Ventilation Effectiveness

&

Indoor Air Quality

Douglas Spratt

February 2001

The Indoor Environment

Air

IndoorEnvironment

HVAC

The Purpose of HVAC Systems

HV

T h erm a l C o n tro l (A S H R A E - 5 5 )

A ir Q u a lity (A S H R A E - 6 2 )(C o n tam in a n t C o n tro l)

AC

HVAC Systems:

Maintain the space within the thermal comfortMaintain the space within the thermal comfort Remove/dilute air contaminantsRemove/dilute air contaminants

Ventilation- Removes Emissions

Source Emissions

Reduce

Remove

Ventilation Approaches

NaturalNatural

DisplacementDisplacement

MechanicalMechanical

Natural Ventilation

Advantages - Low 1st Cost- Low Energy

- Low Maintenance

- Natural- Greater

occupant acceptance

Natural Ventilation

Forces - Buoyancy

- Wind

Effect on Health Symptoms

Impact on Work Environment

Natural Ventilation

Limitations– Limited to Low Occupant Densities– Limited to Mild Climates– Limited Distribution

Natural Ventilation

Considerations– Security– Outdoor air quality– Wind gusts– Noise– Bugs

Displacement Ventilation

Displacement Ventilation

Continuously purges air pollutants

Typical features:

- De-couples ventilation

- Uses natural buoyancy forces

- Low noise

- No drafts

- Fresh air breathing zone

Air Distribution Effectiveness

Source: ASHRAE 62N

Room Air Distribution ERM

Ceiling supply of cool air 1.0

Ceiling supply of warm air and ceiling return 0.8

Ceiling supply of warm air and ceiling return if thesupply air stream reaches floor level

1.0

Floor supply of cool air and ceiling return if thesupply velocity is high enough to induce mixing ofroom air (most underfloor systems)

1.0

Floor supply of cool air and ceiling return if lowvelocity displacement ventilation achieves near-laminar flow and thermal stratification

1.2

Floor supply of warm air and floor return 1.0

Mechanical Ventilation

Practical Help Watch for short circuiting Min. O/A settings for VAV

Systems Ensure continuous fan

operation

Mechanical VentilationMechanical Ventilation

Methods– Exhaust FanExhaust Fan– Exhaust With Make-Up AirExhaust With Make-Up Air– Mixed Supply AirMixed Supply Air

Fixed DampersFixed Dampers Modulating DampersModulating Dampers

The Economizer SectionThe Economizer Section

EXHAUSTRETURN

OUTDOORSUPPLY

MIXED

RECIRCULATED

M-2

M-1

20 ton Roof-top unit9,000 cfm supply1,000 cfm toilet exhaust9,000 ft2 floor spaceOffice space with 150 ft2/person

62-89 O/A = 20 cfm/person x 9,000 ft2

150 ft2/person

= 1,200 cfm

Example

Ignoring Infiltration

1,200 cfm9,000 cfm

200 cfm

7,800 cfm

R/A 8,000 cfm

S/A 9,000 cfm

T/E

% O/A = = 13%

1,200 cfm

1,000 cfm

SourceExtraction

Source Emissions

Removal

Dilution

Natural Mechanical

Ventilation Summary