ANSWERS
TO QUIZ ON BOILERS,HRSGS by
V.Ganapathy
Only
terse answers are provided here. Interested readers may see my book "Steam
Plant Calculations Manual" (page number where
more information is given is shown for many questions) and also the Waste
Heat Boiler Deskbook for detailed discussions,examples.
1.Relation between quality and purity
is simple. Steam quality=100-(0.5/1000)x100=99.95 %
where 0.5 is the steam purity and
1000 ppm is the drum solids (p20).
2.Using the
MM Btu method,quick estimates can be made for air required. For natural
gas, air requires about 730 lb/MM Btu (Higher heating Value basis) and
for fuel oil it is about 745. Hence at 10 % excess air,1 MM Btu gas fired
requires 730x1.1=803 lb of dry air (p69) .
3.The relation between efficiency
on HHV and LHV basis is as follows:
Ehhv
xHHV =ElhvxLHV ,where Ehhv ,Lhhv
are efficiencies on HHV,LHV basis,%(p88).
Typical natural gas has a HHV=23,000 Btu/lb
and LHV=20,800 Btu/lb,with a ratio=1.105. Hence if HHV efficiency=83 %,the
LHV efficiency=83x1.105=91.7 %. Boiler engineers should understand the
difference while evaluating bids. Europeans typically use the LHV basis,while
in the USA,HHV basis is the norm.
4.In an economizer,the
water side heat transfer coefficent is 100 times higher than the gas side
coefficient and hence the tube wall temperature will be close to the water
temperature. Hence it does not matter if gas temperature is 600 F or 300
F,the tube wall temperature will be close to the water temperature. See
my article on Corrosion
in Economizers (p96).
5.The energy absorbed by steam in Btu/h is given by the equation Q=33475xbhp=33475x1000=33.475
MM Btu/h in a 1000 hp.boiler. From steam tables at 150 psig,enthalpy of
vapor=1195.7 and that of feed water at 220 F=188.7 Btu/lb. Hence the steam
generated= 33.475x106/(1195.7-188.7)=33,250 lb/h,neglecting
blow down (p11).
6.See my article on Converting
NOx,CO from mass to volumetric basis for the procedure to convert
from mass to volume basis or vice versa. For natural gas,0.1 lb/MM Btu
NOx=83 ppmvd. hence 56 ppmvd=0.0675 lb/MM Btu fired on HHV basis (p98)
.
7.The heat loss from the casing in Btu/h can be shown to be nearly
same if the ambient temperature,wind velocity are the same. However due
to the lower emissivity (0.15 for aluminium vs 0.8-0.9 for steel),the aluminium
casing will run hotter.The program on insulation performance may be used
to verify this (p335).
8.This is an iterative process. Given the flue
gas flow,inlet temperature,stack dimensions,one can estimate the casing
heat loss,compute the inside heat transfer coefficient and the drop in
gas temperature for a given stack/duct height.See Books,Software
on Boilers,HRSGS,Steam Plant Calculations for information on
a program to perform this calculation
(p 333).
9.Fans for boilers should be sized at the lowest density conditions
or highest ambient/elevation conditions. This is due to the fact that air
for generating a given amount of steam in a boiler in lb/h is nearly constant
for a given fuel input. However the volume will be higher at lower densities
and fans have to be sized to handle this (p363).
10.It can be shown that for a fan there is a
relation between flow,head developed and motor current.
P=1.17x10-4xqH/h
f =0.001732EIh m cosF
where
P=power
consumption of fan,kw
q=flow
of air,acfm
H=head
developed,in wc
E,I=motor
voltage,current
cosF
=power factor
h m ,h f = efficiency of motor,fan,fraction
Thus knowing current,voltage,head developed,one
can estimate the flow. Some iteration may be required to adjust for fan
efficiency,which varies with load. Simialr expression is available for
pumps (p381).
11.The adiabatic combustion temperature is obtained by equating the
net heat released by combustion of the fuel with the product of flue gas
quantity generated and the enthalpy increase.
say fuel oil having a HHV=19,700 and LHV=18,500 Btu/lb is fired with
10 % excess air.The flue gas generated using MM Btu method is: 1+745x1.1x19700/106
=17.15 lb/lb fuel. The enthalpy increase=18500/17.15=1079 Btu/lb. Using
an approximate gas specific heat of 0.32,the combustion temperature=3370
F. A more accurate method involves combustion calculations and estimate
the gas analysis and properties. If air is preheated,it will increase the
combustion temperature (p78).
12.For a deatiled expalnation,see the book(p195).For
bare tubes,inline arrangement gives the lowest gas pressure drop for comparable
duty and surface area. The heat transfer coefficients between inline and
staggered arrangements is not very signifcant ,on the order of 5%;however
the gas pressure drop is much higher,about 30-50 % more for staggered over
inline. Hence use inline arrangements for bare tubes.On the other hand,for
finned tubes,both arrangements are feasible and the differences are not
very significant for the same duty and gas pressure drop.Cost,past practice
and experience more often determine the arrangement.
13.A scale of given thickness will cause more problems in a finned
tube boiler than in a bare tube boiler. The heat flux inside finned tubes
is very high as seen in the article Heat
Transfer in Finned Tubes . If the overall heat transfer coefficient
=7 Btu/ft2hF and the ratio of external to internal surface areas=6,gas
temperature=1000 F and fluid temperature=500 F,heat flux inside tubes=7x6x(1000-500)=21000
Btu/ft2h. If the fouling layer thickness=0.03 in and thermal
conductivity of calcium sulfate scale=16 Btu/ft2hF/in,then the
fouling factor=0.03/16=0.001875. The temperature drop across this layer
= 0.001875x21000=39 F.
For a bare tube boiler,the overall heat transfer coefficient is say
13 Btu/ft2hF; heat flux=13x(1000-500)x2/1.738=7480 Btu/ft2h.
The temperature drop across the scale=7480x0.001875=14 F. Higher the gas
temperature,larger the difference between the two cases. hence one has
to be careful while using finned tubes.
14.True. A tube subject to external pressure
reuqires higher thickness to withstand it.See ASME code for formule.Typically
the pressure can be twice if applied inside comapred to that applied externally.(p41)
15.True. One has to develop the head vs flow curve for the system and
see where it intersects the system resistance to determine the operating
point and it may not,depending on the nature of the curve deliver the desired
flow. Same applies to pumps(p382)
16.It can be shown from fundementals(p103) that
the relation between fuel input (natural gas,distillate oils) and oxygen
is: Q=58.4WO. Q=fuel input on LHV basis,Btu/h,W=exhaust gas
flow,lb/h and O=% volume of oxygen consumed. Hence if O=(15-3)=12,W=100,000,then
Q=12x100,000x58.4=70 MM Btu/h. This would raise the gas temperature by
about:(70x106/100000/0.33)=2100 F.Accurate calculations are
possible using the programs discussed in Books,Software
on Boilers,HRSGS,Steam Plant Calculations
17.More surface area does not mean more duty as discussed in the article
on finned tubes. Even with bare tubes,depending on tube spacing ,gas velocity,tube
size etc,the heat transfer coefficient can vary. One has to look at
the product of overall heat transfer coefficient and Surface area,instead
of simply the surface area alone.
18.The article on finned tubes(see above Q 13)
also shows how with better choice of fins,more energy can be transferred
with lesser surface!
19.Higher fin density or larger ratio of external to tube internal
surface area is preferred when the tube side heat transfer coefficient
is large,as in the case of evaporators. When tube side coefficient is small,as
in the case of superheaters,the large fin surface adds little to the duty
but contributes negatively to heat flux,gas pressure drop and tube wall
temperatures and hence should be avoided. See the example in the book (p254).
20.True. Lower the tube size,higher the heat
transfer coefficient and shorter the tube length,though the labor cost
may be higher as more tubes have to be used (p214).
21.See the article Converting
NOx,CO emissions from mass to volumetric units for examples and
procedure.
22.A higher Circulation Ratio does not mean larger
duty for the evaporator. The duty depends on heat transfer coefficient,surface
area and log-mean temperature difference.
23.Fouling caused by the scale=0.1/10=.01 ft2hF/Btu. Hence
temperature drop across the scale=50,000x0.01=500 F.
24.One cannot arbitrarily select the exit gas
temperature in a gas turbine HRSG,due to limitations of pinch and approach
points,which affect the exit gas temperature. See HRSG
simulation for examples,discussions on this subject.
25.Gas turbine HRSGs have some special characteristics. The exhaust
gas flow increases at lower ambient temperature but the exhaust gas temperature
decreases. This combination generates less steam in the evaporator,which
means less flow through the economizer. However the gas side heat transfer
coefficient,which determines the overall heat tarnsfer coefficient,is the
same or even higher at the economizer compared to the high ambient temperature
or load case.This causes more energy to be transferred to the economizer.
With lesser flow and more duty,the enthalpy absorbed is higher and water
exit temperature approaches the saturation temperature and hence can lead
to steaming. The same situation arises when load of the gas turbine decreases.
The exhaust gas temperature is lower,while the mass flow is unchanged.
Again less steam is generated,more enthaply absorbed in economizer and
so steaming is a possibility.
Hence methods to avoid steaming include bypassing of the gas across
the HRSG or the economizer (lower gas flow helps to reduce the energy
transferred to the economizer),increasing the flow of water by recirculation
or even by using higher blow down. See the Waste Heat Boiler Deskbook for
more deatiled discussions.
26.The gas side heat transfer coefficient is
lower as the fin density increases for the same gas velocity,temperature.
See the article on Finned Tubes above.
27.Yes. A packaged steam generator can have different combinations
of furnace area,convection surface and economizer surface for the same
duty,gas pressure drop. Hence one should not simply go by surface areas
alone.
28.A 100,000 lb/h steam generator handles a flue
gas of about 110 to 130,000 lb/h depending upon the excess air and duty,assuming
no flue gas recirculation. This results in a power consumption of about
4 kw/in wc of pressure drop. Cost conscious plant engineers must evaluate
cost of fan operation along with fuel consumption. It is possible that
a low gas pressure drop design is better in the long run even if the cost
is slightly higher than a high pressure drop design.
29.A 10 % increase in surface area does not translate into 10 % more
duty. Depending upon how the surface is added(by increase/decrease in gas
velocity),the change could be from 2 to 5 %. As more energy is transferred,the
log-mean-temperature-difference decreases. Hence more S does not mean more
Q even if U is unchanged.Note the familiar equation Q=USDT.
30.Finned tubes make the design of water tube
boilers compact as seen in the article above on Finned Tubes.
31.Multiple pressure steam generation is necessary in gas turbine HRSGS
as the exit gas temperature cannot be decreased by a single pressure system
particularly in unfired units. As seen in Generating
Steam Efficienctly in Cogeneration Plants the exit gas temperature
in unfired mode is higher.Exit gas temperature increases as the steam pressure
and steam temperature increase(p286). With 15 F pinch and 20 F approach,the
exit gas temperature in a 600 psig,750 F HRSG is 398 F,while the exit gas
temperature in a 150 psig saturated unit is 313 F. This is a thermodynamic
problem. Hence in high pressure units,multiple pressure modules are required
to lower the exhaust gas temperature. One can run the HRSG simulation program
to obtain these numbers. HRSGS program may also be used to determine if
multiple pressures are necessary.
32.It is desirable to extract as much energy
from the exhaust gas as possible.Hence feed water heating in the HRSG is
a good idea. However one has to evaluate potential corrosion concerns if
water vapor condensation or acid condensation is likely.Note that the water
temperature entering detrmines if condensation is likely as shown in the
article above on corrosion in economizers.
33.As shown in the article above on generating steam efficienctly in
cogeneration plants,it is more efficient to add the fuel in the HRSG rather
than in the steam generator.
34.The exit gas temperature increases with decrease
in steam flow. That is due to the larger ratio of gas to steam in the HRSG.This
is also responsible for steaming in economizers.
35.True. Hydrogen and water vapor increase the gas heat transfer coefficient
by increasing the gas specific heat and thermal conductivity.A high gas
pressure also increases the mass velocity,resulting in higher heat transfer
coefficient as in hydrogen plant reformed gas boilers.
36.Yes. See the HRSGS simulation web site.
37.With air heaters,the air and gas side heat transfer coefficients
are of the same order..Hence it makes no sense to use fins either inside
or outside. See Q 19.Fins are recommended when the tube side coefficient
is several times the gas side coefficient as in the case of economizers,evaporators
and to some extent the superheater.
38.Steam generators at low loads have several
problems. First it is difficult to predict the performance due to poor
gas/steam side mal distrubution. The fan also may operate at an unstable
point,unless variable speed drives are used. If radiant superheaters are
used,they are susceptable to failures due to low tube side pressure drop
and poor tube side flow distribution and associated higher radiant energy
transfer.Good performance prediction can generally be made between 60 to
100 % load.
39.Typical exhaust gas temperature is 850-1000 F. HRSGs can be designed
economically for a pinch and approach point of 10-20 F for these temperatures
and the size/surface areas will be reasonable and feasible. In fired conditions,pinch
and approach points cannot be arbitrarily selected as it can lead to steaming
in the economizer in unfired mode or temperature cross situation can arise
at other fired conditions. If the HRSG is sized in unfired mode,its performance
at other loads,both fired or low loads can be evaluated using the simulation
process to assure soundness of design.See my book Waste Heat Boiler Deskbook
for elaborate discussions on pinch,approach point selection.
40.See the section on Fouling in Waste Heat Boilers
Back to the Quiz!
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