WATER TUBE BOILER DESIGN CONSIDERATIONS
The design procedure for waste heat boilers is quite involved. With a given set of inlet gas conditions such as flow and temperature, we have to see how the various heating surfaces respond. The surfaces could consist of bare or finned tubes. The superheater could have one or more stages; a screen section may or may not be used. Import steam could come from another boiler to be superheated in the boiler in question, or saturated steam may be drawn off the steam drum for deaeration or process purposes. The feedwater temperature or steam pressure could vary depending on plant facilities.
Before attempting to evaluate the performance of a complete waste heat boiler, one must first know how to obtain the performance of individual components such as the superheater, evaporator, and economizer by using the number of transfer units (NTU) method or through trial and error. This is discussed in Q8.29 and Q8.30. Once we know how to evaluate the performance of each surface, evaluating the overall performance of a waste heat boiler is simple. Figure 2.17 shows the logic for a simple waste heat boiler consisting of a superheater, evaporator, and economizer. A few iterations may be required, because we have to first assume a steam flow and completely solve all the other sections and then check on whether the assumed steam flow was fine. A
Figure 2.17 Logic used for evaluating HRSG performance.
Computer program is required, because these calculations become tedious with two-stage superheaters with attemperation, a combination of bare and finned tubes in evaporators, and the use of import or export steam, to mention a few variables. Also the incinerator may operate at different combinations of gas flows, temperatures, and gas analysis. The performance has to be checked at different operating points before finalizing it.
Figure 2.18 Shows the printout of results for a water tube waste heat boiler for a gas turbine exhaust consisting of a furnace section, a screen section, a two — stage superheater, an evaporator consisting of bare and finned tubes, and a finned tube economizer. In the unfired mode this HRSG makes about 45,000 lb/h of steam. The turbine exhaust enters the HRSG at 980°F, which is raised to 2175°F by the burner located at the HRSG inlet to generate 150,000 lb/h of steam at 620psig and 750°F. The oxygen content has decreased from 15% to 8.39% by volume and the burner duty is 123 MM Btu/h on LHV basis. The gas temperature drops to 2063°F in the furnace section and is cooled to 1852°F in the screen section before entering the superheater. The gas pressure drop in the HRSG is about 6 in. WC. To this must be added the burner, selective catalytic reduction (SCR), and duct losses. The printout also shows the tube wall temperatures, fin tip temperatures, heat transfer coefficients at various sections both inside and outside the tubes, and the gas — and steam/water-side pressure drops. The amount of spray water used for attemperation is also computed. Several variables can be changed to check the effect on performance. The evaporator uses different fin configurations. This is done to minimize the heat flux inside the evaporator tubes and also the tube wall and fin tip temperatures. The boiler duty is 177 MM Btu/h. The fuel used is typically natural gas.
Boiler tube sizes typically range from 1.5 to 2.5 in. and fin density can vary from 2 to 6 fins/in. depending upon the design. Bare tube boilers are used in dirty gas applications. Sometimes multipass designs offer a compact design. Whereas with finned tubes, both in-line and staggered arrangements are used, an in-line arrangement is generally used with bare tubes because it is inefficient to use a staggered arrangement, as discussed in Q8.22. Tube spacing can vary depending on gas velocity, dirtiness of the gas stream, and heat transfer considerations. A radiant furnace is also used if the incoming gas is at a high temperature and has the potential to cause slagging problems. Superheaters can be of bare tube or finned tube design, depending upon the gas temperature and cleanliness. Generally a low fin density is preferred for superheaters owing to the low heat transfer coefficient inside tubes, as discussed in Q8.22 and Q8.27. Superheater tubes can be vertical or horizontal depending on size or layout considerations. Economizers are of bare tube design in dirty gas applications and use finned tubes in clean gas applications. In sulfuric acid plants, a few suppliers use cast iron gilled tubes.
WASTE HEAT BOILER PERFORMANCE Project MYBOILER Remarks… units: British case : fired case *** Date : 21-09-01
Gas flow-ib/h=317370 Gas Temp in — F=980 GAS MW =28. Gas pres — psia-14 5
Dmm pres-psia-658 sat temp-F-497 % blow dOwn= 1. fw temp-F=230
Ext duty -MMBtu/h-11.6 % heat loss= 1. foul ftr in-ft2hF/Btu-.001 pros stm -!b/h=
Furnace cools the gases before entering the convection section
Furnace width—ft=9.5 depth=-ft=1t length—ft=l 8. prnj area=-ft2-760
Firing temperature is ~F= 2175
Fuel — GAS tot gas-lb/h-323084 bur duty-MMBtu/h=122.52 fuel LHV-Btu/lb=21439 % gAs Row to scm= 100 sb — 100 evap — 100 econ= 100
C02 H20 N2 02 S02 HCL H2S H2 CO CH4 S03
3. 7. 75. 15…………………………..
TOC o "1-5" h z 6.01 12.8772.71 8.39 .
OD ID TG1 TG2 CPG DUTY U SURFP DELTDELPG MAXVL
2. 1.7382063 1853 . 318 21.39 17.81 824 1458 . 37 93
NW ND L N H B W$ ST SL TFIN TWAL WT
21 7 10.7 … 5.5 4. 567 567 4451
Arrgt=IN corm= 1 foul ftr-ft2hF/Btu=.Q01 duty-MMBtu/h=21.39 gas dp — in wc=.37
TGI TG2 CPG DUTY U SURFP DELT DELPG MAXVL WT
1853 1758 .314 9.51 20.67 415 1109 .59 136 2494
1758 1550 .31 20.65 20.5 949 1062 .831 134 3385
NW ND L N H B WS ST SL PDRP STRMS
22 4 9.. . .075 . 4.5 4. 12.8 22
22 4 9. 1. .5 .075 . 4.5 4. 10.4 22
CONF OD ID DUTY STM IN STM OUT HTC TWAL TFIN CF 2. 1.706 9.5 645 750 344 883 883
CF 2. 1.706 20.6 497 692 375 963 1266
corm= 1 Foul ftr-ft2hF/Btu=.001 stm pr-psia=635 arrgt IN stm flow-lb/h= 146233 spray wat flow — lb/h-3767 spray water temp — F-230 s team veil-ft/s — 116 steam vel2-ft/s — 97
TOC o "1-5" h z
OD ID T&i W2 CPG DUTY U SURFP DELT DELPG MAXVL 2. 1.7381550 1490 . 306 5.84
2. 1.7381490 1299 . 302 18.48
2. 1.7381299 569 .287 67.1
NW ND L N H В WS
21 3 10.7 . . . .
20 3 10. 2. .75 . 075.
20 27 10. 4.5 . 75 . 05 .157
Arrgt-IN corm-1 foul ftr-ft2hF/Btu=.001 duty-MMBtu/h=91.42 gas dp-in wc-3.38 ECONOMISER
OD ID TG1 TG2 CPG DUTY U SURFP DELT DELPG MAXVL WT
2. 1.738569 306 .269 22.57 6.05 29Э57 125 .89 37 41477
NW ND L N H В WS ST SL PRDP STRMS
30 12 16. 4.5 . 75 . 05 .157 4. 4. 13.42 10
Wat temp in — F=230 wat temp out-F=378 wat htc — Btu/ft2bF=1572 water inlet: tube wall temp-F= 241 fin tip temp-F — 261 water exit: tube wall temp-F= 410 fin tip temp-F= 462 corm=.95 foul ftr — ft2hF/Btu-.001 arrgt =IN water vel-ft/s=4.5 econ-counter flow
Blr duty — MMBtu/h=165 54 tot duty-MMBtu/h-177.14 tot gas prdp-in wc-6.06 steam -lb/h-150000 wat flow — tb/h=147695
For clarifications Of for infomtatian. please contact: V. Ganapathy
Figure 2.18 Printout of HRSG performance.