BEM CLASS 5Building Thermodynamics – 2

Air-conditioning Load Calculation – latent heat, solar and internal gains

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 area40,000 x 1/3 x (70-10) = 800,000 BTUH conduction

50 x 150 x 10 x 10 = 750,000 cf volume750,000 x .85 x .018 x (70-10) = 688, 500 BTUH

ventilation

Answer = 800,000 + 688,500 = 1,488,500 BTUH

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 conduction50 x 150 x 10 x 10 = 750,000 cf volume

750,000 x .85 x .018 x (70-10) = 688, 500 BTUH ventilationAnswer = 800,000 + 688,500 = 1,488,500 BTUH

(1) Account for plant efficiency lossif plant is 75% efficient, 1,488,500 / .75 = 1,984,667 BTUH

(2) Convert BTU to FuelNatural gas, 100,000 BTU = 1 therm1,984,667 / 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?

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)

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

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

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

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

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)

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?

LATENT HEAT LOAD

• Humidity in hot air. Enthalpy.

• Psychrometric chart.

LATENT HEAT LOAD

From Tao & Janis Mechanical and Electrical Systems in Buildings

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

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