Lecture 4: Cooling basics Flashcards
what is cooling and pre-cooling?
lowering of produce temp from field to storage conditions
what is the equation of sensible heat?
Q=mCpT
what is the equation of latent heat?
Q=mL
what is the diff between sensible and latent heat?
sensible: results in change of temp
latent: results in a change of phase (state)
define field heat
heat content of harvested produce above storage temp
or heat to be removed to bring produce temp from field to storage
define vital heat
heat of respiration
what is the eqn of vital heat in storage?
resp rate, heat x quantity x time
what is sublimation?
substance going from solid to gas phase, without passing through liquid phase
what are advantages of cooling/precooling?
- 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)
cooling vs cold storage?
cooling: removing heat + vital heat + leak
cold storage: removing vital heat + heat leak
how should you cool produce?
AFAF ( as fast as feasible) given equipment and cost
and ASAP
what are examples of methods available for cooling?
air water vacuum ice dry ice LN
what is the unit for refrigeration capacity (rate)?
ton
a ton of refridgeration is……
the heat needed to freeze one short ton (2000lbs) of water at 32F in 24 hours
what is the cooling time equation?
cooling time (t) = heat (Q) / rate (q)
what does the cooling time/rate depend on in respect to the product?
size shape packing packaging type
what does the cooling time/rate depend on in respect to the system?
size/capacity
medium temp
medium flowrate
medium type
in cooling of a produce, how is conduction and convection heat transferred?
conduction heat is transferred within the produce
convection heat transfer from produce to air; and from air to cooling coil
what are 3 modes of heat transfer? what law’s are they based on?
conduction: fourier’s law
convection: newton’s law
radiation: plank’s law
describe conduction
transfer of heat through molecular vibrations or transfer through compacted particles without involving any gross movement of particles (solids and packaged produce)
what does fourier’s law state?
rate of conduction heat transfer is directly proportional to the heat transfer area and the temperature gradient across a unit thickness
what is the unit of thermal conductivity?
what is the variable used?
unit: W/mC
variable: k
describe convection
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)
what is an example of natural convection?
room heating by radiators
what is an example of forced convection?
forced circulation of air using ans in the case of cold rooms
or pumping a fluid through a heat exchanger
what is the equation for newton’s law?
q = hAT
what is the variable used for heat transfer coefficient?
what is the unit?
variable: h
unit: W/m^2C
which has higher heat transfer coefficients?
a) air
b) steam
steam is much higher
heat transfer across a wall is a combination of what?
what equation shows this?
conduction and convection
Rtotal = Rconduction + Rconvection
what is the variable used for overall heat transfer coefficient?
what is the reciprocal of this?
U
overall resistance = 1/U
describe radiation
- 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
what is an example of radiation
transfer of solar energy
define steady state
temp at any given location does not change with time (heat exchangers, continuous baking oven, water supply in tubes, heat transfer across walls)
define unsteady state
temp at any given location changes with time
the most common heat transfer processes are _____ state
what are examples?
unsteady
heating of water and cooling of produce
describe the simple cooling curve
- non linear behaviour
- drops more rapidly in the beginning then slowly tapers off
how is cooling rate expressed?
as a numerical value (unit with no dimensions) per unit time
cooling rate is an indicator f what?
rate of cooling of a test product under a given situation
larger CR = ____ (higher/lower) rate of cooling
higher
how can the same product have different cooling rates?
while cooling in different mediums (ex water, air, vacuum)
how can the same technique have diff cooling rates?
for different shapes and sizes of produce
CR can be used to compare what?
the effect of process and product related factors
what is half cooling time?
- time interval which reduces the cooling load by one half
- also reduces temp difference between the product and medium by one half
what can half cooling time be used for?
to evaluate the cooling rate of different products or performance of different cooling systems
compare half cooling time vs CR
- 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)
cooling is efficient only up to what percent of half cooling times?
2 (75%) or 3 (88%) half cooling times
compare the HCT of hydrocooling, forced air cooling and air cooling. Rate them from fastest to slowers
hydro –> forced –> air
