The argument has been made that water returning to boiler has to be kept over 140 degrees or you will thermo-shock the steel in the boiler.
In the wood gasification boiler I believe it also possible to thermo-shock the refractory component. Now I have a very small cross section of this field to base this conclusion on, the operation of my own Jetstream, two other Jetstreams and a recent post " Gasification Chamber Refractory" where a unit is seeing refractory cracking after only three years of use.
On the Jetstream I have operated for nearly 30 years, its refractory is nearing the end of its life. Over the years I have seen refractory damage when the boiler water has reached high limit and has shut the blower off with a full charge of wood. There has been a fairly consistent pattern. Any time the blower would shut down at maximum fire and I would find small pieces of refractory when cleaning out the fly ash. When I have sized the finale charge of wood correctly and the storage tank has achieved its desired temperature so that the fire goes out with no cycling of the blower, I have seen no damage.
The two other Jetstreams that I have knowledge of. One worked with no heat storage at all it and went through its original and a second refractory liner inside of five years. The other had heat storage, but the heat exchange to the tank did not work properly, so the blower would constantly cycle on and off. This unit also had a very short life.
My conclusion is at least with the Jetstreams, is if the unit is run where the blower cycles, the preheated blower air that accelerates the burn and elevates burn chamber temperatures also creates a consistent refractory temperature and if this air is instantly cut off, the temperature in the burn chamber is momentarily elevated causing thermal shock to the refractory.
In the wood gasification boiler I believe it also possible to thermo-shock the refractory component. Now I have a very small cross section of this field to base this conclusion on, the operation of my own Jetstream, two other Jetstreams and a recent post " Gasification Chamber Refractory" where a unit is seeing refractory cracking after only three years of use.
On the Jetstream I have operated for nearly 30 years, its refractory is nearing the end of its life. Over the years I have seen refractory damage when the boiler water has reached high limit and has shut the blower off with a full charge of wood. There has been a fairly consistent pattern. Any time the blower would shut down at maximum fire and I would find small pieces of refractory when cleaning out the fly ash. When I have sized the finale charge of wood correctly and the storage tank has achieved its desired temperature so that the fire goes out with no cycling of the blower, I have seen no damage.
The two other Jetstreams that I have knowledge of. One worked with no heat storage at all it and went through its original and a second refractory liner inside of five years. The other had heat storage, but the heat exchange to the tank did not work properly, so the blower would constantly cycle on and off. This unit also had a very short life.
My conclusion is at least with the Jetstreams, is if the unit is run where the blower cycles, the preheated blower air that accelerates the burn and elevates burn chamber temperatures also creates a consistent refractory temperature and if this air is instantly cut off, the temperature in the burn chamber is momentarily elevated causing thermal shock to the refractory.