The old system, a 300 BHP firetube boiler, could not be used to its full capacity due to California's strict limitations on the amount of oxides of nitrogen (NOx) that gas-fired boilers can emit. "We would have had plenty of capacity if we could have used the boiler's full potential," says Roy Ziegenfuss, Valley Fig Growers' maintenance superintendent, "but then we would have been in violation of the local smog laws. We had to derate our old boiler down so much that we weren't able to get 300 BHP. We were getting around 175 BHP, which sometimes made it hard for us to maintain proper steam pressure."
In addition to these emissions issues, Valley Fig's 300 BHP firetube boiler took four hours to heat up after shutdown. The unit ran 24 hours a day, five days a week. On the weekend, it was shutdown to allow it time to cool off. Downtime for the boiler may have been good for the equipment but it wasn't so great for the employees. Every Monday morning, one employee had to arrive at work at 2:00 a.m. to start the boiler and have it ready to produce steam by 6:00 a.m.
The inefficiency, both in steam pressure and in time, led Valley Fig Growers to replace its old boiler with two, 100 BHP Miuras. The boilers use the pre-mix method of NOx control. The pre-mix burners combine gas and air in a manifold. They are burned through a flat burner with several little holes in it, which reduces flame temperature and, ultimately, reduces NOx. Reducing the size of the upper and lower headers, eliminating water from the upper header and partially eliminating water from the tubes by having a two-phase mixture of steam and water in the tubes accomplishes a reduction in water volume. This low water content allows for a faster start-up.
"From dead cold, we can have 125 psi of steam in eight to ten minutes," says Ziegenfuss. "If we were to valve off the boiler, it could heat itself up and be to temperature in about five minutes. But we're heating up the boiler and pressurizing our whole plant all at the same time and it still does it in under 10 minutes."
As part of its rebate program, PG&E certified that the boilers achieved the efficiency promised and awarded the $16,000 rebate to Valley Fig Growers. "We've found that we're running more efficiently, our exhaust gases meet the state requirements and, to date, it looks like we're saving eight to ten percent on our gas bills," says Emigh. Perhaps even better yet, no one has to go to work at 2:00 a.m. u
For more information:
Mark Utzinger, Miura Boiler Inc., 847-465-0001