DISPERSION SYSTEMS AND DRIFT ELIMINATORS FOR OPEN CIRCUIT COOLING TOWERS

Dispersion system

The selection of the most suitable filling is essential for a proper working point and to save materials and money. Substantially the factors affecting the technical selection are:

• - Geographical area of utilization
• - Relevant sector
• - Installation type to which machines are dedicated and then the deriving quantity of dissolved solids TDS (Total dissolved solids) and suspended TSS (Total Suspended Solids).

There are three types of dispersion or “filling” whose distinction is based on the functioning principle, “Splash”,  “Film” or  “Hybrids” wherewith the air and water fluids meet by varying the ratio L/G (ratio between liquid flow and air quantity) and then the efficiency. Generically we will carry out our choice as below:

• - Film dispersion: TSS < 120 PPM
• - Splash dispersion: TSS > 600 PPM
• - Hybrid dispersion: TSS da 120 a 600 PPM

“Splash” functioning (download PDF data sheet)

The water to be cooled created by the distribution system made up of distribution pipes and nozzles (square distribution rotary nozzles, see specific sheet) rebounds on a number of flat concave surfaces (strips, grids), shattering into descending droplets of variable dimensions; these ones artificially increase the contact surface between air and water, moreover their high speed, compared to the air one, increases the exchange coefficient.

The main advantage of this type of functioning is the excellence between efficiency and used material quantity. Other not secondary advantages are a rate empty/full rather high that reduces considerably the engorgement possibilities with a TSS and TDS quantity almost unlimited as it happens in the food industry (e.g the circuits dedicated to concentrators used for the tomato concentrate production).

The main less positive characteristics are two: the high altitude required for the heat exchange (generally at least 1,8 - 3,5 m. of filling height is required) that sometimes may cause problems to the transports and the efficiency that spoils the ratio dimension/prix due to the lower surface development per every cubic meter of filling that is about 70 m2/m3.

Among the materials generally used for this type of filling we can find:

• - PP (polypropylene) with an operating temperature up to 85-90°C which has characteristics of chemical stability and rigidity even at high temperatures but there are defaults such as bonding difficulty and flammability.
• - PP with addition of polymers not flammable, with an operating temperature up to 85-90°C which presents characteristics of chemical stability and rigidity even at high temperatures with qualities of self-extinguishing thanks to the chemical additives used. Unfortunately the purchase price is high and for this reason it is more likely to prefer PP or PVC standard solutions by making choices relevant to plant (and safety) that can minimize the limitations of the chosen material.

“Film” functioning (download PDF data sheet)

The rain, in this case the water to be cooled is distributed on vertical crossed surfaces, covering them with an uniform film, which constitutes the exchange surface with a reduced air resistance (e.g  with a vertical large channels filling we can select up to 4 m/s  speed of the air flowing in the filling with valuable anti clogging qualities).  The plastics introduction led to the development of new fillings, until obtaining systems that represent real evolutions in this field, with high performances and a relevant compact plant.

Another quality, in addition to the above, is the high ratio between surface and occupied volume with a surface development from 140 to 240 m2/m3 (according to the type used) with a dispersion height generally reduced below 1,2 mt.

Some contraindication is underlined in the quantity of TSS circulating suspended solids and TDS  disposable dissolved solids (it is required, as underlined, a water with good chemical and physical characteristics) and reduced hardness. For this purpose it is generally required a careful analysis of the water chemical composition and/or  a specific  treatment in order to prevent possible engorgements. 

Among the materials generally used for this type of filling we can find:

• - PVC-U (polyvinylchloride) with a max. operating temperature of 55-60°C which has characteristics of toughness and strength besides being easily thermo-formable. Other qualities are a good chemical stability, the self-extinguishing and easiness of bonding and extrusion.
• - PVC-C (post-chlorinated polyvinyl chloride) used in place of common PVC-U where there are higher utilization temperatures or corrosive environments. It has an high resistance to traction and compression and it maintains excellent properties of chemical inertness with a max. utilization temperature of 80-85°C.  Other qualities are the self-extinguishing  and easiness of bonding and extrusion. Unfortunately the high specific weight (with a reduced elastic return) reduces its affordability and for this reason we tend to consider the following material.
• - PP (Polypropylene) with an utilization temperature up to 85-90°C, has characteristics of chemical stability and rigidity even at high temperatures, but there are peculiarities such as difficulty in bonding and  flammability.
• - PP with addition of non-flammable polymers and with an utilization temperature up to 90°C, which presents characteristics of chemical stability and rigidity even at high temperatures, with qualities of self-extinguishing thanks to the chemical additives used.

“Hybrid” Functioning (download PDF data sheet)

In this case the water rain is distributed on complex vertical crossed surfaces and rebounds on the oblique and horizontal ones by shattering and distributing itself downhill in droplets of variable dimensions with reduced resistance. The essential quality is to create ideally a “middle ground” between  “Film” and “Slash” filling for all those applications that have a not ideal water quality, but neither clogging (TSS from 120 to 600 PPM).

Another quality besides the one already highlighted, is the high ratio between efficiency and occupied volume with a surface development of 130 m2/m3 (HYBRID SPLASH TCK) with a dispersion height generally reduced under 1,8 mt.

Among the materials commonly used for this type of filling, we can find:

• - PP (polypropylene) with an operating temperature up to 85-90°C which has characteristics of chemical stability and rigidity even at high temperatures.
• - PP with addition of polymers not flammable, with an operating temperature up to 90°C  which presents characteristics of chemical stability and rigidity even at high temperatures with qualities of self-extinguishing thanks to the chemical additives used.

Drift eliminators (download PDF data sheet)

Mainly they are used to hold back the water drops present in the air flowing out the cooling tower dragged vertically. You have to consider that one of the causes of the water consumption in a cooling tower is the one due to “entrainments”. Whereas it is not possible to intervene on the component “evaporation”, since it is inherent in the cooling physical process, as to the entrainments, the modern technology has achieved excellent milestones. Nowadays the percentage of dragging is under 0,002% of the recirculated water volume clearly referred to the cooling tower. We distinguish two types of drift eliminators:

• - Panel drift eliminators
• - Fin drift eliminators

The advantages of the two types of solution are similar:

• - Reduction of the environmental impact phenomena
• - Significant reduction of the water drops coming out the plant
• - Water recovery
• - Energy savings
• - Very low load losses
• - High mechanical resistance
• - Simplified maintenance
• - Strategic panels placement thanks also to their lightness
• - Resistance to almost all the chemical and biological agents (UV included)

Among the materials commonly used for this type of filling, we can find:

• - PVC-U (polyvinylchloride) with a max. operating temperature of 55-60°C which has characteristics of toughness and strength besides being easily thermo-formable. Other qualities are a good chemical stability, the self-extinguishing and easiness of bonding.
• - PP (polypropylene) with an operating temperature up to 85-90°C which presents characteristics of chemical stability and rigidity even at high temperatures.