Mass And Energy Balances Over Oil Fired Boiler Engineering Essay

In engineering industry, boilers play an important role. They are widely used to generate steam with a pressure above the atmospheric pressure; the steam produced is used in various processes, mostly in heating applications such as a heat source in heat exchangers. There are many different types of boilers which are used in different applications. One type of boilers is the Oil-fired steam boilers, generally known as steam generators, are the most common vaporizers. These boilers use raw oil as the fuel; burn it and use the heat to boil the water. The product is steam at high pressure and flue gases.

This experiment involves the use of an oil-fired boiler in the Mechanical Engineering Department in order to produce about 1.4 MW of heating steam when 200 l/hr of oil is combusted. A vacuum condenser is used to condense the produced steam to water at temperatures about 10°C above ambient.

Measurements of temperatures, pressures and other relevant parameters were recorded and averaged in the Appendix Section AIII (Recorded Data). Mass and energy balance was performed using those data. When doing the calculations in this experiment, the uncertainty for a range of data was obtained by the confidence the 95% confidence limit. When calculating errors, the error propagation rules were used. The way uncertainties were calculated in this experiment is explained in more detail in the Appendix.

Aim:

The main objective of this experiment is to perform mass and energy balances over the oil-fired boiler and vacuum condenser. In addition, the thermal efficiency of the boiler when operating at steady state was to be obtained. This experiment involves a large amount of data; therefore, the experience gained when handling and manipulate such large amount of data is sought.

Assumptions

Several assumptions had to be made in order to conduct the experiment.

The oil fired boiler, the vacuum condenser, the ejector and all the components in the plant are assumed to be at steady state. This means that all temperatures, flow rates and pressures are constant with time.

The ideal gas law applies to the flue gas. This assumption seems reasonable because the flue gas has reasonably high temperatures and a partial pressure of less than one atmosphere, the assumption is correct to more than 1% error.

For the combustion stoichiometry, it is assumed that complete combustion occurs for the oil in the air.

In the flue gases analysis on dry basis the Micro GC apparatus was used which cannot analyse for the water-vapour present, only O2 and CO2 is detected and the remaining volume is assumed to be N2.

The air entering the boiler is assumed to be 20.93% O2, 70.94% N2, and 0.03% CO2.

The steam that comes out of the boiler at the boiler gauge pressure is dry and saturated (all vapour, not partially condensed).

The air around the boiler, the vacuum condenser and the pipe works is assumed to be stagnant.

The given values for the heat transfer coefficient have an uncertainty of up to 20%.

All values of constants (i.e. specific heat capacity, GCV) given correspond to the conditions during the day of the experiment.

Theory

The theory is adapted from the Boiler Heat and Mass Balance handout (ENCH 271 homepage).

Mass Balance

(Basis: 1 second)

Steam side

Boiler

From the conservation of mass formula of a process, it is clear that the mass of water fed into the boiler is equals to the mass of water out as a condensate.

General balance formula for the boiler:

Accumulation = In-Out+Generation-Consumption

Mass of water into boiler = mass of water out as condensate

The amount of condensate collected can be compared with the feed water rate and the discrepancy noted.

Steam Ejector

A certain amount of steam is drawn off before the condenser to supply the ejector with stream. This is to produce vacuum for the condenser. This amount is calculated to be found:

M=0.017±0.001 kg/s

Combustion side

Flue Gas Flowrate

Following the assumption that combustion gasses act as ideal gases, the ideal gas law of perfect gas law can be used. Because the flue gas is at relatively high temperature and partial pressure less than one atmosphere, the assumption is correct to more than 1% error. Mass of the flue gas is calculated from its composition and mass flow rate of the oil burned.

Overall Balance

For the overall mass balance on the combustion side, the amount of air entering the boiler needs to be calculated from the flue gas analysis and the oil feed rate.

mass of wet air in + mass of oil in = mass of wet flue gas out

The discrepancy is noted between the two values as well, using the flue gas flow rate obtained from the flue gas analysis.

Water content of Flue Gas

The water in the flue gas arises from two sources: water vapour in the inlet air, and water formed during the combustion of H2 in the oil.

Water in the inlet air can be calculated from the absolute humidity:

Yg, the absolute humidity, can be read off the Grosvoner chart (Page 16 f the briefing sheet).

Therefore,

mass of wet air in = (1 + Yg) x mass of dry air

Water coming from the combustion of H2 can be calculated from stoichiometry.

Conversion of Gas Analysis to Weight Basis

The following can be obtained from the combustion stoichiometry and the weight of oil used basis:

The weights of carbon, hydrogen, and sulphur into the system.

The minimum kmol of dry air needed to burn the carbon, hydrogen and sulphur into CO2, H2O and SO2.

The kmol of dry air in excess of stoichiometric which were used in the combustion process.

From above, the weights of CO2, H2O, SO2, N2, and O2 in the flue gas can be calculated. In addition, if the gas fuel analyses are correct, the ratio of CO2 and O2 in the flue gas measured should correspond to the one calculated by stoichiometry.

Condenser and Ejector Cooling Water Flow

The pressure drop across the orifice plate was measured using a differential pressure gauge and the flow rate was then calculated from the orifice equation:

where:

W = flow rate of water [kg/s]

CD = discharge coefficient (= 0.6)

So = area of orifice [m2]

rw = water density [kg/m3]

rHg = mercury density [kg/m3]

g = acceleration due to gravity [m/s2]

h = height of mercury in manometer [m]

Do = orifice diameter (= 0.0402 m)

D = pipe diameter (= 0.078 m)

Energy Balances (Basis: 1 second)

Reference Basis for Heat Flows

For heat flows on the combustion side, it is convenient to choose the air inlet temperature as the reference basis.

Heat Supplied to the Boiler

Heat supplied to the boiler:

Heat supplied = weight of oil in x gross calorific value of oil

The following corrections must be applied:

2455kJ/kg must be subtracted from the CV value. This is because the CV value is measured at 25°C at constant volume. All water formed during the combustion is condensed; therefore the heat used to evaporate the water produced at the reference temperature must be subtracted.

Qcorrection1 = mwater,combustion x 2455

A correction for non-combustion of CO is calculated from the enthalpy of reaction of CO to form CO2 at the reference temperature.

Qcorrection2 = mCO x ΔH

There is an added quantity of heat put into the boiler because the inlet oil temperature is above the reference temperature:

Qcorrection3 = moil x Cp,oil (Toil,in – Treference)

where:

Cp,oil = 1.95 kJ/kg K. This value is added to the heat of combustion.

Heat Balance over the Boiler

The heat of feed water to produce steam is given by the following equation

The enthalpy data can be obtained from steam tables.

The heat lost in the flue gasses is given by:

where:

M = mass of wet flue gas [kg/s]

= weighted average specific heat of the flue gas =1.07 kJ/kg K

Tgas = flue gas temperature at the back of the boiler [K].

Heat is lost up the flue because the gases are considerable hotter than the reference inlet temperature.

The losses from the boiler surface are calculated from:

Where h = hc + hr = combined heat transfer coefficient for the convection and the radiation losses from the surface. For the platework take h = 15 W/m2 K and for a lagged surface take h = 12 W/m2 K. Assume h is correct within ± 20%.

The overall energy balance for the boiler is performed, and discrepancy noted.

Losses from Pipework to Condenser

The heat loss through pipework between the boiler and condenser is given by:

where:

A: is the surface area of the pipe [m2]

h: is the combined heat transfer for convection and the radiation losses from the surface, which is hc + h r= 12 W/m2 K.

Condition of Steam at Engine Room Header

Enthalpy of steam can be calculated from

where:

Hg,boiler is the enthalpy of steam as it leaves the boiler [kJ/kg].

Ql is the total losses in the pipe work, boiler and engine room header [kJ/kg].

The condition of the steam can be determined using the enthalpy and pressure of the steam:

If Hg > (Hg,sat at P) then the steam is superheated. Amount of superheat can be determined from steam tables.

If Hg = (Hg,sat at P) then the steam is dry and saturated and the dryness fraction x=1

If Hg < (Hg,sat at P) then the steam is wet.

The dryness fraction x can be calculated from

where:

Hf : is the enthalpy of saturated liquid at pressure P [kJ/kg].

hence:

where:

Hf,g is the latent heat of vaporisation at pressure P [kJ/kg].

Energy Balance for Vacuum Condenser and Steam Ejector

Heat loss from the piping between the engine room header and the condenser can be calculated. Enthalpy of inlet steam to condenser (Hg) can then be obtained.

Heat removed from the steam to form water at the condensing pressure from the condenser and then subcool the water is:

where:

m = steam flow rate to the condenser [kg/s]

Hf = enthalpy of the condensate [kJ/kg]

Hg= enthalpy of the inlet steam to the condenser [kJ/kg].

A similar formula is used to calculate the heat removed from the steam due to ejector feed pipe surface losses. However, the Hg value for this steam will be different from that entering the vacuum condenser.

The heat removed in the cooling water of the condenser and ejector is given by:

where:

mw = cooling water flow rate [kg/s]

Cp = specific heat of water = 4.183 kJ/kg K

Tout and Tinlet are given by readings off dial gauges [k]

The heat lost from the outside shell of the condenser can be calculated as for the boiler surfaces.

The Overall balance for condenser and ejector is calculated and discrepancy noted:

Overall Energy Balance on Water and Steam

The heat out in the condensate based on the feed water temperature as the reference is given as:

where:

mc = condensate flow [kg/s]

Cp = specific heat of condensate = 4.187 kJ/kg K

The overall balance is performed, and the discrepancy is noted:

Overall Steam-Raising Efficiency

The efficiency of the boiler can be obtained using:

3.0 Experimental Procedures

Adapted from Boiler Heat and Mass Balance Lab Manual Pages 1&12-16

Plant and Apparatus

The plant consists of two major items. The first and main part of the plant is a boiler that is capable of producing 1.4MW of heat in the form of steam and consuming 200L/hr of oil. The second part is a vacuum condenser which is used to condense the steam produced by the boiler to water at temperatures 10°C above the ambient (or surrounding temperature). The arrangement of the Plant is shown in the figure below.

The boiler is an oil-fired steam boiler which has three passes on the combustion side. It consists of a spinning cup oil feed to spray oil into the combustion chamber, the ignition system which lights up the feed oil and the re- circulation water system. Figure 3.2 shows it’s three passes:

The vacuum condenser is just a multi-pass heat exchanged operating under vacuum. The steam is condensed on the outside of the tubes and the cooling water has six “passes” on the tube side. The vacuum is maintained by a steam ejector throttling steam from approximately 11 bars to 1 bar. The ejector is a two diverging-converging nozzle in a series and requires approximately a flow rate of 60kg/hr of steam to maintain vacuum. The figure bellows shows the vacuum condenser:

The plant operates once fuel and water are fed to the boiler. The fuel is burned with air in a combustion chamber or “fire tube”. The hot combustion gases exchange heat with the water along the “boiler passes” and are taken to atmosphere through the “stack” or chimney. The steam formed passes along the steam lines to the condenser where it exchanges heat with the cold water in the “cold water passes” of the condenser. The area and temperature of theses passes is sufficient to cool the steam to near ambient and the steam ejector prevents the accumulation of any permanent gases in the system. Figure 3.4 below shows a representation of the plant operation.

Measurements

The system was started up by the demonstrators and set up to maintain a steady state as much as possible. There were seven measurement stations where those measurements were taken by nine groups of students who circulated around these stations. Each group took the same measurements and the values were recorded and averaged as shown in the Appendix Section (A.III). Recorded Data. The measurements made in each stations were tabulated and are shown in the Appendix section (A.I).

3.2.1 Flow rate measurements

Boiler feed water flow rate

It was noted that the flow of feed water to the boiler is cyclic. During one period, the pump is replenishing the water level in the boiler while at the same time the water is boiled off to form steam. The pump is off in the other period.

The pump is activated by a level sensor. The flow of feed water to the boiler was timed over a complete cycle which included both periods.

The two feed water tanks were calibrated in gallons which was converted to S.I units by knowing the density of water at inlet temperature. Initial reading of the tank was recorded and just as the pump started whirring, stopwatch was started as well. After the pump finished supplying feed water, the final level of the water was recorded. The stopwatch was not stopped until the pump started again since that is the time needed for a complete cycle. The time was noted down in order to obtain the flow rate.

Oil flow rate

Barrel of oil was weighed using a platform balance. Putting a weight of 4.5 kg on the counter balance and the time needed for the scale to unbalanced, i.e. tipped, was recorded. This was the time needed for consumption of 4.5 kg of oil.

Condensate flow rate

The condensate was collected in a bucket for one minute. The bucket was then weighed on an electronic balance.

Flue Gas flow rate

The flow gas flow rate is determined from the flue gas analysis and the oil feed rate as these two sets of data are determined accurately.

Steam ejector flow rate

Previous tests were done prior to the experiment by the Mechanical Engineering Department and it was shown that the flow rate needed to maintain a full vacuum in the condenser was 0.017±0.001 kg/s.

Condenser and Ejector cooling water flow rate

An orifice plate was used to measure the total cooling water flow rate. The pressure drop across the orifice plate was measured using a differential pressure gauge and the flow rate can be calculated using the orifice equation (refer to Theory section for the formula).

3.2.2 Temperature Measurements

Feed water

Temperature of water was measured in the feed tank by a mercury thermometer. Temperature measurements were done right after time for a cycle was recorded.

Air

The air temperature in front of the boiler was measured and the humidity of air was estimated using a whirling hygrometer.

The hygrometer consists of a dry bulb and wet bulb thermometer. Firstly, the hygrometer was twirled in front (or close to) the boiler for approximately one minute to allow the solution in the wet bulb to evaporate. Then, the temperatures of both bulbs were recorded. The humidity was obtained from a Grosvener chart 17 of the briefing sheet [3]

Oil

The oil temperature was obtained by reading the temperature gauge on the oil feed system after the flow rate measurements was completed.

Boiler surfaces

The temperatures of the boiler surfaces were measured using a copper-constantan thermocouple at areas indicated in the figure below.

Front of the boiler Back and side of the boiler

Figure 3.6: Boiler surface temperature measurements

Note that ambient temperature around each surface was taken was well.

Piping Surface and Condenser Surface

For the piping system temperatures at the surfaces of the boiler outlet, boiler room header, engine room header, engine room feed, condensate and ejector feed, condenser feed, ejector feed and condensate line were recorded using the same copper-constantan thermocouple. The temperature at the surface of the vacuum condensate was taken as well. Note that ambient temperature around each surface was taken was well.

Flue Gas

The temperature of the flue gas was measured using the mercury thermometer at the back of the boiler.

Condenser Cooling Water

The inlet and outlet temperatures of the condenser cooling water were read off the gauges on the control panel of the condenser. The temperature of the combined flow of the ejector and condenser cooling water was measured with a thermometer downstream of the orifice plate.

3.2.3 Composition Measurements

Oil Composition

Analysis of the fuel prior to the experiment shows that the composition is

Carbon 85.9%

Hydrogen 12.5% Sulphur 1.6%

Incombustibles (Inert) –

(All measurements in weight percentages)

Oil Calorific Value

Previous measurements show that the gross calorific value for the fuel oil is 43.6MJ/kg at 25°C.

Flue Gas

The amount of CO2, CO and O2 in the flue gas was measured using Micro GC apparatus on a dry basis. It was noted that the remaining volume analysed is to be assumed to be all N2. The SO2 and SO3 content was measured using a Drager tube. The amount of water can be worked out using flue gas stoichiometry. Checks were made on the CO content using the Drager tube.

Steam

The steam was assumed to be dry and saturated at the boiler gauge pressure as it leaves the boiler. To ensure that the assumption holds throughout the experiment, checks were done by checking the gauges on the boiler, the boiler room header and the Engine Room header. The inlet steam pressure to the steam ejector was also noted down.

Equipment

The following instruments were used by the groups when taking measurements of temperature, pressure, mass and time in the plant:

Ear protection muffs.

Stop watch with resolution of 0.01s.

Platform balance with a resolution of 0.005g.

Electronic mass balance with resolution of 0.01Kg for measuring the mass of condensate over period of time.

Orifice plate to measure the total cooling water flow.

Mercury thermometer.

Whirling hygrometer for estimating the humidity of air (using a crossover chart).

Copper constantan thermocouple with resolution of 0.1°C.

Boiler pressure gauges.

Micro GC apparatus for fuel gas analysis.

Drager tubes for detection of CO content volume analysis.

Results

The uncertainties values are obtained using the 95% confidence limit method for more than 5 data points. On the other hand, half range method was used for less than 5 data points. Further detailed calculations and uncertainty analysis are done in the Appendix.

Mass Balance

Steam Side

Performing a mass balance over the boiler, the boiler feed water and the condensate flow rates are calculated. The discrepancy is found to be:

Boiler water feed rate

=

0.24

±

0.03

kg/s

condensate flow rate

=

0.25

±

0.01

Kg/s

Discrepancy

=

3

%

Combustion Side

Flue Gas Rate

The flue gas is calculated from its composition and the mass-flowrate of the oil burned by applying perfect gas laws and the following is found:

Flue Gas flow rate from Flue Gas Analysis = 0.4 ± 0.01 kg/s

Overall Balance

Table (1): Overall mass balance

Mass of wet air

0.372

±

0.0084

kg/s

kg/s

kg/s

Mass of

wet flue

gas out

0.4

±

0.01

Mass of Oil in

0.0208

±

0.0007

Total mass in

0.419

±

0.04

Discrepancy between Inputs and Outputs

0.06

%

Water Content of Flue Gas

Water in the Inlet Air

Absolute Humidity of Air = 0.007 ±0.0004 kg/kg

Mass flow of Dry Air = 0.36 ± 0.008 kg/s

Mass flow of Wet Air = 0.37± 0.008 kg/s

Therefore,

Water in Inlet Air = 0.003 ± 0.0006 kg/s

Conversion of Gas Analysis to Weight Basis and excess Air analysis

The weights of Carbon, Hydrogen and Sulphur given to the boiler by the feed oil were calculated and the results are shown in the Appendix section (B). The required air is calculated from the stoichiometry using the composition and flow rate of the oil. The excess air is calculated from the stoichiometry using the composition of CO2 in the flue gas from Micro GC analysis. The excess is also calculated as the difference between the oxygen required and the oxygen provided according to the mass balance and the percentage of excess air found to be 20 %. All the data obtained in order preceding the composition of flue gas composition are found in the Appendix section (B)

Table (2): Flue gas Composition.

Composition

Amount wet

 

Mass wet

 

 

kmol

Uncertainty

kg

Uncertainty

O2

0.0005

0.0001

0.02

0.0041

N2

0.0101

0.0003

0.28

0.0080

CO2

0.0015

0.0001

0.07

0.0023

H2O

0.0014

0.0001

0.03

0.0017

SO2

0.000010

0.0000004

0.0007

0.00002

Total

0.0136

0.0003

0.4

0.01

To check if the fuel analysis was correct, the ratio of CO2 to O2 in the flue gas measured was compared to the one calculated by stoichiometry as shown in the table below:

Table (3): Comparsion between CO2/O2ratio

 

Micro GC

Stochiometry

CO2

12.36

±

2.00

12.30

±

0.42

O2

5.26

±

2.00

4.34

±

1.06

Ratio (CO2/O2)

2.35

±

0.97

2.84

±

0.70

Discrepancy

 

20.8

%

 

 

 

Energy Balance

All detailed calculations are shown in the Appendix Section (C).

Reference Basis for Heat Flows

Reference basis for heat flow is the air inlet temperature = 17.6 ± 0.7 °C

Heat Supplied to the Boiler

The heat supplied to the boiler is the heat that is produced by the combustion of fuel (combustion heat). As stated in the theory section, this value needed to be corrected. The corrected value found is:

Qcombustion = 849 ± 10 kW

Heat Balance over the Boiler

The energy balance around the combustion side of the boiler uses the energy available from combustion, the energy required to generate the observed steam flow, the energy lost in the flue gas and the energy lost from the boiler surfaces. These values are listed in Table (4) as well as the discrepancy in the energy balance.

Variables

Values

Uncertainties

 

Units

Qcorrected

849

±

10

kW

Qsteam

656

±

79

kW

Qflue gas

93

±

8

kW

Qboiler surface

29

±

6

kW

Discrepancy

8.4

%

 

 

Table (4): Energy balance around the combustion side

Losses from Pipework to Condenser

The calculated heat loss from the pipework surfaces to the atmosphere

Qpipe work = 7± 1 kW

Condition of Steam at Engine Room Header

The enthalpy lost from the pipe work up to the engine room is found to be:

Hg,sat at the pressure =

2782

kJ/kg

Hg,engine room =

2766

kJ/kg

Since Hg,engine room < Hg,sat at steam pressure, then the steam is wet. By comparing the resulting steam enthalpy to enthalpies of saturated steam and water at this point it is determined that the steam becomes wet during transport. Furthermore, the fraction of the dryness of steam is found 0.993±0.2 dry.

Energy Balance for Vacuum Condenser and Steam Ejector

The energy balance around the condenser and ejector was calculated using the energy lost from the piping between the engine room header and the condenser, the heat removed from the steam and the heat removed due to ejector feed pope surfaces losses. These values are listed in the table below as well as the discrepancy in the energy balance.

Table (5): Heat lost from piping between engine room header and condenser:

Variables

Values

Uncertainties

Units

Qs

598

±

24.1

kW

Qejector

44.4

±

8.7

kW

Qloss

1.87

±

0.38

kW

LSH

644.2

±

25.66

kW

Qw

492.88

±

0.4

kW

Qshell

5.4

±

0.4

kW

RHS

498.3

±

0.6

kW

Discrepancy for overall balance

22.6

 

%

Overall Energy Balance on Water and Steam

Heat out in condensate, Qc= 1.4 ± 0.2 kW

Overall Balance for water and steam:

LHS = Total heat supplied to the water to produce steam,

= Qfeed water = 656 ± 12kW

RHS = Heat out in condensate + Total heat lost form pipwork + Total energy lost in condenser + Energy lost in steam ejector

= Qc + Qpipe work + Qs + Qejector = 651 ± 26kW

Therefore, the Discrepancy found to be = 2%

Overall Steam-Raisin Efficiency

Table (7): Overall steam-Raising Efficiency

Variables

Values

Uncertainties

 

Units

 

 

Qcombustion

849

±

10

kW

1

%

Qfeed water

656

±

12

kW

2

%

Î-

77

±

3

kW

2.2

%

Discussion

Expected Results

The values obtained in this experiment were reasonably accurate with relatively low uncertainties in general. That is because there were 9 sets of measurements taken, one for each group, in each measurement station. The uncertainties values were obtained by the 95% confidence limit and half range methods, so the errors reduced due to the more measurements made.

The Overall Steam Raising Efficiency is expected to be around 80% according to the Briefing Sheet… However, this value is given when the flow rate of feed water to the boiler is around 0.3 kg/s. If the values calculated for this experiment for the flows were different, it is expected to get a different efficiency. Hence, we would expect the efficiency of the boiler to be reasonably different than 80%.

Mass Balance

It is clearly seen in Results Section 4.2. that the mass balance on the steam side was not achieved. The amount of feed water into the boiler was measured to be 0.240 ± 0.003 kg/s (less than the expected 0.3 k/g) while the condensate flow was measured to be 0.257 ± 0.01 kg/s. The two values showed a discrepancy value of 3%. The calculated discrepancy value found to be small and fall within the experimental uncertainties of the two flow rates.

However, several factors can be attributed to the source of error occurred between the difference values obtained of each flow. One possibility is that the feed water to the boiler was not measured very accurately as desired since the level indicator of the tank was fluctuating all the time. This was due to feedback of water to the tank during the experiment since water boils and expands in the boiler. Therefore, the actual water level in the feed tank had to be estimated. In addition, parallax error when reading water level has also contributed to the discrepancy.

The overall balance on the combustion-side showed very insignificant discrepancy of about 0.06%. This discrepancy was calculated using the wet flue gas flow rate obtained from the flue gas analysis. The low discrepancy indicates that the assumption that the flue gas was considered to behave as an ideal gas was valid as the experiment was conducted at relatively high temperatures and partial pressures less than one atmosphere. It also showed that the amount of wet air in the boiler calculated using the combustion stoichiometry and composition of air (20.93% Oxygen, 79.04% Nitrogen and 0.03% Carbon Dioxide) was reasonably accurate.

Composition of flue gases was obtained and analysed in the experiment by using Micro GC apparatus. Despite that, a flue gas analysis using combustion stoichiometry was done as shown in Table 4.2 as a cross check for the gas and fuel analyses. Furthermo