Fundamentals

COOLING TOWER FILM MEDIA  

Numberless structured packing variants have been used since the inception of tower-like, evaporative cooling constructs.  Early arrangements included planks, boards, and bricks.  At the risk of historical inaccuracy perhaps one can point to the 1936 ACB construct, basically assemblies of side-by-side, corrugated asbestos cement board (thus the name) plates, as breaking grounds as regards performance and somehow anticipating later field developments.

The '50s' and the '60s saw probably the greater strides  namely Carl Munters 1957 patent combining corrugated and flat plates, Kohl and Fuller's 1963 vertically corrugated pack patent and ultimately Bredberg's 1966 cross-fluted design.patent which brought [successful] closure as it were to low-profile, highly efficient packing endeavors.

Cross-flute packing, noted for high efficiency and moderate pressure loss, has been used extensively.  Illustrative media include Munters' CF-12060 and CF-19060, Brentwood's CF-1200 and CF-1900, and Marley's MC-67

Offset-flute packing, developed much later splits and reunites gas and liquid streams along the vertical fill section profile.  Typical media include Hamon's ANCS and Balcke-Durrs' FB-20.

Vertical-flute packing, also referred at times as vertical flow packing and a far more recent design, proffers the largest flute openings available for structured packing today.  By permitting higher liquid trickle down rates vertical flute packing helps maintain the packing walls and paths cleaner of foreign, undesirable material such as biofilms or debris. .     

The beauty of cross-flute structured packing is its ability to inherently achieve, impart or impose a high degree of auto or self- liquid phase distribution even though the latter  role is frequently played by/assigned to specialty trays, pressurized nozzles or open, gravity flow distributor channels or basins.  

Structured packing is today widely available from a number of manufacturers.  Typically the packing consists of assemblies or packs of individual corrugated, flat, or both type sheets bound together in some form, e.g.  glued together.  Plastic and metal are probably the most popular materials of construction employed. .

Vendor catalogs frequently indicate surface area, (sq.ft./cu.ft.) and void space.  In practice, the actual or effective surface area can be augmented by an aggregate of sprays, bubbling, dripping, droplet splash, among others.  Effective or actual surface area can be as well diminished by fouling/plugging/blocking, and unused or idle sections. 

COOLING TOWER SPLASH MEDIA 

Typical splash type media include splash bars of various makes most frequently spaced in staggered arrangements held in place by grid hangers.  Splash media is available in PVC, polypropylene, treated wood, aluminum and even stainless steel.  Splash type media, whether in counterflow or crossflow configurations, is not conducive to fouling..

TRANSFER RATES

It is generally understood that greater surface areas promote greater transfer rates.  However this will happen as long as whatever contemplated greater surface area is actually put to use, thus the actual or effective surface area nuance (versus geometric or catalog surface area).  It is wise to scrutinize active to plain geometric surface area ratios.

It must be noted that all too dense, small flute media while proffering highest surface area per cu.ft. may incur so high pressure drops so as to render the media inapplicable.  It may also give rise to exacerbated plugging.      

PRESSURE DROP

Whatever the liquid gas stream arrangement - counterflow, crossflow, cocurrent - packing introduces a level of resistance to the passage of the fluid and pressure drops through the packing media should be appropriately accounted for.  In general one would treasure packing proffering lowest pressure drops naturally assuming full performance can be delivered all other things considered equal.   

In many applications, liquid side pressure drops can be overlooked as it's just some liquid, frequently water, trickling down/gravity flow.   

FOULING

Fouling refers to the undue accumulation and plugging of the media.  Not only will performance noticeably suffer but there can be structural problems, even media/bed collapse as well.  Fouled packing can weight far more than first day, original media.  

GAS AND LIQUID DISTRIBUTION

Probably most if not all structured packing applications require uniform gas and liquid distribution within the unit.  To a certain extent this can translate into a make it or break it situation.  Aside from things like faulty liquid distributors or flow design, media related culprits may include instances of fouling and channeling/by-pass.  Uneven "rain" zones, thermal gradients, dry spots, media holes all add up to under performing packing.   

WETTING 

As with most packings, structured media achieves optimum utilization when most if not all exposed surface area is thoroughly wetted.  So-called wetting point is defined as the recommended or required  minimum liquid loading so as to insure complete media surface "coating" thus permitting design mass transfer rates.  Packings with lower wetting points would configure the more desirable designs. 

FLOODING

During operation most all packings hold up some given amount of the circulating liquid.  When gas or air stream velocity is unduly high, liquid will not be able to trickle or flow through the packing bed at all, a situation called flooding point.  It should be apparent that for certain applications one will want this value to be as high as possible - we would then able to move larger gas phase volumes through smaller face areas.  It should be noted that pressure drops increase as operation, willingly or unwillingly, approaches flooding conditions.

LIQUID DISTRIBUTION 

The even distribution over the fill packing is essential.  This is usually achieved by means of a network of pipes, open troughs or trays.  The various makes may include pressurized spray nozzles, overlapping spill designs or orifices punched on trays

AIR SECTION  

Most fans used in cooling tower applications are equipped with manually adjustable pitch blades.  This permits to use basically the same fan over a range of horsepower requirements.  Most common materials of construction include quality aluminum and fiberglass-reinforced plastic. .

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