PRESENTATION ON DUCT DESIGN

It is an important topic in refrigeration and air conditioning

1. GANDHINAGAR INSTITUTE OF TECHNOLOGY REFRIGERATION AND AIR CONDITIONING ACTIVE LEARNING ASSIGNMENT DUCT DESIGN PREPARED BY VADI RAKSHIT D (150123119055) MECHANICAL 6 D3

2. 2/19/2017 RAC/ALA/6D3 2 Contents :- Function of Duct Classification of Duct Economic factors influencing duct layout Duct design methods dynamic losses & its determination Requirement of air distribution system

3. Function of Duct 2/19/2017 RAC/ALA/6D3 3 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.

4. Classification of Duct 2/19/2017 RAC/ALA/6D3 4 1) 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

5. Economic factors influencing duct layout 2/19/2017 RAC/ALA/6D3 5 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.

6. 2/19/2017 RAC/ALA/6D3 6 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.

7. 2/19/2017 RAC/ALA/6D3 7 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 .

8. 2/19/2017 RAC/ALA/6D3 8 Duct design methods There are mainly three methods which are commonly used for duct design. Velocity reduction method Equal friction loss method Static regain method

9. 2/19/2017 RAC/ALA/6D3 9 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.

10. 2/19/2017 RAC/ALA/6D3 10 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.

11. 2/19/2017 RAC/ALA/6D3 11 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.

12. 2/19/2017 RAC/ALA/6D3 12 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

13. 2/19/2017 RAC/ALA/6D3 13 1 ) Pressure losses due to Enlargement Pr. loss = [ 1-A1/A2 ]² * [ V1/4.04 ] ²

14. 2/19/2017 RAC/ALA/6D3 14 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

15. 2/19/2017 RAC/ALA/6D3 15 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

16. 2/19/2017 RAC/ALA/6D3 16 Requirement of air distribution system 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. The temperature throughout the occupied zone of the room should be within ± 1ºC 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.

17. 2/19/2017 RAC/ALA/6D3 17 Terms used in air distribution 1 ) 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.

18. 2/19/2017 RAC/ALA/6D3 18 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.

19. 2/19/2017 RAC/ALA/6D3 19 Types of air outlets 1 ) Grill outlet :- These outlets have adjustable bar grills which are the most common types with vertical and horizontal vanes.

20. 2/19/2017 RAC/ALA/6D3 20 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.

21. 2/19/2017 RAC/ALA/6D3 21 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.