How to keep hvac condensing water systems free from contamination

June 1, 2000
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Sidestream filtration extracts a portion of the total system volume for filtration on a continuous basis. After several circulation cycles, the total system volume has passed through the sidestream filter.

Field studies have shown that filtration of the system water once per hour will provide for high-quality water in an open system. For a closed system, because the filter is generally only removing system-generated corrosion products, a total volume turnover of once every 4 to 6 hr is all that is required.

In an open system, there are two ways to install a filter: in the basin-sump of the cooling tower, or on a tap off of the water flow line. While both methods are acceptable, the preferred method is a sidestream from the basin. This produces no disruption to the process water flow and, more importantly, results in cleaning of the basin.

The water from the basin is pumped through the filter and the clean water is supplied back through PSJ sweeper jets in such a way as to sweep the dirt across the basin-sump floor and back to the suction side of the filter. Figure 3 shows a typical installation photograph.

For a closed system, filter installation is shown in Figure 4. The inlet to the filter is from the high-pressure side of the pump, while the clean water is returned to the low-pressure side of the system.

Figure 5 shows the closed-loop media filter with double actuators to isolate the filter from the closed loop during backwash.

In some cases, it may be necessary to use a portable sand filter (PSF) in order to clean up a closed-loop system, as shown in Figure 6. One typical application is at a facility with multiple systems, where a continuous preventive maintenance program is employed.

Here, the engineers tie the PSF into each closed-loop system for a period of one to three months. The water in the closed loop is sampled periodically, and the PSF is moved when the dirt load in the system is reduced by greater than 95%.

The PSF has its own pump and controls and is easily connected, as shown in Figure 4. It is an economical approach for a large facility where the PSF can be moved from building to building.

The portable PSF filter is also used as a temporary filter for cleaning up a new closed-loop system prior to tying it into the main loop. This is an important application so as not to contaminate an existing system with additional solids. The PSF is available on a rental basis, which makes it cost-effective for mechanical contractors.

Payback analysis

The installation of a fine media filter has many benefits in both open- and closed-loop applications. The benefits are energy savings, maintenance savings, chemical savings, and equipment and/or operating improvements.

Figure 7 shows before-and-after water reports conducted by an independent laboratory. This manufacturing company realized energy savings of more than $22,000 from the installation of a media filter.

Clean water protected the heat exchange equipment from fouling with solids. The filter reduced the dirt load from 97 to 1.6 lb of dirt in the tower within four weeks, and reduced the foul factor from an initial 0.0007 down to lower than design levels.

A similar result was realized at a cogeneration facility. Here, the media filter reduced the particle loading by 94% in the tower and produced a ten-fold reduction in the foul factor — to 0.0001 — for energy savings of $8,900 per year. This foul factor is less than the current design fouling factor from ARI of 0.00025.

In another example, the Birmingham Airport Authority realized the following benefits with the installation of filters for their hvac systems for the airport terminals:

  • Tower cleaning reduced by 90%;

  • Basin cleaning time reduced to 1.5 workhours;

  • Chiller efficiency improved;

  • Chiller cleaning reduced to every two years;

  • Biocide usage decreased;

  • Conductivity setpoint increased seven-fold; and

  • Setpoint increase resulted in less blowdown, less chemical usage, and greater water savings.

The payback for this application was approximately one year.

Conclusion

Water treatment is a continuing process, not an event. Many variables must be taken into account to have an effective program.

The combination of the correct chemicals and the correct mechanical filtration system will result in an optimized open- or closed-loop system.

The operating benefits include improved heat transfer and energy efficiency, maximized efficiency of the towers and chillers, simplified and reduced-cost chemical treatment program and overall enhanced system cleanliness.

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