AIChE equipment testing procedure continuous direct heat rotary dryers a guide to performance evaluation by american institute of chemical engineers (AIChE) (z lib org)

102.2 This procedure is primarily intended for continuous direct-heat rotary dryers in which both gas and material flow are end-to-end of the dryer cylinder, and in which heat is transfe

Continuous Direct-Heat Rotary Dryers

A Guide to Performance Evaluation

Third Edition

AlChE Equipment Testing Procedure

Continuous Direct-Heat Rotary Dryers

A Guide to Performance Evaluation

Third Edition

Prepared by the

Equipment Testing Procedures Committee

AIChE"

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

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Continuous Direct-Heat Rotary Dryers

Table of Contents

100.0 PURPOSE AND SCOPE 1

102.0 Scope 2

101.0Purpose 1

200.0 DEFINITIONS AND DESCRIPTION OF TERMS 3

20 1 0 Dryer Description 3

202.0 Description of Terms 6

300.0 TEST PLANNING 9

301.0Conditions 9

302.0 Dryer Material and Heat Balances 9

303.0 Test Prepaidan 16

400.0 INSTRUMENTS AND METHODS OF MEASUREMENT 17

402.0 GasFlow 18

403.0 Material Temperature and Moisture Content 19

404.0 Material Flow and Cylinder Fillage 20

405.0 Radiation and Convection Heat Losses 22

406.0 Miscellaneous Measurements 23

40 1 0 Gas Temperature and Humidity 17

500.0 TEST PROCEDURE 25

501.0 Procedures 25

600.0 COMPUTATION OF RESULTS 26

60 1 0 Material Balances 26

602.0 Gas Flow and Heater Output 27

603.0 Heat Balance and Gas Flow 28

604.0 Cylinder Fillage and Time-of-Passage 28

605.0 Dryer Power Consumption 30

700.0 INTERPRETATION OF RESULTS 31

70 1 0 Material, Moisture and Energy Balances 31

702.0 Volumetric Heat Transfer Performance 31

800.0 APPENDIX 34

80 1 0 Nomenclature 34

802.0 Sample Problem-SI Units 35

803.0 Sample Problem-English Units 38

804.0 General References 42

List of Figures

Figure 1: Counter-Flow Rotary Dryer 3

Figure 2 Rotary Dryer with Dust Collection System 4

Figure 3: Drying Rate as a Function of Drying Time 7

Figure 4: Rotary Dryer Test Data Sheet 13 14

Figure 5: Data Measuring Points for Material and Energy Balances 26

Continuous Direct-Heat Rotary Dryers

The purpose of this procedure is to suggest a method for conducting performance tests

on continuous direct-heat rotary dryers

101.2 Reasons for conducting performance tests on commercial-size dryers may be:

To measure the performance of the dryer under typical operating conditions;

To determine optimum dryer capacity under existing operating conditions;

To study alternative operating conditions for increasing dryer capacity and

To provide a record for future troubleshooting;

To gather data for design of new dryers of different capacities, or dryers for

To study specific dryer characteristics that may affect product quality, e.g., speed,

performance;

similar products;

slope, rotation, temperature profile, active loading, and calculated factors, such as the volumetric heat transfer coefficient;

thermal sensitivity of materials;

conservation, and minimum environmental impact, and

dence time, temperature profile, etc

To determine a desirable operating range for routine control of the dryer and the

To determine the optimum operating conditions for cost-effectiveness, fuel

To study the specific drying characteristics that determine product quality, e.g resi-

101.3 Although this procedure could be used as a guide for designing tests to demonstrate dryer capacity under manufacturers’ performance guarantee conditions, it is not intended for this purpose, nor is this procedure adequate to serve as a basis for a performance guar- antee For example:

1) This procedure does not set limits or acceptable deviations between pilot plant test results or manufacturers’ predictions and commercial results

2) It does not address material handling questions, nor feed properties and uniformity other than those of feed rate and moisture content

3) It does not set standards for fabrication quality and mechanical performance

4) Moreover, for any specific product, there may be particular temperature or moisture measurements, sampling techniques, and quality requirements other than dryness which should be included in performance specifications

Continuous Direct-Heat Rotary Dryers:

A Guide to Performance Evaluation, Third Edition

by Equipment Testing Procedures Committee Copyright © 2006 American Institute of Chemical Engineers

102.0 Scope

This procedure applies to continuous direct-heat rotary dryers (See Section 202.8) in

which a wet material being dried is conveyed by slope and rotation of an essentially hori- zontal cylinder Material movement is also either slightly enhanced or impeded by a

stream of gas flowing through the cylinder, depending on the material and the flow direc- tion of the gas stream The gas stream is usually the sole external source of thermal energy for material heating and liquid vaporization, and is also the carrier gas for remov- ing evolved vapors from the cylinder Gas flow direction may be either co-current with, or counter-current to, material flow This procedure excludes situations in which fuel enters with the material and is burned, i.e., the de-oiling of metal chips, turnings, and borings It also excludes cooling operations and special situations, such as drying of sugars and other materials that may change chemical characteristics on being heated Schematically, the flows are as indicated in Figure 1

102.2 This procedure is primarily intended for continuous direct-heat rotary dryers in which both gas and material flow are end-to-end of the dryer cylinder, and in which heat

is transferred primarily by convection from hot gases to wet materials

102.3 This procedure is not intended for any form of indirect-heat rotary dryers, or other types of dryers, such as flash dryers, freeze dryers, vacuum pan dryers, paddle dryers, or high temperature calciners and kilns, where radiation is the primary heat transfer mode

Continuous Direct-Heat Rotary Dryers

201 O Dryer Description (see Figure 1)

Courtesy of Swenson Technology, Inc

Figure 1 : Counter-Flow Rotary Dryer

201.1 A continuous direct-heat rotary dryer consists of a rotating cylinder, which may be slightly inclined to the horizontal to promote or retard material flow The inside of the cylinder may be fitted with material conveying flights and lifting flights of various forms designed to lift and shower the material through the gas stream as both material and gases move through the cylinder, thus enhancing intimate gas-solids contact The ends of the rotating cylinder are joined to stationary breechings that connect to the gas supply and exit gas ducts, the material feed, and product conveyors The annular clearances between the ends of the rotating cylinder and stationary breechings are enclosed by fabric, friction,

or labyrinth rotary seals in order to minimize the effect of air leakage on the operating conditions Figure 1 is an illustration of a typical continuous direct-heat rotary dryer A

sketch of a typical dryer system is shown in Figure 2

Continuous Direct-Heat Rotary Dryers:

A Guide to Performance Evaluation, Third Edition

by Equipment Testing Procedures Committee Copyright © 2006 American Institute of Chemical Engineers

Steam

Condensate +

Figure 2: Rotary Dryer with Dust Collection System 201.2 Feed material is introduced into one end of the rotating dryer cylinder through a

feed screw or chute This screw or chute is supported in the feed breeching, and projects a

short distance into the cylinder Other feed devices include vibrating feeder and progres- sive cavity pumps At the feed point, the cylinder is fitted with a retaining dam and con- veying flights to move the wet product into the active cylinder, and to prevent back spillage into the feed breeching Material is conveyed through the cylinder as a consequence of feed material head and cylinder slope and rotation, aided or retarded by gas flow as the lifting flights repeatedly shower material through the gas stream At the other end

Continuous Direct-Heat Rotary Dryers

of the rotating cylinder, the dry material flows into the discharge-end stationary breeching containing the product conveyor, either by flowing out the end of the cylinder or through

slots around its periphery

201.3 The gas stream may be heated directly or indirectly by any convenient means before entering the dryer cylinder This gas stream provides all the thermal energy needed to:

heat the material and the moisture to be removed (moisture is usually water used in the procedure), heat and evaporate the moisture, and heat the vapors to exhaust tem- perature;

breechings, and ductwork, all of which can be insulated or not insulated;

the exit ducts, and in the downstream off-gas treatment equipment

compensate for conduction, convection, and radiation heat losses from the cylinder, leave the cylinder at a sufficiently high temperature to prevent vapor condensation in

201.4 After leaving the dryer cylinder, all gas and vapor from drying usually passes through dust recovery and exhaust gas treatment equipment before being released to the atmosphere The most common forms of separation equipment employed on continuous direct-heat rotary dryers are dry-type cyclones for the primary recovery of dust, followed

by dry fabric filters or wet scrubbers Modern regulatory requirements encompass volatile

gas components as well as particulate matter, NOx quantities, and the like, in off-gases,

and may dictate further processing and treatment

201.5 Fans are used to direct the gas stream flow through the heater, dryer cylinder, and off-gas treatment equipment The simplest fan arrangement, used on dryers with low pres- sure drop air heaters, is a single, induced-draft exhaust fan, located downstream from the exit gas treatment system so it will operate on dust free gas This one fan must have static pressure capability to accommodate the pressure drop through the entire system, including heat source, dryer, breechings, ductwork, dust removal, and other treatment equipment Alternatively, when the pressure drop through an inlet gas filter, gas heater, and gas inlet duct exceeds a certain level-125-250 Pa, 0.5-3.0 in of water, a second, forced draft fan may be installed upstream from the filter or heater to overcome the drop In this manner,

by balancing the two fans, the pressure inside the cylinder at the rotating seals can be maintained at a level close to, but slightly below, atmospheric pressure, regardless of the pressure drop through the inlet system This will ensure that no process vapors leak to the atmosphere Secondary fans are generally not used in order to conserve cost and reduce complexity of operation

202.4 Bound moisture is liquid held by a material in such a mechanism that the liquid exerts a lower than normal vapor pressure at the same temperature Liquid may be bound

by solution in cell or fiber walls, homogeneous solution throughout the material, and by chemical or physical adsorption on solid surfaces The fraction of bound moisture that can

be removed depends on the specific conditions of humidity temperature in the external surroundings, gas flow rate, and residence time in the dryer

202.5 Cupillaycflotv is flow of liquid through the interstices and over the surfaces of a solid, caused by liquid surface tension resulting from liquid-solid molecular attraction

202.6 Constant-rate period is the drying period during which the rate of liquid removal per unit of material surface, and per unit of time, is constant

202.7 Critical moisture content is the moisture content at which the constant-rate period ends and the falling-rate period begins

202.8 Direct-heat dyer is one type of drying equipment in which heat is transferred to the material being dried by direct contact with the heating medium Usually, the heating medium is a hot gas and the heat transfer mechanism is convection

202.9 By basis expresses the moisture content of a wet material as the weight of mois- ture per unit weight of dry material The advantage of using this basis is that the moisture change per unit weight of dry material is obtained simply by subtracting the moisture con- tent before and after drying

202.10 Dryer eficienv is the fraction of the supplied thermal energy used to heat the material and liquid to evaporation temperature, to vaporize the liquid, and to heat the vapor and the material to their dryer exit temperature

202.1 1 Equilibrium moisture content is the ultimate moisture content to which a given material

Continuous Direct-Heat Rotary Dryers

202.1 2 Evaporative eflcimg of the gas stream flowing through a direct-heat dryer com- pares the amount of evaporation actually obtained to the amount which would be

obtained if the gas stream were saturated adiabatically before leaving the dryer

202.1 3 Falling-rate period is a drying period during which the drying rate per unit of mate- rial surface continually decreases It can also be thought of as the condition where the dif- fusion rate of moisture within the solid particle towards the surface is less than the evapo- ration rate at the surface See Figure 3 below for a graphic representation of these drying

phases

Constant Rate Critical Moisture

Rising Rate Period

Time (hr)

Figure 3: Drying Rate as a Function of Drying Time

202.1 4 Fiber saturation point is the measure of bound moisture content of a cellular mate- rial, such as wood, at which the cell walls are completely saturated, while the intercellular spaces remain liquid free It is the equilibrium moisture content occurring when the

humidity of the surrounding atmosphere approaches saturation

202.1 5 Flash dryer is one in which the material to be dried is carried pneumatically in a hot gas stream through the body of the dryer The product is in contact with the gas stream for only a short time but this feature makes it possible to situate the dryer close to the rest of the process equipment

202.1 6 Free moisture content is the measure of liquid content that is removable at a given temperature and humidity Free moisture may include both bound and unbound moisture, and is equal to the total average moisture content of the material minus the equilibrium moisture content for the prevailing conditions of drying

202.1 7 Funicular state is the condition that occurs while drying a porous body when capil- lary action causes air to be drawn into the pores to replace evaporated moisture

202.18 Humid@ denotes the amount of condensable vapor present in a non-condensable gas, and is usually expressed as weight of condensable vapor per unit weight of dry gas

202.1 9 Indirect-heat d y r is one type of drying equipment in which heat is transferred pri- marily by conduction and radiation, and the heating medium is physically separated by a

wall from the material being dried

202.20 Internal d@on occurs in a material during drying when liquid or vapor flow appears to obey the fundamental laws of diffusion

202.21 Material retention time is a measure of the time it takes for product to flow through the dryer See Section 404.4 for the measurement procedure

202.22 Moisture content of a material is the moisture quantity per unit weight of dry or wet solid

202.23 Moisturegrudht refers to the moisture profile of a material at a given moment during a drying process The nature of the moisture gradient depends on the mechanism

of moisture flow inside the material

202.24 Pendular state is the state of liquid in a porous body when a continuous film of liq- uid no longer exists around and between discrete particles, so that flow by capillarity can- not occur This state follows the funicular state in a drying process

202.25 Percent saturation of a gas containing a condensable vapor is the ratio of the partial pressure of the condensable vapor to the vapor pressure of the pure vapor at the same temperature, expressed as a percentage For water in air, this is also called percent relative humidity

202.26 Unaccomplished moisture change refers to the ratio of free moisture present at any time to that initially present

202.27 Unbound moisture in a hygroscopic material is the moisture in excess of the equilib- rium moisture content corresponding to saturation humidity in the surrounding atmos- phere All water in a non-hygroscopic material is unbound moisture

202.28 lrolumetric heat tran& is a parameter used to assess heat transfer efficiency of the dryer See Section 702.0 for a discussion of this parameter

202.29 Wet basis expresses the moisture content of a wet material as the ratio of mois- ture to the weight of moisture and dry solids

Continuous Direct-Heat Rotary Dryers

During test planning stages, a thorough safety hazards review of the test program and pro- cedures should be completed, and all necessary steps carried out to ensure safe equipment operation, and the safety of all personnel involved, or that could potentially be exposed Special care and study must be given to tests and equipment involving flammable vapors and/or flammable or explosive dust

301.2 Environmental

The test procedure must conform to the latest requirements of all applicable environmen- tal standards, including plant, industry, local, state and federal regulations Environmental standards that apply to the equipment in normal operation should also apply during test- ing

301.3 Liability

See statement on the copyright page at the front of this book

302.0 Dryer Material and Heat Balances

302.1 The performance capability of a continuous direct-heat rotary dryer can be

demonstrated only under conditions of steady-state flow of material and gas For steady- state conditions, the feed material rate, moisture content and temperature, gas velocities, temperatures, and humidities in and out of the dryer cylinder, and product rate, moisture content, and temperature must remain essentially constant during the test period Cylinder rotation speed and cylinder slope must also remain constant during the test

302.2 During the test, gas and material temperatures, moisture contents, flow rates, total heat input to the dryer, and heat losses from the dryer cylinder and breechings must be measured It is necessary to record the quantity of dust, and its temperature and moisture content, separately from the cylinder product because the temperature of dust conveyed to the dust recovery or off-gas treatment equipment is usually the same as the cylinder

exhaust gas temperature, but may be different from the temperature of the cylinder dis- charge product There may also be a difference in material moisture contents Without an accounting of material division, a material balance on moisture content, and an accurate accounting of heat consumed as sensible heat in the material, is not possible

Continuous Direct-Heat Rotary Dryers:

A Guide to Performance Evaluation, Third Edition

by Equipment Testing Procedures Committee Copyright © 2006 American Institute of Chemical Engineers

302.3 Particle size distribution analyses should be made of the feed, product, and

entrained dust materials, so that a reasonable gas velocity, and thus heat input, can be established for the dryer test If the velocity is too high, too much product may be carried out of the dryer as dust into the dust recovery equipment This product may or may not

be fully dry, and thus may not be suitable for mixing with the main cylinder product If the gas velocity is too low, dryer performance will be negatively affected, since the feed rate will have to be adjusted to match the incoming heat Thus, knowing the size distribu- tion of the incoming feed material will enable an adjustment and prediction of the gas velocity and the amount of dust which will be blown to the collectors

302.4 In order to evaluate a dryer thoroughly, all systems, including the feed, air heater, dust collection, and instrumentation should be inspected as part of the test preparations, and verification should be made that each system is working correctly and according to its respective specification

302.5 Because most measurements made on commercial-size, continuous direct-heat rotary dryers are susceptible to human, instrument, and analytical errors, and because small but uncontrollable variations usually occur in material flow, gas flow, temperature and moisture contents during the performance test, heat and material balances should be compared to assure that results are consistent among themselves The balances to be

obtained are as follows:

302.5.1 Moisture Balance:

(feed moisture content) - (product moisture content) -

(dust moisture content) = (evaporation) [measure moisture on these materials by usual methods]

302.5.2 Solids Balance:

(dry material flow in) = (dry material flow out, i.e., the sum of dryer product

discharge + dust recovery + hang-up in the dryer, if any, +

losses from system f back-spillage) [calculate a solids material balance around dryer system after

weighing product, dust recovered, any hang-up in the dryer, and

total dry feed material put into the dryer, using measured moisture

contents Dryer hang-up can be any material stuck or wedged

somewhere in the dryer, between the lifting flights, in the spiral

flights, etc., because it is sticky, wet, or fused, and has not broken

loose to mingle with the rest of the material progressing through

the dryer Such hang-up should be physically removed and

weighed Back-spillage is feed material that spills back out over the

retaining dam of the dryer cylinder at the feed point due to over-

feeding, overloading, and sticking]

Continuous Direct-Heat Rotary Dryers

302.5.3 Dry Gas Balance:

(dry gas flow in) = (dry gas flow out)

[measure gas velocity and humidity of in/out gases]

302.5.4 Humidity Balance:

(evaporation) + (moisture from fuel combustion) =

(gas stream humidity gain)

302.5.5 Heat Balance:

(heat gained by the gas through the heater) = (heat provided by fuel

burned or from other heat source)

[use weight or volume of fuel burned and calorific value]

302.5.6 Dryer Gas Heat Balance:

(heat lost by gas through the dryer) = (material sensible heat gain) +

(vapor sensible heat gain) + (heat of evaporation) + (dryer heat losses) [measure product and dust temperatures, solids balance, calculate total evaporation, and estimate heat losses by calculating from dryer

breeching/duct surface temperatures and areas]

302.6 In order to complete these balances, all of the following data should be obtained

during the test Data units cited are SI, but any consistent system may be employed See

Figures 1 and 2

Dry feed rate pg/s]

Cylinder product rate pg/s]

Recovered dust rate pg/s]

Feed moisture content bg/kg] (dry basis)

Product moisture content pg/kg] (dry basis)

Dust moisture content Fg/kg] (dry basis)

Feed temperature p]

Product temperature [K]

Recovered dust temperature [K]

Inlet gas temperature (ambient) [K]

Inlet gas humidity bg/kg]

Heated gas temperature [K]

Fuel consumption bg/s]

Fuel heating value pJ/kg]

Fuel carbon/hydrogen content pg/kg]

Alternative steam or electricity used Fg/s] or PWI

Alternative steam latent heat value bJ/kg]

Cylinder, breechings, ducts, and cyclone surface temperatures [K]

Cylinder exit gas temperature [K]

Cyclone exit gas temperature [K]

Inlet gas flow [m 3 /s]

Exhaust fan temperature

Exhaust fan humidi? Fg/kg]

Exhaust fan flow [m /s]

Exhaust fan speed [s-l]

Exhaust fan static pressure [pa]

Exhaust fan power consumed FWJ

Supply fan temperature E]

Supply fan humidity [k /kg]

Supply fan speed [s-l]

Supply fan static pressure pa]

Supply fan power consumed FWI

Cylinder rotational speed [s-l]

Cylinder slope in material direction [+ m/m]

Cylinder drive power used FWl

Feed breeching static pressure [pa]

Feed breeching leakage rate [m3/s]

Product breeching static pressure [pa]

Product breeching gas leakage rate [m3/s]

Material retention time [s]

Cylinder material fillage g]

Product bulk density [kg/m3]

Cyclone pressure drop pa]

Entrained dust bulk density Fg/m3]

Supply fan volume [m 3" /s]

Feed bulk density Fg/m P ]

Figure 4 shows a sample test data sheet

Continuous Direct-Heat Rotary Dryers

I

vered Dust Rate

Moisture, Dry Basis

nlet Gas Volume

ient Air Temperature

haust Fan Temperature

haust Fan Humidity

aust Fan Volume

aust Fan Static Pressure

aust Fan Power Consumption

yclone Exit Gas Temperature

kdkg kJ/kg kg/s or kW

Supply Fan Temperature

Supply Fan Humidity

Supply Fan Volume (if used)

Supply Fan Static Pressure

Supply Fan Power Consumption

Feed Breeching Static Pressure

Feed Breeching Leakage Rate

Product Breech, Static Pressure

Product Breech, Leakage Rate

Retention Time

Cylinder Material Fillage

Cylinder Material Fillage

Bulk Feed Density Wet

Product Bulk Density

Cyclone Dust Bulk Density

Cyclone Pressure Drop

Pa m3 /s

Continuous Direct-Heat Rotary Dryers

302.7 The measured fuel consumption of a direct-fired, direct-heat dryer can also be compared with the calculated fuel consumption by carbon dioxide and oxygen analyses of the exit gas If this method is used, a complete fuel analysis should be obtained and the effects of air leakage considered

302.8 Cyclone pressure-drop characteristics can usually be obtained from the cyclone manufacturer Pressure drop should be expressed in terms of a number of gas velocity pressure heads, based on the cyclone inlet gas velocity and density Having this informa- tion, cyclone pressure drop can then be used as an additional measurement of gas flow to compare with other flow measurements

302.9 Fan manufacturer's performance data, or fan curves showing volume flow versus static pressure and power consumed at the operating fan speed, must be available for the performance test In the United States, published fan data are usually based on air at

294K (70' F), and these data must often be corrected for actual fan gas density On the other hand, most manufacturers will provide, if asked to do so, fan curves corrected for

specific applications A Pitot tube traverse of the exit air duct can determine good veloci-

ties for calculating flow volume through the dryer

302.1 0 Inlet gas (air) flow measurements on dryers provided with direct combustion air heaters must include measurement of both the primary air supplied for fuel combustion, and the dilution air supplied for dryer inlet gas temperature control

302.1 1 Inlet gas (air) flow to dryers provided with steam-coil air heaters also can be esti- mated by measuring the static pressure drop of the gas flowing through the heaters Most steam coil manufacturers provide pressure drop data for their specific coils based on gas velocity gas density number of coil rows deep, and fin density Coil pressure drop meas- urements may sometimes be substituted for a direct measurement of inlet gas flow, but

such a measurement is recommended mainly as a way to confirm a direct gas-flow meas-

urement and, perhaps, uncover other measurement errors

ters are in accordance with those originally supplied by the manufacturer; check, note, and reconcile any field changes that may have been made to the dryer, and ascertain that all lifting flights, retaining dams, etc are in place Note the slope and rotational direction of the dryer, and any abnormalities which may impede the flow of material (fused or built-up product or dust), or air through the dryer system (broken or corroded dampers, worn breeching seals, fan belt slippage, excessive pressure drop, combustion system problems, etc.)

303.0 Test Preparation

303.1 During the test planning period, the following preparations should be completed Obtain all burner, fan, heater, steam coil, and cyclone performance data from the Determine fuel analyses and fuel heating values

Verify that all test instruments are installed on the dryer or are available at the test site VeriQ that all instrument connections and taps are provided on the dryer installation, Verify that all test instruments have been tested, calibrated, and are in working order VeriQ that all test data sheets are prepared, and that test personnel are trained in the

manufacturers See NOTE A above

for temporary use

and safe access to all measurement points is provided

safe and proper use of the instruments, the test procedures, and all laboratory analyti- cal procedures for feed and product See Figure 4 for a sample data sheet

Verify that evaluations have been tested and proven, e.g., moisture determinations Verify and plan how feed and product rates will be measured in a full scale dryer

Continuous Direct-Heat Rotary Dryers

401 O Gas Temperature and Humidity

Exposed-junction thermocouples connected to continuous indicator-recorders are pre- ferred (but not absolutely necessary) for rapid response and accuracy Sheathed and ther- mowell-enclosed thermocouples, and gas-filled temperature sensors are also acceptable provided time-constants are known Dial thermometers are a poor third choice, and then only if scale division is sufficiently narrow to provide needed precision The use of glass thermometers for field tests on commercial dryers is generally considered an unsafe prac- tice The single, most significant gas temperature in dryers is the supply gas temperature, since it reflects the process events inside the dryer

401.2 Gas temperature instruments should be installed in gas streams If a temperature sensor is installed in the line of sight of a burner flame, or near a high temperature steam coil gas heater, a radiation shield between the high temperature source and the sensor may

be needed

401.3 A thermocouple temperature is a point measurement and, in any gas conveying duct, the gas velocity and gas temperature may not be uniform across the f d duct section Before the performance test, all duct sections planned for thermocouple or other sensor locations should be thoroughly explored for temperature and velocity variations This work

should be done under dryer operating conditions similar to the proposed test conditions A

proper duct section profile should reveal the best location for a single point measurement,

or reveal whether repeated profiles of the duct section will also be necessary to obtain accurate data during the actual performance test

401.4 The degree of accuracy sought for gas dry-bulb temperature measurements is

k 0.50% of the absolute gas temperature reading, K

401.5 Atmospheric humidity can be determined by comparing dry-bulb and wet-bulb temperatures obtained using a sling psychrometer, or by use of an accurate aluminum-

oxide hygrometer

401.6 Dryer exit humidity should be measured downstream from all dry-type dust collec- tion equipment, to sample the cleanest gas possible Vapor condensation on measuring

devices must be avoided Wet-bulb temperatures up to about 350K can be measured by a

“wet-bulb” thermocouple in clean gas If the gas contains dust, or exceeds 350K in wet-

bulb temperature, a gas sample method which cleans and cools the gas for dew point measurement will be needed In this situation, standard sampling equipment is available and a manufacturer of gas moisture monitoring instruments should be consulted

401.7 The degree of accuracy sought for gas wet-bulb or dew point temperature meas- urements is k 0.50% of the absolute gas temperature reading, K

Continuous Direct-Heat Rotary Dryers:

A Guide to Performance Evaluation, Third Edition

by Equipment Testing Procedures Committee Copyright © 2006 American Institute of Chemical Engineers

402.2 Inlet gas (air) flow at ambient temperature can be measured by:

Making a velocity profile across the total face area of an inlet gas duct or inlet gas fil- Making a velocity profile across a convenient and reasonably straight, uniform section

ter using a hot-wire anemometer

of inlet duct using either a Pitot tube or a hot-wire anemometer

For a discussion of Pitot tube surveys, see Perry’s Chical Ergz*mm~’ Handbook, Reference 804.5

and dusts

402.3 These gas flow measurements can be confirmed by comparing the measured pres- sure-drop across steam coil gas heaters to the coil manufacturer’s published data showing air velocity versus pressure drop Also, the steam condensate rate from the coils can be compared to the gas temperature rise across the coils, or the fuel burned in a combustion gas heater can be compared to gas temperature rise through the heater When measuring condensate flow from steam coils, a weight allowance must be added to account for con- densate flash loss, which occurs when condensate is released from a pressurized condensate system into an atmospheric-pressure condensate collector Both electric and steam heaters

and fuel burners yield an efficiency of about 95%; the 5% loss is due to heater radiation

losses and incomplete combustion

402.4 When making gas flow calculations based on the temperature rise attributable to fuel combustion, the lower heating value of the fuel must be used The higher heating value, which is often cited by the supplier as the fuel’s heating value, includes the latent

heat of condensation of the water produced by the combustion of hydrogen in the fuel

This latent heat is not usable or recoverable in the dryer

402.5 When a gas inlet system also has a supply fan, an additional check of air flow can

be obtained by measuring fan speed, static pressure, and power consumed, and comparing these to the fan manufacturer’s data showing fan volume versus static pressure versus power

402.6 An agreement within & 5% between any two independent methods of inlet gas flow measurements is considered good; & 10% is acceptable, but usually is the maximum flow deviation that should be written off, without explanation, as experimental error on a

commercial-size dryer

Continuous Direct-Heat Rotary Dryers

402.7 The dryer’s exit gas flow volume can also be measured by making a Pitot tube profile of a clean duct section A hot-wire anemometer can be used if the duct gas tem- perature is below the maximum allowable by the product specification, and flammable or explosive dust are not present If it is necessary to measure flow in a somewhat dusty gas

stream, the Type “S” (Stauscheibe) Pitot tube may be preferred over a regular Pitot tube because it is less susceptible to plugging, and gives a higher differential pressure reading for any given velocity

402.8 Cyclone pressure drop measurements can be compared with manufacturers’ data

to confirm exit gas flow measurements

402.9 Measuring the speed, static pressure, and power consumed by a supply fan, and com- paring it to the fan manufacturer’s data, provides a second confirmation of measured gas flow

In the exit gas from a dryer, note that the presence of a large fraction of water vapor and, possibly, carbon dioxide sigruficantly affects the gas density Gas composition, as well as tem-

perature, must be known to determine gas density for use with fan curves and tables

402.10 When a wet scrubber is used for recovery of dust, and when gas flow measure- ment is made downstream from the scrubber, the humidity change that occurs in the scrubber must be accounted for in order to determine its inlet gas flow The large fraction

of water vapor in scrubber exit gas significantly reduces the average gas density compared

to dry air

402.1 1 Agreement between the dryer’s exit gas flow measurement and that indicated by cyclone pressure drop or fan performance is good if within k 10%; a deviation of k 15%

is about the maximum acceptable without explanation

402.12 The dryer’s exit gas flow can be expected to exceed the inlet gas flow by 5-10%

on a constant temperature/constant humidity-adjusted basis This increase is due to in- leakage through rotating seals on the breechings, and through the feed- and the product conveyors If such in-leakage appears to be greater than 10% of total flow, leak sources should be identified and, if possible, eliminated The best method to minimize outside air infiltration into continuous direct-heat rotary dryers is to maintain the rotating cylinder and breeching at a pressure only slightly below atmospheric pressure A negative static pressure in the breechings of 25-50 Pa (0.1-0.2 in of water) is best; 125 Pa (0.5 in water)

is the lowest that should be used in either breeching; otherwise, leakage through the rotat- ing seals and the material openings may be excessive

403.0 Material Temperature and Moisture Content

403.1 The best way to measure the temperature of particulate solids is to place a repre- sentative sample in a closed, insulated container with an exposed-junction thermocouple immersed in the material If the material has either a low bulk density or a low thermal conductivity, occasional shaking of the container and immersed thermocouple may be necessary for a representative reading

403.2 Thermocouples immersed in flowing streams of particulate solids usually yield unreliable data; the indicated temperature is likely to be closer to that of the entrained gas than to the temperature of the solids

403.3 Infrared adsorption instruments may yield acceptable material temperature data provided the field of view can be limited to the material

403.4 The accuracy of material temperature measurements should be f 1.0% of the absolute temperature measured, K

403.5 During the performance test, material moisture samples should be taken at fre- quent intervals, stored in gas-tight and pre-dried containers, and evaluated by the most accurate laboratory moisture test available for the material On-line, continuous moisture meters may be acceptable for routine dryer control; but the purpose of a dryer perform- ance test is usually to test drying capacity, so moisture data should be reliable Variations in moisture measurements which can be considered acceptable depend on material proper- ties and the accuracy of the moisture test employed

403.6 Moisture samples of dust recovered from the dust collector should be analyzed separately from dryer product; sampling precautions and analytical procedures should be followed

404.0 Material Flow and Cylinder Fillage

404.1 For optimum dryer performance, cylinder fillage, the percent of cylinder volume occupied by solids, should be just sufficient to fully load all of the lifting flights Cylinder

fillage in continuous direct-heat rotary dryers with lifting flights may range from 5% to

17% of total cylinder volume, depending on the size, number, and shape of lifting flights Fillage of 10-1Zo/~ is normal A dryer cylinder that has no lifting flights is usually operated

at a cylinder fillage level no greater than 5% of total cylinder volume Such dryers are

generally indirectly heated

404.2 The most accurate method to measure cylinder fillage and cylinder holdup, in terms of both weight and volume, is to stop feed, product, and air flow to and from the dryer simultaneously after a period of steady-state operation, empty the dryer, weigh the contents, and measure its average bulk density Cylinder holdup is the mass of solids in the dryer Cylinder fillage is the cylinder holdup, divided by its bulk density, divided by the cylinder volume, expressed as a percent

404.3 Cylinder fillage can also be estimated by visual inspection when the dryer is not turning, but this method should be used only when no other method is practical