boiler-questions _ answers
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Boiler related Questions and answersTRANSCRIPT
1.The minimum capacity of any closed vessel, which generates steam under Indian Boilers Regulation Act, is ___.
a) 2.275 liters b) 22.75 kilo liters
c) 227.5 liters d) 22.75 liters
2.Steam is preferred medium for heating applications because:
a) high latent heat b) temperature break down is easy
c) does not require any transport facility d) all the above
3.For higher boiler efficiencies, the feed water is heated by ____.
a. recuperator b. convective heater
c. super heater d. economiser
4.The type of firing used for a pulverized coal fired boiler is:
a) over firing b) tangential firing
c) vertical firing d) mixed firing for effective heat transfer
5.The recommended TDS level in boiler drum, that can be safely maintained for the water tube boiler is:
a) 3000 – 3500 ppm b) 2000 ppm
c) 5000 ppm d) It can be anything
6.An evaporation ratio (steam to fuel ratio) of an efficient oil fired boiler is in the range of ___.
a) 5 – 6 b) 13 - 16
c) 1 d) 7 – 9
7.Pick the boiler, which can be considered as most combustion efficient?
a) fluidized bed combustion boiler b) Lancashire boiler
c) Stoker fired boiler d) chain grate boiler
8.The percentage excess air required for pulverised coal fired boiler is:
a) 40 – 50% b) 15 – 20%
c) 60 – 80% d) 30 – 40%
9.Name the predominant loss component for furnace oil fed boiler.
a) losses due to radiation and convention b) loss due to hydrogen in fuel
c) loss due to dry flue gas d) loss due to moisture in fuel
10.Controlled wetting of coal (during the coal preparation) would result in
a) reduction in flue gas exit temperature
b) decrease in the percentage of unburnt carbon
c) improper combustion
d) increase in the fines of coal
11.A rise in conductivity of boiler feed water indicates ____ . a. drop in the contamination of feed water b. greater purity of feed water c. rise in the contamination of feed water
d. it has got no relation with the contamination of feed water
12.Demineralisation of water is the process to remove --------
a) dissolved oxygen b) dissolved salts c) corbondioxide d) chlorine
13.The presence of calcium and magnesium bicarbonates in water to steam boiler would form:
a) acidic solution b) alkaline solution
c) neutral solution d) none of the above
14.Water treatment for steam boiler is generally required to:
a) remove hydrogen b) prevent formation of scales
c) help improve combustion efficiency d) reduce stack temperature
15.In a plant, a boiler is generating a saturated steam of 2 tonnes/hour at a pressure of 7.0 kg/cm2g. The feed water temperature is 70 °C and furnace oil consumption is 138 kg/h. What is the efficiency of the boiler by using direct method of efficiency evaluation? (Calorific value of FO is 10,000 kCal/kg; enthalpy of steam is 660 kCal/kg.
a) 65 b) 75 c) 85 d) 95
16.The ‘indirect method’ of evaluating boiler efficiency is also called as “Heat Loss” method. – True or False?
17. Good opportunity for energy savings from continuous blow down water of boiler is by ___.
a. reusing the hot water so formed as make up water b. using the blow down steam to run steam turbine c. utilisation of flash steam in deaerator
d. none of the above
18.De-aeration of boiler feed water is referred to as:
a) removal of dissolved gases b) removal of silica
c) removal of scales by blow down d) phosphate treatment of feed water
19.The percentage raise in boiler efficiency by a 20 degree centigrade raise in combustion air temperature is ___.
a) 0.1% b) 0.2% c) 10% d) 1%
20.The elements of ultimate analysis of fuel does not include
a) carbon b) Hydrogen
c) oxygen d) volatile matter
Part - II: Short type questions and answers
1.What do you understand by ‘water tube boilers’ and ‘fire tube boilers’?
In water tube boilers the water passes through the tubes and the hot gases passes out side the tubes where as in case of fire tube boiler the hot gases passes through the tubes and the water passes over the tubes.
2.What do you mean by IBR steam boiler?
IBR Steam Boilers means any closed vessel exceeding 22.75 liters in capacity and which is used expressively for generating steam under pressure and includes any mounting or other fitting attached to such vessel, which is wholly or partly under pressure when the steam is shut off.
3.What is the affect of sulphur in coal when used in boiler?
Sulphur will get oxidized to SO2 and fraction of SO3 and will react with water to form sulphuric acid and this occurs at a temperature called the acid dew point which normally is about 120 oC. The sulphuric acid so formed corrodes the steel when it comes in contact with it.
4.Write a short note on IBR steam pipe.
IBR Steam Pipe means any pipe through which steam passes from a boiler to a prime mover or other user or both, if pressure at which steam passes through such pipes exceeds 3.5 kg/cm2 above atmospheric pressure or such pipe exceeds 254 mm in internal diameter and includes in either case any connected fitting of a steam pipe.
5.Why boiler blow-down is required?
As the feed water, evaporate into steam, dissolved solids concentrate in the boiler. Above certain level of concentration, these solids encourage carryover of water into steam. This leads to scale
formation inside the boiler, resulting in localised over heating and ending finally in tube failure. Hence blow-down is very much required for boilers.
6.What are the parameters required to estimate the boiler efficiency by ‘direct method’?
a. Steam flow rate b. GCV of fuel c. Fuel flow rate d. Steam conditions ( pressure and temperature)
e. Feed water temperature
7.What is the principle of mechanical deaeration (pressure type) of boiler feed water?
The pressure-type de-aerators operates by allowing steam into the feed water through a pressure control valve to maintain the desired operating pressure, and hence temperature at a minimum of 105 °C. The steam raises the water temperature causing the release of O2 and CO2 gases that are then vented from the system. This type can reduce the oxygen content to 0.005 mg/litre.
8.What is the effect of boiler loading on boiler efficiency? The maximum efficiency of the boiler does not occur at full load,
but at about two-thirds of the full load. If the load on the boiler decreases further, efficiency also tends to decrease.
As the load falls, so does the value of the mass flow rate of the flue gases through the tubes. This reduction in flow rate for the same heat transfer area, reduced the exit flue gas temperatures by a small extent, reducing the sensible heat loss.
Below half load, most combustion appliances need more excess air to burn the fuel completely. This increases the sensible heat loss.
9.What are the principle heat losses that occur in a boiler?
The principle heat losses that occur in a boiler are:
Loss of heat due to dry flue gas
Loss of heat due to moisture in fuel and combustion air Loss of heat due to combustion of hydrogen Loss of heat due to radiation
Loss of heat due to unburnt fuel
10.What do you meant by tangential firing with respect to pulverized coal fired boiler?
The method of firing used for coal firing in pulverized fuel fired boiler is the tangential firing. In this type of firing four burners are used at the corner to corner to create a fire ball at the center of the furnace.
11.What are the disadvantages of ‘direct method’ of boiler efficiency evaluation over ‘indirect method’?
Direct method
a) Do not give clues to the operator as to why efficiency of system is lower
b) Do not calculate various losses accountable for various efficiency levels
12.List out the data required for calculation of boiler efficiency using ‘indirect method’.
The data required for calculation of boiler efficiency using indirect method are:
Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content)
Percentage of Oxygen or CO2 in the flue gas Flue gas temperature in 0C (Tf) Ambient temperature in 0C (Ta) & humidity of air in kg/kg of dry
air. GCV of fuel in kcal/kg Percentage combustible in ash (in case of solid fuels)
GCV of ash in kcal/kg (in case of solid fuels)
13.Explain the different external water treatment methods.
External treatment is used to remove suspended solids, dissolved solids (particularly the Calcium and Magnesium ions which is a major cause of scale formation) and dissolved gases (oxygen and carbon dioxide). The techniques include:
o Precipitation processes, in which chemicals are added to precipitate calcium and magnesium as compounds of low solubility. The lime-soda process is typical of this class, but other precipitating agents such as caustic soda and sodium phosphate can be used when the composition of the raw water permits.
o Ion-exchange progresses, in which the hardness is removed as the water passes through bed of natural zeolite or synthetic resin and without the formation of any precipitate. Ion exchange processes can be used for almost total demineralization if required, as is the case in large electric power plant boilers.
o De-aeration, in which gases are expelled by preheating the water before entering the boiler system. Water normally contains approximately 10 mg/1 of dissolved oxygen at ambient temperature
o Filtration, to remove suspended solids
14.What are the salient features of a ‘packaged boiler’?
The features of package boilers are:
o Small combustion space and high heat release rate resulting in faster evaporation.
o Large number of small diameter tubes leading to good convective heat transfer.
o Forced or induced draft systems resulting in good combustion efficiency.
o Number of passes resulting in better overall heat transfer.
Higher thermal efficiency at lower capacity (say below 1 ton) levels compared with other boilers.
15.What are the parameters to be monitored for evaluating ‘direct efficiency’ of boilers and what is the empirical relation used?
Parameters to be monitored for the calculation of boiler efficiency by direct method are:
Quantity of steam generated per hour (Q) in kg/hr. Quantity of fuel used per hour (q) in kg/hr. The working pressure (in kg/cm2(g)) and superheat temperature
(oC), if any The temperature of feed water (oC) Type of fuel and gross calorific value of the fuel (GCV) in kcal/kg
of fuel
Boiler efficiency () = :
where, hg – Enthalpy of saturated steam in kcal/kg of steam
hf - Enthalpy of feed water in kcal/kg of water
16.What are the two main classification of a stoker fired boiler?
1. Chain grate or travelling grate stoker
2. Spreader stoker
17.Calculate the blow down rate for a boiler with an evaporation rate of 3 tons/hr, if the maximum permissible TDS in boiler water is 3000 ppm and with 10 % make up water addition. The feed water TDS is around 300 ppm.
Blow down (%) =
Percentage blow down =
If boiler evaporation rate is 3000 kg/hr then required blow down rate is:
=
18.Indicate the different methods of efficiency evaluation of Boiler and describe it. –
i. Direct Method
ii. Indirect Method
Direct Method:
where
Adsorbed heat = Eout -The energy the feedwater has picked up
Energy Input = Ein - The energy going into the boiler.
Indirect Method:
Most performance testing and commissioning of smaller and medium sized boilers is done by the indirect method measuring the losses and calculating the efficiency as
19.Briefly explain the principle involved in ‘reverse osmosis’?
When solutions of differing concentrations are separated by a semi-permeable membrane, water from less concentrated solution passes through the membrane to dilute the liquid of high concentration. If the solution of high concentration is pressurised, the process is reversed and the water from the solution of high concentration flows to the weaker solution.
20.What are the various methods available to control the ‘excess air’ in a boiler?
Various methods are available to control the excess air:
Portable oxygen analysers and draft gauges can be used to make periodic readings to guide the operator to manually adjust the flow of air for optimum operation. Excess air reduction up to 20% is feasible.
The most common method is the continuous oxygen analyzer
with a local readout mounted draft gauge, by which the operator can adjust air flow. A further reduction of 10-15% can be achieved over the previous system.
The same continuous oxygen analyzer can have a remote controlled pneumatic damper positioner, by which the readouts are available in a control room. This enables an operator to remotely control a number of firing systems simultaneously.
Part – III: Long type questions and answers
1.a) What is the benefit of providing Economiser for a boiler?
b) Calculate the fuel oil savings by providing an Economiser for a boiler. The performance data of the boiler are given as below:
o Average quantity of steam generated : 5 T/h o Average flue gas temperature : 315 oC (without
economiser) o Average steam generation / kg of fuel oil : 14 kg o Feed water inlet temperature : 110oC o Fuel oil supply rate : 314 kg/h o Flue gas quantity : 17.4 kg/kg of fuel o Gross calorific value of fuel : 10,000 kCal/kg o Rise in feed water temperature by providing economizer: 26
°C o Annual operating hours : 8600
a. By providing Economiser the exit flue gas losses can be reduced
and hence the boiler efficiency can be increased. b.
Quantity of flue gases : 314 x 17.4 = 5463.6 kg/h
Quantity of heat available in flue gas : 5463.6 x0.23 x(315-200)
: 144512 kCal/h
Rise in the feed water temperature : 26 oC. Heat required for pre-heating the : 5000 x 1 x 26 = 130000
kCal/h
feed water
o Saving in terms of furnace oil : 130000/10000 = 13 kg/h o Annual operating hours : 8600
o Annual savings of fuel oil : 8600 x 13 = 111800 kg
2.Evaluate the option of boiler replacement for the following boiler with a new boiler of 84% efficiency. The cost of new boiler is Rs 30.00 lakh
Data of present boiler:
Average steam generation from the boiler: 5000 kg/h Fuel used: furnace oil Enthalpy gained by the steam in boiler: 600 kcal/kg of steam Cost of furnace oil: Rs 15000 per ton (Rs. 15 per kg) Gross calorific value of the fuel: 10000 kcal/kg Annual operating hours of the boiler: 6000 h Boiler efficiency: 80%
The boiler replacement option can be evaluated by considering the following
Evaporation rate, kg of steam per kg of fuel Cost of steam, Rs. Per kg Annual Cost of steam
Evaporation ratio (kg of steam per kg of fuel) is given by:
=
Cost of steam (Rs. Per kg of steam) is given by:
=
Annual cost of steam, Rs. lakh =
Parameter Present boiler
Proposed boiler
Boiler efficiency 75 84
Steam generation, kg/h 5000 5000
Gain in steam enthalpy, kcal/kg
600 600
Evaporation rate, kg of steam per kg of fuel
12.5 14
Cost of steam, Rs. Per kg 1.2 1.071
Annual cost of steam, Rs. lakh 360 321.0
Annual cost savings by replacing the boiler = Rs. (360-321) lakh
= Rs. 39 lakh
Investment for the new boiler = Rs. 30 lakh
Simple pay period = (Investment /Annual savings)
= (30/39)
= 0.72 years
= 9.2 months
3.Describe ‘chain grate’ and ‘spreader stoker’ type boiler.
Chain-Grate or Travelling-Grate Stoker Boiler
Coal is fed onto one end of a moving steel grate. As grate moves along the length of the furnace, the coal burns before dropping off at the end as ash. Some degree of skill is required, particularly when setting up the grate, air dampers and baffles, to ensure clean combustion leaving the minimum of unburnt carbon in the ash.
The coal-feed hopper runs along the entire coal-feed end of the furnace. A coal grate is used to control the rate at which coal is fed into the furnace by controlling the thickness of the fuel bed. Coal must be uniform in size as large lumps will not burn out completely by the time they reach the end of the grate
Spreader Stoker Boiler
Spreader stokers utilize a combination of suspension burning and grate burning. The coal is continually fed into the furnace above a burning bed of coal. The coal fines are burned in suspension; the larger particles fall to the grate, where they are burned in a thin, fast-burning coal bed. This method of firing provides good flexibility to meet load fluctuations, since ignition is almost instantaneous when firing rate is increased. Hence, the spreader stoker is favoured over other types of stokers in many industrial applications.
4.Explain the reasons for carrying out “blow down” in a boiler?
Water contains certain percentage of dissolved solids. The percentage of impurities found in boiler water depends on the untreated feed water quality, the treatment process used and the boiler operating procedures. As a general rule, the higher the boiler operating pressure, the greater will be the sensitivity to impurities. As the feed
water materials evaporate into steam, dissolved solids concentrate in the boiler either in a dissolved or suspended state. Above a certain level of concentration, these solids encourage foaming and cause carryover of water into the steam. This leads to scale formation inside the boiler, resulting in localised overheating and ending finally in tube failure.
It is therefore necessary to control the level of concentration of the solids and this is achieved by the process of 'blowing down', where a certain volume of water is blown off and is automatically replaced by feed water - thus maintaining the optimum level of total dissolved solids (TDS) in the water. Blow down is necessary to protect the surfaces of the heat exchanger in the boiler.
5.Write short notes on ‘intermittent blow down’ and ‘continuous blow down’ with respect to boilers.
The ‘intermittent blown down’ is given by manually operating a valve fitted to discharge pipe at the lowest point of boiler shell to reduce parameters (TDS or conductivity, pH, Silica and Phosphates concentration) within prescribed limits so that steam quality is not likely to be affected. In intermittent blowdown, a large diameter line is opened for a short period of time, the time being based on a thumb rule such as “once a shift for 2 minutes”.
‘Intermittent blow down’ requires large short-term increases in the amount of feed water put into the boiler, and hence may necessitate larger feed water pumps than if continuous blow down is used. Also, TDS level will be varying, thereby causing fluctuations of the water level in the boiler due to changes in steam bubble size and distribution which accompany changes in concentration of solids. Also substantial amount of heat energy is lost with intermittent blow down.
‘Continuous Blowdown’:
There is a steady and constant dispatch of small stream of concentrated boiler water, and replacement by steady and constant inflow of feed water. This ensures constant TDS and steam purity at
given steam load. Once blow down valve is set for a given conditions, there is no need for regular operator intervention.
Even though large quantities of heat are wasted, opportunity exits for recovering this heat by blowing into a flash tank and generating flash steam. This flash steam can be used for pre-heating boiler feed water or for any other purpose. This type of blow down is common in high-pressure boilers.
Assessment of boilers and thermic fluid heaters
Indirect method of determining boiler efficiency
Methodology
The reference standards for Boiler Testing at Site using the indirect method are the British Standard, BS 845:1987 and the USA StandardASME PTC-4-1 Power Test Code Steam Generating Units.
The indirect method is also called the heat loss method. The efficiency can be calculated by subtracting the heat loss fractions from 100 as follows:
Efficiency of boiler (n) = 100 - (i + ii + iii + iv + v + vi + vii)
Whereby the principle losses that occur in a boiler are loss of heat due to:
i. Dry flue gas ii. Evaporation of water formed due to H2 in fuel
iii. Evaporation of moisture in fuel iv. Moisture present in combustion air
v. Unburnt fuel in fly ash vi. Unburnt fuel in bottom ash vii. Radiation and other unaccounted losses
Losses due to moisture in fuel and due to combustion of
hydrogen are dependent on the fuel, and cannot be controlled by design.
The data required for calculation of boiler efficiency using the indirect method are:
Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content) Percentage of oxygen or CO2 in the flue gas Flue gas temperature in oC (Tf) Ambient temperature in oC (Ta) and humidity of air in kg/kg of dry air GCV of fuel in kcal/kg Percentage combustible in ash (in case of solid fuels) GCV of ash in kcal/kg (in case of solid fuels)
A detailed procedure for calculating boiler efficiency using the indirect method is given below. However, practicing energy managers in industries usually prefer simpler calculation procedures.
Step 1: Calculate the theoretical air requirement
= [(11.43 x C) + {34.5 x (H2 – O2/8)} + (4.32 x S)]/100
kg/kg of fuel
Step 2: Calculate the % excess air supplied (EA)
= O 2% x 100---------------- (21 - O 2%)
Step 3: Calculate actual mass of air supplied/ kg of fuel (AAS)
= {1 + EA/100} x theoretical air
Step 4: Estimate all heat losses
i. Percentage heat loss due to dry flue gas
= m x C p x (T f-T a) x 100 ------------------------------- GCV of fuel
Where, m = mass of dry flue gas in kg/kg of fuel
m = (mass of dry products of combustion / kg of fuel) + (mass of N2 in fuel on 1 kg basis ) + (mass of N2 in actual
mass of air we are supplying).Cp = Specific heat of flue gas (0.23 kcal/kg )
ii. Percentage heat loss due to evaporation of water formed due to H2 in fuel
= 9 x H 2 {584+C p (T f-T a)} x 100----------------------------------------- GCV of fuel
Where, H2 = percentage of H2 in 1 kg of fuel Cp = specific heat of superheated steam (0.45 kcal/kg)
iii. Percentage heat loss due to evaporation of moisture present in fuel
= M{584+ C p (T f-T a)} x 100 --------------------------------- GCV of fuel
Where, M – % moisture in 1kg of fuel Cp – Specific heat of superheated steam (0.45 kcal/kg)
iv. Percentage heat loss due to moisture present in air
= AAS x humidity factor x C p (T f-T a)} x 100 --------------------------------------------------- GCV of fuel
Where, Cp – Specific heat of superheated steam (0.45 kcal/kg)
v. Percentage heat loss due to unburnt fuel in fly ash
= Total ash collected/kg of fuel burnt x GCV of fly ash x 100 ------------------------------------------------------------------
GCV of fuel
vi. Percentage heat loss due to unburnt fuel in bottom ash
= Total ash collected per Kg of fuel burnt x G.C.V of bottom ash x 100 ---------------------------------------------------------------------------- GCV of fuel
vii. Percentage heat loss due to radiation and other unaccounted loss
The actual radiation and convection losses are difficult to assess because of particular emissivity of various surfaces, its inclination, airflow patterns etc. In a relatively small boiler, with a capacity of 10 MW, the radiation and unaccounted losses could amount to between 1% and 2% of the gross calorific value of the fuel, while in a 500 MW boiler, values between 0.2% to 1% are typical. The loss may be assumed appropriately depending on the surface condition.
Step 5: Calculate boiler efficiency and boiler evaporation ratio
Efficiency of boiler (n) = 100 - (i + ii + iii + iv + v + vi + vii)
Evaporation Ratio = Heat utilised for steam generation/Heat addition to the steam
Evaporation ratio means kilogram of steam generated per kilogram of fuel consumed. Typical Examples are:
Coal fired boiler: 6 (i.e. 1 kg of coal can generate 6 kg of steam) Oil fired boiler: 13 (i.e. 1 kg of oil can generate 13 kg of steam)
However, the evaporation ratio will depend upon type of boiler, calorific value of the fuel and associated efficiencies.
Example
Step-1: Calculate the theoretical air requirement
= [(11.43 x C) + [{34.5 x (H2 – O2/8)} + (4.32 x S)]/100 kg/kg of oil = [(11.43 x 84) + [{34.5 x (12 – 1/8)} + (4.32 x 3)]/100 kg/kg of oil = 13.82 kg of air/kg of oil
Step-2: Calculate the % excess air supplied (EA)
Excess air supplied (EA) = (O2 x 100)/(21-O2) = (7 x 100)/(21-7) = 50%
Step 3: Calculate actual mass of air supplied/ kg of fuel (AAS)
AAS/kg fuel = [1 + EA/100] x Theo. Air (AAS) = [1 + 50/100] x 13.82 = 1.5 x 13.82 = 20.74 kg of air/kg of oil
Step 4: Estimate all heat losses
i. Percentage heat loss due to dry flue gas
= m x Cp x (Tf – Ta ) x 100 ---------------------------- GCV of fuel
m = mass of CO2 + mass of SO2 + mass of N2 + mass of O2
0.84 x 44 0.03x64 20.74x77m = ----------- + ---------- + ----------- (0.07 x 32) 12 32 100
m = 21.35 kg / kg of oil
21.35 x 0.23 x (220 – 27)= ------------------------------- x 100 10200
= 9.29%
A simpler method can also be used:Percentage heat loss due to dry flue gas
m x Cp x (Tf – Ta ) x 100 = ------------------------------ GCV of fuel
m (total mass of flue gas)
= mass of actual air supplied + mass of fuel supplied
= 20.19 + 1 = 21.19
= 21.19 x 0.23 x (220-27) ------------------------------- x 100 10200
= 9.22%
ii. Heat loss due to evaporation of water formed due to H2 in fuel
9 x H2 {584+0.45 (Tf – Ta )}
= --------------------------------- GCV of fuel
where H2 = percentage of H2 in fuel
9 x 12 {584+0.45(220-27)}= -------------------------------- 10200
= 7.10%
iii. Heat loss due to moisture present in air
AAS x humidity x 0.45 x ((Tf – Ta ) x 100 = ------------------------------------------------- GCV of fuel
= [20.74 x 0.018 x 0.45 x (220-27) x 100]/10200
= 0.317%
iv. Heat loss due to radiation and other unaccounted losses
For a small boiler it is estimated to be 2%
Step 5: Calculate boiler efficiency and boiler evaporation ratio
Efficiency of boiler (n) = 100 - (i + ii + iii + iv + v + vi + vii)
i. Heat loss due to dry flue gas : 9.29% ii. Heat loss due to evaporation of water formed due to H2 in fuel: 7.10 % iii. Heat loss due to moisture present in air : 0.317 %iv. Heat loss due to radiation and other unaccounted losses : 2%
= 100- [9.29+7.10+0.317+2]= 100 – 17.024 = 83% (approximate)
Evaporation Ratio = Heat utilised for steam generation/Heat addition to the steam
= 10200 x 0.83 / (660-60) = 14.11 (compared to 13 for a typical oil fired boiler)
Advantages of indirect method
A complete mass and energy balance can be obtained for each individual stream, which makes it easier to identify options to improve boiler efficiency
Disadvantages of indirect method
Time consuming
Requires lab facilities for analysis
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Page 1HEAT TRANSFERBoiler circulation calculationsSteam generator studies can be complex. Use these guidelines to perform them effectivelyV Ganapathy, ABCO Industries, Abilene, TexasNatural 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 asteam/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 orSteamFig. 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 heattransfer 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 steamgenerated 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 beexamined 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 beclubbed into one circuit to reduce computing time. Hence, an average CR for the entire system does not give thecomplete 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 bendsTwo-phase friction, acceleration and gravity losses in the heated riser tubesContinuedHYDROCARBON PROCESSING/ JANUARY 1998101
Page 2• 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/144where H is the thermal head, ft (Fig. 1) vlis the specific volume of water, ft3/lbDowncomer losses.Let the average CR for the system = CR and the total steam generation = WS
lb/hr.The total mixture flowing through the system = WSx CRLet the effective length (including bends) of the downcomer piping in ft = LeThe frictional pressure drop, psi = 3.36 X 10-sx f Le vi (Wd)2/di5(Here, it is assumed that the average flow in each downcomer pipe is Wd). di is the innerdiameter 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 innerdiameter is larger than the inner diameter of the
nozzle at the ends, in which case the highervelocity at the nozzles must be used to compute the inlet/exit losses. Velocity V in ft/s = 0.05 Wd vl/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 themixture 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 steamFig. 3. A forced-circulation system showing multiple streams to reduce pressure drop.Steam drum, as in Fig. 1: Wmh+Wfhf=Wmhm+Wsh„ Wm = mixtureflowing through the system, lb/hr =Ws x CRhv, 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)hlwhere hv, h j = enthalpies of saturated vapor and liquid, Btu/lb.From the above, hm is solved for. The boilingheight or the distance the mixture travels beforeboiling starts, Hb, is determined from:Hb= He X WSX CR X (hl- hm)/Qs where He = height of evaporator tubes, ft(For conservative calculations, Hbmay be assumed to be zero.)There are basically three losses in boiling
evaporator tubes:Friction loss.∆pf= 4 X 10-10vl X f L X Gi2X r3/diwhere Gi= mixture mass velocity inside tubes, lb/ft2hrf= fanning friction factor L= effective length, ftdi= 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/v1Thom's multiplication factor for gravity loss, r4 is shown in Fig. 4c.
2HYDROCARBON PROCESSING /JANUARY 1998
Page 3Fig. 4. Thom's two-phase multiplication factors for: a) friction loss, b) acceleration loss, and c) gravity loss.Acceleration loss. ∆pQ=1.664x10-11x vlXGi2xr2where r2, Thom's multiplication factor, is shown in Fig. 4b.External riser losses. These are similar to the downcomer losses except that the specific volume is that of the mixture and not saturated liquid. Mixture specific volume v„,, ft3/lb, iscomputed as:vm= vs/CR + (1- 1/CR) vlRiser gravity loss.∆p, = (H - He)1vm/144where vm is the specific volume of the mixture. Losses in drum internals. These usually consist of losses in the baffles and cyclones if used and range from 0.2 to 1 psi.Total losses are calculated and balanced against the thermal head available. If they balance, the CR assumed is correct, otherwise, the iteration is repeated by assuming another CR until the losses balance with the head available. When there are several boiling circuits, one can split up the total steam flow based on steam generation in each circuit until the losses balance. A simple manual procedure is to compute losses in the circuits as a function of flow and see
where it intersects the available head line, Fig. 5. Since the available head and pressure drops in the riser anddowncomer system are same for all the evapoHYDROCARBON PROCESSING /JANUARY 19983
Page 4Table 1: Boiler data for circulation studyRowsSurface, Fins/in x height x thickness2 Duty,MMBtu/hrSteam flow,Gas temp,Heat flux,Btu/ft2-hr1-4691 bare11.511,5001,65020,5005-72,967 2.5 x 0.75 x 0.07527.127,0001,50083,0008-2020,216 4.5 x .75 x .0539.039,0001,13058,000
Risers:3. 8 in., 8-ft long, 2 bendsDowncomers: 2. 6 in., 24-ft long, 2bends8 in., 12-ft long, 4 bends6 in., 26-ft long, 3bends8 in., 8-ft long, 2 bendsSteam pressure = 645 psia. Total head =18 ft. Drum internal loss 0.3 psi.Evaporator tube ID =1.738 inlength=11 ft.rator circuits, this graphical method is sometimes used, although it is tedious. If the downcomer orexternal riser piping consists of several parallel or parallel-series paths, the electrical analogy is used to determine flow and pressure drop in each circuit. A computer program best handles this problem.EXAMPLE CALCULATIONFig. 1 is a waste-heat boiler operating under the following conditions:Gas flow = 200,000 lb/hrGas inlet temperature = 1,650°F (vol% C02=7, H2O = 18, N2= 69, 02= 6)Steam pressure = 500 psiaFeed water temperature = 230°FThe total steam generated is 63,5001b/hr. Thefirst four rows are bare, followed by six finned tubes and then 10 more with higher fin density. Details of downcomers, riser pipes and other pertinent
information are in Table 1.Determine the system's circulation ratio and the flow in each pipe circuit. Note that the boiler design calculations must be done before circulation studies can be taken up. Also, one must have a good feel for the downcomer and riser pipe sizes and their layout. Often piping layout is changed at the last moment to accommodate other equipment in the plant without reevaluating circulation. A computer program was developed to perform this analysis. The evaporator 4H Y D R O C A R B O NP R O C E S S I N G/J A N U A R Y1 9 9 8For water tube boilers, heat flux, q = Uo x (tg- ts) x At/Ai ,Where Uo= overall outside heat -transfer coefficient, Btu/ft2-hr-°Ftg,is are gas and steam temperatures, °FA
t, Ai are the tube outside and inside surface areas, ft2/ft. This ratio is for bare tubes, while for finned tubes it could be high, say 5 to 12. Hence, one must be careful while analyzing finned tube bundles, as the heat flux can be very high inside the tubes.In fire tube boilers, q = Uo x (tg- ts)Based on preliminary analyses, the CR in each circuit and overall basis seems to be reasonable. The maximum heat flux at the inlet to each section is in Table 1. Corre-lations are available in the literature for allowable heat flux as a function of pressure, quality and tube size, etc. These are mostly based on experimental data conducted inlaboratories and are often used for guidance only. The actual permissible heat fluxes are much lower and are based on industry experience and could be 10% to 30% of the values given in correlations in handbooks.Vertical tubes can handle much higher heat fluxes than horizontal tubes, up to 40% to 50% more. Limits of120,000 to 175,000 Btu/ft2-hr inside tubes are permitted at pressures ranging from 1,000 to 2,000 psig, while in fire tube boilers the limit is around 100,000 Btu/ft2-hr. Thehigher the steam pressure, the lower the allowable heat flux. Similarly, the higher the steam quality (lower CR), the lower the allowable heat flux. As the CR increases, the quality decreases and higher fluxes are permissible. With higher flow, the tube periphery is wetted better and isconsidered safer.Another approach that is widely used is the comparison
between allowable steam quality and actual steam quality. Fig. 6 shows a radiant boiler furnace, where the steam quality, x, (x=1/CR) is plotted against the height. Based on heat flux distribution along the height, the allowable quality is calculated using a correlation similar to that shownbelow. The allowable and actual steam qualities should be apart in order to avoid DNB problems. This type ofanalysis is similar to that using allowable and actual heat fluxes.The McBeth correlation shown below shows the rela-tion among the variables involved in boiling inside vertical tubes: 1q, = 0.00633 x 106x hfg di-0.1(Gi/106)0.51 x (1-x) where q, =critical heat flux, Btu/ft2-hrhfg= latent heat of steam, Btu/lbstreams or paths for evaluating circulation, even though the program can analyze more circuits. Results are shown in Table 2.Analysis of results. Boiler heat -transfer calculations have to bedone before a circulation study can be undertaken. The maximum steam generation caseis
usuallyevaluated. The heat-transfer calculations give the steamgeneration, heat flux and gastemperatures in each section. In water tube boilers, the heat flux inside the tubes is computed, while in fire tube boilers the heat flux outside the tubes is important.
Page 5x= steam quality, fraction (x = 1/CR) Gi Gi= mass velocity lb/ft2-hrdi = tube inner diameter, in.For example, the critical heat flux at a steampressure of 1,000 psi (latent heat = 650 Btu/lb), di = 1.5 in., Gi = 600,000 lb/ft2-hr and x = 0.2 is:qa = 0.00633 x 106x 650 x 1.5-0.1 x 0.6°51(1-0.2) = 2.43 x 106Btu/ft2-hr.As discussed earlier, the above equation may beused to study the effect of various variables involved and not for determining critical heat flux. Actualallowable critical heat fluxes are much lower on the
order of 10% to 30% of the above value.Fire tube boilers. A similar procedure may be adopted for fire tube boilers, Fig. 2. The frictional losses in the evaporator section are usually small. The heat flux at the tube sheet inlet is high and must be considered. CR ranges from 15 to 30 due to the low steam pressures compared to water tube boilers. Generally, there is only one evaporator circuit. The correlation for allowable heat flux by Motsinki is: 1qc = 803 Pcx (Ps/Pe)0.35 x (1 -Ps/Pc)0.9Where PSand Pcare steam pressure and critical pressure of steam, psia. At 400 psia, qc= 803 x 3,208 x (400/3,208)0.35x (1- 400 / 3,208)0.9= 1.1 x 106
Btu/ft2-hr.As mentioned earlier, the actual allowable flux would be 10% to 30% of this value.Forced-circulation boilers. In forced-circulation sys-tems, the losses are determined as indicated above. However, the available head is generally too small, so a circulating pump is added (Fig. 3) to ensure desired CR. The CR may be selected unlike in a natural-circulation system, where it is arrived at through an iterative pro-cess. If the evaporator circuits are of different lengths then orifices may also be added inside tubes to ensure flow stability. CR could range from 2 to 6 in such sys-tems to reduce operating costs.Pump reliability is a must. In gas turbine HRSGs that use horizontal tubes, the pressure drop inside tubes isquite high compared to vertical tubes used in naturalcirculation boilers. To reduce the pressure drop, multiple streams could be considered as shown or the pump may be eliminated bylocating the drum sufficiently high, resulting in a natural-circulation system.Final thoughts. Circulation studies are complex and preferably done using a computer. Theanalysis of results requires experience and is generally based on feedback from operation of similar boilers in the field. Specifying a minimum CR for a boiler is not the right approach since CR varies with different circuits. One has to review the heat fluxes and steam quality at various points in the system to see if there could be problems. Some evaporator circuits could be morecritical than others and require careful analysis. Forexample, Fig. 7 shows the front wall of a packaged water tube boiler with completely watercooled furnace design. This wall has basically two parallel flow systemsbetween the bottom mud drum and the steam drum, namely the tubes that connect the bottom header to the top header and the header itself, which has an L-shape. Flow calculations were done and orifices were used to ensure proper flow distribution in all the heated circuits.LITERATURE CITED
1 Ganapathy, V, Steam plant calculations manual, 2nd edition, Marcel Dekker, New York, 1994.2 Thom, J. R. S., "Prediction of pressure drop during forced circulation boiling of water," International Journal of Heat Transfer No. 7, 1964.3. Roshenow, W, and J. P. Hartnett, Handbook of heat transfer, McGraw-Hill, 1972.The author:V. Ganapathy is a heat transfer specialist with ABCO Industries Inc., Abilene, Texas. He is engaged in the engineering of heat recovery boilers for process, incineration and cogeneration applications and packaged water tube steam generators. He also develops software for engineering of heat recovery systems and components. He holds a B Tech degree in mechanical engineering from Indian Institute of Technology, Madras, India, and an MSc(eng) in boiler technology from Madras University. Mr. Ganapathy is the author of over 175 articles on boilers, heat transfer and steam plant systems and has written five books: Applied Heat Transfer, Steam PlantCalculations Manual, Nomograms for Steam Generation and Utilization, Basic Programs for Steam Plant Engineers (book and diskette), and Waste Heat Boiler Deskbook,copies of which are available from him. He also hascontributed several chapters to the Encyclopedia ofChemical Processing and Designs, Vols. 25 and 26, Marcel Dekker, New York.Fig. 6. Actual vs. allowable quality and heat flux variation with furnace height.