Superheater model was used to analyzing the impact of ash deposits on the outer tube surfaces and scales formed on the inner tube surfaces on the heat flow rate transferred from the flue gas to the steam. The influence of uneven heating of parallel superheater tubes on the flow and temperature maldistribution was also examined. Excessive heating of some tubes reduces the mass flow rate of the steam flowing through these tubes. It may result in overheating of the tube material. Excessive local tube heating by a higher temperature flue gas, in combination with lower steam mass flow rate through each tube, can contribute to the overheating of the tube material. Ash fouling causes a significant decrease in heat flux absorbed by the steam and lowering the temperature of the tube metal. Scale depositions on the inner surface of the tube cause a big increase in wall temperature of superheater tubes despite a decrease in the heat flux transferred from the flue gas to the steam. (C) 2015 Elsevier Ltd. All rights reserved.