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

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Presentation on theme: "BEM CLASS 5 Building Thermodynamics – 2 Air-conditioning Load Calculation – latent heat, solar and internal gains."— Presentation transcript:

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

2 Problem from Class 4 Calculate 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, ,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, ,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 = 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 Relation to lighting
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 watts per sf in typical office space Computers operate at a fraction of rated power data centers 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 How does day-light harvesting work?

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

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15 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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