Complex sentence: The forward boilers were grouped into three compartments with three oil-fired boilers in each while the rear boilers were split into four compartments, each with four coal-fired boilers fitted with oil sprayers.
Split into simpler sentences: The forward boilers were grouped into three compartments with three oil-fired boilers in each compartment. The rear boilers were split into four compartments with each containing four coal-fired boilers fitted with oil sprayers.