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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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Determine Design Equivalent
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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Determine Design Equivalent
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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Cooling LoadsInsert roof values
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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Cooling LoadsInsert roof values
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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Cooling LoadsInsert roof values
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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Cooling LoadsInsert roof values
Insert wall values
Insert glass values
Insert outdoor airvalues
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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Cooling LoadsInsert roof values
Insert wall values
Insert glass values
Insert outdoor airvalues
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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Cooling Degree Hours
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Cooling Degree Hours
Relative indicator of warmth
S: p. 1496, T.B.1
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Cooling Degree Hours
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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