Airborne debris and microbiological contaminants can clog cooling towers and plate heat exchangers, reducing efficiency and creating the potential for process equipment fouling. Effective filtration can minimize the effects of these inevitable invaders.

As government regulations regarding the use of city water continue to restrict its use for once-through cooling, cooling towers have been used extensively as a source of cool water. Cooling towers substantially lower water usage, requiring makeup only for evaporative losses, non-evaporative airborne losses (drift) and bleed off.

The spray-evaporative process is a magnet for all sorts of airborne debris and particulates. Items as large as leaves and as small as microorganisms can find their way into the tower system, jeopardizing the expensive, critical equipment being cooled. Water treatment and blowdown can help with the biological fouling and dissolved solids, but a filtration system is essential for reliable and economic operation.

Many cooling systems rely on the use of sand filters. While these filters are capable of very fine filtration (below 1 micron), the pressure drop that this kind of filtration produces typically limits its use to side streams or partial-flow operation. This leaves the system open to contamination and exposes the condensers and heat exchangers to fouling. A full-flow, self-cleaning filter and strainer, capable of operating over a wide particle size range such as 200 micron up to 1 mm, can be added to the system to fill the gap.

One such filter employs a simple cleaning principle based on the scientific research performed by Swiss scientist Daniel Bernoulli in the 18th century. As the law of conservation of energy states, energy is neither created nor destroyed. Bernoulli found that an increase of energy by higher velocity must be compensated for by lower pressure in order to conserve the energy, a theorem known as the Bernoulli principle.

In systems outfitted with such a filter, water passes through the filter basket and leaves the filter through the outlet. The basket is cleaned by a disc that enters the filter basket. During the cleaning operation, water passes through a defined gap between the disc and the basket. Flow velocity increases locally around the disc and, in accordance with the principle, the static pressure is reduced. The lower static pressure around the edge of the disc has a “vacuum cleaning” effect on the basket that flushes the debris out through the flushing outlet.

The cleaning operation is divided into two phases. During the first phase, the flushing valve opens and larger particles are flushed out. In the second phase, the disc makes two strokes into the basket and vacuum cleans the debris from the basket. Any remaining particles from the first stoke are removed in a second stroke. A cleaning operation is started either by using a preset time interval or a differential-pressure switch that measures the degree of clogging. Target applications include cooling towers and plate heat exchangers as well as ocean and river water filtration.