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Problem from Class 4 Calculate Building Heat Loss

BEM CLASS 5 Building Thermodynamics – 2 Air-conditioning Load Calculation – latent heat, solar and internal gains. Problem from Class 4 Calculate Building Heat Loss.

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Problem from Class 4 Calculate Building Heat Loss

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  1. BEM CLASS 5Building Thermodynamics – 2 Air-conditioning Load Calculation – latent heat, solar and internal gains

  2. Problem from Class 4Calculate Building Heat Loss • A 50’ x 150’ x 10 story free-standing building has an overall R-value of 3 (taking into account all walls, windows, roof). Each story is 10’ tall. Ventilation, as calculated at 15 cfm per occupant at design occupancy, provides .85 air-change per hour. Ignore basement/foundation losses. • Calculate the design heat load at 10 dF outside temperature and 70 dF indoor temperature • [(50 x 2) + (150 x 2)] x 10 x 10 = 40,000 sf surface area • 40,000 x 1/3 x (70-10) = 800,000 BTUH conduction • 50 x 150 x 10 x 10 = 750,000 cf volume • 750,000 x .85 x .018 x (70-10) = 688, 500 BTUH ventilation • Answer = 800,000 + 688,500 = 1,488,500 BTUH

  3. Next Step: Convert to Fuel Use • [(50 x 2) + (150 x 2)] x 10 x 10 = 40,000 sf surface area • 40,000 x 1/3 x (70-10) = 800,000 BTUH conduction • 50 x 150 x 10 x 10 = 750,000 cf volume • 750,000 x .85 x .018 x (70-10) = 688, 500 BTUH ventilation • Answer = 800,000 + 688,500 = 1,488,500 BTUH • (1) Account for plant efficiency loss • if plant is 75% efficient, • 1,488,500 / .75 = 1,984,667 BTUH • (2) Convert BTU to Fuel • Natural gas, 100,000 BTU = 1 therm • 1,984,667 / 100,000 = 19.84 BTUH • If heating is Electric, what is next question? • Next exercise, how much energy would you expect this building to use on an annual basis? How would you calculate?

  4. AC Load Calculation • Cooling Load, Q = conduction + infil/ventil + SG + IG • For cooling design calculation, infil/ventil has two components: (1) Sensible Heat and (2) Latent Heat • SG = solar gain • IG = internal gains (people, equipment/electricity)

  5. Solar gain • Solar Constant: 433 btuh/sf • Actual gain on a surface varies by orientation, season, time-of-day • Desirable in winter but can be excessive • Major instantaneous load in summer – through fenestration; lagging through walls. • Relation to lighting • Day-lighting • Glare

  6. Fenestration treatments for solar control • Architectural features • Adjusting glazed areas, adjusting floor plates, atriums • Overhangs, light shelves, external shading • Active facades • Curtain and shade systems • Electro-chromic

  7. Fenestration treatments for solar control Reflective films and tinting, replaced by spectrally selective coatings solar heat gain coefficient (SHGC) and visible transmittance “low-e” Optimize for heating, cooling

  8. Internal Gains: People • 300 btuh per person at normal office work • Design occupancy. Density by usage • Variable loads in places of assembly • Scheduling and modeling of where people are • big savings in controlling to occupancy • What people DO in their spaces. Relation to ZONES. • Lights • shades • Windows • Thermostats • Diffusers

  9. Internal Gains: Electricity • All electric use converted to heat • 3414 btu / kwh • Lighting, Motors, "plug-loads" • typically 2 - 3 watts per sf in typical office space • Computers • operate at a fraction of rated power • data centers 100 - 150 w/sf (data processing + cooling)

  10. How does day-light harvesting work? Lighting • Comfort & productvity • illuminance levels - IEEE stds by task • lighting quality • Lighting Design, Lighting Modeling - RADIANCE • Lighting Power Density – watts per sf • Basis of code • 1 w/sf and less • CALCULATE • Usage hours • Lighting and lighting retrofit SCHEDULES

  11. LATENT HEAT LOAD • Humidity in hot air. Enthalpy. • Psychrometric chart.

  12. LATENT HEAT LOAD From Tao & Janis Mechanical and Electrical Systems in Buildings

  13. Exercise • It is a 90 dF, 70% RH day outside. You want to deliver air at 65 dF, 50% RH. On the psychrometric chart describe the work that has to be done at the air-handling unit and coils, showing lines for sensible cooling, latent heat removal and re-heat. sensible latent reheat

  14. CONTROL OF OUTSIDE AIR • Fans off at night? OA dampers closed? • "Minimum Outside Air" - fix to code based on full occupancy • Economizer mode - use max OA when conditions are suitable • Dynamic Ventilation Control - CO2 - match OA to occupancy

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