Results research of energetics characteristics of convection drying

Paper received: 15.12.2007 Paper accepted: 07.07.2008

Results Research of Energetics Characteristics of

Convection Drying

Slavica Prvulović1 - D ragiša T olm ač2’* - L jiljana Radovanovič2 1 T echnical Faculty, Bor, Serbia

2 Technical Faculty "M ihajlo Pupin", Zrenjanin, Serbia

E xp erim en ta l a n d theoretic researches are p re se n te d in this p a p e r a n d they are im plem ented on th e re a l industrial equipm ent o f convection d ry e r w ith p n e u m a tic transport o f material. The num eric v a lu e s are given f o r optim um p a ra m eters o f drying, energetic characteristics a n d balances as w ell as the c o e ffic ie n t o f the heat transfer.

The heat transfer in these system s is a c co m p lish ed b a sed on the p rin c ip le o f direct contact o f d r y e d m a teria l a n d w arm air. Then, an intensive transfer o f heat a n d m ass is accom plished.

This p a p e r p re sen ts the results o f researches w hich can be useful in designing a n d construction o f s u c h d ryers in the f o o d a n d agri industry. I t refers to the technological technical characteristics o f the dryer, energetic balances a n d coefficient o f heat transfer.

© 2008 Journal o f M echanical Engineering. A ll rights reserved.

Keywords: energy balance, drying, numeric data, heat transfer

0 IN T R O D U C T IO N

In principle, convection dryers can be used fo r drying o f m eal-like and fine-kernel m aterials. S im ple construction and relatively low consum ption o f energy has enabled successful application o f convection dryers in the above stated industrial branches and starch. A pplication o f convection pneum atic dryers is represented especially in the food industry in the factories for industrial processing o f grains (processing o f w h eat and com based on the w et m illing).

H eat transfer system s o f such o f kind and likew ise are introduced in literature [2] to [4], [6], [8], [9], [12], [14] and [18],

In these dryers, a continuous drying o f loose m aterials is being m ade, the concentration being ck = (0.05 to 2) kg o f m aterial / kg o f air. A verage particle size o f the drying m aterial can be (0.05 to 2) mm. The initial m oisture o f the drying m aterial can be w l= (35 to 40) %, and the rem aining m oisture after drying is usually w 2 = (10 to 15)%. The specific consum ption o f energy is usually (3500 to 5040) kJ / (kg H20 ).

E fficiency o f convection dryers is evaluated according to the therm al degree o f utilization w hich is w ithin the lim its o f (66 to 75)% , depending o f the drying system (indirect or direct drying). The drying tim e in these dryers is very short, only several seconds, therefore they can be used for drying o f the m aterials susceptible

to high tem peratures in the short drying period o f time.

1 M A TERIA L A N D M ETHOD

E xperim ental research is m ade in the convection pneum atic dryer, Figure 1.

Fig. 1. Schem e o f experim ental d rying equipm ent (1 - heat transm iter, 2 - rotation d ozer o f m o ist

m aterial, 3 - d ryer pipe, 4 - dryer head, 5 -

cyclones, 6 - centrifugal ventilator, 7 - bringing o f m oist material))

T able 1. C haracteristics convection p n e u m a tic d ryin g

Position N am e o f equipm ent and characteristics

1 H eat transm iter, heat pow er Q = 2,5 M W

3 D ryer pipe, diam eter d = 1250 m m , height 20 m

5 C yclone separator, diam eter Dc = 1750 m m , height: cylindrical part o f the cyclone is 2620 m m , conical part o f cyclone is 3500 m m

6 C entrifugal ventilator: V = 65000 m n3h"', p = 3500 Pa, N = 90 kW

7 R otating dozer, N = 22 kW , n = 650 min"1

4 D ryer head

D rying is perform ed in the direct contact o f w arm gases w ith the m oist m aterial. Based on that, the principle o f direct drying is being represented here. The drying m aterial is starch.

D osing o f m oist m aterial to the dryer is m ade through the rotation dozer (2) w ith the capacity o f np = 8000 k g h '1, through the screw conveying system , as given in the schem e o f experim ental equipm ent in Fig. 1. In the Table 1, the characteristics o f convection pneum atic dryer are given.

M oist m aterial is being transported via hot air - the drying agent through the dryer pneum atic pipe (3), it passes through the dryer head (4) and goes to the cyclone separators (5) w here separation o f dried m aterial is being m ade, and the hot gases exit w ith the help o f ventilator (6), into the atm osphere (see Fig. 1). The dried m aterial is being transported from the cyclone via screw conveyors and through a separate line o f transport up to the m aterial w arehouse departm ent.

D uring drying, the determ ined steam w ater is mp= 3830 kgh"1. A verage values o f the results o f m easuring the drying tem perature and the m aterial hum idity are: tj= l 50 °C, t2=50 °C,

W ]= 3 6 % , w 2= 13%.

E nergetic balances show appropriate relations betw een the total invested energy, utilized energy and heat losses during the drying process. B ased on the above, the energetic balances can be useful w hen show ing the dryer condition diagnosis. A ccording to the Lam bić [6], P rvulović [10], Tolm ač [17],

The discrepancy in the air enthalpy at the inlet and at the outlet o f the dryer:

A H = H l — H 2 = cp (/, —12 )

Q uantity o f evaporated w ater ( kgh"1 ):

j 100 - Wj_?

o

o

(2) T otal heat quantity:

ß u = ßw + ß s + ßg ₍₃₎

ß u = ,wp ' r M b ' ₍₄₎

Q uantity o f drying air:

vL = —

L A H (5)

Specific consum ption o f energy:

q W

T herm al degree o f utilization:

„ . . h _ ß u _ ßg

t ~

ß u

(6)

(7) T otal heat pow er o f drying:

T o ta l coefficient o f the heat transfer:

K = ö u / ( A At,x) (9) H e a t fo r drying, i.e. its convective part consists o f th e h e a t for w ater evaporation (Qw) and heat for h e a tin g o f the drying m aterial (Q s), m eaning, w ith o u t heat losses (Q g):

ßconv = ß w + ß s (10) D u rin g convective drying the follow ing equation o f th e h eat transfer is applied as well:

ßconv = M A?sr (1 1)

2 R ESU LTS AN D D ISCU SSIO N

Experim ental research on the convection p n eu m atic dryer, Fig. 1, w as aim ed to determ ine th e energetic balance, specific consum ption o f e n erg y , therm al degree o f utilization and other re le v a n t param eters o f drying, according to the re fe re n ce by Prvulović [9]. The results o f the en erg etic balance are being given in the Table 2. T h e total heat force o f drying o f Qd = 2180 kW , is b e in g acquired as w ell as the specific consum ption o f energy q = 3710 kJ kg"1, o f ev aporable w ater. A ccording to H eß [2], P rvulović [8] and Tolm ač [18], a specific consum ption o f energy in convection drying am ounts (3650 to 5040) kJ k g '1, o f evaporable

water. A ccording to the data from reference Islam [3], specific consum ption o f energy am ounts q =

(3642 to 5283) kJ kg o f evaporable water. Based on the results o f energetic balance and results o f the drying param eters m easuring, according to the reference by Prvulović [9], a total coefficient o f the heat transfer is determ ined to be during convection drying ha = 295 WrrUK"1, T able 3. Based on the results o f research, the m ass air flow am ounts 0.450 kg s_1m '2, the drying capacity is 5885 kgh"1, and the air tem perature at the dryer inlet is 150°C. A ccording to the literature Lin [5], the m ass air flow is 0.289 k g s'1! ^ 2, the drying capacity is 1152 k g h '1, at the drying tem perature o f 90°C.

A ccording to the research by Hqß [2] and Tolm ač [12], on the convection pneum atic dryer, the value o f the total coefficient o f heat transfer during drying o f com starch is 308 W m '2K_1 , and during drying o f potato starch the coefficient o f heat transfer is 295 W m '2K _1.

The objective o f this part o f research is to determ ine the character o f heat transfer in such com plex dynam ic m odel, considering that the heat transfer com prises a phenom enon o f heat transfer by convection, conduction and radiation.

Based on the results o f research, the value o f the coefficient o f heat transfer by convection has been determ ined, Table 4.

T ab le 2, E nergetics balance o f convection pneum atic dryer

E nergetics drying param eter Sign M easure

unit Energetics value param eter

A ir tem perature at the inlet o f dryer t\ °C 150

Q uantity o f evaporable w ater W k g h '1 2115

T otal heat quantity ßu k Jh '1 7.846650

D rying heat pow er ßd kW 2180

E nergy specific use q k J k g '1 3710

Q uantity o f drying air vL m „ V 60358

Specific quantity o f evaported w ater k g m 'V 26.90

Specific quantity o f evaported w ater k g m 'V 86.20

A ir tem perature at the outlet o f dryer h °C 50

Table 3. Total coefficient o f heat tra n sfer ( h j

D rying heat pow er V olum e o f pipe drying place

D rying surface* M iddle log. difference o f

tem perature

Total heat transfer coefficient

Öd Vk A Zf/Sr K

kW m 3 m 2

°c

2180 24.53 7850 94 295

*A ccording to [2], [9], [14] drying surface is equal to interior surface o f drying pipe (A = d n h ; d = 1,25 m - pipe diam eter ; h = 20 m - pipe height ).

Table 4. C oefficient o f h eat transfer by convection ( h j

H eat pow er for H eat pow er for H eat pow er o f Surface M ean C oefficient o f w ater m aterial heat transfer drying logarithm ic convection evaporation heating by convection difference o f

tem perature

heat transfer

öw Ös Qconv A Zl/Sr K

kW kW kW m 2

°c

1 2 3 4 5 6

1482 66 1548 78.5 94 210

B ased on the results o f research o f energetic balance and the results o f m easuring the tem perature o f the drying agent, the total coefficient o f the heat transfer is being determ ined in the convection dryer in the am ount o f ha = 295 W m 2K _1, and the coefficient o f the heat transfer by convection hc

= 210 Wm"2K ''. The effects o f the heat losses during drying are expressed through the separate value h u - hc = 85 W m '2K '', so called coefficient o f the heat transfer for the heat losses together w ith the outlet air and the heat transfer by conduction and radiation through the dryer pipe. In such a w ay the effects o f the h eat transfer are being determ ined as w ell as the basic param eters o f the heat transfer.

A coefficient o f h eat transfer under the dynam ic conditions o f the dryer operation (non­ equal dosing o f m aterial to be dryed, oscillations in the initial m oisture content, tem perature o f drying, heat flux, etc.) depends on the greater num ber o f different values w hich characterize the heat transfer. The objective o f this part o f research is to determ ine the character o f heat transfer in such com plex dynam ic m odel, considering that the heat transfer com prises a phenom enon o f heat

transfer by convection, conduction and radiation.

3 C O N C LU SIO N

Energetic balance o f the dryer can serve in evaluation o f energetic condition o f the dryer as w ell as in review ing o f the possibility o f rational consum ption o f energy. This paper presented the experim ental and theoretic research o f relevant param eters o f drying on the convection pneum atic dryer in the agri and food industry. B ased on the analysis o f energetic balance, the heat force o f drying has been determ ined Q d = 2180 kW , specific consum ption o f energy q = 3710 kJkg o f evaporable water, as w ell as the therm al degree o f utilization q = 67% .

for breaking o f connection forces o f m oisture w ith the basis o f the m aterial to be dried.

The results o f research can be used also for: determ ination o f dependence and param eters o f the heat transfer during convection drying, as w ell as in designing and developm ent o f convection dryers. The acquired results o f research are based on the experim ental data from the industrial dryer. B ased on that, the results o f research have a value o f use, i.e. they are useful to the designers, m anufacturers and beneficiaries o f these and sim ilar drying system s as w ell as for the educational purposes.

4 N O M E N C L A T U R E

K W m ' ì '1 total coefficient o f heat

transfer

h c W m '2K _1 coefficient o f convection heat transfer

H k J k g '1 enthalpy

t\ °C air tem perature at the inlet o f

dryer

h

°c

o f dryer

Cp

w k g h '1 quantity o f evaporated w ater

m i k g h '1 quantity o f m oist m aterial

m p k g h 1 quantity o f w ater steam W

j

inlet o f dryer

W2 % m oisture o f the dried

m aterial at the outlet o f dryer

dtsr °C m ean logarithm difference o f

tem perature

Q k Jh '1 heat quantity

A m 2 drying surface

y k m 3 volum e o f dryer pneum atic

pipe

r kJkg"1 specific heat o f evaporation 0 kJkg"1 specific consum ption o f

energy

vL

T h % therm al degree o f utilizatoin

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