Heat Gains

1
Heat Gains

• HVAC 7ab
• CNST 305
• Environmental Systems 1
• Dr. Berryman

2
Cooling Load Components
roof
lights
partition wall
people
infiltration
glass solar
equipment
glass conduction
exterior wall
floor
3
Sensible and Latent Gains
sensible load
latent load
cooling load components
conduction through roof, walls, windows,
and skylights
solar radiation through windows, skylights
conduction through ceiling, interior
partition walls, and floor
people
lights
equipment/appliances
infiltration
ventilation
system heat gains
4
Time of Peak Cooling Load
heat gain
12 6 12 6 12
noon
a.m.
p.m.
mid
mid
5
Sunlit Surfaces
sun rays
solar angle changes throughout the day
6
Time Lag
time lag
B
A
solar effect
12 6 12 6 12
noon
a.m.
p.m.
mid
mid
7
Storage Effect (thermal lag)
8
Conduction Sunlit Surfaces

• A factor called the cooling load temperature
  difference (CLTD) is used to account for the
  added heat transfer due to the sun shining on
  exterior walls, roofs, and windows, and the
  capacity of the wall and roof to store heat. The
  CLTD is substituted for ?T in the equation to
  estimate heat transfer by conduction.

BH U ? A ? ?T
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Roofs

• Based on
• Solar radiation at 40o lat on July 21
• Dark surface
• OA 95oF
• Outdoor mean of 85oF
• Daily Range of 21oF
• No Ventilation

10
CLTD Correction - Roof
CLTDcorr (CLTD LM)k (78 tR) (tO 85)f
to OA (DR/2)
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Latitude Month Adjustment
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Roof Types

• Select closest construction
• Weight
• Construction
• Compare U values
• Additional insulation
• Use a CLTD whose roof weight and heat capacity
  are approximately the same
• Find peak gain during the day
• For each R-7 above selected roof type
• Move value 2 hours later
• 29oF is the lowest adjustment value you can use

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Sunlit Walls

• Based on
• Solar radiation at 40o lat on July 21
• Dark surface
• OA 95oF
• Outdoor mean of 85oF
• Daily Range of 21oF

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Wall Groups

• Select closest wall group
• Compare U-values
• Move up one wall group for each R-7

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Correcting CLTD - Walls

• CLTDcorr (CLTD LM)k (78 tR) (tO 85)

to OA (DR/2)
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Roof Calculation
Find peak cooling load

• Given
• New Orleans, LA
• OA DB93oF WB77oF
• IA 77oF 40 RH
• 30o N. Latitude June 22
• Daily Range of 16oF
• 5000 SF light colored steel sheet roof w/ drop
  ceiling rural area
• No attic ventilation
• Rtotal 21

Closest roof type 1 Peak 1500 hrs CLTDuncorr
78oF
EXAMPLE
Correct for insulation
R7.5 vs 21 (4 hrs) CLTDuncorr 42oF
Correct CLTD
next slide
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CLTD Correction - Roof
CLTDcorr (CLTD LM)k (78 tR) (tO 85)f
42oF
2oF
0.5
77oF
1.0
85oF
CLTDcorr 23oF
93oF
16oF
tO 85oF
(1/21)(5000)(23oF)
5476 BH
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Wall Calculation
Determine Wall Group

• Given
• New Orleans, LA
• OA DB93oF WB77oF
• IA 77oF 40 RH
• 30o N. Latitude June 22
• Daily Range of 16oF
• 12x100 light colored metal curtain wall rural
  area West facing
• Rtotal 19

Type G Metal Curtain Wall
EXAMPLE
Correct for insulation
R values 5.6 - 12.3 vs 19 (up 1 wall group) Use
Type F Wall Group
Correct CLTD
next slide
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CLTD Correction - Walls
CLTDcorr (CLTD LM)k (78 tR) (tO 85)
28oF
0oF
0.65
77oF
85oF
CLTDcorr 19.2oF
(1/19)(12 x 100)(19.2oF)
1213 BTUH
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Sunlit Glass
BH solar gain conduction
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Glass - Conduction
BHconduction U ? A ? CLTD

• Based on
• IA 78oF
• OA 95oF
• Daily Avg 85oF
• DR 20oF

CLTDcorr CLTD (78 tR) (tO 85)
Calculate CLTDcorr like roof/walls
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Solar Heat Gain Factors

• Direction that the window faces
• Time of day
• Month
• Latitude
• Construction of interior partition walls
• Type of floor covering
• Existence of internal shading devices

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Types of Shading Devices
interior blinds
exterior fins
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Glass Solar Gain

• The equation used to predict the solar heat gain
  (radiation) through glass is
• BHglass SHGF x A x SC x CLF where,
• BH heat gain by solar radiation through glass,
  Btu/hr
• SHGF solar heat gain factor, Btu/hrft2
• A total surface area of the glass, ft2
• SC shading coefficient of the window,
  dimensionless
• CLF cooling load factor, dimensionless

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SHGF
Solar energy through fenestration
for Sunlit Glass
use N(shade) for non-sunlit glass
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SC
82 Solar Reduction
Blinds or drapes absorb the solar energy before
it can strike the floor causing a rapid response
in the cooling load
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CLF
Without interior shading
When shading is absent Energy is absorbed by the
more massive elements of the space
Heavier construction larger heat gain delay
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CLF with interior shading
Reduction in the amplitude of the solar heat gain
due to the constructions
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Window Calculation

• GIVEN
• New Orleans, LA
• OA DB93oF WB77oF
• IA 77oF 40 RH
• 30o N. Latitude June 22
• Daily Range of 16oF
• Light venetian blinds
• 30 40 DH Clear Glass
• R 2
• West facing at 1400 HRS

Conduction
BH U ? A ? CLTD
EXAMPLE
CLTDcorr 14oF(78oF77oFF) (85oF85oF)
(½)(3 x 4)(15oF)
90 BH
Solar Gain
BH SHGF x A x SC x CLF
(214 BH)(12)(0.58)(.53)
790 BH
BHtotal 880
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Lighting
1 watt 3.4 BTUH
BHlight watts ? 3.41 ? ballast factor ? CLF

• BH sensible heat gain from lighting, Btu/hr W
• Watts total energy input to lights, W
• 3.41 conversion factor from W to Btu/hr
• Ballast factor 1.2 for fluorescent lights, 1.0
  for incandescent lighting
• CLF cooling load factor, dimensionless

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Lighting Estimates
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LightCLF
Dependent on 1) Occupied Hours and 2)Design
Values
33
Space versus Plenum Loads
Heat absorbed by Return Air
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CLF Design Values (Coefficients)
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Lighting Calculation
BHlight watts ? 3.41 ? ballast factor ? CLF

• GIVEN
• Church w/ 1100 service
• Fluorescent lighting
• Lights on 0800
• Lights off at 1600
• Medium Ventilation Rate
• Supply/return through floor
• Ceiling space not vented
• Ordinary furniture w/ no carpet
• 6 Concrete floor 40x80

(1Wx3200SF)(3.41BH/W)(1.2)(0.75)
1w
Watts per SF
Ballast factor
1.2
Design value of a
0.68
Design value of b
B
CLF
0.75
9821 BH
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People
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Equipment - Office
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Equipment-Restaurant
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Heat Gain in Ductwork

• If insulated Add 1-3 depending of the extent
  of the duct work
• Not insulated Add 10 15 depending on extent
  of duct work or climate (best to calculate gain
  by conduction)

Duct leakage If outside of conditioned space
add 5
BH U ? A ? ?T
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System Heat Gains

• Fan Motor
• Fan Blades
• Duct Friction

fan motor heat gain power input to motor ? (1
motor efficiency)
fan blade heat gain power input to fan ? (1
fan efficiency)
duct friction heat gain power input to fan ?
fan efficiency
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Sample Form Heat Gain

• Space
• Wall,Roof
• Floor, Glass
• Ventilation
• Infiltration
• Internal
• Lights
• People
• Equipment
• Plenum
• Duct gain
• Duct leakage
• System
• Motor, Fan

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Heat GainAssignment

• Use Dinky Office Building
• Calculate total heat gain using your building
  design
• Turn in (in order)
• Assumption sheet
• Hand calculations of room 101
• Excel spreadsheet Heat gain
• Floor plan w/ building orientation
• Corrected Wall Section
• Corrected Heat Loss calculations
• Window/Door data sheets

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Rules of Thumb
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Balancing the System

• 12,000 BTUH / Ton
• CFM

45
Next Time

• Computerized Load Calculations
• Wrightsoft Right-N