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More Passes Does Not Mean More Efficiency

Posted by Ritchie Ware on Aug 28, 2017 9:51:11 AM

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At WARE, we strive to educate you about the intricacies of boiler engineering and product design. One of the common misconceptions is that more passes in a boiler equals more efficiency, but this isn’t always the case.

Boiler efficiency is a measure of how efficient a boiler is in converting its fuel into energy. A boiler operating at 85% is able to convert 85% of its fuel into energy for use in a plant. The other 15% is lost through radiation and convection processes.

Why More Passes?

Boilers with more passes provide more opportunities for hot gases to transfer heat to the water in a boiler. So while it seems easy to say that a 4-pass boiler is more efficient than a 3-pass boiler, it’s not that cut and dry.

Boiler efficiency is highly affected by tube design. If the design, tubing and all other components of a boiler are equal, then yes, a 4-pass boiler will generally be more efficient than a 3-pass boiler. This is because the 4-pass boiler has a greater heat transfer time. However, a 3-pass boiler with a tube design that allows more transfer time can have en equal or higher efficiency rating than a 4-pass boiler with standard tubes.

A 3-pass boiler design allows the boiler to do more work in the furnace because of its larger heating surface area. In fact, in a York-Shipley 3-pass boiler, 55% of the heat input is transferred in the furnace. This type of process makes steam sooner and leads to faster steam production. In comparison, a 4-pass boiler transfers 40% of the heat input in the furnace.

A 4-pass boiler design tends to be shorter than a 3-pass design. 4-pass boilers do more work in the convective section (60%) as opposed to the furnace, where the the fire is. 3-pass boilers do 45% of their work in the convective section. Ideally, a boiler would be a 1-pass design. But this would result in a very long, impractical boiler.   

What Are the Design Differences?

3-pass boilers use a downdraft design that places the coolest water in the coolest part of the boiler. This keeps the cooler water from having an effect on the hot surfaces within the boiler. Conversely, 4-pass boilers operate with a natural draft. Because of this, they they allow cool water to contact the hottest part of the boiler.

At WARE, we use X-ID tubing to provide increased efficiency in boilers with less passes. X-ID tubes are used within the boiler. They have ribs embossed inside of them. These ribs are engineered to a specific height in order to get heat tumbling through the tube and create more productive heat transfer. We do as much work as is practical in the furnace and then use X_ID in the remaining tubes to absorb heat more rapidly in the remaining sections.

The Conclusion?

More does not always mean better in terms of boiler design. More tubes may seem to imply more efficiently, however as with many things in life, it’s about quality — not quantity. With recent innovations in tube design, 3-pass boilers can be just as efficient as their 4-pass cousins.

Topics: Preventative Maintenance Checklist, Boiler Tuning

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