**3. Hood flow rate**

Hoods may be of different shapes and dimension configurations but can be categorized into two general groups, i.e., enclosing and exterior. The type of the hood to be used depends on the physical characteristics of the process equipment, the operator/equipment interface, and the contaminant generation mechanism. Enclosing hoods are those which partially or completely enclose the process or contaminant generation point. A complete enclosure may be a laboratory glove box. Wherever the process configuration and operation permit, enclosing hoods are preferred. Exterior hoods are those which are located adjacent to an emission source without enclosing it. Equation (1) indicates the calculation of flow rate of exterior hood by a general equation.

Calculation of flow rate of exterior hood: Q = KQ (10X2 + A) V (1)

where KQ is the air correction factor, X is the pollution center to hood face (ft), A is the hood face area (ft2 ), V is the capture velocity (fpm), and Q is the hood suction (cfm).

Calculation flow rate of standard VS of ACGIH for example. **Figure 1** shows calculation of flow rate of standard VS of ACGIH.

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

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.

The process of calculation of ventilation systems is as follows:

shows the calculation of velocity pressure by the following formula:

Velocity pressure equation:VP = K<sup>P</sup> (

Total pressure equation:SP (TP = SP + VP) (2)

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

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)

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

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

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

\_\_\_\_ V 4005) 2

(3)

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and for preventing explosive concentrations building up in the duct.

**4.1. Duct segment calculation**

pressure ("wg).

velocity ("wg).

**4.2. Duct losses**

*4.2.1. Loeffler formula*

follows:

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