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
how can you increase the efficiency of evaporative cooling?
- use dry air
- use vacuum
- use mutiple effect
describe night time cooling
- applicable when there is significant temp difference between day and night
- stored commodity is thoroughly ventilated at cool times
- harvesting at night will reduce field heat
describe high altitude cooling
it’s cooler in the mountains
true or false: bringing cold air from the top of the mountain to ground level is a smart idea
not really; the air experiences lower pressure at high altitudes. when it comes to ground level, pressure will increase, compressing the air and heating it up.
what are some other historic methods of refrigeration?
- well water
- naturally occurring ice
- underground storage
thermo-electric cooling is based on the _____ effect
Seebeck
describe the Seebeck effect
by generating an emf between 2 dissimilar metals, you can estimate the temperature difference.
when you cool the hot end, the cool end also gets cooler. if you heat the cold end, the hot end gets hotter.
what’s a vortex tube?
tool that takes compressed air and converts into 2 air streams. one stream is hot and the other is cool.
cold part can reach -50F and the hot side can reach 260F
how does the vortex tube work?
- compressed air injected into vortex tube at extremely high speeds, creating a vortex
- part of the air is forced to spin inward to the center and travels up along the tube where a valve turns the spinning column of air inside itself
- inside of the column gives up its heat to the outside column
- cold air is directed out the cold end of the vortex tube, hot air is directed out the other end
