A4351S09 12 Cooling Loads

8/13/2019 A4351S09 12 Cooling Loads

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Environmental Controls I/IG

Lecture 12

Cooling Loads

Cooling Degree Hours

Energy Performance Ratings

Annual Fuel Consumption


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Cooling LoadsComputed for worstcase scenario:

Late summer afternoonat outdoor design dry

bulb temperature

Include:

Insolation from sun

Heat gain from people,

lights, and equipment Infiltration in residential

buildings

Ventilation innonresidential buildings

SR-3


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Summer Design Conditions

Design Dry Bulb Temperature

Mean Daily Range

S: p. 1496, T.B1


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Determine Design Equivalent

Temperature Difference (DETD)Construction type

Outdoor design temperature

Mean daily range

L: 0-16FM: 16-25FH: 25+F

S: p. 1613, T.F.5


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Determine Envelope U-values

Calculate R and then find U for wallsand roofs.

Note: this method ignores floors, doors,and window U-values


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Determine DCLF

Glazing Type

Design Temperature

Shading

Orientation

S: p. 1615, T.5.6


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Determine Area Quantities

Perform area takeoffs forall building envelopesurfaces on each facade:

gross wall area

window area

door area

net wall area4

Elevation

4

12

100

8

1200 sf

64 sf

368 sf

768 sf

-

-


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Infiltration

S: p. 1617, T.F.7


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Ventilation Analysis

Non-residential buildings use ventilation to providefresh air and to offset infiltration effects.

ASHRAE Standard 62-2001 (S: p. 1598, T.E.25)

Estimates the number of people/1000 sf of usage type

Prescribes minimum ventilation/person for usage type


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ASHRAE 62-2001

Defines space occupancy and ventilation loads

S: p. 1598, T.E.25


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VentilationSensible CLF

S: 1617, T.F.7


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People Sensible Gain

Determine number of people

Activity level

S: p. 1617, T.F.8


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Lights

Determine wattage of lighting/square foot

ASHRAE 90.1 prescriptive levels

Count fixture loads and add together

Note: add 15% for ballasts where applicable

(e.g., fluorescent lights)


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Equipment

Use manufacturers data or otherreferences to obtain heat gain data.

Standby mode

Copiers

Monitors

Printers CPU

energy star

S: p. 1618, T.F.9


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Latent Load

Not calculated separately:

Apply a factor as a percentage of the totalsensible cooling load

Dry climates: 20%Moist/Humid climates: 30%


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Cooling Load Example Problem

Building: Office Building

Location: Salt Lake City

Building: 200 x 100 (2 stories, 12-6 each)

Uwall= 0.054 Btuh/sf-F

Uroof= 0.025 Btuh/sf-F


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Cooling Load Example ProblemDetermine Building Envelope Areas (SF)

Building: 200 x 100 (2 stories, 12-6 each)

N E S WGross Wall 5,000 2,500 5,000 2,500

Windows 1,000 500 2,000 500

Doors 20 20 50 20Net Wall 3,980 1,980 2,950 1,980

Roof/Floor Slab 20,000


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Temperature Difference (DETD)Roof Construction type: Light color, vented, ceilingDesign temperature: 95F

Mean daily range: 32F

L: 0-16FM: 16-25FH: 25+F

DETD=31.0F

S: p. 1613, T.F.5


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Cooling LoadsInsert roof values

SR-3

0.025 20,000 31.0 15,500 15,500


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Temperature Difference (DETD)Wall Construction type (see given)Design temperature: 95F

Mean daily range: 32F

L: 0-16FM: 16-25F

H: 25+F

DETD=11.3F

S: p. 1613, T.F.5


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Insert wall values

SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429

E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645


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Glazing Type

Design Temperature

Shading

Orientation

Determine Window DCLF

S: p.

1615,T.F.6


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Insert wall values

Insert glass values

SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429

E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645

N 1,000 14 14,000E 500 35 17,500S 2,000 20 40,000W 500 35 17,500

89,000


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ASHRAE 62-2001

Defines space occupancy and ventilation loads

S: p. 1598, T.E.25


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Ventilation Load Sensible

40,000 sf x 5people/1,000sf = 200 people

200 people x 17 cfm/person = 3,400 cfm


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Ventilation Load Sensible CLF

Design Temperature: 95F

Commercial Building: Ventilation

S: p. 1617, T.F.7


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Insert wall values

Insert glass values

Insert outdoor airvalues

SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429

E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645

N 1,000 14 14,000E 500 35 17,500S 2,000 20 40,000W 500 35 17,500

89,000

N/A N/A N/A

3,400 22.0 74,800 74,800


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People Sensible Gain

Determine number of people: 280

Activity level: moderately active office work

S: p. 1617, T.F.8


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Insert wall values

Insert glass values


Insert people values

SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429

E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645

N 1,000 14 14,000E 500 35 17,500S 2,000 20 40,000W 500 35 17,500

89,000

N/A N/A N/A

3,400 22.0 74,800 74,800

200 250 50,000


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Lighting Load

Lighting load: 1.5 w/sf

Equipment load: 0.5 w/sf


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Insert wall values

Insert glass values


Insert people values

Insert lighting values

Insert equipmentvalues SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429

E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645

N 1,000 14 14,000E 500 35 17,500S 2,000 20 40,000W 500 35 17,500

89,000

N/A N/A N/A

3,400 22.0 74,800 74,800

200 250 50,000

40,000 1.5 204,780

40,000 0.5 68,260 323,040


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Cooling LoadsSensible Heat Gain:

508985 Btuh

Latent Heat Gain (20%):

101,797 Btuh

Total Heat Gain:

610,782Btuh

or

50.9 Tons

Tons=Q/12,000 SR-3

0.025 20,000 31.0 15,500 15,500

N 0.054 3,980 11.3 2.429

E 0.054 1,980 11.3 1.208S 0.054 2,950 11.3 1,800W 0.054 1,980 11.3 1,208 6,645

N 1,000 14 14,000E 500 35 17,500S 2,000 20 40,000W 500 35 17,500

89,000

N/A N/A N/A

3,400 22.0 74,800 74,800

200 250 50,000

40,000 1.5 204,780

40,000 0.5 68,260 323,040

508,985

101,797

610,78250.9

2.5

1.1

14.6

16.783.3

52.8

12.3

100.0


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Relative indicator of warmth

S: p. 1496, T.B.1


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Balance Point Temperature (BPT):temperature above which cooling is needed

CDH(BPT)= ODBT-BPT

If temperature (ODBT)=91F

CDH74 =ODBT-74=91-74

=17 cooling degree-hours


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Energy Performance Ratings


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Annual Fuel Consumption


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Annual Fuel Usage (E)

E= UA xCDH(BPT)

SEER

where:

UA: cooling load/F

CDH(BPT): degree hours for balance point

SEER: seasonal energy efficiency rating


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Calculating UA

QTotal= UA xT

UA= QTotal/T

From earlier example:

QTotal= Btuh

T= 95-75=20F

UA=610782/20= 30,539 Btuh/F


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Annual Fuel Usage Example

Compare two systems to determine what

is the expected annual electrical usage for

an apartment in Salt Lake City if its peakcooling load is 12,000 Btuh?

UA=Q/T

UA=12,000/20= 600 Btuh/F


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Determine SEER

Obtain SEER from manufacturers data or

Convert COP to SEER

SEER: 5-15

For this example:

SEER1=6.8SEER2=10.2


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Annual Fuel Usage Electricity

E= UA x CDH74

SEER

E1 =(600)(9,898)/(6.8)

=873,353 wh/yr

=873 kwh/yr

If electricity is $0.0735/kwh, then

annual cost = $64


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Annual Fuel Usage Electricity

E= UA x CDH74

SEER

E2 =(600)(9,898)/(10.2)

=582,235 wh/yr

=582 kwh/yr

If electricity is $0.0735/kwh, then

annual cost = $43


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Simple Payback Analysis


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Simple Payback

Cooling SystemCost ComparisonFirst Annual Incremental Incremental Simple

Cost Fuel Cost First Cost Annual Savings Payback($) ($/yr) ($) ($/yr) (yrs)

System 1 500 64 --- --- ---

System 2 600 43 100 21 4.8

Payback exceeds 3 years, select system 1

Other factors?


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A4351S09 12 Cooling Loads