Lecture 6: Heat Penetration Behaviour (Formula Methods) Flashcards
what are uses of general method?
graphical or numerical integration of heat penetration curve
how do you use the formula method
- use heat penetration parameters from the heat penetration curve
- integrate destruction kinetics with heat penetration parameters
what is transient time
temp data obtained at the cold spot location in the container during a process test
in heating and cooling curves, temp and time have a ___ relationship
semi log
in heating and cooling curves, log (temp) vs time have a ____ relationship
linear
what defines the intercept of the heating curve?
log (Tr-Tpih)
what is Tr?
retort temp
what is Tih?
product initial temp
what is T?
product temp at time t
what is Tpih?
pseudo initial temperature (intercept of the straight line)
what defines where the process actually starts?
log (Tr-Tih)
define the slope of the heating curve
[log(Tr-T1) - log (Tr-T2)]/ (t1-t2)
what is fh?
heating rate index (the time to pass one log cycle)
in the heating curve, what is slope defined as?
slope = - 1/fh
what is jch?
lag factor
jch = (Tr-Tpih) / Tr-Tih)
what is g?
Tr - T
what is Ih?
Tr - Tih (temp difference during heating)
what is Tw?
water temperature
what is Tic?
product initial cooling temp
what represents the intercept of straight portion of the cooling curve?
log (Tpic-Tw)
what defines the slope of the cooling curve?
slope = -1/fc
what defines the point where the process actually starts?
log (Tic-Tw)
what is Tpic?
pseudo initial cooling temp (the point where the linear portion of the cooling curve is a zero)
what does Fc = Fh mean?
the product is heating and cooling at the same rate
what is CUT?
come up time
- the time to reach the operating temperature
- not the same as processing time
- contributes to some warming of the product
- using some of the CUT can reduce the process time
when can you ignore CUT?
as a safety factor, CUT can be ignored
how much of the CUT did Ball determine to be effective?
the last 42% of CUT is effective and can be used as processing time
how much of the CUT did Ball determine to be NOT effective?
- the first 58% of CUT
- should be discarded
if your CUT is 10 mins, ____ is not useful
first 6 mins
the come up time means the time scale should be shifted _____
forward
eg. shift the Xzero to the new point at X = 0.58CUT
how are cooling curves affected by CUT?
cooling curves are not adjusted because cooling times are not used in process calculations
what is B? what does this include? what equation represents this?
ball process time
includes: effective portion of the CUT + operators process time
B = Pt + 0.42CUT
what is Pt?
operator’s process time
the total heating time - CUT
what is the ball formula method based on?
heat penetration curve
what is Bb
ball’s process time
what is considered an industry standard?
ball formula method
what are 4 assumptions made in the ball method?
- z = 18F
- heating rate index (fh) = cooling rate index (fc)
- cooling curve lag factor: jcc = 1.41
- CUT effectiveness = 42%
what is used to related heating to lethality?
ball tables related Fh/U to log(g)
when should you not use the ball method? why?
when Fh > Fc
ie. when cooling is faster than heating
using ball method would underestimate
when jcc < 1.41, what method should you use?
ball
when jcc = 1.41, what method should you use?
doesn’t matter
if jcc > 1.41, what method should you use?
stumbo
the ball method would underestimate the processing time
what is the stumbo method based on?
- heating and cooling curves
- jcc and jch values
- assumes Fh = Fc
in the stumbo method, Fh = what?
Fh = Fc
when Fh > Fc, what method should you use?
general methods
