without tears professor eugene silberstein, cmhe suffolk county community college – brentwood, ny...

...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


• 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

“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


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

)


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%


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 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


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


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


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


• 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?


We already have some of our variables in place


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


• 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


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


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!


THE RESULTS:

40ºF 70ºF 55ºF

15ºF 15ºF


2 CUPS 1 CUP

40ºF 70ºF


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


Outside Air: 85ºFDB, 60% r.h.

Mixed Air: 77ºFDB, 52% r.h.

RETURN AIR

OUTSIDE AIR

MIXED AIRSUPPLY AIR

Eugene Silberstein

917-428-0044

[email protected]

without tears professor eugene silberstein, cmhe suffolk county community college – brentwood, ny...

Documents

drybulb combo slide

wetbulb temperature

wet bulb temperature

dry bulb temperature

constant wetbulb temperature

pounds slide

drybulb thermometer

enthalpy slide