Sources of Moisture Emission

Moisture control in industrial buildings is necessary to avoid problems related to

• Human comfort, health, and productivity

• Process requirements (e. g., in pharmaceutical and semiconductor manufacturing, painting processes, plastic injection molding, and breweries)

• Building durability, by preventing decay of wood-based materials, corrosion of metals, and spalling of masonry and concrete caused by freeze-thaw cycles

• Degradation of the thermal resistance of building materials

Principal sources of moisture in the building include

• Evaporation from wet surfaces and open tanks

• Steam leakage from process equipment or pipelines

• Evaporation from people breathing and from perspiration

• Permeation through floors, walls, and ceiling

• Desorption from moist products

• Generation from combustion, i. e., open flame in the space

• Air infiltration through leaks, holes, and door openings

• Untreated outside air supplied by the mechanical or natural ventilation system

Moisture Diffusion through the Building Envelope

The permeation moisture load through building materials can be calcu­lated using

M = pA AVP, (7.22)

Where p = permeance factor, g/(h m2 kPa), A = surface area, m2; AVP = dif­ference in vapor pressure across the material, kPa.

Evaporation from Wet Surfaces and Open Tanks

The amount of water evaporated from wet surfaces (i. e., process equip­ment, floors) or water tanks, M, kg/h, is proportional to the difference in va­por pressure between the surface and air, the surface temperature, and the air velocity across the water surface:11

M = 7.4(a + 0.Ol7V)(P2-Pj)10l.3^, (7.23)

’ b

Where M = evaporation load, kg/h; V ~ air velocity across the surface, m/s; P2 = water vapor pressure in the air above the surface, kPa; Pt = vapor pres­sure of air saturated at the water temperature, kPa; A = total surface area wetted or of the water face, m2; Pb = barometric pressure, kPa; and a = coef­ficient reflecting the influence of air movement. For air temperature between 15 °C and 30 °C, a can be evaluated as shown in Table 7.6.

If the water temperature is held constant and the water is still, Table 7.7 can be used to evaluate the temperature of the water surface (at room air tem­perature 20 °C and RH = 70%). When the water is stirred, the surface tem­perature can be assumed to be equal to the mean water temperature.

TABLE 7.6 Determination of a for Various Water Temperatures

Water

Temperature,

"c:

JO 40

50 60

70 !

?0

90 ) 00

.I

0.022 0.03

0.03 0.04

0.041 0.05

0.051 0.06

TABLE 7.7

Water Surface Temperature Evaluation

Water

Temperature,

"C 20 30

40 50

60 70

80

90 100

Water surface temperature.

, °C 18 28

37 45

51 58

69

82 97

Evaporation load from wet surfaces or floors can be evaluated using the following equation:11

M = (6~6.5)(0O-0U/)A, (7.24)

Where 0O = dry-bulb room air temperature, °C; Bw = wet-bulb room air tem­perature, °C.

Moisture from Air Leaks through Cracks and Apertures

Moisture load from infiltrating air can be evaluated as

M = Ginl(mOM — m0), (7.25)

Where Ginf = infiltrating air flow rate, kg/h; mout and m0 = moisture content in outside and inside air, gr/kg.

Moisture from Personnel

The moisture release rate from people’s respiration and perspiration can be calculated as follows:12

M = (PA x FA) + (PB x FB) + (Pc x Fc) + (PD x FD), (7.26)

Where (PA, Fa), (PB, FB), (Pc, Fc) and (PD, FD) = evaporation rate and number of people seated (A), standing (B), with light activity (C), and with moderate activity (D) (see Fig. 7.5).

Moisture from Combustion

The amount of water vapor resulting from combustion varies with compo­sition of the burned gas. When the value is unknown, one can estimate that each cubic meter of gas burned produces 42 grams (650 grains) of water vapor.

Explosive Gases, Vapors, and Dust Mixtures

Some gases, vapors, and dust mixtures with air or oxygen may produce ex­plosions. For explosion to occur, a flammable gas or combustible material

Sources of Moisture Emission

I emperature

I FIGURE 7.5 Moisture evaporation, gr/’h, per average man.

Mixture with air or oxygen must be in proportion within the explosive (‘‘flammable”) limits, and an ignition source must be present and have suf­ficient energy to ignite the explosive mixture. Ignition can be caused either by electrical arc, spark, or by a hot surface. The surface temperature that can cause ignition varies for different gas mixtures. Ignition of carbon dis­ulfide and ethyl nitrite, for example, requires a surface temperature of onlv 85 °C. * "

Flammable gases and vapors include acetylene, hydrogen, butadiene, eth­ylene oxide, propylene oxide, acrolein, ethyl ether, ethylene, acetone, ammo­nia, benzene, butane, cyclopropane, ethanol, gasoline, hexane, methanol, methane, natural gas, naphtha, and propane.

Combustible dusts include metal dust (e. g., aluminum, magnesium, and their commercial alloys), carbonaceous dust (e. g., carbon black, charcoal, and coal), flour, grain, wood, plastics, and chemicals.

Flammable gases and vapors or combustible dust may be present in quan­tities sufficient to produce explosive or ignitable mixtures due to13

• Leakage from maintenance operations, breakdown of equipment, or faulty operation of equipment

• Escape in the event of an accidental rupture or breakdown of equipment, or in abnormal equipment operation

Among common areas where explosion can occur are coal mines, pet­rochemical plants, chemical plants, paint shops, grain handling industry, etc. Explosive limits for gases and vapors are expressed as percentages (% !, and may be defined as minimum and maximum concentrations of a flam­mable gas or vapor between which ignition occur.14 Concentrations below the lower explosive limit (LEL) are too lean to burn, while those above the upper explosive limit (UEL) are too rich. Table 7.8 lists explosive limits for some common gases.

KB TABLE 7.8 Explosive Limits of Some Gases in Air and Oxygen Mixtures14

Gas

Explosive limits in air

Explosive limits in oxygen

Ammonia

15.50-27.00

13.50-79.00

Carbon monoxide

12.50-4.20

15.50-93.90

Hydrogen

4.00-74.20

4.65-93..90

Methane

5.00-15.00

5.40-59.20

Diethyl ether

1.85-36.50

2.10-82.00

Propylene

2.00-11.10

2.10-52.80

Upper and lower explosive limits ELnllx for mixtures of several gases can be calculated using the Le Chatelier equation15

PL • =_______________________ /7 97)

CljinLX p ’ ‘—’l

Ei^ + + ee; +

Where Pj = proportion of gas i in the gas mixture; EL( = explosive limit for gas i.

Dust particles have a lower explosive limit expressed in mg/m5 and almost no upper limit. Examples of LEL for dusts are polystyrene, 0.02 mg/m3; corn starch, 0.04 mg/m3; and coal, 0.055 mg/m3.

A liquid not considered flammable may still have an explosive potential. An example is dichloromethane or methylene chloride, often used in paint strippers, which evaporates very quickly. It is not flammable, but its vapors may be explosive (explosive limits 12% to 22%).

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