DESIGN OF AC DUCT , FAN FTP

1. “Refrigeration Equipment & its
application”
(RME-604, ME VI SEM, UNIT 5 )
PRESENTED BY:
KRISHNA KUMAR KAROTHIYA
ASSISTANT. PROFESSOR

2. CONTENTS
• RAC Equipments such as compressors.
• Condensers
• Evaporators
• Expansion devices
• Air washers, cooling towers
• Food preservation, cold storage
• Freezers, ice plant, water cooler
• Analysis of ducts & fans
• Basic difference between comfort and
industrial air conditioning

3. Flow of air through ducts:
• fundamental equation to be used in the
analysis of air conditioning ducts is the
Bernoulli’s equation. Bernoulli’s equation
is valid between any two points in the
flow field when the flow is steady,
irrotational, inviscid and
incompressible.

4. Flow of air through ducts:

5. Fan Total Pressure (FTP).
Thus to evaluate FTP, one needs to know the static
pressures at sections 1 and 2 (p1, p2), air velocities at 1
and 2 (V1, V2), datum at 1 and 2 (Z1, Z2) and the head
loss Hl. Normally, compared to the other terms, the
pressure change due to datum is negligible .
Thus to find FTP, one has to estimate the total pressure
loss as air flows through the duct from one section to
other

6. Estimation of pressure loss in ducts
The pressure drop is due to:
• 1. Fluid friction
• 2. Momentum change due to change of direction
and/or velocity
• The pressure drop due to friction is known as
frictional pressure drop or friction loss, Δpf. The
pressure drop due to momentum change is known
as momentum pressure drop or dynamic loss, Δpd.
Thus the total pressure drop Δpt .

7. frictional pressure drop in ducts
• The Darcy-Weisbach equation
where f is the dimensionless friction factor, L is the length of the duct and
D is the diameter in case of a circular duct and hydraulic diameter in case
of a non-circular duct.
•frictional pressure drop in a circular
duct

9. Rectangular ducts
• Even though circular ducts require the least material
for a given flow rate and allowable pressure drop,
rectangular ducts are generally preferred in practice
as they fit easily into the building construction thus
occupying less space, and they are also easy to
fabricate.
Equivalent diameter :

10. Dynamic losses in ducts
• Dynamic pressure loss takes place whenever there is a change in either
the velocity or direction of airflow due to the use of a variety of bends
and fittings in air conditioning ducts
DYNAMIC PRESSURE LOSS THROUGH VARIOUS FITTINGS
1. Turns, bends or elbows

11. 2.BRANCH TAKE OFFS

12. 3. Sudden enlargement
• The pressure loss due to sudden enlargement,
shown in ΔPd,enL is given by

13. 4. Sudden contraction

14. Duct design methods
1.Velocity method
2. Equal Friction Method
3. Static Regain method

15. Velocity method
The various steps involved in this method are:
i. Select suitable velocities in the main and branch ducts
ii. Find the diameters of main and branch ducts from
airflow rates and velocities for circular ducts. For
rectangular ducts, find the cross-sectional area from
flow rate and velocity, and then by fixing the aspect
ratio, find the two sides of the rectangular duct
iii. From the velocities and duct dimensions obtained in
the previous step, find the frictional pressure drop for
main and branch ducts using friction chart or
equation.
From the duct layout, dimensions and airflow rates, find
the dynamic pressure losses for all the bends and
fittings
v. Select a fan that can provide sufficient FTP.

16. Velocity method
• the method is not very efficient as it
requires partial closing of all the dampers
• For example, let the duct run A-C-G-H
be the index run and the total pressure
drop in the index run is 100 Pa. If the
pressure drop in the shortest duct run
(say A-B) is 10 Pa, then the damper in
this run has to be closed to provide an
additional pressure drop of 90 Pa.

17. Equal Friction Method

18. Equal Friction Method

19. • If the ducts are rectangular, then the two sides
of the rectangular duct of each run are obtained
from the equivalent diameter
• v. the total frictional pressure drop of that run
is obtained by multiplying the frictional
pressure drop per unit length and the length,

21. The following figure shows a typical duct layout. Design the duct system using a)
Velocity method, and b) Equal friction method. Take the velocity of air in the main
duct (A) as 8 m/s for both the methods. Assume a dynamic loss coefficient of 0.3 for
upstream to downstream and 0.8 for upstream to branch and for the elbow. The
dynamic loss coefficients for the outlets may be taken as 1.0. Find the FTP required
for each case and the amount of dampening required.

26. Equivalent diameter for each
branch

28. From the example, it is seen that the Velocity method results in larger duct diameters
due to the velocities selected in branch and downstream.
However, the required FTP is lower in case of velocity method due to larger ducts.
Equal Friction method results in smaller duct diameters, but larger FTP.
Compared to velocity method, the required dampering is more at outlet 1 and less at
outlet 2 in case of equal friction method.

29. THANK YOU