DUCTING

DUCT:

A Duct can be described as a device used to provide an isolation path to carry an item

from one place to other place without bringing the product in contact with the

atmosphere before the delivery point.

Types of duct:

1.       Square duct

2.       Rectangular duct

3.       Circular duct

4.       Spiral duct

5.       Oval duct

Duct materials:

1.       Galvanized iron

2.       Aluminum sheet metal

3.       Fiber glass duct

4.       Thick black mild steel

5.       Poly urethane foam

6.       Phenolic  insulation panel

Classification of ducts:

·         Low pressure duct

·         Medium pressure duct

·         High pressure duct

·         Duct construction is classified by application and pressure.

·         HVAC systems in public assembly, business, educational, general factory, and mercantile buildings are usually designed as commercial.

·         Air pollution control systems, industrial exhaust systems, and systems outside the pressure range of commercial system standards are classified as industrial.

Classification standards:

ü  Residences                                   ±0.5 in. of water

                                                                                                ±1 in. of water

                                                                                                ±2 in. of water

ü  Commercial Systems                               ±0.5 in. of water

                                                                                                ±1 in. of water

                                                                                                ±2 in. of water

                                                                                                ±3 in. of water

                                                                                                ±4 in. of water

                                                                                                ±6 in. of water

                                                                                                ±10 in. of water

ü  Industrial Systems                    Any pressure

Pressure velocity classification of the duct (SMACNA)

S	Pressure class	Duct class	Static pressur rating	pressur	Velocity
1	High pressure Duct standard	High pressure	10"	Pos	2000 fpm-up
		Medium pressure	6"	Pos	2000 fpm-up
		Medium pressure	4"	Pos	2000 fpm-up
		Medium pressure	3"	Pos or neg	4000 fpm- dn
2	low pressure Duct standard	low pressure	2"	Pos or neg	2500fpm - dn
		low pressure	1"	Pos or neg	2500fpm – dn
		low pressure	1/2"	Pos or neg	2000fpm - dn

Duct Insulation:

Properly insulating air ducts located in unconditioned spaces such as attics, crawl spaces, garages, or unfinished basements can help improve your home's energy efficiency.

Air ducts supply conditioned air from your space heating and cooling equipment to your living spaces. They also return an equal volume of air back to the equipment to be conditioned again.

Ducts are typically made out of thin metal materials that easily conduct heat. Therefore, uninsulated or poorly insulated ducts in unconditioned spaces can lose through conduction 10%–30% of the energy used to heat and cool your home. The heating and cooling equipment then has to compensate for the heat loss and gain by conditioning additional air. This added conditioning raises a homeowner's energy bills. In addition, when ducts lose heat through conduction, rooms served by long duct runs can experience "cold blow" during the winter because they usually have lower heating-supply temperatures.

Ducts in conditioned spaces experience minimal conductive losses and gains since they are exposed to indoor air temperatures. However, these ducts may also require some insulation to prevent condensation on duct walls and to ensure that conditioned air is delivered at the desired temperature.

Duct system air flow depends on:

1.       Pressure losses as the duct element (filter, dampers, louvers,              grills & others)

2.       The change in the fan speed

3.       Air density

4.       Wind

5.       Branches

6.       The inside duct smoothly

Design methods:-

1.       Equal friction                         -T method

2.       Static regains                         -velocity reduction

3.       Extended plenum                  -total pressure

4.       Constant velocity                   -residential system

CONSIDERATION FOR DUCT DESIGN:

1.       AVAILABLE SPACE

2.       EVEN DISTRIBUTION IN ALL PARTS OF ROOM

3.       SOUND LEVEL - QUIET

4.       NO DRAFTS / SUFFOCATION

5.       APPEARANCE

6.       ADJUSTABLE

7.       NO MOISTURE CONDENSATION / DRIPPING

8.       HEAT GAIN AND LEAKAGE LOSSES

9.       FRICTION LOSS

10.   FIRST COST

Air distribution systems are divided into2 velocity system:-

1.       Low velocity system up to     2500fpm

2.       High velocity system above 2500fpm

 In the duct system the continuous loss of pressure (friction losses) depended on:

1.       Air velocity

2.       Duct size

3.       Interior surface roughness

4.       Duct length.

FORMULA TO FIND FRICTIONAL LOSS:

                      L          V

∆P = 0.03 f {-----} {-------} ¹·82        where

                     D¹·²²    1000

1.       ∆P = friction loss(in.wg)

2.       f    = interior surface roughness(0.9 for galvanized duct)

3.       L    =length of duct(ft)

4.       d   =duct diameter (in)

5.       V    =air velocity(fpm)

The euro vent duct leakage standards define three leakage classes

1.       The lowest class.

Leakage factor = 1.32 liter/(s, m²) at 400 pa

                                                                                  = 0.26cfm/f ²       at 8.4Ib/ f ²

2.       The medium class.

                                                                  Leakage factor = 0.44 liter/(s, m²) at 400 pa

                                                                                              = 0.087cfm/f ²       at 8.4Ib/ f ²

3.       The highest class.

                                                                  Leakage factor = 0.15 liter/(s, m²) at 400 pa

                                                                                             = 0.029cfm/f ²       at 8.4Ib/ f ²

RECOMMENDED MAX. DUCT VELOCITY

FOR LOW VELOCITY SYSTEMS (FPM)

Application	Controlling factor noise generation main duct	Controlling factor-duct friction
		Main duct		Branch duct
		supply	return	supply	return
Residences	600	1000	800	600	600
Apartments Hotel bedrooms Hospital bedroom	1000	1500	1300	1200	1000
Private offices Directors rooms Libraries	1200	2000	1500	1600	1200
Theatres Auditoriums	800	1300	1100	1000	800
General offices High class restaurants High class store Banks	1500	2000	1500	1600	1200
Average store Cafeterias	1800	2000	1500	1600	1200
industrial	2500	3000	1800	2200	1500