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Particulate emissions within foundries — dust arrestment requirements
@BAG FILTRATION INFOUNDRIES Particulate Emissions Within Foundries Dust A r r e s t m e n t Requirements A.J. Bishop Airmaster Engineering Ltd, Limewood Approach, Seacroft, Leeds LS14 1NG, UK Presented at the Filtration Society meeting on 'Pollution Control in the Foundry and Allied Industries' in Birmingham, UK on 10 May 1994 Local exhaust ventilation, together with a means of arrestment and disposal of airborne particulates, provides the basis of the vast majority of dust control solutions in foundries and a multitude of other industries. The paper concentrates on dry bag filtration, and demonstrates how a basically simple and very economical form of dust and fume control can be applied to more arduous conditions to improve the working environment within foundries, as well as reducing emissions to the atmosphere, within the scope of current and anticipated legislation.
o u n d r i e s p r o d u c e a g r e a t deal of dust. In y e a r s gone by t h i s w a s a n accepted fact of life, since social a n d medical science h a d n o t a d d r e s s e d it as a problem. Metal s m e l t i n g a n d c a s t i n g w a s s i m p l y a dirty job, like so m u c h work in heavy industry. However, progressive realisation of h a r m f u l effects, b o t h to h u m a n s a n d to t h e e n v i r o n m e n t at large, backed u p by t h e d e v e l o p m e n t of d r a c o n i a n legislation, h a s t u r n e d t h e face of i n d u s t r y in t h e UK b o t h i n w a r d s (with t h e Control of S u b s t a n c e s H a z a r d o u s to Health Regulations 1988, CoSHH) a n d o u t w a r d s (with t h e E n v i r o n m e n t a l Protection Act 1990). Local e x h a u s t ventilation (LEV), t o g e t h e r with a m e a n s of a r r e s t m e n t a n d d i s p o s a l of airborne particulates, provides t h e b a s i s of t h e v a s t majority of d u s t control s o l u t i o n s in f o u n d r i e s a n d in a m u l t i t u d e of o t h e r industries. In t h i s p a p e r we c o n c e n t r a t e on
dry bag filtration as u s e d to i m p r o v e t h e working e n v i r o n m e n t w i t h i n t h e f o u n d r y , a s well a s r e d u c i n g e m i s s i o n s to t h e a t m o s p h e r e ; this is considered within t h e scope of c u r r e n t a n d a n t i c i p a t e d legislation.
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T h e dry bag filter Dry bag filters c o m e in a wide r a n g e of s h a p e s a n d sizes, each with its p a r t i c u l a r niche applications, its s u p p o r t e r s a n d o p p o n e n t s . Many m a n u f a c t u r e r s p r o d u c e m o s t varieties, to e n s u r e t h a t t h e r e is a d e d i c a t e d solution within t h e i r a r m o u r y of p r o d u c t s . However, t h e y all fall w i t h i n two m a i n groupings: t h o s e t h a t will clean while filtering, a n d t h o s e t h a t will not. T h e s e are t e r m e d o n / off-line cleaning a n d d o w n t i m e cleaning, respectively. 'On/off-line' cleaning is i m p o r t a n t w h e r e a p r o c e s s is largely
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November1994
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@BAG FILTRATION INFOUNDRIES c o n t i n u o u s , since t h e e x t r a c t i o n airflow c a n be m a i n t a i n e d w i t h o u t i n t e r r u p t i o n of t h e process. If t h e d u s t loading is high, on-line cleaning will generally be necessary. In general, on/off-line cleaning bag filters will be of t h e 'reverse jet' or 'reverse air' form, each of w h i c h h a s its perceived benefits. The reverse-jet type m a k e s u s e of s h o r t p u l s e s of c o m p r e s s e d air into t h e bags, in opposition to t h e n o r m a l airflow (typically 6 b a r g for 9 0 - - 1 2 0 m s ) , at r e g u l a r intervals to s u i t t h e cleaning d u t y required. Figure 1 illustrates t h e principle. Usually one row of bags is cleaned with each p u l s e I a n d each row is cleaned in turn, controlled by a s e q u e n c e t i m e r acting on solenoid pilot valves. Reverse-air filters e m p l o y a longer p u l s e of cleaning air at lower p r e s s u r e (typically 4.5 kPa for 1 -- 1.2 s) to blow m a t e r i a l cake off t h e b a g surface. A d j a c e n t b a g s are n o r m a l l y t a k e n offline d u r i n g t h i s progressive process. A mobile carriage carries t h e cleaning air to each bag row in turn, while blanking t h e row each side of t h a t being cleaned. More correctly t h i s w o u l d be t e r m e d 'partial' offiine cleaning, or s i m p l y offiine cleaning. In b o t h t y p e s of filter t h e d u s t t h a t is released d r o p s into a collection c h a m b e r or h o p p e r for removal. 'Downtime cleaning' bag filters also have t h e i r place in t h e f o u n d r y e n v i r o n m e n t . The filter m a y be u s e d on i n t e r m i t t e n t or low d u s t b u r d e n applications, w h e r e t h e fan c a n be s t o p p e d from t i m e to time, to allow t h e m e c h a n i c a l cleaning o p e r a t i o n to function. This is generally a s h a k e r f r a m e s u s p e n d i n g t h e filter b a g a r r a n g e m e n t , a u t o m a t i c a l l y set in m o t i o n after t h e fan h a s been allowed sufficient t i m e to stop. There is no p a r t i c u l a r h i d d e n secret to filter sizing. There are m a n y well proven guidelines to enable selection a n d sizing to be precise a n d therefore efficient. Filter selection m u s t take into a c c o u n t m a n y p a r a m e t e r s , m a n y of w h i c h also apply to t h e collection h o o d s or booths. Filter m e d i a are available with diverse physical a n d chemical properties. A cost-effective d u s t a r r e s t m e n t a n d collection installation will result only from a c o m p e t e n t analysis of c o n d i t i o n s a n d r e q u i r e m e n t s . Some of t h e s e p a r a m e t e r s are as given below (in no p a r t i c u l a r order of i m p o r t a n c e ) -- ff a n y o n e is missed, it m a y prove e m b a r r a s s i n g , a n n o y i n g or expensive:
loss from a workshop, for e x a m p l e ) , t h e n accepted levels are likely to be 1 m g / m 3 or less. [] The a n t i c i p a t e d m e d i u m ' s o p e r a t i n g differential pressure. Many of t h e above c o n s i d e r a t i o n s will have a bearing on this. Clearly t h e total s y s t e m p r e s s u r e m u s t be calculated to d e t e r m i n e t h e size of t h e e x t r a c t i o n fans. [] The e x p e c t e d m o d e of o p e r a t i o n -- will it be 24 h o u r s continuously, or a daily shift p a t t e r n , or occasional use? [] Noise, since f o u n d r i e s are often s i t u a t e d n e a r residential areas. The list of interrelated p a r a m e t e r s is long, b u t a g r a s p of t h e whole s y s t e m is n e c e s s a r y if t h e e n d - r e s u l t is to be a n effective plant.
Bag s h a p e a n d orientation Figures 2 a n d 3 s h o w s o m e possibilities. The c h o s e n f o r m a t is d e p e n d e n t on t h e p a r t i c u l a r application, a n d often -- it m u s t be
[] The q u a n t i t y of p a r t i c u l a t e m a t t e r in t h e airstream; t h i s socalled ' d u s t b u r d e n ' m a y vary from a few milligrams u p to p e r h a p s 1 k g / m s. The d u s t b u r d e n m a y vary d u r i n g a process. [] The d e n s i t y of t h e dust. This bulk d e n s i t y will d e t e r m i n e t h e m a s s of cake a d h e r i n g to t h e bag, a n d will have a bearing on t h e disposal from t h e hopper. The collection bin m a y be impossible to lift, ff oversized. [] The particle size distribution. The efficiency of a r r e s t m e n t a n d filtration is d e p e n d e n t on t h e particle size. I n h a l a b l e d u s t varies from a r o u n d 70 # m downwards, b u t particles g r e a t e r t h a n a b o u t 200 # m (grit, for e x a m p l e ) m a y be carried down t h e d u c t w o r k to t h e filter. [] Mobile v o l u m e of p a r t i c u l a t e s to be collected. The bulk d e n s i t y is useful here to d e t e r m i n e cleaning frequency, in order to p r e v e n t 'bridging' of d u s t between bags. This is chiefly of significance in horizontally a r r a n g e d 'flat' bags. Correct cleaning conditions will reduce t h e t i m e required for p l a n n e d d o w n t i m e maintenance. [] The s h a p e of t h e particle. Some particles (in particular, zinc oxide) have a s t r u c t u r e like b a r b e d wire, w h i c h m a k e s t h e m a t e r i a l self-agglomerating, while o t h e r s a g g l o m e r a t e electrostatically. Some d u s t f o r m s flakes, w h i c h effectively c a u s e 'blind s p o t s ' on t h e media. [] The t r a n s p o r t gas or conveying g a s analysis. It m a y be p u r e dry a m b i e n t air; or it m a y c o n t a i n acidic s u l p h u r or nitrogen oxide g a s e s (SOx, NOx), h y d r o g e n chloride (HC1), or it m a y c o n t a i n v a p o r i s e d oil from s c r a p or possibly w a t e r vapour. [] T e m p e r a t u r e of t h e c o n v e y i n g gas. T h e d e w - p o i n t a n d c o n d e n s a t i o n possibilities, allied to acidic v a p o u r s , m u s t be a d d r e s s e d in t h e filter a n d h o p p e r design. [] E m i s s i o n s t a n d a r d (s) to be achieved. National s t a n d a r d s for e x a m p l e , TA Luft in G e r m a n y requires less t h e n 20 m g / m ; in t h e UK, t h e E n v i r o n m e n t a l Protection Act requires less t h a n 100 m g / m 3 ; while special s t a n d a r d s for lead a n d o t h e r potentially toxic p a r t i c u l a t e s require 5 m g / m a or less. If e m i s s i o n is back to t h e workplace (in order to conserve h e a t
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Figure 2. Vertical bag arrangement. November 1994
Filtration & Separation
@BAG FILTRATION INFOUNDRIES cases, t h e i n t e r m i t t e n t n a t u r e of t h e o p e r a t i o n m e a n s t h a t a n onor offiine filter is n o t required, a n d a simple m e c h a n i c a l shake, d o w n t i m e cleaning filter is ideally s u i t e d (Figure 4). For economy, t h e b o o t h design s h o u l d allow t h e m i n i m u m t h r o u g h p u t of e x t r a c t e d air from t h e fettling process. However, free working s p a c e m u s t be available for t h e largest casting to be encountered. After d e t e r m i n i n g t h e t h r o u g h p u t of air a n d -h a v i n g regard to t h e d u s t t r a n s p o r t velocity, to p r e v e n t deposition w i t h i n t h e d u c t w o r k -- selected t h e correct d u c t cross-section, t h e m a i n c o n c e r n will be abrasive wear. Duct m a t e r i a l selection a n d t h e u s e of 'slow' b e n d s are key factors in m i n i m i s i n g t h i s problem. Also, at t h e filter inlet a n angled wear-plate will reduce i m p a c t abrasive effects on t h e bags. In s o m e c a s e s a p r e - s e p a r a t i o n or d r o p o u t c h a m b e r m a y be fitted to reduce t h e particle inlet velocity, so t h a t m o s t of t h e larger p a r t i c u l a t e s d r o p away w i t h o u t c o m i n g into c o n t a c t with t h e filter media. Vertically located bags fitted in t h e filter housing, cleaned either by a s h a k e r m e c h a n i s m (or for h e a v i e r l o a d i n g s or m o r e c o n t i n u o u s o p e r a t i o n by c o m p r e s s e d air p u l s e j e t s ) m a y be u s e d with confidence.
Sand preparation plant
Figure 3. Horizontal bag arrangement. said -- d e e p - r o o t e d bias on t h e p a r t of t h e e n g i n e e r ( s ) involved. Most m a n u f a c t u r e r s with t h e e x p e r t i s e required to a d d r e s s d u s t a r r e s t m e n t in foundries, will have different f o r m a t s of bag a n d bag cleaning available, to enable a focused decision b a s e d on t h e u s e r ' s specific r e q u i r e m e n t s . In general, online pulse-jet-cleaned, vertically m o u n t e d t u b u l a r b a g s a n d offiine reverse-air-cleaned, horizontally m o u n t e d , envelo p e - s h a p e d b a g s (often in t h e form of c a s s e t t e s ) are offered for m o s t f o u n d r y a p p l i c a t i o n s w h e r e c o n t i n u o u s operation a n d / o r a heavy d u s t b u r d e n is likely.
System design and selection A r m e d with t h e basic principles, filter functions, a n d having a s k e d all t h e p e r t i n e n t questions, it will n o w be possible to a d d r e s s p a r t i c u l a r d u s t a r r e s t m e n t p r o b l e m s within t h e f o u n d r y environm e n t . The e x a m p l e s w h i c h follow are b a s e d on actual applications, a n d w h i c h are k n o w n to be successful. In each case t h e goal w a s t h e solution to a d u s t control p r o b l e m . T h e d u c t w o r k , cooler, filter, f a n a n d all ancillary e q u i p m e n t were merely t h e c h o s e n players.
Fettling This is a s t r a i g h t f o r w a r d a p p l i c a t i o n for dry bag filters. In m a n y
This application is also well s u i t e d to dry bag filtration, particularly m a k i n g u s e of a centralised filter p l a n t c o n n e c t e d via d u c t i n g to t h e v a r i o u s d u s t a r r e s t m e n t a n d collection points. Figure 5 s h o w s a Disa Mark 3 2013 casting s y s t e m with pouring, cooling a n d vibratory or k n o c k - o u t d r u m , t o g e t h e r with a 50 t o n n e / h o u r s a n d p r e p a r a t i o n s y s t e m c o m p r i s i n g conveyor, elevator, ploughs, m i x e r a n d s a n d cooler e x t r a c t i o n points. The e x t r a c t i o n s y s t e m covers t h e whole of t h e s a n d p r e p a r a t i o n function, t o g e t h e r with t h e option of e x t r a c t i o n from t h e p o u r i n g c h a m b e r a n d cooling tunnel. The p r o b l e m for t h e d u s t control e n g i n e e r is t h e relatively h i g h m o i s t u r e c o n t e n t of t h e e x t r a c t e d air, c a u s e d in p a r t i c u l a r by t h e evolution of s t e a m at t h e k n o c k - o u t s t a t i o n or from t h e s a n d cooling process. To e n s u r e effective filtration, it is therefore n e c e s s a r y to keep t h e o p e r a t i n g t e m p e r a t u r e safely above t h e w a t e r d e w - p o i n t t e m p e r a t u r e , typically with a m i n i m u m t e m p e r a t u r e difference of 20°C. To achieve this, t h e correct v o l u m e of air from t h e driest e x t r a c t i o n p o i n t s -- s u c h as t h e p o u r i n g rack, conveyor, elevator, screen, p l o u g h s etc. -- is m i x e d with t h e air from t h e m o i s t e x t r a c t i o n points, s u c h as t h e k n o c k - o u t d r u m , w a t e r injection a n d s a n d cooler. It is a n a d v a n t a g e to reduce d u c t l e n g t h s to a n a b s o l u t e m i n i m u m , a n d if possible to site t h e filtration p l a n t w i t h i n t h e foundry. Some case i n s u l a t i o n m a y be necessary, t o g e t h e r with trace h e a t i n g to t h e header. There m a y -- in e x t r e m e c a s e s -- be a r e q u i r e m e n t to fit a s u p p o r t b u r n e r to increase t h e conveying air t e m p e r a t u r e , to give a safe m a r g i n above t h e w a t e r dew-point. The e x t r a c t i o n p o i n t s at t h e s c r e e n i n g a n d s a n d cooling p l a n t m a y r u n via a cyclone to t h e m a i n e x t r a c t i o n ducting. Larger highly abrasive particles m a y be p r e s e n t at t h e s e points, a n d a preseparator, m o u n t e d as close as practicable to t h e s c r e e n s a n d cooler, will remove s o m e of t h e b u r d e n from t h e whole system. As with t h e fettling application, abrasive 'jagged' d u s t is being h a n d l e d , so t h e filter inlet will c o n t a i n m e a s u r e s to s a f e g u a r d t h e filter m e d i a from its w o r s t effects. T r a n s i t i o n a l d u c t sections serve to reduce t h e inlet velocity at t h e filter, a u g m e n t e d by w e a r shields or c h a i n c u r t a i n s on t h e inlet side of t h e filter.
Induction melting furnaces The i n d u c t i o n m e l t i n g f u r n a c e is often t h e m o s t economical m e a n s of m o l t e n m e t a l production, a n d it is b e c o m i n g m o r e c o m m o n in British foundries. It is c o m p a c t a n d very versatile, b u t is also mobile in its v a r i o u s m o d e s of operation. A static m a c h i n e p o s e s few p r o b l e m s to t h e d u c t a n d h o o d designer. However, t h e i n d u c t i o n f u r n a c e c a n provide a challenge in all its n o r m a l operations, namely. [] Charging. [] Melting. [] Slagging off. [] Pouring. [] Back-pouring.
Figure 4. Fettling booths linked to a mechanical shaker filter. Filtration & Separation
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D u r i n g e a c h of t h e s e o p e r a t i o n s effective d u s t c a p t u r e m u s t be s o u g h t , with d u e regard to t e m p e r a t u r e effects. Many a t t e m p t s have been m a d e to overcome t h e challenge of
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effectively capturing the d u s t during all the furnace operations -b u t with less t h a n total success. However, probably the best allr o u n d solution is to fit a dedicated, 'close capture' hood. Figure 6 shows a close capture hood fitted to a 5 t o n n e furnace. The hood s u r r o u n d s the lid, and can be m o u n t e d on a double-hinged frame. This enables the lid, with the hood, to be tilted in two planes, while following the motion of the furnace w h e n tilted a b o u t its axis. The extraction rate is m a t c h e d to the requirement, which d e p e n d s on the actual distance between the hood and the source of d u s t and fumes. The extraction air is transferred from the hood manifold to the adjacent duct via a 'slide valve' a p e r t u r e (Figure 7), with the extraction duct being guided in a geometrically correct p a t h as the axes of rotation of the lid and furnace change relative to each other. In some furnaces a single axis of rotation can be achieved, which simplifies matters.
During melting, the extraction rate is at a minimum, with the lid closed. When charging and slagging off the lid is partially open, and the extraction rate is increased accordingly. When pouring, a l t h o u g h the lid is closed, m o s t of the emissions occur in the
Figure 6. Close capture hood fitted to a 5 tonne induction melting furnace.
Figure 7, The extraction air is transferred from the hood manifold to the adjacent duct via a 'slide valve' aperture.
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Filtration & Separation
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Figure 8. Working principle of cold blast cupolas.
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locality of t h e p o u r i n g process, so therefore m a x i m u m airflow is provided. Also, d u r i n g back-pouring, w h e n t h e lid is fully open, d u s t c a p t u r e is provided by m a x i m u m airflow volume. M a t c h i n g t h e airflow v o l u m e -- via a p r o g r a m m a b l e logic control centre -- to t h e actual r e q u i r e m e n t s n o t only achieves t h e desired i m p r o v e m e n t s in t h e environm e n t , it a l s o p r o v i d e s e c o n o m i e s in r u n n i n g a n d m a i n t e n a n c e costs, as well as e n s u r i n g a m i n i m u m size of filter. B e c a u s e all e x t r a c t i o n p r o c e s s e s involve a degree of e x c e s s dilution with a m b i e n t air, t h e t e m p e r a t u r e is n o t a problem. A conventional bag filter m a y be employed, a n d t h e r e f o r e n o r m a l m a t e r i a l s m a y be u s e d with t h e i r a s s o c i a t e d benefits. The close-capture h o o d c a n therefore provide t h e economic, c o m p a c t a n d practical solution to t h e challenge p r e s e n t e d by d u s t control in i n d u c t i o n furnaces.
Cold blast cupolas
This is a different challenge for t h e d u s t / f u m e control engineer. However, c o m m o n types of cold b l a s t c u p o l a s can be c a t e r e d for u s i n g a c o m b i n a t i o n of a g a s cooler a n d dry bag filter. W h e t h e r t h e w a s t e gas offtake is above t h e c h a r g i n g door (the s i m p l e s t form), or below a n d fitted with a n a f t e r b u r n e r or c o m b u s t i o n c h a m b e r (Figure 8), t h e c o n d i t i o n s as seen at t h e w a s t e g a s outlet are similar. Very h i g h off-gas t e m p e r a t u r e s a n d varying v o l u m e a n d t e m p e r a t u r e c o n d i t i o n s are t h e n o r m . During n o r m a l o p e r a t i o n , t h e c u p o l a h e a d off-gas t e m p e r a t u r e is 8 Primary blest level typically 600°C, while d u r i n g blowdown t e m p e r a t u r e s in 10 ~ S~phon Recewe~ vessel e x c e s s of 1000°C m a y be e n c o u n t e r e d for 30 m i n u t e s or 11 Waste bucket 12 Gas exhaust more. The off-gas is therefore p a s s e d t h r o u g h a section of duct 13 Combustion h e a t - r e s i s t a n t nickel-chrome steel ducting, t h r o u g h w h i c h chamber 14 Preheat comh e a t is lost rapidly by r a d i a t i o n a n d n a t u r a l convection/ bustton chamber 15 Afterburner conduction. The d u c t w o r k m u s t include e x p a n s i o n j o i n t s 16 emergency door a n d floating s u p p o r t s in o r d e r to m a i n t a i n its d i m e n 17 exhaust pipe sional stability. The t e m p e r a t u r e of t h e air e n t e r i n g t h e g a s cooler is r e d u c e d to a r o u n d 450°C. The g a s cooler is essentially a f a n n e d air-to-air h e a t e x c h a n g e r , in w h i c h t h e air to be cooled p a s s e s over cooling tubes, t h r o u g h w h i c h a m b i e n t air is forced by t h e axial blower fans. The cooler is generally sectionalised to increase t h e n u m b e r of p a s s e s over t h e tubes, or steel envelopes as i l l u s t r a t e d in Figure 9. T h e cooler g a s outlet t e m p e r a t u r e is pegged below 150°C. Normally a cyclone or set of cyclones is m o u n t e d between t h e g a s cooler a n d t h e filter. In t h i s case t h e cyclone is to reduce t h e possibility of s p a r k or fire d a m a g e to t h e filter unit. As a f u r t h e r precaution, t h e cyclone will be fitted with a variable fresh air d a m p e r , to a d m i t a m b i e n t dilution air if t h e n o r m a l filter o p e r a t i n g t e m p e r a t u r e is exceeded. A s e p a r a t e control loop m o n i t o r s t h e actual d u c t static p r e s s u r e d o w n s t r e a m from t h e c u p o l a w a s t e g a s outlet. This static p r e s s u r e is c o n t i n u o u s l y c o m p a r e d with a p r e s e t value. Variations c a u s e a m o t o r i s e d d a m p e r at t h e e x t r a c t i o n fan position to r e s p o n d in o r d e r to keep t h e total airflow w i t h i n prescribed limits. The c u p o l a e x t r a c t i o n s y s t e m is therefore self-regulating.
Conclusion T h e s e a p p l i c a t i o n s of dry b a g filtration d e m o n s t r a t e h o w a basically s i m p l e a n d very economical form of d u s t a n d f u m e control c a n be a p p l i e d to m o r e a r d u o u s conditions. They r e p r e s e n t merely a s m a l l selection of t h e total r a n g e of u s e s for t h e u b i q u i t o u s bag filter. With t h o u g h t , d e t e r m i n a t i o n a n d a little ingenuity, a n d m a k i n g u s e of t h e v a s t d e v e l o p m e n t experience in filter m e d i a a n d m e c h a n i s m s , dry b a g filtration will a l m o s t always p r o v i d e s o l u t i o n s for t h e c h a l l e n g e of a i r b o r n e p a r t i c u l a t e pollution.
~ } ~ r l y air tniet mamfo~d f,~2~Cooler housing Coohng a~r fan (,~) Co~)h~Qa~r inl~t box w~th dmtrtbutc)r Stottcd wall w~th c~arf~pmgfrar~. ~ ) Coo~mg casseHu ,~7") Ther rnocouP~O
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~.~ S~ppott stz~ctdt~ with d~st coJl~;t~or~ hopfu.er and screw conveyor
Filtration & Separation
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November 1994
Figure 9. Cutaway diagram of a gas-air cooler. 741
@TRACTSOFREFEREED PAPERS Comparison of modern beer filters Vergleich moderner Bierfilter von J. Hermia u n d S. Brocheton Bierfiltration wird zur Zeit mit H/Ire yon Kerzenfiltern und Horizontalblatthltern durchgefi)hrt, Der Vergleich zwischen diesen be/den Technologien zeigt, dab der Kerzenfilter gedngere Druckanstiegswerte gestattet, w&hrend der Horizontalblattfilter in e/nero gegebenen Tankvolumen
eine grOBere Filtrationsflache bietet. Der Kerzenfilter erweist sich als elf/z/enter, solange schmale Kerzen verwendet werden. (5 sn., 7 abb., 3 tab., I teL)
Comparaison de filtres il biere modernes p a r J. H e r m i a On utilise couramment les filtres a bougies et les filtres a plateaux horizonta ux pour la filtration de la b/ere. La comparaison entre ces deux technologies montre que les filtres a bougies engendrent des montees en press/on plus faibles mais les filtres a plateaux horizontaux offrent une plus
e t S. B r o c h e t o n grande surface de filtration pour un m#me volume de cuve. II ressort que le filtre a bougies est plus perlormant q u a n d on utilise des bougies de petit diametre. (5 pags , 7 figs., 3 tabs., 1 r#L)
Comparaci6n de filtros modernos para cerveza p o r J. H e r m i a Se filtra cerveza actualmente con filtros a forma vela y con elementos horizontales. La comparaci6n entre las dos tecnicas ind~ca que el filtro a forma vela permite mas peque#as
y S. B r o c h e t o n subidas de presion, pero el filtro con elementos horizontales ofrece una area mas grande para una c/sterna de dado volumen. (5 pags, 7 figs., 3 tabs., 1 reL)
Permeate flux decay during crossflow microfiltration of a cationic surtactant dispersion PermeatfluBzerfall bei Querstrom-MIkrotiltration einer Dispersion kationischer, oberfl~ichenaktiver Stoffe von G. A k a y u n d R.J. W a k e m a n Der Obergangs-PermeatfluB u n d das Sperrungsverhalten e/net Dispersion kationischer, oberflachenaktiver Stoffe be/Querstrom-Mikrofiltration wird a n h a n d von Membranen unterschie dficher Grade von Hydrophobie untersucht, die durch den Loslichkeitsparameter gekennzeichnet /st. Es wird festgestellt, dab das PermeatfluB-(Ibergangsverhalten bei geringer Beschickungskonzentration stark durch die Membran-Hydrophobie beeinfluBt wird. Der Mechanismus der Querstrom-Membranfi/tration wird nicht nut dutch eine Untersuchung des ObergangsPermeatflusses sondern auch dutch eine Untersuchung der Ablagerung des oberfiachenaktiven
Stotls aul der Membran u n d in ihren Poren ausgewertet. Es wird dargelegt, dab die Ablagerung des oberflachenaktiven Stoffs zur Bildung e/net Sekund~rmembran fuhrt, die durch die Primarmembran unterst~)tzt wird. Der Permeatfiu8 und die Sperrung des oberflachenaktiven Stoffs werden grOBtenteils yon der Sekundarmembran bestimmt. Die Sekund~rmembran bildet sich an der Membranoberfl&che und dringt in die Poren e/n, wobei ein Bereich hoher Konzentration des oberflachenaktiven Stoffs entsteht, die um eine Gr(~Benordnung hSher /st als die sogenannte Gelkonzentration. (5 sn., 6 abb., 1 tab., 14 teL)
D(~croissJince du flux de perme~at Iors de la microfiltration tangentielle d'une dispersion de surtactant cationique p a r G. A k a y et R.J. Wakeman On etudie le flux transitoire de permeat et le comportement du relet d'une dispersion de surfactant cationique p e n d a n t la microfiltration tangentielle sur membranes de ditferents degres d'hydrophobicite, caracterisee p a r un parametre de solubilite. On a trouve que le comportement transitoire du flux de permeat est fortement affecte par I'hydrophobicite de la membrane s~ la concentration de I'alimentation est faible On evalue le mecanisme de la filtration tangentielle membranaire en etudiant, outre le flux de permeat transitoire et le rejet, le depSt de surfactant sur
la membra ne et dans les pores. On montre que le depSt de surfactant conduit a la formation d'une membrane secondaire supportee par la membrane/nit/ale. La membrane secondaire conditionne en grande pattie le flux de permeat et le reiet de surfactant. La membrane secondaire est formee sur la surface de la membrane et penetre dans les pores, formant une region de grande concentration en surfactant qui est superieure d'un ordre de grandeur a la concentration dire 'de gel'. (5 pags,, 6 figs., 1 tab., 14 refs,)
Decllnaci6n del flujo de filtrado durante microfiltraci6n a flujo cruzado, de una dispersi6n de surfactante cationica p o r G. A k a y y R.J. W a k e m a n Se han estudiado el fiu]o transitorio de filtrado y comportamiento de rechazo de una dispersion de surfactante cation/ca durante microf/Itraci6n a fiujo tangencial, empleando membranas de varies grados de hidrofobicidad, caracterizada per solubifidad. Se halla que la hidrofobicldad influye fuertemente el flujo transitorio cuando la concentraci6n de afimentaci6n esta bala. Se ha estudiado el mecanismo de filtraci6n a flujo cruzado de membranas, teniendo en cuenta no solamente flujo transitorio del filtrado y rechazo, pete tambien la deposici6n de surfactante sobre
la membrana y dentro los poros. Se ha mostrado que la deposiciSn produce una membrana secundaria apoyada per la membrana original. En gran parte la membrana secondaria regula el flu/o de filtrado y el rechazo de la surfactante La membrana secundaria se formar en la superficie de la membrana y penetra los pores, dando una region de alta concentraci6n surfactante, la cual es order de magnitud mas alta que la intitulada concentracion gel. (5 pags., 6 figs., I tab., 14 refs.)
Hydrocyclone with high sharpness of separation Hydrozyklon mit hoher Abscheidungssch~irfe von Lieng-Yin Chu und Oian Luo Aus experimentellen Forschungsarbeiten, die in jOngster Vergangenheit im Hinblick auf die Zubehorteil ausgerOstet worden. Mit dem neuen Hydrozyklon warden Abscheidungsexpedmente Bewegung von Feststoffpartikeln in e/nero Hydrozyklon durchgefOhrt wurden, ergab sich der durchgeluhrt u n d ein Vergleich mit herkcmmliehen Hydrozyklonen gezogen. Aus den Befund, dab einige Partikel im inneren SpiralfluB sich auf die Hydrozyklonwand im zylindrlschen Experimenten ging hervor, dab die Abscheidungsscharfe be/ dem neuen Hydrozyklon mit Tell zubewegen. In Anbetracht dieses Ergebnisses hat man e/hen neuartigen Hydrozyklon mit zentralem Kegel und Wirbelsucheinrichtung hoher /st als be/ dem herkommlichen Hydrozyklon. zentra!em Kegel entwickelt. Da aufgrund des KufzschluBflusses einige grebe Partikel direkt mit Die optimalen geometrischen Parameter sowie die Position des zentralen Kegels wurden ebenfafls dem Ubedauf abflieBen, /st der neue Hydrozyklon auBerdem noch mit e/hem Wirbelsuchauf experimentellem Wege ermittelt. (4 sn,, 8 abb., 5 tab., 5 reL)
Hydrocyelones a haut pouvoir de coupure p a r Liang-Yin Chu et Q/an Luo Des recherches expebmentales recentes sur le mouvement des particules sol/des dans un hydrocyclone ont montre que quelques particules dans la spirale interne se meuvent vers la parol dans la section cylindrique. A la lure/ere de ces r~sultats, on a developpe un nouvel hydrocyclone avec un c6ne central De plus, etant donna que quelques grosses particules sont entrainees dans la surverse a cause des courts-circuits, le nouvel hydrocyclone a egalement ete dote d'un casseur
de vortex. Des tests de separation ont ete effectues en comparant le nouvel hydrocyclone avec les machines conventionnelles, Les resultats experimentaux ont montr~ que le pouvoir de coupure du nouvel hydrocyclone dote d'un cSne central et d'un casseur de vortex ~tait superieur a celui des hydrocyclones classiques. Les parametres geom~triques optlmaux et la position du cSne central ont egalement ete determines par experience. (4 pags., 8 figs., 5 tabs,, 5 refs,)
Hidrociclon de alto grado de separaci6n p o r Liang- I~n Chu y Q/an Luo Se ha mostrado, en recientes investigaeiones experimentales de/movimiento de particulas s61idas en un hidrocicl6n, que unas particulas en el flujo interior heficoidal mueven hacia el muro del hidrociclon en la seccion cilindrica. Con este resultado se ha desarrollado un nuevo hidrocicl6n con cono central Ademas, porque unas particulas grandes descargan directamente en el liquido derramado, de resultas de flUlO cortocircuito, se ha desehado un accesorio que busca el v6rtice.
Se ha realizado experimentos de separacion con el nuevo hidrocicl6n para hacer comparaci6n con el convencional, los resultados ban mostrado Due la definicion de separaci6n con el nuevo, con cono central y accesorio que busca el vortice, era mayor. Se ha determinado los parametros geometricos y el emplazamiento del cono central pot metodos experimentales. (4 pags., 8 figs., 5 tabs., 5 refs.)
Particulate emissions within foundries -- Dust arrestment requirements Partikelemisslon in GieBerelen -- Staubabfangerfordernlsse yon A.J. Lokal durchgefighrte saugende Bewetterung, in Verbindung rnit e/her Methode zur Abfangung u n d Entsorgung yon in der Luft schwebenden Partikeln stellen die Grundlage einer Vielzahl von Staubbekampfungslosungen in GieBereien und zahlreiehen anderen Industriezweigen dar. Das Referat konzentriert sich auf Trockenbeutelfiltration a n d zeigt, wie eine im Grunde einfache u n d
B~l~p sehr wirtschaftliche Form von Staub- and Rauchgasbekampfung unter schwierigen Bedingungen zur Verbesserung der Arbeitsumwelt in GieBereien sowie zur Minderung von Emissionen in die Atmosphare angewandt werden kann, wobei sowohl die zur Zeit geltende als auch die in Zukunft zu erwartende Gesetzgebung berOcksichtigt wird. (5 sn., 9 abb.)
Emissions particulaire dans les fonderies - Contraintes de retenue des poussi(~res par A.J. B~hop Une ventilation aspirante locale conluguee a une m~thode d'arr~t des part/tales so/ides appliquee pour ameliorer les conditions aux postes de travail dans lea fonderies ainsi que pour entra~nees constitue la base d'une vaste majorite de solutions de depoussierage dans les reduire les ~missions a I'atmosphere dans la perspective de nouvelles reglementations. (5 pags., 9 fonderles et une multitude d'autres industries. Cet article se concentre sur la filtration seehe sur figs.) manche et dernontre comment une forme simple et tres economique de depoussierage peut ~tre
Emisiones de particulas en fundiciones - Requisitos para detener polvos p o r A.J. Bishop Se provee, por ventilaci6n del tubo de escape junto con un mode de detener y recoger particulas Ilevadas pot el a/re, la base de la mayoria de soluciones de control de polvos en fund/c/ones y muchas otras industrias. Esta ponencia trata de filtraci6n seca con saeos y demuestra como se puede aplicar un s/sterna esencialmente sencillo y may economico de control de polvos y humos a
720
condiciones may arduas, con motivo de amejorar el ambiente laboral en fund/c/ones, a s / c o m o reducir era/s/ones a la atm6sfera, todo dentro del aleance de legislacion corriente y anticipada. (5 pags., 9 figs.)
N o v e m b e r 1994
Filtration &
Separation
o u n d r i e s p r o d u c e a g r e a t deal of dust. In y e a r s gone by t h i s w a s a n accepted fact of life, since social a n d medical science h a d n o t a d d r e s s e d it as a problem. Metal s m e l t i n g a n d c a s t i n g w a s s i m p l y a dirty job, like so m u c h work in heavy industry. However, progressive realisation of h a r m f u l effects, b o t h to h u m a n s a n d to t h e e n v i r o n m e n t at large, backed u p by t h e d e v e l o p m e n t of d r a c o n i a n legislation, h a s t u r n e d t h e face of i n d u s t r y in t h e UK b o t h i n w a r d s (with t h e Control of S u b s t a n c e s H a z a r d o u s to Health Regulations 1988, CoSHH) a n d o u t w a r d s (with t h e E n v i r o n m e n t a l Protection Act 1990). Local e x h a u s t ventilation (LEV), t o g e t h e r with a m e a n s of a r r e s t m e n t a n d d i s p o s a l of airborne particulates, provides t h e b a s i s of t h e v a s t majority of d u s t control s o l u t i o n s in f o u n d r i e s a n d in a m u l t i t u d e of o t h e r industries. In t h i s p a p e r we c o n c e n t r a t e on
dry bag filtration as u s e d to i m p r o v e t h e working e n v i r o n m e n t w i t h i n t h e f o u n d r y , a s well a s r e d u c i n g e m i s s i o n s to t h e a t m o s p h e r e ; this is considered within t h e scope of c u r r e n t a n d a n t i c i p a t e d legislation.
F
T h e dry bag filter Dry bag filters c o m e in a wide r a n g e of s h a p e s a n d sizes, each with its p a r t i c u l a r niche applications, its s u p p o r t e r s a n d o p p o n e n t s . Many m a n u f a c t u r e r s p r o d u c e m o s t varieties, to e n s u r e t h a t t h e r e is a d e d i c a t e d solution within t h e i r a r m o u r y of p r o d u c t s . However, t h e y all fall w i t h i n two m a i n groupings: t h o s e t h a t will clean while filtering, a n d t h o s e t h a t will not. T h e s e are t e r m e d o n / off-line cleaning a n d d o w n t i m e cleaning, respectively. 'On/off-line' cleaning is i m p o r t a n t w h e r e a p r o c e s s is largely
i=h manifold
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D i a p h r a g m valve in " p r e s s u r e b a l a n c e " with s p r i n g asserted
NORMAL FILTERING MODE
S o l e n o i ~ valve a c t i v a t e d \ to v e n t d i a p h r a g m valve Diaphragm valve
d =
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~, = = ~ ~ = [
Reverse pulse
vented allowing compressed air to jet t u b e s . REVERSE JET CLEANING OPERATION
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November1994
0015-1882/94/US$7.00 © 1994 Elsevier Science Ltd
737
@BAG FILTRATION INFOUNDRIES c o n t i n u o u s , since t h e e x t r a c t i o n airflow c a n be m a i n t a i n e d w i t h o u t i n t e r r u p t i o n of t h e process. If t h e d u s t loading is high, on-line cleaning will generally be necessary. In general, on/off-line cleaning bag filters will be of t h e 'reverse jet' or 'reverse air' form, each of w h i c h h a s its perceived benefits. The reverse-jet type m a k e s u s e of s h o r t p u l s e s of c o m p r e s s e d air into t h e bags, in opposition to t h e n o r m a l airflow (typically 6 b a r g for 9 0 - - 1 2 0 m s ) , at r e g u l a r intervals to s u i t t h e cleaning d u t y required. Figure 1 illustrates t h e principle. Usually one row of bags is cleaned with each p u l s e I a n d each row is cleaned in turn, controlled by a s e q u e n c e t i m e r acting on solenoid pilot valves. Reverse-air filters e m p l o y a longer p u l s e of cleaning air at lower p r e s s u r e (typically 4.5 kPa for 1 -- 1.2 s) to blow m a t e r i a l cake off t h e b a g surface. A d j a c e n t b a g s are n o r m a l l y t a k e n offline d u r i n g t h i s progressive process. A mobile carriage carries t h e cleaning air to each bag row in turn, while blanking t h e row each side of t h a t being cleaned. More correctly t h i s w o u l d be t e r m e d 'partial' offiine cleaning, or s i m p l y offiine cleaning. In b o t h t y p e s of filter t h e d u s t t h a t is released d r o p s into a collection c h a m b e r or h o p p e r for removal. 'Downtime cleaning' bag filters also have t h e i r place in t h e f o u n d r y e n v i r o n m e n t . The filter m a y be u s e d on i n t e r m i t t e n t or low d u s t b u r d e n applications, w h e r e t h e fan c a n be s t o p p e d from t i m e to time, to allow t h e m e c h a n i c a l cleaning o p e r a t i o n to function. This is generally a s h a k e r f r a m e s u s p e n d i n g t h e filter b a g a r r a n g e m e n t , a u t o m a t i c a l l y set in m o t i o n after t h e fan h a s been allowed sufficient t i m e to stop. There is no p a r t i c u l a r h i d d e n secret to filter sizing. There are m a n y well proven guidelines to enable selection a n d sizing to be precise a n d therefore efficient. Filter selection m u s t take into a c c o u n t m a n y p a r a m e t e r s , m a n y of w h i c h also apply to t h e collection h o o d s or booths. Filter m e d i a are available with diverse physical a n d chemical properties. A cost-effective d u s t a r r e s t m e n t a n d collection installation will result only from a c o m p e t e n t analysis of c o n d i t i o n s a n d r e q u i r e m e n t s . Some of t h e s e p a r a m e t e r s are as given below (in no p a r t i c u l a r order of i m p o r t a n c e ) -- ff a n y o n e is missed, it m a y prove e m b a r r a s s i n g , a n n o y i n g or expensive:
loss from a workshop, for e x a m p l e ) , t h e n accepted levels are likely to be 1 m g / m 3 or less. [] The a n t i c i p a t e d m e d i u m ' s o p e r a t i n g differential pressure. Many of t h e above c o n s i d e r a t i o n s will have a bearing on this. Clearly t h e total s y s t e m p r e s s u r e m u s t be calculated to d e t e r m i n e t h e size of t h e e x t r a c t i o n fans. [] The e x p e c t e d m o d e of o p e r a t i o n -- will it be 24 h o u r s continuously, or a daily shift p a t t e r n , or occasional use? [] Noise, since f o u n d r i e s are often s i t u a t e d n e a r residential areas. The list of interrelated p a r a m e t e r s is long, b u t a g r a s p of t h e whole s y s t e m is n e c e s s a r y if t h e e n d - r e s u l t is to be a n effective plant.
Bag s h a p e a n d orientation Figures 2 a n d 3 s h o w s o m e possibilities. The c h o s e n f o r m a t is d e p e n d e n t on t h e p a r t i c u l a r application, a n d often -- it m u s t be
[] The q u a n t i t y of p a r t i c u l a t e m a t t e r in t h e airstream; t h i s socalled ' d u s t b u r d e n ' m a y vary from a few milligrams u p to p e r h a p s 1 k g / m s. The d u s t b u r d e n m a y vary d u r i n g a process. [] The d e n s i t y of t h e dust. This bulk d e n s i t y will d e t e r m i n e t h e m a s s of cake a d h e r i n g to t h e bag, a n d will have a bearing on t h e disposal from t h e hopper. The collection bin m a y be impossible to lift, ff oversized. [] The particle size distribution. The efficiency of a r r e s t m e n t a n d filtration is d e p e n d e n t on t h e particle size. I n h a l a b l e d u s t varies from a r o u n d 70 # m downwards, b u t particles g r e a t e r t h a n a b o u t 200 # m (grit, for e x a m p l e ) m a y be carried down t h e d u c t w o r k to t h e filter. [] Mobile v o l u m e of p a r t i c u l a t e s to be collected. The bulk d e n s i t y is useful here to d e t e r m i n e cleaning frequency, in order to p r e v e n t 'bridging' of d u s t between bags. This is chiefly of significance in horizontally a r r a n g e d 'flat' bags. Correct cleaning conditions will reduce t h e t i m e required for p l a n n e d d o w n t i m e maintenance. [] The s h a p e of t h e particle. Some particles (in particular, zinc oxide) have a s t r u c t u r e like b a r b e d wire, w h i c h m a k e s t h e m a t e r i a l self-agglomerating, while o t h e r s a g g l o m e r a t e electrostatically. Some d u s t f o r m s flakes, w h i c h effectively c a u s e 'blind s p o t s ' on t h e media. [] The t r a n s p o r t gas or conveying g a s analysis. It m a y be p u r e dry a m b i e n t air; or it m a y c o n t a i n acidic s u l p h u r or nitrogen oxide g a s e s (SOx, NOx), h y d r o g e n chloride (HC1), or it m a y c o n t a i n v a p o r i s e d oil from s c r a p or possibly w a t e r vapour. [] T e m p e r a t u r e of t h e c o n v e y i n g gas. T h e d e w - p o i n t a n d c o n d e n s a t i o n possibilities, allied to acidic v a p o u r s , m u s t be a d d r e s s e d in t h e filter a n d h o p p e r design. [] E m i s s i o n s t a n d a r d (s) to be achieved. National s t a n d a r d s for e x a m p l e , TA Luft in G e r m a n y requires less t h e n 20 m g / m ; in t h e UK, t h e E n v i r o n m e n t a l Protection Act requires less t h a n 100 m g / m 3 ; while special s t a n d a r d s for lead a n d o t h e r potentially toxic p a r t i c u l a t e s require 5 m g / m a or less. If e m i s s i o n is back to t h e workplace (in order to conserve h e a t
738
Figure 2. Vertical bag arrangement. November 1994
Filtration & Separation
@BAG FILTRATION INFOUNDRIES cases, t h e i n t e r m i t t e n t n a t u r e of t h e o p e r a t i o n m e a n s t h a t a n onor offiine filter is n o t required, a n d a simple m e c h a n i c a l shake, d o w n t i m e cleaning filter is ideally s u i t e d (Figure 4). For economy, t h e b o o t h design s h o u l d allow t h e m i n i m u m t h r o u g h p u t of e x t r a c t e d air from t h e fettling process. However, free working s p a c e m u s t be available for t h e largest casting to be encountered. After d e t e r m i n i n g t h e t h r o u g h p u t of air a n d -h a v i n g regard to t h e d u s t t r a n s p o r t velocity, to p r e v e n t deposition w i t h i n t h e d u c t w o r k -- selected t h e correct d u c t cross-section, t h e m a i n c o n c e r n will be abrasive wear. Duct m a t e r i a l selection a n d t h e u s e of 'slow' b e n d s are key factors in m i n i m i s i n g t h i s problem. Also, at t h e filter inlet a n angled wear-plate will reduce i m p a c t abrasive effects on t h e bags. In s o m e c a s e s a p r e - s e p a r a t i o n or d r o p o u t c h a m b e r m a y be fitted to reduce t h e particle inlet velocity, so t h a t m o s t of t h e larger p a r t i c u l a t e s d r o p away w i t h o u t c o m i n g into c o n t a c t with t h e filter media. Vertically located bags fitted in t h e filter housing, cleaned either by a s h a k e r m e c h a n i s m (or for h e a v i e r l o a d i n g s or m o r e c o n t i n u o u s o p e r a t i o n by c o m p r e s s e d air p u l s e j e t s ) m a y be u s e d with confidence.
Sand preparation plant
Figure 3. Horizontal bag arrangement. said -- d e e p - r o o t e d bias on t h e p a r t of t h e e n g i n e e r ( s ) involved. Most m a n u f a c t u r e r s with t h e e x p e r t i s e required to a d d r e s s d u s t a r r e s t m e n t in foundries, will have different f o r m a t s of bag a n d bag cleaning available, to enable a focused decision b a s e d on t h e u s e r ' s specific r e q u i r e m e n t s . In general, online pulse-jet-cleaned, vertically m o u n t e d t u b u l a r b a g s a n d offiine reverse-air-cleaned, horizontally m o u n t e d , envelo p e - s h a p e d b a g s (often in t h e form of c a s s e t t e s ) are offered for m o s t f o u n d r y a p p l i c a t i o n s w h e r e c o n t i n u o u s operation a n d / o r a heavy d u s t b u r d e n is likely.
System design and selection A r m e d with t h e basic principles, filter functions, a n d having a s k e d all t h e p e r t i n e n t questions, it will n o w be possible to a d d r e s s p a r t i c u l a r d u s t a r r e s t m e n t p r o b l e m s within t h e f o u n d r y environm e n t . The e x a m p l e s w h i c h follow are b a s e d on actual applications, a n d w h i c h are k n o w n to be successful. In each case t h e goal w a s t h e solution to a d u s t control p r o b l e m . T h e d u c t w o r k , cooler, filter, f a n a n d all ancillary e q u i p m e n t were merely t h e c h o s e n players.
Fettling This is a s t r a i g h t f o r w a r d a p p l i c a t i o n for dry bag filters. In m a n y
This application is also well s u i t e d to dry bag filtration, particularly m a k i n g u s e of a centralised filter p l a n t c o n n e c t e d via d u c t i n g to t h e v a r i o u s d u s t a r r e s t m e n t a n d collection points. Figure 5 s h o w s a Disa Mark 3 2013 casting s y s t e m with pouring, cooling a n d vibratory or k n o c k - o u t d r u m , t o g e t h e r with a 50 t o n n e / h o u r s a n d p r e p a r a t i o n s y s t e m c o m p r i s i n g conveyor, elevator, ploughs, m i x e r a n d s a n d cooler e x t r a c t i o n points. The e x t r a c t i o n s y s t e m covers t h e whole of t h e s a n d p r e p a r a t i o n function, t o g e t h e r with t h e option of e x t r a c t i o n from t h e p o u r i n g c h a m b e r a n d cooling tunnel. The p r o b l e m for t h e d u s t control e n g i n e e r is t h e relatively h i g h m o i s t u r e c o n t e n t of t h e e x t r a c t e d air, c a u s e d in p a r t i c u l a r by t h e evolution of s t e a m at t h e k n o c k - o u t s t a t i o n or from t h e s a n d cooling process. To e n s u r e effective filtration, it is therefore n e c e s s a r y to keep t h e o p e r a t i n g t e m p e r a t u r e safely above t h e w a t e r d e w - p o i n t t e m p e r a t u r e , typically with a m i n i m u m t e m p e r a t u r e difference of 20°C. To achieve this, t h e correct v o l u m e of air from t h e driest e x t r a c t i o n p o i n t s -- s u c h as t h e p o u r i n g rack, conveyor, elevator, screen, p l o u g h s etc. -- is m i x e d with t h e air from t h e m o i s t e x t r a c t i o n points, s u c h as t h e k n o c k - o u t d r u m , w a t e r injection a n d s a n d cooler. It is a n a d v a n t a g e to reduce d u c t l e n g t h s to a n a b s o l u t e m i n i m u m , a n d if possible to site t h e filtration p l a n t w i t h i n t h e foundry. Some case i n s u l a t i o n m a y be necessary, t o g e t h e r with trace h e a t i n g to t h e header. There m a y -- in e x t r e m e c a s e s -- be a r e q u i r e m e n t to fit a s u p p o r t b u r n e r to increase t h e conveying air t e m p e r a t u r e , to give a safe m a r g i n above t h e w a t e r dew-point. The e x t r a c t i o n p o i n t s at t h e s c r e e n i n g a n d s a n d cooling p l a n t m a y r u n via a cyclone to t h e m a i n e x t r a c t i o n ducting. Larger highly abrasive particles m a y be p r e s e n t at t h e s e points, a n d a preseparator, m o u n t e d as close as practicable to t h e s c r e e n s a n d cooler, will remove s o m e of t h e b u r d e n from t h e whole system. As with t h e fettling application, abrasive 'jagged' d u s t is being h a n d l e d , so t h e filter inlet will c o n t a i n m e a s u r e s to s a f e g u a r d t h e filter m e d i a from its w o r s t effects. T r a n s i t i o n a l d u c t sections serve to reduce t h e inlet velocity at t h e filter, a u g m e n t e d by w e a r shields or c h a i n c u r t a i n s on t h e inlet side of t h e filter.
Induction melting furnaces The i n d u c t i o n m e l t i n g f u r n a c e is often t h e m o s t economical m e a n s of m o l t e n m e t a l production, a n d it is b e c o m i n g m o r e c o m m o n in British foundries. It is c o m p a c t a n d very versatile, b u t is also mobile in its v a r i o u s m o d e s of operation. A static m a c h i n e p o s e s few p r o b l e m s to t h e d u c t a n d h o o d designer. However, t h e i n d u c t i o n f u r n a c e c a n provide a challenge in all its n o r m a l operations, namely. [] Charging. [] Melting. [] Slagging off. [] Pouring. [] Back-pouring.
Figure 4. Fettling booths linked to a mechanical shaker filter. Filtration & Separation
November 1994
D u r i n g e a c h of t h e s e o p e r a t i o n s effective d u s t c a p t u r e m u s t be s o u g h t , with d u e regard to t e m p e r a t u r e effects. Many a t t e m p t s have been m a d e to overcome t h e challenge of
739
BAG FILTRATIONINFOUNDRIES
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effectively capturing the d u s t during all the furnace operations -b u t with less t h a n total success. However, probably the best allr o u n d solution is to fit a dedicated, 'close capture' hood. Figure 6 shows a close capture hood fitted to a 5 t o n n e furnace. The hood s u r r o u n d s the lid, and can be m o u n t e d on a double-hinged frame. This enables the lid, with the hood, to be tilted in two planes, while following the motion of the furnace w h e n tilted a b o u t its axis. The extraction rate is m a t c h e d to the requirement, which d e p e n d s on the actual distance between the hood and the source of d u s t and fumes. The extraction air is transferred from the hood manifold to the adjacent duct via a 'slide valve' a p e r t u r e (Figure 7), with the extraction duct being guided in a geometrically correct p a t h as the axes of rotation of the lid and furnace change relative to each other. In some furnaces a single axis of rotation can be achieved, which simplifies matters.
During melting, the extraction rate is at a minimum, with the lid closed. When charging and slagging off the lid is partially open, and the extraction rate is increased accordingly. When pouring, a l t h o u g h the lid is closed, m o s t of the emissions occur in the
Figure 6. Close capture hood fitted to a 5 tonne induction melting furnace.
Figure 7, The extraction air is transferred from the hood manifold to the adjacent duct via a 'slide valve' aperture.
740
November 1994
Filtration & Separation
BAG FILTRATION INFOUNDRIES @ --+
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/ 3 4 5 7
Figure 8. Working principle of cold blast cupolas.
Port Charging crane wlth charging bucket Level indicator At~nularofftake (below charging doct) Elevator shaft Two rows of tuyeres Secondary t~ast lever
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locality of t h e p o u r i n g process, so therefore m a x i m u m airflow is provided. Also, d u r i n g back-pouring, w h e n t h e lid is fully open, d u s t c a p t u r e is provided by m a x i m u m airflow volume. M a t c h i n g t h e airflow v o l u m e -- via a p r o g r a m m a b l e logic control centre -- to t h e actual r e q u i r e m e n t s n o t only achieves t h e desired i m p r o v e m e n t s in t h e environm e n t , it a l s o p r o v i d e s e c o n o m i e s in r u n n i n g a n d m a i n t e n a n c e costs, as well as e n s u r i n g a m i n i m u m size of filter. B e c a u s e all e x t r a c t i o n p r o c e s s e s involve a degree of e x c e s s dilution with a m b i e n t air, t h e t e m p e r a t u r e is n o t a problem. A conventional bag filter m a y be employed, a n d t h e r e f o r e n o r m a l m a t e r i a l s m a y be u s e d with t h e i r a s s o c i a t e d benefits. The close-capture h o o d c a n therefore provide t h e economic, c o m p a c t a n d practical solution to t h e challenge p r e s e n t e d by d u s t control in i n d u c t i o n furnaces.
Cold blast cupolas
This is a different challenge for t h e d u s t / f u m e control engineer. However, c o m m o n types of cold b l a s t c u p o l a s can be c a t e r e d for u s i n g a c o m b i n a t i o n of a g a s cooler a n d dry bag filter. W h e t h e r t h e w a s t e gas offtake is above t h e c h a r g i n g door (the s i m p l e s t form), or below a n d fitted with a n a f t e r b u r n e r or c o m b u s t i o n c h a m b e r (Figure 8), t h e c o n d i t i o n s as seen at t h e w a s t e g a s outlet are similar. Very h i g h off-gas t e m p e r a t u r e s a n d varying v o l u m e a n d t e m p e r a t u r e c o n d i t i o n s are t h e n o r m . During n o r m a l o p e r a t i o n , t h e c u p o l a h e a d off-gas t e m p e r a t u r e is 8 Primary blest level typically 600°C, while d u r i n g blowdown t e m p e r a t u r e s in 10 ~ S~phon Recewe~ vessel e x c e s s of 1000°C m a y be e n c o u n t e r e d for 30 m i n u t e s or 11 Waste bucket 12 Gas exhaust more. The off-gas is therefore p a s s e d t h r o u g h a section of duct 13 Combustion h e a t - r e s i s t a n t nickel-chrome steel ducting, t h r o u g h w h i c h chamber 14 Preheat comh e a t is lost rapidly by r a d i a t i o n a n d n a t u r a l convection/ bustton chamber 15 Afterburner conduction. The d u c t w o r k m u s t include e x p a n s i o n j o i n t s 16 emergency door a n d floating s u p p o r t s in o r d e r to m a i n t a i n its d i m e n 17 exhaust pipe sional stability. The t e m p e r a t u r e of t h e air e n t e r i n g t h e g a s cooler is r e d u c e d to a r o u n d 450°C. The g a s cooler is essentially a f a n n e d air-to-air h e a t e x c h a n g e r , in w h i c h t h e air to be cooled p a s s e s over cooling tubes, t h r o u g h w h i c h a m b i e n t air is forced by t h e axial blower fans. The cooler is generally sectionalised to increase t h e n u m b e r of p a s s e s over t h e tubes, or steel envelopes as i l l u s t r a t e d in Figure 9. T h e cooler g a s outlet t e m p e r a t u r e is pegged below 150°C. Normally a cyclone or set of cyclones is m o u n t e d between t h e g a s cooler a n d t h e filter. In t h i s case t h e cyclone is to reduce t h e possibility of s p a r k or fire d a m a g e to t h e filter unit. As a f u r t h e r precaution, t h e cyclone will be fitted with a variable fresh air d a m p e r , to a d m i t a m b i e n t dilution air if t h e n o r m a l filter o p e r a t i n g t e m p e r a t u r e is exceeded. A s e p a r a t e control loop m o n i t o r s t h e actual d u c t static p r e s s u r e d o w n s t r e a m from t h e c u p o l a w a s t e g a s outlet. This static p r e s s u r e is c o n t i n u o u s l y c o m p a r e d with a p r e s e t value. Variations c a u s e a m o t o r i s e d d a m p e r at t h e e x t r a c t i o n fan position to r e s p o n d in o r d e r to keep t h e total airflow w i t h i n prescribed limits. The c u p o l a e x t r a c t i o n s y s t e m is therefore self-regulating.
Conclusion T h e s e a p p l i c a t i o n s of dry b a g filtration d e m o n s t r a t e h o w a basically s i m p l e a n d very economical form of d u s t a n d f u m e control c a n be a p p l i e d to m o r e a r d u o u s conditions. They r e p r e s e n t merely a s m a l l selection of t h e total r a n g e of u s e s for t h e u b i q u i t o u s bag filter. With t h o u g h t , d e t e r m i n a t i o n a n d a little ingenuity, a n d m a k i n g u s e of t h e v a s t d e v e l o p m e n t experience in filter m e d i a a n d m e c h a n i s m s , dry b a g filtration will a l m o s t always p r o v i d e s o l u t i o n s for t h e c h a l l e n g e of a i r b o r n e p a r t i c u l a t e pollution.
~ } ~ r l y air tniet mamfo~d f,~2~Cooler housing Coohng a~r fan (,~) Co~)h~Qa~r inl~t box w~th dmtrtbutc)r Stottcd wall w~th c~arf~pmgfrar~. ~ ) Coo~mg casseHu ,~7") Ther rnocouP~O
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Filtration & Separation
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November 1994
Figure 9. Cutaway diagram of a gas-air cooler. 741
@TRACTSOFREFEREED PAPERS Comparison of modern beer filters Vergleich moderner Bierfilter von J. Hermia u n d S. Brocheton Bierfiltration wird zur Zeit mit H/Ire yon Kerzenfiltern und Horizontalblatthltern durchgefi)hrt, Der Vergleich zwischen diesen be/den Technologien zeigt, dab der Kerzenfilter gedngere Druckanstiegswerte gestattet, w&hrend der Horizontalblattfilter in e/nero gegebenen Tankvolumen
eine grOBere Filtrationsflache bietet. Der Kerzenfilter erweist sich als elf/z/enter, solange schmale Kerzen verwendet werden. (5 sn., 7 abb., 3 tab., I teL)
Comparaison de filtres il biere modernes p a r J. H e r m i a On utilise couramment les filtres a bougies et les filtres a plateaux horizonta ux pour la filtration de la b/ere. La comparaison entre ces deux technologies montre que les filtres a bougies engendrent des montees en press/on plus faibles mais les filtres a plateaux horizontaux offrent une plus
e t S. B r o c h e t o n grande surface de filtration pour un m#me volume de cuve. II ressort que le filtre a bougies est plus perlormant q u a n d on utilise des bougies de petit diametre. (5 pags , 7 figs., 3 tabs., 1 r#L)
Comparaci6n de filtros modernos para cerveza p o r J. H e r m i a Se filtra cerveza actualmente con filtros a forma vela y con elementos horizontales. La comparaci6n entre las dos tecnicas ind~ca que el filtro a forma vela permite mas peque#as
y S. B r o c h e t o n subidas de presion, pero el filtro con elementos horizontales ofrece una area mas grande para una c/sterna de dado volumen. (5 pags, 7 figs., 3 tabs., 1 reL)
Permeate flux decay during crossflow microfiltration of a cationic surtactant dispersion PermeatfluBzerfall bei Querstrom-MIkrotiltration einer Dispersion kationischer, oberfl~ichenaktiver Stoffe von G. A k a y u n d R.J. W a k e m a n Der Obergangs-PermeatfluB u n d das Sperrungsverhalten e/net Dispersion kationischer, oberflachenaktiver Stoffe be/Querstrom-Mikrofiltration wird a n h a n d von Membranen unterschie dficher Grade von Hydrophobie untersucht, die durch den Loslichkeitsparameter gekennzeichnet /st. Es wird festgestellt, dab das PermeatfluB-(Ibergangsverhalten bei geringer Beschickungskonzentration stark durch die Membran-Hydrophobie beeinfluBt wird. Der Mechanismus der Querstrom-Membranfi/tration wird nicht nut dutch eine Untersuchung des ObergangsPermeatflusses sondern auch dutch eine Untersuchung der Ablagerung des oberfiachenaktiven
Stotls aul der Membran u n d in ihren Poren ausgewertet. Es wird dargelegt, dab die Ablagerung des oberflachenaktiven Stoffs zur Bildung e/net Sekund~rmembran fuhrt, die durch die Primarmembran unterst~)tzt wird. Der Permeatfiu8 und die Sperrung des oberflachenaktiven Stoffs werden grOBtenteils yon der Sekundarmembran bestimmt. Die Sekund~rmembran bildet sich an der Membranoberfl&che und dringt in die Poren e/n, wobei ein Bereich hoher Konzentration des oberflachenaktiven Stoffs entsteht, die um eine Gr(~Benordnung hSher /st als die sogenannte Gelkonzentration. (5 sn., 6 abb., 1 tab., 14 teL)
D(~croissJince du flux de perme~at Iors de la microfiltration tangentielle d'une dispersion de surtactant cationique p a r G. A k a y et R.J. Wakeman On etudie le flux transitoire de permeat et le comportement du relet d'une dispersion de surfactant cationique p e n d a n t la microfiltration tangentielle sur membranes de ditferents degres d'hydrophobicite, caracterisee p a r un parametre de solubilite. On a trouve que le comportement transitoire du flux de permeat est fortement affecte par I'hydrophobicite de la membrane s~ la concentration de I'alimentation est faible On evalue le mecanisme de la filtration tangentielle membranaire en etudiant, outre le flux de permeat transitoire et le rejet, le depSt de surfactant sur
la membra ne et dans les pores. On montre que le depSt de surfactant conduit a la formation d'une membrane secondaire supportee par la membrane/nit/ale. La membrane secondaire conditionne en grande pattie le flux de permeat et le reiet de surfactant. La membrane secondaire est formee sur la surface de la membrane et penetre dans les pores, formant une region de grande concentration en surfactant qui est superieure d'un ordre de grandeur a la concentration dire 'de gel'. (5 pags,, 6 figs., 1 tab., 14 refs,)
Decllnaci6n del flujo de filtrado durante microfiltraci6n a flujo cruzado, de una dispersi6n de surfactante cationica p o r G. A k a y y R.J. W a k e m a n Se han estudiado el fiu]o transitorio de filtrado y comportamiento de rechazo de una dispersion de surfactante cation/ca durante microf/Itraci6n a fiujo tangencial, empleando membranas de varies grados de hidrofobicidad, caracterizada per solubifidad. Se halla que la hidrofobicldad influye fuertemente el flujo transitorio cuando la concentraci6n de afimentaci6n esta bala. Se ha estudiado el mecanismo de filtraci6n a flujo cruzado de membranas, teniendo en cuenta no solamente flujo transitorio del filtrado y rechazo, pete tambien la deposici6n de surfactante sobre
la membrana y dentro los poros. Se ha mostrado que la deposiciSn produce una membrana secundaria apoyada per la membrana original. En gran parte la membrana secondaria regula el flu/o de filtrado y el rechazo de la surfactante La membrana secundaria se formar en la superficie de la membrana y penetra los pores, dando una region de alta concentraci6n surfactante, la cual es order de magnitud mas alta que la intitulada concentracion gel. (5 pags., 6 figs., I tab., 14 refs.)
Hydrocyclone with high sharpness of separation Hydrozyklon mit hoher Abscheidungssch~irfe von Lieng-Yin Chu und Oian Luo Aus experimentellen Forschungsarbeiten, die in jOngster Vergangenheit im Hinblick auf die Zubehorteil ausgerOstet worden. Mit dem neuen Hydrozyklon warden Abscheidungsexpedmente Bewegung von Feststoffpartikeln in e/nero Hydrozyklon durchgefOhrt wurden, ergab sich der durchgeluhrt u n d ein Vergleich mit herkcmmliehen Hydrozyklonen gezogen. Aus den Befund, dab einige Partikel im inneren SpiralfluB sich auf die Hydrozyklonwand im zylindrlschen Experimenten ging hervor, dab die Abscheidungsscharfe be/ dem neuen Hydrozyklon mit Tell zubewegen. In Anbetracht dieses Ergebnisses hat man e/hen neuartigen Hydrozyklon mit zentralem Kegel und Wirbelsucheinrichtung hoher /st als be/ dem herkommlichen Hydrozyklon. zentra!em Kegel entwickelt. Da aufgrund des KufzschluBflusses einige grebe Partikel direkt mit Die optimalen geometrischen Parameter sowie die Position des zentralen Kegels wurden ebenfafls dem Ubedauf abflieBen, /st der neue Hydrozyklon auBerdem noch mit e/hem Wirbelsuchauf experimentellem Wege ermittelt. (4 sn,, 8 abb., 5 tab., 5 reL)
Hydrocyelones a haut pouvoir de coupure p a r Liang-Yin Chu et Q/an Luo Des recherches expebmentales recentes sur le mouvement des particules sol/des dans un hydrocyclone ont montre que quelques particules dans la spirale interne se meuvent vers la parol dans la section cylindrique. A la lure/ere de ces r~sultats, on a developpe un nouvel hydrocyclone avec un c6ne central De plus, etant donna que quelques grosses particules sont entrainees dans la surverse a cause des courts-circuits, le nouvel hydrocyclone a egalement ete dote d'un casseur
de vortex. Des tests de separation ont ete effectues en comparant le nouvel hydrocyclone avec les machines conventionnelles, Les resultats experimentaux ont montr~ que le pouvoir de coupure du nouvel hydrocyclone dote d'un cSne central et d'un casseur de vortex ~tait superieur a celui des hydrocyclones classiques. Les parametres geom~triques optlmaux et la position du cSne central ont egalement ete determines par experience. (4 pags., 8 figs., 5 tabs,, 5 refs,)
Hidrociclon de alto grado de separaci6n p o r Liang- I~n Chu y Q/an Luo Se ha mostrado, en recientes investigaeiones experimentales de/movimiento de particulas s61idas en un hidrocicl6n, que unas particulas en el flujo interior heficoidal mueven hacia el muro del hidrociclon en la seccion cilindrica. Con este resultado se ha desarrollado un nuevo hidrocicl6n con cono central Ademas, porque unas particulas grandes descargan directamente en el liquido derramado, de resultas de flUlO cortocircuito, se ha desehado un accesorio que busca el v6rtice.
Se ha realizado experimentos de separacion con el nuevo hidrocicl6n para hacer comparaci6n con el convencional, los resultados ban mostrado Due la definicion de separaci6n con el nuevo, con cono central y accesorio que busca el vortice, era mayor. Se ha determinado los parametros geometricos y el emplazamiento del cono central pot metodos experimentales. (4 pags., 8 figs., 5 tabs., 5 refs.)
Particulate emissions within foundries -- Dust arrestment requirements Partikelemisslon in GieBerelen -- Staubabfangerfordernlsse yon A.J. Lokal durchgefighrte saugende Bewetterung, in Verbindung rnit e/her Methode zur Abfangung u n d Entsorgung yon in der Luft schwebenden Partikeln stellen die Grundlage einer Vielzahl von Staubbekampfungslosungen in GieBereien und zahlreiehen anderen Industriezweigen dar. Das Referat konzentriert sich auf Trockenbeutelfiltration a n d zeigt, wie eine im Grunde einfache u n d
B~l~p sehr wirtschaftliche Form von Staub- and Rauchgasbekampfung unter schwierigen Bedingungen zur Verbesserung der Arbeitsumwelt in GieBereien sowie zur Minderung von Emissionen in die Atmosphare angewandt werden kann, wobei sowohl die zur Zeit geltende als auch die in Zukunft zu erwartende Gesetzgebung berOcksichtigt wird. (5 sn., 9 abb.)
Emissions particulaire dans les fonderies - Contraintes de retenue des poussi(~res par A.J. B~hop Une ventilation aspirante locale conluguee a une m~thode d'arr~t des part/tales so/ides appliquee pour ameliorer les conditions aux postes de travail dans lea fonderies ainsi que pour entra~nees constitue la base d'une vaste majorite de solutions de depoussierage dans les reduire les ~missions a I'atmosphere dans la perspective de nouvelles reglementations. (5 pags., 9 fonderles et une multitude d'autres industries. Cet article se concentre sur la filtration seehe sur figs.) manche et dernontre comment une forme simple et tres economique de depoussierage peut ~tre
Emisiones de particulas en fundiciones - Requisitos para detener polvos p o r A.J. Bishop Se provee, por ventilaci6n del tubo de escape junto con un mode de detener y recoger particulas Ilevadas pot el a/re, la base de la mayoria de soluciones de control de polvos en fund/c/ones y muchas otras industrias. Esta ponencia trata de filtraci6n seca con saeos y demuestra como se puede aplicar un s/sterna esencialmente sencillo y may economico de control de polvos y humos a
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condiciones may arduas, con motivo de amejorar el ambiente laboral en fund/c/ones, a s / c o m o reducir era/s/ones a la atm6sfera, todo dentro del aleance de legislacion corriente y anticipada. (5 pags., 9 figs.)
N o v e m b e r 1994
Filtration &
Separation