Air Conditioning Process and Cycles(Colour Version)

8/2/2019 Air Conditioning Process and Cycles(Colour Version)

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Air ConditioningAir Conditioning

Processes and CyclesProcesses and Cycles

2

AB' w

tA tB

hB'

B

A B Sensible heating

A B' Sensible cooling

tB'

hB

Heating / Cooling Coil

ma, hA, tA ma, hB, tB

Sensible heating/cooling

processes


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A

B

wA

wB

tAtB

hA

hB

Cooling and de-humidification

when the coiling coils surface

temperature is below the

dew point temperature

of the entering air;

Cooling Coil

ma, wA, hA, tA ma, wB, hB, tB

Condensate, mw

4

A

B

wA

wB

tAtB

hA

hB

B'

hB'

0

1.0 A

B

sensible / total heat ratio

= (hB - hB) / (hA - hB)


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A

B

wA

wB

tAtB

hA

hB

B'

hB'

BA

BA

w-w

h-h

w

h=

EnthalpyMoisture content

hw=

Sensible HeatTotal Heat

QsQt

=

Line ratio indicator on an

ASHRAE psychrometric chart

6

A

B

wA

wB

tAtB

hA

hB

tC

CwC

CA

CB

CA

CB

CA

CB

tt

tt

ww

ww

hh

hh

ACline

BCline

=

=

Bypass factor =

(1 - ) is called the contact factor.

(apparatus dew point)Cooling Coil(surface temperature = tc

A B

'On' state'Off ' state


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A

B

wA

wB

tAtB

s

r

ts

Heat gain arising from fan power

causing air temperature increase

from tB to ts

/

)(

air)toimparted(energy

/

pa

Bs

pa

CFTP

Cttt

TCV

VFTPenergyFan

=

=

=

=

BA

Fan

Air duct

RoomS

Cooling

coil

8

Adiabatic mixing of two air streams

2

1

w2

w1

t2t1

h2

h1w3

3

t3

h3ma2h2w2

1

2 3

ma1h1w1

ma3h3w3


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2

1

w2

w1

t2t1

h2

h1w3

3

t3

h3

Mass balance: ma1 + ma2 = ma3

Energy balance: ma1 h1 + ma2 h2 = ma3 h3

Moisture balance: ma1 w1 + ma2 w2 = ma3 w3

ma2h2w2

1

2 3

ma1h1w1

ma3h3w3

10

Adiabatic mixing of two air streams2

1

w2

w1

t2t1

h2

h1w3

3

t3

h3

From the three equations:

21

2

21

2

12

13

12

13

VV

V

mm

m

ww

ww

hh

hh

aa

a

&&

&

+

+=

=


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2

1

w2

w1

t2t1

h2

h1w3

3

t3

h3

In the psychrometric chart:

12

13

12

13

)12(

)13(

ww

ww

hh

hh

Segment

Segment

=

=

21

2

21

2

)12()13(

VVV

mmm

SegmentSegment

aa

a

&&

&

+

+=


12

Adiabatic humidification process

2

1

twb

h1

1 2 follows a constant

wet bulb line

ma, t1, h1 , w1 ma, t2, h2 , w2

mw = m (w2 - w1)

h2 = h1 + (w2 - w1) (hf) at twb

h2 h1

Required for winter air conditioning when

there is a cooling load and the weather is dry.


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Adiabatic humidification process

2

1

twb

h1

1 2 follows a constant

wet bulb line

h2 = h1 + (w2 - w1) (hf) at twb

h2 h1

Required for winter air conditioning when

there is a cooling load and the weather is dry.

ma, t2, h2, w2ma, t1, h1, w1

mw = ma (w2 w1)

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Humidification by steam injection

2

1

t1Heat balance: ma h2 = ma h1 + ms hs

Mass balance: ms = ma (w2 - w1)

h2 = h1 + (w2 - w1) hs

t = t2 - t1, which is small

m, h1, t1

m, h2, t2

Steam humidifier


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Tutorial problem:

Saturated water vapour at 100 oC is used to

humidify a stream of moist air. The air enters the

humidifier at 13 oC db and 5 oC wb at a flow rate

of 3 kg/s. The pressure is 101.3 kPa. Determine

(a) the mass flow rate of the steam required to

saturate the air; and

(b) the temperature of the saturated air.

(Ans. 0.0249 kg/s, 14.3 oC)

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Room Space

QLmw

ma

hrwrtr

mahs

wsts Qs

Vs

Vr =Vs

s

rr

Space air conditioning process


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Heat balance:

ma hs + Qt = ma hr

i.e. Qt = space total cooling load

= ma (hr - hs) = Vs (hr - hs)

Space conditioning process

m

c

wm

ws

trtc

s

r

ts

s r : Space conditioning

process

ts , ws , hs tr , wr , hr

Supply air Room air state

ma , Vs

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Heat balance:

hs = cpa ts + ws hfg

hr = cpa tr + wr hfg

Qt = ma (hr - hs) = ma cpa (tr - ts) + ma (wr - ws) hfg

Space conditioning processm

c

wm

ws

trtc

s

r

ts

s r : Space conditioningprocess

Space sensible load, Qs

Space latent load, QL


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SHR = Qs/ (Qs + Ql)

Sensible-total heat ratio (SHR)

m

c

wm

ws

trtc

s

r

ts


process

SHR

Parallel to the line s-r ; calledthe Room Ratio Line

0

1.0

SHR

20

tr

s

r

ts

1.0

0

s'

r'

SHR

s"

Room ratio line

r"

If the supply air state changes at part-load, whilst

tr is maintained, the resultant room RH will shift

away from the design condition.


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Based on the design

space sensible load:

Qs = Vs cpa (tr - ts)

Calculate the requiredsupply air flow rate

m

c

wm

ws

trtc

s

r

ts

s r : Space conditioningprocess

/sm)t-(tC

QV 3

srpa

ss

=

22

Based on the design space

latent load:

QL = Vs (wr - ws) hfg

Calculate the required

humidity ratio of

supply airm

cwrws

trtc

s

r

ts


process

airdrykg/kghV

Q-ww

fgs

Lrs

=


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Summer Air Conditioning Processes

i) Cooling & dehumidification, o so

s

wo

ws

tots

ho

hs r

hr

Coils load, Qc = ma (ho - hs)

= ma (ho - hr) + ma (hr - hs)

ventilation load space cooling load

SO

Room

Cooling

coil

mahsts

maho

O. A. r

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ii) Cooling & dehumidification + Reheat , o c s

Space cooling load

Reheat load

o

c

wo

ws

trtc

sr

ts

ho

hr

hshc

Coil's load

Ventilation load

c

rtr

Conditioned

rooms

Supply air

o

Outdoor air

Cooling coil

Fan

Re-heat coil

Chilled water Hot water


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ii) Cooling & dehumidification + Reheat , o c s

Space cooling load

Reheat load

o

c

wo

ws

trtc

sr

ts

ho

hr

hshc

Coil's load

Ventilation load

Coils load, Qc

= ma (ho - hc)

= ma (ho - hr) + ma (hr - hs) + ma (hs - hc)

ventilation load space load re-heat load

c

rtr

Conditionedrooms

Supply air

o

Outdoor air

Cooling coil

Fan

Re-heat coil

Chilled water Hot water

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Summer Air Conditioning Cycles

iii) Mixing + Cooling & dehumidification + Reheat

(o + r) m c s

a) Mixing outdoor air with return air : (o + r) m

b) Cooling + dehumidification : m c

c) Reheat at off design conditions: c s

c

r

tr

Conditioned

roomm sSupply air

Return Air

o

Exhaust

air

r

Outdoor

air

Cooling

coil

Fan

Re-heat

coil


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Summer Air Conditioning Cycles

iii) Mixing + Cooling & dehumidification + Reheat

(o + r) m c s

Coils load, Qc

= ma (hm - hc)

= mo (ho - hr) + ma (hr - hs) + ma (hs - hc)

ventilation load space load re-heat load

o

c

wo

ws

trtc

sr

ts

ho

hr

hshc

Room

ratio line

wr

hm

m

c

rtr

Conditionedroomm s

Supply air

Return Air

o

Exhaust

air

r

Outdoor

air

Coolingcoil

Fan

Re-heatcoil

28

m

c

wm

ws

trtc

s

r

ts

SHR

0

1.0

SHR

0

1.0

SHR

2

Om

3

r

Room ratio line

SHR

Space cooling with latent load Space heating with latent load

Ls

s

QQ

QSHR

+=


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i) Pre-heat + Humidification + Reheato 1 2 3

2

Room ratio line

ma, to, ho

Re-heater

coil

Preheat coil Water spray

chamber

Heating coils and spray Chamber

ma, t3, h3

O 1

3

r

Sensible-total heat

ratio = Qs / Qt

1 23O

Required in winter situations when:

outdoor air is cold and dry;

net sensible heat loss in space;

the space has moisture gain

Winter Air Conditioning Processes (space heating)

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i) Pre-heat + Humidification + Reheat

o 1 2 3

o 1 : pre-heat outdoor air to twb1 2 : adiabatic humidification at constant twb2 3 : reheat air to t3 ( > tr ) to provide space heating

2

Room ratio line

ma, to, ho

Re-heater

coil

Preheat coil Water spray

chamber

Heating coils and spray Chamber

ma, t3, h3

O 1

3

r

Sensible-total heat

ratio = Qs / Qt

1 23O


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ii) Mixing + Humidification + Reheat(o + r) m 2 3

Preheat can be eliminated or reduced

by allowing recirculation air to mix

with the cold outdoor air.

tr

Room, r

3

Return Air

O

Exhaust air

r

Water spray chamber

m2

Om

3

r

2

Reheat coilOutdoor air


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ii) Mixing + Humidification + Reheat (to give cool supply air)

(o + r) m 2 3

In moderately cold but dry days,

supply air has to be humidified because the O.A. is dry;

space cooling may be required (hence t3 < tr i.e. cool supply air)

because of large internal heat gain;

om

3

r

2

Room

ratio linetr

Room, r

3

Return Air

O

Exhaust air

r

Water spray chamber

m2

Reheat coilOutdoor air

Winter Air Conditioning Processes (space cooling)


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Documents

Air Conditioning Process and Cycles(Colour Version)