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Post harvest technology > Cooling basics > Flashcards

Cooling basics Flashcards

1
Q

Define cooling/precooling:

A

bring produce from field temperature down to storage temperature

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2
Q

Define the different types of heat considered in the cooling process:

A

sensible heat (temperature change): q = m(Cp) delta T
latent heat (state change): q = mL
field heat: heat that needs to be removed to reach storage temp
vital heat: heat of respiration

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3
Q

Advantages of cooling:

A

Quickly lower temperature (less chemical/enzyme/microbe activity, less resp/transp)

Reduced load on cold storage

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4
Q

What is the difference between cooling and cold storage?

A

cooling: quickly remove field heat (+ vital heat, +leaks)
storage: maintain lowered temperature (vital heat, leaks)

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5
Q

What is the temperature limit on cooling?

A

0-5C

or 10-15C for chill-sensitive produce

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6
Q

produce should be cooled ASAP and ____. What do these acronyms stand for? What are the limitations?

A

as soon as possible
AFAF: as fast as feasible

limitations: equipment, cost

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7
Q

How does the refrigeration load compare for cooling vs cold storage?

A

cooling: HEAVY load for SHORT time
storage: LIGHT load for LONGER time

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8
Q

“load” is usually described in what units?

A

kW

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9
Q

What unit is used for refrigeration capacity? What is its definition?

A

ton

heat needed to freeze 2000lbs (short ton) water @ 0C in 24 hrs

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10
Q

What is 1 refrigeration ton in SI units?

A

1 ton = 3.52 kW

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11
Q

Calculation of cooling time:

A

Heat (Q)/rate (q)

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12
Q

Cooling time or rate depends on:

A

product (size/shape/type/package/packing)

system (size/capacity/medium/temperature/flow rate)

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13
Q

What types of heat transfer occur during cooling, and what Laws are used to describe them?

A

conduction (within produce): Fourier’s Law

convection (from produce to medium to cooling coil): Newton’s Law

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14
Q

What type of heat transfer does not occur in cooling processes?

A

Radiation

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15
Q

Conduction heat transfer is due to: _____

It is directly proportional to: _____

A

compacted molecular vibration

proportional to area and temperature gradient, over thickness

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16
Q

what is “R?”

A

resistance

L/kA

17
Q

k represents:_____. The value depends on: ____

A

thermal conductivity

depends on material

18
Q

How can we find the resistance of a multilayered wall?

A

add all the individual R values together

19
Q

The driving force for heat transfer is:

A

temperature difference

20
Q

Aluminum has (good/poor) conductivity. Wood has (good/poor) conductivity.

A

good

poor

21
Q

produce conductivity is similar to: ____

A

conductivity of water

22
Q

Convectional heat transfer is due to: _____, which is caused by: (2)

A

movement of particles/molecules as a whole (liquid or gas)

due to density gradient (temperature) or forced movement (fans)

23
Q

What does “h” represent? Is a higher or lower h value better for a cooling medium?

A

h = heat transfer coefficient

higher: can pick up heat more efficiently

24
Q

How can the h value of a medium be increased?

A

circulation

25
Q

heat transfer across a wall involves what type of heat transfer? What needs to be calculated?

A

both convection and conduction!

need to calculate U (overall heat transfer coefficient)

26
Q

How is U calculated?

A
  1. find R for inner fluid, outer fluid (1/h)
  2. find R for wall materials (thickness/k)
  3. add together to find total R
  4. U = 1/R
27
Q

Compare steady state with unsteady state. What state describes the cooling of produce?

A

steady: no temperature change
unsteady: temperature change with time (cooling)

28
Q

True/False: cooling occurs in a linear fashion

A

False: cooling rate will decrease (curve)

29
Q

The slope of a cooling semi-log curve gives what info?

A

slope = -1/(mCpR) = -CR

Tells cooling rate

30
Q

What is the unit for CR?

A

no dimension: numeric value/unit time

31
Q

What is another way of measuring CR? What is this represented by?

A

half-cooling time: time needed to reduce COOLING LOAD by 50%

z

32
Q

Cooling rate is used to evaluate:

A

process and product related factors (for comparison)

33
Q

The cooling load is described as:

A

Ti-Ta

34
Q

How does the cooling efficiency change as cooling progresses?

A

efficiency decreases

only efficient up to 2 or 3 half-cooling times

Post harvest technology (18 decks)

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