Refractory Maintenance for Furnaces and Boilers. Refractory vendors
recommend a maximum heating and cooling rate of about 200 °F/h to avoid excessive
shrinkage of ceramic fiber. The ceramic fiber blocks are held together by compression
and repeated temperature cycles cause gaps between modules. If these gaps are not filled
back, heat will seep through the gaps and heat up the furnace casing, leading to heat loss
and other maintenance issues. Over the years, hot spots in carbon steel casings, and in
some cases buckling of casings, have been reported by many plants. Buckling will also
lead to uneven surface temperatures and corresponding uneven heat distribution in the
firebox, causing a loss of efficiency.
In order to extend the longevity of furnace refractory and prevent deterioration and
increased heat loss, it is important to inspect the refractory and make minor repairs as
required during every outage and furnace entry. Furnaces are designed typically for a
1.5% heat loss from the refractory. It is not uncommon to see heat loss increase to as
much as 3% over the years due to lack of refractory maintenance, resulting in a
corresponding 1–2% drop in furnace efficiency (Figure 9.15).
One of the major problematic areas in a boiler is refractory failure. Often, refractory
in a boiler is subjected to rough treatment. Seldom do boilers burn clean fuel. It is
important to choose refractory appropriate to the fuel being burned to avoid refractory
failures and loss of efficiency. Equally important is the curing and dry out of refractory
after repair.