White Rust: What it is, and How to Protect Your Project

What is "white rust"? Well, while it is white, it really isn't 'rust' in the normal sense of iron-oxidization. White rust is instead a corrosion product of zinc oxidization that often strikes galvanized surfaces subjected to moisture. In our industry, the most common victims of this corrosion mechanism are cooling towers and fluid coolers. And in these products, white rust can cause thousands of dollars of damage in a relatively short period of time.

White rust damages equipment by allowing a rapid and localized corrosion of the protective zinc coating on galvanized surfaces. Normally, in a galvanized surface, the zinc protects the underlying steel by providing a sacrificial cathodic protection to small areas of exposed steel, and provides bulk protection by providing a durable protective inert zinc oxide coating to prevent exposure of the underlying steel.

In white rust, however, this normal oxidation of the zinc surface goes wrong, and instead of providing a durable dull-gray surface, a porous, powdery or waxy oxide is produced instead. This corrosion product allows a rapid removal of the protective zinc surface--made worse in that the corrosion is generally localized in 'cells' which cause a very quick penetration of the zinc surface, exposing the underlying steel in a pitting process.

In recent years, the incidence of white rust has increased dramatically, leading the industry to study the process in greater depth. The Association of Water Technologies has produced an informative paper (pdf) that investigates the reasons for this increase (essentially changes in the methods used to produce galvanized sheet metal and water treatment methods) and how to prevent its occurrence.

Generally, white rust is more prevalent in soft water areas, which makes it a big problem in the Pacific Northwest. Preventing it entails both design and operational considerations.

White Rust Cells in Basin of Tower

If galvanized surfaces are used in your tower, it is critical that the tower be subjected to a 'passivation' treatment. This is a temporary water treatment regimen in the first few weeks of tower operation that acts to ensure the development of a desirable zinc oxide surface. Evapco discusses this process in this engineering bulletin. If Evapco's non-chemical Pulse~Pure product is provided, passivation is be included in the first year service that is provided with all installations. It is critical that this be performed immediately upon filling the tower with water--if water is left in the tower untreated for a period of time before the passivation treatment begins, white rust cells can develop in the interim. This is a very common cause of white rust corrosion in otherwise well-treated towers.

The other method to avoid problems with white rust in your tower installations is simply to chose your materials of construction wisely. In cooling towers, the most critical portion of the system is the basin--white rust can cause a rapid pinhole leak through the basin of a galvanized basin that would require immediate refurbishment. Providing a 304 ss basin is a very economical way to avoid costly system renovation at a future date. For areas with high chlorides, or when using water treatment methods that operate at high cycles of concentration (thus increasing the low chloride content of the utility water to dangerous levels) 316 ss is also available. Of course, the entire tower can also be constructed from these corrosion-resistant materials if desired.

In fluid coolers, however, the coil is an additional problem area. White rust on this galvanized component can rapidly lead to perforation of the closed-loop side of the system causing loss of cooling water and/or glycol coolant into the open loop side of the system. This can be a triple-threat due to the economic loss of glycol and water, an increased threat of freeze up, and huge water-quality problems due to bacterial growth and plasticization due to glycol exposure in the open side of the cooler.

Plasticized Bacteria/Glycol Slime: Yuck

The coil in your fluid cooler is the single most expensive component in it, by a large margin. And replacing coils can be an extremely costly proposition, especially in coolers without easy access to the coil section.

Until recently, there hasn't been a lot of choice for protection of this critical component of the system. Other than selecting a tower, like Evapco's highly efficient ESWA fluid cooler, that provides easy coil access for coil replacement, the usual option was to ensure a thorough passivation program. However, Evapco has now introduced 304 SS fluid cooler coils to protect your project's investment in this costly and critical component.



White rust is a problem that can cause great economic losses for building owners and operators. Thus it is critical that designers and contractors are aware of the prudent requirements necessary to prevent this damage. But with simple precautions, namely requiring a passivation program or wisely selecting materials of construction, this problem can be avoided in your projects.

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But what if it's too late, and you already have white rust on your tower? Well, there are an array of options, including attempting to re-passivate the galvanized surfaces or a full refurbishment of the basin using a polymer coating like Evapco's Evapliner. The helpful people at Fluid-Tek would be happy to help you determine the best course of action for your project.

Saving Water in Evaporatively Cooled Systems

Water is a limited resource, just like energy. Engineers are very aware of the need to save energy in their designs, and one of the best ways to do this is to take advantage of evaporative heat rejection for their cooling systems. The traditional cooling tower is an extremely effective way to reduce energy use at the compressors in a traditional cooling system. But introducing a cooling tower introduces a need for water to the system. It would be advantageous if this water use could be kept to an absolute minimum.

Especially since, in Seattle, water is expensive. As of this posting, the water utility rate per thousand gallons is $4.48 (summer) and the Sewer costs tack on an additional $9.96. When you consider that a cooling tower consumes a minimum of 1.8gph/ton (evaporation required to reject that heat), you can see that over a 1900 hour cooling season, these costs can really add up for a reasonably-sized cooling tower.



Earlier, I posted an article that highlighted ways to reduce water use in traditional cooling tower systems. For the most part, these recommendations address keeping the actual water use as close to the theoretical 1.8gph/ton evaporation figure as possible. Reducing the water use any further requires reducing the load on the tower, since evaporation is the only way a cooling tower can reject heat.

There are two ways to reduce load on a cooling tower--Reducing the total building load, or rejecting heat through some other method other than the cooling tower. Assuming the first option has already been exhausted through good engineering practices, the only other option is the second.

This is the approach taken by Aaon in their evaporative condenser systems. They essentially use a dry finned coil as the first stage of cooling before the refrigerant is cooled by evaporative methods. This essentially allows the system to reject as much heat as possible through a non-evaporative method before water is used. Every btuh that is rejected in this manner means less water used in the system.


This idea could be borrowed and applied to an open cooling tower by the use of a dry-cooler as a pre-cooler before a cooling tower. This way, the system rejects as much heat as possible in a dry fashion, and only uses water for what the dry-cooler can't do. This system gets to take advantage of the strengths of both methods of heat rejection--the water conserving function of a dry-cooler, and the lower water temperatures and more efficient heat rejection provided by a cooling tower.

Evapco has capitalized on this approach by creating a new, water-saving fluid cooler called the WDW:


This unit is a hybrid between a dry-cooler and an evaporative fluid cooler. It is provided with a control panel that controls both wet and dry sides of the unit, varying fan speeds with a VFD and determining when to run the evaporative pumps to optimize both water efficiency and fan energy.

Cutaway of an Evapco WDW unit

In practice, the evaporative system is only used for a small portion of the year, only when the design condenser water temperatures cannot be met by the dry-cooler side alone. What you see is a major reduction in water use compared to the same system served by a fully evaporative system:


Other advantages of this approach besides reduced water use are reduced chance of tower plume (since there are far fewer hours in which water is being evaporated, and when this does occur, it occurs in warmer temperatures) and the ability to provide some cooling even if city water is lost due to a service disruption.

But since a dry-cooler uses more fan energy per ton of cooling than a cooling tower, this system will inevitably use more energy to save water. Does this approach pay off?

An example from a real project might help demonstrate the economies involved. Below are the utility cost calculations from a project utilizing a 240 ton WDW installed in Seattle on a heat pump system with a portion of the load serving a 24/7 cooling application:



Note that even with the reduced water cost (to approximate the effective cost of using a deduct meter to avoid being charged wastewater charges for evaporated water) the hybrid system saves about 18% of the annual operational utility costs compared to a fully evaporative system. This affords a relatively quick payback for the added equipment costs associated with the hybrid system.

The Importance of Cooling Tower Maintenance

Cooling tower maintenance is not just critical for extending the life of your equipment, but it also can significantly improve the energy performance of your mechanical system.

BetterBricks, a non-profit venture of the Northwest Energy Efficiency Alliance has summarized the energy benefits of cooling tower maintenance in their article, Optimizing Cooling Tower Performance.

This article highlights the negative effects of:

• Scale Deposits
• Clogged Nozzles
• Poor Airflow
• Poor Pump Performance
  

If you own or maintain cooling towers, this article is well-worth reviewing. And remember that Johnson-Barrow's FluidTek tower service group is a certified Evapco Mr. Goodtower service center.

Your Next Energy Conservation Measure May be a Quiet Fan

It might sound strange, but a super low sound axial cooling tower fan is an energy-saving device--But not because it uses less energy than the fan it replaces, because it doesn't. The reason is a little more complicated than that.

But first it makes sense to review a few basics about cooling towers.

The Basics

There are two major types of cooling towers and fluid coolers: Induced Draft and Forced Draft.


Forced-Draft towers utilize centrifugal fans to blow air through the tower. The air is forced into a pressurized plenum inside the tower and then through the fill. This means that access into these towers is limited, since doors must be able to resist pressure without leakage and tend to be small and difficult to use. This also makes it difficult to observe the basin of these towers while operating in order to troubleshoot problems if necessary.



Induced draft towers use an axial fan to pull air through the tower, creating a negative pressure within the tower. This allows the unit to be built in an open configuration, making access and observation far easier. In general, induced draft towers cost less, are easier to maintain and, importantly, require about half the fan horsepower to do the same cooling as a forced draft unit.

In fact, there are only a few reasons why you wouldn't use an induced draft tower in preference to a forced draft tower:
1. Height restrictions
2. Static pressure capacity for ducted installations
3. Noise Control

If you project requires an extremely short cooling tower or needs a tower to be installed indoors with ducted inlets and/or outlets, there is a good chance you will need to use the less efficient forced-draft tower. And, until recently, it used to be that the same was true of sound-critical installations. But not any more.

The acoustical benefit of forced draft units are twofold: First, they are quieter than induced draft units right out of the box. (Low-profile forced-draft units are especially quiet.) And, secondly, they can easily accept sound attenuators to make their already quiet performance even quieter. The price you pay, of course, is fan energy and dollars. Attenuators require that you expend even more money and fan energy than the already more expensive and less efficient bare forced draft unit.



Th super low sound fan (SLSF) changes the playing field. The addition of the SLSF on an Evapco induced draft cooling tower does not affect the efficiency at all--the performance is the same with and without the quieter fan. And since the fan knocks 9-15 dBa off of the sound power of the tower, suddenly induced draft fans are competitive in sound level with a forced-draft unit. Generally speaking (and each application is different) a SLSF induced draft unit is just about as quiet (if not quieter) than a forced-draft unit of the same capacity--and very competitive in first cost. And further sound abatement is available to shave a few more dB off of the sound level.

This development makes it very possible to meet demanding noise criteria and still retain the sizable energy benefits of the axial fan. And with innovative products like the Evapco ESWA, the lowest-sound option can even be the energy leader!

Hearing is believing, so Evapco has provided a few video clips to help you get an idea of how significant this sound improvement is [videos may require Internet Explorer to work properly]:

Video 1
Video 2

More information on low-sound options is also available here (pdf).

Advanced Tower Nozzle Design Eliminates Clogging

Anyone who has operated cooling towers for any significant amount of time knows that a common maintenance point is clearing clogged water distribution nozzles. This is especially a problem for gravity-fed cross-flow towers, where there is very little pressure to force debris through the nozzle orifice, and debris such as leaves, paper and ferrous 'throw' from the pipes can clog the nozzle. This reduces the effectiveness of the water distribution, and in turn the efficiency of the cooling tower.

Typical cross-flow tower gravity distribution pan

Pressurized distribution systems, as are found on Evapco counter-flow towers, eliminate a lot of clogging problems by utilizing pressurized large-orifice nozzles which use the force of the water pressure to keep the nozzles clear. But even these types of systems can clog periodically.

So that is why Evapco developed the Evapjet nozzle.



That's a nozzle?

Yes, it is, but probably the best way to appreciate it is by watching the video of it in action: Evapjet Video (may require Microsoft Internet Explorer to view).

(Pretty cool, huh?)

Importantly, this nozzle can pass a 1" ball, and reduces the total number of nozzles required for a tower by 66%! So you have a much reduced chance of clogging, and many fewer nozzles to maintain.

This nozzle is provided on new Evapco cooling towers, and is available for retrofit on many existing towers of most major manufacturers. If you are interested in retrofit, call Fluid-Tek for a quote!

UPDATE: 9/23/07

Don't be discouraged if you have a fluid cooler and not a cooling tower. Evapco offers their unique ZM (Zero Maintenance) nozzles (pdf) with similar anti-clogging properties as the Evapjet--they just don't make for as cool a video!

Read more about spray header and nozzle replacements here.

Cool Ways to Conserve Water

A few years back, I had an article published in the April 2005 issue of Plumbing Systems and Design Magazine that highlighted the many ways to optimize the water saving performance Cooling towers.



You can read that article right here.