD CHAMBER
- absorbed in another liquid (ex: water)
- heat -> refrigerant evaporates out -> pass through HEAT EXCHANGER (Lose heat to outside, condense)
(cycle repeats)
doesn’t need motor (gas powered), no compression, less efficient, use ammonia/water (not CFCs)
What is a simple form of cooling used by many developing countries? How does it work?
evaporative cooling
input warm (unsaturated) air, warm water
air will pick up water -> cooled to WET BULB TEMP
exit: cold humid air, cooled water
What are the main parts of a vapor absorption system? (5)
Absorber Generator (heat source) Condensor Evaporator Moderator (to maintain pressure)
How can the efficiency of an evaporative cooling system be improved?
lower humidity incoming air (can pick up more water)
vacuum (low pressure, water easier vaporize)
multiple effect (use several systems)
What principle is evaporative cooling based on?
as water vaporizes (picked up by air), latent heat needed - will take energy as sensible heat from air
What is the wet bulb depression? Will it increase/decrease at a high initial RH?
Tdry - Twet
decrease
What are other methods (historic and otherwise) of cooling?
night cooling
high altitude cooling
underground storage
natural ice/underground water
For every 1km increase in altitude, temperature will _____. This is due to:
decrease by 10C adiabatic compression (less pressure at higher altitudes)
What are cooling methods specifically applied to transit? (3)
- Icing - pack with slush ice (contact cooling) - high moisture, drain fluid
- Liquid N cooling (cryogenic): applied as spray, fans distribute N gas
- Dry ice (solid CO2, cryogenic): also release extra CO2
2 examples of high-tech cooling:
- Thermo-electric cooling (thermocouple)
2. Vortex tube
The thermocouple cooling is based on: ______ effect. How is this applied?
Seebeck effect Apply voltage (DC) to circuit with 2 different metals (Cu, Co) -> generates different temperature at each end
use hot end for heating, cold end for cooling (if you remove heat from hot end, it will take heat from cool end)
How does a vortex tube work?
compressed air enter
swirling -> form vortex
75% air expands and cools, remaining air heat up (from churning action)
exit 2 streams (hot end, cold end)
