Psychrometry - Universitas Pendidikan...

A Trane Air Conditioning Clinic

Psychrometry

Air 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 Point

Fog Occurs When Air Is Saturated

Relative Humidity

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

Humidity Ratio

Properties of Air

Dry-bulb temperature

Wet-bulb temperature

Dew-point temperature

Relative humidity

Humidity ratio

Summer Design Conditions

95°F DB (dry bulb)

78°F WB (wet bulb)

Point of Intersection

110.82

131.83

156.38

185.03

218.42

dry-bulbtemp.

humidityratio

dry-bulb temp.

humidityratio

Properties of Saturated Air

Dry-Bulb Temperature (°F)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Plotting Saturation Points218.42

185.03

156.38

131.83

110.82

36.5129.94

24.1919.14

Saturation Curve

100% relative humidity curve

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

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%

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Adding Sensible Heat

100% saturation

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

60°Fdew point

60°Fwet bulb

Relative Humidity Curves

relative humidity

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Properties of Air

humidity ratio

dry bulbrelative humidity

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Determining Wet-Bulb Lines

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Properties of Air

wet bulb

dew point

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

humidity ratio

Specific Volume

specific volume lines

(cubic feet / pound of dry air)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

Effect of Adding Sensible Heat

Effect of Removing Sensible Heat

Effect of Adding Moisture

Effect of Removing Moisture

Removing Sensible Heat and Moisture

Psychrometry

period two

Air Mixtures

supplyfan

mixture outdoorair (OA)

recirculated air (RA)

coolingcoil

Determining Entering Air Conditions

80°F95°F

outdoor air

recirculated air

1,000 cfm

4,000 cfm = 0.25

mixture

4,000 cfmmixed air

1,000 cfmOA

3,000 cfmRA

95°F × 0.25 = 23.75°F

80°F × 0.75 = 60.00°F

mixture = 83.75°F

83.75°F

Psychrometry

period three

Sensible Heat Ratio

Effect of Removing Sensible Heat

Effect of Removing Latent Heat

Removing Sensible and Latent Heat

supply air

sensible

latent

return air

Heat and Moisture Transfer

Removing Sensible and Latent Heat

cooler & damper

warmer & drier

Heat Gain

SHRSensible Heat Gain

Sensible Heat Gain + Latent Heat Gain=

Sensible Heat Ratio (SHR)

78°F DB

65°F WB

index point

Sensible Heat Ratio Scale

Drawing an SHR Line

index point

SHR Dictates Supply Air Condition

Drawing an SHR Line

index point

Psychrometry

period four

Air Quantity

coil curves

Coil Curves

80,000 Btu/hr sensible heat gain

20,000 Btu/hr latent heat gain

STEP 1: Calculate the sensible heat ratio (SHR)

Determining Supply Airflow

= 0.8080,000 Btu/hr

100,000 Btu/hrSHR =

room — 78°F DB, 50% RH

outdoor air (OA) — 95°F DB, 78°F WB

ventilation — 25% OA

95°F × 0.25 = 23.75°F

78°F × 0.75 = 58.50°F

mixture = 82.25°F

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

56.5°F 78°F

STEP 3: Identify supply air conditions

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

3,430 cfm

56.5°F DB

55.2°F WB

82.2°F DB

68.6°F WB

mixture

56.5°F

47,000 Btu/hr sensible heat gain

20,000 Btu/hr latent heat gain

= 0.7047,000 Btu/hr

67,000 Btu/hrSHR =

Arbitrarily Using 55°F Supply Air

Psychrometry

period five

Tons of Refrigeration

What is Enthalpy?

Enthalpy (h) = Sensible Heat + Latent Heat

The total heat energy in one pound of air

(Btu/lb) at its present condition.

Determining Tons of RefrigerationSTEP 1:

Find enthalpies

entering and

leaving coil

Determining Tons of Refrigeration

STEP 2: Solve the total refrigeration load equation

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

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/hr

12,000 Btu/hr/ton

Sensible and Latent Coil Loads

sensibleload

latentload

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

7.8 tons of refrigeration (sensible)

Sensible and Latent Coil Loads

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 at Full-Load Conditions

56.5°F 78°F

80,000 Btu/hr

80,000 Btu/hr + 20,000 Btu/hr

Full-Load SHR:

= 0.80

47,000 Btu/hr

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

SHR Changes with Room Load

Part-Load SHR:

SHR at Part-Load Conditions

47,000 Btu/hr

1.085 × (78°F – Supply DB)= 3,430 cfm

Supply DB = 65.4°F

65.4°F

SHR at Part-Load Conditions

constant quantity of variable-temperature air

modulatingcooling coil

Constant-Volume System

reheat coil

coolingcoil

roomthermostat

adding reheat improves humidity control

Constant-Volume with Reheat

47,000 Btu/hr

1.085 × (78°F – Supply DB)= 3,430 cfm

Supply DB = 65.4°F

Effect of Adding Reheat

65.4°F56.5°F

Effect of Adding Reheat

mixed-air bypass improves humidity control

face-and-bypass

dampers

Mixed-Air Bypass

Effect of Mixed-Air Bypass

82.25°F × 0.455 = 37.4°F

51.40°F × 0.545 = 28.0°F

mixture = 65.4°F

65.4°F51.4°F

Effect of Mixed-Air Bypass

65.4°F51.4°F

variable quantity of constant-temperature air

variable-speed drive

Variable-Air-Volume (VAV) System

47,000 Btu/hr

1.085 × (78°F – 56.5°F)= 2,015 cfm

55.5°F

Calculating Part-Load Airflow

Software Tools

Psychrometry

period seven

Review

Properties of Air

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

wet bulb

dew point

humidity ratio

83.75°F

Determining Mixed-Air Conditions

95°F × 0.25 = 23.75°F

80°F × 0.75 = 60.00°F

mixture = 83.75°F

Determining Sensible Heat Ratio

SHR = Sensible Heat Gain

Sensible Heat Gain + Latent Heat Gain

coil curve

Sensible Heat Gain

1.085 × (Room DB – Supply DB)

SupplyAirflow

Determining Coil Leaving Conditions

supply DB

Determining Tons of RefrigerationRefrigeration Load = 4.5 × Supply Airflow × (h1 – h2)

PsychrometricAnalysis

Psychrometry - Universitas Pendidikan...

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