Comparative Study of Air Flow Measurement Techiques

A Comparative Study of Air Flow Measurement Techniques & How They Compare to Lab Tested Results Patrick W. Jacob, P.E. Williams Gas Pipeline Marcos Sandoval Williams Gas Pipeline Eric R. Dufur ScavengeTech, LLC Eric R. Figge ScavengeTech, LLC Presented at the Gas Machinery Conference in Atlanta, GA. October 5-7, 2009

i

Comparative Study of Air Flow Measurement Techniques

Abstract

While testing a Williams Gas Pipeline Clark HBA-8 turbocharger at the Kansas State University National Gas Machinery Laboratory (NGML), three methods were used to determine the turbocharger air flow rate. These were: (1) a Kurz™ anemometer installed near the turbocharger turbine exhaust; (2) a turbocharger management system that included a pitot tube to measure differential pressure; and (3) an ASME venturi nozzle that is a permanent part of the NGML turbocharger test cell instrumentation.

The turbocharger was then installed in the field and air flow rates were measured using the Kurz™ anemometer and the pitot tube with the turbocharger management system. In addition, emissions data were recorded and the results were used to calculate the air flow rate using a carbon balance method and EPA Method 19. Results showed that all methods were significantly accurate to that of the air flow rate collected on the turbocharger test cell except that of the anemometer.

Implications of this laboratory and field study include the need for field engineers to maintain strict control over the placement of instrumentation and the method of recording data to ensure reliable and repeatable results. In addition, field engineers need to understand instrumentation uncertainty analysis and its impact on collected data to avoid basing decisions upon unreliable or incorrect data

i


Table of Contents

Abstract ..........................................................................................................................................................................i

Table of Contents.......................................................................................................................................................... ii

Introduction ...................................................................................................................................................................1

Background ...............................................................................................................................................................1

Project Overview.......................................................................................................................................................2

Turbocharger Testing and Flow Measurement Collection ............................................................................................2

Turbocharger Test Laboratory...................................................................................................................................3

Performance Data Comparison .............................................................................................................................3

NGML Test Cell ASME Venturi Flow Nozzle.....................................................................................................4

Flow Measurement Uncertainty............................................................................................................................5

Test Data...............................................................................................................................................................6

Turbocharger Management System...........................................................................................................................8

Kurz™ Anemometer ...............................................................................................................................................11

Theoretical Justification......................................................................................................................................12

Sample Data........................................................................................................................................................15

Field Measurement Data Collection ............................................................................................................................17

Turbocharger Management System.........................................................................................................................17

Sample Data........................................................................................................................................................18

Kurz™ Anemometer ...............................................................................................................................................19

Sample Data........................................................................................................................................................19

Issues and Concerns............................................................................................................................................20

Emissions Analyzer.................................................................................................................................................21

EPA Method 19 ..................................................................................................................................................21

Carbon Balance Method .....................................................................................................................................22

Sample Data........................................................................................................................................................22

Results and Conclusions..............................................................................................................................................24

Future Work.................................................................................................................................................................24

References ...................................................................................................................................................................25

APPENDIX A: TRAVERSE DATA...........................................................................................................................26

APPENDIX B: EPA METHOD 19 .............................................................................................................................27

ii


Introduction

A key challenge to determining and monitoring turbocharger performance in the field is the accurate measurement of pressure, temperature, and flow through the compressor and turbine sections of the turbocharger. Of these, air flow measurement may be the most elusive simply because there is typically no permanently installed flow meter. Pressure and temperature can be more readily measured but, of course, accurate measurements are extremely dependent upon the installation of probes in the correct locations and this also can be difficult.

This study compared four methods of turbocharger air flow measurement to each other and to that of the air flow measured in a laboratory setting using a calibrated ASME venturi flow nozzle. The methods are:

(1) Turbo Shield™ (TS)-a turbocharger management system sold by Exterran that uses a pitot tube for air flow measurement;

(2) Kurz™ portable anemometer mass air flow meter;

(3) Calculated values using EPA Method 19; and

(4) Calculated values using a carbon balance method developed by ScavengeTech LLC.

Of these, the EPA Method 19 is widely used for mapping emissions control strategies in the natural gas pipeline industry. The other methods offer possible means for permanent field and remote continuous monitoring, temporary field testing and troubleshooting, and, if approved, an alternate for mapping emissions control strategies.

Background

The turbocharger is an essential component for proper engine performance and compliance with emissions limits. When the required air manifold pressure set point is not being met, a lot of attention can be placed on the turbocharger system. If the waste gate is fully closed, then the prime suspect becomes a turbocharger that is not performing as expected. Several possibilities for a performance loss then come to the forefront, including operating conditions, efficiency reductions from fouling, or damage short of outright failure, manufacturing that falls short of specification, or fundamental sizing issues that did not provide the proper turbocharger to engine match. How can we find out what’s wrong in such a situation? Determining the flow rate along with pressures and temperatures can certainly help correctly troubleshoot problems and direct attention to the proper solution. Why send a turbocharger off to be re-built if there’s nothing wrong with it? Why try to make a system perform in a range that it probably cannot? In some cases it could also be a considerable benefit to continuously monitor a critical turbocharger such that maintenance intervals could be predicted and scheduled prior to exceeding efficiency reduction limits or, perhaps, to avoid serious and expensive mechanical failure. Such a continuous monitoring system would require accurate and repeatable flow measurement.

The work required to be done on the turbocharger compressor by the turbocharger turbine from the energy in the engine exhaust is largely a function of exhaust gas mass flow rate, turbine inlet pressure, and turbine inlet temperature. The relationship of pressures and temperatures are:

1


⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

−= ⎟⎟⎠

⎞⎜⎜⎝

⎛−

•

•

PPTcm

WCin

Coutk

k

p C

C

C

C

CinC

C

1

1η

(1)

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

−= ⎟⎟⎠

⎞⎜⎜⎝

⎛−

••

PPTcmW

Tin

Toutk

k

p

T

T

T TTinTT

1

1η

(2)

As can be shown from these two equations, the mass flow rate is one of the necessary inputs for an accurate determination of work done on the compressor or by the turbine. If we don’t know what the machines are actually doing in the field, we may not properly conclude the cause of the problems and the necessary solution.

Project Overview

A newly overhauled Clark HBA-8 turbocharger owned by Williams Gas Pipeline was tested at the NGML which included precise measurement of the air flow rate using a calibrated venturi nozzle. In addition the air flow rate was also measured using a turbocharger management system sold by Exterran and a Kurz™ anemometer provided by Williams Gas Pipeline. Emissions were also recorded and used to determine air flow via EPA Method 19 and a carbon balance method developed by ScavengeTech. The accuracy of each method is then compared to the test cell measured air flow rate, which thereby highlights the importance of taking uncertainty analysis into consideration.

Next, field data from the turbocharger management system and the Kurz™ anemometer is compared to that of laboratory collected data. To take the analysis a step further, emission data also was used to calculate the air flow rate via the EPA Method 19 method and a carbon balance method that was developed by ScavengeTech LLC. Differences in the measured air flow rates are discussed.

Implications to improve the collection of field data are drawn and suggestions made for future work.

Turbocharger Testing and Flow Measurement Collection

A laboratory test of a turbocharger post-overhaul provides the operator with the knowledge that the turbocharger is free of vibration and is capable of operating at the design point. An optional performance map of a turbocharger provides the operating range of the turbocharger and thereby allows a field engineer to make informed decisions regarding different operating conditions of the turbocharged engine system. This type of information can be provided in a laboratory setting due to the calibrated and redundant instrumentation on the test cell which assures accurate and repeatable results. The test results then provides a baseline operation from which field operation can be compared. It can also provide independent validation that a manufacturer’s design meets specification.

This section reviews the capabilities built into the NGML turbocharger test facility, evaluates the instrumentation uncertainty to determine the accuracy of collected operating data, and then provides the developed performance map for the Clark HBA-8 turbocharger. Laboratory air flow measurement methods are also reviewed for the Kurz™ anemometer, the turbocharger management system, the EPA Method 19, and the carbon balance method.

2


Turbocharger Test Laboratory

The National Gas Machinery Laboratory (NGML) is an institute of the Kansas State University College of Engineering. The NGML is home to the worlds only non-OEM turbocharger testing and research facility for large bore engine turbochargers. The universal design of this test cell allows the capability to mount and test a wide range of turbochargers including axial and radial turbochargers with flow rates from 5,000 SCFM to 30,000 SCFM and compression ratios up to 4.0.

The NGML utilizes a closed loop test cell configuration as shown in Figure 1. This configuration essentially turns the turbocharger into a gas turbine. Air enters the compressor inlet and is compressed by the compressor wheel up to the load valve. The load valve in conjunction with the air heating combustor is used to replicate the backpressure and exhaust power of the engine that is being simulated. The heated air then enters the turbine inlet, passes across the turbine wheel and exits through the turbine exhaust creating shaft horsepower that is used to drive the compressor.

Figure 1: Schematic of Turbocharger Test Cell Configuration.

Instrumentation at the NGML is installed per ASME Power Test Code 10 (1997) and the Gas Research Institute turbocharger testing standard (Chapman et al., 2005). These standards require that instrumentation be installed to provide repeatable and accurate data, and calibrated on a regular schedule. They specify instrument locations for measuring pressure and temperature. For example, the sensors for recording air temperature at the inlet and outlet of the compressor are located a specific number of pipe diameters from the compressor. This specified distance of straight pipe sections ensures the readings are not skewed by circulating flow fields within the piping.

A performance test by necessity is performed with all the components at thermodynamic stability. This stability ensures that component efficiencies can be calculated without the effects of external heat gain or loss skewing the data. The compressor and turbine outlet temperatures are monitored to determine thermodynamic stability. Once the unit is thermodynamically stable (normally 10 to 15 minutes following load changes), all mechanical and performance data is recorded. Thermodynamically stable is quantified by the time rate of change of several parameters is less than a specific metric. This data includes temperatures, pressures, flow rates and other parameters required to create the compressor and turbine performance maps. Four thermocouples, located 90° apart, for each temperature and flow rate are used to assure data integrity. Each collected data point is compared with the other three and is removed from the calculation if it is significantly different.

Performance Data Comparison

Tested conditions such as ambient temperature and pressure could substantially differ from the field condition in which the turbocharger will operate. In order to evaluate test stand performance and analyze how the collected data relates to field conditions on the engine, the data must be reduced to meaningful

3


parameters. For the turbocharger compressor, these parameters are corrected speed, corrected air flow rate, and pressure ratio.

The volumetric flow correction shown in equation (3) and the mass flow correction in equation (4) allow for the comparison of one condition to another, and provide the means to determine how the turbocharger compressor will operate at conditions other than specified on the compressor map.

[ ] [ ],,

SCFMSCFM s act

s corr

VV

θδ

=&

& (3)

actcorr

mm θδ

=&

& (4)

Because of the literal impossibility of maintaining the turbocharger speed at a precise value, the affinity equations can be used to correct the flow rate and the pressure ratio to an exact speed. This correction provides for a more exact compressor performance map. The affinity equations state that the compressor flow rate is linearly proportional to the compressor speed and that the compressor pressure ratio is proportional to the square of the speed. In equation form, the affinity equations are:

2 1

2

1N N

NV VN

⎛= ⎜

⎝ ⎠& & ⎞

⎟ (5)

2

22 1

1

NPR PRN

⎛ ⎞= ⎜ ⎟

⎝ ⎠ (6)

In these equations, the data at point 2 are those at the desired speed and those at point 1 are those at the true, but corrected, speed. The affinity equations are a simplification of the more sophisticated ASME equations that are actually used to correct the test cell data.

At this point, all the information is available to construct a corrected compressor performance map, or at a minimum place data points on a map for comparison.

NGML Test Cell ASME Venturi Flow Nozzle

Fluid flow measurement can be calculated by several methods, but one of the more popular techniques is by measuring the pressure drop over an obstruction inserted in

the flow. This obstruction can be in the form of a venturi tube, orifice plate or flow nozzle among others. A comparison of these three types is shown in Figure 2. The method in use at the NGML is a flow nozzle.

Figure 2a: Venturi Tube Schematic.

Figure 2b: Orifice Plate Schematic.

Figure 2c: Flow Nozzle Schematic.

4


There are well documented advantages and trade-offs of each one which is beyond the scope of this paper. However, all differential pressure flow meters are based on Bernoulli’s equation, where the pressure drop and the further measured signal is a function of the square of fluid flow speed. The three components of Bernoulli’s equation, are; a velocity head (kinetic energy) as shown in equation (7) which is the height from which a mass of liquid would have to fall freely; a pressure head (potential energy) as shown in equation (8) which is the height of liquid equivalent to pressure; and a gravity head (potential energy) which is the elevation above the given base elevation. If the nozzle is installed horizontally with no change in elevation, then gravity head is no longer a factor. Bernoulli’s equation applies the conservation of energy, so any two points in a fluid stream have the same total head H as shown in equation (9).

2

2V

g (7)

Pgρ

(8)

2 2

2 12 2V P V P

g g g gρ ρ⎡ ⎤ ⎡ ⎤⎛ ⎞ ⎛ ⎞⎛ ⎞ ⎛ ⎞

+ = +⎢ ⎥ ⎢⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠⎝ ⎠ ⎝ ⎠⎣ ⎦ ⎣ ⎦

⎥ (9)

By making a few property assumptions and putting V in terms of flow divided by area, equation (9) can be turned into equation (10) at 1p and conditions. In this equation k is a combined term that includes

the flow coefficient, gas expansion factor, and the combined thermal expansion factor. 1T

( )2

42

4 1d kflow g Pπ ρ

β

⎛ ⎞⎛ ⎞⎜ ⎟= ⎜ ⎟⎜ ⎟−⎝ ⎠⎝ ⎠

Δ (10)

Flow Measurement Uncertainty

An inherent amount of uncertainty exists with all measurements. Even the most precise measuring device cannot give the actual value because to do so would require an infinitely precise instrument. A measure of the precision or accuracy of an instrument is given by its uncertainty. This uncertainty is not only a factor with any single measurement, but also propagates into calculations that use those measurements.

For any individual measurement, its uncertainty is a function of all the components that make up that measurement. For example, a single pressure measurement involves a pressure transmitter that converts pressure to a current (or in some cases a voltage), and then that analog current must be converted to a digital number which is usually the number read on a screen. The two items in this example each have their own uncertainty, both of which go into the function for the complete pressure measurement. Statistically, this is performed by finding the standard deviation of the mean.

Using the pressure measurement example from above, the total uncertainty is the square root of the sum of squares of individual uncertainties. If the pressure transmitter has an accuracy of 0.25% of its full scale (30 psi) and the analog to digital input has an accuracy of 16µA, then the pressure measurement uncertainty takes the form of equation (11). If there is more than one measurement (e.g., 4), then the uP

5

http://www.engineeringtoolbox.com/bernouilli-equation-d_183.html


total pressure measurement uncertainty is the square root of the square of all four, as shown in equation (12).

[ ]2302(0.25%)(30 ) (16 ) 0.081

16psiP psi A psiu mA

μ⎡ ⎤⎛ ⎞= + =⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦ (11)

24(0.081 ) 0.162.P psiu total = = psi (12)

Once the uncertainties for all the components are evaluated, then the uncertainty for the calculated value of flow can be found. This is accomplished by taking the square root of the squares of each term. However, when more than one unit variable is used, then the partial derivative of each term must be used, as shown in equation (13). For a percentage, flow divides out of equation (9), which leaves each factor as a unit vector as shown in equation (14).

2 2

flow flow flowu P T

P T P

Flow u u uΔΔ

∂ ∂ ∂⎛ ⎞ ⎛ ⎞ ⎛= + +⎜ ⎟ ⎜ ⎟ ⎜∂ ∂ ∂⎝ ⎠ ⎝ ⎠ ⎝

2

P

⎞⎟⎠

(13)

2 2

u P T PFlow U T U 2

Flow P T P⎛ ⎞ ⎛ ⎞ ⎛ ⎞= + +⎜ ⎟ ⎜ ⎟ ⎜ ⎟Δ⎝ ⎠ ⎝ ⎠ ⎝ ⎠

(14)

In the case of the NGML, the uncertainty for flow calculation comes out to be just over 4%, which is smaller than many other methods. In this calculation, temperature significantly contributes to the overall uncertainty since a slight change in temperature has a big effect in gas density. Also of note, when specifications are received from the nozzle manufacturer, parameters such as measured dimensions have an associated error, but generally it can be assumed they are accounted for in the flow coefficient, which is the ratio of theoretical flow to actual flow through the nozzle.

Test Data

A performance test was conducted on a newly overhauled Clark HBA-8 that was owned by Williams. Information from the performance map would allow personnel to collect field performance data and accurately identify how the system was operating.

To complete the performance map, the five speed lines selected that were within normal operating possibilities for this turbocharger ranged from 6,000 rpm to 9,000 rpm at 750 rpm increments. To gather points at each speed line, data was taken at the minimum possible differential between the compressor discharge and the turbine inlet. These points were very close to the turbocharger stonewall, which means that for each speed the maximum possible air flow was achieved. Then, while maintaining the designated speed, the load valve was closed which increased the differential close to compressor surge. Surge is the point at which the compressor can no longer overcome the discharge backpressure, and is the point at which the minimum possible air flow is achieved.

Once stonewall and surge were distinguished, points between were collected to complete the performance data for that speed line. In this case, five points per speed line were collected to accurately capture the operation of the turbocharger at that speed. Following this method, the rest of the speed lines were

6


collected which resulted in the performance map shown in Figure 3. This map can be used by field engineers to interpolate fresh overhaul or baseline performance as long as any two of factors – speed, pressure ratio, or flow – are known. What this does not take into account, however, is performance degradation over time, which usually decreases flow and pressure ratio for any given speed. This is the necessitating factor for requiring robust field air flow measurement.

Figure 3: Clark HBA‐8 Compressor Performance Map.

7


Turbocharger Management System

Turbocharger performance, especially on SIP-call engines, remains essentially unmeasured even though key turbocharger data and tracking information would provide critical information that could help natural gas compressor station operators lower emissions and lower the cost of operations.

Turbocharger degradation is caused by multiple factors, including: • Fuel quality • Engine load • Ambient conditions • Engine condition • Misfires • Engine start/stop frequency • Lubricants/filtration

Without monitoring turbocharger degradation rates, the only indication of degraded air flow rate is the waste gate position. This method, however, provides neither the means to calculate turbocharger performance and NOX emissions, nor the means to forecast turbocharger performance at various ambient temperatures. Furthermore, without monitoring turbocharger degradation rates, most field turbochargers may experience either:

1. A premature and, therefore, not cost-beneficial overhaul; or 2. Continued operation in a degraded state, which needlessly increases the cost of operations.

Most engine control strategies will retard ignition timing when necessary to maintain adequate exhaust energy to the turbocharger if the waste gate is closed. This results in a higher fuel consumption rate for the engine which will increase fuel costs and may also increase emissions. While ignition timing retard does not increase NOx it may increase other emissions parameters including greenhouse gas and this will likely be a greater concern in the near future.

Figure 4: Turbocharger Management System Parameters.

In 2006, with the assistance of funding from the Pipeline Research Council International Inc., ScavengeTech LLC developed and field tested a turbocharger management system that consisted of a set of sensors to measure key turbocharger performance parameters and hardware to interface the sensor outputs to a computer. These measured parameters shown in Figure 4 include the following:

• Compressor Inlet temperature and pressure • Compressor discharge temperature and pressure • Compressor discharge mass air flow rate • Turbine inlet temperature and pressure

8


• Turbine discharge temperature and pressure • Turbocharger speed

The most important and most difficult parameter to accurately measure is the compressor air flow rate. Without the ability to precisely measure the air flow rate from the turbocharger to the engine, accurately forecasting turbocharger degradation and engine emissions compliance is impossible.

Figure 5: Compressor Inlet Temperature (Top) and Pressure (Bottom) Tap Locations.

Before installing the turbocharger management system, which is exclusively sold by Exterran under the name Turbo Shield™, at Williams Station 150, the unit was calibrated in a laboratory setting. The Turbo Shield™ unit was installed on the NGML turbocharger test cell and operated under anticipated field conditions to ensure proper operation and to calibrate the flow measurement device.

As shown in Figure 5, the taps for the compressor inlet temperature and pressure sensors were located directly in the turbocharger and not in a pipe upstream or downstream from the turbocharger. These locations assured a more accurate and robust measurement.

Figure 6 shows the locations for the flow and temperature taps on the compressor discharge. The pressure measurement cannot be seen in the figure, but is located opposite of the temperature location. As with the compressor inlet, the measurement locations are located directly on the turbocharger. This is extremely important when collecting the flow measurement from the compressor discharge to ensure accurate measurement of the flow device. Chapman and Mohsen (2002) identified the location within the compressor discharge scroll to precisely place the pitot tube.

Figure 7 and Figure 8 show the tap locations for the turbine inlet and the turbine discharge, respectively. Because of the high temperatures measured at the turbine, the thermocouples used with the Turbo Shield™ are type K thermocouples with an operating range up to 1500°F. Again, for increased accuracy and reliability of the measurements, the tap points are located directly in the turbocharger turbine inlet and outlet.

Figure 6: Compressor Discharge Flow (Top) and Temperature (Bottom) Tap Locations.

9


Data were collected from each measurement device and logged to a data file. Data points were collected at exactly the same time as the NGML test cell data to ensure that the collected data could be compared. This direct comparability is critical to precisely calibrate the flow instrument.

Figure 7: Turbine Inlet Pressure (Left) and Temperature (Right) Tap Locations.

The Turbo Shield™ flow measurement device was calibrated to the test cell flow readings based on four parameters:

• Compressor discharger pressure • Compressor discharger temperature • Compressor pressure ratio • Differential pressure across the flow device

By using these four parameters and a proprietary equation developed by ScavengeTech, the flow device can be accurately calibrated to the turbocharger test cell results.

After the pitot tube was calibrated, the measured flow from the Turbo Shield™ was compared to the NGML flow measurements. This provided a check of the Turbo Shield™ flow equation and shows the accuracy of the calibration. Figure 9 compares the NGML test cell flow measurements and the Turbo Shield™ flow measurements. As shown in the figure, the Turbo Shield™ flow measurements compare very accurately to the NGML test cell flow measurements.

Figure 9: Comparison between NGML Test Cell Flow Device Calibration Results.

Figure 8: Turbine Discharge Pressure (Top) and Temperature (Bottom) Tap Locations.

10


Kurz™ Anemometer

The anemometer used to measure flow standard velocity in the exhaust side of the turbocharger was manufactured by Kurz™ Instruments Inc. and is shown in Figure 10. The probe was inserted into the exhaust side exit of the turbocharger, as shown in Figure 11.

Choosing the location of probe insertion was driven mainly by availability. A port was already available at the time in the turbocharger exhaust side transition piece (Figure 11 and 12). The transition piece has straightening vanes in its cross-section which suggested that flow in this location would be steady. During the first day of laboratory testing, one velocity and temperature profile were collected, that was comprised of seven data points. The probe was traversed across the cross-section of the exhaust outlet with each data point collected at a different location. These locations are represented in Figure 13 as areas A-G. The collected data were then compared with the test cell data to select the location that best matched the measured flow from the test cell.

Figure 10: Kurz™ InstrumentsAnemometer.

Figure 11: Anemometer Probe Inserted in Turbocharger Exhaust Outlet.

Figure 12: Schematic of the Side View of Insertion Location.

During the second day of laboratory testing, a full performance map was collected, which was comprised of five speed lines with five points per line for a total of 25 data points. The location of the probe

11


remained at the selected location – center of the cross-sectional area – throughout the test with one exception. When the turbocharger reached 7,500 rpm, the probe was once again traversed across the exhaust outlet and seven data points were collected while the speed remained steady. This was done to verify the insertion location of the probe.

Figure 13: Kurz™ Anemometer Traverse Schematic.

Theoretical Justification

The anemometer has two resistive thermal devices (RTD) at the head of the probe, as shown in Figures 14 and 15. The shorter RTD measures the temperature of the flow (Rt) and the longer RTD (Rp) is heated to a temperature above Rt and hence, above the temperature of the flow. The temperature difference between Rp and Rt is kept constant. The current necessary to keep the Rp temperature above that of Rt temperature is also measured.

The heat transfer occurring between Rp and the flow can be modeled as:

(15) ( sQ hA T T∞= −

Figure 14: Anemometer Probe.

)

Where:

Q = Heat transfer between the heated RTD and the flow

h = Heat transfer coefficient

A = Surface area of RTD.

Ts = RTD surface temperature

T∞ = Fluid flow temperature

Surface area and temperatures are readily available but the heat transfer coefficient must be derived by applying heat transfer and fluid flow fundamental correlations. The heat transfer coefficient will eventually be:

mCK Vdhd

ρμ

⎛ ⎞= ⎜

⎝ ⎠⎟ (16)

Figure 15: Anemometer Probe Schematic.

Substituting equation (16) into equation (15) yields:

12


(

m

sCK VdQ Ad

ρμ

)T T∞

⎛ ⎞= −⎜ ⎟

⎝ ⎠ (17)

Where:

C = Constant

Κ = Thermal conductivity of fluid.

ρ = Density

V = Velocity

µ = Fluid dynamic viscosity

m = Coefficient

This fundamental heat transfer correlation between the RTD and the flow around allows for the calibration of ρV versus electric current.

Note from Figure 16 that the heat transfer rate is known by measuring the electric power transferred to the heated RTD (alternatively current or voltage required). For every increase/decrease in the product ρV there is a corresponding increase/decrease in heat transfer while all other parameters remain constant including ( at a given temperature. Because

this correlation exists, ρV versus electric power can be calibrated. Alternatively, ρV versus electric current can be calibrated to an accepted true velocity value in a laboratory. Parameters such as Κ, ρ, µ, etc. that should remain constant, will actually vary as flow temperature varies. To resolve this issue, calibration curves can be generated for various temperatures. In other words, generate flow at a specific temperature (i.e., 60°F), and while that temperature is maintained, the flow velocity can be varied to calibrate ρV versus current. This calibration is repeated for few other temperatures and the resultant calibration curves will be like the ones shown in Figure 17.

Figure 16: Schematic of Kurz™ Anemometer.

)sT T∞−

The Kurz™ Anemometer gives velocity readings as standard velocity in the units of standard feet per minute (SFPM) which is defined as:

s

VSFPM ρρ

≡ (18)

Where:

ρ = Density of flowing fluid

ρs = Standard density of air

13


V = Flow velocity

Even though the Kurz™ Anemometer is calibrated for air, the heat transfer model shows that none of the thermodynamic/fluidic properties (Κ, µ, etc.) of the flow should cause significant errors at high temperatures, which allows the anemometer to be used in exhaust gas stacks for internal combustion engines.

To obtain the mass flow rate, the following equation is applied using the SFPM value read from the Kurz™ anemometer:

s exhausts

Vm A SGρ ρρ

= && (19)

Where:

= Mass flow rate

A = Cross sectional area of flow boundary

SG = Specific gravity of exhaust (can be obtained from exhaust sampling)

Figure 17: Calibration Curves for Kurz™ Anemometer.

14


Figure 18: Velocity and Temperature Profiles for the First Data Set at 7,500 rpm.

Figure 19: Velocity and Temperature Profiles for Second Data Set at 7,500 rpm.

Sample Data Table 1: Comparison between Kurz Anemometer

Data and the Lab Data.

Data Points

Kurz™ (lb/sec)

Lab (lb/sec)

Error (%)

1‐7 9.91 10.64 6.83%

8‐14 8.05 9.63 16.47%

Figures 18 and 19 show the velocity and temperature profiles of the anemometer data collected during the two traverses across the exhaust outlet. While the general shapes were the same from one data set to the next, there were definitely variations for the collected data. When compared to the data collected by the turbocharger test cell, the center of the cross-sectional area was selected as the location for the most stable flow.

To compare the anemometer data to that of the test cell data for the two traverses, the data were averaged and are shown in Table 1. As illustrated by the high rates error of 6.83% and 16.47% for tests 1 and 2 respectively, the importance of selecting a point where flow is steady is critical.

For the performance map, the probe remained at the center of the outlet cross section. As shown by the data in Table 2, a consistent measure of flow was not attainable as the speed of the turbocharger was varied.

15


Average error rates were especially high at the lower speeds. For example, the average error rate at 6,000 rpm and 6,750 rpm was 12.37% and 11.20%, respectively. At higher speeds, however, the error rate decreased. For example, the average error rate at 8,250 rpm and 9,000 rpm was 6.98% and 8.36%, respectively. Standard deviations for these samples, however, were high at 12.19 and 10.15 for 6,000 rpm and 6,750 rpm samples, respectively. At the higher speed samples, the standard deviations were lower at 4.73 and 7.62 for 8,250 rpm and 9,000 rpm, respectively.

Table 2: Performance Test Data.

Data Point

Speed (rpm)

Differential Pressure

(inHg)

Lab Mean

(lb/sec)

Kurz™ Mean

(lb/sec)

Error (%)

1 6,000 2.81 5.79 5.41 6.56%2 6,000 0 7.87 5.19 34.05%3 6,000 0.71 7.23 6.86 5.12%4 6,000 1.47 6.73 7.31 8.62%5 6,000 2.20 6.26 6.73 7.51%6 6,750 3.99 6.48 5.80 10.49%7 6,750 0 9.19 6.61 28.01%8 6,750 1.00 8.46 7.60 10.17%9 6,750 2.00 7.81 8.28 6.02%

10 6,750 7.17 7.17 7.26 1.26%11 7,500 5.44 7.27 6.31 13.20%12 7,500 0 10.43 8.13 22.05%13 7,500 1.36 9.64 9.27 3.84%14 7,500 2.72 8.98 10.02 11.58%15 7,500 4.10 8.17 8.22 0.62%16 8,250 6.00 8.73 8.41 3.67%17 8,250 0 11.73 10.14 13.55%18 8,250 1.50 10.89 10.75 1.29%19 8,250 3.00 10.23 11.08 8.08%20 8,250 4.50 9.40 10.16 8.31%21 9,000 7.00 9.80 9.81 0.10%22 9,000 0 13.22 13.15 0.53%23 9,000 1.8 12.18 13.53 11.08%24 9,000 3.6 11.61 13.16 13.35%25 9,000 5.4 10.75 12.55 16.74%

If all data collected at zero differential pressure were removed, then the average error rate decreases to 7.38% with a standard deviation of 4.68 as compared to an overall average error of 9.83% with a standard deviation of 8.43.

These high error rates and standard deviations can be attributed to the unsteady and inconsistent flow at the measurement location. During field use of the Kurz™, the probe location had a consistent flow profile, and the error rate and standard deviation was drastically reduced. This demonstrates a concern with using the anemometer and the importance of having a location with steady and consistent flow.

16


Field Measurement Data Collection Field testing was performed on the same turbocharger that was tested at the NGML after it was installed at Williams Station 150 in Mooresville, NC. The Kurz™ data was collected specifically for this project which was then coordinated with the emissions and Turbo Shield™ field data collection. The Kurz™ instrument and the emissions analyzer used for data collection were temporary installations, while the Turbo Shield™ was a permanent installation and is still in use. Figure 20: Turbo Shield™ Mounting Location

Additional testing was done on a Cooper GMW-10 at Williams Station 110 in Wadley, AL but this data is not included in this paper.

Figure 21: Compressor Inlet Field Temperature Tap Location

Figure 22: Compressor Discharger Field Tap Locations

Turbocharger Management System

As shown in Figure 20, the Turbo Shield™ was installed at an easily accessible location on the turbocharger skid. The Turbo Shield™ was mounted in a location slightly above the turbocharger. Moisture in the air can condense in the pressure lines which could cause inaccurate readings by the pressure sensors. In addition, the water can freeze and damage the pressure tubing or the pressure sensors. By mounting the unit above the turbocharger, these potential problems can be avoided.

The tap locations for the compressor inlet are the only locations that we different in the field than they were during the lab testing. As shown in Figure 21, while not being directly on the turbocharger, the tap locations for the compressor inlet were in a straight transition piece leading directly into the compressor. Therefore, the readings should be the same as if the taps were in the turbocharger.

As shown in Figure 22, the tap locations for the compressor discharger in the field are the same as they were during lab testing. Because the flow device is calibrated during the turbocharger testing,

17


properly installing the device in the field is extremely important. If the flow device is not installed in the same location, the calibration equation will not be valid and the flow measurement will be incorrect.

All pressure differential pressure based flow meters are based on having a known geometry from which the differential pressure and flow rate can be related. The pitot tube flow meter must be calibrated to the specific geometry of a given turbocharger, or family of turbocharger, installations.

As with the compressor discharge, the tap locations for the turbine inlet and turbine discharger were the same in the field as they were in the lab. The field tap locations for the turbine inlet and turbine discharger are shown in Figures 23 and 24 respectively.

Sample Data Turbocharger data from the Turbo Shield™, was collected in conjunction with flow data from the Kurz™ instrument. In addition to collecting temperatures and pressures, the Turbo Shield™ collected air flow data at the compressor discharge. The flow data collected by the Turbo Shield™ has been proven to be very accurate in field measurement and because the flow device was calibrated to lab results, the Turbo Shield™ flow data was accepted as the standard for comparison in the field. However, because of resource scheduling issues, emissions data and Turbo Shield™ data were not collected at the same time, therefore an accurate comparison between the two methods of flow measurement cannot be made. Table 3 below shows the comparison of the Kurz™ and Turbo Shield™ flow data. The difference between the Kurz™ and the Turbo Shield™ data is very high,

between 18% and 20%. The Kurz™ flow data was taken at the exhaust stack and therefore, the comparison of the data must be made by adding the fuel flow to the Turbo Shield™ flow. This reduced the difference between the measurements by approximately 3%. The Kurz™ instrument and the large differences in flow measurement are further discussed in the following section.

Figure 23: Turbine Inlet Field Tap Locations

Figure 24: Turbine Discharger Field Tap Locations

Table 3: Turbo Shield™ and Kurz™ Flow Data

Flow Data Set Kurz™ (lb/sec)

TS (lb/sec)

Percent Difference

TS Data 1 8.32 6.94 19.9%

TS Data 2 8.28 7.00 18.3%

TS Data 1 w/fuel 8.32 7.13 16.7%

TS Data 2 w/fuel 8.28 7.19 15.2%

18


Figure 25: Exhaust Stacks with Ports for Probe Insertion.

Kurz™ Anemometer

For the field test of the Kurz™ anemometer, the probe was inserted in the exhaust stack as shown in Figure 25. The anemometer is shown in Figure 26 as held in the manlift for field data collection. Along side, an emissions trailer was used to collect emissions data (O2, NOX, CO2, etc.) and the Turbo Shield™ unit collected flow data.

Sample Data

Table 5 provides a summary of the data that was collected during engine testing for each of the three test days. The traverse log column indicates the data point collected by the Kurz™ anemometer; the Turbo Shield™ column indicates whether or not data was available from Turbo Shield™; and the EPA Method 19 column indicates whether or not data was available from the emissions trailer to calculate EPA Method 19 flow.

An example of the traverse flow and temperature data is shown in Figure 27. The remaining traverse data figures can be found in Appendix A. Note that a traverse was only available for one sampling port. Hence, the profile for a traverse at another port 90o from the first traverse is unknown. The data in Table 4 portray a discrepancy between the Kurz™ anemometer readings and the readings from EPA Method 19 and the Turbo Shield™ unit with discrepancies up to 15%. Assuming that the EPA Method 19 values and

Figure 26: Kurz™ Instrument Unit.

Table 4: Averaged Field Test Data by Day.

Test Day Kurz™ Traverse Log #

Load (%)

Kurz™ (lb/sec)

Turbo Shield™ (lb/sec)

EPA Method 19(lb/sec)

ScavengeTech Carbon Balance

(lb/sec)

1 1 100% 8.61 n/a n/a n/a

2 1 93.45% 8.38 n/a 6.92 6.97

2 2 91.42% 8.68 n/a 6.73 6.75

2 3 90.01% 8.06 n/a 6.60 6.66

2 4 97.52% 8.78 n/a 7.40 7.44

3 1 100% 8.32 6.94 n/a n/a

3 1 100% 8.28 7.00 n/a n/a

19


Turbo Shield™ values are accurate then the Kurz™ anemometer values are not.

Initially, the research team suspected that the air calibrated Kurz™ values were wrong because of the thermodynamic parameter differences between pure air and exhaust gas. This possibility was explored by some research and a model, which proved that these parameters should cause no more than 5% differences.

Another possibility is the missing data from another sampling port’s profile. Because the location 90o

from the tested port was not available, the research team could not prove or disprove this possibility. Hence, two reasonable assumptions were made as to what the missing profile would look like and both assumptions bring the discrepancy down to within about 2% of the “true” values.

Figure 27: Exhaust Velocity and Temperature Traverse at 97.52%.

Issues and Concerns

As shown in the above material, measuring flow in the exhaust of an engine with a portable anemometer may prove to be a plausible method. An issue with this method, though, is the necessity of taking traverse measurements at two sampling ports 90o from each other for a complete overall measurement.

More testing still needs to be conducted to confirm that obtaining the second profile will actually get the values within a reasonable error margin.

20


Emissions Analyzer The final method used for measuring air flow was by using emissions data. Emissions data was collected during field testing of the Kurz™ anemometer. Two different methods to calculate flow rate were used; EPA Method 19, and a carbon balance method. These methods require the measurement of fuel gas composition, oxygen concentration in the exhaust and engine fuel consumption rate. The fuel gas composition was retrieved from a chromatograph. The oxygen concentration was measured by an emissions analyzer installed in the exhaust. The analyzer probe is shown in Figure 28. The engine fuel consumption rate was measured from a permanently installed orifice plate meter run.

Figure 28: Emissions analyzer probe.

EPA Method 19 The first method used in the field for obtaining mass flow rate from emissions data was EPA Method 19. The flow rate calculated by using EPA Method 19 is a function of the measured values above and is shown in equation (20).

2

1920.9

(20.9 ) 60std exhaust

Method d fuelO

SGm F V HHVy

ρ=

−&& (20)

Where the result is in lbm/s and:

EPA Method 19 drybasis F Factor ddscfmF

mmBtu⎡ ⎤= − ⎢ ⎥⎣ ⎦

2= Oxygen concentration ofexhaust, dry basisOy

[ ]Flow rate of the fuelfuelV s=& cfm

Higher heating value of the fuel mmBtuHHVdscf

⎡ ⎤= ⎢ ⎥

⎣ ⎦

Standard density of air at 14.696 psi and 68 F stdlbmscf

ρ⎡ ⎤

= ° ⎢ ⎥⎣ ⎦

Specific gravity of the exhaustexhaustSG =

21


Development of the F-Factor and the flow equation, as developed using the chemical kinetic equation for methane combustion, are further discussed in Appendix B.

Carbon Balance Method

The second method used for calculating air flow from emissions was a carbon balance method developed by ScavengeTech. The carbon balance method of determining air flow rate is directly derived from balancing the combustion equation. The final derivation is shown in equation (21).

( )2

2

O

O

4.773 14

1 4.773air

carbonbalance fuel fuelfuel

ba yMWm

y MWV ρ

⎡ ⎤+ −⎢ ⎥⎣ ⎦= ×−

&& × (21)


Total moles of carbon a =

Total moles of hydrogen b =

Molecular weight of air airMW =

Molecular weight of fuel fuelMW =

Density of fuelfuellbmcf

ρ⎡ ⎤

= ⎢ ⎥⎣ ⎦

Since the carbon balance equation is a direct derivation, the total moles of carbon and hydrogen must be known. In the EPA Method 19 F-Factor, the heating value of the fuel is used to assume the total moles of carbon and hydrogen. Therefore, the carbon balance method is a more thermodynamically accurate way to calculate flow rate from emissions data.

Sample Data

The three measurements collected were fuel component concentrations, oxygen concentration of the exhaust, and engine fuel consumption rate. Table 5 shows the fuel component data from a chromatograph. This data only needs to be collected once because the gas composition is not expected to change over a short period of time. As illustrated in Appendix B, all chromatograph data is broken down into elemental components (%H, %C, %O, etc.) and converted to %weight to plug into the EPA Method 19 flow equation.

The O2 concentration of the exhaust collected from the emissions trailer and the engine fuel consumption rates are shown in Table 6. By having the fuel composition and knowing the oxygen concentration and the fuel flow, EPA Method 19 and the carbon balance method could then be used to calculate the air flow rate. The flow rates from each method are shown in Table 6. The EPA Method 19

Table 5: Chromatograph fuel composition data.

Component mol %

1.1098 Carbon Dioxide

0.5059 Nitrogen

95.7168 Methane

2.0961 Ethane

0.3109 Propane

0.0754 I Butane

0.0767 N Butane

0.0000 Neo-Pentane

0.0318 I Pentane

0.0199 N Pentane

0.0567 C6+

1026.14 BTU/dscf

SG 0.5857

22


flow rate and the carbon balance flow rate were very similar, less than 1% difference. While the carbon balance method is more thermodynamically accurate, this shows the F-Factor in EPA Method 19 can be effectively used as a surrogate for the total moles of carbon and hydrogen in the fuel.

Because the data were taken on separate days, the emissions flow calculations cannot be directly compared to the Turbo Shield™ data. However, if a similar Kurz™ flow rate is used as a common connection between the two, the emissions flow calculations are within 1% of the Turbo Shield™ data. This comparison is shown in Table 7. Although this is not a direct comparison, this shows the ability of both the EPA Method 19 and the carbon balance method to accurately calculate flow rate from emissions data. The connection can be made because the Kurz™ flow rates are nearly identical for the two data sets as were operating conditions.

Table 6: EPA Method 19 and carbon balance flow rate calculations

Data Point

O2 (%) Fuel (mscfh)

EPA Method 19

(lb/sec)


(lb/sec)

1 12.24 14.96 6.92 6.97

2 12.14 14.64 6.73 6.75

3 12.12 14.48 6.60 6.66

4 12.51 15.50 7.40 7.44

Table 7: Comparison of Turbo Shield™ data to emissions calculations

Kurz™ (lb/sec)

Turbo Shield™ (lb/sec)

EPA Method 19

(lb/sec)


(lb/sec)

8.38 n/a 6.92 6.97

8.32 6.94 n/a n/a

23


Results and Conclusions

The Turbo Shield™ turbocharger monitoring system is an effective means to provide continuous performance monitoring of the complete turbocharger system. The data is collected and stored so that it is available for review routinely or in response to an event. The complete turbocharger system can be monitored locally or remotely. Laboratory testing has demonstrated that the flow measurement matches a known standard and, therefore, if properly installed and maintained in the field can be considered accurate.

A case can be made for simple performance monitoring that would relate a base line waste gate position to given operating conditions whereby decreased waste gate margin would be an indicator of turbocharger performance degradation. However, such a simplified system could not provide the broader and deeper detail of the Turbo Shield™ turbocharger monitoring system which would more clearly point to the specific cause of performance degradation. Such degradation may be from components of the system other than just the turbocharger compressor or turbine.

The Kurz™ anemometer may be an effective means to provide field verification of other results or as a troubleshooting tool. However, more work is needed to determine the proper measurement points and methods before this device can be considered to have reasonable accuracy. The key to being able to successfully apply this instrument is finding the correct point of measurement. Repeatable results seem to be attainable but matching the exact magnitude of measurements to known standard was elusive. It will certainly be necessary to install adequate sampling ports into which the anemometer can be inserted. The targeted purpose for this device is an additional tool for the maintenance toolbox.

Initial test results suggest that the EPA Method 19 may be much more accurate than was widely thought. Obviously, more testing and comparison is needed to prove or disprove the initial observations. These observations suggest readings may be within 1% of actual as opposed to the 5% or greater often given.

The ScavengeTech method are a simpler and more direct calculation to perform in the field than the EPA Method 19 and are directly related to natural gas engine applications. Not having to compute an F-Factor may be seen as an advantage in the field. The testing results show a very close agreement between the two methods. The difference was about 0.25% for the testing done. Industry and regulatory agency acceptance to use this method as part of an emissions compliance testing protocol may be needed.

Future Work

Additional testing and comparison of all four methods is proposed to obtain more confidence with the results and conclusions.

Additional field testing is planned within the next few months to simultaneously compare Kurz™ anemometer, EPA method 19, and ScavengeTech carbon balance method.

24


References

American Society of Mechanical Engineers, Test Code on Compressors and Exhausters, ASME Power Test Code 10, New York: 1997.

Chapman, K.S., Kesharvarz, A., Shultz, J., and Sengupta, J., “Test Standard for Testing Large-Bore Engine Turbochargers,” GRI-05/ 0026, 2005, Gas Research Institute, Des Plaines, Illinois.

Chapman, K.S. and Mohsen, O., “Common Turbocharger Performance Measurement Locations based on an Elliott H-Series Turbocharger,” GRI-02/0162, 2002, Gas Research Institute, Des Plaines, Illinois.

Kurz™ Instruments Inc. Anemometer Model 2445 Brochure 1997.

Environmental Protection Agency, 40 CFR Part 60: Method 19, EPA-HQ-OAR-2005-0031, June 13, 2007

25


APPENDIX A: TRAVERSE DATA



Figure 29: Exhaust Velocity and Temperature Traverse at 100%.


26


APPENDIX B: EPA METHOD 19

EPA Method 19 for calculating exhaust flow rate can be explained by starting with a basic formulation of flow rate based on a stoichiometric combustion process. Consider, for example, the complete combustion of one mole of Methane:

CH4 + a(O2 + 3.78N2) bCO2 + cH2O + dN2

Where:

a, b, c and d = number of moles for each molecule

Air = 79.1%N2 + 20.9%O2 , so that, 2

2

# 3.78#

molesNmolesO

=

Solving a, b, c and d yields:

CH4 + 2(O2 + 3.78N2) 1CO2 + 2H2O + 7.56N2

A ratio of how many moles of exhaust product you can achieve per mole of fuel can be defined.

# 1 2 7.56 10.56# 1 1

Exhaust molesExhaust molesExhaustfFuel molesFuel moleFuel

+ += = = =

EPA Method 19 also provides a dry basis definition of this ratio which simply excludes the H2O term in the products. In this methane fuel example, the ratio would be 8.56 moles of exhaust per mole of fuel. For the continuation of the formulation, the dry basis will be followed for simplicity. In addition, the units can actually be replaced by cubic feet instead of moles because of a simple ratio. Hence:

Next, SCF of fuel should be converted to lb of fuel as follows:

Where:

Ideal gas Molar Volume = 385.31 scf/lb-mol

MW of Natural Gas = 16.042 lb/lb-mol

This example only shows how to determine f for one fuel type at a time. A more general method is needed to determine f. A more general method can be summarized as so:

Where:

EP = Exhaust Product attributed to component n.

C = Component (ie. Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur, etc.)

27


N = Total number of components in fuel mixture.

Wt% = Concentration by weight of component n.

This equation allows a fuel source to be broken up into elemental components and then defines a fixed amount of exhaust product per mol of fuel component. For example, for every mole of Carbon (C1) in the fuel gas, a certain amount of moles of [CO2 + N2] (EP1) will result as a product, hence, EP1/C1. Because this reaction is only a fraction of sum of all the other reactions, that corresponding fraction should be multiplied by the EP1/C1 ratio, which happens to be the wt% concentration of Carbon (C1). The same must be applied to the other components.

The EPA Method 19 assumes that only Carbon, Hydrogen, Nitrogen, Oxygen and Sulfur appear in the fuel. The method also defines the EPn/Cn ratios as follows:

Kc = EPfromCarbon/CCarbon

Khd = EPfromHydrogen/CHydrogen

Ko = EPfromOxygen/COxygen

Kn = EPfromNitrogen/CNitrogen

Ks = EPfromSulfur/CSulfur

Hence,

Now that a general formula for f has been presented for all fuel types, the EPA 19 Fuel Factor can be defined as:

Kc , Khd , Ko , Kn , Ks are derived from performing a combustion analysis for each. For example, to determine Kc, it should be noted that during the combustion process, 1 mole of Carbon is combined with air to produce 1 mole of CO2. Since this reaction takes 1 mole of O2, then it follows that N2 will also be a resultant because N2 comes with O2 in air. The resultant number of moles of N2 will be 3.78 moles. Ultimately the products of combustion due to Carbon only are [1 mole CO2 + 3.78 mole N2] which is a total of 4.78 mol-Products per 1 mol-Carbon. Alternatively,

Another interesting example of this application is the determination of Oxygen contributions to the products of combustion when Oxygen exists in the fuel source (ie. CH3OH). To determine Ko it must be realized that when combustion occurs, the Oxygen in the fuel source will be consumed in the reaction which will avoid using some of the air. If 1 mole of Oxygen exists in the fuel, then that’s 1 mole which will not be used from the air mixture and 3.78 moles of Nitrogen which will not be used in the combustion, which is depicted by the following:

28


Applying the same logic to the other components and setting as a per % basis:

Kc = (1.53

Khd = (3.64

Ko = (-0.46

Kn = (0.14

Ks = (0.57

The reason they are in a per % basis is because each component contributes by a fraction to the whole combustion process, and if the wt% of each component is multiplied by its corresponding factor above, it will yield the actual scf/lb for each component. Plugging in the factors and wt% to the flow factor will yield:

By analyzing the units of Fd, it becomes clear that multiplying Fd by the heat input (fuel rate consumption) will yield the stoichiometric Exhaust flow rate. Because all engines will have un-burnt oxygen in the exhaust due to trapped equivalence ratios greater than 1, a measurement of oxygen concentration in the exhaust will allow you to obtain a ratio of excess oxygen, hence, excess air. By combining these factors, the flow equation will ultimately be:

2

190

20.9(20.9 ) 60

std exhaustMethod d fuel

SGm F V HHVy

ρ=

−&&


EPA Method 19 drybasis F Factor ddscfmF

mmBtu⎡ ⎤= − ⎢ ⎥⎣ ⎦

2

Oxygen concentration ofexhaust, dry basisOy =

[ ]Flow rate of the fuelfuelV s=& cfm

Higher heating value of the fuel mmBtuHHVdscf

⎡ ⎤= ⎢ ⎥

⎣ ⎦

Standard density of air at 14.696 psi and 68 F stdlbmscf

ρ⎡ ⎤

= ° ⎢ ⎥⎣ ⎦

Specific gravity of the exhaustexhaustSG =

29


The F-Factor is calculated by taking the ratio of the volume of the gas components to the heat content of the fuel.

6(10 [3.64 % 1.53 % 0.57 % 0.14 % 0.46 % ])d

fuel

H C S NF HHVρ

∗ + ∗ + ∗ + ∗ − ∗=

O

Where:

%H = Concentration of Hydrogen in the fuel by weight.

%C = Concentration of Carbon in the fuel by weight.

%S = Concentration of Sulfur in the fuel by weight.

%N = Concentration of Nitrogen in the fuel by weight.

%O = Concentration of Oxygen in the fuel by weight.

Density of the fuel lbmscf

ρ⎡ ⎤

= ⎢ ⎥⎣ ⎦

It is important to note that the component concentrations must be in %weight but the chromatograph readings are in %mol. In addition, the chromatograph does not measure each of the above components separately; rather, it measures the concentrations of the various compound molecules such as CH4, CO2, N2, etc. Hence, to obtain the above concentrations in their elemental form and in %weight, a break down must be accomplished along with a conversion to %weight.

30

Documents

Comparative Study of Air Flow Measurement Techiques