Heat Density : This term is commonly applied to the heat release through the cross section of the firetube, expressed as BTU/hour/square inch of cross sectional area. (Spec 12K).
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}