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INTERMEDIATE COURSE IN FOOD DEHYDRATION AND DRYING
CHAPTER 5: DRYING CURVES
5.1 Introduction
We have now familiarized ourselves witha number of aspects
involved in thedrying of food products. We haveexamined the basics
of drying, wet anddry basis moistures, drying mechanisms,and the
thermal properties of foodmaterials being dried. Using
thisknowledge, we have performedcalculations of the amount of heat
thatwould be required to dry a specific weightof product under a
given set of dryingconditions.
Before we proceed to look at the varioustypes of dryers that are
available, I wouldlike to examine methods of handlinginformation
obtained from drying tests.This information will provide you with
anunderstanding of how a material dries andwill assist you in
determining how youmight want to approach the commercialscale
drying of that material. Once theinformation from a drying test run
hasbeen gathered, it can be organized andused to compile a series
of graphs whichwe refer to as “drying curves”.
Drying curves are very useful inunderstanding the “kinetics” of
how aparticular product dries under a specificset of conditions.
Basically what thismeans is that you will know how thedrying
process changes over the time thematerial is being dried. Very wet
productwill certainly dry differently than the samematerial when it
has a lower watercontent. Drying curves will alert you tothe
changes that are taking place and will
allow you to adjust the drying processaccordingly.
A personal observation that I have madein the field of food
product drying is thatmost dryer operators want to treat theproduct
as if it were something like wetchunks of broken stone, or some
otherequally as indestructible material. Theyfail to realize that
most food products arerather delicate and require a great deal
ofcare when reducing their moisturecontent. These operators use
theapproach that all you have to do is get asmuch heat into the
product as rapidly aspossible and you can keep pushingproduct
through the dryer. The result ofthis fallacy is that they end up
withproduct toasted or burnt on the outsideand wet in the middle.
However, theaverage moisture most frequently meetstheir target
specifications.
Keep in mind that you cannot speed upthe drying process of many
food productswithout doing serious harm to their qualityand
appearance. Knowing how yourproduct responds to the input of heat
overthe course of the drying process is criticalfor achieving the
desired quality andfinished product performance.
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5.2 What Are Drying Curves?
Drying curves are generally graphs of themoisture of a food
product versus time, orplots of the rate of water removal
versustime. However, there are some ways ofmanipulating the drying
information whichyou have that are more informative andenlightening
than others.
Perhaps the best way to study this topic isthrough the use of a
Case Study.
5.3 Case Study #3: Drying Curve Exercise
5.3.1 Drying Scenario
A food processor wanted to dry appleslices for use in a snack
product. Fearingthat improper drying of the apple slices inan
actual production dryer would result inlarge amounts of wasted
product, theprocessor decided to do a some smallpilot-scale or
bench-scale tests.
For these tests, a small cabinet dryer wasused as shown in
Figure 5-1. Cabinetdryers offer a great degree of flexibilityand
require only a small amount ofproduct.
After a few failed attempts, the processorfinally obtained the
desired quality in thedried apple slices. Data from thesuccessful
pilot-scale run were thenanalysed to determine how the productwas
behaving under these dryingconditions.
It should be noted here that by using onlysmall amounts of the
apple slices in thesmall cabinet dryer rather than doing thetests
on a large production-scale dryer, agreat deal of expense was
avoided. Thelarge dryers require much greaterquantities of raw
materials and a largeamounts of waste product can beproduced while
trying to identify the bestdrying conditions.
When doing drying tests such as these,you may often learn more
from your“failures” than you do from your“successes”. In designing
a set of testruns, you may need to push the limits ofyour drying to
determine the conditionsunder which the final dried product fails
tomeet your finished product specifications.By knowing the drying
conditions underwhich your tests fail, you can set up adryer
operating strategy that avoids theseundesirable conditions and
stays withinwhat you consider to be a safe range ofdrying
conditions.
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5.3.2 Dryer Operating Data
We will only concern ourselves here witha portion of the data
obtained. We willnot worry about air velocities and
relativehumidities etc.
In our exercise, 450 g of apple slices withan initial moisture
content of 84.4% (wetbasis) were placed in the dryer .
The air was heated to 65EC in the dryerand then blown through
the dryingchamber containing the apple slices. Theapple slices were
placed on a small wiremesh rack suspended from a balancemounted on
top of the dryer for weighingthe sample. The weight of the wire
meshrack was determined to be 85 gramsbefore the apple slices were
placed on it.
The weight of the apple slices and thewire mesh rack were
recorded every 15minutes throughout the course of the trial.By
subtracting the weight of the rack (85g) from the total weight, the
weight of theapple slices could be found. In addition,
the temperature of the exit air leaving thedryer was recorded.
This information ispresented in Table 5-1. I have personallydone
numerous test runs with differenttypes of small dryers (including
solardryers which take the energy of the sunas their heat source)
and have used thisbasic approach each time.
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Table 5-1: Drying Data for Apple Case Study
Time(minutes)
Exit AirTemperature (EC)
Weight of Apple Slices+ Tray (grams)
Weight of Apple Slices (grams)
0 — 535 450
15 51 513 428
30 51 487 402
45 51.5 460 375
60 51 433 348
75 51.5 407 322
90 52 380 295
105 52 354 269
120 53 328 243
135 54.5 305 220
150 56 285 200
165 58.5 266 181
180 59.5 251 166
195 60.5 237 152
210 61 225 140
225 62 214 129
240 62.5 205 120
255 63 195 110
270 63.5 187 102
285 63.5 179 94
300 64 174 89
315 64 169 84
330 64 165 80
345 64 163 78
360 64 160 75
375 64 159 74
390 64 158 73
405 64 158 73
420 64 158 73
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5.3.3 Working with the Data
One of the first things that you may wantto do with the raw data
in Table 5-1 isprepare plots to show how the weight ofthe apple
slices changes over time andhow the temperature of the exit
airchanges over the same time period.These plots are presented as
Figures 5-2and 5-3, respectively.
From Figure 5-2, we can see that there isnot a great deal of
scatter in the data andthat the shape of the curve of weightversus
time follows a trend that we wouldtypically expect to see. We also
see thatthe weight of the dried apple slices “levelsoff” at about
73 grams. In spite ofadditional exposure to drying conditions,its
weight does not change. In addition, itis evident that the weight
of the appleslices decreases at a relatively uniform orconstant
rate during the first 120 minutesto 150 minutes (i.e., 2 to 2.5
hours) of thetest run. After this, the line in Figure 5-2begins to
decrease its slope. Thisobservation will become significant as
weexamine the data more thoroughly. Rightnow, we should suspect
that there issomething causing a change in the rate ofdrying at a
time around 120 minutes.
Figure 5-3 shows us that the temperatureof the exit air
increases over time. Wecan compare this to the weight changedue to
moisture loss in Figure 5-2. Itappears as if the temperature of the
air isincreasing as more and more of the waterin the apple slices
is removed. Once theapple slices stop losing weight, the exit
airtemperature remains constant at 64EC.This is not surprising,
since thetemperature of the air going into the dryeris being
controlled at 65EC and there isusually some small loss of heat
throughthe walls of the dryer, even though they
are insulated. Once again, we notice thatthere is something
happening in our dryerat approximately the 120 minute mark ofour
test run. From the start of the testuntil 120 minutes, the exit air
temperatureis relatively low, being about 51EC to52EC prior to this
time. After 120minutes, the air temperature begins torise. We will
come back to thisobservation shortly.
For the food processor, the firstmanipulation that could be
performed onthe data would be to determine themoisture content of
the apple slices fromthe weight data gathered during the trialrun.
We know that we have 450 grams ofapple slices at the start with
84.4%moisture. This translates to a solidscontent of 70.2 grams,
and a moisturecontent of 379.2 grams at the start of thedrying
test. As the drying proceeds, theweight of solids will remain
constant,assuming no losses due to such things asair blowing pieces
away etc. The onlything that will change is the weight of
thewater.
As stated previously, the weight of theapple slices can be found
at eachsampling time by subtracting the weight ofthe wire mesh tray
(85 grams) from thecombined tray and apple slice weightswhich were
taken at 15 minute intervals.Table 5-2 shows the data used
forFigures 5-2 through 5-7 inclusive. Anexplanation of how the
various valueswere calculated is included for clarity
andunderstanding of the procedures involved.Spreadsheet programs
are ideal fororganizing raw data and calculatingderived data as
shown here.
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TABLE 5-2: APPLE DRYING CASE STUDY - VALUES USED IN GRAPHS
Time(min)
Exit AirTemp(EC)
Apple SliceWeight
(g)
SolidsWeight
(g)
Moisture Weight
(g)
Wet BasisMoisture
(%)
Dry BasisMoisture
2(g H O/g solids)
WaterRemoved
(g)
Water RemovalRate
(g/g solids/min)
0 450 70.2 379.8 84.4 5.41
15 51 428 70.2 357.8 83.6 5.10 22 0.0209
30 51 402 70.2 331.8 82.5 4.73 48 0.0247
45 51.5 375 70.2 304.8 81.3 4.34 75 0.0256
60 51 348 70.2 277.8 79.8 3.96 102 0.0256
75 51.5 322 70.2 251.8 78.2 3.59 128 0.0247
90 52 295 70.2 224.8 76.2 3.20 155 0.0256
105 52 269 70.2 198.8 73.9 2.83 181 0.0247
120 53 243 70.2 172.8 71.1 2.46 207 0.0247
135 54.5 220 70.2 149.8 68.1 2.13 230 0.0218
150 56 200 70.2 129.8 64.9 1.85 250 0.0190
165 58.5 181 70.2 110.8 61.2 1.58 269 0.0180
180 59.5 166 70.2 95.8 57.7 1.36 284 0.0142
195 60.5 152 70.2 81.8 53.8 1.17 298 0.0133
210 61 140 70.2 69.8 49.9 0.99 310 0.0114
225 62 129 70.2 58.8 45.6 0.84 321 0.0104
240 62.5 120 70.2 49.8 41.5 0.71 330 0.0085
255 63 110 70.2 39.8 36.2 0.57 340 0.0095
270 63.5 102 70.2 31.8 31.2 0.45 348 0.0076
285 63.5 94 70.2 23.8 25.3 0.34 356 0.0076
300 64 89 70.2 18.8 21.1 0.27 361 0.0047
315 64 84 70.2 13.8 16.4 0.20 366 0.0047
330 64 80 70.2 9.8 12.2 0.14 370 0.0038
345 64 78 70.2 7.8 10.0 0.11 372 0.0019
360 64 75 70.2 4.8 6.4 0.07 375 0.0028
375 64 74 70.2 3.8 5.1 0.05 376 0.0009
390 64 73 70.2 2.8 3.8 0.04 377 0.0009
405 64 73 70.2 2.8 3.8 0.04 377 0.0000
420 64 73 70.2 2.8 3.8 0.04 377 0.0000
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By definition, the Wet Basis Moisture ofa product is:
Weight of water x 100% Total weight of material
Dividing the weight of water in the appleslices at each sampling
time by theweight of the wet apple slices at that timewill give us
the wet basis moisture.
We can find the weight of water presentin the apple slices at
each sampling pointby subtracting the weight of solids (i.e.,70.2
grams) from the total weight of thesample at that time.
Wet basis moistures are plotted againsttime in Figure 5-4.
However, there isnothing that really stands out and grabsour
attention in this graph.
Even though we have calculated the wetbasis moisture, it is the
dry basis moisturewhich tends to be more informative.
By definition, the Dry Basis Moisture ofa product is:
Weight of water Weight of dry solids present
= grams water / gram dry solid (or other appropriate weight
units)
We have already determined the weightof water present in the
apples at each 15minute interval. We can divide it by theweight of
solids (i.e., 70.2 grams) presentin the sample at that time to get
the drybasis moisture.
Dry basis moistures are plotted againsttime in Figure 5-5.
Figure 5-5 is remarkably similar in shape
to Figure 5-2. This is due to the fact thatthey are both based
on a constant weightof dry solids. We can observe the sametrends in
Figure 5-5 as we did in Figure 5-2 with respect to the slope of the
curvebeing constant or linear up until about120 to 150 minutes in
the trial run andthen becoming less and less as time goeson after
that.
The most revealing information for theprocessor will be obtained
by determininghow fast the water is removed from theapple as the
drying progresses. Todetermine the rate of water removal, wecould
take the slope of tangents to thecurve of dry basis moisture versus
time(Figure 5-5), or we could use the raw dataand do some
calculations.
The dimensions associated with the rateof water removal will
be:
“grams of water per gram of drymaterial per minute”
Other appropriate dimensions of weightand time could also be
used.
To calculate the water removal rate fromthe observed data, we
will first determinehow much water is removed in each 15minute
period between sample weighings.For example, 22 grams of water
werecalculated to have been removed duringthe first 15 minutes of
the drying process.Dividing this value by the number ofgrams of
solids present (i.e., 70.2 g) aswell as the number of minutes that
it tookto remove this amount of water (i.e., 15minutes), will give
the following:
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Water removal rate =
22 g water removed | | | 70.2 g solids | 15 min
= 0.0209 g water / g solid / minute
or: 0 .0209 grams of water areremoved from each gram of drysolid
material per minute
In the time interval from 15 minutes to 30minutes, an additional
26 grams of waterwere removed (i.e., 48 g water loss at 30minutes
minus the 22 gram water loss at15 minutes). Dividing this water
loss bythe weight of dry solids and the 15minutes time that it took
to evaporate the26 grams of water gives us a waterremoval rate of
0.0247 grams of waterremoved per gram of dry solids perminute.
A plot of “Water Removal Rate vs Time”appears as Figure 5-6.
From Figure 5-6, we can see that the rateof water removal
remains relativelyconstant for the first 120 minutes or so.This
corresponds to the constant ratedrying period where the water that
isevaporating is on the surface of theproduct or is just at the
surface in crevicesand larger capillaries in the product (i.e.,free
water).
The water removal rate then begins todecrease or “fall” until it
levels off at about375 minutes. Drying from about 120minutes to
about 375 minutes is in thefalling rate drying period. Here, water
isbeing brought to the surface through thepores and capillaries in
the product.Some of this water is physically trapped inthe product
capillaries while other water isloosely bound by the product. In
eithercase, it takes time for this moisture to
work its way to the surface of the apple,where it can then be
removed by thedrying air. During the falling rate period,the rate
at which water is removed followsa gently decreasing slope
downwards andto the right in Figure 5-6. This shows howdiffusion of
moisture to the surface of theapple slices is becoming more and
morecontrolling in the drying process.
Finally, all of the free water, loosely boundwater, and
physically trapped water isremoved from the apple slices. At
thisstage, only the more tightly bound water inthe water
“monolayer” is left. Since it istightly bound, it will not be
removed by thegentle drying conditions to which the appleis
exposed. A vacuum oven and muchhigher temperatures are required
toremove it, as would be done in a moisturedetermination test.
The moisture of the product at the point atwhich the drying
mechanism changesfrom the constant rate drying period to thefalling
rate period is known as the criticalmoisture content. In Figure
5-6, we canclearly see that the water removal ratechanges from a
relatively constant valueof approximately 0.025 grams of waterper
gram of dry solids per minute tocontinuously decreasing lower
values after120 minutes of drying. From Figure 5-4,we can estimate
that the moisture contentof the apple slices 120 minutes into
thedrying test was just over 70% on a wetbasis. From Figure 5-3, we
can see thatthe dry basis moisture content was about2.5 grams of
water per gram of dry solids.
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The change from constant rate drying tofalling rate drying was
what gave us thetemperature change in the air leaving thedryer as
shown in Figure 5-3. Sinceevaporation of water was not as
rapidafter the constant rate drying periodended, there was not as
muchevaporative cooling of the air in the dryerbecause not as much
heat was requiredper unit time to remove the moisture fromthe
product. This change in the dryingmechanism is also what is
responsible forthe change in the rate of weight changenoticed in
Figure 5-2. You can see howthe slope of the curve in Figure
5-2changes at approximately 120 minutes ofdrying time, which is
when we have thechange to the falling rate drying periodfrom the
constant rate drying period.
In Figure 5-7, we have plotted the waterremoval rate against the
dry basismoisture of the product. This is aninteresting, although
at times somewhatconfusing curve. The easiest way to readit is to
start at the right side and work yourway towards the left. In this
way, we cansee that when the water content is at itshighest, the
water removal rate isconstant. At a certain point (i.e., about
2.4or 2.5 g water per g dry product) the waterremoval rate begins
to fall. It drops untilthe moisture content is extremely low.This
clearly shows us the criticalmoisture content as a dry basis
moisturevalue.
In summary, Figure 5-6 and Figure 5-7 arethe two drying curves
that allow us to mostclearly understand the apple dryingoperation.
From Figure 5-6, we can seethat after 120 minutes drying shifts
fromits constant rate period to its falling rateperiod. From Figure
5-7, we can tell thatthe constant rate period ends when themoisture
content of the apple slices hits
about 2.5 g water per g dry product.
We then know that during the initial 120minutes of drying, water
removal isprimarily from the surface of the appleslices. However,
after 120 minutes,moisture is being pulled from inside theapple
slices. If we heat the apple tooharshly during the falling rate
period, theapple material could be damagedseverely. Therefore, we
must tailor ourdrying process to address the dryingmechanism that
is occurring within theapple itself. This could be done with
amultiple zone continuous belt dryer thathas higher temperatures in
the first zoneand lower temperatures in subsequentzones (more on
this later). We could alsodesign a dryer that had different
zonelengths to accommodate the variousdrying periods.
We can see in Table 5-2, that the watercontent of the apple
slices at 120 minuteswas 71.1% wet basis moisture or 2.46grams of
water per gram of dry solids drybasis moisture.
Therefore, in our drying work, we wouldneed to be aware of the
critical moisturecontent of the apple slices being about71% wet
basis moisture and we wouldhave to control our drying and
theapplication of heat accordingly.
From Figures 5-4 , 5-5, and 5-6, we canalso see that drying the
product beyond375 or 390 minutes will not give us anyadditional
advantages. Therefore, wecould stop the drying process at this
point.Figures 5-4 and 5-5 show no change inthe wet or dry basis
moisture content ofthe apples after this time; and Figure
5-6indicates that the water removal rate isessentially “zero”
beyond this time.
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5.4 Caveats in Scaling up
BEWARE: Every caution must be takenin “scaling up” the apple
dryer based onthe data generated in the cabinet dryertests.
Even though tests have been done todetermine the drying
characteristics of theproduct being dried, there are otherfactors
that play a major role in drying.Many of these factors can change
as youscale up a dryer from a small unit to alarger size.
The following factors must be considered:
C bed loading characteristics C thickness C uniformity C
permeability C changes during drying C throughput rates C etc.
C air distribution patterns
C product attributes C uniformity C seasonal variation C apple
varietal differences C etc.
Once the conditions from the cabinetdryer have been established
that bestmeet the needs of the processor,expanded testing should be
consideredbefore committing to a larger dryer. Suchtests might be
done on a small-scale beltdryer at a dryer manufacturer’s
testingfacility. The dryer manufacturer wouldthen be able to offer
advice on the finalscale-up to a production-scale unit.
Do not think that just because you knowhow your product behaves
in a small dryerthat you will know how it behaves in alarger dryer.
If you do fall into this trap,you may be setting yourself up for a
nastyor unpleasant surprise later when youattempt to go into
commercial production.
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5.5 Practice Problems (with answers)
Question 1:
If a sample contains 10.5 grams of waterper gram of dry solids
at the start of adrying process and after 3 hours of drying,its
moisture content is down to 2.7 gramsof water per gram of dry
solids, what is itsrate of water loss? Express your answerin units
of grams of water per gram of drysolids per minute.
Answer: 0.043 grams of water per gramof dry solids per
minute.
Question 2:
A sample of apple starts out at 84%moisture. Two hours later its
moisture is75%. What is the rate of moistureremoval? Express your
answer in units ofgrams of water per gram of dry solids perminute
and grams of water per gram ofdry solids per hour.
Answer: 0.0188 grams of water per gramof dry solids per minute;
or 1.125 grams ofwater per gram of dry solids per hour.
Be sure to convert the percent moisturesto a dry basis as the
first steps in yourcalculations. Then take the differences inthe
water content and divide by the time inthe appropriate units.
Question 3:
2.5 kg of tomatoes with a moisture contentof 93% by weight are
dried in the sun.After 6 hours, they weigh 0.70 kg. What isthe
final moisture content on a wet basis,and on a dry basis? How fast
was themoisture removed ? Express the answerto the last part of the
question as “gramsof water per gram of dry solids per hour”.
Answer: After six hours, the wet basismoisture content = 75%,
and the dry basismoisture content = 3.0 g water per gramof dry
solids.
Water removal rates:0.0286 grams of water per gram of drysolids
per minute or 1.714 grams of waterper gram of dry solids per
hour.
In your calculations, you need todetermine the weights of solids
and waterat the start of the drying (0.175 kg solidsand 2.325 kg
water). After drying, 0.175kg of solids would still be present. Of
the0.70 kg of dried tomatoes, 0.525 kg wouldbe water (i.e., 0.70 kg
- weight of the drysolids). The percent moisture on a wetbasis
would be the weight of the water(i.e., 0.525 kg) divided by the
total weight(i.e., 0.70 kg) times 100%. For the waterremoval rates,
you remove 1.800 kg ofwater (i.e., weight of water at the start
-weight of water at the end) from 0.175 kgof solids in 6 hours.
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INTERMEDIATE COURSE IN FOOD DEHYDRATION AND DRYING
CHAPTER 6: TYPES OF DRYERS
6.1 Introduction:
Drying is an incredibly diverse activitycovering a wide variety
of applications andproducts. As a result, many differenttypes of
dryers are now available for foodprocessing and other applications.
Inaddition, there are several new conceptsbeing developed that
address highlyspecific needs for specialized
dryingapplications.
The purpose of this chapter is to introducea number of these
dryers to you and giveyou a brief overview of how they operateas
well as their areas of application. It iscertainly beyond the scope
of a coursesuch as this to provide detailedinformation regarding
any individual typeof dryer.
We will begin by looking at variousmethods of applying heat to
foodmaterials, and sources of heat commonlyused. Then we will look
at batch andcontinuous methods of drying. Followingthis, we will
discuss airflow in dryers,before we begin to look at individual
typesof dryers. While we are examining thetypes of dryers, please
keep in mind thatdevelopments are always happening inthe areas of
design and operation. Wecannot possibly provide a
in-depthexamination of each type of dryer. Forthis reason, you
should consult othersources, such as the Internet andespecially
dryer suppliers, for any detailsthat you may require for your
particularapplications.
6.2 Direct and Indirect Heating
A key feature that separates oneclassification of dryers from
another is themethod by which heat is delivered to theproduct being
dried. Even freeze dryersutilize some heat in combination with
avacuum at low temperatures to dryproducts.
The most common method of deliveringheat to materials in a dryer
is referred toas “direct” heating. Here, the dryingmedium is air
which has been heated priorto entering the drying chamber. It may
beheated by passing it through the flames ofa burner (such as a
natural gas burner,etc.), or by passing it across heated
metalsurfaces where it picks up heat which itthen carries and
transfers to the materialbeing dried.
There may be cases where it is notsuitable to dry materials with
the directapplication of heat from hot air. In theseinstances, the
product may be broughtinto contact with heated surfaces and theheat
can then be transferred to thematerial in this manner. Hot
surfacessuch as those on the outside of rotatingmetal “drums” with
steam circulatingthrough them are one method of indirectheating
that may be used.
Figure 6-1 shows diagrams of direct andindirect heating for a
food material in twotypes of dryers.
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6.3 Batch and Continuous Dryers
Another way in which we can classifydryers is through the manner
in which theyare used.
Consider the example of when we drysmall quantities of materials
in thelaboratory (or even in our kitchen). Weoften put the material
inside a bench-topdryer (or our kitchen oven); start the dryerto
remove moisture; and finally, we takethe dried material out of the
dryer oncethe desired final moisture has beenreached. This process
is referred to as“batch drying”. The dryers are called“batch
dryers” since we have dried thematerial in small batches.
Figure 5-1 in the previous chapter showsa cabinet dryer which is
a type of batchdrying apparatus.
Small food dehydrators that function asbatch dryers are
available commercially.They allow the user to place several
kilograms of material on trays inside thedehydrator and remove
the moisture byblowing hot air through the unit.
For larger, commercial scale dryingapplications, it is not
really practical orefficient to use a batch dryer. You cannotkeep
putting in small amounts of materialand removing them after they
are dry.This is much too labour-intensive; far tooslow; and just
not practical. In caseswhere you may have many kilograms ofmaterial
to dry and you will be working atit for long periods of time,
“continuousdryers” are best suited for the task.
Consider a farmer who has largequantities of grain to dry. The
dryingcould be done by spreading the grain in athin layer on a wire
mesh conveyor beltand passing it through a drying chamberwhere hot
air removes the excessmoisture from the grain. After
travellingthrough the drying chamber, the grain fallsoff the end of
the conveyor belt and iscollected in storage bins or sent to
storage
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silos for later use. Many industries usecontinuous dryers in
their processes dueto their convenience, reliability, and
waterremoving capabilities.
Figure 6-2 shows a diagram of acontinuous belt dryer.
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6.4 Airflow in Dryers
Air is probably the most commonly useddrying medium in the food
processingindustry.
Before we begin to examine the differenttypes of dryers
available for variousapplications, it would be a good idea tolook
at the ways in which air can beintroduced into the dryers.
If you look at the continuous belt dryershown in Figure 6-2, you
can see that theheated air is being introduced into thebottom of
the dryer and travels upwardsthrough the bed of material being
dried.This is referred to as “updraft”. The airthen exits through
the top of the dryer.The exhaust air is cooler and containsmore
moisture than the air entering thedryer because it has given up
some of itsheat to evaporate the moisture from thematerial in the
dryer. Having air flowingupwards through the bed of material is
agood way of avoiding problemsencountered with soft wet products.
If theair was flowing downwards through thematerial, it could
literally push the softmaterial into the wire mesh of theconveyor
belt where it would dry andharden. In a very short time, the
beltwould become plugged and no air couldflow through it to dry the
product. With airtravelling in the upward direction, the
wetmaterial is dried on the bottom of the bedwhich may harden it
slightly and prevent itfrom being mashed into the wire mesh ofthe
conveyor belt.
Once the bottom portion of the materialhas been dried somewhat,
the flow of aircan be directed downwards to dry the topportion of
the product bed. This“downdraft” can be accomplished byhaving the
conveyor belt pass through
various “zones” in a dryer. The zones areseparated by walls or
partitions in thedryer. More information regarding dryerzones will
be presented later in thischapter.
In cases where very light fluffy productsare being dried, it
might not be desirableto have air flowing in the upward
directionsince this may blow the product aroundinside the dryer.
Care must be taken tomatch the direction of air flow to thematerial
being dried.
There may also be cases when it is notdesirable to have updraft
or downdraftflow of air in a dryer. You may want tohave the air
flowing along or across thesurface of the material in the dryer.
Thereare several different options available thatmay be used. We
will look at each one ofthese options in turn.
Counter-current air flow is the term usedto describe the
situation where product isintroduced into one end of the dryer
andheated air is introduced into the oppositeend, as shown in
Figure 6-3 on the nextpage. In Figure 6-3, the wet material
isentering the dryer from the left and leavesthe dryer at the
right-hand side of thediagram. Heated air is blown into thedryer
from the right and leaves the dryeron the left side of the diagram.
This is avery good way to maximize the efficiencyof the drying
operation.
The air entering the dryer in Figure 6-3 isat its highest
temperature just as it entersthe dryer. It also has its lowest
watercontent at this point. The combination ofbeing at its highest
temperature andlowest water content means that the airhas its
highest capacity to remove water.
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This is also the point where it is mostdifficult to remove
moisture from theproduct which is almost finished beingdried, but
is in its falling rate drying periodwhere diffusion is slow .
As the air travels to the left of the dryer inFigure 6-3, it
continues to lose heat as itevaporates moisture. As it is leaving
thedryer at the left-hand side of the diagram,it still has
sufficient heat to warm theincoming cool wet material. At this
point,the air has its lowest water removalcapacity, but since it is
in contact with verywet product, it may still be able to pick
upsome moisture before it leaves the dryer.
While counter-current air flow maximizesthe driving forces of
temperature andmoisture difference between the air andthe material
being dried, it may pose aproblem in some applications. For
thisreason, we should consider co-current airflow.
Co-current air flow describes thesituation where the heated air
for dryingand the material to be dried are bothintroduced into the
dryer and flow throughthe dryer in the same direction. Lookingat
Figure 6-4, we can see how this takesplace. With co-current
airflow, we arebringing the hottest, driest air into contactwith
the wettest, coolest material. Thisavoids the danger of
over-heating theproduct before it leaves the dryer, whichcan happen
with counter-current airflow.Excessive heat may damage
delicateproducts and it should be avoided if thereis a danger of
doing harm to the productquality by exposing it to excessive
heat.While there are not the most optimumdifferences in temperature
and moisturecontent helping to dry the product, theeffects may be
more gentle on the productitself.
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Some drying applications may use acombination of counter-current
air flowand co-current air flow by having them intwo separate
sections of the dryer asshown in Figure 6-5.
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Another option for the direction of air flowis across the
surface of the material beingdried from one side of the dryer to
theother. This would be referred to as“cross-current air flow” and
is shown inFigure 6-6. One potential danger here ishaving material
on one side of the dryingbed becoming dryer than material on
theother side of the bed. This would besimilar to the flow of air
in a batch dryerwhere the bed of material is not moving.The air is
simply blown across the bed ofmaterial from one side of the dryer
to theother.
Special Note:
Whatever direction we have for the airflowin a dryer, having a
uniform distribution ofthe air is absolutely essential in order
toget a uniformly dried final product. We willcome back to this
topic when we discusscontinuous belt dryers.
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6.5 Types of Dryers
Now that we have had a look at how weget the heat to the product
in dryers usinghot air, we should examine several of thedifferent
types of dryers that are availableto the food processor. These
dryers maybe divided into two main groups:“traditional” dryers and
“emergingtechnology” dryers. Even though we mayonly be able to take
a quick look at just afew different types of dryers, there aremany
variations of dryer types that havebeen designed to meet various
needs infood drying. You should consult a dryingspecialist for a
dryer to meet your specificdrying requirements. We will consider
the“traditional dryers first.
6.5.1 Traditional Dryers
6.5.1.1 Continuous Through-Circulation Dryers
This type of dryer is often called by namessuch as a “continuous
belt dryer” or“conveyor belt dryer” etc. It is similar tothe dryers
shown in Figures 6-2 through 6-5. Basically, the material to be
dried isspread evenly on a wire mesh belt whichtravels through the
drying chamber. Thetime that the material spends in the dryeris
controlled by the speed of the belt. Theamount of heat delivered to
the dryer isdetermined by the temperature of theheated air, and the
volume of air blowninto the dryer in a given period of time.
Figure 6-7 shows a side view of acontinuous through-circulation
dryer withfour zones for drying the product. Zones1 and 3 have
updraft and zones 2 and 4are downdraft zones. In some belt
dryers,the final zone may be used for cooling theproduct before it
leaves the dryer as is the
case in Figure 6-7. This helps to “set up”the product. If the
product happens to bestarchy in nature, cooling it will make
thestarch become more solid rather thanbeing soft and pliable. This
can be veryimportant if the starchy product is going tobe held in a
bulk storage bin prior to beingpackaged. Hot or warm products
may“sweat” during storage and give offmoisture which collects on
surfaces of thestorage bin. Later this moisture maycause mold
growth to occur. Hot productsalso tend to be somewhat soft, which
cancause them to alter their shape or changetheir structure as they
cool.
Each zone in the continuous throughcirculation dryer can have
its air flow andair temperature controlled independentlyfrom the
other zones. Care must be takenin each drying zone to match
theapplication of heat to the drying needs ofthe product.
In zone 1, the material may be in itsconstant rate drying
period, so moisture isbeing evaporated from the surface. Thismeans
that it may be possible to usehigher air temperatures without
damagingthe product. The updraft direction of airflow prevents the
air from pushing themoist material into the mesh belt.
Havingmaterial pushed into the belt can causethe wire mesh to
become plugged and notallow air to pass through it during thedrying
process.
In zone 2, a lower temperature might beused since the material
may be in itsfalling rate drying period where moisturemust diffuse
to the surface before it canbe removed by the drying air. If high
airtemperatures were used, the materialcould heat up and become
damaged bythe heat. Even if the product was stillundergoing
constant rate drying, it may be
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considered to be a good idea to changethe direction of the
airflow so that the topof the drying bed becomes dry and thebottom
does not become overly dry.
Zone 3 is a second updraft zone. It wouldgenerally have a much
lower air flow thanzone 1 because the product is lighter thanit was
when it contained a lot of water asit did in zone 1. If high air
flows wereused, the speed of the air could besufficient to lift
pieces of material off thedrying belt and blow them around
insidethe dryer. This would create uneven
drying and could also result in productbeing blown out of the
dryer.
Continuous through-circulation dryers canhave any number of
zones. The actualnumber depends on the nature of theproduct being
dried and other suchconsiderations including air temperaturesduring
drying and the air flowrates.
These dryers are used in many dryingapplications where the
particles ofmaterial are easily handled and can bespread on a belt
for drying. They may beused for drying grain and cereal
products,animal feed, etc.
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6.5.1.2 Tunnel Dryers
Tunnel dryers are similar in many respectsto continuous
through-circulation dryers.The big difference is that the material
isnot placed on a moving conveyor belt.Instead, the material to be
dried is placedon trays or racks that are then placed oncarts which
are pulled through longtunnels where heated air is blown acrossthe
material. Figure 6-8 shows such adryer.
The carts are manually loaded andpushed into the “front end” of
the dryer.They can either be fastened to a chainthat will pull them
through the tunnel, or
the wheels of the cart may be grabbed inan assembly that will
pull them throughthe tunnel. The speed at which they arepulled
determines the time the materialspends in the dryer. Once the carts
reachthe end of the dryer, they are pushed outand unloaded. The
empty carts are thenreturned to the start of the dryer to
bereloaded and sent through the dryer witha fresh load of wet
product.
These dryers require much more labour tooperate than a
continuous belt dryer andare not as commonly used as they
oncewere.
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6.5.1.3 Cabinet Dryers
Cabinet dryers represent a basic type ofbatch style dryer. We
have already seena cabinet dryer in Figure 5-1. It isreproduced
here as Figure 6-9 for thesake of completeness.
Cabinet dryers are useful in drying smallquantities of food
material or forlaboratory-scale drying studies.
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6.5.1.4 Tray Dryers
Tray dryers are another type of batchdryer that is often used in
small tomoderate scale food drying operations. Inmany respects,
they resemble cabinetdryers.
In a tray dryer, the material to be dried(e.g., sliced fruit or
vegetables) is placedon large trays, generally made of metal,
orwire mesh. The tray itself can be a solidsheet of metal, or it
may have slots orholes in it to allow drying air to passthrough the
material being dried.
Once they are loaded, the trays areplaced on supports inside a
large dryingcabinet or compartment. The trays looklike shelves
inside a large box that isactually the dryer. After all the trays
have
been loaded, the drying chamber is closedand air is blown
through the dryingcompartment.
By monitoring the humidity of the airleaving the tray dryer, the
progress of thedrying process can be followed. Once thedrying is
completed, the air flow is stoped;the dryer is opened; and the
trays areremoved. The dried contents of the traysare then dumped
and the trays arereloaded for the next load of product thatis to go
into the dryer.
Figure 6-10 shows a diagram of a traydryer.
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6.5.1.5 Fluidized Bed Dryers
Fluidized bed dryers recognize the factthat drying can be done
much moreefficiently if all surfaces of the productbeing dried are
in contact with the dryingmedium, which is usually heated air.Such
dryers can be either batch orcontinuous in their design and
operation.For our example purposes, we willexamine a batch
fluidized bed dryer.
Consider a chamber with small openingsin its bottom and top. The
openings arelarge enough to permit air to pass throughthem but do
not allow particles of materialbeing dried to escape.
Figure 6-11 is a schematic representationof such a dryer.
Heated air is blown into the dryingchamber through the openings
in thebottom. By using a sufficient volumetricflowrate of air, a
velocity can be achievedthat is sufficient to lift the wet
productpieces and keep them suspended in theair that is drying
them. While in theirfluidized state, it appears as if theparticles
are “dancing” in the air that isdrying them. As the process
continues,the product particles lose moisture andbecome less dense.
This means that theair flowrate must be reduced so as not tolift
the particles too much and pack themagainst the openings in the top
of thedrying chamber. When drying iscompleted, the batch of product
can beremoved from the drying chamber and afresh batch of wet
product can be insertedfor drying.
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6.5.1.6 Vibrating Bed Dryers
Vibrating bed dryers also recognize that itis important to
expose as much of thesurface of a product to the drying medium.When
dealing with starchy products thatcan have a sticky surface, these
dryersmay be used in series with another dryer,such as a conveyor
belt dryer. In theconveyor belt dryer, the surface of theproduct is
dried to the point where theproduct is no longer sticky.
Essentially,this will be at the end of the constantdrying rate
period. If the material is left onthe conveyor dryer, the points
where eachparticle touches another particle willexperience slower
drying than fullyexposed surface areas.
Figure 6-12 is a schematic representationof a vibrating bed
dryer.
The bed of partially dried material leavingthe conveyor belt
dryer can be broken upand fed into a second dryer which has
avibrating surface onto which the productparticles are spread. Air
is introducedthrough small openings in the vibratingbed, or is
blown into the dryer from aboveor from the sides. As the particles
arethrown a short distance upwards, they arein full contact with
the heated air. As soonas they come back down onto the dryerbed or
“deck”, they are once again thrownupwards into the drying air.
Thisprocedure is repeated many times as theparticles travel from
the feed end of thedryer to the discharge end where they goon to
further processing, storage, orpackaging, etc.
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6.5.1.7 Spray Dryers
Spray dryers are typically used in caseswhere solids are
required to be recoveredby drying liquid streams. An example ofthis
would be in the recovery of wheysolids from liquid whey in a
cheese-making operation. See Figure 6-13 for adiagram of a spray
dryer. There are manydifferent designs and configurations.However,
their basic operation is quitesimilar.
In spray drying, the liquid is pumpedthrough an atomizing nozzle
that createssmall droplets which are then distributeduniformly into
a large drying chamber, ortower, where they are allowed to fall
through heated air that is circulating in anupwards direction.
As the droplets fallthrough the heated air, they lose moisture.By
adjusting conditions such as dropletsize, air temperature, and air
velocity, etc.,the desired degree of drying can beachieved so that
by the time they reachthe bottom of the dryer, the droplets
havebecome small particles of powder. Thispowder can be collected
at the bottom ofthe spray drying tower. Any powder beingcarried out
of the spray dryer in theexhaust air can be recovered through
theuse of one or several cyclone separators.
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6.5.1.8 Drum Dryers
Drum dryers use heat to warm a set ofmetal drums to the desired
temperature.The drums rotate in opposite directions(as shown in
Figure 6-14). The materialto be dried is introduced into the
narrowgap, or “nip” between the drums. Thismaterial can be a
viscous liquid or a“mushy” solid. An example of a solidbeing dried
on a drum dryer would be theconversion of mashed potatoes into
driedflakes for use as instant mashed potatoes.After it goes
through the “nip”, the materialsticks to the surface of the
rotating heateddrum and moisture evaporates from it.
Just before it travels back to the top ofthe dryer, the dried
material is removedfrom the drum using a “doctor blade” that
continuously scrapes the drum surface.As it falls, the dried
product is caught in ahopper and is removed for
furtherprocessing.
There are numerous variations in theoperation of drum dryers.
Each method isdesigned to suit a particular drying needor product
characteristic. Some dryershave only one of the drums heatedinstead
of both of them, and others havedifferent ways of getting the wet
materialonto the drum for drying. Figures 6-15and 6-16 show two
additional drum dryerconfigurations. Such variationsdemonstrate how
adaptable certain dryersare, and how creative dryer
manufacturerscan be at meeting specific drying needs.
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6.5.1.9 Other Types of TraditionalDryers
It is basically impossible to discuss in anygreat detail the
full range of dryers thatare available to food processors today.Not
only is it beyond the scope of a worksuch as this, but there are
far too manyspecific applications of dryers that wouldhave to be
considered to do justice to thetreatment of each dryer type.
Completetextbooks have been written on thesubject of dryers. New
styles of dyers arebeing developed continuously to meetnew drying
demands.
In addition to those dryers describedabove, the following types
of dryers arealso available for consideration in foodprocessing
applications.
C Freeze dryersC Flash dryersC Plate dryersC Rotary dryersC
Vacuum dryersC Solar dryersC Roto-louvre dryersC etc.
Should you wish to study any of theseadditional types of dryers,
you may findthe Internet to be of particular assistance.
Before deciding upon a specific dryer fora processing task, care
should be taken toinvestigate all suitable types of dryers andto
pick the one most appropriate fro theproduct. You should work with
a dryerspecialist or drying company andrecognize the needs to match
the dryer tothe product being dried. The purchase ofa dryer is
often a major capital expense.Mistakes made in the selection of a
dryercannot be easily corrected in most cases.
6.5.2 Emerging Technologies
In a chapter of the book “Food Scienceand Food Biotechnology”
(edited by G.F.Gutierrez-Lopez and G.V. Barbosa-Canovas; Food
Preservation TechnologySeries, by CRC Press, 2003), Dr.
ArunMujumdar, discusses a number of newdevelopments in the field of
dryingtechnology.
He lists the following new dryer designs:
C Heat pump dryersC Intermittent batch dryersC Vacuum fluid-bed
dryersC Sorption dryersC Pulse combustion dryersC Cyclic pressure /
vacuum dryersC High electric field dryersC Superheated steam at low
pressure dryers
Each one of these dryers addresses aspecial concern in the
drying of aparticular product.
While it is not possible to examine eachone of these dryers
here, it is important tomention them in order to show how newdryers
are being developed to meet theneeds of various processors.
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INTERMEDIATE COURSE IN FOOD DEHYDRATION AND DRYING
CHAPTER 7: DRYER OPERATION - CASE STUDY
7.1 Introduction
Setting up a food drying process is not amatter of simply
running a few small-scaletests and having a supplier design a
dryerthat meets your requirements. Once thedryer is installed and
operating, conditionsmust be maintained to keep the
dryerfunctioning properly.
In order to illustrate some of the situationsthat can arise
during a food dryingprocess, a case study example involvinga
continuous through-circulation dryer willbe used.
7.2 Case Study:Continuous Through-circulationDryers
7.2.1 Mode of Operation
The continuous through-circulation dryeris used in this chapter
as a means ofintroducing drying or dehydration on anindustrial
scale and to provide material fora “Case Study”. As previously
stated,there are many types of dryers availableand each application
must be assessedon an individual basis to optimize thedrying
process.
7.2.2 Design Features
To ensure the best possible performancefor any type of dryer, it
is essential to havea uniform bed of material on the dryerbelt. The
material being dried must then
be exposed to a uniform, controlled dryingenvironment.
7.2.2.1 Creating a Uniform Product Bed
Methods of establishing a uniform productbed are varied and
often imaginative.They are dependent upon the propertiesof the
material being dried, and on thenature of the discharge stream from
theprevious unit operation in the processsequence. Some materials
may beconveyed in a water slurry and spread onthe dryer belt by
dams or weirs, anddrained prior to entering the dryer itself.Other
materials may be “airveyed” (i.e.,blown in a stream of
high-velocity air) andblown onto the belt through tubes thatsweep
from side-to-side across the dryerbelt.
7.2.2.2 Creating Uniform Air Flow
Delivering the drying medium (usuallyheated air) to the product
is a majorchallenge. “Air distribution plates” are themost commonly
used method incontinuous through-circulation dryers.These plates
are simply large sheet-metalpanels with small holes (typically 1 to
2cm) spaced at regular intervals to give anappropriate open area
(perhaps 25% to50%). A schematic diagram of an airdistribution
plate appears as Figure 7-1.Sufficient back pressure must be
createdby the air distribution plates to establish auniform flow of
air through the holes in
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the plate. The flow must be uniform bothacross the product bed,
and along thetotal length of the dryer zone. Non-uniform air
patterns will result in non-uniform drying.
Figures 7-2a and 7-2b show thepositioning of air distribution
plates inupdraft and downdraft zones of a dryer,respectively.
In the case of airflow in an updraft zone(Figure 7-2a), air
enters the dryingchamber below the product bed. Due tothe large
open volume and the highvelocity of the air, the air flow patterns
arevery chaotic. Air from the fan may beblown across to the far
side of the dryerand be deflected in a random mannerfrom the wall
of the dryer. If nothing waspresent to even out the air flow,
therewould quite probably be a highly non-
uniform distribution of air going upwardsthrough the product
bed. However, withthe air distribution plates in place, thechaotic
flow of air is essentially trappedbelow the distribution plates and
cannotreach the product bed until the air flowpattern is made more
uniform. The airdistribution plates create a back-pressureby
allowing only a portion of the air topass through the small holes.
Thiscreates a uniform flow at all locationsbeneath the product bed
so that when theair does travel upwards, all product spreadon the
dryer belt receives the samedegree of exposure to the drying
air.
In the downdraft zones of the dryer(Figure 7-2b), the same
arrangement isused for the air distribution plates. Theplates are
placed between the source ofthe air (i.e., the fans) and the
product bed
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to prevent chaotic air flow from reachingthe product bed. As can
be seen, the airdistribution plates are located above theproduct
bed in this case. The air travelsdown through the holes in the
platesbefore it strikes the product bed. In somecases a second set
of distribution platesor air deflectors could be used to
furtherensure the uniform distribution of air in thedowndraft
zones. This might benecessary due to the large volume ofspace above
the product bed in mostdryers of this type.
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7.2.2.3 Volume of Air to the Dryer
The volume of air being sent to the dryermay be controlled in
several differentways. If the dryer is so equipped,variable-speed
drives can be used toadjust the speed of the fans. The fasterthe
fan is spinning, the more air that willbe delivered to the dryer.
Thespecifications of the manufacturer and theappropriate fan curves
(i.e., curvesdefining the air delivery of a fan under aset of given
conditions such as speed,temperature of the air,
back-pressure,etc.) must be used to determine the actualdelivery
rates. In addition, tests todetermine air velocities should
beconducted to verify results. This is a topicbest left for other
courses, or hands-ontraining.
A second way to control the amount of airdelivered by a fan is
through the use oflouvres or dampers to control the
opencross-sectional area of the plenumthrough which the air is
flowing.
A third way to adjust the air delivered by afan is by using
volume control disksmounted on the central shaft of the fan.These
disks can be moved along the shaftto control the percentage of the
blades ofthe fan available to blow air into the dryer.This concept
is somewhat more complexthan the other two and is also
moredifficult to use since the fans must be shutoff and the dryer
cooled down to enablecrews to go in and physically adjust thevolume
control disk positions.
7.2.3 Assessing Dryer Performance
7.2.3.1 Drying Uniformity
How well a dryer does its job isdetermined by a wide variety of
factors. Ingeneral, however, the success of drying aproduct comes
down to how well you as aprocessor understand the behaviour ofyour
product while it is being dried, andhow well you match the
operation of thedryer to your product’s drying needs.
The first thing that you must realize is thatthere is a limit to
how much water canbe removed from a particular type ofmaterial by a
given dryer in a specifiedperiod of time.
Suppose you buy a dryer that is designedto dry grain and remove
a certain amountof water from it on an hourly basis. Let’sassume
the dryer can remove 1,500 kg ofwater per hour from a specified
input rate.You should not expect to dry more grainwhich requires
that you remove 2,000 kgof water per hour. Some operators try todo
this by turning the temperaturecontrollers up to their maximum
settings toget the air as hot as possible. They alsoturn the fans
up to their maximum settingsto deliver as much air as possible.
Inspite of these measures, they still fail toget an acceptable
product, since theyhave not taken into account the time ittakes for
the moisture inside the kernels ofgrain to diffuse out to the
surface and beremoved. Even if the conveyor belt isslowed down to
allow the grain to spendmore time in the dryer, the results
areusually not encouraging because thethickness of the drying bed
increases andthe air cannot penetrate through it andremove the
desired moisture.
Consider the following factors:
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C Time: Kinetic factors (such as diffusion)control the removal
of moisture. Itis not simply matter of blasting thematerial with
abundant amounts ofhot air.
C Nature of the Product:This is critical. Not all products dryin
the same way. You cannotexpect to have grain dry in thesame manner
as flakes of parsleyor other leafy plant material.
C Temperature:Excessive temperatures candamage your product. You
cannotkeep increasing the heat to driveoff moisture without
scorching orburning your product or withoutdecreasing its
nutritional orfunctional properties.
C Air Flow:The air entering a dryer must bedistributed uniformly
to all productin a particular drying zone. Its rateof delivery
(linear and volumetric)must be such that it does notdisrupt the
product bed. The airmust also have a relatively lowmoisture content
to maximize itsability to remove moisture from theproduct in the
dryer.
C Material Bed Characteristics:The product must be
distributedevenly from side-to-side and alongthe dryer belt. Its
thickness mustbe sufficient to ensure that it is notdisrupted by
the air passingthrough it and it cannot be so thickas to be
impenetrable to the air.
C Other Factors:Other factors exist that are specific
to each dryer that impact itsoperation. The operator of thedryer
must identify and understandhow these factors relate to thedrying
of his or her specificproduct.
All of these factors are usually taken intoaccount by the dryer
manufacturer. It israther amazing that dryer manufacturersare often
blamed for problems when thedryer is not used in the manner in
which itwas designed to be run. Processors maybe running a product
that the dryer wasnot designed to dry; and they may beusing
improper drying conditions.
No dryer can be expected to operateproperly if it is not run
under itsappropriate design conditions.
7.2.3.2 Aspects to Consider
In operating a commercial-scale dryersuch as a continuous
through-circulationdryer, keep in mind the following points:
C No matter what you do to try toduplicate drying on a small
scale,nothing can be done to reproduceac tua l cond i t i ons du r
i ngcommercial production.
C Small scale t r ia ls in lab unitsimpose wall effects and fail
toduplicate air flow patterns (Note:some dryer types are
scaleable).
C Lab tests can give very goodinformation about the
dryingproperties of the material itself onan individual "chunk" or
particlebasis.
e.g.: TGA - thermal gravimetric
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analysis can detail howmoisture loss proceedswith temperature
andtime
DSC - differential scanningcalorimetry can show howproperties of
a materialchange over time as heat isapplied
C To assess the true operat ingcapacity of a dryer, you need
tohave a test sequence that includesoverall water removal and
waterremoval uniformity across andalong the dryer bed.
C Single or even mult ip le grabsamples of product cannot
providesufficient data as to a dryer'soverall operation. We will
discussthis more in the “Advanced Coursein Food Dehydration and
Drying”.
We will look at a relatively simple methodof determining the
uniformity of dryingtogether with the water removal capacityof a
dryer later in this case study.
7.2.4 An Approach to Water RemovalCapacity Determination
7.2.4.1 Definition
We can define “water removal capacity”as:
The amount of water a dryer is capableof removing from a given
product in agiven period of time (usually per hour).
It is highly dependent on a number offactors which are listed
below.
7.2.4.2 Factors Influencing Water Removal
Factors influencing the water removalcapacity of a dryer
include:
C characteristics of the product to bedried
C characteristics of the product bed(on the dryer belt)
C condition of the dryer C age of the dryer C characteristics of
the drying air C etc.
You cannot always rely on themanufacturer’s rated capacity of
thedryer.
Manufacturers of dryers build theirequipment to deliver a
certain level ofperformance that can be demonstratedwhen the dryer
is newly installed. Withage and other factors, the performance
ofthe dryer can change. Insulation in thedryer can deteriorate and
more heat canbe lost when the dryer is old than when itwas new.
Burner performance candeteriorate over time and processors caneven
change things on their own without
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the manufacturer’s knowledge.
7.2.4.3 Water Removal Capacity Testing
One method of determining how muchwater the dryer is actually
capable ofremoving and to determine how uniformthe drying is, is to
do a series of simpletests.
Imagine yourself standing at the dischargeend of a conveyor belt
dryer with the driedproduct coming towards you. The bed ofdried
product may be two or three metreswide and will fall off the belt
into acollection hopper of some description justin front of you.
What you want to do isdetermine the uniformity of moistureacross
the dryer bed. If you do this at aseries of time intervals, you
canalso determine the uniformity in moistureover time.
A procedure that I have used is to take aset of six samples
across the end of thedryer at a particular time. Two otherhelpers
are needed to get the samples atthe same time. Each sample is
placedinto a labelled plastic bag and tied forfuture testing. The
time of these sampleswill be “Time t = 0". Five minutes later,
asecond set of samples is taken at thesame six locations and are
labelled “Timet = 5 minutes”. Five minutes later, a thirdset of
samples labelled “Time t = 10minutes” is taken; and five minutes
afterthat, a fourth set of samples labelled“Time t = 15 minutes” is
taken. Eachsample is then tested for moisture using arecognized
moisture determinationmethod. Figure 7-3 shows how thesampling
pattern would look.
The results of the 24 moisture tests, inwhat is referred to as
“Scenario 1", arethen arranged in a table format such asthat shown
in Table 7-1. The first set ofsamples is in the bottom row to
duplicatethe view of the dryer bed as if seen fromabove. Averages
of moistures arecalculated across each row of six samplesand along
each group of four samplestaken at each sample site. The
overallaverage for the 24 samples is alsocalculated.
In order to get some idea of the variationor spread of the
moisture results, standarddeviations are also taken for each row
ofsix samples, each set of four moisturesfrom each sample location
and for theentire 24 samples. While it is recognizedthat a standard
deviation based on foursamples may not be statistically valid,
theobjective is to have the standard deviationas small as possible
to indicate a lowdegree of variability among any set ofsample
results. The standard deviation isessentially used for qualitative
purposes.
In Table 7-1, we can see the individualmoisture values and the
calculatedaverages and standard deviations. Itappears as if the
product along the leftside of the dryer (average moisture =7.2%) is
much dryer than that along theright hand side of the dryer
(averagemoisture in position 5 = 21.6% and inposition 6, average
moisture = 21.3%).The degree of variation in the moistures isalso
lower on the left hand side of thedryer than it is on the right
hand side.
The average moisture across the dryer isreasonably uniform over
time. It rangesfrom an average of 14.2% to 16.3%. Theoverall
average moisture for the 24samples was found to be 14.97%, or
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TABLE 7-1: Scenario #1: Initial Moisture Profile Along and
Across the Dryer
Time Position1
Position2
Position3
Position4
Position5
Position6
Averageand
Std. Dev.
15 min.7.6 11.4 14.3 16.7 18.3 21.3 14.9
4.93
10 min. 5.9 10.9 13.7 16.1 21.2 17.4 14.25.35
5 min. 7.1 6.3 13.9 14.1 27.4 29.2 16.39.85
0 min. 8.0 12.2 14.0 15.6 19.5 17.2 14.44.03
AverageStd. Dev.
7.20.91
10.22.65
14.00.25
15.61.11
21.64.05
21.35.61
14.976.07
Confidence Limits Standard Deviations Average Moisture Range
68.3% ± 1.0 14.97% 8.90% to 21.04%
95.5% ± 2.0 14.97% 2.83% to 27.11%
99.7% ± 3.0 14.97% -3.24% to 33.18%
Note: Target moisture = 14.0% ± 1.5%
The use of standard deviations with only four data points may be
considered to have somestatistical deficiencies. They are used here
to show directional or qualitative trends and aidin the assessment
of overall dryer performance.
Large standard deviation values indicate a wide scatter of the
data. Low standard deviationsshow a small degree of scatter in the
data.
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15.0% when rounded off to one decimalplace. If our target
moisture is 14% andwe have an acceptable range of ±1.5%, amoisture
value between 12.5% and 15.5%would be considered acceptable. On
thebasis of these results, the averagemoisture of 15.0% would be
acceptable,but this does not tell the whole story.There are
individual moistures as low as5.9% and as high as 29.2%. These
arecertainly outside the range ofacceptability. From experience, we
mayknow that mold can grow on our product ifmoisture levels rise
beyond a certainthreshold level. For this example, let’sconsider
that value to be 20% moisture.
A chart of confidence limits has also beenincluded as part of
Table 7-1. With anoverall standard deviation of 6.07% and99.7%
confidence limits, we know that themoisture will lie in a range of
threestandard deviations below the mean tothree standard deviations
above themean. This tells us that moisture levelswill lie between
-3.24% (this is impossible,so we’d call this 0%) and 33.18%.
Basically, Table 7-1 is telling us that wehave a widely
fluctuating moisture contentin our product and our dryer is
notfunctioning very uniformly at all.
Now let us suppose that we do majormodifications to our dryer.
Perhaps wefind that there are no air distribution platesin it; so
we install some, etc.
We now repeat our set of tests undersimilar operating conditions
and tabulatethe results. Table 7-2 shows the datafrom the tests
done in Scenario #2 afterthe dryer modifications were made.
We can see that average moistures forthe six positions across
the product bedrange from 13.4% to 14.7%, with very lowstandard
deviations (0.13% to 0.28%).Average moistures over time range
from13.9% to 14.1% and the standarddeviations range from 0.47% to
0.56%.This shows a great improvement over thefirst test
results.
Now, if we take the mean of 13.98% plusor minus three standard
deviations, wecan say with 99.7% confidence that themoisture of any
sample taken from thedryer will lie between 12.54% moistureand
15.43% moisture. Since this moisturerange is within our allowable
moisturerange of 12.5% to 15.5% moisture and ouraverage moisture
value (i.e., 13.98%) isbasically right on the target value of
14%;we can say that the dryer is operating welland moisture
fluctuations are acceptable.We may still want to work on the dryer
toimprove its operation, but we are certainlyin much better shape
now than we wereoriginally.
In order to determine the water removalcapacity of the dryer, we
should do aseries of similar tests using different waterloadings
for the dryer. We could increasethe moisture content of the wet
productentering the dryer and determine themoisture of the product
leaving the dryer.Once the dryer is no longer able toremove the
necessary amount of water togive us an average moisture that is
withinour specifications, we can say that thedryer has exceeded its
water removalcapacity.
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TABLE 7-2: Scenario #2: Second Moisture Profile Along and Across
the Dryer
Time Position1
Position2
Position3
Position4
Position5
Position6
Averageand
Std. Dev.
15 min. 13.2 13.8 14.3 14.2 14.6 14.2 14.10.49
10 min. 13.4 13.6 13.9 14.1 15.0 13.9 14.00.56
5 min. 13.3 13.4 14.1 14.1 14.7 13.7 13.90.52
0 min. 13.5 13.4 14.0 14.4 14.5 14.3 14.00.47
AverageStd. Dev.
13.40.13
13.60.19
14.10.17
14.20.14
14.70.22
14.00.28
13.980.48
Confidence Limits Standard Deviations Average Moisture Range
68.3% ± 1.0 13.98% 13.50% to 14.46%
95.5% ± 2.0 13.98% 13.02% to 14.94%
99.7% ± 3.0 13.98% 12.54% to 15.43%
Note: Target moisture = 14.0% ± 1.5%
The use of standard deviations with only four data points may be
considered to havesome statistical deficiencies. They are used here
to show directional or qualitativetrends and aid in the assessment
of overall dryer performance.
Large standard deviation values indicate a wide scatter of the
data. Low standarddeviations show a small degree of scatter in the
data.
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7.2.4.4 Summary of Points for Water Removal Capacity
Determination
You should begin by conducting a seriesof tests similar to those
outlined in Table7-1 to determine a bench-mark for howthe dryer is
operating under currentconditions. You can run this procedureunder
increasing moisture loads anddetermine at what point the dryer
fails tomeet the desired target moisture anduniformity.
The maximum dryer capacity is thepoint at which the dryer last
meets theperformance criteria.
To establish an operating strategy for adryer, you may want to
operate at about90% of the maximum water removal whichyou
determined. This will allow someextra capacity in the event of
anemergency and does give an added rangeof control.
In actual fact, many processors seem tooperate a dryer at 110%
(or more) of itsrated or designed maximum waterremoval capacity. In
spite of demandingthat the dryer perform above its designcapacity,
the operators still want the dryerto function perfectly and to
still be capableof handling any “spikes” of high moisturein the
incoming product.
As stated previously, the typical approachis to turn up the
burners and maximize thevolumetric air flowrate. The dryer belt
canthen be sped up to make a thinner bed ofmaterial or slowed down
to make a thickerbed but give the material more exposuretime.
Regardless of the approach, theresults are always the same
-catastrophic. To make matters worse, thedryer manufacturer usually
bears themajor portion of the unjustified blame for
this.
7.2.5 Typical Problems
If a dryer is not operated properly, thefollowing problems can
be expected:
C wet pockets of material (moldgrowth may occur in the
productlater).
C toasting / browning of product.
C case hardening + wet centres ofparticles. This means that
theoutside of the product is dried toform a hard crusty shell
around amoist wet centre of the material.Water cannot readily
escape and iscaught inside the case-hardenedparticle.
C stress cracking due to unevenmoisture or temperature profiles
ina product particle.
C holes in the dryer bed due to airlifting the product.
C dry top and bottom surfaces of thematerial bed but wet centre
layer.
C non-uniformity of drying betweenproduct on one side of the
dryerand the other.
C poor final product performance dueto changes in product
propertiesand functionality.
C economic losses (fuel and productwaste).
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7.2.6 Changes in Moisture After Drying:
If poorly dried product is packaged, thefollowing problems may
arise. The actualpackaging material will certainly have animpact on
how the product is affectedduring storage.
C Establishment of equilibrium withambient atmosphere. You
wantthe product to be dried to a level ofmoisture that is as close
aspossible to the moisture at which itwill be stored. In this way,
theproduct will not experienceexcessive moisture losses or
gainsthat can alter its properties andperformance.
C Moisture equilibrium of stored orpackaged product. You do
notwant to have excessively wet anddry product in the same
package.Moisture changes during storagewill affect the product in a
negativemanner.
C Structural collapse (if improperlyprocessed). Moist product
may besoft and collapse in on itself as itdries in the package.
C Spoilage. Mold growth can result ifmoisture levels are
excessive.
C Product shrinkage. Some productsactually begin to “shrivel up”
if theydry slowly under uncontrolledconditions.
C Nutritional degradation. Nutrientscan be lost during storage
due toexcessive moisture levels.
7.2.7 Factors to Remember AboutProduct Drying:
Once you have established a dryer's waterremoval capacity and
have optimized itsperformance, there are still some things tokeep
in mind:
C Every product has its own dryingcharacteristics.
C In the case of agricultural crops,there will be crop to crop
variationand seasonal changes based onfresh versus stored
material:
There may be years when kernelsof grain are quite large and
plump.In other years the kernels may notbe as plump. This
difference indiameter can affect how fast thegrains take up water
in a hydrationprocess and how fast they losewater in a drying
process.
You must always be aware of thecharacteristics in your product
tobe successful in any dryingoperation.
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INTERMEDIATE COURSE IN FOOD DEHYDRATION AND DRYING
CHAPTER 8: SOURCES OF INFORMATION
8.1 Introduction
The Internet / World-Wide-Web is avaluable source of information
and shoulddefinitely be considered as a primaryresource.
When looking for detailed information ondrying a particular
product, dryermanufacturers and equipment suppliersare a valuable
source of information.They often have useful websites withdetails
on how to contact them for furtherinformation. Since website
addresses areconstantly changing while new onesappear and others
may disappear, nospecific website addresses are given here.
Scientific journals also offer in-depthstudies of food drying.
These papers areoften highly specific and complex in
theirmathematical treatment of a particulardrying phenomenon. For
these reasons,scientific journal articles are not listedhere.
The following is a list of general referencebooks relating to
food drying and foodprocessing in general. It is not intended tobe
an exhaustive listing of availabletextbooks, etc. Once again, the
Internetcan provide up-to-date information on newpublications and
journal articles aboutdrying.
8.2 General References:
“Dehydration of Foods”; Gustavo V.Barbosa-Canovas and Humberto
Vega-Mercado; Chapman and Hall; New York,1996. (ISBN
0-412-06421-9)
“Food Preservation and Safety -Principles and Practice”; Shirley
J.VanGarde and Margy Woodburn; IowaState University Press, Ames,
Iowa, 1994. (ISBN 0-8138-2133-9)
“Food Science - Fifth Edition”; NormanN. Potter and Joseph H.
Hotchkiss;Chapman and Hall; New York, 1995. (ISBN
0-412-06451-0)
“ F o o d S c i e n c e a n d F o o dBiotechnology”; Gustavo F.
Gutierrez-Lopez and Gustavo V. Barbosa-Canovas,editors; CRC Press;
New York, 2003. (ISBN 1-56676-892-6)
“Food Process Engineering - Theoryand Laboratory Experiments”;
Shri K.Sharma, Steven J. Mulvaney, and Syed S.H. Rizvi;
Wiley-Interscience; New York,2000. (ISBN 0-471-32241-5)
“Food Processing Technology:Principles and Practice”; P.J.
Fellows;Taylor and Francis, New York, 2000. (ISBN
0-8493-0887-9)
“Fruit and Vegetable Processing”; WJongen; Taylor and Francis;
New York,2002. (ISBN 0-8493-1541-7)
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”How to Dry Foods”; Deanna DeLong;HP Books, Division of Berkley
PublishingGroup; New York, 1992. (ISBN 1-55788-050-6)
“Introduction to Food Engineering -Second Edition”; R. Paul
Singh andDennis R. Heldman; Academic Press;New York, 1993. (ISBN
0-12-646381-6)
“Perry’s Chemical Engineers’Handbook - Seventh Edition”;
RobertH. Perry and Don W. Green editors;McGraw-Hill, New York,
1997.(ISBN 0-07-049841-5, InternationalEdition ISBN
0-07-115448-5)
“Principles of Food Processing”;Dennis R. Heldman and Richard
W.Hartel; Chapman and Hall, New York,1997. (ISBN 0-412-99451-8)