AIR-HANDLING EQUIPMENT AND SYSTEMS-How do they work

AIR-HANDLING EQUIPMENT AND SYSTEMS-How do they work

1. CHAPTER 7. AIR-HANDLING EQUIPMENT AND SYSTEMS
7.1 Air-Handling Equipment 7.2 Heat Transfer
7.3 Air Cleaning
7.4 Air Mixing 7.5 Fans 7.6 Duct Systems 7.7 Air Devices 78G lG idli f D tS t D i 7
.
8
G
enera
l

G
u
id
e
li
nes
f
or
D
uc
t

S
ys
t
em
D
es
i
gn
7.9 Under-Floor Air Systems

2. 7.1 Air-Handling Equipment
Components of air-handling units:
-
Fan section(s)
-
Heat
-
exchange section
Heat
-
exchange

section
-
Humidification section
-
Filter section
-
Air-mixing section
-
Discharge air plenum

5. 7.2 Heat Transfer
Heat transfer occurs at the heat-exchange section of the air handling unit.
Commonly used heating and cooling media include water, steam, refrigerant, and
electric.
7.2.1 Water Coils 
The water coils are normally constructed of copper tubes and aluminum fins

The

water

coils

are

normally

constructed

of

copper

tubes

and

aluminum

fins
.
Drain pans are required under cooling coils to collect condensate.
The construction of a typical water coil

6. The
p
erformance of a heatin
g
or coolin
g
coil de
p
ends on the desi
g
n of its tubes and fins
,

pggpg,
as well as the size of the coil, including the depth and face area.
The depth of the coils is expressed in rows, which represent layers of tubes that conduct
the heating or cooling fluid.
Tubes of adjacent rows are staggered to gain more contact between air and coil.

7. Depths of heating coils and cooling coils:
-Heating coils: 1 to 4 rows (because of high temperature difference between
the heating fluid and the heated air)
-Cooling coils: 4 to 8 rows (because of low temperature difference between
the coolin
g
fluid and the cooled air
)

g)

Facevelocitiesofheatingcoilsand coolingcoils: Face
velocities
of
heating
coils
and
cooling
coils:
-Heating coils: up to 6m/s -Cooling coils: below 3m/s (to avoid carryover of condensate)

8. 7.2.2 Steam Coils
In steam coils, the tubes are designed for easy drainage of the condensate.

Tow types of steam coil: Tow
types

of

steam

coil:
- Conventional type (single-tube design): supply(steam) and return(condensate) at
different ends of the coil.
- Steam-distribution type (tube-in-tube desi gn): steam is distributed evenly from an
inner orifice tube within the outer heat-
transfer tube.

9. 7.2.3 Electrical Coils Electrical coils may be designed as a part of the air-handling unit or installed on the
ductwork exterior to the air-handling unit.
The heating elements are usually made of a nickel-chromium alloy.
Electric coils have ver
y
low resistance to airflow
,
so hi
g
her velocities can be used than
y,g
with water or steam coils.

10. 7.2.4 Direct Expansion (DX) Coils When the cooling medium is a refrigerant, the cooling coil is designed to allow the
refrigerant to vaporize in the coil.
A typical DX coil consists of a refrigera nt header and many distribution tubes.
Refrigerant Refrigerant inlets
Header
Refrigerant
outlets

11. 7.3 Air Cleaning
Air in urban environment contains many im purities, in the form of gas, liquid, and
solid particulates.
Many of these particulates are classified as pollutants, such as smog, smoke, and
pollen.
In addition, air may contain bacteria and viruses.
Ptilt
Normal Size ()
Ptilt
Normal Size ()
Common suspended particulates in urban air
P
ar
ti
cu
l
a
t
e
(
µm
)
P
ar
ti
cu
l
a
t
e
(
µm
)
Fumes 0.001-1 Tobacco smoke 0.01-1
Smo
g
0.001-2 Bacteria 0.3-30
g
Dust 0.001-20 Pollen 10-100
Viruses 0.003-0.05 Human hair 40-200
Source : ASHRAE Handbook of Fundamentals, 1993.

12. 7.3.1 Means of Cleaning Air Air can be cleaned by passing it through a liquid curtain or spray or through a dry
filter medium.
A liquid curtain or spray may use water or chemical solutions to remove the air
particulates, but these solutions usually serve other functions, such as cooling or
humidification humidification
.
The dry type of filtration is by far the most commonly used method for cleaning air.
Classification criteria of air filters:
-Fil
t
r
at
i
o
n
p
rin
c
i
p
l
e:
Fil
t
r
at
i
o
n
by

t
h
e
m
ed
i
u
m
o
r
by

e
l
ect
r
ostat
i
c

p
r
ec
i
p
i
tat
i
o
n
tato p cpe:
t at o by t e ed u o by e ect ostat c p ec p tat o
- Impingement: Dry medium or viscous medium
- Configuration: Flat or extended surface (pockets, V-shaped or radial pleats)
- Service life: One-time dis
p
osable or renewable
p
- Performance: Low and medium efficiency, high efficiency particulate air
(HEPA), or ultrahigh efficiency (UEPA)
- Special features: Order absorption, disposal of radioactive material, etc.

13. 7.3.2 Typical Air Filters

15. 
Inelectrostaticfilters,dustand In
electrostatic
filters,
dust
and

fumes first are positively charged
at 14,000 V and then enter a
secondelectricfield where they second
electric
field
,
where

they

are pushed by positively charged
plates and attracted to the collector
lt h d ti l
pl
a
t
es c
h
arge
d
nega
ti
ve
l
y.

16. For odor removal, adsorption-type
filters are used to remove gaseous
contaminants from the airstream.
These filters rely on extremely
porous activated charcoal to
collect the contaminant.

17. 7.3.3 Application of Air Filters
For residential and commercial buildings:
- Low-efficiency and medium-efficiency filters are adequate. - Bag-type or pleated filters are used for higher efficiency.

Forhealthcarefacilitiesand laboratories: For
health
care
facilities
and

laboratories:
- The filtration requirements are often dictated by codes and regulations.
-
HEPAfilters are used for clean rooms and special laboratories to maintain very clean HEPA
filters

are

used

for

clean

rooms

and

special

laboratories

to

maintain

very

clean

environments or to remove hazardous particles.
- HEPA filters are expensive and exhibit a high resistance to airflow, so their use is
limit
ed
t
o
th
ese

spec
ial a
pp
li
c
ati
o
n
s.
ed o ese spec pp c o s.
For hazardous materials:
- The filter housing is designed to pull the contaminated filter directly into a plastic bag
so that the filter can be replaced without exposing it to the environment. This feature is
called
“
bag out.
”
called

bag

out.

18. 7.4 Air Mixing
Outside air required for a building is usually ducted to the inlet of
an air-handling unit by mixing with the return air.
The two airstreams(OA, RA) must be balanced with dampers to introduce sufficient
outside air for ventilation, but not so much as to require excessive energy for conditioning
durin
g
extremes of weather.
g
Typical air-mixing box

19. Introducing large quantities of
outside air for ventilation or outside

air

for

ventilation

or

free cooling will tend to over-
pressurize a building.
Therefore provisions are made
for relieving certain portion of
the return air to outside. The choice between a return
air fan and a relief air fan
depends on the relative resistance of the air paths.
A long relief air path that
causes a high resistance to
airflow would favor the use of
li f i f
a re
li
e
f
a
i
r
f
an.
Otherwise, a return air fan is
ffi i t
su
ffi
c
i
en
t
.

20. 7.5 Fans
A fan moves air used in HVAC systems to ventilate or transport heating or cooling.
All fans have a rotating impeller with blades ; this increases the kinetic energy of air by
changing its velocity. The increased velocity is then converted to pressure.
Two basic fan designs: centrifugal and axial.

21. 7.5.1 Application of Fans
Centrifugal fans are generally used for ai
r
-handling application.
Axial fans have the advantage of being compact when installed in line with ductwork.

22. 7.5.2 Controls of Air Volume (Flow Rate)
Outlet dampers are common for small units.
Inlet guide vane dampers are more energy-efficient and are used on larger air-handling
units
Variable-frequency motor speed controllers are the most energy-efficient and have become
it
common
i
n recen
t
years.

23. M t b l d ithi th f bi t i
h dli it b t d
7.5.3 Fan Drives

M
o
t
ors can
b
e p
l
ace
d
w
ithi
n
th
e
f
an ca
bi
ne
t
or a
ir
-
h
an
dli
ng un
it
or can
b
e moun
t
e
d

externally.

Fan wheels can be coupled directly to the motor or can be driven by belts and

Fan
wheels

can

be

coupled

directly

to

the

motor

or

can

be

driven

by
belts

and

pulleys (sheaves).

Direct
-
drive fans operates at the same speed as the motor and are not adjustable

Direct
-
drive

fans

operates

at

the

same

speed

as

the

motor

and

are

not

adjustable
.
Sheaves and belts allow fans to be designe d for slower, quieter operation, and selection
of pulley diameter. of
pulley

diameter.

24. 7.5.4 Fan Performance and Fan Laws
-
Volume of air delivered per unit
The performance of a fan is measured by the following characteristics: -
Volume

of

air

delivered

per

unit

time (airflow rate): m
3
/s (cfm)
-
Pressure created (static, velocity, Pressure
created

(static,

velocity,

and total pressures): Pa (inches of
water column)
- Power input: W (horsepower, hp)
- Mechanical and static efficiency:
percentage
- Other factors, such as sound level in
noise criteria(NC) or decibels(dB),
etc

25. The most common procedures for developing the characteristics of a fan:
- The performance of a fan is tested from tested from shutoff conditions to nearly free delivery
conditions.
At h t ff the d t i letel bl ked ff; t f ee deli e the tlet e i t e i ed ed t
-
At
s
h
u
t
o
ff
,
the

d
uc
t
i
s comp
letel
y
bl
an
ked
o
ff;
a
t

f
r
ee

deli
v
e
ry,
the
ou
tlet
r
e
s
i
s
t
anc
e

i
s r
ed
uc
ed
t
o
zero.
- Between these two conditions
,
various flow restrictions are
p
laced on the end of the duct to
,p
simulate various conditions on the fan. Sufficient points are obtained to define the curve between
shutoff and free delivery conditions.
Pitttbt ftht td t
-
Pit
o
t

t
u
b
e
t
raverses o
f
th
e
t
es
t

d
uc
t
are
performed with the fan operating at
constant speed. The point of rating may
b
e an
y
p
oint on the fan
p
erformance
yp p
curve.
- For each case, the specific point on the
c r e m st be defined b referring to the c
u
r
v
e
m
u
st

be
defined
b
y
referring

to

the

flow rate and the corresponding total
pressure.

27. 
The fan laws in the previous table relate the performance variables for any

The

fan
laws

in

the

previous

table

relate

the

performance

variables

for

any

dynamically similar series of fans. The variables are fan size D; rotational speed N;
gas density r; volume flow rate Q; pressurep
tf
or p
sf
; power W; and mechanical
efficiency
η
t.
efficiency
η
t.

Fan Law 1 shows the effect of changing size, speed, or density on volume flow rate,
p
ressure
,
and
p
ower level.
p,p
Fan Law 2 shows the effect of changing size, pressure, or density on volume flow rate,
speed, and power.
Fan Law 3 shows the effect of changing size, volume flow rate, or density on speed,
pressure, and power.
The fan laws apply only to a series of aerodynamically similar fans at the same point
of rating on the performance curve.
They can be used to predict the performance of any fan when test data are available for
any fan of the same series.

28. An example of the application of the fan laws for a change in fan speed N for a specific
size fan.
For example, point E (N
1
= 650) is
computed from point D (N
2
= 600) as
follows: follows: At D, Q
2
= 3 m
3
/s and = 228 Pa
Using Fan Law 1a at point E,
Q1 = 3×650/600 = 3.25 m
3
/s
Using Fan Law 1b,
P
tf1
= 228(650/600)
2
= 268 Pa
The total pressure curve at N = 650 The
total

pressure
curve
at

N
=
650

may be generated by computing
additional points from data on the base
curve, such as point G from point F.

29. 
If equivalent points of rating are joined as shown by the dashed lines they form

If
equivalent

points

of

rating

are

joined
,
as

shown
by

the

dashed

lines
,
they

form

parabolas, which are defined by the relationship:
Th li i ll d SYSTEM LINE

Th
e
li
ne
i
s ca
ll
e
d

SYSTEM

LINE
.

31. 7.5.5 Examples of Fan Performance Given conditions:
1)An air
-
handling system is designed to circulate 15 000 cfm at a system pressure 2 7 in
1)
An

air
handling

system

is

designed

to

circulate

15
,
000

cfm

at

a

system

pressure

2
.
7

in
.
w.c.
2) A fan is selected on the basis of the previous graph (Barry Blower Model Versacon).
Problems:
1
)
With this
p
articular choice of fan, what should be the fan s
p
eed?
)
pp
2) What is the Brake Horsepower (BHP) of th e electric motor to drive the fan operating at
this condition?
3) If the same fan is operating at 825 rpm, what will be the fan delivery, the air-handling
system static pressure, and the BHP required?
BHP (Brake Horsepower) is the amount of power generated by a motor without considering
any of the various auxiliary components that ma y slow down the actual speed of the motors.
S ti f d t h BHPi d ithi th i ’ t t S
ome
ti
mes re
f
erre
d

t
o as pure
h
orsepower,
BHP
i
s measure
d
w
ithi
n
th
e eng
i
ne
’
s ou
t
pu
t

shaft.

32. Unitsusedinair
-
conditioningengineering
Units
used
in
air
conditioning
engineering
1 hp = 0.7457 kW (approximately 0.75 kW)

33. Answers:
1) From the graph, the intersection of 15,000 cfm with the system curve demands that the
fan run at 1125 rpm.

34. 2) The fan should be driven by a motor having a minimum of 8.55 BHP.

35. 3) At 825 rpm, the fan can deliver 11,000 cfm, operating at 1.46 in. w.c., and drawing
3.39 BHP.

36. 7.6 Duct Systems Ductwork is part of the air-handling system.
Duct system includes ducts for supply air, retu rn air, outside air, relief air, and exhaust
airair
.
Ducts are usually fabricated from sheet metal, such as galvanized steel, aluminum, or
stainless steel.
Some ducts are made with nonmetals
,
such as
p
lastics.
,p

37. 7.6.1 General Classifications for Ductwork
D t k t f HVAC l ifi d b t ti

D
uc
t
wor
k
sys
t
ems
f
or
HVAC
are c
l
ass
ifi
e
d

b
y s
t
a
ti
c pressure.
Recommended air velocities:

38. 7.6.2 Symbols for Sheet Metal Work

There are various standard There
are
various

standard

symbols for sheet metal work
to be used in drawings.
One of the commonly accepted
standards is the SMACNA
Symbols.

39. 7.6.3 Duct Shapes and Insulation Methods
Rectangular ducts
- Mostly used for low-velocity applications.
- Insulation is often applied to the interi or of the duct for both acoustical absorption
and thermal insulation.
- The insulation applied internally is called duct liner.
I li li ti li l t d ti th t f f
-
I
n coo
li
ng app
li
ca
ti
ons,
li
ner a
l
so preven
t
s con
d
ensa
ti
on on
th
e ou
t
er sur
f
ace o
f

the duct.

40. Rdfltldt
7.6.4 Duct Shapes and Insulation Methods

R
oun
d
or
fl
a
t
ova
l

d
uc
t
s:
- Can be used for low-velocity, but these shapes are generally reserved for medium-
and high
velocity ductwork
and
high
-
velocity

ductwork
.
- If insulation is required, it is usually applied externally in the form of a fiberglass
blanket wrap with an external vapor barrier for cooling applications.
-
The vapor barrier is intended to prevent the migration of humid air through the
-
The

vapor

barrier

is

intended

to

prevent

the

migration

of

humid

air

through

the

insulation, which could result in condensation on the surface of a cold duct.
- Internal insulation is also available for round or flat oval ductwork.

41. Gl idtlith tidl d til
7.6.5 Materials of Construction

G
a
l
van
i
ze
d
s
t
ee
l

i
s
th
e mos
t
w
id
e
l
y use
d
ma
t
er
i
a
l
.
Aluminum, stainless steel, and plastic may be used for ducts installed in humid
environments or ducts that carry moist air, such as a dishwasher exhaust.
Fire-resistant steel ductwork is used for kitchen hoods.
Assembly of high-pressure/velocity round ducts
with low-pressure rectangular ducts

42. Rddthth t
ti l it f h t t l d th
7.6.6 Duct Assembly and Air Leakage

R
oun
d
d
uc
t
s
h
ave
th
e mos
t
cross-sec
ti
ona
l
area per un
it
area o
f
s
h
ee
t
me
t
a
l
an
d

th
us are
most economical.
When ceiling space is limited, oval ducts fit the need.

Round or oval ducts are available in double
wall construction to reduce the air

Round
or

oval

ducts

are

available

in

double
-
wall

construction

to

reduce

the

air
-
transmitted and duct-radiated noises.
Ducts should be sealed in compliance with the recommended duct seal levels for
different applications different

applications
.
Exposed supply ductwork in conditioned spaces should be seal level A to prevent dirt
smudges.

Responsibility for proper assembly and sealing belongs to the installing contractor. Responsibility

for

proper

assembly

and

sealing

belongs

to

the

installing

contractor.
Working on duct assembly

43. Duct seal

44. Transverse joints
Longitudinal seams

45. Plastic film and fabric ductwork is good a lternative to sheet metal duct work in
hi h
hidit h i i l d h
hi
g
h
-
h
um
idit
y spaces suc
h
as sw
i
mm
i
ng poo
l
s an
d
green
h
ouses.
The rigidity and tubular shape of these ducts is achieved by internal air
p
ressure, and air is diffused through holes (plastic duct) or through pores
(fabric).
Pl ti
fil d t k( h )
Pl
as
ti
c-
fil
m
d
uc
t
wor
k(
green
h
ouse
)
Different applications of fabric ductwor
k

46. I t i l b ildi d t k h b th ili ith th
7.6.7 Coordination of Ductwork with Other Building Elements

I
n mos
t
commerc
i
a
l

b
u
ildi
ngs,
d
uc
t
wor
k
s
h
ares space a
b
ove
th
e ce
ili
ng w
ith
o
th
er
elements including structural supports, fi reproofing, electrical conduits, sprinkler
piping, and light fixtures.
Clearances must be provided between ductwork and the lighting fixtures below them
to allow the fixtures to be removed (3 inches, approximately 8cm).
Congested ceiling space and the importance of coordination between systems

48. 7.6.8 Materials and Fittings for Sound Control
S d tt t l ll d d t i l d t fitti t i i d

S
oun
d
a
tt
enua
t
ors, a
l
so ca
ll
e
d
soun
d

t
raps, are spec
i
a
l

d
uc
t

fitti
ngs con
t
a
i
n
i
ng soun
d
-
absorbing material faced with perforated metal.
Inner wall is constructed of perforated metal

49. 7.7 Air Devices 7.7.1 General Classifications
Air devices, used for supplying air and removing air from space, are among the few
p
arts of the HVAC system that are visible to the occupants of a building.
Air devices include diffusers, grills and re gisters, flow control devices, and other
i
accessor
i
es.
Grills simply contain vanes; registers contain a
control damper behind the vanes control

damper

behind

the

vanes
.

51. Coanda Effect Ceiling diffusers rely on a phenomenon called the Coanda effect, in which cold air clings
to the ceiling upon being discharged from the diffuser.
The Coanda effect allows the airstream to fall gradually and mix with the room air over a
large area.

52. 7.7.2 Special Concerns for Warm-Air Supply
If ceiling diffusers are used to deliver warm ai r, the warm air has a tendency to stay at
the ceiling, owing to its low density.

Th l it t b ffi i t t t t b l d i i ith ld i

Th
e ve
l
oc
it
y mus
t

b
e su
ffi
c
i
en
t

t
o crea
t
e
t
ur
b
u
l
ence an
d
m
i
x
i
ng w
ith
co
ld
a
i
r.
7.7.3Spacing,Distribution,andArea ofCoverage
The area that can be effectively served by a diffuser is affected by a parameter called
throw
7.7.3
Spacing,
Distribution,
and
Area

of
Coverage
throw
.
If the velocity is too small, complaints of stuffiness will result.
If the velocity is too high, drafts will result.
Throw: The horizontal or vertical axial distance an airstream travels after leaving an air outlet before the maximum stream velocity is reduced to a specified terminal velocity
(e.g., 0.25, 0.5, 0.75, or 1.0 m/s), defined by ASHRAE Standard 70. Room air velocities
less than 0.25 m/s are generally preferred.

53. Ai di t ib ti tt f ili i diff Ai
r
di
s
t
r
ib
u
ti
on pa
tt
ern
f
rom ce
ili
ng a
i
r
diff
users

54. 7.8 General Guidelines for Duct System Design
A well-designed duct system will result in the lowest installation cost and most energy-
efficient operation of the air-handling system.
Duct sizes are selected that will produce the highest possible velocity consistent
with a reasonable friction loss.
Duct installations are most economical when maximum use is made of straight duct runs ,
and the number of fittings is minimized .
Duct systems that are not properly designed will create objectionable noise. Insulation
placed inside the ductwork will attenuate noise radiating from the system.
Doors should be provided in the duct walls to allow access for cleaning inside the duct
and for maintaining components in the ductwork.
Coordination of the ductwork with lighting layout and structural elements is an
important criterion in the architectural design process.

55. Ductwork plan of a simple HVAC system
showing supply air ducts and heating water piping

56. Ceiling grid and lighting fixtures are added for coordination
between different space elements

57. 7.9 Underfloor Air Systems
Delivering air from underfloor is an alternative to conventional overhead air
distribution.

Underfloor distribution for commercial buildings generally uses 60x60cm access
-
floor
Underfloor
distribution

for

commercial

buildings

generally

uses
60x60cm

access
floor

panels supported by pedestals on the structure slab to form an air plenum.
Air is introduced to the space through floor-mounted diffusers which are specially
desi
g
ned with cleanable rece
p
tors for dust and debris.
gp

58. Air is introduced to the space through floor-mounted diffusers.
Ai l i i b l ll 15
℃
18
℃

Ai
r ve
l
oc
i
t
i
es must
b
e
l
ow, temperatures are genera
ll
y
15
℃
to
18
℃
Floor diffuser on access floor
with car
p
et tiles
Specially designed with cleanable
rece
p
tors for dust and debris
p
p

60. Air-handling devices for underfloor HVAC system