without tears professor eugene silberstein, cmhe suffolk county community college – brentwood, ny...
TRANSCRIPT
...WITHOUT TEARS
Professor Eugene Silberstein, CMHESUFFOLK COUNTY COMMUNITY COLLEGE – BRENTWOOD, NY
CENGAGE DELMAR LEARNING – CLIFTON PARK, NY
HVAC EXCELLENCE INSTRUCTOR CONFERENCELAS VEGAS, NEVADA
MARCH 20-22, 2011
What Makes Psychrometrics so Painful for our Students?
Unfortunately, most of the time it’s us!
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
• 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?
THE ANSWER MIGHT SURPRISE YOU...
(I Hope It Does!)
0 pounds? 10 pounds? 50 pounds?
100 pounds? 250 pounds?
500 pounds? 1000 pounds? 4500 pounds?
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
The First Four Things...
Dry-Bulb Temperature
Wet-Bulb Temperature
Absolute Humidity
Relative Humidity
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?
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
DRY-BULB TEMPERATURE
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
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 wet-bulb temperature line, the wet bulb temperature does not change
WET BULB TEMPERATURE
The red arrow indicates an increase in the wet bulb temperature reading
The blue arrow indicates a decrease in the wet bulb temperature reading
DRY-BULB TEMPERATURE
WET B
ULB T
EMPERATURE
WET-BULB, DRY-BULB COMBO
SLING PSYCHROMETER
75
70
68
65
65 69 70 71 73 75
65
70
75100%
80%
60%
DRY BULB TEMPERATURE
WE
T B
ULB
TE
MP
ER
AT
UR
E
WET B
ULB T
EMPERATURE
---- 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
• 7,000 grains of moisture = 1 pound
1 POUND
60 GRAINS
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 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
MO
IST
UR
E C
ON
TE
NT
(B
TU
/LB
AIR
)
DRY-BULB TEMPERATURE
WET B
ULB T
EMPERATURE
WET-BULB, DRY BULB & MOISTURE CONTENT
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?
% FULL = # of CARS
# of SPACESX 100% % FULL =
10 CARS
20 SPACESX 100%
% FULL = 0.5 X 100% % FULL = 50%
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
DRY-BULB TEMPERATURE
WET B
ULB T
EMPERATURE
WET-BULB, DRY BULB, MOISTURE CONTENT & RELATIVE HUMIDITY
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
DRY-BULB TEMPERATURE
WET-BULB, DRY BULB, MOISTURE CONTENT, RELATIVE HUMIDITY & ENTHALPY
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
ft 3/lb
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
DRY-BULB TEMPERATURE
WET-BULB, DRY BULB, MOISTURE CONTENT, RELATIVE HUMIDITY & ENTHALPY
RETURN AIR
SUPPLY AIR
Return Air: 75ºFDB, 50% r.h.
Supply Air: 55ºFDB, 90% r.h.
Airflow: 1200 cfm
RETURN AIR
SUPPLY AIR
Return Air: 75ºFDB, 50% r.h.
Supply Air: 55ºFDB, 90% r.h.
Airflow: 1200 cfm
55ºF 75ºF
ΔT = Return Air Temp – Supply Air Temp
ΔT = 75ºF - 55ºF = 20ºF
64 grains/lb
60 grains/lb
h = 28.1 btu/lbAIR
h = 21.6 btu/lbAIR
ΔW = Return grains/lbAIR – Supply grains/lbAIR
ΔW = 64 Grains – 60 Grains = 4 grains/lbAIR
Δh = Return btu/lbAIR – Supply btu/lbAIR
Δh = 28.1 btu/lbAIR - 21.6 btu/lbAIR = 6.5 btu/lbAIR
AIR FORMULAE
QL = 0.68 x cfm x ΔW
QT = QS + QL
QT = 4.5 x cfm x Δh
Qs = 1.08 x cfm x ΔT
Yeah, yeah, but where do they come from?
ON PLANET ENEGUE...
100 MILES
HOURX
24 HOURS
DAYX
365 DAYS
YEARX
5280 FEET
MILE
100 x 24 x 365 x 5280 FEET
YEARX
12 IN
FTX
2.54 cm
INCHX
10 mm cm
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
• QT (btu/hour)= 4.5 x CFM x Δh
• Units on the right must be the same as the units on the left
Total Heat Formula
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
CFM?ΔT?SPECIFIC VOLUME?
60 MIN = 1 HOUR?
SPECIFIC HEAT?
ΔW?
Δh?
We have btu/hour on the left...
btu/hour = ? x ? x ? x ? x ?
Total Heat Formula
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?
btu/hr = 60 x (btu x ft3)/hour x lbAIR x ?
Total Heat Formula
btu/hr = Δh (btu/lbAIR) x ? x ? x ? x ?
btu/hr = Δh (btu/lbAIR) x ft3/min x ? x ?
Airflow
btu/hr = Δh (btu/lbAIR) x ft3/min x 60 min/hr
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 ?
btu/hr = 60 x (btu x ft3)/hour x lbAIR x lb/ft3
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
Total Heat Formula
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
btu/hour = cfm x 60 x 0.075 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
btu/hour = 4.5 x cfm x lb/hour x ?
Which factor, Δh, ΔW, or ΔT, is associated with sensible heat?
Sensible Heat Formula
We already have some of our variables in place
Sensible Heat Formula
btu/hour = 4.5 x lb/hour x 0.24 btu/lb
The specific heat of air is 0.24 btu/lb/ºF
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
• 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
btu/hour = cfm x 60 x 0.075 x lb/hour x ?
btu/hour = 4.5 x cfm x lbAIR/hour x ?
Which factor, Δh, ΔW, or ΔT, is associated with sensible heat?
Latent Heat Formula
We already have some of our variables in place
ΔW = Change in moisture in grains/lbAIR
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 ENTHALPY
TEMP °F TEMP °F Saturated
Vapor Btu/Lb Saturated
Vapor Btu/Lb Saturated
Liquid Btu/Lb Saturated
Liquid Btu/Lb
38 1078 6 40 1079 8 42 1080 10 44 1081 12 46 1081 14 48 1082 16 50 1083 18 52 1084 20 54 1084 22 56 1085 24 58 1086 26 60 1087 28 62 1088 30 64 1089 32 66 1090 34
68 1091 36 70 1092 38 72 1093 40 73 1093 41 74 1094 42 75 1094 43 76 1095 44 77 1095 45 78 1096 46 80 1096 48 82 1097 50 84 1098 52 86 1099 54 88 1100 56 90 1100 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
1094 btu/lb - 18 btu/lb - 1076 btu/lb
From the steam table we get:
btu/hour = [(4.5 x 1076) ÷ 7000] x cfm x lb/hour x btu/lb
QL (btu/hr) = 0.68 x Airflow x ΔW
www.efunda.com/Materials/water/steamtable_sat.cfm
You can find automated steam tables at:
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
• 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 WATERCLASSROOM DEMONSTRATION or EXPERIMENT
1 CUP 1 CUP
40ºF 70ºF
Have students predict final mixed temperature.... Then combine, mix, measure and confirm..... Then change the rules!
LAW OF THE TEE FOR WATERCLASSROOM DEMONSTRATION or EXPERIMENT
THE RESULTS:
40ºF 70ºF 55ºF
15ºF 15ºF
LAW OF THE TEE FOR WATERCLASSROOM DEMONSTRATION or EXPERIMENT
2 CUPS 1 CUP
40ºF 70ºF
LAW OF THE TEE FOR WATERCLASSROOM DEMONSTRATION or EXPERIMENT
THE RESULTS:
40ºF 70ºF
10ºF 20ºF
50ºF
LAW OF THE TEE FOR MIXED AIR
AIR HANDLER
OUTSIDE AIR
RETURN AIR
MIXED AIR
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 AIRSAMPLE 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.
RETURN AIR
OUTSIDE AIR
MIXED AIRSUPPLY AIR