MEBS6010 -Indoor Air Quality

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MEBS6010 -Indoor Air Quality
Chapter 7 Indoor Air Quality and HVAC

• Yuguo Li

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Two basic air distribution systems - mixing
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Two basic air distribution systems - Displacement
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Basic idea
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Air flow from these diffusers is not a jet, but a
gravity current
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Rule of thumb
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Overview of Indoor Air Quality
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The purpose of this lecture

• To provide practical guide for the evaluation of
  ventilation systems, control of pollutant flows
  in buildings and sources of air contaminants
  related to HVAC systems with respect to providing
  and maintaining good indoor air quality.
• To consider the impact of outdoor air quality

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Before
Before
After
Before
After
After
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clean
dirty
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Topics

• 1 Evaluation of the ventilation system
• 2 Air movement pathways and pressure
  relationships
• 3 Sources of air Contaminants related to HVAC
• 4 A simple macroscopic model considering the
  effect of recirculation and outdoor air quality
• 5 I/O ratios

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1 Evaluation of the Ventilation System

• A review of the occupants complaints or concerns
• A review of the design documentation
• The measurement of the outdoor air quantity
  entering the HVAC system
• The measurement of delivered quantities of
  outdoor
• A determination of air pathways through the
  building
• Inspection for sources of air contaminants

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Prof. Fangers approach
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Prof. Fanger
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Prof. Fanger
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Prof. Fanger
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Review of the Occupants Complaints or Concerns

• The occupants are important sources of
  information.
• Determine if the pattern of complaints
  corresponds to any particular pattern of HVAC
  system.
• Distribution of the complaints by location.
• Try to interpret the significance of the
  complaints.
• While peoples ability to detect odors exceeds
  that of most analytical equipment, their ability
  to explain and describe these odors is typically
  not as refined.

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Review of the Design Documentation

• The design documentation provides the basis for
  determining the original intent of the
  specification of the HVAC system.
• A very useful source of information is the
  Equipment Schedule.
• Two useful calculations relate to the density of
  people in the building and the predicted supply
  air flow per square metre of occupied area of the
  building, or litre per person.

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The Measurement of the Outdoor Air Quantity

• An important starting point in a
  ventilation-based IAQ evaluation.
• Two issues, the first is about the OA quantity
  entering the HVAC system, and the second is about
  the OA quantity delivered to the occupants.
• The techniques for determining the OA quantity
  entering an HVAC system include
• either a direct measurement of the volume of this
  air stream,
• or measurement of the total air volume in
  combination with a determination of the portion
  of this total supply air volume that is outdoor
  air.

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RELATIONSHIP BETWEEN OUTDOOR AIR QUANTITY AND THE
HVAC SYSTEM

• (1) its design,
• (2) its installation,
• (3) its operation and
• (4) its maintenance.

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Design Factors Affecting Outdoor Air Quantity
Figure 1. HVAC system with fixed minimum outdoor
air, without return air fan
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Figure 2. HVAC system with economizer control of
outdoor air quantity.
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Table 1 Estimate of OA as a function of outdoor
air temperatures
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Installation Factors Affecting Outdoor Air
Quantity

• For example, in one building, the HVAC system was
  based on a packaged system located on the roof.
  In this package system, all of the wiring had
  been done at the factory except for the wiring
  for the building relief fan, which was performed
  in the field. This on-site portion of the
  installation, however, was performed incorrectly
  and the building relief fans failed to operate.
• Other installation errors include the placement
  of VAV (variable air volume) mixing boxes upside
  down from their intended arrangement. Since the
  damper position was intended to work with gravity
  to keep it open, its being upside down led to the
  damper remaining closed when it wasn't supposed
  to be.

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Operational Factors Affecting Outdoor Air
Quantity

• In one building, the OA dampers were observed to
  be open only to their minimum position. It turned
  out that, unknown to his bosses, the individual
  responsible for that particular building had
  overridden the design intent of the controls of
  the HVAC system.
• Another building, the determining factor for the
  position of the OA dampers was not the outdoor
  air temperature, but in fact was the need to keep
  a certain minimum load on the chillers.
  Therefore, numerous opportunities to utilize
  "free cooling" and increase the ventilation rate
  in the building were not taken advantage of
  because the OA dampers were being manually kept
  at their minimum position to maintain the load on
  the chiller.
• In another building, complaints of inadequate
  ventilation were high on Mondays and Tuesdays,
  but dropped off dramatically for the rest of the
  week. - the time clock used to provide reduced
  ventilation during the weekend days was not set
  up correctly, so that Monday and Tuesday received
  the reduced condition intended for Saturdays and
  Sundays

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Maintenance Factors Affecting Outdoor Air
Quantity

• The maintenance issues that have the potential
  for affecting the OA quantity entering the
  dampers are more limited than those categorized
  as due to design, installation, or operation.
• Maintenance activities are more likely to affect
  the air contaminants side of the equation rather
  than the ventilation quantity.
• There is, however, one maintenance activity -
  that of keeping the OA intake unclogged - which
  can directly affect the OA quantity entering the
  HVAC system. Therefore, the presence of
  obstructions in grilles or screens of air intakes
  should be visually assessed. Dirty coils will
  also reduce the overall quantity of supply air
  and with it, potentially the OA quantity as well.

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Determination of the quantity of outdoor air
entering via the outdoor air dampers

• The actual determination of the OA quantity
  entering an AHU can be achieved by
• either a direct measurement of the air flow,
• or indirectly using the measurement of either the
  return air or mixed air volume plus the
  measurement of some attribute of the three air
  streams involved, the OA, RA (return air), and MA
  (mixed air).

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Temperature-Based Determination of Percentage
Outdoor Air
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Figure 4. Calculation of outdoor air from
temperature measurements
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Carbon Dioxide-Based Determination of Percentage
Outdoor Air
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Figure 7 Carbon dioxide re-circulation of
outdoor air
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Figure 8 Carbon dioxide determination of
reentrainment.
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Sulfur Hexafluoride-Based Determination of
Percentage Outdoor Air
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Other pathways of outdoor air into the HVAC
equipment

• Mechanical rooms are typically under negative
  pressure with respect to the outdoors because the
  ductwork on the suction side of the fan is not
  air-tight.

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Infiltration and Natural Ventilation

• Infiltration and natural ventilation are really
  one and the same phenomenon
• The driving forces
• Mostly neither thermally conditioned nor filtered
  (airborne pollutant treated)

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2 Air Movement Pathways and Pressure Relationships

• It is pressure differences that provide the
  driving forces to transport air from one location
  to another.

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How air moves through buildings

• Natural forces
• Mechanical fans
• A network of flows

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Airflow through leakages and openings
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A simplified model of the air flows between the
stairwell and the outside.
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Figure 3.2 Air and smoke movement in a high-rise
building due to stack effect (a) airflow due to
stack effect, (b) where is the fire?, (c) where
is the fire?, and (d) Same as (c), but what
happened?
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Figure 3.2 Air and smoke movement in a high-rise
building due to stack effect (a) airflow due to
stack effect, (b) fire below the neutral plane,
(c) fire above the neutral plane, and (d) fire
above the neutral plane with smoke entering a
shaft due to buoyancy.
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Discussion air flow direction in the Metropole
Hotel
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Discussion air flow direction in the Metropole
Hotel
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Reentrainment of exhausts from stacks
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Figure 11. 3-Dimensional recirculation pattern
around building
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Solutions

• Reducing the source strength by process
  modification
• 2. Increasing the stack height
• 3. Increasing the stack discharge velocity
• 4. Relocating the stack
• 5. Relocating the air intake
• 6. or diluting the source with outdoor air prior
  to its release into the atmosphere

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HVAC equipment-related pressure relationships

• In a building with a properly functioning HVAC
  system, the maintenance of the occupied areas of
  the building at a positive pressure prevents or
  minimizes the infiltration of unconditioned air
  into the occupied areas of the building.
• The pressurization of the occupied spaces with
  respect to the outdoors is typically achieved by
  operating the supply fan such that it attempts to
  introduce a larger volume of air into the
  building than the return air (RA) fans and
  exhaust fans are attempting to remove from the
  building.

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Figure 12. Outdoor air intake functioning as
building exhaust.
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Assessment techniques for identifying air
movement pathways

• Visual assessment of the evidence
• Test with your finger on a visible gap
• Use of air current tubes (or any small smoke
  generators)
• Tracer gas techniques

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Existence of bi-directional flows
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Any air leakage from this SARS ward?
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Example
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Odors from Loading Dock at Health Care Facility
Figure 14. Tracer measurement from releases at
loading dock.
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Figure 15. Building section showing pathways from
loading dock.
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3. Sources of air contaminants

• The HVAC system may be a source of air
  contaminants.
• The HVAC system may be functioning as a transfer
  mechanism to transport air contaminants from
  their source to where the people are.
• The HVAC system may be failing to adequately
  dilute and remove the air contaminants that are
  arising from a source that is located in the
  occupied space.

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Sources of air contaminants by location of origin

• Air contaminants from outdoors via outdoor air
  intake
• Air contaminants from outdoors via unintentional
  intakes
• Air contaminants arising from within the HVAC
  system
• Air contaminants arising from the building itself

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Air Contaminants Arising from Outdoors

• Air contaminants can be transported via the HVAC
  system by being captured by the outdoor air
  intake at the air handling units (AHUs).
• Air contaminants can be transported via the HVAC
  system by being captured via less obvious
  pathways into the HVAC system, such as via the
  mechanical room or somehow into the return flow
  to an AHU.
• Air contaminants can be transported directly to
  an occupied area by infiltration through the
  building shell.

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Air Contaminants Entering at the Outdoor Air
Intake

• If the air intakes are situated near the top of
  the building, the sources to be concerned about
  include cooling towers, plumbing vents, stagnant
  roof puddles?? , and building exhausts, such as
  from kitchens or laboratory fume hoods.
• For lower-level air intakes, this list of
  potential sources includes vehicle emissions from
  roadways, parking lots, garages, loading docks,
  dumpsters??? , trash compactors, etc.
• For below-grade air intakes, those located in
  areaways, there is also the potential for
  microbial contamination from decaying leaf litter
  or even pigeon droppings.

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Others

• Air Contaminants Entering via Unintentional Air
  Intakes
• Air Contaminants Entering via Infiltration

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Air Contaminants Arising from within the HVAC
System

• Outdoor Air Intake Chamber
• Air Filters
• Condensate Drain Pan
• Heat Exchangers
• Supply Air Fan
• Internal Duct Liners

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Air Contaminants Arising from Activities in the
Building

• Air contaminants from construction or renovation
• Air contaminants from building support activities
• Air contaminants arising from maintenance of the
  HVAC system
• Air contaminants arising from maintenance of the
  building

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4. A Simple Macrosopic Model
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Solution

• Questions
• What happens if the infiltration rate is very
  high?
• Which parameter is important a, K, n or nr

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Table 1. Occupant and MVAC details for the three
measured offices
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Table 2. Operation modes of air-conditioning
systems in Office A
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Measured concentration profile of carbon dioxide
during three consecutive working days in the two
tested offices in Hong Kong.
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Measured concentration profile of respirable
suspended particles during three consecutive
working days in the two tested offices in Hong
Kong.
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Measured concentration profile of carbon dioxide
during three consecutive working days in Office A
with different operation modes of the
air-conditioning system.
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Measured concentration profile of respirable
suspended particles during three consecutive
working days in Office A with different operation
modes of the air-conditioning system.
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Table 4. Parameters used in the office model
study
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Three cases

• Case 1 high recirculation flow rate (4 ACH),
  high infiltration flow rate (0.4 ACH).
• Case 2 low recirculation flow rate (2 ACH),
  high infiltration flow rate (0.4 ACH).
• Case 3 high recirculation flow rate (4 ACH),
  low air infiltration flow rate (0.1 ACH).

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Hourly averaged indoor particulate concentrations
in an office building with mixed-mode
air-conditioning at a constant outdoor PM10
particle concentration of 100 mg/m3.
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5. I/O Ratios
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• For pollutants predominantly generated outdoors,
  I/O is determined by the filtration(1-penetration
  ) efficiency, which can be anything less than 1.
• For pollutants predominantly generated indoors,
  I/O is determined by the source strength and the
  ventilation rate.
• For pollutants generated both indoors and
  outdoors, I/O is determined by the combined
  effects of filtration efficiency, indoor source
  strength and the ventilation rate.

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Particulate Matters
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Sulphur dioxide
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Carbon monoxide
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Carbon dioxide
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Nitrogen dioxide
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Formaldehyde
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I/O ratios - summary

• Particles 0.5-2.5
• SO2 0.5-0.9
• CO 1.0-2.0
• CO2 1.5-3.0
• NO2 0.7-1.4
• Formaldehyde 5-30

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3. Effect of Natural Ventilation on Indoor
Particle Concentrations

• When to open windows???

Exfiltration removal
Penetration
Other sinks
Deposition
Indoor source
Resuspension
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Macroscopic equation for a building
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Balance-point outdoor concentration
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Parameters used in the model study for I/O and
I/O concentration relationship
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Buildings with no activity and without indoor
sources
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Buildings with normal activity and without major
indoor sources
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I/Os for buildings (B) with normal human activity
and without major indoor sources.
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Buildings with normal activity and major indoor
sources
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The effect of smoking and cooking on the daily
average I/O ratios at an outdoor PM10
concentration of (a) 15 µg/m3 (b) 25 µg/m3 (c)
50 µg/m3 and (d) 100 µg/m3.
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The effect of smoking and cooking on the daily
average I/O ratios at an outdoor PM10
concentration of (a) 15 µg/m3 (b) 25 µg/m3 (c)
50 µg/m3 and (d) 100 µg/m3.
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The effect of smoking and cooking on the daily
average I/O ratios at an outdoor PM10
concentration of (a) 15 µg/m3 (b) 25 µg/m3 (c)
50 µg/m3 and (d) 100 µg/m3.
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The effect of smoking and cooking on the daily
average I/O ratios at an outdoor PM10
concentration of (a) 15 µg/m3 (b) 25 µg/m3 (c)
50 µg/m3 and (d) 100 µg/m3.
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Topics IAQ and HVAC
1 Evaluation of the Ventilation System 2 Air
Movement Pathways and Pressure Relationships 3
Sources of Air Contaminants related to HVAC 4 A
Simple Macroscopic Model considering the effect
of recirculation