Transcript Psychrometrics Without Tears

...WITHOUT TEARS
Professor Eugene Silberstein, CMHE
SUFFOLK COUNTY COMMUNITY COLLEGE – BRENTWOOD, NY
CENGAGE DELMAR LEARNING – CLIFTON PARK, NY
HVAC EXCELLENCE INSTRUCTOR CONFERENCE
LAS VEGAS, NEVADA
MARCH 14-16, 2010
What Makes Psychrometrics so
Painful for our Students?
Unfortunately, most of the time it’s us!
How Do We Introduce the Topic?
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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
You’re not going to understand this but it’s
okay because I don’t either
• You guys are going to hate this
• I hate it, so you will also
“This is really going to hurt!”
TEACHING PSYCHROMETRICS IS A LOT LIKE COMMERCIAL FISHING...
How Much Does the Air in this
Room Weigh?
0 pounds?
10 pounds?
100 pounds?
50 pounds?
250 pounds?
500 pounds? 1000 pounds? 1500 pounds?
THE ANSWER MIGHT SURPRISE YOU...
(I Hope It Does!)
Room Dimensions...
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Length: 42 feet
Width: 42 feet
Ceiling Height: 16 feet
Room Volume: 42 x 42 x 16 = 28,224 ft3
Based on this volume, the air in this room
weighs approximately:
28,224 ft3 x 0.075 lb/ft3 =
2,117 POUNDS
The First Four Things...
Dry-Bulb Temperature
Wet-Bulb Temperature
Absolute Humidity
Relative Humidity
TEMPERATURES: WET & DRY
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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?
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
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
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
WET-BULB TEMPERATURE SCALE
As we move up and down along a wetbulb 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, DRY-BULB COMBO
DRY-BULB TEMPERATURE
SLING PSYCHROMETER
100%
75
WET BULB TEMPERATURE
75
80%
70
70
68
65
60%
65
65
69 70 71
73
DRY BULB TEMPERATURE
75
---- 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
ABSOLUTE HUMIDITY
• Amount of moisture present in an air sample
• Measured in grains per pound of air
60
• 7,000 grains of moisture
GRAINS = 1 pound
1 POUND
The moisture scale on the
right-hand side of the chart
provides information regarding
the absolute humidity of an air
sample
MOISTURE CONTENT SCALE
As we move up, the moisture
content increases
As we move down, the moisture
content decreases
MOISTURE CONTENT (BTU/LBAIR)
As we move from side to side, the
moisture content does not change
WET-BULB, DRY BULB & MOISTURE CONTENT
DRY-BULB TEMPERATURE
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
RELATIVE HUMIDITY
Example #1
HOW FULL IS THE PARKING LOT?
#10
ofCARS
CARS
X 100%
% FULL
=
% FULL
%#20
FULL
=SPACES
0.5= X
50%
100%
of
SPACES
RELATIVE HUMIDITY
Example #2
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%
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
WET-BULB, DRY BULB, MOISTURE CONTENT & RELATIVE
HUMIDITY
DRY-BULB TEMPERATURE
The lines that represent
constant wet-bulb temperature
also represent the enthalpy of
the air
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
WET-BULB, DRY BULB, MOISTURE CONTENT, RELATIVE
HUMIDITY & ENTHALPY
DRY-BULB TEMPERATURE
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
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
WET-BULB, DRY BULB, MOISTURE CONTENT, RELATIVE
HUMIDITY & ENTHALPY
DRY-BULB TEMPERATURE
Return Air: 75ºFDB, 50% r.h.
Supply Air: 55ºFDB, 90% r.h.
Airflow: 1200 cfm
RETURN AIR
SUPPLY AIR
ΔT = Return Air Temp – Supply Air Temp
ΔT = 75ºF - 55ºF = 20ºF
ΔW = Return grains/lbAIR – Supply grains/lbAIR
ΔW = 64 Grains – 60 Grains = 4 grains/lbAIR
Return Air: 75ºFDB, 50% r.h.
Δh = Return btu/lbAIR – Supply btu/lbAIR
Supply Air: 55ºFDB, 90% r.h.
Δh = 28.1 btu/lbAIR - 21.6 btu/lbAIR = 6.5 btu/lbAIR
Airflow: 1200 cfm
h = 28.1 btu/lbAIR
h = 21.6 btu/lbAIR
RETURN AIR
64 grains/lb
60 grains/lb
SUPPLY AIR
55ºF
75ºF
AIR FORMULAE
QT = QS + QL
QT = 4.5 x cfm x Δh
Qs = 1.08 x cfm x ΔT
QL = 0.68 x cfm x ΔW
Yeah, yeah, but where do they come from?
ON PLANET ENEGUE...
100 MILES
24 HOURS
365 DAYS
X
X
HOUR
DAY
YEAR
100 x 24 x 365 x 5280 FEET
YEAR
5280 FEET
X
MILE
12 IN
2.54 cm
10 mm
X
X
X
cm
FT
INCH
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!
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?
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
Total Heat Formula
• We all know QT = 4.5 x CFM x Δh
• 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
Total Heat Formula
• QT (btu/hour)= 4.5 x CFM x Δh
• Units on the right must be the same as the
units on the left
Let the students “BUILD” the Sensible
Heat Formula...
Heat Formulae Variables
So, ask your students what variables and
factors will have an effect on the amount
of heat transferred by the process
Δh?
Total Heat Formula
We have btu/hour on the left...
btu/hour = ? x ? x ? x ? x ?
Which factor, Δh, ΔW, or ΔT, is associated with the total heat?
btu/hour = Δh (btu/lbAIR) x ? x ? x ? x ?
Which other factors are associated with the total heat?
Total Heat Formula
btu/hr = Δh (btu/lbAIR) x ? x ? x ? x ?
Airflow
btu/hr = Δh (btu/lbAIR) x ft3/min x ? x ?
btu/hr = Δh (btu/lbAIR) x ft3/min x 60 min/hr
btu/hr = 60 x (btu x ft3)/hour x lbAIR x ?
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
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 Δh
Sensible Heat Formula
• We all know QS = 1.08 x CFM x ΔT
• 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
Sensible Heat Formula
Which factor, Δh, ΔW, or ΔT, 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
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 ΔT
Challenges with the Sensible
Heat Formula
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It doesn’t 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
Latent Heat Formula
• We all know QL = 0.68 x CFM x ΔW
• 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
Latent Heat Formula
Which factor, Δh, ΔW, or ΔT, is associated with sensible heat?
ΔW = 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 ?
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
RETURN AIR
SUPPLY AIR
Water Vapor at 75ºF
Water at 50ºF
STEAM TABLES ACCOMPLISH ONE THING!
Pertinent Enthalpy Information
ENTHALPY
TEMP °F
38
40
42
44
46
48
50
52
54
56
58
60
62
64
66
Saturated
Vapor Btu/Lb
1078
1079
1080
1081
1081
1082
1083
1084
1084
1085
1086
1087
1088
1089
1090
Saturated
Liquid Btu/Lb
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
ENTHALPY
TEMP °F
68
70
72
73
74
75
76
77
78
80
82
84
86
88
90
Saturated
Vapor Btu/Lb
1091
1092
1093
1093
1094
1094
1095
1095
1096
1096
1097
1098
1099
1100
1100
Saturated
Liquid Btu/Lb
36
38
40
41
42
43
44
45
46
48
50
52
54
56
58
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 ΔW
You can find automated steam tables at:
www.efunda.com/Materials/water/steamtable_sat.cfm
Enter Temperature Here
Read Cool Stuff Here
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
LAW OF THE TEE
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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
?
5 AMPS
2 AMPS
?
5 GPM
2 GPM
5 GPM @ 100ºF
5 GPM @ 140ºF
?
5 GPM @ 100ºF
3 GPM @ 140ºF
?
Here’s The Math...
(5 GPM x 100ºF) + (3 GPM x 140ºF) = (8 GPM x YºF)
500 + 420 = 8YºF
920 = 8YºF
Y = 115ºF
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!
LAW OF THE TEE FOR WATER
CLASSROOM DEMONSTRATION or EXPERIMENT
THE RESULTS:
15ºF
40ºF
15ºF
55ºF
70ºF
LAW OF THE TEE FOR WATER
CLASSROOM DEMONSTRATION or EXPERIMENT
40ºF
2 CUPS
70ºF
1 CUP
LAW OF THE TEE FOR WATER
CLASSROOM DEMONSTRATION or EXPERIMENT
THE RESULTS:
10ºF
40ºF
20ºF
50ºF
70ºF
LAW OF THE TEE FOR MIXED AIR
OUTSIDE AIR
RETURN AIR
MIXED AIR
AIR
HANDLER
LAW OF THE TEE FOR MIXED AIR
PERCENTAGE OF RETURN AIR
+ PERCENTAGE OF OUTSIDE AIR
100% of MIXED AIR
OUTSIDE
RETURN
LAW OF THE TEE FOR MIXED AIR
SAMPLE PROBLEM
AIR CONDITIONS:
RETURN AIR (80%): 75ºFDB, 50%RH
OUTSIDE AIR (20%): 85ºFDB, 60%RH
MIXED AIR = 80% RETURN AIR + 20% OUTSIDE AIR
MIXED AIR = (.80) RETURN AIR + (.20) OUTSIDE AIR
MIXED AIR = (.80) (75ºFDB, 50%RH) + (.20) (85ºFDB, 60%RH)
MIXED AIR = 60ºFDB, 40%RH + 17ºFDB, 12%RH
MIXED AIR = 77ºFDB, 52%RH
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
Eugene Silberstein
917-428-0044
[email protected]