01 POWER ISLAND / 01 CCPP / V. Ganapathy-Industrial Boilers and HRSG-Design (2003)

6.S Drennen, V Lifshits. Developmental issues of ultra low NOx burners for steam generation. Paper presented at the Fall Meeting of the Western States Section of the Combustion Institute, Diamond Bar, CA, Oct 23–24, 1997.

7.T Webster. Burner technology for single digit NOx emissions in boiler applications, CIBO NOx Control Conference, San Diego, CA, Mar 13, 2001.

8.Use of SCR for control of NOx emissions from power plants in the US. Prepared by Synapse Energy Economics, Inc., Cambridge, MA, for the Ontario Clean Air Program, Canada (ONTAIR), campaign, February 2000.

9.S Naroozi. Urea enhances safety in SCR applications. Power Engineering, December 1993.

10.L Czarnecki. SCONOx—ammonia-free NOx removal technology for gas turbines. International Joint Power Generation Conference (IJPGC)-2000-15032, Florida, July 2000.

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5

Basic Steam Plant Calculations

5.01Converting liquid flow in lb=h to gpm, and vice versa; relating density,

specific gravity, and specific volume

5.02Relating head of liquid or gas column to pressure; converting feet of liquid

to psi; relating inches of water column of gas to psi and feet of gas column

5.03Estimating density of gases; relating molecular weight and density; effect of elevation on gas density; simplified formula for density of air and flue

gases at sea level

5.04Relating actual and standard cubic feet of gas per minute to lb=h

5.05Computing density of gas mixture; relating mass to volumetric flow;

computing velocity of gas in duct or pipe

5.06Relating mass and linear velocities

5.07Calculating velocity of wet and superheated steam in pipes; computing specific volume of wet steam; use of steam tables

5.08Relating boiler horsepower to steam output

5.09Calculating amount of moisture in air; relative humidity and saturation vapor pressure

5.10Water dew point of air and flue gases; partial pressure of water vapor

5.11Energy absorbed by wet and superheated steam in boilers; enthalpy of wet and dry steam; use of steam tables; converting MM Btu=h (million Btu=h) to kilowatts

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5.12Relating steam by volume, steam by weight, and steam quality; relating circulation ratio and quality

5.13a Determining steam quality using throttling calorimeter

5.13b Relating steam quality to steam purity

5.14Water required for desuperheating steam; energy balance in attemperators, desuperheaters

5.15Water required for cooling gas streams

5.16Calculating steam volume after throttling process; use of steam tables

5.17Determining blowdown and steam for deaeration

5.18Calculating flash steam from boiler blowdown; economics of flash steam recovery

5.19a Estimating leakage of steam through openings; effect of wetness of steam on leakage

5.19b Estimating air flow through openings

5.20Estimating leakage of gas across dampers; calculating energy loss of leakage flow; sealing efficiency of dampers on area and flow basis

5.21Economics of waste heat recovery; annual cost of energy loss; simple payback period calculation

5.22Life-cycle costing applied to equipment selection; interest and escalation factors; capitalized and life-cycle cost

5.23Life-cycle costing applied to evaluation of heat recovery systems

5.24Calculating thickness of boiler tubes to ASME Code; allowable stresses for various materials

5.25Calculating maximum allowable working pressures for pipes

5.26Sizing tubes subject to external pressure

5.27On sound levels: OSHA permissible exposure levels

5.28Adding decibels

5.29Relating sound pressure and power levels

5.30Effect of distance on noise level

5.31Computing noise levels from engine exhaust

5.32Holdup time in steam drum

5.01

Q:

Convert 50,000 lb=h of hot water at a pressure of 1000 psia and 390 F to gpm.

A:

To convert from lb=h to gpm, or vice versa, for any liquid, we can use the following expressions:

where

W ¼ flow, lb=h

q ¼ flow, gpm (gallons per minute) r ¼ density of liquid, lb=cu ft

s ¼ specific gravity of liquid

v ¼ specific volume of liquid, cu ft=lb

For hot water we can obtain the specific volume from the steam tables (see the Appendix). v at 1000 psia and 390 F is 0.0185 cu ft=lb. Then, from Eq. (1),

q ¼ 50;000 0:0185 ¼ 115:6 gpm 8

For water at temperatures of 40–100 F, for quick estimates we divide lb=h by 500 to obtain gpm. For example, 50,000 lb=h of water at 70 F would be 100 gpm.

5.02A

Q:

Estimate the head in feet developed by a pump when it is pumping oil with a specific gravity of 0.8 through a differential pressure of 150 psi.

A:

Conversion from feet of liquid to psi, or vice versa, is needed in pump calculations. The expression relating the variables is

5.02B

Q:

If a fan develops 8 in. WC (inches of water column) with a flue gas density of 0.05 lb=cu ft, what is the head in feet of gas and in psi?

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TABLE 5.1 Gas Molecular Weights

A simplified expression for air at atmospheric pressure and temperature t at sea level is

For a gas mixture such as flue gas, the molecular weight (MW) can be obtained as discussed in Q5.05. In the absence of data on flue gas analysis, Eq. (7) also gives a good estimate of density.

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When sizing fans, it is the usual practice to refer to 70 F and sea level as standard conditions for air or flue gas density calculations.

5.04A

Q:

How is acfm (actual cubic feet per minute) computed, and how does it differ from scfm (standard cubic feet per minute)?

A:

acfm is computed using the density of the gas at given conditions of pressure and temperature, and scfm is computed using the gas density at 70 F and sea level (standard conditions).

q ¼ gas flow in acfm (at 70 F and sea level, scfm and acfm are equal; then q ¼ W =4:5)

rg ¼ gas density in lb=cu ft (at standard conditions rg ¼ 0:075 lb=cu ftÞ W ¼ gas flow in lb=h ¼ 4:5q at standard conditions

5.04B

Q:

Convert 10,000 lb=h of air to scfm.

A:

Using Eq. (6), it can be shown that at P ¼ 14:7 and t ¼ 70, for air rg ¼ 0:075 lb=cu ft.

Hence, from Eq. (8),

5.04C

Q:

Convert 3000 scfm to acfm at 35 psia and 275 F. What is the flow in lb=h? The fluid is air.

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A:

Calculate the density at the actual conditions.

35

rg ¼ 29 492 359 735 14:7 ¼ 0:129 lb=cu ft

From the above,

W ¼ 4:5 3000 ¼ 13;500 lb=h

Hence

acfm ¼ 13;500 ¼ 1744 cfm 60 0:129

5.05

Q:

In a process plant, 35,000 lb=h of flue gas having a composition N2 ¼ 75%, O2 ¼ 2%, CO2 ¼ 15%, and H2O ¼ 8%, all by volume, flows through a duct of cross section 3 ft2 at a temperature of 350 F. Estimate the gas density and velocity. Because the gas pressure is only a few inches of water column, for quick estimates the gas pressure may be taken as atmospheric.

A:

To compute the density of a gas, we need its molecular weight. For a gas mixture, molecular weight is calculated as follows:

MW ¼ PðMWi yiÞ where

yi ¼ volume fraction of gas i MWi ¼ molecular weight of gas i

Hence

MW ¼ 0:75 28 þ 0:02 32 þ 0:15 44

þ 0:08 18 ¼ 29:68

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where

Vg ¼ velocity, fpm (feet per minute)

A ¼ cross section, ft2

Hence

35;000

Vg ¼ 60 0:05 3 ¼ 3888 fpm

The normal range of air or flue gas velocities in ducts is 2000–4000 fpm. Equation (9) can also be used in estimating the duct size.

In the absence of flue gas analysis, we could have used Eq. (7) to estimate the gas density.

5.06

Q:

A term that is frequently used by engineers to describe the gas flow rate across heating surfaces is gas mass velocity. How do we convert this to linear velocity? Convert 5000 lb=ft2 h of hot air flow at 130 F and atmospheric pressure to fpm.

A:

Use the expression

5.07A

Q:

What is the velocity when 25,000 lb=h of superheated steam at 800 psia and 900 F flows through a pipe of inner diameter 2.9 in.?

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A:

Use expression (11) to determine the velocity of any fluid inside tubes, pipes, or cylindrical ducts.

V ¼ velocity, fps

v ¼ specific volume of the fluid, cu ft=lb di ¼ inner diameter of pipe, in.

For steam, v can be obtained from the steam tables in the Appendix.

The normal ranges of fluid velocities are

Water: 3–12 fps

Steam: 100–200 fps

5.07B

Q:

Estimate the velocity of 70% quality steam in a 3 in. schedule 80 pipe when the flow is 45,000 lb=h and steam pressure is 1000 psia.

A:

We need to estimate the specific volume of wet steam.

v ¼ xvg þ ð1 xÞvf

where vg and vf are specific volumes of saturated vapor and liquid at the pressure in question, obtained from the steam tables, and x is the steam quality (see Q5.12 for a discussion of x). From the steam tables, at 1000 psia, vg ¼ 0.4456 and vf ¼ 0.0216 cu ft=lb. Hence the specific volume of wet steam is

v ¼ 0:7 0:4456 þ 0:3 0:0216 ¼ 0:318 cu ft=lb

The pipe inner diameter di from Table 5.3 is 2.9 in. Hence, from Eq. (11),

0:318

V ¼ 0:05 45;000 2:92 ¼ 85 fps

Copyright © 2003 Marcel Dekker, Inc.