HVAC - Duct Designing

This is the part 8th of the full HVAC course.. In this part we are going to see about the duct designing.

Duct Designing and Installation :

Duct :

It is a conduit through which air flows. Ducts are functions of a duct system to transmit air from Air Handling Unit (AHU) or Fan Coil Unit (FCU) to space or room which is to be conditioned, and this passage of air through which air travels is known as "Duct".

To fulfill this function in a practical manner duct systems must be designed within pre-chilled limits of available space (height), friction loss, velocity, sound level, leakage loss etc. As Velocity V increases, Area decreases, Friction loss increases and sound increases. As Velocity decreases, area increases, Friction loss decreases and sound decreases, where area is the cross sectional area of the duct.

Classification of Ducts :

Ducts are classified based on various aspects.

* According to the shape of duct.

* According to the type of air passage.

* According to Velocity passing through ducts.

* According to pressure inside the ducts.

* According to duct designing methods.

* According to materials used to manufacture ducts.

* According to shape of ducts :

Rectangular ducts : 100% application in actual practice with false ceiling. Some of the advantages of Rectangular ducts are, Height required to install rectangular ducts is less. Easy to fabricate and can be fabricated on site itself. Fabrication cost is low. The disadvantage of rectangular duct is, friction losses are more in rectangular ducts as compared to round or circular ducts.

Round or circular ducts : Some of the advantages of Round or circular ducts are, It has the lowest friction loss. Less material is required for fabrication. Looks nice from interior perspective. Some of the disadvantages of Round or circular ducts are, Height required to install round duct is more. More fabrication time is required. Factory fabrication is required. Cost of fabrication is kind of high.

Square ducts : Some of the advantages of square ducts are, Less friction losses when compared to rectangular ducts, but not when compared with round ducts. Less material expected for manufacture when contrasted with rectangular duct. Some of the disadvantages of square ducts are, Height required to install square duct is more when compared with the rectangular duct.

* According to the type of air passage :

* According to Velocity passing through ducts :

* Low velocity Duct system &

* High velocity Duct system.

* Low velocity Duct system :

Velocity ranges from 900 fpm to 2500 fpm. Application of these types of duct systems are in Residential buildings, offices, hospitals, supermarkets, public places etc.

* High velocity Duct system :

Velocity ranges from 2500 fpm to 4000 fpm.

1 m/s = 196.8 fpm.

* According to pressure inside the ducts :

* Low pressure duct :

Here in low pressure ducts, pressure lies up to 95 mm of water gauge, and velocity of air through ducts or in ducts is less than 600 fpm and static pressure is less than or equal to 5 cm of water gauge. Class 1 fans are applicable in this case. No specified requirements of class of cleanliness often. space does not require any specific positive pressure. General air conditioning is done under low pressure, which is often surrounded by return air over the false ceiling.

* Medium pressure duct :

Here in this, pressure lies between 95 mm to 170 mm of water gauge. Here velocity of air is smaller or or equal to 600 fpm and static pressure is 5.15 cm of water gauge. Specified requirement of cleanliness is often used and these are used in filters, coils, supply, supply air duct, return air duct, extract air duct, fresh air duct etc. Clean room applications / clean room operations (hospitals, labs) require medium pressure duct because, heavy pressure losses takes place through high efficiency filters. Class 2 fans are applicable in this case.

Here in this, care has to be taken to seal all duct joints to prevent leakages. Joints should be sealed with good quality sealant. Sheets used should be free from flaking or any such defects, as this is likely to add corroded or eroded metal particles to the conditioned air.

* High Pressure duct :

In these type of ducts, pressure lies between 170 mm to 310 mm of water gauge. Velocity of air through ducts is greater than or equal to 600 fpm and static pressure is greater than 16 cm of water gauge. These types of ducts are used for specific class of applications such as in marine applications. Pressure drop in supply air duct is 10 cm to 15 cm of water gauge. Pressure drop in return air duct is 5 cm to 10 cm of water gauge. Here in this, circular or spiral ducting (machine fabricated) is recommended, as helically wound longitudinal joints of circular or spiral ducts provide adequate mechanical strength. Class 3 fans are applicable in this case.

* Flexible Duct :

It is used to connect between main duct and terminal duct (only for branch ducts ). Some of the advantages of flexible ducts are, Less / Negligible leakage occurs in flexible ducts because, the plenum box can be adjusted as per false ceiling or diffuser level. They are easy to install. Low friction losses occur because of the round size of flexible duct in nature (as per installation). These ducts can be rotated up to 180°.

Some of the disadvantages of flexible ducts are, They can be used up to 1.5 m / 5' (5 ft) length ("Stretchable"), because it will bend due to its own weight. Materials used are fiber glass with aluminum foil.

Ducting Materials :

* Galvanized Iron (GI) / Galvanized steel sheet :

Almost 80% of ducting application is used in actual practice. Ducts are usually made from Galvanized Iron (GI) sheet metal, Aluminum sheet metal / Black sheet metal. Most commonly used duct material in air conditioning is Galvanized sheet metal because the fine zinc coating of metal prevents it from rusting and avoids the cost of painting. Sheet thickness of GI duct varies from 26 gauge to 16 gauge or 0.55 mm to 1.5 m. It has low cost and also easy to fabricate. Some of the applications are, Commercial offices, Residential apartments, houses, hospitals, supermarkets etc.

* Aluminum Sheet Metal / Black Steel :

Used for Light weight and resistant to moisture applications, and simultaneously used in medical facilities also, such as in X-ray and MRI rooms, where the magnetic properties of machines get affected with metals. Especially like GI. So, aluminum duct is preferred here in medical facilities, radiation centers etc.

* Carbon steel / Black Iron :

It has resistance to heat and used in high temperature applications. Such as in kitchens or exhaust system ducts.

* Stainless Steel Duct :

It is used in High strength applications. Galvanized steel is Used in marine Applications. Marine Galvanize steel sheet has heavy deposition of zinc, which is used in internal ducting and stainless steel 316L for external ducting.

* Copper ducts :

These are used at very high humidity areas.

* PUF (Poly Urethane Foam) :

These ducts are flexible in nature and do not require insulation, as PUF is insulated material already. But it is costlier, and generate toxic gases in case of fire hazard. So, it is very less preferable ducting material.

* Fiber Glass Duct :

It is reinforced with plastics and are used in low velocity applications only. Otherwise they will generate heavy vibrations.

* Concrete or Asbestos cement ducts :

It is used for underground ducts. These are very less preferable for Modern building/Energy efficient buildings for supply air ducts.

Well here completes the 8th part of the full HVAC Course. Hope you guys got to learn something from this article.. we will continue the ducting session in the next part.. till then, keep learning, keep growing.

Access Previous parts of this course, if you have missed it, by clicking Below..

Part 1 -- Click Here!!

Part 2 -- Click Here!!

Part 3 -- Click Here!!

Part 4 -- Click Here!!

Part 5 -- Click Here!!

Part 6 -- Click Here!!

Part 7 -- Click Here!!