Special Duct Systems

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C H A PT E R 13SPECIAL D U C T SYSTEMS

Ductwork requiring special attention is sometimesencountered by the designer. I t is of utmost impor-tance that the duct designer be aware of th e differentrequirements existing in regards to th e geographicalarea of the installation.

Before design of special systems, th e designer mustacquaint himself thoroughly with local practices andconcerned governing authorities.

Industrial process or material handling systems are

appropriately covered in other S M A C N A publicationsan d w i l l no t be considered herein. This section con-

tains a general description of some of the special ductsystems frequently encountered in H V A C work.

KITCHEN AN D MOISTURELADEN SYSTEMS

1 . D ishwasher Exhaust andM oisture Laden Ducts

Exhausting moist air should be accomplished throughducts fabricated from non-corrosive materials. Theseducts should b e sloped toward th e source of moistureor provided with proper drains. A l l seams an d jointsmust be sealed watertight. The temperature of thevapor may be excessively high and, therefore, mayrequire the use of duct insulation or other treatment.A ll duct penetrations should b e avoided.

2 . Range and Grease HoodExhaust Ducts

Vapors from cooking equipment must b e exclusivelyhandled through ducts designed specifically for thatpurpose. Ca re must b e taken to assure that these

ducts will contain fire an d smoke. M aterials usedmust be heavier than standard an d are usually con-tinuously welded to provide a liquid-tight system.

Cleanouts should b e provided at each change of di-rection in th e duct. The system should b e constructedsuch that grease cannot b e trapped an d th e ductshould be sloped toward th e hood or a grease res-ervoir. Ducts within the building should lead as directly

as possible to th e exterior. Where ducts pass through

combustible walls, partitions, etc., adequate clear-ance or protection must be provided. I n the event ofa fire, temperatures in excess of 2000°F (1100oC)may b e experienced. Fire extinguishing systemsmayb e required by local codes. Long, straight runs of

duct should have a means for expansion.

Local codes governing range an d grease hood ductsystems vary widely; therefore, i t is imperative thatthe designer be familiar with these codes an d con-struction requirements of NFPA 96.

SYSTEMS HANDLINGSPECIAL GASES

1 . Corrosive Vapors and NoxiousGases

Ducts which convey thesegases should be fabricatedfrom materials impervious to all the gases that maybe handled, an d must be sealed air tight. They must

terminate outside the building, maintaining adequateclearances from walls, roof, adjacent buildings, trafficareas or equipment. The discharge airflow should no tcontaminate outside air intakes an d other buildingopenings under an y conditions.

2 . Flammable VaporsDucts conveying these vapors must b e sealed airtight an d terminate outside the building, maintainingadequate clearances from building construction an dother objects. Nonflammable materials must b e used

for the ducts and duct supports.

S O L A RC SYSTEMS

1 . Solar System Sizing

Successful application of solar heating systems re-quires careful selection an d sizing of components.

Collectors, heat storage units, fans an d pumps, con-

1 3 . 1



SPECIAL DUCT SYSTEMS

trols, heat exchangers, and auxiliary heaters must be

effectively integrated. Unlike the selection of a fur-

nace or boiler, a solar space heating system may be

sized to provide a selected portion of the annual heat-

ing load. Generally 30 to 70 percent is reasonable.

The size of the solar system basically depends on

the collector area. The collector area then determines

the quantity of solar heat delivery or the amount of

fossil fuel savings.

Guidelines fo r sizing components of integrated air-

based solar systems fo r space and potable water

heating are listed in Table 13-1. A typical arrangement

fo r which the guidelines apply is shown in Figure 13-

1.

2 . D uc t System Layout

A layout of the duct distribution system should beprepared and sizing of all ductwork should be accom-

plished using the method that the designer is most

comfortable with fo r the air volume required. How-

ever, the designer shall be responsible fo r correctly

sizing the duct system so that its total external static

pressure (ESP) shall not exceed the manufacturer's

ESP rating fo r the air handling equipment.

Ducts connecting solar air collector inlets and outlets

shall be sized to meet the air quantities that are re -

quired by the airflow characteristics of the collector.

Review the collector manufacturer's literature to de-

termine the correct flow rates. Connections to the

collectors shall be in accordance with the manufac-

turer's recommendations.

When auxiliary heating equipment is used, the airflow

volume of the duct distribution system must provide

an air temperature rise through the equipment that is

below the maximum temperature rise noted on the

equipment nameplate.

Table 13-1 G U I DELI N ES FO R SIZING C O M P O N E N T S O F AI R- B ASED S O L A R SYST EMS

FO R S PA CE AND PO TAB L E W A T E R HEATING

13.2

*For potable water heating only the collector slope should be at latitude angle, and the recommended range is

La t - 5° to La t + 5°

**For potable water heating only systems, pebble bed storage is not required.



CHAPTER 13

Figure 13-1 TYP ICAL AIR-HEATING SYSTEM

3 . Solar Collecting SystemsThe duct system between th e solar collectors an d the

thermal storage containers, an d the ductwork con-

necting to the space distribution system shall beknown as the primary solar duct system (PSDA). The

P S D A shall be designed using the criteria described

above. Care shall be used to assure balanced airflow

in th e P S D S fo r th e various operational modes of the

system.

A l l ducts an d duct linings composing th e P S D S shall

be installed in strict conformance with th e S M A C N A

H V A C Duct Construction Standards and Fibrous

Glass Duct Construction Standards. A l l materials

used in th e P S D S shall be able to withstand temper-atures up to 250°F (121°C) without degradation or

release of odor-causing or noxious gasses.

Air leakage from P S D S should no t exceed 5 percent.

I t is not th e intent of these Standards to test th e P S D S

fo r compliance with th e 5 percent duct leakage re-

quirement, but simply to assure construction stan-

dards which will essentially provide the required de -

gree of air tightness in th e P S DS .

Ducts may be sealed using mastic, or mastic plus

tape or gasketing as appropriate. The selection of the

most appropriate sealant depends on joint configu-

ration, clearances, surface conditions, temperature,th e direction of pressure an d preassembly or post

assembly placement. Tapes should not be applied to

bare metal no r to dry sealant. Foil tapes are not suit-

able. Liquids an d mastics should be used in well-

ventilated areas an d th e precautions of manufactur-

ers followed. O il base caulking and glazing com-

pounds should not be used. Gasketing should be ma-

terial with long l i f e an d suitable for th e service.

4 . Solar System Dampers

a. CONTROL DAMPER S (M otorized)

Dampers that open or close to divert, direct, or shut-

off airflow in th e Primary Solar Duct System shall

have "sealing" edges on th e blades with a suitable

material such as felt, rubber, etc., to insure tight cutoff

of th e air stream when closed.

b. SHUT -OFF DAMPER S(Not M otorized)

Shut-off dampers installed to prevent air flow, as in

th e summer by-pass duct in th e Primary Solar Duct

System, shall be sealed tightly to prevent air flow

when pressurized from either side of th e damper.

Slide dampers shall have suitable seals on th e guides

to prevent leakage around th e blade an d through the

guide.

Table 13 - 2 SOLAR A I R DISTRIBUTION SY STEMS

13.3



SPECIAL DUCT SYSTEMS

c. V O L U M E D A M P E R S

Volume control of balancing dampers shall be in-stalled in each branch or zone duct. Single leaf dam-pers which are a part of a manufactured air grille do

no t mee t the requirements of the SMACNA solar in-stallation standards found in the SMACNA

"Installa-tion Standards fo r Residential Heating and A ir Con-ditioning Systems." Opposed blade dampers whichare a part of a manufactured air grille meet the re -quirements of the Standards f sufficient space is pro-vided behind the grille face fo r proper operation of thedamper.

Figure 13-2 MULTI-BLADE VOLUME DAMPERS

1 3 . 4

Where space prohibits th e use of an opposed bladedamper behind the grille face, an opposed bladedamper may be installed in th e register stack at alocation where i t is accessible from th e grille opening.

Volume dampers installed i n branch ducts where thetotal estimated

system static pressure is less than0.5 in. w.g. (125 Pa) should be of a single leaf type.Volume dampers installed in ductwork where the totalestimated system static pressure exceeds 0.5 in. w.g.(125 Pa) shall be manufactured i n accordance withFigure 13-2 .

d. BACK - DRAF T D A MPE R S

Back draft dampers shall be installed to close underthe action of gravitational force when there is no airflow, and open when there is a drop in pressureacross the damper n the direction of desired air f l o w .

M ulti-bladed back-draft dampers shall have suitableseals on the blade edges, and appropriate sealsalong the sides. Light-weight rubberized fabric dam-pers of the type shown in Figure 13 -3 shall be tiltedsufficiently to ensure closure when there is no airflow.Single blade dampers shall have seals along the seatand the pivot shall be off-center and horizontal to

ensure closure when there is no airflow.

Figure 13-3 RUBBERIZED FABRICBACK-DRAFT DAMPER



CHAPTER 14

Table 14-15 LOSS COEFFICIENTS , ENTRIES (Cont.)

Use th e velocity pressure ( V p ) of th e downstream section. Fitting loss (TP) = C x Vp

E . Conical, Converging Bellmouth, Round and Rectangular, with End Wall ( 1 5 )

F. Intake Hood (15 )

G . Hood , Tapered , F langed or Unflanged(2)

Note 9: With screen in opening at D s, Cs = C (from table) + C (Screen coef . Table 14-17)

where : A = Area at D ; A s = Area at D s A s ) 2

14.43



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Special Duct Systems