Service Ducts

8/12/2019 Service Ducts

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BUILDING CONSTRUCTION VISERVICE DUCTS

Prepared by Rajendrasingh Pardeshi


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INTRODUCTION


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INTRODUCTION Although the Electrical supply intakecan be terminated in a meter boxsituated within a dwelling, mostsupply companies prefer to use theexternal meter box to enable themeter to be read without the need toenter the premises.

The local company is responsible

for providing electricity up to andincluding meter, but the consumer isresponsible for safety and protectionof the companys equipment.

The supplier will install the servicecable up to the meter position wheretheir termination equipment isinstalled


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INTRODUCTION

Rewire able systems housed in

horizontal conduits can be castinto the structural floor slab orsited within the depth of the floorscreed.

To ensure that such a system isrewire able, draw in boxes mustbe incorporated at regularintervals and not more than tworight angle boxes to be includedbetween draw-in points.

Vertical conduit can be surfacemounted or housed in a chasecut in to a wall provides depth of

the chase is not more than 1/3 rd of the wall thickness


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INTRODUCTION

Gas Service Pipes

1. Whenever possible the servicepipe should enter the building onthe side nearest to the main.

2. A service pipe must not passunder the foundation of a building.

3. No service pipe must be runwithin a cavity but it may passthrough a cavity by the shortestroute.

4. Service pipes passing through awall or solid floor must beenclosed by a sleeve or ductwhich is end sealed with mastic.

5. No service pipe shall be housed

in an unventilated void.6. Suitable material for service pipes

are copper and steel.Polyethylene is normally usedunderground.


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INTRODUCTION


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INTRODUCTIONFire Protection or Service Openings:Penetration of compartment walls and floorsby service pipes and conduits is very difficult

to avoid. An exception is where purpose builtservice ducts can be accommodated.Wherever a pipe passes through acompartment interface, it must be providedwith a proprietary seal. Seals are collars ofintumescent material which expands rapidlywhen subjected to heat, to form acarbonaceous charring. Expansion issufficient to compress warm plastic andsuccessfully close a pipe void for up to 4hours.

In some circumstances fire stopping aroundthe pipes will be acceptable, providing thegap around the pipe and hole through the

structure are filled with non combustiblematerial. Various materials are acceptable,including reinforced mineral fibre, cement andplasters, asbestos rope and intumescentmastics.

Pipes of low heat resistance, such as PVC,lead, aluminium alloys and fibre cement mayhave a protective sleeve of non combustiblematerial extending at least 1m either side of


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INTRODUCTIONTelephone Installations: Unlikeother services such as water, gasand electricity, telephones cannot be

connected to a common main supply.Each telephone requires a pair ofwires connecting it to the telephoneexchange.

The external supply services andconnection to the lead-in socket iscarried out by telecommunicationengineers. Internal extensions canbe installed by the site electrician.


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INTRODUCTIONElectronic Installations: In addition tostandard electrical andtelecommunication supplies in to a

buildings, there is a growing demand forcable TV, security cabling andbroadband access to the internet.

Previous construction practice has notforeseen the need to accommodatethese services from distributionnetworks into buildings, andretrospective installation throughunderground ducting is both costly anddisruptive to the structure andsurrounding area, particularly whenrepeated for each different service.Ideally there should be a commonfacility integral with new construction to

permit simple installation of thesecommunication services at any time. Atypical installation will provideconnection from a common externalterminal chamber via undergroundducting to a terminal distribution boxwithin the building. Internal distributionis through service voids within thestructure or attached trunking.


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INTRODUCTIONCOMMERCIAL, industrial, andresidential air duct system design must

consider

1.space availability2.space air diffusion3.noise levels,4.air distribution system (duct andequipment)5.duct heat gains and losses,6.balancing,7.fire and smoke control8.initial investment cost, and9.system operating cost.


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INTRODUCTIONHVAC DESIGN Combining comfortwith efficiency

When defining an HVAC installation,comfort is the most obvious primaryconsideration, as it is invariably themain reason for installing an HVACsystem in first place.

The air duct distribution network is animportant component of HVACinstallations, helping to significantlyreduce energy costs and reduce noisenuisance generated by the systemsequipment.

An HVAC installation is designed toensure the thermal and acoustic comfort

of a buildings occupants, use energyefficiency and comply fully with safetyrequirements.

The shape of ducts usually installed asrectangular, because dimensions caneasily be changed to maintain therequired area


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INTRODUCTIONAdvantages of Installation HVACDuct work

Installing an HVAC duct work systemduring a buildings construction oftenmeans reduced costs in the long run, asit avoids any future necessary structuralmodifications.

Installation typically consists ofdesigning space for the distributionnetwork and the location of AHUs.


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DESIGN CONSIDERATIONSSpace Pressure Relationship

Space pressure is determined by fan location and duct system arrangement.

For example, a supply fan that pumps air into a space increases space pressure; anexhaust fan reduces space pressure.

If both supply and exhaust fans are used, space pressure depends on the relativecapacity of the fans. Space pressure is positive if supply exceeds exhaust andnegative if exhaust exceeds supply .

System pressure variations caused by wind can be minimized or eliminated by carefulselection of intake air and exhaust vent locations


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DESIGN CONSIDERATIONSFire and Smoke Management

Because duct systems can convey smoke, hot gases, and fire from one area toanother and can accelerate a fire within the system, fire protection is an essential partof air-conditioning and ventilation system design. Generally, fire safety codes requirecompliance with the standards of national organizations. fire safety requirements for

1.ducts, connectors, and appurtenances;2.plenums and corridors;3.air outlets, air inlets, and fresh air intakes;4.air filters5.fans;6.electric wiring and equipment;7.air-cooling and -heating equipment;8.Building construction, including protection of penetrations; and

9.Controls , including smoke control.


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DESIGN CONSIDERATIONSDuct Insulation

In all new construction (except low-rise residential buildings), air-handling ducts andplenums that are part of an HVAC air distribution system should be thermallyinsulated in accordance with ASHRAE Standard 90.1. Duct insulation for new low-riseresidential buildings should comply with ASHRAE Standard 90.2. Existing buildingsshould meet requirements of ASHRAE Standard 100.

In all cases, thermal insulation should meet local code requirements. Insulationthicknesses in these standards are minimum values; economic and thermalconsiderations may justify higher insulation levels. Additional insulation, vapourretarders, or both may be required to limit vapour transmission and condensation.Duct heat gains or losses must be known to calculate supply air quantities, supply airtemperatures, and coil loads.


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DESIGN CONSIDERATIONSDuct System Leakage

It is recommended that all transverse joints, longitudinal seams, and ductworkpenetrations be sealed. Longitudinal seams are joints oriented in the direction ofairflow. Duct wall penetrations are openings made by screws, non-self-sealingfasteners, pipe, tubing, rods, and wire. All other connections are consideredtransverse joints , which are connections of two duct or fitting elements orientedperpendicular to flow. System (ductwork and equipment) leakage should be tested toverify the installing contractors workmanship and sealing practices.

Leakage in all unsealed ducts varies considerably withthe fabricating machinery used,material thickness,assembly methods, andinstallation workmanship.

For sealed ducts, a wide variety of sealing methods and products exists


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DESIGN CONSIDERATIONSSystem and Duct Noise

The major sources of noise from air-conditioning systems are diffusers, grilles, fans,ducts, fittings, and vibrations. Sound control for terminal devices consists of selectingdevices that meet the design goal under all operating conditions and installing themproperly so that no additional sound is generated.

The sound power output of a fan is determined by the type of fan, airflow, andpressure. Sound control in the duct system requires proper duct layout, sizing, and

provision for installing duct attenuators, if required. Noise generated by a systemincreases with both duct velocity and system pressure.


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DUCT DESIGN PROCEDURESThe general procedure for HVAC system duct design is as follows:

1.Study the building plans, and arrange supply and return outlets to provide properdistribution of air in each space. Adjust calculated air quantities for duct heat gains orlosses and duct leakage. Also, adjust supply, return, and/or exhaust air quantities tomeet space pressurization requirements.2. Select outlet sizes from manufacturers data 3.Sketch the duct system, connecting supply outlets and return intakes with the air-handling units/air conditioners. Use rigid round ducts, minimize the number of fittings,

and avoid close-coupled fittings because little is known about the resulting losscoefficients. If space is restricted and a properly designed round duct is too large, thenext best option to minimize leakage and pressure losses is to use flat oval ductwork.Multiple runs of round duct should also be considered. Limit flexible duct to the final 5ft of connections to diffusers and terminal boxes, with no more than 5% compression.4.Divide the system into sections and number each section. A duct system should bedivided at all points where flow, size, or shape changes. Assign fittings to the section

toward the supply and return (or exhaust) terminals.5.Size ducts by the selected design method.6.Lay out the system in detail. If duct routing and fittings vary significantly from theoriginal design, recalculate pressure losses. Reselect the fan if necessary7.Resize duct sections to approximately balance pressures at each junction8.Analyse the design for objectionable noise levels, and specify lined duct, double-wall duct, and sound attenuators as necessary.


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DUCT INSULATIONThe need for duct insulation is influenced by the

1.Duct location (e.g., indoors or outdoors; conditioned, semi conditioned, orunconditioned space)2.Effect of heat loss or gain on equipment size and operating cost3.Need to prevent condensation on low-temperature ducts4.Need to control temperature change in long duct lengths5.Need to control noise transmitted within the duct or through the duct wall

All HVAC ducts exposed to outdoor conditions, as well as those passing throughunconditioned or semi conditioned spaces, should be insulated. Analyses oftemperature change, heat loss or gain and other factors affecting the economics ofthermal insulation are essential for large commercial and industrial projects.

ASHRAE Standard 90.1 and building codes set minimum standards for thermal

efficiency, but economic thickness is often greater than the minimum. Additionally, thestandards and codes do not address surface condensation issues. Theseconsiderations are often the primary driver of minimum thickness in unconditioned orsemi conditioned locations subject to moderate or greater relative humidity


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INSULATION MATERIAL FOR HVACDUCTS

Insulated ducts in buildings can consist of insulated sheet metal, fibrous glass, orinsulated flexible ducts, all of which provide :combined air barrierThermal insulation, andsome degree of sound absorption.

Ducts embedded in or below floor slabs may be of compressed fiber, ceramic tile, orother rigid materials. Depending on the insulation material, there are a number ofstandards that specify the material requirements.

Duct insulations include semirigid boards and flexible blanket types, composed oforganic and inorganic materials in fibrous, cellular, or bonded particle forms.Insulations for exterior surfaces may have attached vapor retarders or facings, orvapor retarders may be applied separately.

When applied to the duct interior as a liner, insulation both insulates thermally andabsorbs sound. Duct liner insulations have sound-permeable surfaces on the sidefacing the airstream capable of withstanding duct design air velocities or duct cleaningwithout deterioration.


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INSULATION MATERIAL FOR HVACDUCTS

Abuse Resistance

One important consideration in the choice of external insulations for air ducts is abuseresistance. Some insulation materials have more abuse resistance than others, butmost rigid insulations will not withstand high abuse such as foot traffic. In high-trafficlocations, a combination of insulation and protective jacketings is required.

In areas where the insulation is generally inaccessible to human contact, less rigidinsulations are acceptable. One important consideration for internal insulation abuseresistance is that, in large commercial units, it is common for maintenance personnelto enter and move around. In these instances, it is critical that structural elements beprovided to keep foot traffic off the insulation.


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DUCT TYPESTypes of duct are:

1.Metal ducts

2.Glass wool duct boards3.Plastic ducts4.Flexible ducts

Metal Duct work

Such ducts are made from sheet metal (galvanized or stainless steel, copper,aluminium), cut and shaped to the required geometry for the air distribution system.

Since metal is a good thermal conductor, such ducts require thermal insulation, thecommonest material for which is glass wool, usually in roll form (known as wraps orwrapped insulation), wrapped around the outer duct wall. Wraps incorporate analuminium foil facing that acts as a vapour barrier.

Insulation can also be installed on the inner wall of the duct (duct liners), as glasswool duct wraps or duct slabs faced with a glass matting or mesh providing acousticinsulation and strengthening the inner face of the duct.


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DUCT TYPESGlass wool duct boards

These are ducts made from high density glass wool board. Ducts are shaped from the

boards, by cutting and folding in These are ducts made from high density glass woolboard. Ducts are shaped from the boards, by cutting and folding in order, to obtain therequired geometry required.

The face of the original board in contact with the air stream when assembled as aduct is called the internal face, i.e. inside the duct. The other surface of the originalboard is called the external face.

Panels are supplied with double facing such that: The external face of the duct is faced with a robust reinforced aluminium foil whichacts as a vapour barrier and confers air tightness on the duct. The internal face of the duct has either an aluminium coating, a glass mat or fabriclayer, depending on the properties required of the duct

Flexible ducts

These usually consist of two aluminium and polyester concentric tubes. A glass woollayer is inserted between the two tubes as thermal insulation.

Their use is generally limited to short lengths, due to high pressure drop-off and theacoustic problems they create: they are mainly used to connect main air duct andterminal units (diffusers, grids). In most countries the regulations do not permit theiruse in lengths greater than 1.5 m.


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DUCT TYPESPlastic ducts

This category includes ducts made from plastic or foam boards, shaped by cutting

and folded to produce the required cross-sectional geometry. Boards are facedusually with an aluminium coating both internally and external.

The main drawback of this type of ducting is their fire classification. Even if theycomply with local standards, when exposed to fire they often exhibit poor performancein terms of the production of both smoke and flaming droplets.

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Service Ducts