ashrae tech conference 2008 psychrometrics presentation · 2012-05-26 · learning objectives...

Terms, Chart, Air Mixing, and Basic Calculations

Tim Junker, P.E.

Psychrometric Fundamentals

Learning ObjectivesUnderstand the significance of psychrometrics to “air conditioning”Understand common psychrometric terms including: dry bulb and wet bulb temperature, relative humidity, specific humidity, dew point, enthalpy, flow rate, & “standard air”Identify all lines and units of measure on the psychrometric chartGiven any 2 properties of air, use a psychrometric chart to determine all other other psychrometric propertiesUnderstand the relationship between sensible, latent, and total heatUnderstand air mixing and be able to calculate the properties of the air resulting from the mixture of two airstreamsUse the basic psychrometric equations to calculate sensible, latent, and total cooling/heating

What is air conditioning?TemperatureMoistureCleanlinessVentilationSound

Simultaneous control

Simultaneous control

Of Of ……....

What is Psychrometrics?The study of the state of the atmosphere with regard to its temperature and moisture content.

AIR

1

23

5

60

Pounds

21% Oxygen

78% Nitrogen

1% Other

What is air?

Standard Conditions“Standard conditions at SL”

70° F29.921 in. Hg (atmospheric pressure)Dry (no water vapor)

Spec. vol. of air at std conditions = 13.35 cu. ft. / lbAir flow rate expressed as: cubic feet / minute (CFM)Air flow rate at standard conditions expressed as (SCFM)Specific volume changes with temperature, moisture content, and atmospheric pressure

At 5,000 ft atmospheric pressure is 24.896 in. HgDensity of dry air at 70°F at 5,000 ft is 16.04 cu. ft. / lb

Standard Conditions

SCFM = 13.35 cu. ft. / lbActual sp. vol. X Actual CFM

At 5,000’ Elevation

0.83 SCFM = 13.35 cu. ft. / lb16.04 cu. ft. / lb X 1 CFM

Water VaporNot present in large quantitiesSignificant factor in air conditioning

40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028

Dry-bulb temperature

Specific Hum

idity (lbs. moisture/ lb. air)

40°

50°

70°

60°

80°

40° 50° 70°60° 80° 90° 100° 110° 120°Dry-bulb temperature

Saturat

ion Line

40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


Specific Hum

idity (lbs. moisture/ lb. dry air)

Saturat

ion Line

1 pound7000Grains

Moisture - Units of Measure

40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


75° Dry-bulbSpecific Humidity of0.010 lbs/lb

Saturat

ion Line

Specific Hum

idity (lbs. moisture/ lb. dry air) or (grains/lb. D

ry air)

40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


Suppose this air is cooled

Saturat

ion Line

Specific Hum


40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


Saturat

ion Line

Suppose this air is cooled

Does its moisture Content Change?

Specific Hum


Dew Point

40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028

80%

Rela

tive H

umid

ity

60%

Rela

tive H

umid

ity

40% R

elativ

e Humidity

20% Relative Humidity


100%

Rela

tive H

umid

ityR.H. =Actual moisture

Maximum moisture

Specific Hum


Sling PsychrometerThe sling psychrometer is a simple and practical device for measuring the moisture content of air.

40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


Wet-bulb is a practical indicator of the moisture content in a sample of air

40°

50°

70°

60°

80°

Specific Hum


40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


40°

50°

70°

60°

80°

Enthalp

y -H (B

TU / lb.

of dry

air)

15.0

20.2

34.1

43.7

Specific Hum


40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


40°

50°

70°

60°

80°

Specific Volume (Cubic ft / lb dry air)

15.0

14.5

14.0

13.5

13.0

Specific Hum


40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


Specific Hum

idity (lbs. moisture/ lb. air)

40°

50°

70°

60°

80°

Enthalp

y (BTU / l

b. of

dry ai

r)

15.0

34.1

43.7

20.2

80%

Rela

tive H

umid

ity

60%

Rela

tive H

umid

ity

40% R

elativ

e Humidity

20% Relative Humidity10

0% R

elativ

e Hum

idity 15.0

14.5

14.0

13.5

13.0

SpecificHumidity

Dew Point

Relative

HumidityWet-bulb

Dry-bulb

Given any two properties...All other properties can be determined Specific

Volume

40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


90° F Dry-bulb70° F Wet-bulbGiven any two properties...All other properties can be determined

40°

50°

70°

60°

80°

Specific Hum


80%

Rela

tive H

umid

ity

60%

Rela

tive H

umid

ity

40% R

elativ

e Humidity

20% Relative Humidity10

0% R

elativ

e Hum

idity 15.0

14.5

14.0

13.5

13.0

15.0

34.1

43.7

20.2

40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


A - HUMIDIFYING ONLYB - HEATING AND HUMIDIFYINGC – SENSIBLE HEATING ONLYD – CHEMICAL DEHUMIDIFYINGE – DEHUMIDIFYING ONLYF – COOLING AND DEHUMIDIFYINGG – SENSIBLE COOLING ONLYH – EVAPORATIVE COOLING

40°

50°

70°

60°

80°

Specific Hum


A

B

C

D

E

F

G

H

Mixing Air StreamsThe result of mixing two airstreams always falls on a line connecting the two points on a psychrometric chart.

Mixing Air Streams

RA

OA

MA

OA = 95DB/78WB 1000 CFMRA = 78DB/65WB 4000 CFM

1) Draw a line between OA & RA

2) Determine the relative percentage of OA & RATotal CFM = OA + RA = 5000 CFMOA = 1000/5000 = 20%RA = 4000/5000 = 80%

3) Determine the DB or WB temperature of the mixed point using ratios. Using DB:Delta T = 95-78 = 17FRA Delta = 17F X 0.20 = 3.4 FMixed Air DB = 78 +3.4 = 81.4 F

4) The mixed air point lies on the mixing line at the 81.4F pointWB = 67.3 F

40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


Dew PointSatu

ration Line

Specific Hum


SENS COOLINGSENS AND LATENT COOLING

Fan

Cooling C

oil

SA

MA

FAN HEAT

Sensible Heat Ratio (SHR)

28

Room design for: 78° / 50% RH Sensible heat gain = 150,000 BTUHLatent heat gain = 30,000 BTUH

78°

50% RHSHR = 150,000

150,000 + 30,000

SHR = 0.833

SHR = QSensible

QSensible + QLatent= QSensible

QTotal

Sensible Heat Ratio (SHR)Sensible Heat Ratio (SHR)

29


SHF

.833

78°

50% RHSHR = 150,000

150,000 + 30,000

SHR = 0.833

SHR = QSensible


QTotal

Sensible Heat Ratio (SHR)

30


SHF

.889

78°

50% RH

40% RH

75°

ParallelSHR = 150,000

150,000 + 30,000

SHR = 0.833

SHR = QSensible


QTotal

40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


Dew PointSatu

ration Line

Specific Hum



Fan

Cooling C

oil

SA

MA

FAN HEAT

SENS AND LATENT GAIN (ROOM)

Room

RA

FormulasQsensible = SCFM (ft3/Min) x ∆ T (°F) x 1.085 (BTU*Min)/(ft3*Hr * °F) (Sometimes also see 1.1, 1.09, or 1.08)

Qsensible (5000 ft) = CFM (ft3/Min) x ∆ T (°F) x 0.90 (BTU*Min)/(ft3*Hr * °F)

QTotal = QSensible + QLatent

QTotal = SCFM (ft3/Min) x ∆ h (BTU/lb) x 4.5 (Min * lb/ ft3)QTotal (5000 ft) = CFM (ft3/Min) x ∆ h (BTU/lb) x 3.74 (Min * lb/ ft3*Hr)

Q is in BTU/h

Total Heat

Total H

eat ∆

h

Sensible Heat

Latent Heat

40° 50° 70°60° 80° 90° 100° 110° 120°

.004

.008

.012

.016

.020

.024

.028


Dew PointSatu

ration Line

Specific Hum



Fan

Cooling C

oil

SA

MA

FAN HEAT

SENS AND LATENT GAIN (ROOM)

Room

RA

Enthalp

y -H (B

TU / lb.

of dry

air)

15.0

27.6

34.1

30.0

18.0

43.7

FormulasQroom = 5,000 (ft3/Min) *(27.6 BTU/LB-18.0BTU/LB)*3.74 (Min * lb/ ft3*Hr)Qroom =179,600 BTU/h

Qcoil = 5,000 (ft3/Min) *(30.0 BTU/LB-15.0BTU/LB)*3.74 (Min * lb/ ft3*Hr)Qcoil =280,500 BTU/h

Thank You

Questions?

ashrae tech conference 2008 psychrometrics presentation · 2012-05-26 · learning objectives...

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