Reference Fluid & Gas - Gilson Eng

Reference - Fluid & Gas Properties

Fluid & Gas Properties

FLUID DENSITY

Density is the ratio of mass to volume. In English, units density is expressed in pounds mass/cubic foot (lbm/ft3). The symbol for density is . Density is usually written as:

= lbm/ft3

The density for a liquid is normally taken from a table. Table 1 lists the densities of various liquids. For steam, the density is typically read from the steam tables for the desired pressure and temperature. For a gas, density is usually calculated using the ideal gas law. The ideal gas law is:

= p/RT

where p = pressure R = gas constant T = temperature

SPECIFIC VOLUME

Specific volume is the reciprocal of density and is the volume occupied by 1 lbm of fluid.

v = 1/

Table 2 gives the specific volume for saturated steam at various temperatures and pressures.

SPECIFIC GRAVITY

Specific gravity is the ratio of a fluid's density to some reference density. For liquids, the reference density is the density of pure water. Strictly speaking, specific gravity of a liquid cannot be given without specifying the reference temperature at which the water's density was evaluated. The density of water in English units is normally referenced at 32?F and is 62.43 lbm/ft3. Using this as the reference density, specific gravity is given as:

density S.G. =

62.43lbm/ft3

RELATIVE DENSITY

Relative density is the ratio of the density of one substance to that of another, both at the same temperature. The use of specific gravity to describe this quantity is discouraged, partially due to the fact there is no stipulation that the temperatures be equal in specific gravity measurements.

For gasses, the relative density is generally the ratio of the density of the gas to that of air, again both at the same temperature, and also at the same pressure and dryness.

Relative densities of petroleum products and aqueous acid solutions can be found using a device called a hydrometer. In addition to the hydrometer scale that references water, there are two basic hydrometer scales, the Baume scale and the API (American Petroleum Industry Scale). The Baume scale was widely used in the past but the API scale is now recommended for use with all liquids.

The API scale can be used with all liquids:

Relative Density = 141.5

131.5+?API

Table 3 lists the relative densities corresponding to the API scale.

For liquids lighter than water, their specific gravity can be found from the Baume hydrometer reading using this equation:

Relative Density = 140.0

130.0+?Baume

For liquids heavier than water, their specific gravity can be found from the Baume hydrometer reading using this equation:

Relative Density = 145.0

145.0-?Baume

Relative densities can also be given for gases. The reference density is that of air at specified temperature and pressure. Since both the gas and air are evaluated at the same pressure and temperature, the relative density is the inverse of the ratio of the gas constants.

330

Moore Process Automation Solutions moore-

Reference - Fluid & Gas Properties

TABLE 1 Density of Liquids

Liquid Acetaldehyde Acetone Acetic Anhydride Acetic Acid (conc) Ammonia Aniline Benzene Benzoic Acid Brine, 10% CaCl Brine, 10% NaCl Butyric Acid (conc) Carbon Disulfide Carbon Tetrachloride Chlorobenzene Chloroform Cresol, Meta Diphenyl Distillate Fuel Oil #6 (min) Furfural Gasoline Gasoline (natural) Glycerin Heptane Hydrochloric Acid (42.5%) Hydrocyanic Acid Kerosene Mercury

Methylene Chloride Milk Nitric Acid (conc) Olive Oil Ortho-phosphoric Acid Pentane Phenol Toluene Xylene

Temperature ?F 64 60 68 68 10 68 32 59 32 32 68 32 68 68 68 68 163 60 60 68 60 60 112 68 64 64 60 20 40 60 80 100 68 -64 59 65 59 77 68 68

Density lbm/ft3

48.9

Specific Gravity redl H2O @60?F

0.784

49.4

0.792

67.5

1.083

65.5

1.050

40.9

0.656

63.8

1.023

56.1

0.899

79.0

1.267

68.1

1.091

67.2

1.078

60.2

0.965

80.6

1.292

99.6

1.597

69.1

1.108

92.9

1.489

64.5

1.035

61.9

0.993

53.0

0.850

61.9

0.993

72.3

1.160

46.8

0.751

42.4

0.680

78.6

1.261

42.7

0.685

92.3

1.400

43.5

0.697

50.8

0.815

849.7

13.623

848.0

13.596

846.3

13.568

844.6

13.541

842.9

13.514

83.4

1.337

64.2 - 64.6

--

93.7

1.502

57.3

0.919

114.4

1.834

38.9

0.624

66.8

1.072

54.1

0.867

55.0

0.882

Americas +1 215 646 7400 ext. 6613 Asia Pacific +65 299 6051 Europe +44 1935 706262

331

Reference - Fluid & Gas Properties

TABLE 2 Properties of Saturated Steam

Pressure PSIA

0.50 1.0 2.0 3.0 4.0 5.0 10.0 14.7 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 65.0 70.0 75.0

Temp ?F

79.58 101.74 126.08 141.48 152.97 162.24 193.21 212.00 213.03 227.96 240.07 250.33 259.28 267.25 274.44 281.01 287.07 292.71 297.97 302.92 307.60

Sp. Vol. ft3/lbm

641.4 333.6 173.73 118.71 90.63 73.52 38.42 26.80 26.29 20.089 16.303 13.746 11.898 10.498 9.401 8.515 7.787 7.175 6.665 6.206 5.816

Pressure PSIA

80.0 85.0 90.0 95.0 100.0 105.0 110.0 115.0 120.0 125.0 130.0 135.0 140.0 145.0 150.0 160.0 170.0 180.0 190.0 200.0 225.0

Temp ?F

312.03 316.25 320.27 324.12 327.81 331.36 334.77 338.07 341.25 344.33 345.32 350.21 353.02 355.76 358.42 363.53 368.41 373.06 377.51 381.79 391.79

Sp. Vol. ft3/lbm

5.472 5.168 4.896 4.652 4.432 4.232 4.049 3.882 3.728 3.587 3.455 3.333 3.220 3.114 3.015 2.834 2.675 2.532 2.404 2.288 2.042

Pressure PSIA

250.0 275.0 300.0 350.0 400.0 450.0 500.0 550.0 600.0 650.0 700.0 750.0 800.0 850.0 900.0 950.0 1000 1250 1500 1750 2000

Temp ?F

400.95 409.43 417.33 431.72 444.59 456.28 467.01 476.93 486.21 494.90 503.10 510.85 518.23 525.26 531.98 538.42 544.61 572.42 596.23 617.09 635.82

Sp. Vol. ft3/lbm

1.8438 1.6804 1.5433 1.3260 1.1613 1.0320 0.9278 0.8422 0.7698 0.7085 0.6554 0.6094 0.5687 0.5328 0.5006 0.4718 0.4456 0.3450 0.2765 0.2267 0.1878

Degree API

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

TABLE 3 Relative Density at 60o/60oF Corresponding to the API Scale

Relative Density

1.0000 0.9930 0.9861 0.9792 0.9725 0.9659 0.9593 0.9529 0.9465 0.9402 0.9340 0.9279 0.9218 0.9159 0.9100 0.9042 0.8984 0.8927 0.8871 0.8816 0.8762 0.8708 0.8654 0.8602 0.8550 0.8498 0.8448 0.8398 0.8348 0.8299 0.8251

Pound US Gal.

8.328 8.270 8.212 8.155 8.099 8.044 7.989 7.935 7.882 7.830 7.778 7.727 7.676 7.627 7.578 7.529 7.481 7.434 7.387 7.341 7.296 7.251 7.206 7.163 7.119 7.076 7.034 6.993 6.951 6.910 0.870

Degree API

41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70

Relative Density

0.8203 0.8155 0.8109 0.8063 0.8017 0.7972 0.7927 0.7883 0.7839 0.7796 0.7753 0.7711 0.7669 0.7628 0.7587 0.7547 0.7507 0.7467 0.7428 0.7389 0.7351 0.7313 0.7275 0.7238 0.7201 0.7165 0.7128 0.7093 0.7057 0.7022

Pound US Gal.

6.830 6.790 6.752 6.713 6.675 6.637 6.600 6.563 6.526 6.490 6.455 6.420 6.385 6.350 6.316 6.283 6.249 6.216 6.184 6.151 6.119 6.087 6.056 6.025 5.994 5.964 5.934 5.904 5.874 5.845

Degree API

71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100

Relative Density

0.6988 0.6953 0.6919 0.6886 0.6852 0.6819 0.6787 0.6754 0.6722 0.6690 0.6659 0.6628 0.6597 0.6566 0.6537 0.6506 0.6476 0.6446 0.6417 0.6388 0.6360 0.6331 0.6303 0.6275 0.6247 0.6220 0.6193 0.6160 0.6139 0.6112

Pound US Gal.

5.817 5.788 5.759 5.731 5.703 5.676 5.649 5.622 5.595 5.568 5.542 5.516 5.491 5.465 5.440 5.415 5.390 5.365 5.341 5.316 5.293 5.269 5.246 5.222 5.199 5.176 5.154 5.131 5.109 5.086

332

Moore Process Automation Solutions moore-

Reference - Fluid & Gas Properties

ACFM vs. SCFM

It is often desirable to express a gas flow in terms of a "standard" volumetric flowrate. One of the most common forms encountered is standard cubic feet per minute (scfm). To convert an actual flowrate reading, here referred to as actual cubic feet per minute (acfm), it is necessary to multiply the actual flowrate by the ratio of the standard condition's specific volume over the actual condition's specific volume.

QS = QA [ vS / vA ] where: QS = flowrate in scfm

QA = flowrate in acfm vS = standard conditions specific volume vA = actual conditions specific volume The ratio vS / vA can be written as: [ TSPA/PSTA ]

IDEAL GAS BEHAVIOR

A gas can be considered ideal if it exhibits ideal gas behavior. Typically, gases at low pressures and at temperatures much higher than their critical temperatures can be treated as ideal gases. When the volume occupied by the gas molecules is negligible in comparison to the total volume, it is acceptable to treat the gas as an ideal gas. The benefit of treating gases as ideal is that it greatly simplifies the math required to evaluate their behavior.

There are two basic laws that define the behavior of ideal gases; they are Boyle's and Charle's Law. Boyle's law states that the volume and pressure of an ideal gas vary inversely when the temperature is held constant and is written as:

p1V1 = p2V2 The second law is Charle's law that states when pressure is held constant, volume and temperature vary proportionally. Charle's law is written:

T1 = T2 V1 T2 The ideal gas law relates the pressure, temperature, and volume to the amount of gas present. The ideal gas law states that equal volumes of different gases at the same temperature and pressure contain the same number of molecules. The ideal gas law is written:

pV = nR*T

R* is known as the universal gas constant. It is universal because the same number can be used with any gas. Due to the different units that can be used for pressure, temperature, and volume, there are different values of R*. Table 4 lists values for R*.

The ideal gas law can be used with more than 1 mole of gas. If there are n moles, the equation is written as:

n = m/M

TABLE 4 Values of Universal Gas Constant

1545.33 0.08206 1.986 1.986 8.314 0.730

ft-lbf/pmole-?R atm-liter/gmole-?K BTU/pmole-?R cal/gmole-?K joule/gmole-?K atm-ft3/pmole-?R

The ideal gas law can be rewritten taking into account the molecular weight of the specific gas. The specific gas constant is unique for each gas.

pV = mR*t/M

pV = mT(R*/M)

pV = mRT

Table 5 lists the properties of common gases.

Since density is the reciprocal of specific volume, the ideal gas law can be used to determine the density of an ideal gas. If m=1, then the ideal gas law can be rewritten as:

p = 1RT v

p = RT

= p

RT

PROPERTIES OF REAL GASES

Unfortunately, it is not always possible to achieve acceptable results by using the ideal gas law. It is very common for gases at low temperatures and/or high pressures to exhibit real gas behavior.

When the spacing between the gas molecules is small, they tend to attract each other. These attractive forces are called Van der Waals forces. Van der Waal's equation of state can be used to describe the behavior of real gases. Van der Waal's equation of state is written as:

(p+a)(V-b) = nR*T

V2

For an ideal gas, a and b are zero and Van der Waal's equation reduces to the familiar ideal gas law.

Since real gas molecules tend to attract each other, the actual pressure exerted by a real gas is less than that predicted by the ideal gas law. The reduction in pressure is corrected for in the Van der Waal equation by the term (a/V2). The constant b is dependent on the volume occupied by the gas molecules in the dense state. Table 6 gives the values for a and b of common gases. The Van der Waal equation is typically used only when the gas is below critical pressure.

Americas +1 215 646 7400 ext. 6613 Asia Pacific +65 299 6051 Europe +44 1935 706262

333

Reference - Fluid & Gas Properties

TABLE 5 Properties of Common Gases

Gas

Symbol MW

R

Density

k

Acetylene Air Ammonia Argon Carbon Dioxide Carbon Monoxide Chlorine Ethane Ethylene Freon (R-12) Helium Hydrogen Isobutane Krypton Methane Neon Nitrogen Oxygen Propane Steam** Sulfur Dioxide

Xenon

C2H2 --NH3 A CO2 CO Cl2 C2H6 CH

24

CCl F 22

He H2 C4H10 Kr CH4 Ne N2 O2 CH

38

HO 2

SO2 Xe

26.0 29.0 17.0 39.9 44.0 28.0 70.9 30.07 28.0 120.9 4.0 2.0 58.12 82.9 16.0 20.18 28.0 32.0 44.09 18.0 64.1

130.2

59.4 53.3 91.0 38.7 35.1 55.2 21.8 51.3 55.1 12.6 386.3 766.8 26.6 18.6 96.4 76.4 55.2 48.3 35.0 85.8 24.0

11.9

0.07323 1.30

0.08071 1.40

0.04813 1.32

0.11135 1.67

0.12341 1.28

0.07806 1.40

0.2006

1.33

0.08469 1.18

0.07868 1.22

---

1.13

0.01114 1.66

0.00561 1.41

---

1.09

0.2315

1.67

0.04475 1.32

0.05621 1.64

0.07807 1.40

0.08921 1.40

0.1254

1.12

---

1.28

0.1827

1.26

---

1.67

** Values for steam are approximate and may be used only for low pressures and high temperatures. R is in ft-lbf/lbm-?R, Density is in lbm/ft3 at 32?F and 14.7 PSIA.

TABLE 6 Values of a and b for Common Gases

TABLE 7 Approximate Critical Properties

Air

CO2 H2 O

2

Steam

a(atm-ft6/pmole) 345.2 926 62.8 348 1400

b(ft3/pmole) 0.585 0.686 0.427 0.506 0.488

There is another correction factor that is applied to real gases. This factor is known as the compressibility factor, and is sometimes used for correction of gas flows through orifices. The compressibility factor is denoted by Z, and is dependent on pressure, temperature, and the type of gas. The modified ideal gas law is:

pV = ZRT

Correction factors for gases can be plotted against pressure and temperature. By using the principle of corresponding states, it is possible to create one graph that covers multiple gases. The principle of corresponding states says that all gases behave alike whenever they have the same reduced variables. The reduced variables that the law refers to are the ratios of pressure, temperature, and volume to their critical values. Table 7 gives critical properties for selected gases.

P = P/P

r

c

Tr = T/Tc

V = v/v

r

c

gas

Air Ammonia Argon Carbon Dioxide Carbon Monoxide Chlorine Ethane Ethylene Helium Hydrogen Mercury Methane Neon Nitrogen Oxygen Propane Sulfur Dioxide Water Vapor Xenon

T (?R) c

235.8 730.1 272.2 547.8 242.2 751.0 549.8 509.5 10.0 60.5 2109.0 343.9 79.0 227.2 278.1 666.3 775.0 1165.4 521.9

P (psia) C

547.0 1639.0 705.0 1071.0 508.2 1116.0 717.0 745.0 33.8 188.0 2646.0 673.3 377.8 492.5 730.9 617.0 1141.0 3206.0 855.3

334

Moore Process Automation Solutions moore-

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download