A Brush with Death (to fouling problems)
The West Los Angeles Veterans Administration Medical Center, a 350-bed hospital, has an average patient load of about 300 people. This creates a demand for cooling in the hospital of a minimum 1,300 tons year-round, with an additional 1,300 tons required in the summer months.
In 1998, Lee Hayes, Director of Plant Operations and Maintenance, purchased two new 1,300-ton Trane chillers with “helix” style internally enhanced condenser tubes for the West LA VAMC. The advanced tube design offers increased heat transfer and therefore decreased operating costs, providing the tubes are kept scale and deposit free. But as with any chiller, when the fouling in tubes exceeds the .00025 design fouling factor, chiller capacity decreases while kw/ton requirements increase. To maximize the benefit of the new condenser tube, Hayes needed to find a solution to tube fouling.
For over ten years, the Washington DC engineering office for the VAMC has included an option in new chiller specifications that is aimed at eliminating tube fouling. The Automatic Tube Brushing (ATB) System, an on-line condenser tube cleaning system that guarantees to maintain condenser tubes at the .00025 design fouling factor, is available to all VA Medical Centers installing new chillers. Hayes decided to give the system a try.
A Simple Solution
The ATB System consists of a single diverter valve set on an automatic timer to reverse the flow through the condenser every four to six hours for a period of about 30 seconds. During the flow reversal, small nylon-bristled brushes shuttle through the condenser tubes to wipe away foulants before they have a chance to form scale on the tube walls. Open-ended catch baskets are installed in the tube ends to capture the brushes at each end of the tube sheet.
The ATB System control panel is set from the factory to reverse the valve every six hours, but an end-user can easily change the settings if desired. Some hospitals, for example, choose to leave the valve in normal flow during the peak load hours each day, while reversing it every three hours during the cooler part of the day.
Jesse Barfield, Plant Supervisor at the West Los Angeles VAMC, states, “Because our chillers have the brush system, the head pressures are as good as when the machines were new four years ago. We automatically clean tubes every four hours, six times a day, instead of once a year manually.”
Rick George, Plant Operator at the West LA VAMC, says, “I would recommend the brush system to anyone who wants a clean chiller.”
Maintenance savings are obvious at the West LA VAMC. When coupled with the energy savings the system provides by keeping the condenser tubes at their design fouling factor, the ATB System easily paid for itself during the first year of operation.
The heavy load requirements (at least 1,300 tons year round) and high power costs ($0.08/kwh) make the Return on Investment much better at the West LA VAMC than it might be on an average, but most every installation will provide a payback of less than two years. Using average fouling (.0018) as a baseline, compared to the design fouling factor of .00025, the estimated load hours, power costs and kw/ton rating of the chiller are taken into account.
Each system at the West LA VAMC had a material cost of about $33,000, which includes the on-site installation of the nylon-bristled brushes and open catch-baskets in the tube ends by the manufacturer. Adding in a contractor’s costs to install the reversing valve and controls makes the entire system cost just over $50,000 on 1,300-ton units.
This is minimal when compared to the energy costs required to produce the hospital’s load requirements each year, about $350,000 per unit. By keeping fouling factors at the design .00025 rating, the system is able to help the chiller maintain its design kw/ton rating of .56, rather than operating at the .65 kw/ton rating to which it would normally increase with average fouling.
The $350,000 energy cost drops to $293,000 each year, a savings of $57,000 with a total investment of just over $50,000. With these numbers, it is easy to see why the Washington DC VAMC engineering office recommends the ATB System to its hospitals.