Fundementals of Drying
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Transcript of Fundementals of Drying
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Fundamentals of Drying
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What effect does the design have onthe three critical factors?
1.Uniform Process Time2.Uniform Process Airflow3.Uniform Process Temperature
Non-uniformity in any, or all of these keyvariables will result in non-uniform drying.
(the Software)
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Dryer Summary CostsAquatic Food System (12 mt / hr)
Depreciation
12.1%
Interest5.9%
Dryer Maintenance5.2%
Electricity5.9%
Gas62.9%
Labor6.9%
Example: 12 Mt/h Dog Food
Gas Costs determined assuming an
efficiency of 1420 btu / lb (856 cal / g)
of water removed
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Drying / Cooling
System Configuration
Various components of dryersDrying Theory
How does a dryer work?Processing Conditions
What parameters effect drying?
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Conveyor Dryers / Coolers
Gentle handling ofproduct
Air flow Through Over On
Positive conveying Uniform drying time
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Primary Components
Spreader Conveyor Beds Fans Heat Source Makeup Air Inlets Exhaust Outlets Optional Devices
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Spreaders Purpose: Deliver product uniformly to conveyor bed
Across Conveyor Bed Along length of Conveyor Bed
Uniform spreading is essential to good product drying
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SpreadersOscillating Spout
Advantages Sanitation, versatile, cost,
length
Disadvantages Sticky / fragile products, low
bed depth / short retentiontime products, height
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Conveyor Beds
Purpose: Transport product through heated air flow Establish product retention time
Higher speed on upper bed - shallow depth Lower speed on lower bed - deeper bed
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Conveyor Design Trays
Hinged Overlapping Slotted or perforated
Galvanized or stainless steel
Screen Wire mesh Polyester Screen PEEK Screen
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Tray Openings Perforations:
9/64 (3.6 mm) - 31% net open area 3/32 (2.4 mm) - 23% net open area
Slotted: 3/32 x 1/2 (2.4 x 13 mm) - 23% net open area
Polyester: 1,000 micron - 45% net open area 300 micron - 36% net open area
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Recirculation Fans
Purpose:
Provide uniform heated airflow to the dryerAcross the bed Through the bed (up and down)Uniform airflow is essential to product quality
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Heat Source
Purpose: Produce heated process air
Uniform air temperature is essential to product quality
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Direct Heat
Advantages Energy exchange, high temperature, efficiency, economics
Disadvantages Combustion by-products, limited to clean burning fuels
required
Indirect Heat
Advantages
No combustion by-products, non-clean burning fuels Disadvantages
Temperature, energy exchange, efficiency, heat exchanger
Heat Source
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Makeup Air Inlets
Purpose: Replace spent exhaust air in dryers Replace cooler exhaust air
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Exhaust Air Outlets
Purpose:
Remove spent process air in dryer
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Optional Devices Brushes
Remove product clinging to conveyor Fines system
Improve sanitation Productbreaker / agitator
Break up product clumps / stir product Spray wands
Clean conveyor beds - extend process time Humidity control
Artificially lower the drying rate
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Product Cooling
Eliminate moisture condensation during storage 90C air - holds 454 g water 20C air - holds 8 g water
Normally cool 5 - 7C above storage temperature
High ambient temperature Mechanically cool process air
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Conveyor Dryer with Attached Cooler
Product Inlet
DryerCooler
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Counter Current Coolers
Product discharges continuously Reduced bridging
Trays push up into product Product constantly moving
No moving conveyers Air flow
Through Products
Coated Feeds
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Vertical Dryers(Wenger Cascade)
Continuous turning of theproduct
No moving conveyers No fines system needed Allows for efficient use of
floor space
Continuous batchoperation
Air flow
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Drying Theory
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"Black Box" approach to drying
PRODUCT
AIR
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Effects on Product
Moisture level decreases Product temperature
Increase Decrease Remain constant
PRODUCT
AIR
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Effects on Air
Moisture level of air increases Temperature of air decreases Energy content of the air may or may not
change
PRODUCT
AIR
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Aquatic Food Drying Curve
40
50
60
70
80
90
100
110
120
0 5 10 15 20 25 30
Drying Time (Minutes)
MeasuredTemperature(C)
0
5
10
15
20
25
30
Pro
ductMoistureCon
tent(MCWB)
Product Temperature
Product Moisture
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Basic Factors Controlling Drying
Temperature Airflow Time Bed Depth Spreading Product Characteristics
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Temperature
Defined by product characteristics Rule of Thumb: Lower Always Better Reality: Run to the maximum the product
will allow
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Water Holding Capacity of Air
Temperature
(Deg. C)
2530
40
50
60
Moisture in Air
(kg H2O / kg dry air)
0.0200.028
0.049
0.0870.150
Air heated to 1350C has 70X water holding capacity of 200C air
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AIRFLOW
Most Important Factor Rule of Thumb:
Last Variable to be Adjusted Reality:
Instant Experts Make Life Interesting
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Effect of Air Velocity on Drying Time
0
5
10
15
20
25
30
0.5 0.7 0.9 1.1 1.3 1.5 1.7Air Velocity (m/s)
DryingTim
e(min)
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Effect of Air Velocity on Drying Time
Low Air Velocity High Air Velocity
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Time
Defined by product characteristics Rule of Thumb: Longer Always Better Controlled by:
Conveyor Speed Bed Depth
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Bed Depth
Increasing bed depth decreases dryer size Deeper beds affect air flow Bed depth limited by
Soft or fragile products Sticky products Very wet products Product Size
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Spreading - Across the Bed
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A commercial case study was undertaken
to examine the cost benefits associatedwith decreased moisture variation.
The following slides are the results of the
study which directly compared theWenger Series III and Series VII.
The two dryers tested were in commercial production,equally sized, and running like products
at similar evaporative loads.
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Case Study of Moisture Variation inSeries III Single Plenum Dryer
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Case Study of Moisture Variation in
Airflow II Dual Plenum Dryer
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Case Study of Moisture VariationOverlaying Series III and Airflow II
Data
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A lot of data, so what does it mean?
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What is the 3.8% Moisture LossCosting the Operation?
This cost is in two forms:
1. The loss in profit potential from nothaving the ability to market moisture
at Finished Product Value.
2. The additional energy cost neededto remove the excess moisture.
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What is the 3.8% Moisture LossCosting the Operation?
1. Lost Profit from Excessive Drying
Interactive
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What is the 3.8% Moisture Loss
Costing the Operation?
2. The additional energy cost needed to
remove the excess 668 lb of moisture.
668 lb / hr X 1450 BTU / lb evaporation = 968,000 BTU / hr
968,000 BTU / hr X 5.0 6 $ / BTU** = 4.84 $ / hr
** assumption of $ 5.00 / MCF natural Gas Cost
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The Silent Thiefs Annual Reward.
Is a dryer replacement justifiable?
Interactive
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Payback PeriodDefinition
The minimum period of time required for thebenefits to equal the cost of the investment.
Payback
Period
=Depreciable Fixed Capital Investment
Avg. Profit / year
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Dryer Replacement Expenses
(based on 8 ton / hr system)
* Depreciable Item
** Non-Depreciable itemsInteractive
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Actual Payback Period
Payback
(Period in
Years)
=Depreciable Fixed Capital Investment**
Avg. Profit / year
For this example Depreciable and Non-Depreciablecosts are included **
Interactive
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What is your Moisture Uniformity?
ExtremeLeft
LeftCenter
CenterRightCenter
ExtremeRight
HighReading
LowReading
Time
1 % % % % %
2 % % % % %
3 % % % % %
4 % % % % %
5 % % % % %
Date: _____________________
Product: ___________________
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Dryer Process Trouble
Shooting
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Two Basic Areas:Process Issues
Maintenance / Hardware Issues
TROUBLE SHOOTING
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Guidelines: Remember Primary Factors
Time
TemperatureAirflow
Effect of Retention Time One At A Time
TROUBLE SHOOTING
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ONE AT A TIME
Most process corrections will involvemultiple steps
Make one process change at a timeAny change made to the process will take the
retention time of the process to be noted at thedryer discharge
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PROCESS ISSUES
Consistent OccurrencesUnder-drying (Product too wet)Over-drying (Product too dry)
Random Occurrences Case Hardening
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CONSISTENT UNDER-DRYING
Factors:TemperatureRetention TimeBed DepthIncoming Product MoistureAir Flow
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CONSISTENT UNDER-DRYING
Temperatures Too Low:
Verify temperatures for all zones Confirm operation of all burners Check steam pressure Plugged filters or steam coils
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CONSISTENT UNDER-DRYING
Retention Time Too Short:
Confirm speed of both conveyors Based on previous runs Based on pre-set levels Upper beds are designed to travel faster than lower beds
35 percent of total time on top pass 65 percent of total time on bottom pass
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Bed Depth Too Deep:
Conveyor speed too slow for incoming feed rate Increased feed rate into dryer Reduced product density at same production rate.
CONSISTENT UNDER-DRYING
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Bed Depth
Bed depth limited by Sticky products Very wet products Soft or fragile products Product size
Increasing bed depthdecreases dryer size
Deeper bed depthsaffect air flow
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CONSISTENT UNDER-DRYING
Feed Product Moisture Too High: Same Feed Product Outside Moisture Specification
Change in Feed Product: Product Size small pieces dryer more quickly than large
pieces due to the surface to volume ratio
Product Density increase in density results in an increase indrying time
Recipe change
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CONSISTENT UNDER-DRYING
Insufficient Exhaust Volume Excess Spent Process Air
Exhaust Fans Belts Rotation Louvers
DuctingAdditional equipment Rusted or damaged duct
Major Seasonal Change
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NEED FOR SEPARATE EXHAUST
Majority of water removed occurs infirst half of top pass - MaximumExhaust needed.
Zone 2 Exhaust handles balance
0 5 10 15 20 25 30
Drying Time
0
5
10
15
20
25
30
M
oisture
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CONSISTENT UNDER-DRYING
Insufficient Make Up Air Dryer is Starved for fresh air
Louvers Filters / steam coils plugged Recirculation Fans
Belts Rotation Product buildup on wheels
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CONSISTENT UNDER-DRYING Improper Air Flow Air short circuits the product bed
Missing or damaged air locks or air curtains Missing Panels Missing or Damaged Flashing Door seals
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CONSISTENT OVER-DRYING
Factors: Temperature Retention Time Bed Depth Incoming Product Moisture
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CONSISTENT OVER-DRYING
Temperatures Too High:
Verify Set Points for all temperature zones Recently Cleaned Conveyors Major Season Change (wet to dry)
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CONSISTENT OVER-DRYING
Retention Time Too Long:
Confirm Speed of both Conveyors Based on previous runs Based on Pre-Set levels Upper Beds are designed to travel faster than Lower Beds
35 percent of total on top pass 65 percent of total on bottom pass
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CONSISTENT OVER-DRYING
Bed Depth Too Shallow:
Retention Time Too Short Decreased Feed Rate into Dryer
Lower Temperature Decrease Fan Speed
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CONSISTENT OVER-DRYING
Incoming Product Moisture Moisture Too Low
Change in Feed Product: Product Size small pieces dryer more quickly than
large pieces due to the surface to volume ratio
Product Density increase in density results in anincrease in drying time
Recipe change
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Factors: Non-Uniform Spreading Inconsistent Incoming Product Unbalanced Airflow Blocked perforations on conveyor Product Clumping
RANDOM UNDER / OVER DRYING
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Non-Uniform Spreading: Across the Bed:
Hump pattern most common Edge Overflow second most commonAdjust Oscillation Frequency
Along the Bed: Ocean Wave, or S patternAccelerate oscillation frequency
RANDOM UNDER / OVER DRYING
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Spreading - Across the Bed
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Spreading - Along the Bed
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Inconsistent Incoming Product Density Moisture
RANDOM UNDER / OVER DRYING
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Unbalanced Airflow: Equal time exposure Top and Bottom Equal time exposure Right to Left Majority of Exhaust from Zone 1
Verify Internal Doors in place Confirm Make-up Air louver settings Confirm Exhaust Damper settings
RANDOM UNDER / OVER DRYING
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Blocked Conveyor: Restricts uniform airflow Fines and Fat will plug bed slots
May require more frequent bed cleaning
RANDOM UNDER / OVER DRYING
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Product Clumping: Product agitator
Use only if clumping is anissue
Clumps of product actthe same as largediameter pellets
RANDOM UNDER / OVER DRYING
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CASE HARDENING
Outside of product dries too rapidly whichcloses all pores in skin and blocks
moisture transfer from interior of pellet Reduce initial heat exposure If random search for airflow restrictions
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Maintenance List
1. Improper spreader adjustment2. Improper gas pressure3. Unbalanced exhaust and make up air4. Combustion blower fan wheels
clogged with dust.
5. Combustion air duct rusted out.
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Maintenance List
6. Steam coils clogged with fines.7. Missing air panels8. Side flashings not secure.9. Missing/damaged air locks / curtains.10.Internal filters clogged.
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Dual PlenumCross Air Flow
Balances airflow
throughout dryerFans isolated by zoneAirflow UP or DOWN by zone
Dryer Air Flow Design
+/- 0.5% Moisture Variation
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Wenger Airflow II Dryers What is this new dryer?
Dual Plenum Dryer That is NOT a Series VI or VII.Airflow is same as ET Air Flow II although construction is NOT.
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New Dryer Utilizes a Center-Airflow
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Center-Airflow
Design AdvantagesBalanced Airflow
Increased Process Airflow
Sub-floor between conveyors
Temperature Profiling
Process Air to Product BedFresh Make-up AirExhaust Air
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Process Airflow Control Sub-floor
M t i l f C t ti
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Materials of Construction:
Unchanged from previous models:Mild Steel (w/304 SS in high corrosion areas)Mild Steel (w/304 SS in product contact areas)Complete 304 SS
Time Temperature - Airflow
sub-floor 304 SS
H t S O ti
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Indirect steam heat coils inside heatplenum include removable / airfilters for dust removal.
Heat Source Options:
Direct fired gas note unlike
the Series VI dryers -- heat unitis mounted exterior to dryer.
Factory assembly & testing
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Factory assembly & testingdryer access / insulation
Unchanged from previous models:
Factory assembly & testing
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Factory assembly & testingdryer access / insulation
When compared to the North Carolina company Wengeroffers unparallel dryer access for sanitation / maintenance.
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Polyester Screen in the Dryer
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Leveling Auger Advantages
Bed depth variation, height, depth,great for small products
DisadvantagesCost, more mechanism
Special Inlet Spreaders
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Wenger or Extru-Tech AFII Dryers
-Guarantee +/- .5% Moisture Variance. -Watch the Water and Increase profits. -Calculations can show that this may equal
depending on capacity upwards of 250,000.00 US
each year in increased profits. 10MT/hr over 5000 hours is 50,000 tons
@.5% Moisture variance lost 250 tons of water sold @ 2.5% Moisture variance lost 1250 tons of water sold ( difference is 1000 tons)
@500 US per ton feed sold at 1000 tons is a loss of 500,000.00 US @ 250 US per ton feed raw material cost loss is 250,000.00 US
An efficient and modern dryer can increase profits on this example by 750,000.00 US per year not includingthe loss if energy over drying when drying to a variance of 2.5%
ASR System will Automatically Sense and Control the
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Exhaust Air Values
Total Gas Cost @ Highest Evaporative Load$ / hr 36.34
Total Gas Cost @ Moderate Evaporative Load$ / hr 26.21
Unnecessary Gas Cost$ / hr 10.13
Time Temperature - Airflow
$ 26.21 $ 10.13
ASR System will Automatically Sense and Control theHumidity in the Exhaust to a Pre-Set Level
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Side fines auger connecting tocross auger at Return section
Cross auger with extensionfor side auger inlet
Normal side plenum floorunder fan side of Dryer
Midpoint of Dryer length
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Floors not used, trough and augerinserted. Side of Dryer opposite fans.
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Protective grate