Presentation on theme: "...WITHOUT TEARS Professor Eugene Silberstein, CMHE SUFFOLK COUNTY COMMUNITY COLLEGE – BRENTWOOD, NY CENGAGE DELMAR LEARNING – CLIFTON PARK, NY HVAC EXCELLENCE."— Presentation transcript:
...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 20-22, 2011
What Makes Psychrometrics so Painful for our Students? Unfortunately, most of the time its 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 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
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 = 60,720ft 3 Based on this volume, the air in this room weighs approximately: 60,720 ft 3 x lb/ft 3 = 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 BULB TEMPERATURE WET-BULB, DRY-BULB COMBO
% 80% 60% DRY BULB TEMPERATURE WET BULB TEMPERATURE
---- 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 MOISTURE CONTENT (BTU/LB AIR )
DRY-BULB TEMPERATURE WET BULB TEMPERATURE 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 SPACES X 100% % FULL = 10 CARS 20 SPACES X 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 BULB TEMPERATURE 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
SPECIFIC VOLUME & DENSITY Specific volume and density are reciprocals of each other Density = lb/ft 3 Specific volume = ft 3 /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
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/lb AIR h = 21.6 btu/lb AIR ΔW = Return grains/lb AIR – Supply grains/lb AIR ΔW = 64 Grains – 60 Grains = 4 grains/lb AIR Δh = Return btu/lb AIR – Supply btu/lb AIR Δh = 28.1 btu/lb AIR btu/lb AIR = 6.5 btu/lb AIR
AIR FORMULAE Q L = 0.68 x cfm x ΔW Q T = Q S + Q L Q T = 4.5 x cfm x Δh Q s = 1.08 x cfm x ΔT Yeah, yeah, but where do they come from?
ON PLANET ENEGUE...
100 MILES HOUR X 24 HOURS DAY X 365 DAYS YEAR X 5280 FEET MILE 100 x 24 x 365 x 5280 FEET YEAR X 12 IN FT X 2.54 cm INCH X 10 mm cm So, my rate of speed was 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 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 Q T = 4.5 x CFM x Δh Where does the 4.5 come from? Work with the units –Q T (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
Q T (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/lb AIR ) x ? x ? x ? x ? Which other factors are associated with the total heat?
btu/hr = 60 x (btu x ft 3 )/hour x lb AIR x ? Total Heat Formula btu/hr = Δh (btu/lb AIR ) x ? x ? x ? x ? btu/hr = Δh (btu/lb AIR ) x ft 3 /min x ? x ? Airflow btu/hr = Δh (btu/lb AIR ) x ft 3 /min x 60 min/hr
We need to get rid of the ft 3 in the numerator and the lb AIR in the denominator... What factor relating to air has ft 3 in the denominator and lb in the denominator? Density btu/hr = 60 x (btu x ft 3 )/hour x lb AIR x ? btu/hr = 60 x (btu x ft 3 )/hour x lb AIR x lb/ft 3
Density = lb/ft 3 at atmospheric conditions btu/hr = 60 x btu/hour Q T (btu/hr) = 4.5 x Airflow x Δh Total Heat Formula
Sensible Heat Formula We all know Q S = 1.08 x CFM x ΔT Where does the 1.08 come from? Work with the units –Q S (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 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: Q S (btu/hr) = 1.08 x Airflow x ΔT
Challenges with the Sensible Heat Formula It doesnt always give accurate results The 1.08 is only an estimate The lb/ft 3 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 Q L = 0.68 x CFM x ΔW Where does the 0.68 come from? Work with the units –Q L (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 x lb/hour x ? btu/hour = 4.5 x cfm x lb AIR /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/lb AIR 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
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 btu/lb From the steam table we get: btu/hour = [(4.5 x 1076) ÷ 7000] x cfm x lb/hour x btu/lb Q L (btu/hr) = 0.68 x Airflow x ΔW
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 100ºF ? 5 140ºF
5 100ºF ? 3 140ºF
Heres The Math... (5 GPM x 100ºF) + (3 GPM x 140ºF) = (8 GPM x YºF) = 8YºF 920 = 8YºF Y = 115ºF
LAW OF THE TEE FOR WATER CLASSROOM DEMONSTRATION or EXPERIMENT 1 CUP 40ºF70ºF 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: 40ºF 70ºF 55ºF 15ºF
LAW OF THE TEE FOR WATER CLASSROOM DEMONSTRATION or EXPERIMENT 2 CUPS 1 CUP 40ºF70ºF
LAW OF THE TEE FOR WATER CLASSROOM DEMONSTRATION or EXPERIMENT THE RESULTS: 40ºF 70ºF 10ºF 20ºF 50ºF
LAW OF THE TEE FOR MIXED AIR AIR HANDLER OUTSIDE AIRRETURN AIRMIXED 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 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. RETURN AIR OUTSIDE AIR MIXED AIR SUPPLY AIR