06_psychrometrics

International MSc Programme Sustainable Energy EngineeringInternational MSc Programme Sustainable Energy Engineering

SUSTAINABLE ENERGY UTILIZATION

Lecture:- PSYCHROMETRICS

Assist. Prof. Igor BALEN

Psychrometrics

Basic terms- moist air typical atmospheric air which contains a certain amount of water vapor (mixture of two ideal gases - dry air and water vapor)

- dry air O2 & N2 mixture that contains no water vapor- assumed to consist of:Nitrogen N2 : 78 % of volume, M=28 kg/kmolOxygen O2 : 21 % of volume, M=32 kg/kmolArgon Ar: 1 % of volume, M=40 kg/kmolmean molecular mass M=28.96 kg/kmol

PSYCHROMETRICS deals with the thermodynamic properties of moist air and uses these properties to analyze conditions and processes involving moist air.

- considering temperature range 40 to 50C

Psychrometrics

Basic terms- dry-bulb temperature - usually referred to as air temperature, is the air property that is most common used.- when people refer to the temperature of the air, they are normally referring to its dry bulb temperature.- dry-bulb temperature - Tdb, can be measured by using a normal thermometer; it is an indicator of heat content.

- wet-bulb temperature - associated with the moisture content of the air.- Twb can be measured with a thermometer that has the bulb covered with a water-moistened bandage with air flowing over the thermometer.- wet-bulb temperatures are always lower than dry bulb temperatures but they will be identical with 100% relative humidity in the air.

- dew point - Tdp, is the temperature at which water vapor starts to condense out of the air, the temperature at which air becomes completely saturated; above this temperature the moisture will stay in the air.

Psychrometrics

Basic parameters- humidity ratio (moisture content, specific humidity) - the ratio between the actual mass of water vapor present in moist air - to the mass of the dry air- normally expressed in kilogram water vapor per kilogram dry air:

a

w

mmx = [kgwater/kgdry air]

mass of dry air

mass of water

- assuming ideal gas behavior, humidity ratio can also be expressed with the partial pressure of water vapor and total pressure of air:

w

w

ppp.x = 6220 [kgwater/kgdry air]

Partial pressure of water vapor [Pa]

Total pressure of moist air [Pa]

- hard to measure, but very useful in calculations

Psychrometrics

Basic parameters- relative humidity - is the ratio of the partial pressure of water vapor in a given moist air sample to the partial pressure of water vapor in an air sample saturated at the same temperature and pressure:

)T(pp

s

w=the saturation pressure of water vapor in the absence of air at the given temperature T

the partial pressure of water vapor

- relation between humidity ratio and relative humidity:

)T(pp)T(p.x

s

s

=

6220

- easy to measure and useful in some contexts, but often need to know temperature as well

6220.xx

)T(pp

s +=

Psychrometrics

Basic parameters- specific volume function of temperature, pressure and moisture content:

)622.0(6.461 xpTva += [m3/kgdry air]

- density of the moist air:

a

ss v

xTpp +=+= 1

1.287 [kgmoist air / m3]

- the moist air saturation:

)T(pp)T(p.x

s

ss = 6220

1= - for dry air: x =0 kg/kg; =0[kgwater/kgdry air]

Psychrometrics

Basic parameters- specific enthalpy - consist of sensible heat and latent heat; it is important for calculating cooling and heating processes.- enthalpy of moist and humid air includes:

the enthalpy of the dry air - the sensible heat the enthalpy of the evaporated water - the latent heat

- specific enthalpy - h (kJ/kg) of moist air is defined as the total enthalpy of the dry air and the water vapor mixture per kg of dry air.

- specific enthalpy of dry air:

tch p = [kJ/kgdry air]- specific enthalpy of the water vapor:

tcrh v,p += 0 [kJ/kgdry air]

t temperature of air [C]cp specific heat capacity of

air [kJ/(kgK)]cp,v specific heat capacity of

water vapor [kJ/(kgK)]r0 evaporation heat of water

at 0C [kJ/kg]

Psychrometrics

Basic parameters- specific enthalpy of the water - fog:

tc)xx(h ws = [kJ/kgdry air]specific heat capacity ofliquid water [kJ/(kgK)]

- specific enthalpy of the ice:

)qtc)(xx(h iis = [kJ/kgdry air]specific heat capacity ofice [kJ/(kgK)]

melting heat of ice [kJ/kg]

- specific enthalpy of the moist air (unsaturated):

)t.(xt.h ++= 8612501011 [kJ/kgdry air]

Psychrometrics

Basic parameters- specific enthalpy of the moist air containing water fog (saturated):

t.)xx()t.(xt.h ss +++= 1948612501011 [kJ/kgdry air]- specific enthalpy of the moist air containing ice (saturated):

)t.)(xx()t.(xt.h ss 3340928612501011 +++=- mass flow of dry air:

xVma +=

1&&

volume flow of the moist air [m3/s]

density of the moist air [kg/m3]

[kgdry air/s]

Psychrometrics

Psycrometric chart and Mollier (h-x) diagram

DRY BULB TEMPERATURE (F)

80

40

40

60

Wet B

ulb (F

)

50

50

60

70

70

40%

80 90 100 120

20%

80% 60%

90

.004

.016

.012

.008

HUMIDITY RATIO

(Lbv/Lba)

.028

.024

.020

Psychrometric chart Mollier diagram

Psychrometrics

Psycrometric chart and Mollier (h-x) diagram the difference

FLIP

ROTATE

Mollier (h-x) diagram

Saturated - fog

Saturated - ice

Unsaturated

Saturation line


- need two quantities for a state point

- can get all other quantities from a statepoint

- can do all calculations without a chart, but sometimes it requires iteration

- pressure must be specified

- charts available for a range of pressures

- dew point right below the state point onthe saturation line

State point

Dew point

Psychrometer

- adiabatic saturation temperature can be determined (at atmospheric pressure) by using a thermometer whose bulb is covered with a wet bandage

- measuring: webbulb temperature drybulb temperature

- sling psychrometer (the picture)

- electronic humidity sensors availabletoday

Psychrometer

1

stdbtwb

- determination of the state point 1: graphically mathematically:

1

1

xxhhtch

dxdh

s

swbwws

===- i.e. if measured tdb, twb; xs, hs read from the table for saturation line:

wbdb

dbswbs

t.t.t.xt.hx

1948612501011194

1 +=

dbtt =1


s

- for all points on the saturation line -=1, dry-bulb temperature tdb, wet-bulb temperature twb and dew point tdp are the same!


Adiabatic mixing of two air streams of different properties

1

2

M

L1

L2- graphical solution:measure the distancebetween 1 and 2, thencalculate L1 or/and L2

LmmL

LmmL

LLLmmm

M

M

M

&&&&

&&&

22

11

21

21

=

=+=

+=


Adiabatic mixing of two air streams of different properties- mathematical solution:

22

11

22

11

21

hmmh

mmh

xmmx

mmx

mmm

MMM

MMM

M

&&

&&

&&

&&

&&&

+=

+=+=

[kgwater/kgdry air]

[kJ/kgdry air]

[kgdry air/s]


Sensible heating and cooling

1

2

- humidity ratio stays unchanged while temperature increases (heating) or decreases (cooling) and relative humidity changes

)tt(.hh)hh(mQ a1212

1221

011 == && [kW]

h2-h1

h2 =const.

h1 =const.

[kJ/kgdry air]


Cooling and dehumidification

- temperature and humidity ratio decrease1

2h1-h2

)hh(mQ a 2121 = &&

2

[kW]

- the state point 2 determined graphically- the theoretical point 2 read from the table for saturation line


1

2w

s

- liquid water:

- water vapor:

Humidification by injection of water (liquid or vapor)

2v

x2

)xx(mm aw 12 = &&

12

12

xxhhtch

dxdh w

www ===

)xx(mm av 12 = &&

12

12

xxhhh

dxdh v

v ==

from the table for saturated vapor [kJ/kg]


Evaporative cooling (air washing)

1

2

s

- wet-bulb temperature of the entering air stream limits direct evaporative cooling (the diagram) efficiency 80-95%

- wet-bulb temperature of the secondary airstream limits indirect evaporative cooling efficiency 60-75%

Psychrometric chartCondition line for a space

)hh(r)xx(qS 1212 =+&sensible load [kJ/kg]

latent load [kJ/kg]

enthalpy change [kJ/kg]

- condition line:

12

12

12

12

xxr)xx(q

xxhh

xh S

+=

= &

LS

S

QQQSHR &&&+=sensible-to-total heat ratio

06_psychrometrics

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