Boiler circulation calculations, Study notes of Engineering

Download Boiler circulation calculations and more Study notes Engineering in PDF only on Docsity! HEAT TRANSFER Boiler circulation calculations Steam generator studies can be complex. Use these guidelines to perform them effectively V Ganapathy, ABCO Industries, Abilene, Texas Natural circulation water tube and fire tube boilers (Figs. 1 and 2) are widely used in the chemi cal process industry. These are preferred to forced circulation boilers (Fig. 3) where a circulation pump ensures flow of a steam/water mixture through the tubes. In addition to being an operating expense, a pump failure can have serious consequences in such systems. The motive force driving the steam/water mixture through the tubes (water tube boilers) or over tubes (fire tube boilers) in natural-circulation systems is the difference in density between cooler water in the downcomer circuits and the steam/water mixture in the riser tubes. This flow must be adequate to cool the tubes and prevent overheating. This article explains how circulation ratio or the ratio of steam/water mixture to steam flow may be evaluated. Circulation ratio (CR) by itself does not give a complete picture of the circulation system. Natural-circulation boiling circuits are in successful operation with CRs ranging from 4 to 8 at high steam pressures (1,500 to 2,100 psig) in large utility and industrial boilers. In waste-heat boiler systems, CR may range from 15 to 50 at low steam pressures (1,000 to 200 psig). CR must be used in conjunction with heat flux, steam pressure, tube size, orientation, roughness of tubes, water quality, etc., to understand the boiling process and its reliability. Tube failures occur due to conditions known as departure from nucleate boiling (DNB) when the actual heat flux in the boiling circuit exceeds a critical value known as critical heat flux-a function of the variables mentioned above. When this occurs, the rate of bubble formation is so high compared to the rate at which they are carried away by the mixture that the tube is not cooled properly, resulting in overheating and failure. Circulation process. Fig. 1 shows a typical water-tube, natural-circulation waste-heat boiler with an external steam drum and external downcomers and riser pipes. Feed water enters the drum from an economizer or Steam Fig. 1. Schematic of a water tube boiler. deaerator. This mixes with the steam/ water mixture inside the drum. Downcomers carry the resultant cool water to the bottom of the evaporator tubes while external risers carry the water/steam mixture to the steam drum. The heat transfer tubes also act as risers generating steam. The quantity of mixture flowing through the system is determined by calculating the CR. This is a trial-and-error procedure and is quite involved when there are multiple paths for downcomers, risers and evaporator circuits. Each boiling circuit has its own CR depending on the steam generated and system resistance. One can split up any evaporator into various parallel paths, each with its own steam generation and CR. Splitting up is done based on judgment and experience. A particular circuit may be examined in detail if the process engineer feels that it offers more resistance to circulation or if it is exposed to high heat flux conditions. Several low heat flux circuits can be clubbed into one circuit to reduce computing time. Hence, an average CR for the entire system does not give the complete picture. Circulation ratio. CR is defined as the ratio of the mass of steam/water mixture to steam generation. The mass of the mixture flowing in the system is determined by balancing the thermal head available with various system losses, including: • Friction and other losses in the downcomer piping, including bends Two -phase friction, acceleration and gravity losses in the heated riser tubes Continued HYDROCARBON PROCESSING/ JANUARY 1998 101 • Friction and other losses in the external riser piping • Gravity loss in the riser piping • Losses in drum internals. COMPUTING THE VARIOUS LOSSES Total thermal head. The total thermal head available in psi = H/vl/144 where H is the thermal head, ft (Fig. 1) vl is the specific volume of water, ft3/lb Downcomer losses. Let the average CR for the system = CR and the total steam generation = WS lb/hr. The total mixture flowing through the system = WS x CR Let the effective length (including bends) of the downcomer piping in ft = Le The frictional pressure drop, psi = 3.36 X 10-s x f Le vi (Wd) 2 /di 5 (Here, it is assumed that the average flow in each downcomer pipe is Wd). di is the inner diameter of the downcomer pipe in inches. f is the friction factor. If there are several parallel paths or series -parallel paths, then the flow and pressure drop in each path is determined using electrical analogy. This calculation may require a computer. In addition to the frictional drop, the inlet (0.5 x velocity head) and exit losses (1 X velocity head) must be computed. Sometimes the pipe inner diameter is larger than the inner diameter of the nozzle at the ends, in which case the higher velocity at the nozzles must be used to compute the inlet/exit losses. Velocity V in ft /s = 0.05 Wd v l/die and velocity head, psi = V2/2 g vl/144. Heated riser losses. The boiling height must first be determined. This is the vertical distance the mixture travels before the boiling process begins. It can be shown by calculation that the mixture's enthalpy entering the evaporator section is usually less than that of saturated liquid. The following is the energy balance around the steam Fig. 3. A forced-circulation system showing multiple streams to reduce pressure drop. Steam drum, as in Fig. 1: Wmh+W fhf=Wmhm+Wsh„ Wm = mixtureflowing through the system, lb/hr =Ws x CR hv, hm, hf, and h are the enthalpies of saturated steam mixture leaving the drum, feed water entering the drum and mixture leaving the drum, Btu/lb. h=(hv/CR)+(1-1/CR)hl where hv, h j = enthalpies of saturated vapor and liquid, Btu/lb. From the above, hm is solved for. The boiling height or the distance the mixture travels before boiling starts, Hb, is determined from: Hb = He X WS X CR X (hl - hm)/Qs where He = height of evaporator tubes, ft (For conservative calculations, Hb may be assumed to be zero.) There are basically three losses in boiling evaporator tubes: Friction loss. ∆pf= 4 X 10-10 vl X f L X Gi2 X r3/di where Gi= mixture mass velocity inside tubes, lb/ft2hr f= fanning friction factor L= effective length, ft di= tube inner diameter, in. r3= Thom's multiplication factor for two- phase friction loss (Fig. 4a). Gravity loss in tubes. ∆Pg = 0.00695 (He - Hb) r4/v1 Thom's multiplication factor for gravity loss, r4 is shown in Fig. 4c. 2 HYDROCARBON PROCESSING /JANUARY 1998