Cooling basics Flashcards
Define cooling/precooling:
bring produce from field temperature down to storage temperature
Define the different types of heat considered in the cooling process:
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
Advantages of cooling:
Quickly lower temperature (less chemical/enzyme/microbe activity, less resp/transp)
Reduced load on cold storage
What is the difference between cooling and cold storage?
cooling: quickly remove field heat (+ vital heat, +leaks)
storage: maintain lowered temperature (vital heat, leaks)
What is the temperature limit on cooling?
0-5C
or 10-15C for chill-sensitive produce
produce should be cooled ASAP and ____. What do these acronyms stand for? What are the limitations?
as soon as possible
AFAF: as fast as feasible
limitations: equipment, cost
How does the refrigeration load compare for cooling vs cold storage?
cooling: HEAVY load for SHORT time
storage: LIGHT load for LONGER time
“load” is usually described in what units?
kW
What unit is used for refrigeration capacity? What is its definition?
ton
heat needed to freeze 2000lbs (short ton) water @ 0C in 24 hrs
What is 1 refrigeration ton in SI units?
1 ton = 3.52 kW
Calculation of cooling time:
Heat (Q)/rate (q)
Cooling time or rate depends on:
product (size/shape/type/package/packing)
system (size/capacity/medium/temperature/flow rate)
What types of heat transfer occur during cooling, and what Laws are used to describe them?
conduction (within produce): Fourier’s Law
convection (from produce to medium to cooling coil): Newton’s Law
What type of heat transfer does not occur in cooling processes?
Radiation
Conduction heat transfer is due to: _____
It is directly proportional to: _____
compacted molecular vibration
proportional to area and temperature gradient, over thickness
what is “R?”
resistance
L/kA
k represents:_____. The value depends on: ____
thermal conductivity
depends on material
How can we find the resistance of a multilayered wall?
add all the individual R values together
The driving force for heat transfer is:
temperature difference
Aluminum has (good/poor) conductivity. Wood has (good/poor) conductivity.
good
poor
produce conductivity is similar to: ____
conductivity of water
Convectional heat transfer is due to: _____, which is caused by: (2)
movement of particles/molecules as a whole (liquid or gas)
due to density gradient (temperature) or forced movement (fans)
What does “h” represent? Is a higher or lower h value better for a cooling medium?
h = heat transfer coefficient
higher: can pick up heat more efficiently
How can the h value of a medium be increased?
circulation
heat transfer across a wall involves what type of heat transfer? What needs to be calculated?
both convection and conduction!
need to calculate U (overall heat transfer coefficient)
How is U calculated?
- find R for inner fluid, outer fluid (1/h)
- find R for wall materials (thickness/k)
- add together to find total R
- U = 1/R
Compare steady state with unsteady state. What state describes the cooling of produce?
steady: no temperature change
unsteady: temperature change with time (cooling)
True/False: cooling occurs in a linear fashion
False: cooling rate will decrease (curve)
The slope of a cooling semi-log curve gives what info?
slope = -1/(mCpR) = -CR
Tells cooling rate
What is the unit for CR?
no dimension: numeric value/unit time
What is another way of measuring CR? What is this represented by?
half-cooling time: time needed to reduce COOLING LOAD by 50%
z
Cooling rate is used to evaluate:
process and product related factors (for comparison)
The cooling load is described as:
Ti-Ta
How does the cooling efficiency change as cooling progresses?
efficiency decreases
only efficient up to 2 or 3 half-cooling times
