Scale never sleeps

The New Orleans Airport Hilton opened in 1989, and since its debut, scale-related troubles have been increasingly problematic.

The hotel’s cooling plant originally was comprised of three centrifugal chillers rated at 135, 185, and 235 tons. The condenser water loop was conditioned using a magnetic water-treatment device to prevent scaling. Chemical treatment was used for biological control.

Nonetheless, as cooling tower water was warmed in the chillers’ condensers, dissolved mineral ions precipitated out of the water and adhered to the condenser tube walls. Over time, soft deposits formed that eventually turned into hard scale. This accumulation decreased the effective heat transfer surface and restricted water flow.

In short, the old magnetic system proved to be ineffective for scale control in this application. What’s more, this system’s failure to alleviate scale formation in the condenser tubes resulted in higher-than-design condenser pressures, which created other operating concerns.

“High condenser pressure increases the chiller’s energy consumption, and if the pressure increases enough, it will force the compressor into surge. A surge condition limits a chiller’s ability to produce cooling, and if left unchecked, can lead to significant compressor mechanical failures,” commented Ned Hebert, zone service manager at York’s Saint Rose, LA office.

Chiller tube fouling can also increase chiller energy consumption. For example, with a fouling factor of 0.003 (0.036-in. scale thickness), the additional energy cost per year for a 500-ton chiller is $25,300.

“Energy use was rising, and maintenance and water treatment costs were going up as well. The magnetic system just wasn’t working and we had to do something to protect our investment for Hilton and its guests,” stated Glenn Ziegler, the hotel’s chief engineer.

Traditional cleaning

Once scaling occurs, traditional tube cleaning involves acid, steel brushes, and abrasives. This method may shorten tube life which, in turn, leads to tube replacement. Also, acid disposal may pose environmental concerns.

Ziegler acted quickly to halt the mineral deposits. In April 1998, an ED2000 electronic anti-tube- fouling system (from York International Corp., York, PA) was installed on each of the condenser water supply lines to the three chillers; the units were also added to the cooling towers’ makeup water supply inlet piping.

The system’s electronic process is based on patented technology, inducing dissolved mineral ions to precipitate into larger insoluble crystals, which pass through the condenser without adhering to the tube walls.

The process is called solenoid-induced molecular agitation (SIMA™). The insoluble crystals settle at the bottom of the cooling tower sump, where they are flushed from the system during blowdown.

Using such a system, “You eliminate the hassles of cleaning tubes with acid, steel brushes, and abrasives. You also save the wear and tear on tubes caused by these cleaning methods,” Ziegler noted.

The system is installed without welding or other invasive procedures, according to the manufacturer. The hardware includes an electronic control box attached to a prewrapped solenoid coil in a weather-resistant enclosure. The solenoid coil is externally fitted to the condenser inlet pipe. Installation takes about 20 min. There is no downtime.

Clean machine

The condenser water loop continues to be treated by a combination approach: the anti-fouling unit for scale control and chemicals for biological control.

About 10 months after the anti-fouling devices were installed, the condenser heads were removed from the three chillers. When the tubes were inspected, no evidence of scale was detected.

In December 1999, a post-installation water chemistry analysis performed by an independent laboratory supported the physical results. The tests showed a significant increase in the amount of calcium ions precipitating out of the tower water.

When compared to the raw makeup water, which was determined to have respective chloride and calcium levels of 84 and 134 parts per million (ppm), the tower water showed chloride and calcium readings of 500 and 400 ppm.

“The ED2000 prematurely precipitated scale, causing the system to take longer to reach the TDS [total dissolved solid] blowdown setpoint. The result was an increase in cycles of concentration and water savings,” noted Ziegler.

Less calcium in the tower water created a proportional increase in chloride concentration, he explained, allowing increased concentration cycles from 3.4 to 5.8, saving up to $3,157 per year in water expenses.

In all, cleaner condenser tubes are predicted to reduce chiller energy usage by about $7,303 per year by improving the chillers’ efficiency from about 0.80 to 0.75 kW/ton.

“Even if these symptoms aren’t evident yet, if you are in a hard-water area like us, scale is likely to be lurking in your tube bundles,” the chief engineer warned.