If you are a plant engineer or a consultant or process engineer involved in developing specifications for steam generators,HRSGS, or involved in their operation or selection (and you have some time ) you may try to answer these questions.
disclaimer: The purpose of this page is to purely have technical discussions on boilers and to involve and educate consultants,plant engineers and process engineers specializing in steam generator specifications,selection or operation. These discussions,statements or surmises in no way reflect policies or views of any particular organization. They are my own. V.Ganapathy

Questions on HRSGS & Steam generators

1. How do you relate steam purity with steam quality? A boiler drum has 1000 ppm dissolved solids and the steam purity is 0.5 ppm. What is the steam quality? 2.Can you tell approximately how much combustion air is required to burn natural gas at 10 % excess air in lb/lb fuel ? A calculator may be used...no reference books. 3.If the boiler efficiency on Higher Heating Value basis is 83 % for natural gas firing what is it approximately on Lower Heating Value basis? 4.The sulfuric acid dew point for your flue gases is say 285 F. Is your economizer safer if the feed water temperature is 230 F and flue gas is above 600 F or when the feed water is at 275 F and flue gas temperature is 350 F? Why? 5.How do you convert Boiler Horse Power to lb/h of steam? Say feed water is at 220 F and steam pressure is 150 psig saturated. 6.My boiler burning natural gas is generating 56 ppmv NOx corrected to 3 % oxygen dry. What is the NOx level in lb/MM Btu fired? 7.For the same insulation thickness, hot face temperature and ambient conditions,is there a difference between using aluminium for the outer casing rather than steel? Which runs hotter? 8.If the boiler stack is not insulated,do you know how to estimate the stack wall temperature if the flue gas temperature and ambient conditions are known? 9.Should a boiler fan be sized at the highest or lowest ambient temperature conditions? 10.Can we get an idea of fan air flow (or pump water flow) from motor current readings? 11.What is the adaibatic combustion temperature for fuel oil burning at 15 % excess air? 12.Is inline arrangement of bare tubes preferred to staggered arrangement in boiler banks? What about finned tubes? 13.A scale of given thickness will cause more problems in a bare tube boiler compared to a finned tube boiler at the same gas temperature and velocity-True or False? 14.A tube requires more thickness when subject to external pressure than when it is subject to same internal pressure-True or False? 15.2- 50 % capacity fans when operated in parallel may not give 100 % capacity-True or False? 16.The oxygen content in a gas turbine HRSG exhaust gas is 15 % volume. If the exhaust gas flow is 100,000 lb/h how much fuel energy in the form of natural gas can be added by reducing oxygen to say 3 %? What is the approximate firing temperature? 17.More surface area in a boiler always means more energy transferred. True or False? 18. Can I transfer more energy to a boiler using lesser surface area? How? 19.In HRSGs,a higher fin density is preferred for evaporator tubes than for superheater tubes. Why? 20.The length of fire tube waste heat boilers can be generally reduced if I used smaller tube sizes-True or False?

21.The burner in my gas turbine HRSG is adding 100 lb/h of NOx. How do I convert this to ppmv corrected to 15 % oxygen dry? Assume you know the total exhaust gas flow and analysis after combustion. 22.I can transfer more energy to my evaporator if the Circulation Ratio were higher- True or False? 23.How much higher would the tube wall temperature be if a scale of thickness 0.1 in with thermal conductivity 10 Btu/fthF formed inside tubes of water tube boiler operating at a heat flux of 50,000 Btu/ft2h? (about 40 F or 480 F ?) 24.I can select exit gas temperature from a single pressure gas turbine HRSG at will in order to compute the steam generation-True or False? 25.Why is it that economizers are likely to steam in gas turbine HRSGs (at low steam flows) rather than in conventional steam generators? 26.The gas side heat transfer coefficient is higher with higher fin densities for the same gas velocity ,tube size,tube spacings and temperature-True or False? 27.A steam generator can have different combinations of radiant and convective surface areas and transfer the same energy to steam with same gas side pressure drop-True or False? 28.How much is an additional inch (of water) pressure drop worth approximately in a 100,000 lb/h packaged steam generator? 0.5 kw or 4 kw or 15 kw ? 29.A 10 % more surface area in a boiler evaporator is equivalent to 10 % more energy -True or False? 30.Why are water tube boilers with finned tubes preferred over bare tube boilers in gas turbine applications? Also,why are water tube boilers preferred to fire tube boilers for this application? 31.Why do we use multiple pressure steam generation in gas turbine HRSGS and not in conventional steam generators? 32.If you had a gas turbine HRSG and you are at present heating feed water using extraction steam from the steam turbine,would you be better off heating the feed water in the HRSG or not? 33.Is it more fuel efficient to generate additional steam in a gas turbine HRSG rather than in conventional steam generators? What is the approximate difference in fuel consumption in % ? Assume that the HRSG is capable of generating the steam and you also have a steam generator. 34.In a single pressure gas turbine HRSG does the exit gas temperature increase or decrease with steam flow? How does it compare with conventional boilers and why? 35.Waste gas streams containing hydrogen,water vapor have higher heat transfer coefficients compared to those with lesser water vapor,hydrogen-True or False? 36.Can we predict the performance of a gas turbine HRSG at different steam/gas conditions without knowing its mechanical details such as tube size,surface areas etc ? 37.Why shouln't I use finned tubes in boiler air heaters with air inside tubes and gas outside tubes? 38.Is it a good idea to operate steam generators at low loads such as 15 to 25 % percent? What are the concerns? 39.Why is it a good idea to select pinch and approach points for gas turbine HRSGs in unfired mode? 40.A fire tube waste heat boiler is designed to operate at 300 psig with 75,000 lb/h of gas at 1600 F with an exit gas temperature of 600 F. If it is operating at 250 psig with 40,000 lb/h and same inlet gas temperature and the exit gas is 550 F. Is the boiler operating well or is there fouling or underperformance? Is there a simple way to determine this? (Same question for water tube boilers)

Note:Answers to above questions may be found in my books.
Books,Software,Papers on Boilers,HRSGs

What is HRSG simulation
Course on Waste Heat Boilers and Steam Generators by V.Ganapathy
Partial List of Ganapathy's Papers on Boilers,HRSGs
Answers to Quiz Questions!!
email Ganapathy
If you are tired of boiler questions,try this!There are 12 marbles,all similar in looks,but one of them is either heavier or ligher than the rest,you are told. You are given a balance and in 3 weighings you should find out which ball is the odd one and whether it is heavier or lighter than the rest!!