Ventilation for Indoor Air Quality (IAQ)

The quality of air inside the conditioned space should be such that it provides a healthy and comfortable indoor environment. Air inside the conditioned space is polluted by both internal as well as external sources. The pollutants consist of odours, various gases, volatile organic compounds (VOCs) and particulate matter. The internal sources of pollution include the occupants (who consume oxygen and release carbon dioxide and also emit odors), furniture, appliances etc, while the external sources are due to impure outdoor air. Indoor Air Quality (IAQ) can be controlled by the removal of the contaminants in the air or by diluting the air. The purpose of ventilation is to dilute the air inside the conditioned space. Ventilation may be defined as the “supply of fresh air to the conditioned space either by natural or by mechanical means for the purpose of maintaining acceptable indoor air quality”. Generally ventilation air consists of fresh outdoor air plus any re-circulated air that has been treated. If the outdoor air itself is not pure, then it also has to be treated before supplying it to the conditioned space.

Since people have to spend a major part of their time within the building, without much exposure to outdoors, the concept of Indoor Air Quality (IAQ) has become very important. There are a large number of pollutants that are emitted by the materials used in the construction of buildings and brought into the buildings. IAQ addresses to these issues and gives recommendation for their reduction to safe limits. Sick building syndrome is very common in poorly designed air conditioned buildings due to inadequate ventilation and use of improper materials. The sick building syndrome is characterized by the feeling of nausea, headache, eye and throat irritation and the general feeling of being uncomfortable with the indoor environment. In developed countries this is leading to litigation also.

Though the minimum amount of air required for breathing purposes is small (about 0.2 litres per second per person), the actual ventilation air required is much larger as in addition to supplying oxygen to the occupants, the ventilation air must:

a) Dilute the odours inside the occupied space to a socially acceptable level

b) Maintain carbon dioxide concentration at a satisfactory level

c) Pressurizing the escape routes in the event of fire

Estimation of minimum outdoor air required for ventilation:

Ventilation is one of the major contributors to total cooling and heating load on the system. From energy conservation point of view, it is important select the ventilation requirements suitably. The amount of air required for ventilation purposes depends on several factors such as: application, activity level, extent of cigarette smoking, presence of combustion sources etc. After several studies stretched over several years, standards for minimum ventilation requirements have been formulated. For example, ASHRAE standard 62-1989 provides a guideline for minimum ventilation requirements. Table 33.3 provides typical outdoor (OD) air requirement for the purpose of ventilation:

It can be observed from the above table that the ventilation requirement increases with the occupancy. It can also be seen that the required amount of OD air increases significantly if smoking is permitted in the conditioned space.

Infiltration

Infiltration may be defined as the uncontrolled entry of untreated, outdoor air directly into the conditioned space. Infiltration of outdoor air into the indoors takes place due to wind and stack effects. The wind effect refers to the entry of outdoor air due to the pressure difference developed across the building due to winds blowing outside the building. The stack effect refers to the entry of outdoor air due to buoyancy effects caused by temperature difference between the indoor and outdoors. Though infiltration brings in outdoor air into the building similar to ventilation, in many commercial buildings efforts are made to minimize it, as it is uncontrolled and uncertain. Some of the means employed to control infiltration include use of vestibules or revolving doors, use of air curtains, building pressurization and sealing of windows and doors. It is very difficult to estimate the exact amount of infiltration as it depends on several factors such as the type and age of the building, indoor and outdoor conditions (wind velocity and direction, outdoor temperature and humidity etc.). However, several methods have been proposed to estimate the amount of infiltration air. Sometimes, based on type of construction, buildings are classified into loose, average or tight, and infiltration is specified in terms of number of air changes per hour (ACH). One ACH is equal to the airflow rate equal to the internal volume of the occupied space per hour. The ACH values are related to the outside wind velocity and the temperature difference between the indoor and outdoors. Infiltration rates are also obtained for different types of doors and windows and are available in the form of tables in air conditioning handbooks.

Heating and cooling loads due to ventilation and infiltration:

Due to ventilation and infiltration, buildings gain energy in summer and loose energy in winter. The energy gained or lost consists of both sensible and latent parts, as in general the temperature and moisture content of indoor and outdoors are different both in winter and winter.

The sensible heat transfer rate due to ventilation and infiltration, Qs,vi is given by:

The latent heat transfer rate due to ventilation and infiltration, Ql,vi is given by:

In the above equations, m and V are the mass flow rate and volumetric flow rates of outdoor air due to ventilation and infiltration, Cp,m is the average specific heat of moist air, hfg is the latent heat of vaporization of water, To and Ti are the outdoor and indoor dry bulb temperatures and Wo and Wi are the outdoor and indoor humidity ratios. Thus from known indoor and outdoor conditions and computed or selected values of ventilation and infiltration rates, one can calculate the cooling and heating loads on the building. The sensible and latent heat transfer rates as given by the equations above will be positive during summer (heat gains) and negative during winter (heat losses).

Though the expressions for heat transfer rates are same for both ventilation and infiltration, there is a difference as far as the location of these loads are considered. While heat loss or gain due to infiltration adds directly to the building cooling or heating load, heat loss or gain due to ventilation adds to the equipment load.