A Trane Air Conditioning Clinic Psychrometry Air Conditioning Clinic TRG-TRC001-EN © American...

A Trane Air Conditioning Clinic

Psychrometry

Air Conditioning Clinic TRG-TRC001-ENAir Conditioning Clinic TRG-TRC001-EN© American Standard Inc. 1999

Psychrometry

period one

The Psychrometric Chart


PsychrometricChart



Properties Of Air

Dry-bulb temperature Wet-bulb temperature Dew-point temperature Relative humidity Humidity ratio


Dry-Bulb Thermometer


Wet-Bulb Thermometer


Condensation Occurs At Dew PointCondensation Occurs At Dew Point

Air Conditioning Clinic TRG-TRC001-EN© American Standard Inc. 1999


Fog Occurs When Air Is Saturated

Air Conditioning Clinic TRG-TRC001-EN© American Standard Inc. 1999


Relative Humidity…describes the degree of saturation

RelativeHumidity

Amount of moisture that a given

amount of air is holding=

Amount of moisture that a given

amount of air can hold


50% 100%(saturated)

Relative Humidity…compares moisture content to saturation


Humidity Ratio…compares water vapor to dry air, by weight


Knowing Two Properties…lets you determine the remaining three

Dry-bulb temperature Wet-bulb temperature Dew-point temperature Relative humidity Humidity ratio


EXAMPLESummer Design Conditions

95°F DB (dry bulb) 78°F WB (wet bulb)


“X Marks The Spot”…

95°F95°F

78°F78°F

50%

50%

40%

40%

72°F72°F


25°F

30°F

35°F

40°F

45°F

50°F

55°F

19.14

24.19

29.94

36.51

44.34

53.63

64.63

60°F

65°F

70°F

75°F

80°F

85°F

90°F

77.61

92.89

110.82

131.83

156.38

185.03

218.42

DBTemp.

HumidityRatio

DBTemp.

HumidityRatio

Saturated Air…grains of moisture per pound of dry air

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Dry-Bulb Temperature (°F)

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Plotting Saturation Points218.42

185.03

156.38

131.83

110.82

92.89

77.61

64.63

53.6344.34

36.5129.94

24.1919.14


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Saturation Curve

100% Relative Humidity Curve


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relationship between…

Dry Bulb, Wet Bulb And Dew Point

60°FDew Point

60°FWet Bulb

A B C D Dry Bulb 60°F 62°F 75°F 90°F

Wet Bulb 60°F 60.8°F 65.2°F 70°F

Dew Point 60°F 60°F 60°F 60°F

Rel Humidity 100% 92% 60% 37%

DDCCBB

AA


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moisture content doesn’t change by…

Adding Sensible Heat

100% Saturation

60°FDew Point

60°FWet Bulb


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Relative Humidity Curves

Relative Humidity

0%

10%

20%

30%

40%50%

60%70%80%90

%100%


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intersection represents…

Three Air Properties

Humidity RatioHumidity Ratio

Dry BulbDry BulbRelative HumidityRelative Humidity


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Determining Wet-Bulb Lines

AA

BIBICICI

DIDI

BB CC DDAIAI


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intersection of two known air properties…

Determines The Remaining Three



Wet BulbWet Bulb

Dew PointDew Point


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A Final Property…Specific Volume

14.014.013.0

13.0

13.513.5

14.514.5

Specific Volume Lines (cubic feet / pound of dry air)Specific Volume Lines (cubic feet / pound of dry air)


Effect Of Adding Sensible Heat


Effect Of Removing Sensible Heat


Effect Of Adding Moisture


Effect Of Removing Moisture


effect of changing…

Sensible Heat And Moisture Content


effect of changing…

Sensible Heat And Moisture Content


Psychrometry

period two

Air Mixtures


Air ConditionerAir Conditioner

Supply FanSupply Fan

MixtureMixture OutdoorOutdoorAir (OA)Air (OA)

Recirculated Air Recirculated Air (RA)(RA)

AA

CC

BB

Cooling CoilCooling Coil

Determining Entering Air Conditions


80°F80°F95°F95°F


BB

AA

Outdoor Air

Outdoor Air

Recirculated Air

Recirculated Air


1,000 cfm4,000 cfm

= 0.25

OARAMixture

===

25%75%

100%

AA

CC BB4,000 cfm4,000 cfmMixed AirMixed Air4,000 cfm4,000 cfmMixed AirMixed Air

1,000 cfm1,000 cfmOAOA

3,000 cfm3,000 cfmRARA




95°F × 0.25 = 23.75°F 80°F × 0.75 = 60.00°F

Mixture = 83.75°F

Mixed Air Temperature


83.75°F83.75°F

70°F70°F


BB

80°F80°F

95°F95°F

CC

AA


Psychrometry

period three

Sensible Heat Ratio


Effect Of Removing Sensible Heat


Effect Of Removing Latent Heat


effect of…

Removing Sensible And Latent Heat


Supply AirSupply Air

Sensible HeatSensible Heat

Latent HeatLatent Heat

Return AirReturn Air

supply air conditions determine…

Heat And Moisture Transfer


sensible and latent heat…

Effects Of Improper Removal

Cooler& DamperCooler& Damper

Warmer& DrierWarmer& Drier


A Look At “Heat Gain” . . .



SHR Sensible Heat Gain

Sensible Heat Gain + Latent Heat Gain=

determining the…

Sensible Heat Ratio (SHR)

78°F DB65°F WB78°F DB65°F WB

Index PointIndex Point

Sensible Heat Ratio Scale


0.80 SHR Line0.80 SHR Line

Drawing An SHR Line

AAIndex PointIndex Point


Sensible Heat Ratio LineSensible Heat Ratio Line

Room SHR dictates…

Supply Air Conditions

AA


CC

DDBB

80°F80°F

drawing an SHR line for…

Other Room Conditions

0.60 SHR Line0.60 SHR Line

Index PointIndex Point


Psychrometry

period four

Air Quantity


Coil CurvesCoil Curves

Coil Curves…show changes in air properties through a cooling coil



80,000 Btu/hr sensible heat gain

20,000 Btu/hr latent heat gain

= 0.80

STEP 1: Calculate the sensible heat ratio (SHR)

80,000 Btu/hr100,000 Btu/hrSHR =

Determining Supply Airflow


Room — 78°F DB, 50% RH

OA — 95°F DB, 78°F WB

Ventilation — 25% OA


Design Conditions:

95°F × 0.25 = 23.75°F78°F × 0.75 = 58.50°F

Mixture = 82.25°F

Determining Supply AirflowSTEP 2: Plot room, outdoor, and entering conditions

0.80 SHR0.80 SHRAA

CC

BB


56.5°F56.5°F 78°F78°F

STEP 3: Identify supply air conditions

0.80 SHR0.80 SHRAA

CC

BB

DD




Sensible Heat Gain

1.085 × (Room DB – Supply DB)

SupplyAirflow=

STEP 4: Solve the supply airflow equation



80,000 Btu/hr

1.085 × (78°F – 56.5°F)3,430 cfm=

STEP 4: Solve the supply airflow equation


D

A

C B3,430 cfm

56.5°F DB55.2°F WB

RA

OA82.2°F DB68.6°F WB

Mixture


SA

56.5°F56.5°F

DD AA

CCBB



effect of…

Arbitrarily Using 55°F Supply Air

47,000 Btu/hr sensible heat gain

20,000 Btu/hr latent heat gain

= 0.7047,000 Btu/hr67,000 Btu/hrSHR =

AA

CC

BB

DD

55°F55°F

DIDIAIAI

effect of…

Arbitrarily Using 55°F Supply Air

0.70 SHR0.70 SHR

51°F51°F


Psychrometry

period five

Tons Of Refrigeration



What is Enthalpy?

Enthalpy (h) = Sensible Heat + Latent Heat

The total heat energy in onepound of air (Btu/lb) at its presentcondition:

Determining Tons Of RefrigerationSTEP 1: Find enthalpiesentering andleaving coil

AA

BB

CC

DD32.732.7

23.523.5



Determining Tons Of Refrigeration

STEP 2: Solve the total refrigeration load equation

4.5 × Supply Airflow × (h1 – h2)Refrigeration Load (Btu/hr)=

where:h1 = enthalpy of air entering coil (Btu/lb)h2 = enthalpy of air leaving coil (Btu/lb)


= 11.8 tons of refrigeration

Determining Tons Of Refrigeration

STEP 2: Solve the total refrigeration load equation

4.5 × 3,430 cfm × (32.7 – 23.5) = 142,000 Btu/hr

142,000 Btu/hr12,000 Btu/hr/ton

determining cooling coil…

Sensible And Latent Loads

AA

BB

CC

DD

Sensible LoadSensible Load

Latent LoadLatent Load

32.732.7

23.523.5

29.629.6



Sensible Coil Load:

4.5 × 3,430 cfm × (29.6 – 23.5) = 94,150 Btu/hr

7.8 tons of refrigeration (sensible)

determining cooling coil…

Sensible And Latent Loads

Latent Coil Load:

4.5 × 3,430 cfm × (32.7 – 29.6) = 47,850 Btu/hr

4.0 tons of refrigeration (latent)

Psychrometry

period six

Psychrometric Analyses


SHR Changes Throughout The Day

Full-load conditions

56.5°F56.5°F 78°F78°F

DDAA

BB

CC



Full Load:

80,000 Btu/hr80,000 Btu/hr + 20,000 Btu/hr SHR =

Part Load:

= 0.80

47,000 Btu/hr47,000 Btu/hr + 20,000 Btu/hr SHR = = 0.70

SHR Changes With Room Load

DD

AA

BB

CC

New SHR Requires New SA Condition

0.80 SHR0.80 SHR

0.70 SHR0.70 SHR

Part-load conditions


DD

AA

BB

CC

at part-load, with constant supply airflow …

Raising SA Temp. Increases Room RH

0.80 SHR0.80 SHR

0.70 SHR0.70 SHRAIAI

DIDI

50%

50%

50%

50%60

%60

%60

%60

%

47,000 Btu/hr

1.085 × (78°F – Supply DB)= 3,430 cfm

Supply DB = 65.4°F

65.4°F65.4°F


constant quantity of variable-temperature airconstant quantity of variable-temperature air

OA

RA

SA

Constant-Volume System

BBDD CC

AAModulatingModulatingCooling CoilCooling Coil


Reheat Reheat CoilCoil

CoolingCoolingCoilCoil

RoomRoomThermostatThermostat

adding reheat improves adding reheat improves humidity controlhumidity control

BB

EE

DD CC

AA

Constant-Volume With Reheat


DD AA

C

BB

47,000 Btu/hr

1.085 × (78°F – Supply DB)= 3,430 cfm

Supply DB = 65.4°F

Effect Of Adding Reheat…

EE

0.80 SHR0.80 SHR

65.4°F65.4°F56.5°F56.5°F


Effect Of Adding Reheat…

DDAA

CC

BB

EE

0.80 SHR0.80 SHR

65.4°F65.4°F56.5°F56.5°F

0.70 SHR0.70 SHR


mixed air bypass improves humidity controlmixed air bypass improves humidity control

Mixed Air Bypass

OA

RASA

BB

DDCC

AA

Face-and-Face-and-BypassBypassDampersDampers


Effect Of Mixed Air Bypass…

DDAA

CC

BB

EE

82.25°F × 0.455 = 37.4°F51.40°F × 0.545 = 28.0°F

Mixture = 65.4°F

65.4°F65.4°F51.4°F51.4°F


Effect Of Mixed Air Bypass…

0.70 SHR0.70 SHR

AA

CC

BB

AIAI

65.4°F65.4°F51.4°F51.4°F

DDEE



Variable-Air-Volume (VAV) System

OA

RA

SA

BBDD

AA

CC

variable quantity of constant-temperature airvariable quantity of constant-temperature air

VSDVSD

DD AA

CC

BB

Calculating Part-Load Airflow

47,000 Btu/hr

1.085 × (78°F – 56.5°F)= 2,015 cfm

0.80 SHR0.80 SHR

0.70 SHR0.70 SHRAIAI

55.5°F55.5°F



PC-based software tools…

Simplify Psychrometric Analysis

Psychrometry

period seven

Review


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review…

Five Properties Of Air



Wet BulbWet Bulb

Dew PointDew Point


80°F80°F

95°F95°F

83.75°F83.75°F

70°F70°F

CC

BB

AA

review…

Determining Mixed Air Conditions

95°F × 0.25 = 23.75°F80°F × 0.75 = 60.00°F

Mixture = 83.75°F


AA

Sensible Heat Ratio LineSensible Heat Ratio Line

review…

Determining Sensible Heat Ratio

SHR = Sensible Heat Gain

Sensible Heat Gain + Latent Heat Gain


AA

CC

BB

DD

Coil Curve

Sensible Heat Gain

1.085 × (Room DB – Supply DB)SupplyAirflow

review…

Determining Coil Leaving Conditions

SHR LineSHR LineSHR LineSHR Line

=

Supply DBSupply DB


AA

CC

BB

DD

review…

Determining Tons Of Refrigeration4.5 × Supply Airflow × (h1 – h2) = Refrigeration Load


CC

OA

RA

BB

AADD

SA

review…

PsychrometricAnalysis

SHR LineSHR Line

AA

BBCCDD



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