Ventilation and Indoor Environment Quality

Ventilation and IEQ

1. Kuwait University Mechanical Engineering Department Ventilation and IEQ ME 424 Lecture Notes Dr. Raed Bourisli

2. Ventilation: What? Simply speaking, ventilation is the intentional movement of air from outside a building to the inside. It is the “ V ” in HVAC . Should not be confused with “vents,” found in clothes dryers, and combustion equipment such as water heaters, boilers, fireplaces, and woodstoves. ASHRAE defines ventilation air to be that air used for providing acceptable indoor air quality .

3. Ventilation: Why? Ventilation is necessary when people or animals are present in buildings. Ventilation air is necessary to: Dilute odors. Limit the concentration of carbon dioxide and airborne pollutants such as respirable suspended particles and volatile organic compounds. In certain applications, such as submarines, pressurized aircraft, and spacecraft, ventilation air is also needed to provide oxygen, and/or to dilute carbon dioxide for survival. We will not focus on these applications in this course.

4. Ventilation: How? Ventilation air is often delivered to spaces by mechanical systems which may also heat, cool, humidify and dehumidify the space. In other words, as part of the AC system. Air movement into buildings can occur due to uncontrolled infiltration of outside air through the building fabric. The ventilation rate is normally expressed by the volumetric flow rate of outside air introduced in the building. The typical units used are L/s or CFM. Often expressed per person or floor area , L/s/p, L/s/m 2 , CFM/p, CFM/ft 2 .

5. Ventilation: How Much? In buildings, most of the conditioned air is recirculated, but a percentage of the return air is normally exhausted and replaced by outside (fresh) air. Most residential buildings rely on infiltration to meet ventilation needs. Measured in Air Changes per Hour ACH , typical values are between 0.5 and 1.5 ACH. If smoking is allowed indoors, much more ventilation is needed to dilute airborne contaminants. (Secondhand smoking however, cannot be eliminated.)

6. IEQ Indoor Environment Quality . (IEQ) deals with the content of interior air that could affect health and comfort of building occupants. Industrial Ventilation . Industrial ventilation is a method of controlling worker exposure to airborne toxic chemicals or flammable vapors by exhausting contaminated air away from the work area and replacing it with clean air. Complicated jobs are usually handled by specialized ventilation engineers. Industrial Hygiene . It is defined as the recognition, evaluation, and control of workplace hazards. Occupational exposure to chemicals was the original basis but is still considered one of the most wide spread hazards in industry. Dilution and capture ventilation are two important methods to control occupational exposure—the design and position of hoods and vents, and amount of air infiltration can substantially change exposure conditions, all of which are HVAC related techniques.

7. Sick Building Syndrome Sick Building Syndrome (SBS). Linked to situations in which building occupants experience acute health and comfort effects that appear to be linked to time spent in a building, but no specific illness or cause can be identified. Causes are frequently pinned down to flaws in the heating, ventilation, and air conditioning (HVAC) systems. Other causes have been attributed to contaminants produced by out-gassing of some types of building materials, or improper exhaust ventilation of light industrial chemicals used within. These cause are often cured by boosting the overall turn-over rate in fresh air exchange with the outside air (ACH). Building occupants complain of symptoms associated with acute discomfort, e.g., headache; eye, nose, or throat irritation; dry cough; dry or itchy skin; dizziness and nausea; difficulty in concentrating; fatigue; and sensitivity to odors. The cause of the symptoms is not known. Most of the complainants report relief soon after leaving the building. Indicators

8. Filters are placed in return air ducts. Cleanable filters are usually used. Filters are also needed for outside (fresh) air. As a minimum, filters should be cleaned once every season and after each major dust storm . Air Filter: Types and Locations Pictures courtesy of Dr. Ahmad Al-Sahhaf ©

9. A long list of guidelines and rules are available for optimum performance of air intake and exhaust grilles. These include, General stack design strategies Effective stack height Plume and re-entrainment Effect of building shape References: ASHRAE 2003 Applications Manual, Chapter 44, “Building Air Intake and Exhaust Design” ASHRAE 2001 Fundamentals Manual, Chapter 16, “Air Flow Around Buildings” Air Intake and Exhaust Considerations

10. Effective Stack Height

11. Purpose of Each Intakes: Bring in ‘fresh’ outdoor air for ventilation of interior space Need to avoid bringing in contaminants from other nearby sources Exhaust: Transport ‘contaminated’ exhaust air away from building and surroundings “ Dilution is the solution to pollution”

12. Plume trajectory with downwash Effective stack height Height ‘deducted’ for downwash Low velocity exhaust may result in downwash created by negative pressures immediately downwind of stack Low Stack Exhaust Velocity

13. Basic criteria: 2000 feet per minute to provide adequate plume rise and dilution Higher velocity leads to better dilution, but above about 3000 feet per minute noise and vibration from the exhaust fans becomes a concern To avoid stack wake downwash (previous slide) Recommended Stack Exhaust Velocity

14. Stack Height and Location to Avoid Re-entrainment

15. Newer Stack Concept Inducing airflow from surrounding adds to total mass in exhaust stream = more momentum = higher effective carry of the exhaust away from roofline Adding Induced Airflow

16. Air Intake Locations Even if no potential contamination from exhaust stacks, locate intakes as to avoid contamination from: Vehicle traffic Kitchens Wind blown dust from streets Evaporative cooling towers Emergency generators Plumbing vents Pay attention to cosmetics !