s g@ps ps g@ 1.88 kJ/kg K - Weebly · 2018. 10. 14. · Slide Nr. 3 of 14 Slides Specific Enthalpy...

Slide Nr. 1 of 14 Slides

Specific Enthalpy of Moist Air

Mon 2:04:28 PM

The enthalpy of a mixture is the sum of the enthalpies of theindividual constituents

ssaa hmhmhm +=

Enthalpy of mixture per unit mass of dry air

Chapter 2:

Psychrometry and Air-Conditioning

Dr. Eng. Sameh Shaaban

3 November 2009


sas

a

sa

a

hhhm

mh

m

hmω+=+=

At low partial pressures the specific enthalpy of water vapor can be expressed as

( )ss p@gpsp@gs ttchh −+=

1.88 kJ/kg K



Mon 2:04:28 PM

Since the specific enthalpy of a vapour from steam tables isexpressed above a datum of approximately 0 °C then the specificenthalpy of dry air in the mixture is also expressed above thesame datum

tch paa = cpa = 1.005 kJ/kg K

Chapter 2:



3 November 2009

( )[ ]ss p@gpsp@gpa

a

ttchtcm

hm−+ω+=


Over the temperature range encountered in air-conditioning the term(hg@ ps

)- cps(tg@ ps

) may be taken as a constant, C



Mon 2:04:28 PM

[ ]tcCtcm

hmpspa

a

+ω+=

cpa = 1.005 kJ/kg K C = 2500 kJ/kg cps = 1.88 kJ/kg K

Chapter 2:



3 November 2009

Alternatively, since for low pressures the enthalpy of superheatedvapour is approximately equal to the saturation value at the sametemperature, then

t@gpa

a

htcm

hmω+=


Specific Heat Capacity of Moist Air

Mon 2:04:28 PM

ssaa hmhmhm +=

tcmtcmtcm s,psa,pap +=

Specific heat capacity of mixture per unit mass of mixture:

Chapter 2:



3 November 2009

s,ps

a,pa

p cm

mc

m

mc +=

Specific heat capacity of mixture per unit mass of mixture:

Specific heat capacity of mixture per unit mass of dry air:

s,pa,ppma ccc ω+=


Specific Volume of Dry Air

Mon 2:04:28 PM

aa

aa

1

p

TRv

ρ==

Chapter 2:



3 November 2009

aa

a

aa V

v

Vm &

&

& ρ==


Air Conditioning Systems

Mon 2:20:10 PM

a. Conventional

the air is processedin a central plant andis distributed to theconditioned spacesvia ducts

Chapter 2:



3 November 2009

via ducts



Mon 2:20:10 PM

b. Terminal reheat

air supply to the units in theroom is from ducting as in

Chapter 2:



3 November 2009

room is from ducting as in(a) and provides the coolingand dehumidification load;the units in the room,provided with water coils,supply the necessaryreheat.



Mon 2:20:10 PM

c. Induction

similar to (b) but only asmall quantity of primaryconditioned air is suppliedto each unit in the rooms

Chapter 2:



3 November 2009

to each unit in the roomswhere it expands throughnozzles thus inducing alarge volume of secondaryroom air into the unit; thesecondary air passes over acoil before mixing with theprimary air, the mixture thenbeing delivered to the room



Mon 2:20:10 PM

d. Fan-coil

Chapter 2:



3 November 2009

air is drawn from the room and from outside the building into, theroom units and passed over coils as necessary, the coils carryingchilled or hot water supplied from a central plant.



Mon 2:20:10 PM

e. Dual-duct

twin ducts of high-velocity air, one with hot air the other with coldair, are supplied to room units from a central plant as in (a); mixing

Chapter 2:



3 November 2009

air, are supplied to room units from a central plant as in (a); mixingat the units gives the required condition of the room air.



Mon 2:20:10 PM

f. Variable air volume (VAV)

a high-velocity flow of cooled airis supplied in a single duct andthe change of load is met by

Chapter 2:



3 November 2009

the change of load is met byvarying the air volume whilemaintaining the same supplytemperature; winter conditionscan be catered for using aterminal reheat unit as (b) or byadding a second duct carryinghot air with a mixing box as in(e) or by providing a perimetersystem of LP hot-water heating.



Mon 2:20:10 PM

g. Panel air systems

chilled water is circulated

Chapter 2:



3 November 2009

chilled water is circulatedthrough radiant ceiling panels ata temperature above the roomdew point.



Mon 2:20:10 PM

h. Integrated environmental design

this is an all-embracing term to cover complex systems

Chapter 2:



3 November 2009

this is an all-embracing term to cover complex systemsincorporating some of the features described above, but with theemphasis on energy recovery

Slide Nr. 14 of 14 SlidesThu 8:46:56 AM

Chapter 2:



3 November 2009

s g@ps ps g@ 1.88 kJ/kg K - Weebly · 2018. 10. 14. · Slide Nr. 3 of 14 Slides Specific Enthalpy...

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