Having open space between the individual coils and between the coils and the wall of the combustion chamber also reduces localized heat flux. To prevent the fluid from exceeding its maximum film temperature, full convection heaters are available. These units separate the burner from the heat exchanger, eliminating the radiant section. Look at Oil Color . Most oils are rated for similar bulk and film temperatures.
Mineral oils consist of napthenic and paraffinic carbon atoms in various proportions.
From a durability standpoint, these molecules are the most desirable. Less refined oils retain certain heterocyclic derivatives of nitrogen, sulfur and oxygen as well as other polar compounds. These compounds are more susceptible to degradation at high temperatures. Mineral oil color is an indication of the types of molecules used in the formulation. Those with the least impurities are water white.