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Los Alamos National Laboratories

National defense research lab
fights silica in heat transfer

Los Alamos National Laboratories (LANL) in Los
Alamos, New Mexico, is the primary research lab for national security research and development. For years LANL searched for a solution to the buildup of silica in its chiller condenser tubes. The concentration of silica in the water, about 90 parts per million, contributed to tube scaling and great losses in energy efficiency until the research agency implemented a program wherein the tubes
are automatically cleaned while the chillers remain on-line. When it forms on the tubes, the silica acts as an insulator to heat transfer, which in turn increases the operational costs of the chillers. To keep the silica in suspension LANL has incorporated the use of a polymer base inhibitor and maintained low cycles of concentration (2½-3) in the cooling towers. Still, the amount of buildup
in the tubes requires regular manual cleaning, which is costly in terms of maintenance, downtime and additional labor expenses since the cleaning has to be scheduled over the weekends.
In April 2001, LANL had two new 1200-ton and two new 600-ton chillers installed in its Super Computer Center (SCC). These chillers are equipped with the on-line Automatic Tube Brushing (ATB) System in the condenser tubes to control the silica problem. LANL’s history with the ATB System goes back to 1988, when a new 800-ton chiller was fitted with the system. Designed to keep condenser tubes continuously clean and thereby provide energy savings through increased heat transfer, the system has been very effective in eliminating the silica problem at LANL.

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NOTHING TO BRISTLE AT

     The ATB System consists of a small nylon-bristled brush installed in each condenser tube, open catch baskets that are epoxied into the tube ends in order to suspend the brushes in the water flow, and the ATB System 4-way backflushing valve. The valve automatically reverses the flow through the tubes several times per day for a period of 30 seconds, which propels the brushes through the tubes and cleans the tube walls many times each day. The system is manufactured by Water Technology in Pensacola, Florida.
     No special challenges came up during the SCC installation. The only requirement for the system is that the supply and return piping for the condenser have a parallel run large enough to accommodate the 4-way backflushing valve, from 24-48” in length depending on the piping size.
For retrofits, Water Technology custom manufactures the valves to fit into existing piping.
     The ATB System Control Panel is equipped with a 24-hour clock and adjustable relays that control the duration of the valve reversal. Control options include a flow-switch bypass and an automated chiller-unloading feature (required in chillers over 1200 tons). A system cycle counter allows the end-user to monitor valve reversals.

BRUSHING UP ON SAVINGS

     Solid data that takes into account load factors, chiller efficiency ratings and local power costs indicate less than a two-year payback on the four SCC systems, which cost just under $100,000. Savings are derived from reduced maintenance requirements and from the energy savings
that result from increased heat transfer when fouling is eliminated in the condenser tubes.
     “We have a couple of 500-ton chillers that were installed without the brush system in 1986,” states Doug Hefele, P.E., Facility Engineer at the LANL Super Computer Center. “We have to clean the condenser tubes regularly in those, but we have never once had to rod out the tubes in the 800-ton unit installed in 1988 with the ATB System.”
     “I’ve purchased a number of chillers in the last few years,” Hefele says, “and after seeing the results, I’ve made sure all of them are equipped with the brush system.” Other buildings at LANL that have the brush system include the Target Fab center and the Laboratory Data Communication Center. According to Hefele, those chiller condensers, like the ones at the SCC, never require manual cleaning.