National Defense Research Lab Fights Silica In Heat Transfer
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 incorporates the use of a polymer base inhibitor and maintained low cycles of concentration (2.5 to 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 early 2001, two 1,200-ton and two 600-ton chillers were installed in the LANL Super Computer Center (SCC). These chillers are equipped with an on-line Automatic Tube Brushing (ATB) system manufactured by Water Technology of Pensacola, Inc. (Pensacola, FL) in the condenser tubes to fight the silica problem.
LANL's history with the ATB System goes back to the late 1980s, 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.
Nothing To Bristle AtThe 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 four-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.
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 four-way backflushing valve, from 24 to 48 in. 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 1,200 tons). A system cycle counter allows the enduser to monitor valve reversals.
Brushing Up On SavingsSolid 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," stated 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 said, "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. ES