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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 CONFERENCE LAS VEGAS, NEVADA MARCH 20-22, 2011

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**What Makes Psychrometrics so Painful for our Students?**

Unfortunately, most of the time it’s us!

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**How Do We Introduce the Topic?**

You guys are going to hate this This stuff is really difficult This involves a ton of math You’re not going to understand this but it’s okay because I don’t either 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? pounds? pounds? 100 pounds? pounds? 500 pounds? pounds? 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 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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65 70 75 100% 75 80% WET BULB TEMPERATURE WET BULB TEMPERATURE 70 68 60% 65 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**

DRY-BULB TEMPERATURE WET BULB TEMPERATURE

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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 % FULL = 10 CARS 20 SPACES X 100% % FULL = # of CARS**

Example #1 HOW FULL IS THE PARKING LOT? % FULL = 10 CARS 20 SPACES X 100% % FULL = # of CARS # of SPACES X 100% % 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**

DRY-BULB TEMPERATURE WET BULB TEMPERATURE

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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**

DRY-BULB TEMPERATURE

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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**

DRY-BULB TEMPERATURE

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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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**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. Supply Air: 55ºFDB, 90% r.h. Airflow: 1200 cfm Δh = Return btu/lbAIR – Supply btu/lbAIR Δh = 28.1 btu/lbAIR btu/lbAIR = 6.5 btu/lbAIR h = 28.1 btu/lbAIR h = 21.6 btu/lbAIR RETURN AIR 64 grains/lb 60 grains/lb SUPPLY AIR 55ºF ºF

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**Yeah, yeah, but where do they come from?**

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?

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ON PLANET ENEGUE...

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**100 x 24 x 365 x 5280 x 12 x 2.54 x 10 mm/year, which is....**

100 MILES 24 HOURS DAY 365 DAYS YEAR 5280 FEET MILE X X X HOUR 100 x 24 x 365 x 5280 FEET YEAR 12 IN FT 2.54 cm INCH 10 mm cm X X X 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 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 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

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**Let the students “BUILD” the Sensible Heat Formula...**

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...

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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 ΔW? 60 MIN = 1 HOUR? CFM? ΔT? Δh? 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, 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?

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**Total Heat Formula btu/hr = 60 x (btu x ft3)/hour x lbAIR x ?**

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 ?

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**Density 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 btu/hr = 60 x 0.075 btu/hour**

Density = lb/ft3 at atmospheric conditions btu/hr = 60 x btu/hour QT (btu/hr) = 4.5 x Airflow x Δh

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**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

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**Sensible Heat Formula btu/hour = 4.5 x cfm x lb/hour x ? Specific Heat**

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 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 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 = 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

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**Challenges with the Sensible Heat Formula**

It doesn’t always give accurate results The 1.08 is only an estimate The 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 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

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**Latent Heat Formula btu/hour = cfm x 60 x 0.075 x lb/hour x ?**

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 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**

TEMP °F Saturated Vapor Btu/Lb Saturated Liquid Btu/Lb

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**Latent Heat Formula QL (btu/hr) = 0.68 x Airflow x ΔW**

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 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

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**You can find automated steam tables at:**

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 100ºF 5 140ºF

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? 5 100ºF 3 140ºF

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**(5 GPM x 100ºF) + (3 GPM x 140ºF) = (8 GPM x YºF)**

Here’s The Math... (5 GPM x 100ºF) + (3 GPM x 140ºF) = (8 GPM x YºF) = 8YºF 920 = 8YºF Y = 115ºF

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**CLASSROOM DEMONSTRATION or EXPERIMENT**

LAW OF THE TEE FOR WATER CLASSROOM DEMONSTRATION or EXPERIMENT 40ºF 70ºF 1 CUP CUP Have students predict final mixed temperature.... Then combine, mix, measure and confirm..... Then change the rules!

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**CLASSROOM DEMONSTRATION or EXPERIMENT**

LAW OF THE TEE FOR WATER CLASSROOM DEMONSTRATION or EXPERIMENT THE RESULTS: 40ºF 70ºF 55ºF 15ºF

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**CLASSROOM DEMONSTRATION or EXPERIMENT**

LAW OF THE TEE FOR WATER CLASSROOM DEMONSTRATION or EXPERIMENT 40ºF 70ºF 2 CUPS CUP

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**CLASSROOM DEMONSTRATION or EXPERIMENT**

LAW OF THE TEE FOR WATER CLASSROOM DEMONSTRATION or EXPERIMENT THE RESULTS: 10ºF 20ºF 40ºF 50ºF 70ºF

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**LAW OF THE TEE FOR MIXED AIR**

OUTSIDE AIR MIXED AIR AIR HANDLER RETURN AIR

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**LAW OF THE TEE FOR MIXED AIR**

PERCENTAGE OF RETURN AIR + PERCENTAGE OF OUTSIDE AIR 100% of MIXED AIR OUTSIDE RETURN

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**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

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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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**917-428-0044 silbere@sunysuffolk.edu**

Eugene Silberstein

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