dryer types. It is based on the patented that increases the heat input. If the increased, without exceeding the
predictive model seen below: temperature drop is lower than the upper specification limit.
MC=K p q
1 (∆T) – K2/S set point value, the heat input is Ascontrolisimproved,theimproved
This equation relates the moisture reduced. Any change in water load control system continually calculates
content (MC) of the petfood leaving entering with the feed will be detected the MC standard deviation and moves
the dryer to the temperature drop and corrected before it leaves the the mean (see dashed line shifting
(Delta T = ∆T) of hot air after contact dryer. This predictive model-based upward) MC value upward, always
with the wet product and the dryer MC control system is automatically keeping it three standard deviations
production rate or speed (S). engaged by a “cruise control” method below the upper specification limit to
Figure 3 illustrates the manner in that requires no learning time or ensure that the maximum amount of
which the improved control system tedious and costly calibration. water is left in the product.
Criteria overview
Improved control allows moisture increase As demonstrated in this article, the
Old control method
Improved control method
recommended selection criteria for a
MC control system:
Stage 1
Stage 2
Stage 3
•Includes a MC sensor that operates
inside a dryer to reduce dead time;
UCL •Has a large sample size, including
New target the effect of bed depth MC gradient;
Spec target •Includes a MC sensor that is simple
to operate, rugged and reliable;
LCL •Has model-predictive control
needing no calibration or learning
time;
•Is universally applicable;
•Utilizes “cruise control” operation
to make it more user friendly; and
Figure 4. Mean MC continually increased by better control leaves more water •Uses statistical methods to
in product. maximize water content of exiting
senses the MC inside a single- or Figure 4 shows the advantage of product from 0.5-1.0% without
multiple-pass conveyor dryer, or the “improved” moisture control exceedingtheupper MCspecification
a vertical dryer using temperature system in continually forcing the limits.
sensors. After the hot air supply to mean value of the exiting petfood MC It is our view that due to the reduction
thedryercontactsthewetproduct, the upward toward an operating value in moisture content variation, the
temperature drop of the air is used by that is always + 3 standard deviations improved MC system will pay back the the above model to predict the exiting below the upper MC specification investment in less than one year. ●
product MC. limit (UCL). The installation of the Mr. Robinsonispresidentof Drying
As the petfood passes the improved control method reduces
Technology, Inc. Mr. Douglas is director
temperature sensors, if the temperature the wider variation (higher standard
drop is higher than the set point deviation) produced by the current ofEngineeringatDryingTechnology,Inc.
value, the controller uses the Delta control (left side of curve) and allows Theycanbereachedat+1.409.385.6422,
T value to calculate an output signal the mean value of the MC to be E-mail: john@moisturecontrols.com,
Website: www.moisturecontrols.com.
NEW!
DRIED
CHICKEN
Natural Chicken Flavor
and
Palatability Enhancement
www.petfoodingredients.com
Coming in June:
Packaging
update
