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GANDHINAGAR INSTITUTE OF TECHNOLOGYREFRIGERATION AND AIR CONDITIONINGACTIVE LEARNING ASSIGNMENTDUCT DESIGN PREPARED BY VADI RAKSHIT D (150123119055)

MECHANICAL 6 D3

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2/19/2017RAC/ALA/6D32Contents :-

Function of DuctClassification of DuctEconomic factors influencing duct layoutDuct design methodsdynamic losses & its determinationRequirement of air distribution system

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Function of Duct2/19/2017RAC/ALA/6D33The function of duct is to transmit air from the air handling apparatus to the space to be air conditioned.To fulfil the above function, the system must be designed within the prescribed limits of available space, friction loss, velocity, sound level and heat losses.Duct system are also used to bring cooled air to the proper distribution points where several rooms are to be conditioned.

Classification of Duct2/19/2017RAC/ALA/6D341) According to velocity of air :Low velocity ductHigh velocity duct

2) According to pressure :Low pressureMedium pressureHigh pressure

3) According to type of air :Supply airReturn airFresh air

Economic factors influencing duct layout

2/19/2017RAC/ALA/6D35Heat gain or loss from the duct :

The heat gain or loss in the supply and return duct system is considerable.This occurs due to the duct passing through the unconditioned space.The allowance must be made for duct heat loss or gain in load.

2/19/2017RAC/ALA/6D362) Aspect ratio of the duct :Aspect ratio is defined as the ratio of longer side of the rectangular duct to shorter side of rectangular duct.Mathematically, Aspect ratio = a/b

b aThis ratio is an important factor to be considered in the initial design of duct.

2/19/2017RAC/ALA/6D373) Duct friction :If duct friction rate is higher, higher will be the size and aspect ratio.This will be increase the initial and running cost.

4) Type of fittings :The fittings can be divided into class-A and class-B .It is desirable to use fittings of class-A as fabrication cost of class-A is 1/2.5 to that of claas-B .

2/19/2017RAC/ALA/6D38Duct design methods

There are mainly three methods which are commonly used for duct design.

Velocity reduction methodEqual friction loss methodStatic regain method

2/19/2017RAC/ALA/6D391) Velocity reduction method :

In this method the duct designed in such a way that the velocity decreases as flow proceeds.The pressure drops are calculated for this velocities for respective branches and main duct.The duct size are determined for assumed velocities and known quantities of air to be supplied through the respective ducts.

2/19/2017RAC/ALA/6D3102) Equal friction loss method :

In this method, the frictional pressure drop per unit length of duct is maintained constant throughout the duct system.The procedure is to be select a suitable velocity in the main duct from the sound level consideration.Knowing the air flow rate and the velocity in the main duct, the size and friction loss are determined from the friction chart.

2/19/2017RAC/ALA/6D3113) Static regain method :For the perfect balancing of the air duct layoiut system, the pressure at all outlets must be made same.This can be done by equalizing the pressure losses in various branches.This is possible if the friction loss in each run is made equal to the pressure gain due to reduction in velocity.

Advantages :It is possible to design long run as well as short run for complete regain.It is sufficient to design the main duct for complete regain.

2/19/2017RAC/ALA/6D312Dynamic losses in duct1 ) Pressure losses due to Enlargement

2 )Pressure loss due to contraction

3 ) Pressure loss at inlet and outlet of a duct

2/19/2017RAC/ALA/6D3131 ) Pressure losses due to Enlargement

Pr. loss = [ 1-A1/A2 ] * [ V1/4.04 ]

2/19/2017RAC/ALA/6D314A1 = Cross section area of the duct at section 1-1

A2 = Cross section area of the duct at section 2-2

V1 = Velocity of air at section 1-1

C1 = [ 1-A1/A2 ] = Enlargement Coefficient

Pr. Loss = Cr*C1* [V1/4.04] = Cr*C2* [V2/4.04]

2/19/2017RAC/ALA/6D3152 )Pressure loss due to contraction

Pr. Loss = [ 1-A1/A2 ] * [ V1/4.04 ] = C1 [ V1/4.04 ] = C2 [ V2/4.04 ]

2/19/2017RAC/ALA/6D316Requirement of air distribution systemThere should be enough entrainment of room air with the supply air , so that upon reaching the occupied zone, the air stream attains desired temperature.The temperature throughout the occupied zone of the room should be within 1C of the design temperature.Only minor horizontal or vertical temperature variation should be there in occupied zone.Noise level should be below the objectionable level.Effect of natural convection and radiation within the room should be minimum.

2/19/2017RAC/ALA/6D317Terms used in air distribution1 ) Outlet :- It is an opening through which air is supplied to the conditioned space.

2 ) Intake :- It is an opening through which air is return from the space.

3 ) Grills :- Grills provide decorative covering for an outlet or inlet.

2/19/2017RAC/ALA/6D3184 ) Diffuser :- It is an outlet grille designed to guide the direction of the air.

5 ) Register :- It is a grille provided with a damper or control valve.

6 ) Drop :- It is the vertical distance that the lower edge of the horizontally projected air stream drops between the outlet and the end of its throw.

2/19/2017RAC/ALA/6D319Types of air outlets1 ) Grill outlet :- These outlets have adjustable bar grills which are the most common types with vertical and horizontal vanes.

2/19/2017RAC/ALA/6D3202 ) Slot diffuser :- It is an elongated outlet with an aspect ratio 25 : 1 and maximum height of 7.5 cm. They are used in side walls but at a higher height of the floor.

3 ) Ceiling outlets :- They are mounted in the ceiling. Multi-passage round, square or rectangular are most common type.

2/19/2017RAC/ALA/6D3214 ) Perforated ceiling :- In this case, confined space above the ceiling is used as supply plenum.The air from the plenum is supplied to the room through small holes or slots.The air is supplied at the rate of 0.3 to 4.5 m/m of the floor area.They are specially suited to large zones.