EXHAUST SYSTEM DESIGN PROCEDURE

5.1 INTRODUCTION .. · 5-2

5.2 PRELIMINARY STEPS. · 5-2

5.3 DESIGN PROCEDURE. · 5-2

5.4 DUCT SEGMENT CALCULATIONS · 5-3

5.5 DISTRIBUTION OF AIR FLOW .. · 5-4

5.5.1 Balance by Design Method . · 5-4 5.5.2 Blast Gate Method · . . . . ^5-10 5.5.3 Choice of Methods · . . . . ^5-10 5.5.4 Balance by Design Procedure. 5-10

5.5.5 Blast Gate Procedure 5-10

5.5.6 System Redesign . . . 5-10

5.6 AIDS TO CALCULATION . . . . 5-1I

5.7 PLENUM EXHAUST SYSTEMS 5-11

5.7.1 Choice of Systems 5-11

5.7.2 Design. 5-11

5.8 FAN PRESSURE CALCULATIONS. 5-11

5.8.1 Fan Total Pressure · . . . 5-11 5.8.2 Fan Static Pressure · . . . . 5-12 5.8.3 Completion of the Example on Figure 5-3 5-12 5.9 CORRECTIONS FOR VELOCITY CHANGES 5-12 5.9.1 Branch Entries to Main Ducts 5-12 5.9.2 Contractions and Expansions . 5-13

5.10 SAMPLE SYSTEM DESIGN .. 5-13

5.11 DIFFERENT DUCT MATERIAL FRICTION

LOSSES .. . . . 5-13

Figure 5-1 Figure 5-2 Figure 5-3 Figure 5-4 Figure 5-5 Figure 5-6 Figure 5-7 Figure 5-8 Figure 5-9 Figure 5-10 Figure 5-11 Figure 5-12 Figure 5-13 Figure 5-14 Figure 5-15

System Duct Calculation Parameter Location . . 5-3

Figure 5-16

Problem I . . .. . . . . . Velocity Pressure Method Calculation Sheet ..

5-5 5-6 Plenum Vs. Conventional System

Types of Plenums . . . . Branch Entry Velocity Correction Expansions and Contractions . . . Problem 2 . . . . .. . . . . Balanced Design Method Calculation Sheet Blast Gate Method Calculation Sheet.

System Layout . . . . Psychrometric Chart for Humid Air Hood Entry Loss Coefficients . . . Duct Design Data Elbow Losses . Duct Design Data (Branch Entry Losses/

Weather Cap Losses) . . . . Duct Design Data (Static Pressure

Regains/Losses) . . . .

5-8 5-9 .5-13

· 5-14

· 5-16

· 5-17 .. 5-23

· 5-28

· 5-30 .5-40

· 5-41

· 5-42 5-47

5.12 FRICTION LOSS FOR NON-CIRCULAR DUCTS .5-13 5.13 CORRECTIONS FOR NONSTANDARD DENSITY 5-15

5.13.1 Variable Temperature and/or Different

Altitude . . . . 5-27

5.13.2 Elevated Moisture .. .5-27

5.13.3 Psychrometric Principles · 5-27 5.13.4 Density Determination .. .5-28 5.13.5 Hood Flow Rate Changes with Density · 5-28

5.14 AIR CLEANING EQUIPMENT. · 5-32

5.15 EV ASE' DISCHARGE. 5-32

5.16 EXHAUST STACK OUTLETS · 5-33

5.16.1 Stack Considerations .5-34

5.17 AIR BLEED-INS .. .5-35

5.18 OPTIMUM ECONOMIC VELOCITY · 5-35

5.19 CONSTRUCTION GUIDELINES FOR LOCAL

EXHAUST SYSTEMS · 5-35

5.19.1 Materials . . . · 5-35

5.19.2 Construction . · 5-35

5.19.3 System Details · 5-38

5.19.4 Codes . . . · 5-38

5.19.5 Other Types of Duct Materials · 5-38

5.19.6 Testing .5-38

REFERENCES. . . . . . .5-38

Figure 5-17 Figure 5-18 Figure 5-19 Figure 5-20 Figure 5-21 Figure 5-22 Figure 5-23 Figure 5-24 Figure 5-25 Figure 5-26 Figure 5-27 Figure 5-28 Figure 5-29 Figure 5-30

Psychrometric Chart-Normal Temperature .. 5-54 Psychrometric Chart-Low Temperatures .. 5-55 Psychrometric Chart for High Temperatures 5-56 Psychrometric Chart for Very High

Temperatures . . 5-57

Principles of Duct Design Elbows 5-58

Heavy Duty Elbows . 5-59

Cleanout Openings 5-60

Blast Gates . . . . 5-61

Principles of Duct Design. 5-62

Principles of Duct Design Branch Entry 5-63 Principles of Duct Design Fan Inlets 5-64 Airflow Around Buildings .. 5-65 Effective Stack Height and Wake Downwash 5-66 Stackhead Designs . . . 5-67

5.1 INTRODUCTION

The duct system that connects the hoods, air cleaning device(s), and fan must be properly designed. This process is much more involved than merely connecting pieces of duct.

]f the system is not carefully designed in a manner which inherently ensures that the design flow rates will be realized, contaminant control may not be achieved.

The results of the following design procedure will deter- mine the duct sizes, material thickness, and the fan operating point (system flow rate and required pressure) required by the system. Chapter 6 describes how to select a fan based on these results.

5.2 PRELIMINARY STEPS

Coordinate design efforts with all personnel involved, in- cluding the equipment or process operator as well as mainte- nance, health, safety, fire, and environmental personnel. The designer should have, at a minimum, the following data available at the start ofthe design calculations:

I. A layout of the operations, workroom, building (if necessary), etc. The available location(s) for the air cleaning device and fan should be determined. An important aspect that must be considered at this time is to locate the system exhaust point (where the air exits the system) so that the discharged air will not re-enter the work space, either through openings in the building perimeter or through replacement air unit intakes. (See Figures 5-28 and 5-29.)

2. A line sketch ofthe duct system layout, including plan and elevation dimensions, fan location, air cleaning device location, etc. Number, letter, or otherwise iden- tify each branch and section of main duct on the line sketch for convenience. The examples show hoods numbered and other points lettered.

Locate the fan close to pieces of equipment with high losses. This will facilitate balancing and may result in lower operating costs.

Flexible duct is susceptible to sagging and excessive bending, which increases static pressure losses. Usually, these additional System Pressure (SP) losses cannot be predicted accurately. Use hard duct whenever possible and keep flexible duct lengths as short as possible.

3. A design or sketch of the desired hood for each opera- tion with direction and elevation of outlet for duct connection.

4. Information about the details of the operation(s), spe- cifically toxicity, ergonomics, physical and chemical characteristics, required flow rate, minimum required duct velocity, entry losses, and required capture ve- locities.

5. Consider the method and location of the replacement

air distribution devices on the hood's performance. The type and location of these fixtures can dramatically lower contaminant control by creating undesirable turbulence at the hood (see Chapter 7). Perforated plenums or perforated duct provide better replacement air distribution with fewer adverse effects on hood performance.

5.3 DESIGN PROCEDURE

All exhaust systems are comprised of hoods, duct seg- ments, and special fittings leading to an exhaust fan. A com- plex system is merely an arrangement of several simple exhaust systems connected to a common duct. There are two general classes of duct system designs: tapered systems and plenum systems. The duct in a tapered system gradually gets larger as additional flows are merged together, thus keeping duct velocities nearly constant. If the system transports par- ticulate (dust, mist, or condensable vapors), the tapered sys- tem maintains the minimum velocity required to prevent settling. The duct in a plenum system (see Section 5.7) 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. Figures 5-4 and 5-5 illustrate design alternatives. Regardless of which system is used, the follow- ing procedure will result in a workable system design.

I. Select or design each exhaust hood based on the tox- icity, physical, and chemical characteristics of the material and the ergonomics of the process and deter- mine its design flow rate, minimum duct velocity, and entry losses (see Chapters 3 and 10). Note that mini- mum duct velocity is only important for systems trans- porting particulate, condensing vapors, or mist and to prevent explosive concentrations building up in the duct (see Section 5. I 8 for a discussion on economic velocities for non-particulate systems).

2. Start with the duct segment that has the greatest number of duct segments between it and the fan. A duct segment is defined as the constant diameter round (or constant area rectangular) duct that separates points of interest such as hoods, entry points, fan inlet, etc.

3. Determine the duct area by dividing the design flow rate by the minimum duct velocity. Convert the resul- tant cross-sectional area into a tentative duct diameter.

A commercially available duct size (see Table 5-8) should be selected. If solid particulates or condensable vapors are being transported through the system, a minimum velocity is required (see Chapters 3 and 10).

If the tentative duct diameter is not a standard size, select the next smaller size to ensure that the actual duct velocity is equal to or greater than the minimum required.

4. Using the line sketch, determine the design length for each duct segment and the number and type of fittings

(elbows, entries, and other special fittings) needed.

Design length is the centerline distance along the duct (the distance between the intersection of the center- lines of the straight duct components).

5. Calculate the pressure losses for the duct segments that merge at a common junction point. (See Section 5.4 for the details on how to calculate these losses.) 6. Directly after each junction point, there must be one

and only one

sr,

regardless of the path taken to reach that point. If not ensured by the design process, the system will "self-balance" by reducing the flow rate in the higher-resistance duct segment(s) and increasing the flow rate in the lower-resistance duct segment(s) until there is a single

sr

in the duct downstream of each junction point.

SP balance at any junction point can be achieved by either one of two fundamental design methods: I) Adjust the flow rate through the hood(s) until the SPs at each junction point are the same. 2) Increase the resistance in the low resistance duct segment(s) by means of some artificial device such as a blast gate, orifice plate, or other obstruction in the segment.

Section 5.5 discusses the details of these procedures.

7. Select both the air cleaning device and fan based upon final calculated system flow rate, temperature, moisture

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condition, contaminant loading, physical and chemical characteristics, and overall system resistance.

8. Check the duct sizes designed against the available space and resolve any interference problems. (For example, will the elbow size desired actually fit in the available space?) This may cause a redesign of part of the system.

9. Determine the material type and thickness (gauge) for each duct segment based on the air stream characteristics.

5.4 DUCT SEGMENT CALCULATIONS

The Velocity Pressure (VP) Method is based on the fact that all frictional and dynamic (fitting) losses in ducts and hoods are functions of the velocity pressure and can be calculated by a loss coefficient multiplied by the velocity pressure. Loss coefficients for hoods, straight ducts, elbows, branch entries, contractions, and expansions are shown in Figures 5-13 through 5-16. Figure 5-1 shows the application of these coefficients. For convenience, loss coefficients for round elbows and entries are also presented on the calculation sheet (see Figure 5-3).

Friction data for this method are presented as Tables 5-5 and 5-6. These tables give the loss coefficients per foot of galvanized and commercial steel, aluminum, PVC, and stain- less steel duct. The equations for these tables are listed on

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these tables and also on the calculation sheet (see Figure 5-3)YI) These equations and the resultant tables have been designed to be no more than 4% different from the "exact"

values of the Colebrook-White equation and were designed to err on the high side ofthe normal velocity range of exhaust venti lation systems.

For convenience, two data sets determined from the same equations were used to generate the friction tables. These tables are possible because, for a specific diameter, the friction loss coefficient changes only slightly with velocity. Each table lists the friction coefficient as a function of diameter for six different velocities. The error in using these data with veloci- ties plus or minus 1000 fpm is within 6%. If desired, a linear interpolation between velocity values can be performed.

In Chapter 1, an equation was presented for flexible duct with the wires covered. No data are presented here for this type of material due to the wide variability from manufacturer to manufacturer. Perhaps an even more important reason is that these data are for straight duct losses, and flexible duct, by its very nature, is seldom straight. Typically, bends in flexible duct can produce extremely large losses which cannot be predicted easily. Be very careful to keep the flexible duct as straight and as short as possible.

The following steps will establish the overall pressure loss of a duct segment that starts at a hood. Figure 5-2 shows a simple one-hood ventilation system. The use of a calculation sheet can be very beneficial when performing the calculations manually. Figure 5-3 shows the details of the calculations for each component of the system. There is also a profile through the system showing the magnitude and relationships of total, static, and velocity pressures on both the "suction" and the

"pressure" sides of the fan on Figure 5-2. It should be noted that YP is always positive. Also, while total and static pressure may be either negative or positive with respect to atmospheric pressure, Total Pressure (TP) is always greater than SP (TP =

SP

+

YP).

NOTE: The numbers in the problems presented in this chapter were generated using one of the available computer programs (see Section 5.6). The values pre- sented in the calculation sheets may be different from those determined by other methods.

1. Determine the actual velocity by dividing the flow rate by the area of the commercial duct size chosen. Then determine the corresponding velocity pressure from Table 5-7 or the equations in Chapter 1. In the example, the diameter chosen was 4" (line 5), the actual velocity is given on line 7 and the YP corresponding to this actual velocity is given on line 8.

2. Determine the hood static pressure from the equations in Chapter 3. In this example, there are no slots, so the duct entry loss is as given on lines 17 through 22.

3. Multiply the design duct length by the loss coefficient from the tabulated data of Tables 5-5 or 5-6 (lines 23 through 25.) The use of galvanized sheet metal duct was assumed throughout this chapter.

4. Determine the number and type of fittings in the duct segment. For each fitting type (see Figures 5-13, 5-14, 5-15, and 5-16), determine the loss coefficient and multiply by the number of fittings (there were none in this example.)

5. Add the results of Steps 3 and 4 above and mUltiply by the duct YP. This is the actual loss in inches of water for the duct segment (given on line 34).

6. Add the result of Step 5 to the hood suction. If there are any additional losses (expressed in inches of water), such as for an air cleaning device, add them in also. This establishes the cumulative energy required, expressed as static pressure, to move the design flow rate through the duct segment (line 37). Note that the value on line 37 is negative.

The calculations listed in the last three columns of Figure 5-3 will be discussed in Section 5.8.3.

5.5 DISTRIBUTION OF AIR FLOW

As discussed previously, a complex exhaust system is actually a group of simple exhaust systems connected to a common main duct. Therefore, when designing a system of multiple hoods and branches, the same rules apply. In a multiple branch system, however, it is necessary to provide a means of distributing air flow between the branches either by balanced design or by the use of blast gates.

Air will always take the path of least resistance. A natural balance at each junction will occur; that is, the exhaust flow rate will distribute itself automatically according to the pres- sure losses of the available flow paths. The designer must provide distribution such that the design air flow at each hood will never fall below the minimums listed in Chapter 3 and/or Chapter 10. To do so, the designer must make sure that all flow paths (ducts) entering a junction will have equal calcu- lated static pressure requirements.

To accomplish this, the designer has a choice of two methods. The object of both methods is the same: to obtain the desired flow rate at each hood in the system while main- taining the desired velocity in each branch and main.

The two methods, labeled Balance by Design Method and Blast Gate Method, are outlined below. Their relative advan- tages and disadvantages can be found in Table 5-1.

5.5.1 Balance by Design Method: This procedure (see Section 5.10) provides for achievement of desired air flow (a

"balanced" system) without the use of blast gates. It is often called the "Static Pressure Balance Method." In this type of design, the calculation usually begins at the hood farthest from

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