7/25/2019 Grinding Mill Scale-up Problems by CC Harris N Arbiter

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Gr i i id i i i g illSco fe yp Prob lem s

C .

C . H a r r i s an d N. A r b i t e r

Bal l and rod mi lls at An ac on da

C o.

s Carr Fork coppe r mine outs ide Salt Lake City , UT . At lef t , Harding e

16.5 x 29

f t 5 x 8 . 8 m) ball

mill. A t r ight , Hardinge

14 x 20

f t

(4.3 x 6.1

m) rod mi l l . Th e mi l ls were produ ced by the Mine ral Processing Div is ion of Koppers Co.

Inc. Photo courtesy of Koppers.

T h e eco n o m ic ad van tage .s o f large

d i a m e t e r

b a l l

m i l l s

c a n b e c o m -

o r o m i . s e d

b y u n e . \ p e c t e d c a p a c i t y

i m i t i n g c o n d i t i o n s .

These arise

b e

cause

m e d i a r o t a t i o n a l

flow

to ore

a.xial flow r a t i o s , a n d t h e n u m b e r o f

m i l l

r e v o l u t i o n s t h a t o r e is s u b j e c t e d

to d u r i n g residence,

a r e b o t h i n

v e r s e l y

p r o p o r t io n a l

to

m i l l

d i a m e t e r .

Because o f

t h i s , m i x i n g

e f f i c i e n c y a n d

g r i n d i n g

k i n e t i c s m a y

decrease

a n d

become

c a p a c i t y l i m i t i n g w h e n

m i l l

d i a m e t e r s r e a c h a c r i t i c a l r a n g e .

T h e r e a r e t w o p o s s i b l e

scale-up

p r o b l e m s i n v o l v e d

i n t h e d e s i g n a n d

use o f

m i n e r a l

p r o c e s s i n g m a c h i n e r y :

S e l e c t i n g t h e

size

a n d o p e r a t i n g

c o n d i t i o n s

f o r a v a i l a b l e l a r g e r e q u i p

m e n t ,

t o i n s u r e t h a t o p e r a t i n g r e s u l t s

w i l l

m a t c h

those

o b t a i n e d

w i t h

s m a l

le r u n i t s ;

o r

E x t e n d i n g t h e

size ranges

o f

e q u i p m e n t b e y o n d e x i s t i n g l i m i t s .

P r e v i o u s

research

a t C o l u m b i a

U n i v e r s i t y

in to flotation

m a c h i n e

h y d r o d y n a m i c s

h a s d e m o n s t r a t e d t h e

C. C. Harr is is professor of mineral

engineer ing and N. Arbi ter is emer i tus

professor wi th Henry Krumb School of

M ines , Co lumbia Un ivers i t y , New York ,

N Y 10027.

i m p o r t a n c e

i n

scale-up

o f i n t e r n a l

flow r e l a t i o n s h i p s ; these p e r f o r m

a

s i m i l a r

r o l e i n

g r i n d i n g

( A r b i t e r

an d

H a r r i s ,

1980).

Problems at oug ainv il le

T h e l a r g e s t

t u n i b l i n g m i l l s

in use in

1 9 4 3, a c c o r d i n g t o T a g g a r t , w e r e 9 f t

( 2 .7 m) f o r ro d

m i l l s

an d 10.5 f t (3 .2 m)

fo r

b a l l

m i l l s . Since

t h e n , r o d

m i l l

d i a m e t e r s h a v e i n c r e a s e d t o 1 5 f t

( 4 . 6 m ) a n d

b a l l

m i l l

d i a m e t e r s t o

16.5 f t (5 .0 m ) ,

w i t h

t w o p l a n t s u s i n g

1 8 - f t ( 5 . 5 - m )

m i l l s .

T h e m o r e r e c e n t l y

d e v e l o p e d p r i m a r y

a u t o g e n o u s

m i l l s

h ave d iam eters up to 36 f t ( 11 m) .

I n

s p i t e o fthese r e l a t i v e l y l a r g e i n

creases,

th ere h as

been o n l y

o n e

p u b

l i s h e d

r e p o r t r e g a r d i n g

scale-up

p r o b

l e m s : t h e B o u g a i n v i l l e i n s t a l l a t i o n o f

e i g h t 18 ft x 2 1 ft (5.5 m x 6.4 m )

b a l l

m i l l s .

D es ig n e d f o r 90 000 s t/d ( 82

kt/d) , t h e c i r c u i t

o r i g i n a l l v

t r e a t e d

a b o u t

72,000

st/d (66

kt/d).

A c c o r d i n g

to

t h e o p e r at o r ( H i n k f u s s ,

1976),

. .

t h e

m i l l s

u se a b o u t o n e - t h i r d m o r e

p o w e r p e r t o n n e o f o r e

g r o u n d

t h a n

w o u l d

b e e x p e c t e d

f r o m

s m a l l e r ,

3 .7 - m ( 12- f t )

m i l l s .

C i r c u l a t i n g loads

up

to 650 % are

necessary,

a n d

coarser

f e e d

sizes

a r e a p r o b l e m .

V a r y i n g b a l l

l o a d s ,

b a l l sizes,

a n d l i n e r p r o fi l e s

b r o u g h t

n o i m p r o v e m e n t . I n c r e a s i n g

m i l l

speed

i n c r e a s e d p o w e r p r o p o r

t i o n a t e l y ,

b u t c a p a c i t y

less

t h a n

p r o

p o r t i o n a t e l y .

T o r e a c h d e s i g n c a

p a c i t y , an

a d d i t i o n a l

18 x 2 1 ft (5.5 x

6.4 m)

m i l l

w a s i n s t a l l e d ,

f o l lo w e d

b y

a 18 x 24 ft (5.5 x 7.3 m )

m i l l Steane

an d

H i n k f u s s ,

1979).

Because o fthese

p r o b l e m s , t h e

m i l l

m a n u f a c t u r e r r e c o m m e n d e d t h a t n o

b a l l m i l l s

lar ge r tha n 16.5 f t (5 m) in

d i a m e t e r b e c o n s t r u c t e d

u n t i l

th e

B o u g a i n v i l l e

p r o b l e m c o u l d b e b e t t e r

u n d e r s t o o d ( K j o s ,

1979).

H o w e v e r ,

m i l l s of

t h e

samesizep e r f o r m satisfac-

t o r i l v

a t C i t i e s

Service

Co . ' s P in to

\'al-

le y

p l a n t ( H u l s e b o s , 1 9 79 ,

1981),

an d

a u t o g e n o u s

m i l l s

n e a r l y t w i c e th a t

d i a m e t e r a re i n r e g u l a r s e r v i c e .

S c a l e E f fe c t s i n T u m b l i n g M ills

S t u d y o f a v a i l a b l e d a t a

f r o m

B o u g a i n v i l l e ,

P i n t o V a l l e y , a n d o t h e r

p l a n t s ,

a n d a d e t a i l e d a n a l y s i s o f

s c a l e - u p , i n d i c a t e t h a t t h e r e s h o u l d b e

a

c r i t i c a l

d i a m e t e r r a n g e f o r

b a l l m i l l s *

* The sam e proble m can be expected

with any tum bl in g m i l l , but not necessar-

i ly at the same diameter.

M I N I N G E N G I N E E R I N G

J A N U A R Y 1 9 8 2 ^

7/25/2019 Grinding Mill Scale-up Problems by CC Harris N Arbiter

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above

w h i c h

scale-up

p r o b l e m s c a n

b e e x p e c t e d . T h e

d i f f i c u l t y

i s n o t i n

th e

a b i l i t y

t o p r e d i c t o r p r o v i d e t h e

p o w e r

necessary

t o

d r i v e

t h e m i l l

(Kjos,

1979),

b u t r a t h e r i n t h e

exis

tence

o f a p r e v i o u s l y u n r e c o g n i z e d

d i m e n s i o n a l a n d d y n a m i c f a c t o r a s

sociated w i t h m i l l scale-up.

T h i s h a s

tw o

i n t e r r e l a t e d c o m p o n e n t s :

A decrease i n m i x i n g w i t h i n

c r e a s i n g m i l l d i a m e t e r , d u e t o t h e d e

pendence

o f

m i x i n g effectiveness

o n

t h e n u m b e r o f

m i l l

r e v o l u t i o n s t o

w h i c h

a

u n i t

o f f e e d i s s u b j e c t e d . T h i s

w i l l

b e c o m p o u n d e d

w i t h

t h e d i s p e r

s i o n

o f

residence

t i m e s ( H e r b s t a n d

F u e r s t e n a u , 1 98 0; F i n c h a n d R a m i

rez-C astro, 1981) to be

expected

in a

s i n g l e r e a c t o r e v e n i f

w e l l

m i x e d

( L e v e n s p i e l ,

1972);

A

g r i n d i n g

k i n e t i c s f a c t o r , r e p r e

sented

b y t h e r a t i o o f t h e

g r i n d i n g

m e d i a c i r c u l a t i o n r a te t o o r e f e e d r a t e ;

t h i s also decreases w i t h

i n c r e a s i n g

m i l l d i a m e t e r s .

These hectors

ar e d i f f e r e n taspects o f

t h e i n t e r a c t i o n b e t w e e n m e d i a c i r c u

l a t i o n

a n d f e e d

flow.

T h e i r

systematic

d e v e l o p m e n t i s g i v e n i n

Table

1 .

Mixing Considerat ions

T h e B o u g a i n v i l l e s ta f f ( H i n k f u s s ,

1 97 6; H i n k f u s s a n d Steane, 1979) and

o t h e r

observers

( K j o s , 1 9 7 9 )

have

suggested

t h a t p o o r m i x i n g a n d

flow

p r o b l e m s a r e

i n v o l v e d .

C a l c u l a t i o n s

y s

o n B o u g a i n \ i l I e a n d P i n t o

V a l

l e y o p e r a t i n g d a t a ,

i n c l u d i n g

a p p l i c a

t i o n

o f e q u a t i o n s 8 a n d 9 , l e a d t o d a t a

i n Table

2 . A ls o i n c l u d e d f o r c o m p a r i -

Notation

D M i l l d i a m e t e r m e a s u r e d i n s l d ^ l i n e r s

F r a c t i o n c r i t i c a l s p e e d (= N \ 3 w h e r e N

i s i n r pm and D i n f ee t )

L M i l l l e n g t h m e a s u r e d i n t e r n a l l y

L , L o a d i n g : f r a c t i o n o f m i l l v o l u m e o c c u p i e d by

g r i n d i n g m e d i a , m e a s u r e d a t r e s t

N M i l l r o t a t i o n a l s p e e d : r e v o l u t i o n s p e r u n i t t i m e

n A v e r a g e n u m b e r o f r e v o l u t i o n s d u r i n g t h e r e

s i d e n c e o f a n e l e m e n t o f o r e i n t h e m i l l {= Nr

P M i l l power c o n s u m p t i o n : n e t p o w e r = c o n

s u m e d p o w e r - i d l i n g p o w e r

O , M a s s fe e d r a t e o f o r e t h r o u g h m i l l : a x i a l m a s s

f l o w r a t e [ = n e w f e e d r a t e x ( 1 -i - c i r c u l a t i n g

l o a d r a t i o ) . N o t e : Q / V ^ D ' ]

Q , M a s s r o t a t i o n a l f l o w r a t e : m a y r e f e r t o s t e e l , o r

p u l p , o r d r y o r e , o r a n y c o m b i n a t i o n , d e p e n d

i n g o n d e n s i t y t e r m , p N V ^ . S e e a l s o T a b l e

2 , f o o t n o t e j . N o t e : 0,. V ^ D ' )

t N o m i n a l r e s i d e n c e t i m e o f o r e e l e m e n t i n m i l l

( = V^or/Q,)

V M i l l v o l u m e ( = 7 r L D /4)

V V o l u m e o f m i l l o c c u p i e d b y m e d i a ( = V L , )

V o l u m e o f p u l p

(

= V e )

W W e i g h t o f m i l l c o n t e n t s : m a y r e f e r t o s t e e l o r

p u l p o r d r y o r e d e p e n d i n g o n d e n s i t y t e r m ,

p ( = V p )

f G r i n d i n g m e d i a v o i d r a t i o : v o i d v o l u m e / b u l k

v o l u m e - 0 . 4 1 , n e w b a l l c h a r g e : e - - 0 . 3 8 .

s e a s o n e d b a l l c h a r g e ( T a g g a r l 5 - 3 2 ) ; ~ 0 . 4 t o

0 . 5 , e x p a n d e d d u e t o m i l l r o t a t i o n ( t o b e p u b

l i s h e d )

A u t o g e n o u s m i l l s : < = 1 ; = 1 .2 , e x p a n d e d d u e

t o m i l l r o t a t i o n

e H a l t a n g l e s u b t e n d e d a t m i l l c e n t e r b y g r i n d i n g

med i a a t r es t [ ( 9 -

sin6cose

/7r

= L,]

p D e n s i t y o f m i l l c o n t e n t s o r o f a c o m p o n e n t o f

c o n t e n t s : b u l k d e n s i t y o f b a l l l o a d - 2 9 0 l b s /

c u b f t ; s t e e l d e n s i t y 4 8 0 l b s / c u b f t

c r O r e de ns i t y

son are data

f r o m

a s m a l l e r

b a l l

m i l l

( T a g g a r t ,

194-5)

a n d a l a r g e a u t o g e n o u s

m i l K L o v e d a y .

1979).

T h e

18-ft 5.5-m)

m i l l d a t a s h o w t h a t a t P i n t o V a l l e y a

u n i t

o f f e e d i s

exposed

to 2 .4 t i m es as

man\l r e v o l u t i o n s as at B c 2 U ga in -

v i l l e .

T h e s i g n i f i c a n t l y l o w e r n v a l u e

f or B o u g a i n x

i l l e

t h a n f o r P i n t o V a l l e y

( a n d e s p e c i a l l y f o r t h e o t h e r t w o

p l a n t s ) a n d s t a f f o b s e r v a t i o n s a t

B o u g a i n v i l l e

raise

q u e s t i o n s a b o u t

t h e n u m b e r o f m i l l r e v o l u t i o n s r e

q u i r e d

f o r e f f e c t i v e

m i x i n g

o f f e e d

w i t h g r i n d i n g m e d i a .

O r r discussed m i x i n g

g r a n u l e s o f

t h r e e c o l o r s i n a r o t a t i n g m i x e r a n d

c o n c l u d e d t h a t t h e q u a l i t y o f

m i x i n g

based o n a

test)

d e p e n d e d o n t h e

n u m b e r o f r e v o l u t i o n s as

f o l l o w s :

11 ,

DOor; 2 3 , fiiir ; 3 5 , v e r y g o o d ; 5 6 ,

excel-

e n t . W h i l e t h e r e a r e t o o m a n y d i s

s i m i l a r i t i e s

b e t w e e n

systems,

r e -

( j u i r e m e n t s , a n d c r i t e r i a t o a p p l y t h i s

i n f o r m a t i o n

d i r e c t l y t o c o m m i n u t i o n ,

these

r e s u l t s s u p p o r t t h e i d e a t h a t

p o o r

m i x i n g

i s o n e p r o b a b l e

cause

o f

t h e B o u g a i n v i l l e p r o b l e m .

T h e 1 8 - f t

5.5-m) m i l l s

i n q u e s t i o n

operate

a t s u b s t a n t i a l l y t h e

same

speed

a n d

w i t h

s i m i l a r l o a d i n g s . T h e

p e r f o r m a n c e d i f f e r e n c e s m u s t b e d u e

p r i m a r i l y

to d i f f e r e n c e s i n

residence

t i m e s d e t e r m i n e d b y t h e h i g h e r f e e d

rates

a n d m u c h h i g h e r c i r c u l a t i n g

l o a d s a t B o u g a i n \ ' i l l e .

These

r e s u l t i n

a 4 0 % l o w e r n o m i n a l

residence

t i m e

a nd

n v a l u e c o m p a r e d

w i t h

P i n t o

V a l

l e y . F o r m i l l s o f d i f f e r e n t d i a m e t e r , a l l

o t h e r f a c t o r s b e i n g t h e

same,

t h e

d i f

ferences

c a n b e e v e n l a r g e r . T h i s i s

i l l u s t r a t e d

i n

Table

3 , c a l c u l a t e d

f r o m

B o u g a i n v i l l e

d a t a p r o p o r t i o n e d d o w n

to

a 5 .9 - f t

1.8-m)

m i l l , w h i c h

i s t h e

size u s e d i n t h e i r

p i l o t

s t u d i e s . C a l c u

l a t e d v a l u e s a r e

also

i n c l u d e d f o r a

16.5-ft

( 5 - m )

m i l l ,

i n t e r n a l d i a m e t e r

1 5 .9 f t ( 4 .9 n i ) ( R o w l a n d a n d K j o s ,

1978),

w h i c h

i s t h e l a r g e s t r e c o m

m e n d e d d i a m e t e r m e n t i o n e d e a r l i e r

( K j o s ,

1979).

T h e f i g u r e s f o r t h e

5.9-ft 1.8-m) m i l l

a re r o u g h l y c o m p a r a b l e w i t h

those

f o r

t h e t h r e e s a t i s f a c t o r i ly p e r f o r m i n g

m i l l s i n

Table

2 ; t h e h i g h e r n v a l u e s

suggest

a h i g h e r r o t a t i o n a l

m i x i n g

ef

fectiveness

f o r t h e o t h e r

m i l l s

c o m

p a r e d

w i t h

B o u g a i n \ ' i ll e . O t h e r

m i l l

dataprocessed

i n t h i s s t u d y s h o w t h a t

t h e n v a l u e f o r t h e B o u g a i n v i l l e m i l l is

b y f a r t h e s m a l l e s t f o r a n o p e r a t i n g

m i l l , a l t h o u g h s m a l l e r v a l u e s have

been

c a l c u l a t e d

f r o m

m a n u f a c t u r e r s '

e s t i m a t e d

capacities

f o r 1 8 - f t - d i a m

( 5 . 5 - m - d i a m )

m i l l s

p r o d u c i n g v e r y

coarse

g r i n d s .

A l t h o u g h m i x i n g

d e f i c i e n c i e s a re

u s u a l l y a t t r i b u t e d t o r o t a t i o n a l

flow

inadequacies,

a n o t h e r m e c h a n i s m

c a n also r e s u l t i n s e g r e g a t i o n o f

p u l p

a n d g r i n d i n g

m e d i a

w i t h

r e d u c e d

g r i n d i n g e f f i c i e n c y . T h e a x i a l c o m p o

n e n t o f o re

flow

w i t h

average

v e l o c i t y ,

L/ t

oc

L D - ^

increases

s t r o n g l y o n

scale-up. Table

2 s h o w s t h a t t h i s v e l

o c i t y

i n t h e B o u g a i n v i l l e

m i l l

i s c o n

s i d e r a b l y

greater

t h a n i n t h e o t h e r

m i l l s :

n e a r l y 2 . 5 t i m e s t h a t f o r P i n t o

V a l l e y , a n d m o r e t h a n 1 0 t i m e s t h a t f o r

B u t t e a n d S u p e r i o r . I n s p i t e o f t h e

v ry l a r g e size o f th e a u t o g e n o u s m i l l ,

it s

a x i a l c o m p o n e n t o f o r e v e l o c i t y is

l o w .

H o r i z o n t a l flow

v e l o c i t i e s t h r o u g h

p o r o u s m e d i a a r e g o v e r n e d b y t h e

pressure

g r a d i e n t

f r o m

f e e d t o d i s

charge;

s c a l e d - u p g e o m e t r i c a l l y , t h i s

i s c o n s t a n t . I f t h i s a p p l i e s e v e n a p

p r o x i m a t e l y to f l ow-throu gh gr i nd i ng

m e d i a , a

p o i n t w i l l

b e

reached

i n

scale-up

w h e r e t h e i m p o s e d f e e d r a t e

T a b l e

1General

M i l l E q ua t i ons

P o w e r - I n t e r n a l R o t a t i o n a l F lo w E q u a t i o n s

1. P ^ N L D ^ T o r q u e - a r m e q u a t i o n . T h e o r e t i c a l / e x p e r i m e n t a l c o n f i r m a t i o n .

2.

O ,

^ NID

=cN V I n t e r n a l r o t a t i o n a l f l o w ( V =

n LD U^

= VL , )

3 . P 0 , D F r o m e q u a t i o n 1 /2 . P u m p e q u a t i o n : h e a d i s s y n o n y m o u s w i t h D .

O p e r a t i o n a l C o n s t r a i n t s C u r r e n t P r a c t i c e )

4

c o n s t a n t

5 . Q , =t P

D e r i v e d E q u a t i o n s

6, P ^ LD - ^

7, P, 'V, ^ Q/ V . D -^

8- Q, , /Q, ^ D - '

9 . ( Q , / V ) { V / Q , ) N t D - '

C o n s t a n t f r a c t i o n c r it i c a l s p e e d ( C o n s t a n t F r o u d e n u m b e r ) . K i n e m a t i c s i m i l a r i t y .

T h r o u g h p u t s c a l e u p . C o n s t a n t s p e c i f i c e n e r g y . P r o p o r t i o n a l i t y c o e f f i c i e n t i n c r e a s e s

f o r c o a r s e r g r i n d s , s o f t e r o r e s , a n d v i c e v e r s a .

F r o m 1 a n d 4 . P o w e r v e r s u s m i l l s i z e e q u a t i o n f o r s i m i l a r o p e r a t i n g c o n d i t i o n s .

F r o m 5 a n d 6. S p e c i f i c p o w e r a n d s p e c i f i c t h r o u g h p u t i n c r e a s e s i m i l a r l y w i t h m i l l

d i a m e t e r .

F r o m 3 a n d 5 . F l o w r a t i o ( r o t a t i o n a l / a x i a l ) d e c r e a s e s s t r o n g l y w i t h i n c r e a s i n g m i l l

d i a m e t e r .

F r o m 2 a n d 8 . N o m i n a l r e s i d e n c e t i m e , t = V c r /0, =t D~' ' ^ Vp = fV^.

N o t e s

I n e q u a t i o n s 1 , 2 . a n d 3 m i l l l o a d i n g i s a s s u m e d t o b e c o n s t a n t i n s c a l e u p .

E q u a t i o n s 1 , 2 , a n d 3 a p p l y t o a l l r o t a t i o n a l m a c h i n e r y o p e r a t i n g u n d e r g r a v i t a t i o n a l c o n s t r a i n t .

C u r r e n t s c a l e u p p r o c e d u r e s i n v o l v e o n l y e q u a t i o n s 1 . 4 , 5 , 6 , a n d 7 .

E q u a t j ^ o n s 2 , 3 , 8 , a n d 9 i n v o l v e i n t e r n a l r o t a t i o n a l f l o w a n d p r o v i d e n e w i n s i g h t i n t o m i l l d y n a m i c s .

N r = n in e q u a t i o n 9 i s a v e r a g e n u m b e r o f m i l l r e v o l u t i o n s d u r i n g r e s i d e n c e o f o r e .

Current scale-up pract ice involves equat ions 1 4 and 5 with equat ions 6and 7 derived from them. Equat ion 1 is

the torque-arm relat ionship for power consumption whi le equat ions 4 and 5

respectively

express the imposed

constancy-or

near constancy-of fract ion cr i t ical speed and of applied energy per

unit

of ore in the mill. Equat ion 2

internal rotat ional flow leads to equat ion 8 f low rat io and to equat ion 9 giving the average number of revolut ions

during ore

residence

in the mill. While all the equat ions are capable of

some

refinement this cann ot significantly

affect

the strong inverse relationship to

mill

d iameter shown by equat ions 8 and 9 from which the concept of a

critical diameter follows.

4 4 J A N U A R Y 1 9 8 2

M I N I N G E N G I N E E R I N G

7/25/2019 Grinding Mill Scale-up Problems by CC Harris N Arbiter

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Table2Mil l Spec ifi cat ions, Operat ing and Perf orman ce Data, and Derived Parameters

Bouga inv i l le P in to Va l ley ' '

Butte and Super ior '^ ' Palabora * '

18 ft (5.5 m) over flo w mill 18 ft (5.5 m) overf low mill

8 ft (2,4 m) over flow mill 32 ft (9.7 m) aut oge nous mill

Porphyry Copper

Quartz monozi te Zinc blende in granite

Carbonati te

Design specif ication

Current operation

-1927 Operat ion

Current Operation

D X L ft '

17.4 X 21

17.4 X 21 7.4 X 5.8

31.5 X 21.4 X 14 '

V cu ft 4994 4994

2494 10476'

L,%: (9 + sine)'

4 0 ; 2.40

3 7 ;

2.34

3 6 ;

2.32

3 5 ; 2.30

W St '

289.6

267.9 13

297.6

V cu ft 998.7

923.9

44.9

4400

N rpm

12.5

12.3

20

10

68 67

71

73.3

P hp inst al led/ consumed

4250/4410

4000/ 200/246

/6970

New feed st/h

483

365 12.5

623

Circu lat ing load ratio 4 1.5 2 1.67

Q, st/h

2415 912.5

37.5

1663

CT

Ib/ cu ft 172 ' 172 ' 172