BEM CLASS 4 Building Thermodynamics 2 Convection Overall class - - PowerPoint PPT Presentation

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BEM CLASS 4 Building Thermodynamics 2 Convection Overall class - - PowerPoint PPT Presentation

Jan 28, 2023 267 likes •425 views

BEM CLASS 4 Building Thermodynamics 2 Convection 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

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SLIDE 1

BEM CLASS 4

Building Thermodynamics – 2 Convection

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SLIDE 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.
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SLIDE 3

Forms of Heat Transfer

Conduction Convection Radiation

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SLIDE 4

CONVECTIVE EFFECTS

  • Effect on conduction (surface effects)
  • Air movement into and out of buildings
  • Latent heat in outside air
  • Interior air flows and comfort
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SLIDE 5

AIR MOVEMENT INTO & OUT OF BUILDINGS

  • Wind and Stack

effects

  • Variable impacts
  • CFD
  • Ventilation exhaust
  • Supply air & building

pressurization

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SLIDE 6

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 fan ratings
  • Code for ventilation
  • NYC Building Code.
  • ASHRAE 62.1
  • "15 CFM PER PERSON"
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SLIDE 7

HOW MUCH OUTSIDE AIR?

  • ASHRAE 62.1 – NOT SO SIMPLE
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SLIDE 8

HOW MUCH HEATING LOAD?

  • Let’s look at heating only
  • Infiltration/ventilation can be 50% or more
  • f building heat load

dT

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SLIDE 9

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

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SLIDE 10

[(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

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SLIDE 11

LATENT HEAT LOAD

  • Humidity in hot air.

Enthalpy.

  • Psychrometric chart.
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SLIDE 12

LATENT HEAT LOAD

From Tao & Janis Mechanical and Electrical Systems in Buildings

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SLIDE 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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SLIDE 14

CONTROL OF OUTSIDE AIR

  • Fans off at night? OA dampers closed?
  • "Minimum Outside Air" - fix to code based on full
  • ccupancy
  • Economizer mode - use max OA when conditions

are suitable

  • Dynamic Ventilation Control - CO2 - match OA to
  • ccupancy
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SLIDE 15

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?