What is Thermal Balance in firefighting terms?

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In firefighting terms - what is Thermal Balance? There appear to be varying definitions in relation to this term but the earliest reference appears in work completed by Keith Royer and Floyd Nelson of Iowa University during the late 1950s.

The degree of thermal balance existing in a closed room during a fire's development is dependant upon fuel supply and air availability as well as other factors. The hot area over the fire (often termed the fire plume or thermal column) causes the circulation that feeds air to the fire. However, when the ceiling and upper parts of the wall linings become super-heated, circulation slows down until the entire room develops a kind of thermal balance with temperatures distributed uniformly horizontally throughout the compartment. In vertical terms the temperatures continuously increase from bottom to top with the greatest concentration of heat at the highest level.

John D. Wiseman writes (In the Iowa State Story) that there are several meanings to the term 'thermal balance'. First it means that the energy coming into the fire must equal the energy being released. Keith Royer explains "The amount of air that comes into the fuel is directly proportional to the amount of combustion that goes away from the fuel. So basically what a fire is trying to do is to seek some sort of equilibrium between products getting away and products coming in. Anything that disrupts either flow throws it out of balance".

Royer further stated that "the importance of maintaining thermal balance in the area of extinguishment is critical. Without thermal balance operations become delayed". He went on to say that "thermal imbalance occurs through turbulent circulation of steam and smoke" in the fire area and this leads to decreased visibility and uncomfortable conditions.

It has been noted, through monitoring of compartmental temperatures at varying levels, that temperature inversions can result during over zealous use of water in either fog or straight stream (smooth-bore) form. This can lead to temperatures at floor level exceeding those at ceiling level for short periods, creating an extremely uncomfortable environment for firefighters occupying the space. In this state, the compartment could be said to be thermally imbalanced.

The US Navy referred to thermal balance when reporting on their research into 3D gas-cooling applications as follows -

"The offensive (3D) fog attack also resulted in the least amount of disturbance to the thermal layer. The disturbances of the thermal balance within the fire compartment were best shown by comparing the total heat flux measured by the calorimeters mounted 0.9 m and 2.4 m (3 ft and 8 ft) above the deck in the fire compartment. The key indicator of significant disturbances in the thermal balance was the upward spike in the 0.9-m (3-ft) heat flux that approached or met the heat flux for the 2.4-m (8-ft) calorimeter. This indicated total compartment mixing with steam.

During the offensive fog test, the initial attack actually cooled the upper layer enough to result in a 14.2-kW/sq m (1.25 Btu/sq ft-) drop in the 2.4-m (8-ft) heat flux level. While steam was produced, it was described as more of a moist "sweaty" type of steam rather than a hot penetrating steam. In contrast, for the straight-stream test, there were several instances in which the thermal balance was disturbed sufficiently to cause the upward spikes on the 0.9-m (3-ft) heat flux plot (see Figure 3). For all of the straight-stream evolutions, the thermal balance was disturbed sufficiently to impose a serious heat and steam threat to the attack team members. To highlight the significance of this finding, it was noted that none of the attack team members suffered burns during the offensive fog attack tests, whereas many of the straight-stream attacks resulted in burns to the hands, wrists, face, neck, and back".

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Thermal Balance
A fire seeks some sort of equilibrium between products flowing towards and away from the fire itself.

Maintaining Thermal Balance is Critical
Over zealous applications of water will create turbulent circulation of steam, heat and smoke leading to thermal imbalance.

Temperature Inversions
Have been noted where too much water is applied - leading to  temperatures at floor level exceeding those at the ceiling. This is a form of thermal imbalance.