Unit 9 Air Conditioning System

Unit Nine

Air Conditioning System

I.

Text

1. The meaning of air conditioning

An

air

conditioning

system

is

an

assembly

of

various

components

which

operate

in

a

controlled

manner

to

produce

a

specified condition of the air within a space or building. To provide

complete

air

conditioning

a

system

must

accomplish

all

the

following:

heat,

humidify,

cool,

dehumidify,

ventilate,

filter,

and

circulate.

1.1 Comfort conditioning

The

design

specification

for

a

comfort

conditioning

system

is

intended

to

be

the

framework

for

providing

a

comfortable

environment for human beings throughout the year, in the presence

of sensible heat gains in summer and sensible heat losses in winter.

Dehumidification

would

be

achieved

in

summer

but

the

relative

humidity in the conditioned space would be allowed to diminish as

winter

approached.

There

are

two

reasons

why

this

is

acceptable:

first,

human

beings

are

comfortable

within

a

fairly

large

range

of

humidity, from about 65 percent to about 20 percent and, secondly,

if single glazing is used it will cause the inner surfaces of windows

to stream with condensed moisture if it is attempted to maintain too

high a humidity in winter.

1

Thus, a system

might be specified as capable of maintaining an

internal

condition

of

22

℃

dry-bulb,

with

50

percent

saturation, in

the

presence

of

an

external

summer

state

of

28

℃

dry-bulb,

with

20

℃

wet-bulb,

declining

to

an

inside

condition

of

20

℃

dry-bulb,

with an unspecified relative humidity, in the presence of an external

state of -2

℃

saturated in

winter.

1.2 Industrial conditioning

Here

the

picture

is

quite

different.

An

industrial

or

scientific

process may, perhaps, be performed properly only if it is carried out

in an environment that has values of temperature and humidity lying

within well defined limits. A departure from these limits may spoil

the

work

being

done.

It

follows

that

a

choice

of

the

inside

design

condition

is

not

based

on

a

statistical

survey

of

the

feelings

of

human beings but on a clearly defined statement of what is wanted.

Thus, a system might be specified to hold 21

℃

±

0.5

℃

, with 50

percent saturation ±

2.5 percent, provided that the outside state lay

between 29.5

℃

dry-bulb, with 21

℃

wet- bulb and

–

4

℃

saturated.

2. System Categories

Air

conditioning

systems

may

be

classified

as

central-station

or

unitary

systems.

A

central-station

system

is

one

in

which

the

components

of

the

system

are

all

grouped

together

in

one

central

mechanical

room

and

the

conditioned

air

is

sent

from

there

to

the

spaces to be conditioned through extensive ductwork.

Unitary

systems

make

use

of

factory-assembled

and

tested

air

conditioning

units.

These

are

usually

installed

in

or

immediate

adjacent to a zone or space to be conditioned. The source of the heat

and

the

refrigeration

may

be

in

each

air

conditioning

unit

or

in

a

“

central

”

mechanical room.

Another

possible

classification

divides

air

conditioning

systems

into cooling systems and heating systems. Cooling systems may in

turn

to

be

classified

in

terms

of

the

cooling

medium

and

cooling

surface used. Heating systems are classified as warm-air systems or

radiant-heat system.

The basic systems and their several variants will be discussed in

the following section.

2.1 Central-station System

A central-station year- round air conditioning system is one which

usually

serves

a

single

large

building

or

space.

The

term

“

central- station

”

is

ordinarily

not

applied

to

systems

of

less

than

about 25 tons of

cooling capacity, circulating about 10,000 cfm of

conditioning

air.

Buildings

whose

demand

is

less

than

this

are

usually

more

economically

air

conditioned

by

unitary

equipments,

all field-assembled.

A general arrangement of the component parts of a central system

suitable

for

year-round

air

conditioning

with

close

control

of

temperature

and

humidity

is

illustrated

in

Figure

usually

consisted of following components:

(1) Coils for cooling and dehumidifying, either for chilled water or

for direct expansion ofthe refrigerant

(2) Coils for heating, supplied with steam or hot water

(3) Blower and driving motor

(4) Sprays for cooling and dehumidifying or for washing the air

(5) Air-cleaning equipment such as filters, electrostatic precipitators,

odor-removing equipment, and germicidal lamps

(6) A wide variety of control devices.

Fig.9.1 Equipment Arrangement for Central Systems

The

central- station

air

conditioning

plant

must

be

supplied

with

either chilled water or refrigerant for the cooling coils and with hot

water or steam for the heating coils. Refrigeration condensing units

and

steam

or

hot-water

boilers

for

these

purposes

may

be

in

the

same

mechanical

room

with

the

air

conditioner

or

may

be

in

an

adjoining room or in the basement.

2.2 Capacity considerations.

It

will

be

recalled

that

heat

transfer

between

air

and

metal

surfaces

requires

rather

large

surfaces

areas.

Heating

and

cooling

coils handling air at face velocities of around 500 fpm must present

about 2 sq ft of face area (four-row coil) for each 1,000 cfm of air

handled.

Physical

size

of

coil

filter

banks

is,

then,

one

of

the

limiting

factors

influencing

the

design

of

central-station

systems.

Space between

floors

of

a

building

usually

does

not

exceed

14

ft,

and

since

floor

area

utilized

is

a

justifiable

charge

against

the

operating expense of the system, it is not good practice to expand

horizontally much beyond 12 to 14 ft, which means that a practical

limitation of somewhere around 40,000 cfm or 100 tons of cooling

capacity is set for single air conditioning units

.

2

If the job requires

more

total

capacity

than

this,

two

or

more

“

zones

”

should

be

planned

with

an

air

conditioning

system

installed

to

handle

each

zone.

Blowers

are

manufactured

commercially

in

capacities

up

to

200,000

cfm,

but

the

associated

equipment

for

units

handling

this

much air is heavy, difficult to fabricate and install, and difficult to

service. Supply and return ductwork and outside air intakes have to

be

extremely

large

for

units

handling

over

50,000

cfm,

and

the

problem

of

locating

these

ducts

in

the

building

structure

is

extremely difficult.

The nature of the building itself frequently determines the extent

of zoning to be specified, a theatre or department- store installation,

where spaces of large cubical content are the rule, may usually be

most

economically

conditioned

by

only

a

few

(perhaps

only

one)

central- station air conditioning units. Office buildings, schools, and

similar buildings with many rooms lend themselves to more zoning,

with

air

conditioning

units

of

10,000

to

30,000

cfm

being

in

common use. As a general rule, considering all the factors involved,

it

is

more

economical

to

pipe

chilled

water,

hot

water,

steam,

or

refrigerant to several smaller air conditioning units than it is to erect

massive supply and return ducts to serve the extended runs from one

extremely large unit.

3

2.3 Filter and mixing plenums

Return air and fresh outside air should be brought into a plenum

for mixing prior to being filtered. Motorized dampers usually central

the amount of outside air introduced. The damper may be controlled

by

a

thermostat

to

admit

less

outside

air

when

extreme

outdoor

temperatures

occur

or

by

smoke

and

odor-sensing

devices

in

the

conditioned space in order to increase the amount of ventilation air

as needed, owing to the nature of the occupancy.

Outside-air

quantity

should

periodically

be

markedly

increased

for

short

periods

of

time

in

all

buildings

to

prevent

a

situation from developing. In seasons between heating and cooling,

it is desirable to operate on 100 per cent outside air.

Filter banks are ordinarily assembled vertically, and on large jobs

the

filters

are

usually

of

the

cleanable

type

for

use

over

and

over

again. Disposable filters are occasionally used for central system of

less

than

10,000

cfm

and

are

rather

universally

used

for

smaller

capacity unitary equipment.

2.4 Refrigeration and heating equipment

These

equipments

are

certainly

factors

in

the

design.

In

systems

requiring

only

one

or

two

zones,

where

the

air

conditioning

units

can be located close to the cooling and heating equipment, the usual

practice

is

to

use

direct

expansion

refrigerant

coil

and

hot-water

coils. If, on the other hand, many zones at remote locations are to be

served

by

a

single

cooling

and

heating

plant,

chilled

water

is

the

cooling medium circulated to the air conditioning coils. Either hot

water

or

steam

may

be

used

as

the

heating

medium.

The

water

is

pumped to the mechanical rooms which serve the several zones.

2.5 Always be insulated

The entire housing for the central-station air conditioning unit is

usually

insulated

over

its

outer-surface

with

at

least

l-in-thick

insulating

board.

All

steam,

hot

water,

chilled

water,

and

suction

refrigerant

piping

should

be

insulated

to

prevent

heat

losses

and

gains.

2.6 Noise and vibration

These

two

factors

are

of

extreme

importance.

If

air

conditioning

central systems are located on upper floors, it must be ascertained

that

the

building

stress

design

has

taken

these

loads

into

account.

Quiet-operating

fan motors should

be

specified,

vibration dampers

should

be

installed

under

all

rotating

equipment,

and

acoustic

linings specified for mechanical rooms. Ducts and duct turns should

be

lined

at

critical

places

with

acoustic

board,

and

extreme

care

must be exercised in duct design so that high velocity whistle will

not occur. Selection of grilles and registers must also be done with

great care or undue air noises will result.

3. Unitary Air Conditioning Systems.

There are three broad categories of unitary systems: the fan-coil

unit system, the duct-type system, and the high-velocity conduit unit

system.

3.1 fan-coil unit system