6 Sources of Refrigeration Flashcards
early refrigeration mostly made use of _____
ice
describe ice refrigeration
ice held within an insulated chamber above produce (since cool air sinks); melting ice absorbs heat from produce;
today, _____ refrigeration is the primary source of refrigeration
mechanical
describe mechanical refrigeration
a refrigerant absorbs heat from produce; refrigerant is now warmer than the air outside and releases out the heat
(air goes from cold room to warm room)
describe the properties of a refrigerent
usually a gas that can compress
- increases temp
- increases boiling point
- under pressure it can be liquefied at a moderate temp
- compressed liquid will cool when decompressed
- decompressed refrigerant has a lower boiling point and will remain a gas
compression decreases _____, but increases _____, _____, and _____
decreases volume
increases pressure, temperature, and boiling point
decompression increases _____, but decreases _____, _____, and _____
increases: volume
decreases: pressure, temperature, and boiling point
the refrigerant moves within a _____ system
closed
what happens inside the cooling coils?
- refrigerant expands, lowering pressure, temp, and BP
- product temp is > refrigerant temp
- refrigerant absorbs heat
what happens outside? (air/water cooled condensors)
- refrigerant is compressed, increasing pressure, temp, and BP
- refrigerant temp > outside temp
- outside absorbs heat
- refrigerant becomes a high pressure liquid at ambient temp
what are the 5 components of a mechanical refrigerator?
1) evaporator
2) compressor
3) condenser
4) receiver
5) expansion valve
draw the diagram of a refrigerator
it’s on slide 16 :)
name some refrigerants
ammonia, freons/fluorocarbons
describe ammonia as a refrigerant
- used in early times
- has very high latent heat
- non-corrosive to iron and steel but corrodes copper
- irritates mucous membranes and eyes
- toxic at >0.5% concentration
- leaks are easily detected (smells bad)
- leaks can damage produce
describe freons/fluorocarbons as refrigerants
- mostly replaced ammonia
- Freon 12 (CCl2F2) is aka Refrigerant-12 or R-12; one of most commonly used
- Freon-22 or R-22 = chloro-difluoro-methane
- freon-30 or R-30 is methylene chloride (CH2Cl2)
R-12 = ? R-22 = ? R-30 = ?
12 = dichloro-difluoro-methane (CCl2F2) 22 = chloro-difluoro-methane (CHClF2) 30 = methylene chloride (CH2Cl2)
describe desirable characteristics of refrigerants (theres like 10, if you can name 6 i’ll be happy)
- high L (higher = better)
- moderate condensing pressures are better
- freezing point far below evaporator temp
- critical temp should e high, much higher than ambient
- low toxicity
- inflammable
- low corrosiveness
- chemically stable
- easily detectable leaks
- low cost
what are some of environmental impacts of refrigerants?
- CFCs are stable and can destroy the ozone
- CFCs are cleaved by UV light which reacts with the ozone, depleting its concentration
what are some alternatives to CFCs? why are they better?
- hydrofluorocarbons (HFC)
- hydrochlorofluorocarbons (HClFC
why:
- H containing FCs have weak C-H bonds that are more susceptible to cleavage/breakdown (less stable)
- degraded on earth
describe the vapor absorption refrigeration system
- gas refrigeration system, no compressor
- burning of gas provides energy for moving refrigerant from low temp to high temp region
- no moving parts in system
- movement of refrigerant is facilitated by a medium which absorbs refrigerant from evaporator and releases it into
what is the most common vapor absorption refrigeration system?
ammonia-water
ammonia = refrigerant water = absorbant
what are the main components of a vapor absorption system?
- absorber
- generator (external heat source)
- condenser
- evaporator
- moderator (maintains system pressure)
describe the mechanism of the vapor absorption system
- burning gas vaporizes refrigerant
- gas is moved from generator to condenser
- water returns to absorber
- ammonia absorbs heat in evaporator and vaporizes
- ammonia is dissolved in water in absorber
- has (hydrogen) is used as a pressure moderator
pros and cons of evaporative cooling
pros:
- energy efficient and economical
- highly suitable for developing countries
- can produce cold air with RH > 90%
cons:
- can only cool air to its wet bulb temp
