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Heat Gains

Heat Gains. HVAC 7ab CNST 305 Environmental Systems 1 Dr. Berryman. Cooling Load Components. roof. lights. partition wall. people. infiltration. glass solar. equipment. glass conduction. exterior wall. floor. Sensible and Latent Gains. sensible load. latent load.

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Heat Gains

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  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. east-facing window roof Time of Peak Cooling Load heat gain 12 6 12 6 12 mid a.m. noon p.m. 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 mid a.m. noon p.m. mid

  7. Storage Effect (thermal lag)

  8. CLTD 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

  9. 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)

  11. Latitude Month Adjustment

  12. 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

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

  14. G F E D C B A Each R-7 Wall Groups • Select closest wall group • Compare U-values • Move up one wall group for each R-7

  15. Correcting CLTD - Walls CLTDcorr = (CLTD + LM)k + (78 – tR) + (tO – 85) to = OA – (DR/2)

  16. Roof Calculation Find peak cooling load: • Given: • 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 Closest roof type: 1 Peak: 1500 hrs CLTDuncorr: 78oF EXAMPLE Correct for insulation: R=7.5 vs 21 (+4 hrs) CLTDuncorr: 42oF Correct CLTD: next slide

  17. Average outside temperature tO = (OA – (DR/2) Heat gain through roof: BH = U  A CLTD CLTD Correction - Roof CLTDcorr = [(CLTD + LM)k + (78 – tR) + (tO – 85)]f 42oF 2oF 0.5 77oF 85oF 1.0 CLTDcorr = 23oF 93oF 16oF tO = 85oF (1/21)(5000)(23oF) = 5476 BH

  18. Wall Calculation Determine Wall Group: • 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 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

  19. Heat gain through wall: BH = U  A CLTD 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

  20. sun rays solar gain (radiation) transmitted energy conduction reflected energy glass window Sunlit Glass BH = solar gain + conduction

  21. 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

  22. 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

  23. Types of Shading Devices interior blinds exterior fins

  24. 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/hr•ft2 • A = total surface area of the glass, ft2 • SC = shading coefficient of the window, dimensionless • CLF = cooling load factor, dimensionless

  25. SHGF Solar energy through fenestration for Sunlit Glass* *use N(shade) for non-sunlit glass

  26. 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

  27. 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

  28. CLF with interior shading Reduction in the amplitude of the solar heat gain due to the constructions

  29. Window Calculation • GIVEN: • 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 BH = U  A CLTD EXAMPLE CLTDcorr= 14oF+(78oF–77oFF) + (85oF–85oF) (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

  30. 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

  31. Lighting Estimates

  32. LightCLF Dependent on: 1) Occupied Hours and 2)Design Values

  33. roof exterior wall plenum return air lights ceiling Space versus Plenum Loads Heat absorbed by Return Air

  34. CLF Design Values (Coefficients)

  35. Lighting Calculation BHlight = watts  3.41 ballast factor  CLF • GIVEN: • 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’ (1Wx3200SF)(3.41BH/W)(1.2)(0.75) Watts per SF = 1w Ballast factor = 1.2 Design value of “a” = 0.68 Design value of “b” = B CLF = 0.75 = 9821 BH

  36. People

  37. Equipment - Office

  38. Equipment-Restaurant

  39. 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

  40. air handler fan motor 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

  41. Sample Form Heat Gain • Space • Wall,Roof • Floor, Glass • Ventilation • Infiltration • Internal • Lights • People • Equipment • Plenum • Duct gain • Duct leakage • System • Motor, Fan

  42. 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

  43. “Rules of Thumb”

  44. RSH 1.08 x T Balancing the System • 12,000 BTUH / Ton • CFM =

  45. Next Time Computerized Load Calculations • Wrightsoft Right-N

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