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Two Men and a Burner (A Midco saga)
Last month JStar came out to help diagnose a hard light-off issue I'd been occasionally experiencing with the Midco burner. Long story short, we ended up rotating the burner head so that the spark rod assembly was on top of the burner, rather than below as supplied from the factory. Once we could watch the spark thru the firebox viewport (couldn't see it when it was underneath the burner), we observed that the spark would jump between the ground rod and the burner mesh. After repositioning and re-gapping the spark rod, we saw good ignition every time. I sent a detailed report to Midco describing our observations and changes, and they agreed with what we'd done. In over 200 firings since, there have been no hard light-offs.
In August, when Midco replaced the 500 MBH burner head with a 300 MBH version they made up custom for this boiler, stack temps were still running high at 780 degrees at high fire (260 MBH), efficiency was around 72% (78% at low fire 75 MBH), and excess air was 42%. It was pretty clear that a lot of heat was being lost up the stack, but there wasn't much else that could be done to the burner to fix it.
Today, JStar brought some ideas to try to bring down the stack temperature and increase the efficiency. First he installed some angle iron T baffles between the boiler sections. On the very next firing we saw the stack temperature drop by 100 degrees and the high fire efficiency increase to 78%, with excess air at 34%.
The Midco has a variable speed blower so there's no other adjustment for combustion air. Joe brought in a HUGE PVC ball valve and attached it to the blower inlet. We tried several valve positions to restrict the combustion air to the blower and took analyzer readings for each. With the valve 5/8ths closed, Joe got excess air down to 11.5% with 81.1% efficiency at low fire, but CO went through the roof. The "sweet spot" seems to be with the valve half closed, with the following readings at low and high fire: stack temp = 575 / 685, excess air = 19 / 26.5, efficiency = 80.6 / 78, CO = 25 / 8. If you're interested, here's a link to all of the combustion results: Combustion Results
In addition to the combustion analyzer readings, I was also watching the stack temp and steam pressure readings on my control system. With 1 ounce of pressure at the boiler and a constant firing rate, we saw steam pressure increase almost 2/10ths of an ounce when we introduced the 50% combustion air restriction, and half an ounce when 5/8 restricted. At first I thought it was a fluke but the results were repeatable, a clear demonstration of the actual impact changes like this make to system performance. Pretty neat.
Next Sunday I'll be posting a summary report of the Midco's performance data for October and November.
And finally, a picture of the reconfigured Midco burner with the combustion air restrictor valve in place. Kids, don't try this at home.
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