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Water Velocity

The interest to reduce energy usage and achieve “Green” LEED status and stature now drives most building operations.  Leading the list of high energy consumers are related to the HVAC systems – the high horsepower water circulating pumps, refrigeration chiller machines, cooling towers, and air handling fans.

On the water side, one immediate energy saving step is to reduce pump speed by switching to variable frequency motor controller drives.  For most building properties where demand loads will typically vary, reduced pump speeds to match cooling or heating demands produces an immediate energy savings, and elevates LEED status.

The true negative impact to the piping system years later, however, is rarely considered, or even anticipated.

For open cooling tower systems, the constant capture of particulate matter and microbiological organisms only adds to the volume of iron oxide rust product naturally created.  The combination of such potential contaminates varies widely over different environmental conditions, but can be guaranteed to exist.

In terms of rust produced, even a low corrosion rate will translate into a large volume of material every year.  For condenser water cooling system having a low corrosion rate of 1 mill per year (MPY), rarely achieved today, approximately 24 pounds of steel is lost per year per 100 feet for pipe of 24 in. diameter.  Reverted into less dense iron oxide this loss of steel then produces 0.6 ft.³ of rust and mud respectively.  For any moderately sized facility, potentially hundreds of cubic feet of rust debris can be created in just a few years of poor corrosion control.  Under even the best corrosion conditions, significant rust is produced.

Microbiological contamination may include common elements to the makeup water as well as those captured by the air scrubbing effects of the cooling tower.  Combined with a high volume of iron oxide debris, common microbiological growths can evolve into microbiological influenced corrosion, MIC, and can produce the most severe form of pipe wall loss known

The true impact of added rust particulates and microbiological growth is typically minimized in theory and intent.  Water filtration is commonly installed to address the dirt particular problem but in virtually all examples is wholly ineffective.  Tower pan washing systems are generally not effective in the design.  Sidestream filtration defines that 90 to 95% of particulates will escape capture to potentially settle elsewhere within the piping system.  Since most water filters are installed incorrectly, the impact toward water quality improvement is minimal.

In fact, total filtration of 100% flow is required to ensure a clear open condenser water system, but very rarely is installed.  If model units are available to accommodate both low particulate retention of under 50 μ as well as the larger flow rates of most condenser water systems.  In terms of high-capacity automatic backwashing units, we consider the zero gravity filters as excellent.

© Copyright 2023 – William P. Duncan, CorrView International, LLC