Typically hot water is produced in natural-gasfired boilers, which have an efficiency of 75–85% at best. Several factors limit this efficiency. First, to avoid problems with acid condensation, the exhaust gas from these indirect-fired boilers is kept above 300°F. The benefit of the additional heat-transfer surface area (which requires stainless steel construction) to recover more heat may not justify the expense. Second, the water produced from combustio is kept in the vapor state — an additional tap on the heat of combustion. Finally, heat lost to the atmosphere causes further reduction of efficiency despite efforts to insulate all external surfaces.

Enter submerged combustion, a relatively obscure technology that offers improved thermal efficiency for hot water heating. One author compared submerged combustion to blowing air through a straw in a glass of water. If the air is heated, the rising bubbles will cause the water temperature to increase (1).

The Technology
In reality, submerged combustion is more complex than simply blowing hot air through a straw. A fuel/air mixture is ignited in a cylindrical chamber, which is submerged in a liquid solution or slurry. Positive pressure evacuates the chamber of liquid, thereby allowing adequate
liquid-free volume for essentially complete combustion before the hot exhaust gases contact the liquid.

When the gases reach the end of the cylinder, they form tiny bubbles that are in intimate contact with the liquid medium. Upon contact with the liquid, sensible heat transfer from the gas

bubbles causes a rapid drop in gas temperature and an increase in liquid temperature. At the same time, the exhaust gas becomes saturated in vapor. As the bubbles move toward the surface of the liquid, the hot saturated vapors continue to transfer
latent heat to the liquid. At the surface of a unit, the temperature of the liquid and the saturated exhaust gas are within 10°F.
“Perry’s Chemical Engineers’ Handbook” (2) describes submerged-combustion evaporators as “well suited to use with severely scaling liquids.” In one application,

submerged combustion was shown to be effective when used to concentrate either an ethylene glycol or propylene glycol solution after it removed ice from airplanes. The solution was evaporated from its spent condition of 10–20 wt.% concentration to above 50%, where the solution could be used again. Previously,
the glycol solutions were sent for disposal at a cost to the user. The cost savings of replacing and disposing of the glycol more than offset the cost of the fuel to evaporate the solution.

In addition to evaporation, submerged combustion can also be used for sensible heating. In one application, it is used to heat water to recover crude oil from tar sands. The water can be recirculated and reheated to minimize water treatment costs. With submerged combustion, the concern of tube bundle fouling is eliminated. Perry’s Handbook suggest that installations are limited to regions with low fuel cost. In reality, it is much easier to justify a more-efficient heating system when fuel costs are high (2). > CONTINUE >