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 |
