Lecture 6: Cooling Methods Flashcards
what does t=z mean?
half cooling time
describe room cooling technique
- 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
what are advantages of room cooling?
- 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
what are disdvantages of room cooling?
- very slow = takes long time to cool
- unsuitable for produce which does not tolerate slow cooling
- desiccation occurs if higher air speeds are used
what are special considerations of room cooling?
- 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
how can room cooling be modified? how does this improve it?
- adding ceiling gets
- used to improve air circulation around containers
- air is directed downwards into the stack rather than over it - open cones are attached to a pressurized false ceiling
- cold air moves down from jets
- allows faster cooling - 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
what is differential air flow rate?
higher rate during precooling and lower rates during storage
what is forced air cooling?
what principle is it based on?
- 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)
what are main features of forced air coolers?
- air flows through container, not around (more efficient contact and faster cooling)
- uses fan/blower to pull air through the produce, which is then pushed to cooling coils
- stacking and blocking open spaces results in negative pressure between fan and container. Results in suction of cold air
what are features of hydrocooling?
- 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
why is cold water a more efficient cooling medium than air?
q = hAT
h of air = 5
h of fast moving water = 85
what are disadvantages of hydro cooling?
- 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)
describe package icing in transit cooling
- 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
describe vacuum cooling
- cooling achieved by rapid water evaporation
- needs large capital investment
- best for produce with large ratio of surface to volume
in vacuum cooling, how does water evaporation cause fast cooling?
- evaporation results in cooling and moisture loss
- evaporation rate is driven by WVPD, and is very slow
