HTW Building Stack Velocity Testing

Prepared by

Spencer Owen and Geoff Silcox

Department of Chemical Engineering

University of Utah

Prepared for

Michael D. Brehm

Environmental Health and Safety

University of Utah

June 2015

Introduction

The Office of Environmental Health and Safety (EHS) at the University of Utah inquired

whether the students in Chemical Engineering 5305, Air Pollution Control Engineering,

could help resolve an apparent inconsistency in the measured stack velocity in the High

Temperature Water Plant (HTW). The velocity measured by DMK Environmental

Engineering was higher than what would be expected based on the reported firing rate of

the turbine and boilers in the HTW. The air pollution course is offered spring semester

and in 2015 the students were asked to pick a research project related to air pollution. For

his project, one of the students, Spencer Owen, offered to work with EHS to resolve the

inconsistency noted above.

The HTW is located at 1705 E South Campus Dr. The plant burns natural gas in a simple-

cycle gas turbine and occasionally in supplemental boilers. The turbine is connected to a

generator to supply the campus with electric power and the hot exhaust gases pass

through a heat recovery unit to produce hot water for heating. The hot gases are

exhausted through a stack on the roof of the HTW.

The stack data collected by DMK were consistent except for the velocity. Through a

material balance the stack flow rate and velocity are related to the amount of natural gas

burned. The amount that DMK had calculated [1] was up to 30 million BTU’s (MBTU)

more than what the plant was reporting. DMK also measured apparent negative static

pressures at the sampling locations in the stack. This report explains these two features of

the measurements and recommends ways to improve future measurements and analysis.

Technical Approach

The velocity of the exhaust gases in the stack was measured with an S-type pitot tube

supplied by DMK. Pressure differences were measured with a magnehelic gauge that was

also supplied by DMK. The experimental procedure followed the standard EPA Method

2 [2]. Prior to making stack measurements, qualitative tests of the pitot tube and the

magnehelic gauge were performed in a small wind tunnel located in the Senior Projects

Lab in the Department of Chemical Engineering.

The velocity measurement locations in the stack were chosen based on EPA Method 1

[3]. The cylindrical stack and its dimensions are given in Figure 1 [1]. The inside

diameter was 53.5 inches and the sampling locations were 34 inches above the roof.

Method 1 specifies how to pick traverse points in measuring stack velocities. We used 16

points to traverse the stack and an additional 16 points perpendicular to those.

The flow in the stack was turbulent and the pressure differences (P) obtained with the

pitot tube and magnehelic were uncertain to 0.5 inches of water. Appendix 1 shows the

recorded values of . The average exhaust temperature, based on measurements at four

points, was 388.5 . To estimate the stack velocity and the average molar mass of the

exhaust gases the natural gas was approximated as pure methane. This simplified the

stoichiometric and heating value calculations.

Figure 1. Schematic of stack and its dimensions. The measured inside diameter was

53.5 in. [1].

The gas velocities were calculated as specified in Method 2 [2]. The average was

1.19 inches of water (in. H2O). EPA Method 2 recommends

∑ √

√

(1)

to calculate the average velocity; where is the velocity equation constant, is the

constant for S-type pitot tubes, is the average absolute temperature in the stack,

is the absolute pressure in the stack, is the pressure difference from the S-type pitot

tube at location i, n is the number of sampling points, and is the average molar mass

of the combustion gases. The latter is calculated by assuming complete combustion with

2 24 2 2 2 2 2 2

2 2

1 12 2 2 2 2 1O O

O O

y yCH SR O SR N H O CO SR N SR O

y y

(2)

where SR is the stoichiometric ratio, and yO2 is the mol fraction of oxygen in air. Air was

assumed to be 21 mol % oxygen, balance nitrogen and the stoichiometric ratio was

assumed to be 2.0.

The stack velocity is related to molar flow rate ( n ), gas density () and the cross-

sectional area of the stack (A) by

(3)

The molar flow rate is related to the number of moles of methane burned through (2). The

flow rate of methane was calculated from the given firing rate (decatherms/h or MBtu/h)

of the HTW and the higher heating value (HHV) of methane. Appendix 2 contains the

values and calculations related to firing rate, molar mass, and stack velocity.

The absolute stack pressure, Ps, appears in (2). The absolute pressure is the sum of the

stack static pressure, Pg, and the barometric pressure, Pbar,

Ps = Pbar + Pg (4)

EPA Method 2, Part 6.4, suggests measuring Pg by placing one of the openings of the s-

type pitot tube parallel to the gas flow. The measured value of Pbar was 0.84 atm. The

University of Utah and DMK performed this test and measured a negative value. A

negative value is physically impossible. As an alternative, the engineering form of

Bernoulli’s equation provides an estimate of the absolute stack pressure:

(5)

where is the density of the gas and the friction heating term is given by

(6)

In (6), D is the diameter, is the change in height from the top of the stack to the plane

of measurement. The friction factor is estimated by

[ (

)

] (7)

where is the roughness, and the Reynolds number is given by

(8)

where µ is the viscosity. Equations 5 – 8 show that the static pressure must be positive.

Numerical values are given in Appendix 3.

Results and Discussion

We made our stack measurements on April 10, 2015 and for this day the HTW staff

reported that the power was 80 MBtu/hr. This “firing rate” and Equations 2 - 8 give an

average stack velocity of 55 ft/s. The average pressure difference (P) measured in the

stack with the pitot tube, and used in (1), was 1.5 in. water. Using this for (P) in (3), the

resultant velocity is 93 ft/s, which is equivalent 130 MBtu/hr. This is quite different from

the reported firing rate.

This discrepancy prompted another visit with the staff at the HTW building to ask how

they measured power in the plant. The HTW staff calculates power by the product of the

heat capacity of water, the temperature increase of the water, and the mass flow rate of

water leaving the plant. This approach underestimates the firing rate of the turbine

because it neglects the electric power produced by the generator and the energy losses

through the hot exhaust gases leaving the stack. This explains why the stack velocities

measured by DMK and the University of Utah appear to be too high.

Conclusion and Recommendations

The power output report by the HTW is based on the properties of the hot water: the

product of its heat capacity, its temperature increase, and its mass flow rate. This

approach underestimates the firing rate of the turbine because it neglects the electric

power produced by the generator and the energy losses through the stack. If the firing rate

is calculated in this way the measured stack velocities will always appear to be too high.

Two key recommendations have resulted from this study: (1) future calculations of the

stack velocity should be based on the firing rate of natural gas and (2) when using s-type

pitot tubes the stack static pressure, Pg, should be compared to that calculated from

Equations 4 – 8. Negative stack static pressures are impossible and are probably an

artifact of the s-type pitot tube.

In addition, the accuracy of future University of Utah measurements can be improved by

(1) using a digital manometer to obtain averaged values of P, (2) closing the opening

around the probe to prevent the escape of the hot gases, and (3) using a support to hold

the probe and magnehelic when making traverses.

Acknowledgements

The authors gratefully acknowledge the assistance of David Kopta, President of DMK

Environmental Engineering, Inc., for supplying the equipment used in this study and for

explaining how to use it. Michael Brehm, Environmental Protection Section Leader at the

University of Utah gave generously of his time in helping to make the stack

measurements. Finally, Terry Walters, supervisor at the High Temperature Water Plant,

made us welcome in his facility and provided us with the tools and information that made

this project possible.

References

[1] Kopta, David, “U of U Central Heat Plant Gas Turbine,” January 2015

[2] “Method 2 – Determination of Stack Gas Velocity and Volumetric Flow Rate (S-Type

Pitot Tube),” Internet Posting at http://www.epa.gov/ttn/emc/promgate/m-02.pdf, Updated December 5, 2014, Environmental Protection Agency.

[3] “Method 1 – Sample and Velocity Traverses For Stationary Sources,” Internet Posting

at http://www.epa.gov/ttn/emc/promgate/m-01.pdf, Last Updated December 5, 2014,

Environmental Protection Agency.

Appendix 1 Stack Measurement and Velocities

Appendix 2. Stoichiometry

Appendix 3 Static Pressure Calculations