MV Rane - Psychrometry

Heat Pump Laboratory, IITB 1 of 32

PSYCHROMETRY

AND AIR-CONDITIONING PROCESSES

• Moist Air as Mixture of Ideal Gases

Dalton’s law of additive pressures

Amagat’s law of additive volumes

• Psychometric Chart

Properties of Moist Air

DBT, WBT, DPT, RH

Humidity Ratio w kg H2O/kg DA

Specific Enthalpy of moist air h kJ/kg DA

Specific Volume of moist air v m3/kg DA

• Air-Conditioning Processes

Sensible heating and cooling

Cooling and dehumidifying

Heating and humidifying

Mixing of two air streams

ME 306 Applied Thermodynamics Lecture Presented by Prof M V Rane at IIT Bombay

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T-v DIAGRAM FOR WATER VAPOURMoran & Shapiro (2006) Figure 12.4

in an Air-Water Mixture



Heat Pump Laboratory, IITB 2

DALTON’S LAW OF ADDITIVE PRESSURES




AMAGAT’S LAW OF ADDITIVE VOLUMES




RELATING p, V & T

for Ideal Gas Mixtures



p nR T

V

n number of moles of mixture

R Universal Gas Constant, 8.314 kJ/kmol.K

Dalton Model

pi niR T

V

pi yi p

p

1

j

i

pi

Amagat Model

Vi niR T

p

Vi yi V

V

1

j

i

Vi


RELATING p, V & T

Moist Air & Partial Pressures of Dry Air and Water Vapour



Moist Air

pn R T

V

m R M1

T

V

n number of moles of mixturem mass of mixture, kgM molecular weight of mixture, kg/kmol

each mixture component is considered as if it existed alone in the volume V at themixture temperature T while exerting part of the pressure

Partial Pressures of Dry Air and Water Vapour

pa

na R T

V

ma R Ma1

T

V

pv

nv R T

V

mv R Mv1

T

V


RELATING p, V & T

Humidity Ratio and Relative Humidity



mv

ma

mv

ma

Mv pv V R T( )1

Ma pa V R T( )1

Mv pv

Ma pa

0.622pv

p pv

where pg is the saturation pressure of

water at mixture temperature T

yv

yv.sat

pv

pg


RELATING p, V & T

Enthalpy of Moist Air, Water vapour and Dry Air



Enthalpy of Moist Air

H Ha Hv ma ha mv hv kJ

H

ma

ha

mv

ma

hv ha hvkJ

kgdry.air

Enthalpy of Water Vapour in the Moist Air

Reference to steam table data or Mollier diagram for water shows that the enthalpy

of superheated water vapour at low vapour pressures is very closely given by thesaturated vapour value corresponding to the given temperature.

hv hg T( ) an approximationkJ

kgwater

Enthalpy of Dry Air

ha

273.15 K

T

Tcp.a

dkJ

kgdry.air


MOIST AIR COOLED AT CONSTANT PRESSUREMoran & Shapiro (2006) Figure 12.6




DEW POINT TEMPERATUREMoran & Shapiro (2006) Figure 12.6

State of Water for Moist Air Cooled at Constant Mixture Pressure




ADIABATIC SATURATORMoran & Shapiro (2006)

(a) Schematic and (b) Process Representation




WBT MEASURING DEVICESMoran & Shapiro (2006) Figure 12.8

(a) Sling Psychrometer and (b) Aspirating Psychrometer




PSYCHROMETRIC CHARTMoran & Shapiro (2006) Figure 12.10




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PSYCHROMETRIC CHARTMoran & Shapiro (2000)

LINES OF CONSTANT PROPERTY VALUE




WBT & ENTHALPY LINES

Why are they not Exactly Parallel?

• Adiabatic Saturation Process

Adiabatic saturation temperature, tas

Energy balance

• Wet Bulb Temperature

Adiabatic saturation temperature, twb

Energy balance

• What Contributes to the Deviation?

Contribution of the energy entering the adiabatic saturator with the makeup water is

normally much smaller than that of moist air

Enthalpy of entering moist air is very nearly equal to the enthalpy of the saturated

mixture exiting

This slight effect is ignored in some psychometric chart




DIRECTION OF INCREASING PROPERTY VALUES




ASHRAE SUMMER AND WINTER COMFORT CONDITIONASHRAE Fundamentals 1997, Fig 4, pp 8.21

• Air Temperature

22 to 27 C

• Mean Radiant Temperature

Air temperature +10 C

• Humidity

4.5 to 11 g moisture/kg DA

• Air velocity

0.1 to 0.25 m/s

• Summer Design Comfort Conditions are Higher than that in the Winter

This reduces Air Conditioning Load and Save Energy




DEHUMIDIFICATIONMoran & Shapiro (2006) Figure 12.11

(a) Equipment Schematic and (b) Representation on Psychrometric Chart




HUMIDIFICATIONMoran & Shapiro (2006) Figure 12.12

Steam Injection and Water Injection




EVAPORATIVE COOLINGMoran & Shapiro (2006) Figure 12.13

(a) Equipment Schematic and (b) Representation on Psychrometric Chart




ADIABATIC MIXING OF TWO MOIST AIR STREAMS Moran & Shapiro (2006) Figure 12.14

Equipment Schematic

Mass Balance of Dry Air

ma3 = ma1 + ma2

Mass Balance of Water Vapour

ma3 w3 = ma1 w1 + ma2 w2

Energy Balance

ma3 (ha3 -w3 hg3) = ma1 (ha1 -w1 hg1) + ma2 (ha2 -w3 hg2)




ASSIGNMENT PROBLEMMoran & Shapiro (2006) P 12.34 1 of 2

Exploring Psychrometric Principles

Heat Pump Laboratory, IITB 23ME 306 Applied Thermodynamics Lecture Presented by Prof M V Rane at IIT Bombay




Heat Pump Laboratory, IITB 24 of 18ME 306 Applied Thermodynamics Lecture Presented by Prof M V Rane at IIT Bombay


ASSIGNMENT PROBLEMMoran & Shapiro (2006) P 12..48




ASSIGNMENT PROBLEMMoran & Shapiro (2006) P 12.50 1of 2




MV Rane - Psychrometry

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