Lecture 5: Principles and Sources of Refridgeration

Question 1

describe Kohlrabi

Answer 1

• 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

Question 2

early refrigeration mostly involved the use of ____

what is this referred to as?

how does this function?

Answer 2

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

Question 3

what is the primary source of refrigeration today?

how does it function?

Answer 3

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)

Question 4

how does heat get transferred from a cold room to a warm room?

Answer 4

• 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)

Question 5

describe properties of refrigerant

Answer 5

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

Question 6

what are the most common refrigerants?

Answer 6

NH4

freons

Question 7

how does a refrigeration system work inside the cold room (cooling coils)?

Answer 7

• refrigerant expands and is at lower pressure, lower temp and lower BP
• product temp is higher than refrigerant T
• refrigerant absorbs the heat

Question 8

how does a refrigeration system work outside the cold room (air/water cooled condensers)

Answer 8

• 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

Question 9

what are the 5 imp parts of vapor compression mechanical refrigeration?

Answer 9

1. evaporator
2. compressor
3. condenser
4. receiver
5. expansion valve

Question 10

what was the earliest refrigerant used?

what are some properties?

Answer 10

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%

Question 11

what happens when Ammonia leaks?

Answer 11

can damage produce

can be detected by smell of sulfur

Question 12

what has replaced ammonia?

Answer 12

freons or fluro carbons

Question 13

describe Freon-12

Answer 13

• aka refrigerant-12 or R-12
• one of the most commonly used refrigerants
• dichloro-difluoro-methane (CCl2F2)

Question 14

what is freon-22? other names?

Answer 14

R-22

chlorodifluor-methane

CHClF2

Question 15

what is freon 30? other names?

Answer 15

R-30

methylene chloride

CH2Cl2

Question 16

what are desirable characteristics of refrigerants?

Answer 16

• 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

Question 17

describe the environmental impacts of CFC

Answer 17

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

Question 18

what are alternatives to CFCs?

describe their characteristics

Answer 18

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)

Question 19

describe the vapor absorption refrigeration system

Answer 19

• 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)

Question 20

what is the most common vapor absorption refrigeration system?

what is the refrigerant? what is the absorbent?

Answer 20

ammonia water

ammonia: refrigerant
water: absorbent

Question 21

what are the main components of the vapor absorption system?

Answer 21

absorber
generator (external heat source)
condenser
evaporator
moderator (to maintain system pressure)

Question 22

describe evaporative cooling

Answer 22

• 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

Question 23

what are limitations of evaporative cooling?

Answer 23

• only possible to cool air to its wet bulb temp

Question 24

how can efficiency of evaporative cooling be increased?

Answer 24

by using dry air, vacuum or multiple effect

Question 25

describe night time cooling

Answer 25

• for when there is significant temp difference between day and night
• stored commodity is throughly ventilated at cool times
• harvesting at night time in these areas will reduce the field heat

Question 26

describe high altitude cooling

Answer 26

• generally, air temp decreases by 10C for every 1000m increase in altitude
• hence, temp is cooler in valleys than hills
• adiabatic compression: brings cold air from top of mountain to create cold storage at ground level
• or produce is transported up to cold storage on hills
• used in california

Question 27

what are 3 techniques used in transit cooling?

Answer 27

• package icing
• cooling with liquid N (cryogenic)
• cooling with dry ice (cryogenic)

Question 28

describe package icing in transit cooling

Answer 28

• product is packaged with slush ice
• rapid direct contact cooling
• excess H2O drained
• high RH environment

Question 29

describe liquid N cooling in transit cooling

Answer 29

• LN has low boiling point (-196C)
• commercially available
• environment friendly
• mainly used as a spray into the chamber: N2 gas is distributed with a fan
• widely used for transit

Question 30

describe cooling with dry ice in transit cooling

Answer 30

• solid CO2 (-80C)
• dry environment
• limited use in transit

Question 31

describe thermoelectric cooling. what is it based on?

Answer 31

based on Seebeck effect:

• when 2 junctions of dissimilar metals are placed at diff temps, an EMF is generated that flows through the circuit
• the generated EMF is proportional to temp difference.
• if one temp is known, the other can be estimated

Question 32

describe thermocouple cooling

Answer 32

• opposite of thermometry principle
• when a DC voltage is applied to a the Co-Cu circuit, it causes 2 junctions to be at diff temp
• hot end used for heating; cold end for cooling
• if heat is removed at hot end, colder end gets cooler
• used in air crafts and portable coolers/heaters

Question 33

describe a vortex tube

Answer 33

• a tool that can take normal compressed air and convert it into 2 air streams (one stream is hot and the other is cold)
• has no moving parts (hence, little maintenance)

Question 34

how does a vortex tube work?

Answer 34

• compressed air is injected into tube at high speeds, which creates a vortex
• part of the air is forced to spin inward and travels up a tube where a valve turns the spinning column of air inside itself
• the inside column of air gives heat to the outside column
• cold air goes out cold end of tube; hot air goes out other end