ACTIVE LEARNING ASSIGNMENT ON REFRIGERATION & AIR CONDITIONING SYSTEM

ANUPAM CHATURVEDI ( 14ME701 )MILAN PATEL ( 14ME702 )SUNIL RATHAVA ( 14ME503 )GAURANG RAVAL ( 14ME504 )RAJ SHAH ( 14ME505 )

DUCT DESIGN & AIR DISTRIBUTION

Contents :-

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

FUNCTION OF DUCT

The 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 DUCT1) According to velocity of air : Low velocity duct High velocity duct

2) According to pressure : Low pressure Medium pressure High pressure

3) According to type of air : Supply air Return air Fresh air

ECONOMIC FACTORS INFLUENCING DUCT LAYOUT

1) Heat 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) 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 a This ratio is an important factor to be

considered in the initial design of duct.

3) 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 .

DUCT DESIGN METHODS

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

1) Velocity reduction method2) Equal friction loss method3) Static regain method

1) 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) 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.

3) 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.

DYNAMIC LOSSES IN DUCT

1 ) Pressure losses due to Enlargement

2 )Pressure loss due to contraction

3 ) Pressure loss at inlet and outlet of a duct

1 ) Pressure losses due to Enlargement

Pr. loss = [ 1-A1/A2 ]² * [ V1/4.04 ]²

A1 = 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 ]² = Enlargement Coefficient

Pr. Loss = Cr*C1* [V1/4.04]² = Cr*C2* [V2/4.04]²

2 )Pressure loss due to contraction

Pr. Loss = [ 1-A1/A2 ]² * [ V1/4.04 ]² = C1 [ V1/4.04 ]² = C2 [ V2/4.04 ]²

Pr. Loss = Cr*C1* [V1/4.04]² = Cr*C2* [V2/4.04]²

3 ) Pressure loss at inlet and outlet of a duct :-

Pr. Loss = C * [ V / 4.04 ]² Where, V = Velocity of Air in Duct C = Loss coefficient

REQUIREMENT OF AIR DISTRIBUTION SYSTEM1) There should be enough entrainment of room

air with the supply air , so that upon reaching the occupied zone, the air stream attains desired temperature.

2) The temperature throughout the occupied zone of the room should be within ± 1ºC of the design temperature.

3) Only minor horizontal or vertical temperature variation should be there in occupied zone.

4) Noise level should be below the objectionable level.

5) Effect of natural convection and radiation within the room should be minimum.

TERMS 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.

4 ) 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.

TYPES OF AIR OUTLETS1 ) Grill outlet :- These outlets have adjustable

bar grills which are the most common types with vertical and horizontal vanes.

2 ) 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.

4 ) 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

m³/m² of the floor area. They are specially suited to large zones.