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FundamentalsFundamentalsofof
PsychrometricsPsychrometrics
By James L. Murphy, Colorado Building Environments Sales Engineer
LetLet’’s Start with Airs Start with Air
Air is Made up of Two Main GasesAir is Made up of Two Main Gases–– Nitrogen (77%)Nitrogen (77%)–– Oxygen (23%)Oxygen (23%)
If that is all there was HVAC Calculations would be easyIf that is all there was HVAC Calculations would be easy–– Cooling BTUCooling BTU’’s = (Ts = (T22--TT11)*1.085*CFM)*1.085*CFM
But there is One other elementBut there is One other element–– Water VaporWater Vapor
How do we account for Water Vapor?How do we account for Water Vapor?–– Psychrometric ChartPsychrometric Chart
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DRY BULB TEMPERATURE - °F
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40
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50
EN
THA
LPY
-BTU
PER
PO
UN
D O
F D
RY
AIR
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20
25
30
35
40
45
ENTHALPY - B
TU PER P
OUND OF D
RY AIR
SATURATION TE
MPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
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10% RELATIVE HUMIDITY
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75 WET BULB TEMPERATURE - °F
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80
15.0
15.5
16.0
16.5 VOLU
ME- C
U.FT. PER
LB. DRY AIR
17.0
17.5
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TUR
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PO
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AIR
BAROMETRIC PRESSURE: 24.896 in. HG
PSYCHROMETRICCHARTNormal TemperatureI-P Units
5000 FEET
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
DRY BULB TEMPERATURE - °F
10 15 20 25
30
35
40
45
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50
EN
THA
LPY
-BTU
PER
PO
UN
D O
F D
RY
AIR
15
20
25
30
35
40
45
ENTHALPY - B
TU PER P
OUND OF D
RY AIR
SATURATION TE
MPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
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75 WET BULB TEMPERATURE - °F
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80
15.0
15.5
16.0
16.5 VOLU
ME- C
U.FT. PER
LB. DRY AIR
17.0
17.5
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TUR
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PO
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D O
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AIR
BAROMETRIC PRESSURE: 24.896 in. HG
PSYCHROMETRICCHARTNormal TemperatureI-P Units
5000 FEET
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
Dry Bulb Temperature Lines
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DRY BULB TEMPERATURE - °F
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EN
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-BTU
PER
PO
UN
D O
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AIR
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20
25
30
35
40
45
ENTHALPY - B
TU PER P
OUND OF D
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SATURATION TE
MPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
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75 WET BULB TEMPERATURE - °F
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80
15.0
15.5
16.0
16.5 VOLU
ME- C
U.FT. PER
LB. DRY AIR
17.0
17.5
HU
MID
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RAT
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GR
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S O
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OIS
TUR
E P
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PO
UN
D O
F D
RY
AIR
BAROMETRIC PRESSURE: 24.896 in. HG
PSYCHROMETRICCHARTNormal TemperatureI-P Units
5000 FEET
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
Wet Bulb Temperature Lines
30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
DRY BULB TEMPERATURE - °F
10 15 20 25
30
35
40
45
50
50
EN
THA
LPY
-BTU
PER
PO
UN
D O
F D
RY
AIR
15
20
25
30
35
40
45
ENTHALPY - B
TU PER P
OUND OF D
RY AIR
SATURATION TE
MPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
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10% RELATIVE HUMIDITY
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90%
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35 40
40 45
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5055
5560
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75 WET BULB TEMPERATURE - °F
75
80
15.0
15.5
16.0
16.5 VOLU
ME- C
U.FT. PER
LB. DRY AIR
17.0
17.5
HU
MID
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TUR
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PO
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D O
F D
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AIR
BAROMETRIC PRESSURE: 24.896 in. HG
PSYCHROMETRICCHARTNormal TemperatureI-P Units
5000 FEET
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
Relative Humidity Lines
30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
DRY BULB TEMPERATURE - °F
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EN
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-BTU
PER
PO
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D O
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AIR
15
20
25
30
35
40
45
ENTHALPY - B
TU PER P
OUND OF D
RY AIR
SATURATION TE
MPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
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5560
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75 WET BULB TEMPERATURE - °F
75
80
15.0
15.5
16.0
16.5 VOLU
ME- C
U.FT. PER
LB. DRY AIR
17.0
17.5
HU
MID
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TUR
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PO
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D O
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AIR
BAROMETRIC PRESSURE: 24.896 in. HG
PSYCHROMETRICCHARTNormal TemperatureI-P Units
5000 FEET
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
Saturation Temperature
30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
DRY BULB TEMPERATURE - °F
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EN
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-BTU
PER
PO
UN
D O
F D
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AIR
15
20
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30
35
40
45
ENTHALPY - B
TU PER P
OUND OF D
RY AIR
SATURATION TE
MPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
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75 WET BULB TEMPERATURE - °F
75
80
15.0
15.5
16.0
16.5 VOLU
ME- C
U.FT. PER
LB. DRY AIR
17.0
17.5
HU
MID
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RAT
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TUR
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PO
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D O
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AIR
BAROMETRIC PRESSURE: 24.896 in. HG
PSYCHROMETRICCHARTNormal TemperatureI-P Units
5000 FEET
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
Grains Of Moisture Lines
EnthalpyEnthalpy
What Is EnthalpyWhat Is Enthalpy–– The Total Heat Content of the air and water VaporThe Total Heat Content of the air and water Vapor
Why is it Important?Why is it Important?–– The more Heat Content in the air the Harder it is to cool The more Heat Content in the air the Harder it is to cool –– The Less Heat Content in the air the easier it is to coolThe Less Heat Content in the air the easier it is to cool
30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
DRY BULB TEMPERATURE - °F
10 15 20 25
30
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40
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50
50
EN
THA
LPY
-BTU
PER
PO
UN
D O
F D
RY
AIR
15
20
25
30
35
40
45
ENTHALPY - B
TU PER P
OUND OF D
RY AIR
SATURATION TE
MPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
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35 40
40 45
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5560
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75 WET BULB TEMPERATURE - °F
75
80
15.0
15.5
16.0
16.5 VOLU
ME- C
U.FT. PER
LB. DRY AIR
17.0
17.5
HU
MID
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RAT
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S O
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TUR
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PO
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D O
F D
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AIR
BAROMETRIC PRESSURE: 24.896 in. HG
PSYCHROMETRICCHARTNormal TemperatureI-P Units
5000 FEET
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
Enthalpy Lines
Plotting Points On the Psychrometric Plotting Points On the Psychrometric ChartChart
Only need two of the following to Plot a pointOnly need two of the following to Plot a point–– DB Temp, WB Temp, RH%, Grains Of MoistureDB Temp, WB Temp, RH%, Grains Of Moisture
Once the Point is plotted, You can find all the Once the Point is plotted, You can find all the other informationother information
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DRY BULB TEMPERATURE - °F
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30
35
40
45
50
50
EN
THA
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-BTU
PER
PO
UN
D O
F D
RY
AIR
15
20
25
30
35
40
45
ENTHALPY - B
TU PER P
OUND OF D
RY AIR
SATURATION TE
MPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
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75 WET BULB TEMPERATURE - °F
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15.0
15.5
16.0
16.5 VOLU
ME- C
U.FT. PER
LB. DRY AIR
17.0
17.5
HU
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AIR
BAROMETRIC PRESSURE: 24.896 in. HG
PSYCHROMETRICCHARTNormal TemperatureI-P Units
5000 FEET
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
Mixed Air TemperaturesMixed Air Temperatures
Plot both Outdoor air and Return Air on Plot both Outdoor air and Return Air on ChartChart Draw a line between the two PointsDraw a line between the two Points What ever the percentage of RA is, it is the What ever the percentage of RA is, it is the
same percentage down the line from RA to same percentage down the line from RA to OAOA
AIR MIXING PROCESSAIR MIXING PROCESS
AD
C
B¾
½¼
½
Heating and Cooling Heating and Cooling
Sensible Heating, CoolingSensible Heating, Cooling–– No moisture in the air Gained or LostNo moisture in the air Gained or Lost–– Sensible Heat= 1.085*CFM(dT)Sensible Heat= 1.085*CFM(dT)OROR–– Sensible Heat = 4.5*CFM*(EnthalpySensible Heat = 4.5*CFM*(Enthalpy11--EnthalpyEnthalpy22)) Enthalpy = HEnthalpy = H
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DRY BULB TEMPERATURE - °F
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40
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EN
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-BTU
PER
PO
UN
D O
F D
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AIR
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20
25
30
35
40
45
ENTHALPY - B
TU PER P
OUND OF D
RY AIR
SATURATION TE
MPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
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75 WET BULB TEMPERATURE - °F
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80
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16.0
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ME- C
U.FT. PER
LB. DRY AIR
17.0
17.5
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BAROMETRIC PRESSURE: 24.896 in. HG
PSYCHROMETRICCHARTNormal TemperatureI-P Units
5000 FEET
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
Sensible Cooling
SENSIBLE HEATINGSENSIBLE HEATING
A B
hA
hBA
70 DB
54 WB
40 DP 40 DP
65 WB
100 DBAirFlow
B
HeatingCoil
SENSIBLE COOLINGSENSIBLE COOLING
CoolingCoil
AB
hA
hB
A
65 WB
40 DP 40 DP
54 WB
100 DB AirFlow
B
70 DB
Latent HeatLatent Heat
Occurs when Water is evaporated or Occurs when Water is evaporated or Condensed and the dry bulb temperature Condensed and the dry bulb temperature does not changedoes not change Adds to cooling needed to produce the dry Adds to cooling needed to produce the dry
bulbbulb
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DRY BULB TEMPERATURE - °F
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TU PER P
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SATURATION TE
MPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
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75 WET BULB TEMPERATURE - °F
75
80
15.0
15.5
16.0
16.5 VOLU
ME- C
U.FT. PER
LB. DRY AIR
17.0
17.5
HU
MID
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AIR
BAROMETRIC PRESSURE: 24.896 in. HG
PSYCHROMETRICCHARTNormal TemperatureI-P Units
5000 FEET
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
Latent Cooling
Total Heating, CoolingTotal Heating, Cooling
Sensible + Latent = TotalSensible + Latent = Total
Total = 4.5*CFM*(HTotal = 4.5*CFM*(H11--HH22))
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DRY BULB TEMPERATURE - °F
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TU PER P
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SATURATION TE
MPERATURE - °F
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
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65
65
70
70
75 WET BULB TEMPERATURE - °F
75
80
15.0
15.5
16.0
16.5 VOLU
ME- C
U.FT. PER
LB. DRY AIR
17.0
17.5
HU
MID
ITY
RAT
IO -
GR
AIN
S O
F M
OIS
TUR
E P
ER
PO
UN
D O
F D
RY
AIR
BAROMETRIC PRESSURE: 24.896 in. HG
PSYCHROMETRICCHARTNormal TemperatureI-P Units
5000 FEET
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
Total Cooling
Total CoolingTotal Cooling
E
A
SHF =0.67
qs
B
q
C
Dqt
Why is the Psychrometric Chart Why is the Psychrometric Chart usefuluseful
LetLet’’s Say an Engineers Say an Engineer’’s Schedule calls for a s Schedule calls for a cooling coil to cool 1000 CFM from 80 cooling coil to cool 1000 CFM from 80 degrees dry bulb and 67 degrees wet bulb degrees dry bulb and 67 degrees wet bulb to 55db degrees and wants 42 MBH Total to 55db degrees and wants 42 MBH Total Cooling at 5000Cooling at 5000’’ Altitude. Why is this a Altitude. Why is this a problem?problem? Total Cooling=(60/16.5)*1000*(35Total Cooling=(60/16.5)*1000*(35--
24.6)=38.4 MBH 24.6)=38.4 MBH
Why is the Psychrometric Chart Why is the Psychrometric Chart usefuluseful
LetLet’’s Say an Engineer wants to put a humidifier in s Say an Engineer wants to put a humidifier in the duct work of a VAV unit down stream of the the duct work of a VAV unit down stream of the VAV box. The air is supplied at 55 degrees and VAV box. The air is supplied at 55 degrees and the space needs to be kept at 80 degrees and the space needs to be kept at 80 degrees and 60% RH Why is this a problem?60% RH Why is this a problem?
At 55 degrees the max grains of moisture per lb of At 55 degrees the max grains of moisture per lb of dry air is 65 grains. At 80 degrees at 60% RH the dry air is 65 grains. At 80 degrees at 60% RH the air has 93 Grains/Lb of Dry airair has 93 Grains/Lb of Dry air
Real world applicationsReal world applications
Cooling coilCooling coil–– Total CFM = 10,000Total CFM = 10,000–– OA =25% At 95/63OA =25% At 95/63–– RA = 75% at 75/58RA = 75% at 75/58–– LA = 55LA = 55–– Find Mixed air TempFind Mixed air Temp–– Find LAT WB, RH%, LA EnthalpyFind LAT WB, RH%, LA Enthalpy–– Find Sensible and Latent LoadFind Sensible and Latent Load
Real World AplicationReal World Aplication
Direct Evap CoolingDirect Evap Cooling–– Unit 1 is 100% OA in Denver, at 1000 CFM with Unit 1 is 100% OA in Denver, at 1000 CFM with
a temperature of 95/63 and is 90% effectivea temperature of 95/63 and is 90% effectiveFind LAT Find LAT
–– Unit 2 is 100% OA in Nashville, at 1000 CFM Unit 2 is 100% OA in Nashville, at 1000 CFM with a Temperature of 95/77 and is 90% with a Temperature of 95/77 and is 90% effectiveeffectiveFind LATFind LAT
Direct Evap CoolingDirect Evap Cooling
CompleteSaturation
AirFlow
Sprays
Humidifier
63.3 DP
73 WB
95 DB
74.1 DB
A B
B C
D E73 WB
60% R.H.
A
A'B' D'
Real World ApplicationsReal World Applications
Indirect/Direct Evap CoolingIndirect/Direct Evap Cooling–– How can you get colder Temperatures out of How can you get colder Temperatures out of
this system this system –– Lets do a rough plot to seeLets do a rough plot to see
