Heat Gains

HVAC 7ab

CNST 305 Environmental Systems 1Dr. Berryman

Cooling Load Componentsroofroof

lightslights

equipmentequipment

floorfloor

exteriorexteriorwallwall

glass solarglass solar

glassglassconductionconduction

infiltrationinfiltrationpeoplepeople

partitionpartitionwallwall

Sensible and Latent Gainssensible

loadlatentload

conduction through roof, walls, windows, and skylightssolar radiation through windows, skylightsconduction through ceiling, interior partition walls, and floorpeoplelightsequipment/appliancesinfiltrationventilationsystem heat gains

cooling load components

Time of Peak Cooling Loadh

eat

gai

nh

eat

gai

n roofeast-facing

window

12 6 12 6 1212 6 12 6 12noonnoona.m.a.m. p.m.p.m. midmidmidmid

Sunlit Surfacessun

rayssun

rayssolar angle changes throughout the daysolar angle changes throughout the day

Time Lagso

lar

effe

ctso

lar

effe

ct

12 6 12 6 1212 6 12 6 12noonnoona.m.a.m. p.m.p.m. midmidmidmid

AA BB

time lagtime lag

Storage Effect (thermal lag)

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 TCLTD

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

CLTD Correction - RoofCLTDcorr = [(CLTD + LM)k + (78 – tR) + (tO – 85)]f

to = OA – (DR/2)

Latitude Month Adjustment

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

Sunlit Walls

Based on:• Solar radiation at

40o lat on July 21• Dark surface• OA 95oF• Outdoor mean of

85oF• Daily Range of

21oF

Wall Groups

•Select closest wall group

•Compare U-values

•Move up one wall group for each R-7

G F E D C B A

Each R-7

Correcting CLTD - Walls

CLTDcorr = (CLTD + LM)k + (78 – tR) + (tO – 85)

to = OA – (DR/2)

Roof Calculation

EXAMPLEGiven:

•New Orleans, LA

•OA DB=93oF WB=77oF

•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

Find peak cooling load:

Closest roof type: 1 Peak: 1500 hrs CLTDuncorr: 78oF

Correct for insulation:

R=7.5 vs 21 (+4 hrs) CLTDuncorr: 42oF

Correct CLTD:

next slide

CLTD Correction - RoofCLTDcorr = [(CLTD + LM)k + (78 – tR) + (tO – 85)]f

42oF 2oF 0.5 77oF 1.0

93oF 16oF

tO = 85oF

tO = (OA – (DR/2)Average outside temperature

85oF

CLTDcorr = 23oF

Heat gain through roof:BH = U A CLTD

(1/21)(5000)(23oF) = 5476 BH

EXAMPLE

Wall Calculation

Given:

•New Orleans, LA

•OA DB=93oF WB=77oF

•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

Determine Wall Group:

Type G Metal Curtain Wall

Correct for insulation:

Correct CLTD:

next slide

R values 5.6 - 12.3 vs 19 (up 1 wall group)Use Type F Wall Group

CLTD Correction - WallsCLTDcorr = (CLTD + LM)k + (78 – tR) + (tO – 85)

28oF 0oF 0.65 77oF 85oF

CLTDcorr = 19.2oF

Heat gain through wall:BH = U A CLTD

(1/19)(12’ x 100’)(19.2oF) =

1213 BTUH

sun rayssun rayssun rayssun rays

reflectedreflectedenergyenergy

reflectedreflectedenergyenergy

transmittedtransmittedenergyenergytransmittedtransmittedenergyenergy

glassglasswindowwindowglassglasswindowwindow

conductionconduction

solar gain solar gain (radiation)(radiation)

Sunlit Glass

BH = solar gain + conductionBH = solar gain + conduction

Glass - Conduction

BHconduction = U A CLTD

Based on:

•IA = 78oF

•OA = 95oF

•Daily Avg – 85oF

•DR = 20oF

Calculate CLTDcorr like roof/wallsCLTDcorr = CLTD + (78 – tR) + (tO – 85)

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

Types of Shading Devices

interiorinteriorblindsblinds

interiorinteriorblindsblinds

exteriorexteriorfinsfinsexteriorexteriorfinsfins

Glass – Solar GainThe 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/hr•ft2

A = total surface area of the glass, ft2

SC = shading coefficient of the window, dimensionless

CLF = cooling load factor, dimensionless

SHGF

Solar energy through fenestration

for Sunlit Glass*

*use N(shade) for non-sunlit glass

SC

Blinds or drapes absorb the solar energy before it can strike the floor causing a rapid response in the cooling load

82% Solar Reduction82% Solar Reduction

CLFWithout interior shading

When shading is absent: Energy is absorbed by the more massive elements of the space

Heavier construction = larger heat gain delay

CLF with interior shading

Reduction in the amplitude of the solar heat gain due to the constructions

Window Calculation

EXAMPLEGIVEN:

•New Orleans, LA

•OA DB=93oF WB=77oF

•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

Solar Gain

BH = U A CLTD

BH = SHGF x A x SC x CLF

(½)(3 x 4)(15oF)CLTDcorr= 14oF+(78oF–77oFF) + (85oF–85oF)

= 90 BH

(214 BH)(12)(0.58)(.53)= 790 BH

BHtotal = 880

Lighting

BHlight = watts 3.41 ballast factor CLF

1 watt = 3.4 BTUH

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

Lighting Estimates

LightCLF

Dependent on:

1) Occupied Hours

and

2)Design Values

Space versus Plenum Loadsroofroof

lightslights

plenumplenum

exteriorexteriorwallwall

return airreturn air

ceilingceiling

Heat absorbed by Return Air

CLF Design Values (Coefficients)

Lighting CalculationGIVEN:

•Church w/ 11:00 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 40’x80’

BHlight = watts 3.41 ballast factor CLF

= 1w

Design value of “a”= 0.68

Design value of “b”= B

Watts per SF

Ballast factor= 1.2

CLF= 0.75

(1Wx3200SF)(3.41BH/W)(1.2)(0.75)

= 9821 BH

People

Equipment - Office

Equipment-Restaurant

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)

BH = U A T

Duct leakage – If outside of conditioned space add 5%

System Heat Gainsair handlerair handlerair handlerair handler

fan motorfan motorfan motorfan motor

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

Fan Motor Fan Blades Duct Friction

Sample Form Heat Gain

Space• Wall,Roof• Floor, Glass• Ventilation• Infiltration

Internal• Lights• People• Equipment

Plenum• Duct gain• Duct leakage

System• Motor, Fan

Heat GainAssignment

Use Dinky Office Building Calculate total heat gain using your

building designTurn 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

“Rules of Thumb”

Balancing the System 12,000 BTUH / Ton CFM =

T

RSH

1.08 x

Next Time

Computerized Load Calculations Wrightsoft Right-N