PSYCHROMETRICS

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PSYCHROMETRICS
...WITHOUT TEARS
Professor Eugene Silberstein, CMHE
SUFFOLK COUNTY COMMUNITY COLLEGE BRENTWOOD, NY
CENGAGE DELMAR LEARNING CLIFTON PARK, NY
HVAC EXCELLENCE INSTRUCTOR CONFERENCELAS VEGAS,
NEVADA MARCH 20-22, 2011
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What Makes Psychrometrics so Painful for our
Students?
Unfortunately, most of the time its us!
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How Do We Introduce the Topic?

• You guys are going to hate this
• This stuff is really difficult
• You guys are going to hate this
• This involves a ton of math
• You guys are going to hate this
• Youre not going to understand this but its okay
  because I dont either
• You guys are going to hate this
• I hate it, so you will also

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This is really going to hurt!
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TEACHING PSYCHROMETRICS IS A LOT LIKE COMMERCIAL
FISHING...
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How Much Does the Air in this Room Weigh?
0 pounds? 10 pounds? 50 pounds?
100 pounds? 250 pounds?
500 pounds? 1000 pounds? 4500 pounds?
THE ANSWER MIGHT SURPRISE YOU... (I Hope It Does!)
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Room Dimensions...

• Length 66 feet
• Width 46 feet
• Ceiling Height 20 feet
• Room Volume 66 x 46 x 20 60,720ft3
• Based on this volume, the air in this room weighs
  approximately

60,720 ft3 x 0.075 lb/ft3 4,554 POUNDS
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The First Four Things...

• Dry-Bulb Temperature

Wet-Bulb Temperature
Absolute Humidity
Relative Humidity
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TEMPERATURES WET DRY

• Are all temperatures created equal?
• Are all pressures created equal?
• What is the difference between psia and psig?
• How do we teach our students the difference?
• How are wet/dry bulb temperatures similar?
• How are wet/dry bulb temperatures different?
• Can we create visual examples?

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Dry Bulb Temperature

• Measured with a dry-bulb thermometer
• Measures the level of heat intensity of a
  substance
• Used to measure and calculate sensible heat and
  changes in sensible heat levels
• Does not take into account the latent heat aspect
• Room thermostats measure the level of heat
  intensity in an occupied space

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DRY-BULB TEMPERATURE SCALE
As we move up and down, the dry bulb temperature
does not change
As we move from left to right, the dry bulb
temperature increases
As we move from right to left, the dry bulb
temperature decreases
DRY-BULB TEMPERATURE
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Wet Bulb Temperature

• Measured with a wet-bulb thermometer
• Temperature reading is affected by the moisture
  content of the air
• Takes the latent heat aspect into account
• Used in conjunction with the dry-bulb temperature
  reading to obtain relative humidity readings and
  other pertinent information regarding an air
  sample

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WET-BULB TEMPERATURE SCALE
As we move up and down along a wet-bulb
temperature line, the wet bulb temperature does
not change
The red arrow indicates an increase in the wet
bulb temperature reading
The blue arrow indicates a decrease in the wet
bulb temperature reading
WET BULB TEMPERATURE
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WET-BULB, DRY-BULB COMBO
WET BULB TEMPERATURE
DRY-BULB TEMPERATURE
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SLING PSYCHROMETER
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100
75
80
WET BULB TEMPERATURE
WET BULB TEMPERATURE
70
68
60
65
65 69 70 71 73
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DRY BULB TEMPERATURE
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---- HUMIDITY ----ABSOLUTELY RELATIVE

• There are two types of humidity
• ABSOLUTE
• RELATIVE
• AH and RH are not the same
• Cannot be used interchangeably
• All humidities are not created equal

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ABSOLUTE HUMIDITY

• Amount of moisture present in an air sample
• Measured in grains per pound of air
• 7,000 grains of moisture 1 pound

60 GRAINS
1 POUND
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The moisture scale on the right-hand side of the
chart provides information regarding the absolute
humidity of an air sample
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MOISTURE CONTENT SCALE
As we move from side to side, the moisture
content does not change
As we move up, the moisture content increases
As we move down, the moisture content decreases
MOISTURE CONTENT (BTU/LBAIR)
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WET-BULB, DRY BULB MOISTURE CONTENT
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RELATIVE HUMIDITY

• Amount of moisture present in an air sample
  relative to the maximum moisture capacity of the
  air sample
• Expressed as a percentage
• Can be described as the absolute humidity divided
  by the maximum moisture-holding capacity of the
  air

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RELATIVE HUMIDITY
Example 1
HOW FULL IS THE PARKING LOT?
FULL 0.5 X 100
FULL 50
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RELATIVE HUMIDITY
Example 2
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RELATIVE HUMIDITY
Example 3
60 GRAINS
If capacity is 120 grains, then the relative
humidity will be
RH (60 grains 120 grains) x 100 50
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RELATIVE HUMIDITY SCALE
As we move along a relative humidity line, the
relative humidity remains the same
As we move up, the relative humidity increases
As we move down, the relative humidity decreases
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WET-BULB, DRY BULB, MOISTURE CONTENT RELATIVE
HUMIDITY
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The lines that represent constant wet-bulb
temperature also represent the enthalpy of the air
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ENTHALPY SCALE
As we move up and down along an enthalpy line,
the enthalpy does not change
The red arrow indicates an increase in enthalpy
The blue arrow indicates a decrease in enthalpy
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WET-BULB, DRY BULB, MOISTURE CONTENT, RELATIVE
HUMIDITY ENTHALPY
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SPECIFIC VOLUME DENSITY

• Specific volume and density are reciprocals of
  each other
• Density lb/ft3
• Specific volume ft3/lb
• Density x Specific Volume 1
• Specific volume can be determined from the
  psychrometric chart, density muse be calculated

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LINES OF SPECIFIC VOLUME
As we move along a line of constant specific
volume, the specific volume remains unchanged
As we move to the right, the specific volume
increases
As we move to the right, the specific volume
increases
ft3/lb
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WET-BULB, DRY BULB, MOISTURE CONTENT, RELATIVE
HUMIDITY ENTHALPY
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Return Air 75ºFDB, 50 r.h.
Supply Air 55ºFDB, 90 r.h.
Airflow 1200 cfm
RETURN AIR
SUPPLY AIR
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?T Return Air Temp Supply Air Temp ?T 75ºF
- 55ºF 20ºF
?W Return grains/lbAIR Supply grains/lbAIR ?W
64 Grains 60 Grains 4 grains/lbAIR
?h Return btu/lbAIR Supply btu/lbAIR ?h
28.1 btu/lbAIR - 21.6 btu/lbAIR 6.5 btu/lbAIR
RETURN AIR
64 grains/lb 60 grains/lb
SUPPLY AIR
55ºF 75ºF
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AIR FORMULAE

• QT QS QL

QT 4.5 x cfm x ?h

• Qs 1.08 x cfm x ?T

• QL 0.68 x cfm x ?W

Yeah, yeah, but where do they come from?
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ON PLANET ENEGUE...
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100 MILES
HOUR
So, my rate of speed was...
100 x 24 x 365 x 5280 x 12 x 2.54 x 10 mm/year,
which is....
1,409,785,344,000 mm/year!
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Try These Ideas for Your Students

• If your car get 30 miles per gallon, how many
  inches per ounce will you be able to travel?
• If you earn 15/Hour, how many pennies per year
  will you earn in a year if you work 40 hours per
  week and 50 weeks per year?
• If air weight 0.075 lb per cubic foot how many
  ounces per cubic inch is that?

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Let Students Take Ownership

• Ask the right questions
• Let the students create a formula
• Let students identify relevant factors that
  should be included in the formula
• Let students identify relevant conversion factors
  that should be included

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Total Heat Formula

• We all know QT 4.5 x CFM x ?h
• Where does the 4.5 come from?
• Work with the units
• QT (btu/hour)
• What factors will contribute to get this result
• Factors must be relevant to sensible heat
• For example, grains/pound is not a relevant term
  as it applies to latent heat

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Total Heat Formula

• QT (btu/hour) 4.5 x CFM x ?h
• Units on the right must be the same as the units
  on the left

Let the students BUILD the Sensible Heat
Formula...
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Heat Formulae Variables
So, ask your students what variables and factors
will have an effect on the amount of heat
transferred by the process
?W?
CFM?
60 MIN 1 HOUR?
?T?
?h?
SPECIFIC VOLUME?
SPECIFIC HEAT?
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Total Heat Formula
We have btu/hour on the left...
btu/hour ? x ? x ? x ? x ?
Which factor, ?h, ?W, or ?T, is associated with
the total heat?
btu/hour ?h (btu/lbAIR) x ? x ? x ? x ?
Which other factors are associated with the total
heat?
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Total Heat Formula
btu/hr ?h (btu/lbAIR) x ? x ? x ? x ?
btu/hr ?h (btu/lbAIR) x ft3/min x ? x ?
btu/hr ?h (btu/lbAIR) x ft3/min x 60 min/hr
btu/hr 60 x (btu x ft3)/hour x lbAIR x ?
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btu/hr 60 x (btu x ft3)/hour x lbAIR x ?
We need to get rid of the ft3 in the numerator
and the lbAIR in the denominator...
What factor relating to air has ft3 in the
denominator and lb in the denominator?
Density
btu/hr 60 x (btu x ft3)/hour x lbAIR x lb/ft3
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Total Heat Formula
Density 0.075 lb/ft3 at atmospheric conditions
btu/hr 60 x 0.075 btu/hour
QT (btu/hr) 4.5 x Airflow x ?h
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Sensible Heat Formula

• We all know QS 1.08 x CFM x ?T
• Where does the 1.08 come from?
• Work with the units
• QS (btu/hour)
• What factors will contribute to get this result
• Factors must be relevant to sensible heat
• For example, grains/pound is not a relevant term
  as it applies to latent heat

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Sensible Heat Formula
Which factor, ?h, ?W, or ?T, is associated with
sensible heat?
We already have some of our variables in place
btu/hour cfm x 60 x 0.075 x lb/hour x ?
btu/hour 4.5 x cfm x lb/hour x ?
We need to add the btu to the right side and
get rid of the lb on the right side
Specific Heat
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Sensible Heat Formula
The specific heat of air is 0.24 btu/lb/ºF
btu/hour 4.5 x lb/hour x 0.24 btu/lb
btu/hour 1.08 x btu/hour
Adding in our other variable values gives us
QS (btu/hr) 1.08 x Airflow x ?T
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Challenges with the Sensible Heat Formula

• It doesnt always give accurate results
• The 1.08 is only an estimate
• The 0.075 lb/ft3 is not correct most of the time
• The density comes from the specific volume
• Specific volume must be determined
• Specific volume estimate is the average of the
  values before and after the heat transfer coil

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Latent Heat Formula

• We all know QL 0.68 x CFM x ?W
• Where does the 0.68 come from?
• Work with the units
• QL (btu/hour)
• What factors will contribute to get this result
• Factors must be relevant to latent heat
• For example, grains/pound is definitely a
  relevant term as it applies to latent heat

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Latent Heat Formula
Which factor, ?h, ?W, or ?T, is associated with
sensible heat?
?W Change in moisture in grains/lbAIR
We already have some of our variables in place
btu/hour cfm x 60 x 0.075 x lb/hour x ?
btu/hour 4.5 x cfm x lbAIR/hour x ?
btu/hour 4.5 x cfm x grains/hour x ?
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Latent Heat Formula
1 pound of water contains 7000 grains
btu/hour 4.5 x cfm x grains/hour x lb/7000
grains
btu/hour (4.5 7000) x cfm x lb/hour
We need to add the btu to the right side and
get rid of the lb on the right side
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SUPPLY AIR
RETURN AIR
Water Vapor at 75ºF
Water at 50ºF
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STEAM TABLES ACCOMPLISH ONE THING!
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Pertinent Enthalpy Information
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Latent Heat Formula
btu/hour (4.5 7000) x cfm x lb/hour
We need to add the btu to the right side and
get rid of the lb on the right side
From the steam table we get
1094 btu/lb - 18 btu/lb - 1076 btu/lb
btu/hour (4.5 x 1076) 7000 x cfm x lb/hour
x btu/lb
QL (btu/hr) 0.68 x Airflow x ?W
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You can find automated steam tables at

• www.efunda.com/Materials/water/steamtable_sat.cfm

Enter Temperature Here
Read Cool Stuff Here
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MIXED AIR SYSTEMS

• Return air is mixed with outside air
• Heat transfer coil does not see return air from
  the occupied space exclusively
• Percentage of outside air changes with its heat
  content
• Process is governed by an enthalpy control
• The heat transfer coil sees only the mixture of
  the two air streams

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LAW OF THE TEE

• Also known as nodal analysis
• What goes into a tee, must go out!
• Electric circuit applications
• Water flow applications
• Hot water heating applications
• Mixed air applications

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?
5 AMPS
2 AMPS
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?
5 GPM
2 GPM
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?
5 GPM _at_ 100ºF
5 GPM _at_ 140ºF
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?
5 GPM _at_ 100ºF
3 GPM _at_ 140ºF
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Heres The Math...
(5 GPM x 100ºF) (3 GPM x 140ºF) (8 GPM x YºF)
500 420 8YºF
920 8YºF
Y 115ºF
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LAW OF THE TEE FOR WATER
CLASSROOM DEMONSTRATION or EXPERIMENT
40ºF
70ºF
1 CUP 1 CUP
Have students predict final mixed temperature....
Then combine, mix, measure and confirm..... Then
change the rules!
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LAW OF THE TEE FOR WATER
CLASSROOM DEMONSTRATION or EXPERIMENT
THE RESULTS
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LAW OF THE TEE FOR WATER
CLASSROOM DEMONSTRATION or EXPERIMENT
40ºF
70ºF
2 CUPS 1 CUP
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LAW OF THE TEE FOR WATER
CLASSROOM DEMONSTRATION or EXPERIMENT
THE RESULTS
10ºF
20ºF
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LAW OF THE TEE FOR MIXED AIR
AIR HANDLER
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LAW OF THE TEE FOR MIXED AIR
PERCENTAGE OF RETURN AIR PERCENTAGE OF
OUTSIDE AIR
100 of MIXED AIR
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LAW OF THE TEE FOR MIXED AIR
SAMPLE PROBLEM
AIR CONDITIONS RETURN AIR (80) 75ºFDB,
50RH OUTSIDE AIR (20) 85ºFDB, 60RH
MIXED AIR 80 RETURN AIR 20 OUTSIDE AIR
MIXED AIR (.80) RETURN AIR (.20) OUTSIDE AIR
MIXED AIR (.80) (75ºFDB, 50RH) (.20)
(85ºFDB, 60RH)
MIXED AIR 60ºFDB, 40RH 17ºFDB, 12RH
MIXED AIR 77ºFDB, 52RH
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Return Air 75ºFDB, 50 r.h.
Outside Air 85ºFDB, 60 r.h.
Mixed Air 77ºFDB, 52 r.h.
OUTSIDE AIR
MIXED AIR
SUPPLY AIR
RETURN AIR
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Eugene Silberstein

• 917-428-0044
• silbere_at_sunysuffolk.edu