Fundementals of Drying

7/29/2019 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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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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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.



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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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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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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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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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Temperatures Too High:

Verify Set Points for all temperature zones Recently Cleaned Conveyors Major Season Change (wet to dry)


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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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Bed Depth Too Shallow:

Retention Time Too Short Decreased Feed Rate into Dryer

Lower Temperature Decrease Fan Speed


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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



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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



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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



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Blocked Conveyor: Restricts uniform airflow Fines and Fat will plug bed slots

May require more frequent bed cleaning



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Product Clumping: Product agitator

Use only if clumping is anissue

Clumps of product actthe same as largediameter pellets



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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

Documents

Fundementals of Drying