THE BOILING PROCESS
When thermal energy is applied to furnace tubes, the process of boiling is initiated. However, the fluid leaving the furnace tubes and going back to the steam drum is not 100% steam but is a mixture of water and steam. The ratio of the mixture flow to steam generated is known as the circulation ratio, CR. Typically the steam quality in the furnace tubes is 5-8%, which means that it is mostly water, which translates into a CR in the range from about 20 to 12. CR is the inverse of steam quality. Circulation calculations and the importance of heat fluxes are discussed in Q7.29.
Nucleate boiling is the process generally preferred in boilers. In this process, the steam bubbles generated by the thermal energy are removed by the flow of the mixture inside the tubes at the same rate, so the tubes are kept cool. Boiling heat transfer coefficients are very high, on the order of 50008000 Btu/ft2 h °F as discussed in Q8.46. When the intensity of thermal energy or heat flux exceeds a value known as the critical heat flux, then the process of nucleate boiling is disrupted. The bubbles formed inside the tubes are not removed adequately by the cooler water; the bubbles interfere with the flow of water and form a film of superheated steam inside the tubes, which has a lower heat transfer coefficient and can therefore increase the tube wall temperatures significantly as illustrated in Fig. 3.13. It is the designer’s job to ensure that we are
Figure 3.13 Boiling process and DNB in boiler tubes.
Never close to critical heat flux conditions. Generally, packaged boilers operate at low pressures compared to utility boilers and therefore DNB is generally not a concern. The actual heat fluxes range from 40,000 to 70,000 Btu/ft2 h, while critical heat flux could be in excess of 250,000 Btu/ft2 h. However, one has to perform circulation calculations on all the parallel circuits in the boiler, particularly the front wall, which is exposed to the flame, to ensure that there is adequate flow in each tube. In the ABCO D-type boiler, carefully sized orifices are used to limit the flow of mixture through the D headers while ensuring flow through all the tubes in the front wall. Ribbed or rifled tubes are sometimes used as evaporator tubes. These tubes ensure that the wetting of the tube periphery is better than in plain tubes. They have spiral grooves cut into their inner wall surface. The swirl flow induced by the ribbed tubes not only forces more water outward onto the tube walls but also promotes general mixing between the phases to counteract the gravitational stratification effects in a nonvertical tube. Ribbed or twisted tubes can handle a much higher heat flux, often 50% higher than plain tubes. They are expensive to use but offer a safety net in regions of high heat flux, particularly in very high pressure boilers.
In fire tube boilers, the critical heat flux may be estimated as shown in Q8.47. Again owing to the low pressure of steam, the allowable heat flux to avoid DNB is much higher than the actual values; hence tube failures are rare unless tube deposits or scale formation is severe. As discussed later in this chapter, maintaining good boiler water chemistry, ensuring proper blowdown, and adding chemicals to maintain proper alkalinity and pH in the boiler should minimize scale formation and thus prevent tube failures.