(4) is ducting that simple (r1)

8/13/2019 (4) is Ducting That Simple (r1)

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3rd December 2012 Is ducting that simple? 1


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Typical answer to this question is a big

Why, then, do we have so many issues in thefield related to ducting?

The normal field experience indicates that70% of the field problems relate primarily to

ducting issues. It could be either an issue with the duct

design or an issue with installation.



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And all this happens because ducting and airdistribution is one of the most neglected andunder estimated discipline in entire air-conditioning gamut.

It is taken for granted with changes made atsite without taking any design considerationinto account to accommodate interiors andother utilities.

The end result is that while all othercontractors on site walk away with completioncertificates, we keep struggling at site.



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There are many reasons, why these thingshappen.

The prime reason for this is ducting is takenas one of the simplest things in the world.

And the in fact it is !

But we forget the basic GEPs (goodEngineering Practices) related to ducting,both in the design phase as well asinstallation phase.

In other words it is the most ignoreddiscipline in the entire HVAC field.

We therefore, today, look at some classicdosand don'ts of ducting.



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It is very common tohave plenums like these.

Such plenums createmore problems than

solving any. Such plenums impose

high pressure drops onthe fan

This either impacts the delivery of airquantity or consumes more power.

Difficulties with air balancing and unevenair flow are observed.


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It also creates more noise. In other words it defies the very purpose of

ducting.

In case plenums have to be used they must beused in unidirectional configuration wherethe air does not change primary direction offlow.

The cross section of plenum in direction ofairflow should be such that the air velocity is

preferably restricted to 500 fpm. The depth of the plenum in direction of the

airflow should be at least 2.5~3 times that ofAHU connection size.



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Many a times the AHU isconnected to the ducting systemusing a elbow turning in thereverse direction of the fan

rotation. In such configuration again the air needs to

change its direction and momentum, thattoo at high velocity and turbulent state. This

imposes high and unpredictable pressuredrops on the blower.

This results in either the drop in airflow orhigher power consumption.


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The noise levels too increase due toturbulence.

We must tackle this issue at design stage.

One solution is to change the orientation of

either the AHU or connecting ductwork. The other solution is to mirror the AHU

orientation within the AHU.

Even if, the elbow is installed in the directionof the fan rotation, it still does not complywith GEP of ducting.



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Even if this elbow is correctlyinstalled in terms of direction offan rotation, the mountingelbow on the fan outlet itself is

not recommended.

It is recommended that the astraight piece of duct of length

equal to 2.5 to 3 times the fanoutlet dimension be installedbefore any transition or bend.


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This is because the main duct is carrying airat reasonably high velocity. Since the ductsare large, air does not turn into the collar,hence there is no airflow.

How many timeshave we encounteredthe problem of noisewhen the diffuser

collar is takendirectly from themain duct but noairflow?


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Since the collar directly opens into the ductwith air travelling at reasonable highvelocities, the noise is transmitted to theoccupied zone.

It is highly recommended the collars forgrilles and diffusers must be drawn from thebranches where the air velocities are lowerthan 1,000 fpm.

In case of constraints we can alwaysincorporate linear diffusers on the sides of thecorridors or spaces.



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As much as we try to balance the air, no airseems to be coming out of the grille.

This happens because the air travelling and

turning tends to travel along the heal radius. This causes lack of airflow on the throat

radius side.

How many times havewe seen a grille collar onthe duct just after anelbow on throat radius

side?


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This actually creates a vacuum in the elbow onthe throat radius side.

This is why the air never comes out of thegrille in such a location.

In fact, if we observe carefully in the field,such grilles suck air out of the occupied zone,rather than pushing the air into the zone.

All we need to do is to ensu7re that no grillecollar is taken out on the throat radius sideimmediately after the elbow.



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The grille collar or branches can always betaken out after the distance of 2.5~3 times theturning dimension of the elbow.

2.5 to 3 D

D


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Have we ever noticed that the room

temperatures are higher that the return airtemperature?

This is a sure shot indication of the leakagesin the supply air ductwork.

And why do leakages happen?

Sheer negligence at installation phase !

This happens due to many reasons :

We do not close the seams properly. We do not nut bolt the flanges adequately.

We space the clamps at too far a distance fromeach other in case of TDF joints.



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In case of C and standing S seams, the C cleat isnot properly closed over the standing S seam.

Use of sealants is an unknown activity.

All such problems can be mitigated if we arevigilant enough.

Following care must be taken at installationphase : The seam closing should be done at the factory.

If done at site, it must be done using the seam

closing machines. In case of flanged ducting nut bolts should be

done ay not more than 10 0mm distance withrivets at distance not exceeding 75 mm.



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In case of TDF joints the gap between twocleats should not exceed 75 mm.

All transverse and longitudinal joints must besealed using silicone sealants.

Having taken care of all these issues at the

installation phase, it is advisable to carry outduct pressure testing as per SMACNA or DW-143 specifications. This must perticularly bedone for ducts where duct pressures are likely

to exceed 1 static, for example, ductsupstream of VAV terminals.

Duct pressure testing ensures that leakagesare in permissible limits.



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But is case of duct leakages, what takes thecake is the way the connections are donebetween the collars and grilles or diffusers.

Invariably most of the grille collars or diffuser

collars are cut short, creating a gap betweenbottom edge of collar and top edge of diffuser.

This make the air bypass directly to the returnair path. Hence, the air does not get pumped

into the conditioned area. Adequate care must be taken at installation

phase to ensure that such gaps do not exist.



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These days Variable Air Volume systems have

become extremely popular. There is no doubt that VAV systems save us a

lot of energy.

But in zest to use these systems many times

we install hybrid systems. In part of the air distribution systems we

install VAV terminals, whereas in rest of thepart we install grilles/diffusers directly from

the main duct without any VAV terminals. This leads to uneven temperatures in the

conditioned space, some areas not coolingadequately while as other cooling excessively.



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While as VAV terminals are pressureindependent, the areas where we mountgrilles/diffusers directly on the supply ductsbecomes pressure dependent system.

As a whole the system works as pressuredependent system. As some of the VAVs close,more air travels thru directly mountedgrilles/diffusers and vice versa.

This leads to uneven and unpredictabletemperatures in the areas served without VAVterminals.



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Also, normally the VAV terminals imposehigher pressure drops as compared togrilles/diffusers.

This tends to oversupply the air to areasserved without VAV terminals and starve the

areas served with the VAV terminals. This results in higher temperatures in areas

served by VAV terminals and much lowertemperatures in other areas.

The grille/diffuser dampers have such lowauthority, that these dampers can hardlycompensate for the pressure drops imposedby the VAV terminals.



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Therefore, such hybrid systems should neverbe used.

Whenever, VAV systems are used, every grilleor diffuser must be fed only through a VAV

terminal. Also many times, we install the duct pressure

sensor in a VAV system right at the start ofductwork at a AHU room location.

While the VAV system works at more than 85%of its volumetric capacity, the system worksperfectly.



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However, when the system starts to work atvolumetric capacities lower than 80~85%, thesystem starts to starve the VAV terminals atthe end of the ductwork.

This creates the high temperatures in areas

served by ductwork towards its tail end. This can very easily be averted by placing the

duct pressure sensors towards the end of theduct, ideally place at a distance of 75 to 80%

of total duct length from the AHU. This however needs prior planning and

alertness, rather than waking up when thefalse ceiling work is already completed and

we are about to commission the system.3rd December 2012 Is ducting that simple? 23


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What we have talked about so far, all of ushave learnt, some in hard way by makingmistakes and some by learning from otherpeoplesmistakes.

However, a due diligence on duct design ismostly found missing on drawing boards,since it has become draughtsmans activityand very little engineering goes into it.

The site execution team only makes it worseby accepting changes a drop of hat,introducing bends, tapers etc.



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All we really need is a careful scrutinyof duct design and follow somesimple dosand don'ts

On the drawing board At site.


(4) is ducting that simple (r1)

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