Water-cooled furnaces have a number of advantages over other types:

1. The front, rear, and side walls are completely water-cooled and are of membrane construction, resulting in a leakproof enclosure for the flame, as shown in Fig. 3.2. The entire furnace expands and contracts uniformly, thus avoiding casing expansion problems. When refractory is used on the front, side, or rear walls, the sealing between the hotter membrane walls and the cooler outer casing is a concern and hot gases can sometimes leak from the furnace to the outside. This can cause corrosion of the casing, particularly if oil fuels are fired.

2. Problems associated with refractory maintenance are eliminated. Also, there is no need for annual shutdown of the boiler plant to inspect the refractory or repair it, thus lowering the cost of owning the boiler.

3. Fast boiler start-up rates are difficult with refractory-lined boilers because of the possibility of causing cracks in the refractory. However, with completely water-cooled furnaces, start-up rates are limited only by thermal stresses in the drums and are generally quicker. The tubes may be welded to the drums instead of being rolled if the start-ups are frequent. With boilers maintained in hot standby conditions using steam-heated coils located in the mud drum, even 10-15 min start-ups are feasible. With a separate small burner whose capacity is 6-8% of the total heat input in operation during boiler standby conditions, the boiler can be maintained at pressure and can be ramped up to generate 100% steam within 3-5 min.

4. Heat release rate on an area basis is lower for the water-cooled furnace by about 7-15% compared to the refractory-lined boiler. Some gas — fired boilers designed decades ago still use refractory on the floor; replacing this with a water-cooled floor will increase the effective heating surface of the furnace and lower the heat flux inside the tubes even further. The furnace exit gas temperature also decrease slightly due to the increased effective cooling surface of the furnace. A lower furnace exit gas temperature decreases the radiant energy transferred to a superheater located at the furnace exit and thus reduces the potential for superheater tube failures. A lower area heat release also helps reduce NOx, as can be seen from the correlations developed by a few burner suppliers.

5. Reradiation from the refractory on the front wall, side walls, and a floor increases the flame temperatures locally, which results in higher NOx formation. Of the total NOx generated by the burner, a significant amount of NOx is formed at the burner flame base, so providing a cooler environment for the flame near the burner helps minimize NOx to some extent.

6. Circulation was one of the concerns about the use of refractory on the floor of even gas-fired boilers because the D tubes are longer than partition tubes of the dividing wall. Heat fluxes in packaged boilers are generally low compared to those of utility boilers. To further protect the floor and roof tubes, a small inclination to the horizontal is used; also, considering the low steam pressure, tube-side velocities, heat flux, and steam quality, departure from nucleate boiling (DNB) has never been an issue, as evidenced by the operation of hundreds of boilers at pressures as high as 1000-1500 psig. The tube-side velocities are also adequate to ensure that steam bubbles do not separate from the water. Hence refractory is not required on the floor or front or rear walls for oil and gas firing.

7. Packaged boilers use economizers as the heat recovery equipment instead of air heaters, which only serve to increase the flame tempera­ture, thus increasing the NOx formation. The gas — and air-side pressure drops are also higher with air heaters, thus adding to the fan size and power consumption. The heat flux inside the furnace tubes is also reduced owing to the smaller furnace duty.