Lecture 5: Principles and Sources of Refridgeration Flashcards
(34 cards)
describe Kohlrabi
- low stout cultivar of the cabbage
- swollen and spherical shape
- aka Sputnik
- taste and texture is similar to broccoli stem or cabbage heart, but milder and sweeter
- high ratio of flesh to skin
- can be eaten raw or cooked
early refrigeration mostly involved the use of ____
what is this referred to as?
how does this function?
ice in an insulated chamber
ice refrigeration
melting ice absorbs heat from the produce. Cold air is heavier than warm air, thus the ice chest is placed above the produce to permit natural convection
what is the primary source of refrigeration today?
how does it function?
mechanical refrigeration, uses refrigerant to absorb heat in the cooling chamber (which is at a lower temp) and releases it outside (which is at a higher temp)
how does heat get transferred from a cold room to a warm room?
- refrigerant absorbs heat in the cold room (where the cooling coil is at a lower temp than the food; refrigerant can absorb heat naturally) and ejects the heat outside (where the refrigerant will be warmer than the surrounding air; refrigerant can give up heat)
describe properties of refrigerant
usually a gas that is compressible. When compressed:
- the temp is increased
- it has a higher boiling point
- it can be liquified at a moderate temp
- volume is decreased
when decompressed
- expands (volume increase)
- has lower temp
- has lower boiling point
- has lower pressure
- remains a gas
what are the most common refrigerants?
NH4
freons
how does a refrigeration system work inside the cold room (cooling coils)?
- refrigerant expands and is at lower pressure, lower temp and lower BP
- product temp is higher than refrigerant T
- refrigerant absorbs the heat
how does a refrigeration system work outside the cold room (air/water cooled condensers)
- refrigerant is compressed
- temp and BP elevated
- outside temp is lower than refrigerant temp
- refrigerant rejects heat and it becomes high pressure liquid at ambient temp
what are the 5 imp parts of vapor compression mechanical refrigeration?
- evaporator
- compressor
- condenser
- receiver
- expansion valve
what was the earliest refrigerant used?
what are some properties?
ammonia
- very high latent heat
- non-corrosive to iron and steel but corrodes copper
- irritating to mucous membranes and eyes
- toxic at conc above 0.5%
what happens when Ammonia leaks?
can damage produce
can be detected by smell of sulfur
what has replaced ammonia?
freons or fluro carbons
describe Freon-12
- aka refrigerant-12 or R-12
- one of the most commonly used refrigerants
- dichloro-difluoro-methane (CCl2F2)
what is freon-22? other names?
R-22
chlorodifluor-methane
CHClF2
what is freon 30? other names?
R-30
methylene chloride
CH2Cl2
what are desirable characteristics of refrigerants?
- high latent heat of vaporization
- moderate condensing pressures
- freezing point far below evaporator temp
- high critical temp (higher than ambient)
- low toxicity
- not inflammable
- low corrosiveness
- chemically stable
- easily detectable leaks
- low cost
describe the environmental impacts of CFC
CFC (chlorofluorocarbon)
- very stable
- poses threat to environment due to potential to damage the protective layer of atmopshere/ozone
- chlorine from CFCs is cleaved by UV light and reacts with ozone by depleting ozone concentration
what are alternatives to CFCs?
describe their characteristics
hydrofluorocarbons (HFC)
hydrochloroflurocarbons (HCIFC)
- hydrogen containing fluorocarbons have a weak carbon hydrogen bonds and are more susceptible to cleavage/breakdown
- less stable than conventional CFCs (hence why they get degraded on earth)
describe the vapor absorption refrigeration system
- gas refrigeration system with no compressor (burning gas provides external energy for moving the refrigerant from the low temp region to an external high temp region)
- no moving parts (mvmt of refrigerant is favilitated by a medium which absorbs the refrigerant from the evaporator and releases it in to condensor)
what is the most common vapor absorption refrigeration system?
what is the refrigerant? what is the absorbent?
ammonia water
ammonia: refrigerant
water: absorbent
what are the main components of the vapor absorption system?
absorber generator (external heat source) condenser evaporator moderator (to maintain system pressure)
describe evaporative cooling
- cooling is achieved due to evaporation of water
- evaporation results due to heat transfer and the air temp will fall along the wetbulb line
air will simultaneously get humidified. Cool humidified air will be coming off the cooler to cool produce - energy efficient and economical
what are limitations of evaporative cooling?
- only possible to cool air to its wet bulb temp
how can efficiency of evaporative cooling be increased?
by using dry air, vacuum or multiple effect
