Principles/Sources of Refrigeration

Question 1

What did early forms of refrigeration depend on?

Answer 1

Ice (stored in insulated container)

Question 2

How does ice provide cooling in ice-refrigerators?

Answer 2

melting ice (latent heat change) -> absorb heat
NATURAL CONVECTION:
stored above chamber so cool air flows down

Question 3

Why is the ice stored above the chamber?

Answer 3

cold air is denser -> will sink down; warm air rises

Question 4

What innovation in fridges was made in the early 1900s?

Answer 4

electric fridges

Question 5

What is the primary source of refrigeration today? What is the principle?

Answer 5

mechanical

heat absorbed from cooling chamber -> released outside

Question 6

What makes the heat transfer from cold to hot air possible in a refrigeration system?

Answer 6

use of a refrigerant and compression/decompression

Question 7

Describe how the refrigerant is used for cooling:

Answer 7

DECOMPRESS -> cools down -> ABSORBS heat when inside cooling chamber

COMPRESS -> heat up -> RELEASE heat outside

Question 8

How does compression/decompression affect state?

Answer 8

compress -> higher boil pt -> liquid form

decompress -> lower boil pt -> gas

Question 9

What are common refrigerants?

Answer 9

freons, ammonia

Question 10

Compression will decrease ____, and increase _____, ______, _____

Answer 10

volume

pressure, temperature, boil point

Question 11

decompression will increase ____, and decrease ___, ____, and _____

Answer 11

volume

pressure, boil pt, temperature

Question 12

the 5 parts of a vapor compression refrigeration system:

Answer 12

evaporator
compressor
condensor
receiver
expansion valve

Question 13

The condensor is located _____.

The evaporating coil is located ____.

Answer 13

outside fridge

inside

Question 14

The earliest refrigerant used is:

What are advantages/disadvantages?

Answer 14

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

Question 15

The smell of sulfur in a old fridge system indicates:

Answer 15

ammonia leak

Question 16

True/False: ammonia is the most commonly used refrigerant

Answer 16

false; mostly replaced by FREONS

Question 17

What is an alternative name for freons? What are some common types?

Answer 17

fluorocarbons
Refrigerant 12 (difluro-dichloro-methane)
R-22 (chloro-difluoro-methane)
R-30 (methylene-chloride)

Question 18

What are characteristics of a good refrigerant?

Answer 18

high latent heat of vaporization
moderate condensing pressure

freezing pt «< evaporator temperature
critical temp&raquo_space;> ambient temperature

low toxicity/corrosiveness, stable (SAFE)
NOT inflammable

cheap

leaks easily detected

Question 19

What is the environmental concern about refrigerants?

Answer 19

CFCs:

• very stable (don’t break down)
• cleaved by UV, react with and damage ozone layer

Question 20

What is an alternative to CFCs, and why are they better?

Answer 20

hydrofluorocarbons (HFC)
hydrochlorofluorocarbons (HClFC)

less stable -> more easily degraded on earth

Question 21

The 2 types of modern refrigeration:

Answer 21

vapor-compression

vapor absorption

Question 22

How does vapor absorption refrigeration work? How does it compare to vapor compression?

Answer 22

refrigerant with LOW BOIL PT (ex: ammonia)

1. evaporate -> COOLS FOOD CHAMBER
2. absorbed in another liquid (ex: water)
3. 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)

Question 23

What is a simple form of cooling used by many developing countries? How does it work?

Answer 23

evaporative cooling

input warm (unsaturated) air, warm water
air will pick up water -> cooled to WET BULB TEMP
exit: cold humid air, cooled water

Question 24

What are the main parts of a vapor absorption system? (5)

Answer 24

Absorber
Generator (heat source)
Condensor
Evaporator
Moderator (to maintain pressure)

Question 25

How can the efficiency of an evaporative cooling system be improved?

Answer 25

lower humidity incoming air (can pick up more water)
vacuum (low pressure, water easier vaporize)
multiple effect (use several systems)

Question 26

What principle is evaporative cooling based on?

Answer 26

as water vaporizes (picked up by air), latent heat needed - will take energy as sensible heat from air

Question 27

What is the wet bulb depression? Will it increase/decrease at a high initial RH?

Answer 27

Tdry - Twet

decrease

Question 28

What are other methods (historic and otherwise) of cooling?

Answer 28

night cooling
high altitude cooling
underground storage
natural ice/underground water

Question 29

For every 1km increase in altitude, temperature will _____. This is due to:

Answer 29

decrease by 10C
adiabatic compression (less pressure at higher altitudes)

Question 30

What are cooling methods specifically applied to transit? (3)

Answer 30

1. Icing - pack with slush ice (contact cooling) - high moisture, drain fluid
2. Liquid N cooling (cryogenic): applied as spray, fans distribute N gas
3. Dry ice (solid CO2, cryogenic): also release extra CO2

Question 31

2 examples of high-tech cooling:

Answer 31

1. Thermo-electric cooling (thermocouple)

2. Vortex tube

Question 32

The thermocouple cooling is based on: ______ effect. How is this applied?

Answer 32

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)

Question 33

How does a vortex tube work?

Answer 33

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)