**4. Ducts**

Ducts are connected from hoods to fan and transfer air pollutions; therefore, the most velocity of air is in the duct. Then, the most losses are in the duct, and ducts show the static pressure of ventilation system. This process is more involved than merely connecting pieces of duct. Contaminant control may not be achieved if the system is not carefully designed in a manner which inherently ensures that the design flow rates will be realized. The results of the following design procedure will determine the material thickness, duct sizes, and the fan operating point required by the system.

All exhaust systems contain hoods, duct segments, and special fittings leading to an exhaust fan. A complex system is an arrangement of some simple exhaust systems which are connected to a common duct. There are two general types of duct system designs: tapered systems and plenum systems. The duct in a tapered system gradually gets larger since additional flows are merged together, thus keeping duct velocities nearly constant. The tapered system will maintain the minimum velocity required to prevent settling if the system transports particulate. The duct in a plenum system is generally larger than that in a tapered system, and the velocity in it is usually low. Any particulate in the air stream can settle out in the large ducts. Select or design each exhaust hood on the basis of the process, toxicity, and physical and chemical characteristics of the material and the ergonomics of the process, and then determine its minimum duct velocity, design flow rate, and entry losses. Note that minimum duct velocity is only important for systems transporting particulate, condensing vapors, or mist and for preventing explosive concentrations building up in the duct.

#### **4.1. Duct segment calculation**

The velocity pressure method is based on the fact that all frictional and dynamic (fitting) losses in ducts are functions of the velocity pressure and can be calculated by a loss factor multiplied by the velocity pressure. Loss factors are for straight ducts, elbows, and branch. Note that velocity pressure is always positive. Also, total pressure is always greater than static pressure when static and total pressures are negative at suction zoon and positive at air drift to atmosphere. Determine the hood static pressure. By the loss coefficient from the tabulated data, multiply the design duct length. Using galvanized sheet metal duct was assumed throughout this article. Determine the number and type of fittings in the duct segment. Add the results of and multiply by the duct VP. This is the actual loss in inches of water for the duct segment. Finally add the result to the hood suction. Add them in also if there are any additional losses (expressed in inches of water), such as for an air cleaning device. This establishes the cumulative energy required, expressed as static pressure, to move the design flow rate through the duct segment. It should be noted that the final value is negative. Equation (2) indicates the total pressure equation.

$$\text{Total pressure equation:}\\\text{SP (TP = SP + VP)}\tag{2}$$

where SP is the static pressure ("wg), VP is the velocity pressure ("wg), and TP is the total pressure ("wg).

The process of calculation of ventilation systems is as follows:

By the following formula, determine flow correction for air psychrometric and sea level elevation. By dividing the actual flow rate by the area of the commercial duct size chosen, determine the minimum duct design velocity, and then calculate actual velocity. Equation (3) shows the calculation of velocity pressure by the following formula:

$$\text{Velocity pressure equation:} \text{VP} = \text{K}\_p \left(\frac{\text{V}}{4005}\right)^2 \tag{3}$$

where KP is the pressure correction factor, VP is the velocity pressure (fpm), and V is the duct velocity ("wg).

#### **4.2. Duct losses**

**4. Ducts**

point required by the system.

**Figure 1.** Calculation of flow rate of standard VS of ACGIH.

148 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

Ducts are connected from hoods to fan and transfer air pollutions; therefore, the most velocity of air is in the duct. Then, the most losses are in the duct, and ducts show the static pressure of ventilation system. This process is more involved than merely connecting pieces of duct. Contaminant control may not be achieved if the system is not carefully designed in a manner which inherently ensures that the design flow rates will be realized. The results of the following design procedure will determine the material thickness, duct sizes, and the fan operating

All exhaust systems contain hoods, duct segments, and special fittings leading to an exhaust fan. A complex system is an arrangement of some simple exhaust systems which are connected to a common duct. There are two general types of duct system designs: tapered systems and plenum systems. The duct in a tapered system gradually gets larger since additional flows are merged together, thus keeping duct velocities nearly constant. The tapered system will maintain the minimum velocity required to prevent settling if the system transports particulate. The duct in a plenum system is generally larger than that in a tapered system, and the velocity in it is usually low. Any particulate in the air stream can settle out in the large ducts. Select or design each exhaust hood on the basis of the process, toxicity, and physical

Air movement has friction to the inside wall of the duct, so it creates loss that is calculated as follows:

#### *4.2.1. Loeffler formula*

Equation (4) shows the calculation of Loeffler, shown in **Table 1**, choices *a*, *b*, *and c*:


**Table 1.** Correlation equation constants

$$\text{Loeffler equation:}\newline\text{H}\_{\text{f}} = \text{a}\frac{\text{V}^{\text{b}}}{\text{Q}^{\text{c}}}\tag{4}$$

where h<sup>f</sup>

pressure ("

**5.4. Entrance**

friction factor [1–3].

**Figure 3.** Entrance.

is the loss of elbow ("

**R/d Friction factor**

1.5 0.24 2.0 0.19 2.5 0.17

WG).

**Table 2.** Elbow friction factor

**Figure 2.** Standard elbows.

WG), K is the elbow friction factor ("

This is the branch of the flow of air flow from one duct to another duct distribution (see **Figure 3**) of air flow. Equation (6) shows the pressure drop in the entrance, and **Table 3** shows the entrance

WG), and VP is the velocity

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where H<sup>f</sup> is the Loeffler coefficient; Q is the air flow rate (cfm); V is the duct velocity (fpm); l is the straight duct length (ft); VP is the duct velocity pressure ("wg); and a, b, and c are the coefficients of duct material.

## **5. Fittings**

The fittings are the pieces that are mounted on the duct to redirect the path and branch. The standard fitting in industrial ventilation include:

#### **5.1. Elbow**

Elbows are used to redirect the air stream. In industrial ventilation, the standard elbows include 90°, 60° and 45°. It is practically impossible to curl the sheets, given that the elbow is made of metal sheets (black iron, galvanized, stainless steel, etc.); therefore, elbows must be made of pieces. Minimum segments in elbows 90° must be five segments, in elbows 60° must be four segments, and in elbows 45° must be three segments. **Figure 2** shows the standard elbows.

#### **5.2. Radius of elbow**

Distance center of elbow arc with longitudinal axis line in standard elbows in industrial ventilation has three types of circle radius. The elbow is as follows in the radius of rotation and its diameter: R = 1.5 d or R = 2.0 d or R = 2.5 d. The elbow R = 2.0 d is optimized for low-pressure drop and low turn radius, but R = 1.5 d is economical according to our experiences.

#### **5.3. Elbow losses**

The pressure drop in the elbows is calculated in Eq. (5), and **Table 2** indicates the elbow friction factor.

Calculation of the pressure drop in the elbows: h<sup>f</sup> = K ∙ VP (5)

**Figure 2.** Standard elbows.

Loeffler equation:H<sup>f</sup> <sup>=</sup> <sup>a</sup> <sup>V</sup><sup>b</sup> \_\_\_

150 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

standard fitting in industrial ventilation include:

is the Loeffler coefficient; Q is the air flow rate (cfm); V is the duct velocity (fpm); l

is the straight duct length (ft); VP is the duct velocity pressure ("wg); and a, b, and c are the

**Duct materials a b c** Galvanized-PVC-PE 0.0307 0.533 0.612 Iron-steel-aluminum 0.0425 0.465 0.602 Flexible duct wires covered 0.0311 0.604 0.639 Flexible duct wires exposed 0.0428 0.649 0.683

The fittings are the pieces that are mounted on the duct to redirect the path and branch. The

Elbows are used to redirect the air stream. In industrial ventilation, the standard elbows include 90°, 60° and 45°. It is practically impossible to curl the sheets, given that the elbow is made of metal sheets (black iron, galvanized, stainless steel, etc.); therefore, elbows must be made of pieces. Minimum segments in elbows 90° must be five segments, in elbows 60° must be four segments, and in elbows 45° must be three segments. **Figure 2** shows the standard elbows.

Distance center of elbow arc with longitudinal axis line in standard elbows in industrial ventilation has three types of circle radius. The elbow is as follows in the radius of rotation and its diameter: R = 1.5 d or R = 2.0 d or R = 2.5 d. The elbow R = 2.0 d is optimized for low-pressure

The pressure drop in the elbows is calculated in Eq. (5), and **Table 2** indicates the elbow fric-

Calculation of the pressure drop in the elbows: h<sup>f</sup> = K ∙ VP (5)

drop and low turn radius, but R = 1.5 d is economical according to our experiences.

where H<sup>f</sup>

**5. Fittings**

**5.1. Elbow**

**5.2. Radius of elbow**

**5.3. Elbow losses**

tion factor.

coefficients of duct material.

**Table 1.** Correlation equation constants

Qc (4)


**Table 2.** Elbow friction factor

where h<sup>f</sup> is the loss of elbow (" WG), K is the elbow friction factor (" WG), and VP is the velocity pressure (" WG).

#### **5.4. Entrance**

This is the branch of the flow of air flow from one duct to another duct distribution (see **Figure 3**) of air flow. Equation (6) shows the pressure drop in the entrance, and **Table 3** shows the entrance friction factor [1–3].

**Figure 3.** Entrance.


**Table 3.** Entrance friction factor

$$\text{Pressure drop in the entrance equation:} \text{h}\_{\text{f}} = \text{K} \cdot \text{VP} \tag{6}$$

where h<sup>f</sup> is the loss of entrance (" WG), K is the entrance friction factor (" WG), and VP is the velocity pressure (" WG).
