I think I can answer. Without a mixer, and with the boiler being underfired, moisture is falling out of the flue gas stream at the boiler outlet. This moisture enters with both the primary air and with the fuel. Perhaps this scenerio (boiler low firing rate) is utilized to increase efficiency. In a perfect world, flue gas temp would never be allowed to reach dewpoint.
Without a mixer, flue gas temp would need to be increased at boiler outlet (increased firing rate). During the off cycle, boiler would need to be "bottled up", using dampers to isolate the fireside. And, the water flow would need to be diverted away from boiler.
I would consider the need for the mixer a bandaid approach to minimize equipment at the setting.
But, this is but a small operation, so I am probably okay with it. I guess. BUT, the mixer does reduce NPSH at the pump inlet because of elevated temp.
