Fire tube boiler design considerations
The sizing procedures for fire tube boilers are discussed in Q8.10. It may be noted that the tube size plays a significant role in minimizing the length of the boiler. With small gas flows, one may consider multi-gas-pass design, which can reduce the overall length. Tube sizes vary from 1.5 to 2.5 in. OD; smaller tubes generally have lower tube wall temperatures and also require less surface area and shorter tube length. Hence a comparison of surface areas of two or more designs should be made with caution. Heat fluxes are quite low in fire tube boilers owing to low gas-side heat transfer coefficients, an exception being gas streams in hydrogen plants, as discussed earlier. SA 178a carbon steel tubes are typically used for evaporators handling common flue gases. In reformed gas applications, T11 or T22 tubes are preferred. Gas pressure drop can range from 3 to 6 in. WC in flue gas heat recovery boilers and about 1 psi in high gas pressure applications such as reformed gas boilers.
Boiler circulation may be checked using methods discussed in Q7.32. With poor water quality, fouling and scale formation are of concern, and tube wall temperatures can increase significantly with scale thickness as discussed in Q8.13.
Elevated steam drum design is generally used if the steam purity has to be less than 1 ppm. External downcomers and risers help cool the tubes and tube sheet by circulating the water-steam mixture over them. If the flue gas temperature is below 1500°F, then an elevated drum design may be dispensed with and a single-shell fire tube boiler may be used. The steam purity without internals is low, on the order of 5-15 ppm, which may be adequate for low pressure process heating applications.
Owing to the large inventory of water, fire tube boilers respond slowly to load changes compared with water tube units. However, the pressure decay on loss of heat input will also be smaller.