Aggregates cooling for hot weather concreting

7/28/2019 Aggregates cooling for hot weather concreting

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2/10

2 Copyright 2011 by ASME

The Arab peninsula weather conditions are associated

with hot weather for approximately eight months per year. The

weather fluctuates between summer temperatures that approach

50oC and winter temperatures that sink to 18oC. Relative

humidity follows a similar pattern ranging between 5% and90% from inland to coastal regions, respectively. Ready-mixed

concrete manufacturers have to accommodate these extreme

highs and lows in climate fluctuation [10].Aggregate temperature plays a very important role in

defining the concrete mix temperature [11]. It is generally

believed that the aggregate temperature has to be less than 2oCto have a concrete mix temperature less than 10oC. However, it

has been shown that keeping the aggregate temperature about

15oC is adequate in achieving proper results [11-24].

Cooling the concrete aggregate is one of the most

effective methods to reduce the concrete mix temperature.Different equipments have been developed for that purpose.

Among those are belt conveyors, cooling drums, chilled storage

rooms, and a mix of those methods [23-25]. In theseequipments, cooled air, liquid nitrogen, chilled water, or ice are

used to reduce the concrete mix temperature [19]. Ice andchilled water are usually added to the mix to keep water

balance of the concrete mix under control. Liquid nitrogen issometimes used to reduce the cement temperature with special

precautions to avoid concrete aggregate thermal shocks.

Cooled air is the preferred candidate, although this requires

huge flow rates and extensive cooling systems.

Despite the criticality of the previously described problemfor concrete makers, there is very little work in the literature to

address the numerical or the experimental simulation of the

problem and to optimize the cooling facility. Most of theavailable work presents industrial experiences and

developments [23-25] and this is mostly for food cooling

systems [26].The cooling drums are designed on the basis of well

mixing the aggregates using radial webs [24]. The cooling

process in the existing drums in the market is based on mixing

the aggregates to enlarge the contact area between the

aggregates and the cooling air. Research shows that cooling

drums are very compact, efficient, and provide a high

production rate. Cooling drums entail, although, the

disadvantages of being quite expensive, consume high

mechanical power, and are difficult to maintain.

On the other hand, cooling using air jets over belt

conveyors, similar to those reported in [14], is the cheapest and

simplest method. However, the concrete industry reports long

cooling time, low cooling efficiency, large occupied space anda low production rate.

Combination between both methods is generally

recommended. It was reported that, three-stage cooling of

return sand is effective and efficient when flash cooling andpremixing are accomplished on a belt conveyor and final

cooling is performed in a rotary sand blending, cooling,

screening drum [18].

Convection to the cooling air is the main heat transfer

theme in these previous designs. Cooling through the therma

contact between the aggregates and the cold belt conveyor or

drum body as well as the mixing process was neither analyzed

nor optimized.

Cooling aggregates in general is a challenging task due to

its low thermal conductivity. Due to the porosity of the

concrete aggregates (10%-50%) the thermal conductivity woulddeteriorate at a very high rate; e.g., the thermal conductivity of

limestone decreases by about 70% for a porosity of 10% [27-

28]. Same behavior applies to sand in which a 25% increase in

its porosity results in 52% reduction in the thermal conductivity

[32-33]. This level of low thermal conductivity of these

materials inhibits heat flux toward the cooler surfaces as the

materials porosity increases.

The heat flow during the cooling process, either by belt

conveyors or drums, needs to be analyzed and optimized to

achieve short and optimum cooling time with low cooling

power. The main objectives of the current work are; to propose

an optimized design for belt conveyors systems to be used in

cooling the concrete aggregates and to present a numericasimulation for the cooling process using the finite elemen

method with the objective of optimizing the overall system

performance.

DEVELOPMENT OF THE PROPOSED DESIGN

The current work is focused on sand cooling. The therma

conductivity of sand in general is very low. This requires, for

thick sand beds over the conveyor, very long time for the hea

flow to be conducted from the deep layer to the cooled sand

surface. This problem is explored using a simple finite elemen

simulation model, Fig. 6 for cooling 15cm of sand initially at

45oC by cooled air jet at -15oC for 30 minutes. The sand

effective conductivity is assumed 0.5W/m.C [32]. The lowelayer is assumed adiabatic while the upper layer is assumed

under forced convection with heat transfer coefficien

20W/m2.C and bulk temperature -15oC.

This test case is a 1D heat transfer problem. A closed

form solution for the surface temperature Ts at sand bed

thickness l meters after time t seconds is given by [38] for

sand bed with initial temperature Ti, conductivity K

density , and specific heat c as follows;


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The surface temperatures of the sand bed are calculated using

the above equations using up to 10 roots of Eq. 1b. These

results are compared to finite element solutions using up to 30

elements through the height of the sand.

The temperature distribution after 30 minutes through the

sand bed thickness is highly nonlinear as shown in Fig. 2. This

requires, as shown in Fig. 3, at least 7 elements through thesand bed thickness for the finite element simulation model to

converge to the exact solution. As shown in Fig. 2, the most

upper layer temperature is severely reduced while the inside

layers are still very hot. Furthermore, the average temperature

of the sand after 30 minutes cooling is 38.5oC. This reflects the

importance of developing special techniques to overcome this

cooling problem that arises from the severely low sand bed

effective conductivity.

Fig 2. Finite element model and the resulted temperature

distribution in deg. C of the first test case

Fig 3. Effect of the number of elements through the sand bed

thickness and the effect of the number of closed form used

roots on the convergence of the final temperature at the surface

of the sand bed after 30 minutes cooling

A. Sand MixingConveyors with multi-stages are generally recommended

[18]. This implies that frequent mixing of the sand within the

cooling air enhances the cooling conditions by, randomly

moving out part of the hot sand and moving inside part of the

cooled sand. Sand mixing can be simulated in the finite

element simulation model with an aided subroutine. In thissubroutine, each nodal temperature is reassigned randomly to

another node in the sand domain each certain period of time

Figure 4 shows a schematic diagram illustrating the random

mixing technique. This technique simulates well mixing o

sand before being moved to another stage [34].

Element size smaller than 0.02m (7 elements through the

sand bed thickness) shows good result in the 1D test case

However, the mixing technique is examined with three mesh

sizes 0.03m, 0.015m, and 0.01m. The nodal temperatures

distribution after 30 mixings within 30 minutes for the same

boundary conditions of the previous case is presented in Fig. 4

The sand mixing, as shown in Fig. 5, has significantly reduced

the average temperature of the sand bed. Results obtained with

element size 0.01m (15 elements through sand bed thickness)

are appropriate to be considered for the rest of finite element

simulations in the current work.

Fig 4. The random reallocation of the nodal temperature and the

nodal temperature distribution in deg. oC after 30 mixings

B. Effect ofHeat Sink AdditionAlthough the mixing has significantly reduced the amoun

of heat stored in the sand, the final sand temperature after 30

minutes cooling is still high. Adding a heat sink to the belt i

then essential. A simple finite element model is used to tes

adding heat sinks as separators over the belt, Fig. 6. The hea

sinks are assumed aluminum walls with height 30cm and width

10cm and the sand height is assumed 25cm. The distance

between each two successive walls is investigated between

10cm to 50cm. The sand is assumed initially at 45oC and the

cooling boundary conditions are similar to the previous test

cases except for the heat sinks sides which are considered

symmetry boundary conditions. The cooling time is assumed

two hours with no mixing.

Convection,h=20W/m2.oC,T=15

oC


4/10

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LING SYSToling system

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M DESIGNis designed

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N MODELof the cooli

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5/10

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Copyright

output avera

and different

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6/10


No heat flow (adiabatic or natural boundary):

This boundary condition is called the adiabatic or natural

boundary condition, and may be expressed as:

0=

n

T, on surface Si (4)

Convection heat exchange:When there is a convective heat transfer on a part on the body

surface, Si, due to contact with a fluid medium, it can be writtenas:

)( fsn TThn

Tk =

, on surface Si (5)

where h is the convection heat transfer coefficient, which

may be temperature dependent (non-linear), Ts is the surface

temperature on Si and Tf is the fluid temperature, which may be

constant or a function of boundary coordinate and / or time and

kn is the thermal conductivity in direction n.

Prescribed temperature (Dirichlet BC):

The temperature may be prescribed on a specific boundary of

the body. The prescribed temperature may be constant or a

function of boundary coordinate and/or time:

),,,( tzyxTT s= , on surface Si (6)

As seen in Fig. 10, half of the belt is considered for the

simulation model due to the symmetry. The boundary

conditions applied to this simulation are defined for the four

surfaces S1, S2, S3, and S4 as follows:

( )1 1

1

S S

S

TK h T T

y

=

(7)

( )4 4

4

S S

S

TK h T T

n

=

(8)

2,3

0S

TK

x

=

Due to Symmetry (9)

( )5 5

5

S S

S

TK h T T

n

=

(10)

Following the normal finite element discretization and

assembly procedures for Equation (1), the model ends up with

the global finite element equations in the form [39-41]:

[ ] [ ] [ ][ ] { } { } { }hshc QQTKKTC +=++&

(11)

where [ ]C is the thermal capacity matrix given by:

[ ] [ ] [ ]= VT

dVNNcC ; (11a)

[ ]cK is the thermal conductivity matrix given by:

[ ] [ ] [ ][ ]= VT

c dVBKBK ; (11b)

[ ]hK is the additional thermal conductivity matrix due toconvection B.C. given by:

[ ] [ ] [ ]= SsTs

h dSNNhK ; (11c)

[ ]hK is the additional thermal conductivity matrix due toconvection B.C. given by:

[ ] [ ] [ ]= SsTs

h dSNNhK ; (11d)

{ }SQ is the heat flux vector due to input surface flux B.C.given by:

{ } [ ]= STsss dSNqQ ; (11e)

and, finally, { }hQ is the heat flux vector due to convection B.Cgiven by:

{ } [ ]= STs

eh

dSNhTQc

; (11f)

It should be noted that Equation (11) doesnt involve

radiation boundary conditions and internal heat generation

which are not a factor in the current simulation. Time

discretization of equation (11) using the -method results in the

following final equation to be solved for the linear transien

simulation:

( ) [ ] TKKQQTCKK thchStthct

++=

++

+

1 (12)

where is a constant which is between 0 and 1 depending on

the solution method used. The following methods are used

frequently:

= 0 explicit Euler forward method.

= implicit trapezoidal rule. = 1 implicit Euler backward method.

The current analysis is solved using the implicit Euler

backward method.

C. The simulation model of the whole plantFigure 11, shows the simulation model of the eigh

conveyers. The model shows eight belts with sand and anotheeight belts without sand. The upper model is for the working

belts and the lower model is for the returning belts. Each belt i

solved for a load step with time that depends on the belt speedand then the sand temperatures are calculated, randomized and

then are applied as initial conditions for the next belt in the nex

load step. Also, the temperatures of the belt and aluminum finnodes are calculated and applied to the returning belt and

aluminum fins as initial condition for the next load stepAdding, the calculated temperatures of the returning belt are

calculated and then applied to the loaded conveyor as initia

condition for the next load step. Each belt transfers the

calculated temperatures to the next till the last belt, while thefirst belt gets new sand set with a Tin (Fig. 11).

The surface of the sand, the bottom of the belt, and the

surrounding areas of the aluminum fins are loaded with forced


7/10

conv

simultemp

Ac

estim

600

800

RES

temp

temp

80 m

the sbelt.

gaine

shortin te

Fi

ction bound

ated patternratures are th

cording to [

ated to be 2

/m2.C betwe

/m

2

.C betwee

ULTS ANDThe cooling

ratures vary

ratures are si

inutes. Figur

nd over the 8This short ti

d heat during

cooling timeperature drop

g 12. Temper

ry condition

are considen considered

6, 35], the

00W/m2.C be

n the alumin

n the sand and

ISCUSIONSplant is te

rom 10oC to

ulated at co

e 12 shows t

belts due to 2e is not eno

the return stro

educe the cooof 30oC at th

Fig 11. Finite el

ture distributi

sy

s. The pa

ed adiabatic.or the next co

ontact therm

tween the sa

m fins and

aluminum fin

sted for inp

0oC with step

ling times; 8,

e temperature

minutes cooligh for the be

e. This resid

ling rate of the last stage.

ement model fo

ons in deg. oC

stem with Tin=

titions of t

The resulteveyor.

l resistance

d and rubbe

he rubber, an

s.

ut sand wi

of 5oC. The

16, 32, 48, a

distribution

g time for eact to release t

al heat and t

e sand resultinigure 13 sho

the eight conv

for the results

55 oC, cooling

7

e

d

is

r,

d

h

e

d

f

he

e

gs

the te

minuthelps

durin

45oCfor ea

totall

the la

tempetempe

result

condisumm

cooli

input

coolireco

yors showing t

obtained from

time tc= 16mi

perature dist

es cooling timost of the

the return str

at the last stach belt, as s

released whi

t stage.

or full vie

ratures arerature for 7 co

are the fi

ions after 4 harized further

g time on the

sand temper

g time basedmended outp

e loaded conve

the finite ele

n (2 minutes f

ibution of the

e for each begained heat i

oke. This is r

e. For very sown in Fig. 1

h is resulted i

of the pl

plotted againoling rates as

al average

ours of operatin Fig. 16 w

final average

atures. This

n the input tet temperature

yors and the ret

ent simulatio

or each belt)

Copyright

sand over the

lt. This longthe belt to

sulted in tem

low belt, 104, the gained

temperature

nt performa

st the finalshown in Fig.

emperature a

on. These rehich shows t

sand temperat

chart define

perature of tas indicated in

rning ones

of belt conve

2011 by ASM

8 belts due to

r cooling time released o

erature drop o

inutes coolinheat has bee

drop of 52oC

ce, the inp

stage outp15. The show

t steady stat

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ure at differe

s the require

e sand and ththe figure.

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6

e

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8/10

Fi

Fi

g 13. Temper

g 14. Temper

Fig 15.

ture distributisy

ture distributisys

elation betwe

Re

ons in deg.oC

stem with Tin=

ons in deg. oCtem with Tin=

en the average

commende

for the results55 oC, cooling

for the results5 oC, cooling

output tempe

Zone

8

obtained fromtime tc= 48mi

obtained fromtime tc= 80mi

ature and the

the finite elen (6 minutes f

the finite ele(10 minutes

input temperat

ent simulatioor each belt)

ent simulatiofor each belt)

ure for differe

Copyright

of belt conve

of belt conve

t cooling tim

2011 by ASM

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yor cooling

s tc


9/10

CONCo

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REF[1]

[2]

[3]

Fig 16.

CLUSIONSoling concrete

like the A

th. Existingized for powe

st of the exigates in a col

ll advantagesextended bel

nted simulatio

ixing processg system de

nt simulation

factors.

NOWLEDMresearch was

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Aggregates cooling for hot weather concreting