Knowing what the composition of the deposited material is is very important in order to decide what process and chemical cleaning solution to use. If you have the ability to analyze the deposits I suggest you do so.
The Betz (now GE Water) and Nalco industrial water conditioning handbooks have good general information on chemical cleaning of industrial boilers. Specific parameters such as the type of agents, solution concentration, recirculation time, cleaning system configuration, recirculation rate, and solution temperature will have to be determined by someone with expertise in this area.