Psychrometrics Without Tears

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...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?

• 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? 50 pounds?

100 pounds? 250 pounds?

500 pounds? 1000 pounds? 1500 pounds?

THE ANSWER MIGHT SURPRISE YOU...

(I Hope It Does!)

Room Dimensions...

• Length: 42 feet

• Width: 42 feet

• Ceiling Height: 16 feet

• Room Volume: 42 x 42 x 16 = 28,224 ft 3

• Based on this volume, the air in this room weighs approximately:

28,224 ft 3 x 0.075 lb/ft 3 =

2,117 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

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

75

75

100%

80%

70

70

68

65

65

65 69 70 71 73 75

DRY BULB TEMPERATURE

60%

---- 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 = 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 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

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?

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/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

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: 75 ºFDB, 50% r.h.

Supply Air: 55 ºFDB, 90% r.h.

Airflow: 1200 cfm

SUPPLY AIR

RETURN AIR

ΔT = Return Air Temp – Supply Air Temp

ΔT = 75ºF - 55ºF = 20ºF

ΔW = Return grains/lb

AIR

– Supply grains/lb

AIR

ΔW = 64 Grains – 60 Grains = 4 grains/lb

AIR

Return Air: 75

Supply Air: 55

ºFDB, 50% r.h.

ºFDB, 90% r.h.

Airflow: 1200 cfm h = 28.1 btu/lb

AIR h = 21.6 btu/lb

AIR

Δh = Return btu/lb

AIR

– Supply btu/lb

AIR

Δh = 28.1 btu/lb

AIR

- 21.6 btu/lb

AIR

= 6.5 btu/lb

AIR

RETURN AIR

SUPPLY AIR

64 grains/lb

60 grains/lb

55 ºF 75ºF

AIR FORMULAE

Q

T

= Q

S

+ Q

L

Q

T

= 4.5 x cfm x Δh

Q s

= 1.08 x cfm x ΔT

Q

L

= 0.68 x cfm x ΔW

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...

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 Q

T

= 4.5 x CFM x Δ h

• 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

Total Heat Formula

• Q

T

(btu/hour)= 4.5 x CFM x Δ h

• 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/lb

AIR

) x ? x ? x ? x ?

Which other factors are associated with the total heat?

Total Heat Formula

btu/hr = Δh (btu/lb

AIR

) x ? x ? x ? x ?

Airflow btu/hr = Δh (btu/lb

AIR

) x ft 3 /min x ? x ?

btu/hr = Δh (btu/lb

AIR

) x ft 3 /min x 60 min/hr 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 ?

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 lb/ft 3

Total Heat Formula

Density = 0.075 lb/ft 3 at atmospheric conditions btu/hr = 60 x 0.075 btu/hour

Q

T

(btu/hr) = 4.5 x Airflow x Δh

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

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:

Q

S

(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/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

Latent Heat Formula

Which factor, Δh, ΔW, or ΔT, is associated with sensible heat?

ΔW = Change in moisture in grains/lb

AIR

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

AIR

/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

Water Vapor at 75ºF

SUPPLY AIR

Water at 50ºF

STEAM TABLES ACCOMPLISH ONE THING!

Pertinent Enthalpy Information

TEMP °F

ENTHALPY

Saturated

Vapor 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

TEMP °F

ENTHALPY

Saturated

Vapor Btu/Lb

Saturated

Liquid Btu/Lb

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

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

Q

L

(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

• 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 @ 100 ºF ?

5 GPM @ 140 ºF

5 GPM @ 100 ºF ?

3 GPM @ 140 ºF

Here’s The Math...

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

500 + 420 = 8Y ºF

920 = 8YºF

Y = 115ºF

LAW OF THE TEE FOR WATER

CLASSROOM DEMONSTRATION or EXPERIMENT

40 ºF 70 ºF

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:

15 ºF 15 ºF

40 ºF 55 ºF 70 ºF

LAW OF THE TEE FOR WATER

CLASSROOM DEMONSTRATION or EXPERIMENT

40 ºF 70 ºF

2 CUPS 1 CUP

LAW OF THE TEE FOR WATER

CLASSROOM DEMONSTRATION or EXPERIMENT

THE RESULTS:

10 ºF 20 ºF

40 ºF 50 ºF 70 ºF

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%): 75 ºFDB, 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) (75 ºFDB, 50%RH) + (.20) (85ºFDB, 60%RH)

MIXED AIR = 60 ºFDB, 40%RH + 17ºFDB, 12%RH

MIXED AIR = 77 ºFDB, 52%RH

Return Air: 75 ºFDB, 50% r.h.

Outside Air: 85 ºFDB, 60% r.h.

Mixed Air: 77 ºFDB, 52% r.h.

SUPPLY AIR

OUTSIDE AIR

MIXED AIR

RETURN AIR

Eugene Silberstein

917-428-0044 silbere@sunysuffolk.edu

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