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Home , Air changes per hour

BEM CLASS 4 Building – 2 Overall class review

• Discussion - Where are we? What does BEM do? What is it good for?

• Conduction discussion – does it matter whether insulation is on the inside or outside of a masonry building?

• Housekeeping - engineers & architects. Lab teams. Project teams. Forms of

Conduction

Convection

Radiation CONVECTIVE EFFECTS

• Effect on conduction (surface effects)

• Air movement into and out of buildings

• in outside air

• Interior air flows and comfort AIR MOVEMENT INTO & OUT OF BUILDINGS

• Wind and Stack effects • Variable impacts • CFD

• Ventilation exhaust

• Supply air & building pressurization HOW MUCH OUTSIDE AIR?

• All-natural draft ventilation • Function of wind, draft and openings • estimation by crack-length or by air-changes per hour. • 0.5 - 2 ACH

• Mechanically-driven systems • Count up CFM from ratings

• Code for ventilation • NYC Building Code. • ASHRAE 62.1 • "15 CFM PER PERSON" HOW MUCH OUTSIDE AIR?

• ASHRAE 62.1 – NOT SO SIMPLE HOW MUCH HEATING LOAD?

• Let’s look at heating only

• Infiltration/ventilation can be 50% or more of building heat load

dT Overall Building Heat Loss

Exercise

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 BTU LATENT HEAT LOAD

• in hot air. .

• 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

• mode - use max OA when conditions are suitable

• Dynamic Ventilation Control - CO2 - match OA to occupancy INTERIOR AIR FLOWS

• HVAC “throw and spread". • Coanda effect. • Short-circuits. • Stratification. • "droop" at low flows in variable volume systems. • Mixing vs Displacement • Use of CFD.

• What is the response to comfort complaints?