instructions - weeklyjoys · ashrae psychrometric chart no.1 normal temperature barometric...

CE311 Fall 2016 Mid-Term Exam 1 Name: __________________________________

! 1/6

Instructions

1.! Please solve the problems using the specified SI or IP system of units.

2.! Please show all of your calculations to get full credit and pay attention to assumptions.

3.! Neatly mark points and outline processes on the ASHRAE SI or IP psychrometric chart. �

4.! Final solutions should be clearly summarized and marked/boxed. � 5.! Use only the attached equation sheet.

6.! Please attach additional pages of your work, clearly marking each problem.

Problem 1 (5/100pts)

Moist air exists at 85°F dry-bulb temperature, 40% relative humidity, and a total pressure of 1 atm. Use the psychrometric chart to determine the following values (with units, neatly mark points on chart):

(a)!Dew-point temperature (1pt)

(b)!Wet-bulb temperature (1pt)

(c)! Humidity ratio (1pt)

(d)!Mixture enthalpy (1pt)

(e)! Specific volume (1pt)

Problem 2 (2.5/100pts)

Would you recommend an evaporative cooling system for use in the summer in Florida? In Arizona?

Briefly explain why.


! 2/6


It’s February here in West Lafayette, Indiana. Describe how you would condition the outdoor air in an

air handling unit (AHU) of a building in order to meet the thermal comfort preferences of the occupants.

Please list the AHU components you would use and neatly illustrate the process on the psychrometric

chart to the right.


Briefly explain Dalton’s Law of Partial Pressures (Dalton’s Model) through both words and equation(s).


What is the difference between the humidity ratio and relative humidity? Explain their relationship to

temperature.


! 3/6


It’s winter and you’re in a large classroom with a volume 150 m3 at a dry-bulb temperature of 22°C, a total pressure of 96 kPa, and a relative humidity of 45%. Determine:

(a) Humidity ratio (5pts)

(b) Mass of dry air in the room (5pts)

(c) Mass of water vapor in the room (5pts)

(d) Will condensation occur on a window with a surface temperature of 2°C? (5pts)


4/6


An AHU is operating to heat and humidify the outdoor air before it is supplied to a room. The air first

passes over a pre-heat coil, then through an evaporative cooler, and finally, a re-heat coil. The pre-heat

coil is used to prevent the water in the evaporative cooler from freezing. The air flows through a perfectly

insulated rectangular galvanized steel duct of 40cm by 30cm.

Conditions:

Total pressure of 1atm at all points.

Point 1: Dry-bulb temperature of 8°C, relative humidity of 75%, velocity of 30 m/s. Point 3: Relative humidity of 90%.

Point 4: Dry-bulb temperature of 23°C, relative humidity of 50%.

Determine:

(a) Neatly sketch the process on a psychrometric chart and label all points (5pts)

(b) The amount of heat added by the pre-heat (10pts) and re-heat coils (10pts). What is the ratio of the

heat added by the pre-heat coil to the re-heat coil (4pts)?

(c) The rate at which water is added in the evaporative cooler (6pts) (!")


! 5/6


It’s Dubai in July. You need to provide cool air to occupants in a building. First, you adiabatically mix

outdoor air with recirculation air. The mixed air stream is then cooled and dehumidified, followed by

re-heat, before it is supplied to occupied spaces.

Conditions:

Total pressure of 1atm at all points.

Outdoor air: dry-bulb temperature of 44°C, relative humidity of 50%, and mass flow rate is !$% = 400 kg dry air/min.

Recirculation air: wet-bulb temperature of 19°C, specific volume of 0.8675 m3/kg dry air, and mass flow rate, !$&, is 33% greater than !$%.

Supply air: dry-bulb temperature of 20°C, relative humidity of 50%.

Collected condensate: temperature of 8°C.

(a)!Draw a schematic of an AHU for this process. Label each point and the components (5pts)

(d)!Neatly sketch the process on a psychrometric chart and label all points (5pts)

(b)!Determine the heat removed by the cooling coil (7.5pts) and heat added by the re-heat coil (7.5pts).


! 6/6

(c)! Determine the amount of water removed from the air during the cooling and dehumidification

process as kg water/kg dry air (5pts)

!" =$"$ !

!":&'()*+,-./0'$!$": $1&2),*'+*&'()3*'+*.'&4'$)$/*5-3*0* &'( !$: /'/-(*$1&2),*'+*&'()3*'+*&06/1,)* &'( !!

7" =&"$"

!

789:*;"9 = 28.97!7A;BC9*D;EF9 = 18.02!I = 8.314!

I" =I7"

!

7":&'().1(-,*L)05ℎ/*'+*5-3*0*(5/&'()!&":&-33*'+*5-3*0*(5)!789:*;"9 = &'().1(-,*L)05ℎ/*'+*Q,!*-0,*(g/mol)!7A;BC9*D;EF9 = &'().1(-,*L)05ℎ/*'+*L-/),*(g/mol)!I: 1$0V),3-(*5-3*.'$3/-$/*(WX/W&'( − Z)!I" = 5-3*.'$3/-$/*'+*5-3*0!!

!

ideal*gas*equation*of*state:!PV = nIg!

T: temperature*!V: volume! !P: pressure! !

ω = &D&;

!

ω = 0.622 nDn;!

φ = nDnp!

φ = qn(0.622 + q)np

!

ω: humidity*ratio! (e.g.!kg!wv/kg!da)!&D:mass*of*water*vapor! !&;:mass*of*dry*air! !nD: partial*pressure*of*water*vapor!n;: partial*pressure*of*dry*air! !φ: relative*humidity! (-)!np: saturation*vapor*pressure! !!

h = ℎ; + qℎD! h:moist*air*mixture*enthalpy*(kJ/kg*dry*air)!ℎ;: )$/ℎ-(4!*'+*Q,!*-0,*(kJ/kg*dry*air)!ℎD: )$/ℎ-(4!*'+*L-/),*V-4',*(kJ/kg*water*vapor)!

!conservation*of*mass:!

&; ="

&;C

!

&D +"

&A"

= &D +C

&AC

!

e:*exit*******i: inlet!&;:&-33*+('L*,-/)*'+*Q,!*-0,*(W5*Q,!*-0,/&0$)!&D:&-33*+('L*,-/)*'+*L-/),*V-4',*(W5*L-/),*V-4',/&0$)!&A:&-33*+('L*,-/)*'+*(0z10Q*L-/),*(W5*(0z10Q*L-/),/&0$)!!

conservation*of*energy:!

{ −| = &CℎC −C

&}ℎ}"

!

!

!

RR

ASHR

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9.92

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199

2AM

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OF

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1015

2025

303540455055

55

60

60

EN

THA

LPY

- B

TU P

ER

PO

UN

D O

F D

RY

AIR

15

20

25

30

35

40

45

50

ENTHALPY - B

TU PER POUND OF D

RY AIR

SATURATION TEMPERATURE - °

F

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

DRY BULB TEMPERATURE - °F

.002

.004

.006

.008

.010

.012

.014

.016

.018

.020

.022

.024

.026

.028

10%

REL

ATIV

E H

UM

IDIT

Y

20%

30%

40%

50%

60%

70%

80%

90%

35

3540

4045

4550

5055

5560

6065

65

70

70

75

75

80

80

85 W

ET B

ULB

TEM

PERA

TURE

- °F

85

90

12.5

13.0

13.5

14.0 VOLUME- CU.FT. PER LB. DRY AIR

14.5

15.0

HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR

0

1.0

1.0

:-

2.0

4.0

8.0

-8.0

-4.0

-2.0-1.0

-0.5-0.4

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

0.5

0.6

0.8

-200

0-1

000

0

500

1000

1500

2000 3000

5000

- ::

SENS

IBLE

HEA

TQ

sTO

TAL

HEAT

Qt

ENTH

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AL T

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ESSU

RE: 2

9.92

1 IN

CHES

OF

MER

CURY

Copy

right

199

2AM

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AN S

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ETY

OF

HEAT

ING

, REF

RIG

ERAT

ING

AND

AIR

-CO

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, INC

.

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AMER

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1020

3040

5060708090100

110

110

120

120 E

NTH

ALP

Y -

KJ

PE

R K

ILO

GR

AM

OF

DR

Y A

IR

10

20

30

40

50

60

70

80

90

100

ENTHALPY - K

J PER KILO

GRAM OF D

RY AIR

SATURATION TEMPERATURE - °

C

5

10

15

20

25

30

35

40

45

50

DRY BULB TEMPERATURE - °C

.001

.002

.003

.004

.005

.006

.007

.008

.009

.010

.011

.012

.013

.014

.015

.016

.017

.018

.019

.020

.021

.022

.023

.024

.025

.026

.027

.028

.029

.030

10%

REL

ATIV

E H

UM

IDIT

Y

20%

30%

40%

50%

60%

70%

80%

90%

5

5

10

10

15

15

20

20

25

25

30 W

ET B

ULB

TEM

PERA

TURE

- °C

30

0.78

0.80

0.82

0.84

0.86 VOLUME - CUBIC METER PER KG DRY AIR

0.88

0.90

0.92

0.94

HUMIDITY RATIO - KILOGRAMS MOISTURE PER KILOGRAM DRY AIR

0

1.0

1.0

:-

1.5

2.0

4.0

-4.0

-2.0-1.0

-0.5

-0.2

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-5.0

-2.0

0.0

1.0

2.0

2.5

3.0

4.0

5.0

10.0

- ::

SENS

IBLE

HEA

TQ

sTO

TAL

HEAT

Qt

ENTH

ALPY

HUM

IDIT

Y RA

TIO

Dh

DW

RR

ASHR

AE P

SYCH

ROM

ETRI

C CH

ART

NO.1

NORM

AL T

EMPE

RATU

REBA

ROM

ETRI

C PR

ESSU

RE: 1

01.3

25 k

PaCo

pyrig

ht 1

992

AMER

ICAN

SO

CIET

Y O

F HE

ATIN

G, R

EFRI

GER

ATIN

G A

ND A

IR-C

OND

ITIO

NING

ENG

INEE

RS, I

NC.

SEA

LEVE

L

Name __________________________

1

Name __________________________

2

instructions - weeklyjoys · ashrae psychrometric chart no.1 normal temperature barometric...

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