Thermal Fluid Heating Systems Sigma Thermal offers various types of thermal fluid heaters as well as complete thermal fluid systems. Exhaust Stack Exhaust Stack Responsible for removing the combustion gases from the heat exchangers. Thermal Fluid Heater Thermal Fluid Heater Indirect heating in which a liquid phase heat transfer medium is heated and circulated to one or more heat energy users within a closed loop system. Pump Skid Pump Skid A pump skid includes a pump and a driver such as an electric motor or diesel engine mounted on a common steel base. Expansion Tank Expansion Tank Vessel designed to contain thermal expansion within a closed loop thermal fuid system. expansion within a closed loop thermal fuid system.