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5) Post-Harvest > Lecture 4: Cooling basics > Flashcards

Lecture 4: Cooling basics Flashcards

1
Q

what is cooling and pre-cooling?

A

lowering of produce temp from field to storage conditions

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2
Q

what is the equation of sensible heat?

A

Q=mCpT

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3
Q

what is the equation of latent heat?

A

Q=mL

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4
Q

what is the diff between sensible and latent heat?

A

sensible: results in change of temp
latent: results in a change of phase (state)

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5
Q

define field heat

A

heat content of harvested produce above storage temp

or heat to be removed to bring produce temp from field to storage

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6
Q

define vital heat

A

heat of respiration

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7
Q

what is the eqn of vital heat in storage?

A

resp rate, heat x quantity x time

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8
Q

what is sublimation?

A

substance going from solid to gas phase, without passing through liquid phase

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9
Q

what are advantages of cooling/precooling?

A
  • cooling quickly brings down: resp rate, transpiration rate, ethylene, chem activity
  • cooling helps in cold storage design ( reduces heat of resp and reduces load on cold storage)
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10
Q

cooling vs cold storage?

A

cooling: removing heat + vital heat + leak

cold storage: removing vital heat + heat leak

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11
Q

how should you cool produce?

A

AFAF ( as fast as feasible) given equipment and cost

and ASAP

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12
Q

what are examples of methods available for cooling?

A 
air
water
vacuum
ice
dry ice
LN
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13
Q

what is the unit for refrigeration capacity (rate)?

A

ton

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14
Q

a ton of refridgeration is……

A

the heat needed to freeze one short ton (2000lbs) of water at 32F in 24 hours

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15
Q

what is the cooling time equation?

A

cooling time (t) = heat (Q) / rate (q)

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16
Q

what does the cooling time/rate depend on in respect to the product?

A 
size
shape
packing
packaging
type
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17
Q

what does the cooling time/rate depend on in respect to the system?

A

size/capacity
medium temp
medium flowrate
medium type

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18
Q

in cooling of a produce, how is conduction and convection heat transferred?

A

conduction heat is transferred within the produce

convection heat transfer from produce to air; and from air to cooling coil

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19
Q

what are 3 modes of heat transfer? what law’s are they based on?

A

conduction: fourier’s law
convection: newton’s law
radiation: plank’s law

20
Q

describe conduction

A

transfer of heat through molecular vibrations or transfer through compacted particles without involving any gross movement of particles (solids and packaged produce)

21
Q

what does fourier’s law state?

A

rate of conduction heat transfer is directly proportional to the heat transfer area and the temperature gradient across a unit thickness

22
Q

what is the unit of thermal conductivity?

what is the variable used?

A

unit: W/mC
variable: k

23
Q

describe convection

A

results from the gross mvoement of molecules or particles either by density gradients (temp) or by forced movement of particles by external means (pump or fan)

24
Q

what is an example of natural convection?

A

room heating by radiators

25
Q

what is an example of forced convection?

A

forced circulation of air using ans in the case of cold rooms

or pumping a fluid through a heat exchanger

26
Q

what is the equation for newton’s law?

A

q = hAT

27
Q

what is the variable used for heat transfer coefficient?

what is the unit?

A

variable: h
unit: W/m^2C

28
Q

which has higher heat transfer coefficients?

a) air
b) steam

A

steam is much higher

29
Q

heat transfer across a wall is a combination of what?

what equation shows this?

A

conduction and convection

Rtotal = Rconduction + Rconvection

30
Q

what is the variable used for overall heat transfer coefficient?

what is the reciprocal of this?

A

U

overall resistance = 1/U

31
Q

describe radiation

A
  • does not involve any medium
  • travels like other electromagnetic radiation
  • can be reflected, deflected, absorbed or transmitted
  • travels uninhibited in vacuum and released only when it hits an absorbing surface
32
Q

what is an example of radiation

A

transfer of solar energy

33
Q

define steady state

A

temp at any given location does not change with time (heat exchangers, continuous baking oven, water supply in tubes, heat transfer across walls)

34
Q

define unsteady state

A

temp at any given location changes with time

35
Q

the most common heat transfer processes are _____ state

what are examples?

A

unsteady

heating of water and cooling of produce

36
Q

describe the simple cooling curve

A
  • non linear behaviour

- drops more rapidly in the beginning then slowly tapers off

37
Q

how is cooling rate expressed?

A

as a numerical value (unit with no dimensions) per unit time

38
Q

cooling rate is an indicator f what?

A

rate of cooling of a test product under a given situation

39
Q

larger CR = ____ (higher/lower) rate of cooling

A

higher

40
Q

how can the same product have different cooling rates?

A

while cooling in different mediums (ex water, air, vacuum)

41
Q

how can the same technique have diff cooling rates?

A

for different shapes and sizes of produce

42
Q

CR can be used to compare what?

A

the effect of process and product related factors

43
Q

what is half cooling time?

A
  • time interval which reduces the cooling load by one half

- also reduces temp difference between the product and medium by one half

44
Q

what can half cooling time be used for?

A

to evaluate the cooling rate of different products or performance of different cooling systems

45
Q

compare half cooling time vs CR

A
  • half CT is simpler and more easily recognizable than CR
  • both can be used to calculate degree of cooling required. They are mathematically the same (one can be obtained from the other)
46
Q

cooling is efficient only up to what percent of half cooling times?

A

2 (75%) or 3 (88%) half cooling times

47
Q

compare the HCT of hydrocooling, forced air cooling and air cooling. Rate them from fastest to slowers

A

hydro –> forced –> air

5) Post-Harvest (17 decks)

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