Lecture 6: Cooling Methods

Question 1

what does t=z mean?

Answer 1

half cooling time

Question 2

describe room cooling technique

Answer 2

• widely used and simple
• refrigerated room where cold air is discharged from ceiling sweeps past the produce
• draws heat and carries it back to the cooling coils where heat is discharged
• air gets cooled again and comes back to pick up more heat

Question 3

what are advantages of room cooling?

Answer 3

• produce is cooled and stored at same place = less handling
• design and operation is simple
• based on moderate refrigeration load
• moderate air speeds = faster cooling

Question 4

what are disdvantages of room cooling?

Answer 4

• very slow = takes long time to cool
• unsuitable for produce which does not tolerate slow cooling
• desiccation occurs if higher air speeds are used

Question 5

what are special considerations of room cooling?

Answer 5

• need storage maintenance
• need proper stacking to allow air to pass around container
• use pallet frames for systematic loading
• need proper ventilation for air to contact produce

Question 6

how can room cooling be modified? how does this improve it?

Answer 6

1. adding ceiling gets
   - used to improve air circulation around containers
   - air is directed downwards into the stack rather than over it
2. open cones are attached to a pressurized false ceiling
   - cold air moves down from jets
   - allows faster cooling
3. cooling bays
   - the large cooling room is divided into smaller cooling bays, which is each independently controlled for temp and air flow rate
   - lowers amount of product handling needed
   - but more expensive due to structural modification

Question 7

what is differential air flow rate?

Answer 7

higher rate during precooling and lower rates during storage

Question 8

what is forced air cooling?

what principle is it based on?

Answer 8

• uses forced circulation air flow (high velocity cold air forced through container) to promote faster cooling times
• aka pressure or suction cooling
• based on suction principle (which is easier than forcing air through holes)

Question 9

what are main features of forced air coolers?

Answer 9

1. air flows through container, not around (more efficient contact and faster cooling)
2. uses fan/blower to pull air through the produce, which is then pushed to cooling coils
3. stacking and blocking open spaces results in negative pressure between fan and container. Results in suction of cold air

Question 10

what are features of hydrocooling?

Answer 10

• uses cold water, which is more efficient than air
• no moisture loss or desiccation during cooling
• can be immersion or shower/spray system
• batch or continuous operation
• facilitates produce cleansing

Question 11

why is cold water a more efficient cooling medium than air?

Answer 11

q = hAT
h of air = 5
h of fast moving water = 85

Question 12

what are disadvantages of hydro cooling?

Answer 12

• less adaptable to packaged commodities
• water must be cleaned often (incr manual labor)
• packages must be tolerant of wetting
• produce must be tolerant to chlorine (since water is used with chlorination)

Question 13

describe package icing in transit cooling

Answer 13

• product is packaged with slush ice
• rapid direct contact cooling = rapid cooling
• excess water drained
• high RH
• produce must be compatible
• commercially adapted
• needs enough holes in package to drain water
• packages should be water tolerant

Question 14

describe vacuum cooling

Answer 14

• cooling achieved by rapid water evaporation
• needs large capital investment
• best for produce with large ratio of surface to volume

Question 15

in vacuum cooling, how does water evaporation cause fast cooling?

Answer 15

• evaporation results in cooling and moisture loss

- evaporation rate is driven by WVPD, and is very slow

Question 16

evaporation of 1% moisture reduces the temp by _____ C

Answer 16

6C

Question 17

what is the evaporation rate in air driven by?

Answer 17

WVPD

Question 18

how can you increase the WVPD, which in turn increases the evaporation rate? how?

Answer 18

using a vacuum

1decreases BP of water

Question 19

as pressure decreases, BP of water ____ (incr/dec)

Answer 19

decreases

Question 20

what are the basic components of a vacuum cooler?

Answer 20

1. vacuum chamber: cylindrical vessel that can withstand at least 1 atm
2. vacuum device: mechanical or steam injection
3. condenser: water or refrigeration coils

Question 21

what are the two main types of vacuum coolers?

Answer 21

1. mechanical rotary vacuum pump

2. steam jet vacuum cooling system

Question 22

what are factors that influence vacuum cooling?

Answer 22

surface area
porosity
degree of vacuum
availability of water

Question 23

what needs to be considered when choosing a cooling system?

Answer 23

• produce to be cooled
• compatibility with existing facilities
• costs: capital investment, operation cost, labor, maintenance
• harvest in cool parts of day