Ventilation dilution

In industrial air-conditioning systems, harmful environmental gases, vapors, dusts, and fumes are often encountered. These contaminants can be controlled by exhaust systems at the source, by dilution ventilation, or by a combination of the two methods. When exhaust... 

Ventilation ean be provided as general dilution ventilation or loeal extraetion ventilation. 

For continuous release of gas or vapour the steady-state dilution ventilation required to reduce the atmospheric pollutant to a level below its hygiene standard is given by... 

Figure 12.1 Dilution ventilation inlet air may require tempering according to outside conditions...

Air jets are used to create enough air movement inside the room to circulate and mix the whole room air. This strategy is often called dilution ventilation as the contaminants created inside the room are mixed into the whole room volume thus reducing the local peak concentrations. 

A dilution ventilation rate of at least 6 air changes per hour (ach) is recommended, with 12 or more ach recommended for new construction or renovation. This may not provide sufficient dilution to allow workers to enter without respiratory protection, but it is considered a feasible dilution rate that will reduce the risk of infection for those workers who must enter the room with respiratory protection. Dilution also reduces the contaminant concentration and therefore the risk when temporary leakage from the room occurs such as when doors are opened or closed. 

Dilution ventilation or general exhaust ventilation A mixed airflow designed to dilute the contaminants within a space to required safe concentration... 

Automatic fire ventilation See Smoke ventilation. Dilution ventilation A ventilation strategy whereby contaminants are allowed to escape into the ventilated space and are then diluted to an acceptable level by means of the ventilation system. 

The starting point for design of a dilution ventilation scheme is normally a hygiene survey in which levels of pollutants and the ventilation rate are measured under worst conditions. The required ventilation rate is then calculated by ratio of level of contamination measured and required, where... 

Local extract is used to remove contaminants directly from a process to the exterior without passing through personnel breathing zones. It thus provides a high degree of safety and because small volumes of air are extracted relative to a dilution ventilation system it is energy efficient. 

In general, dilution ventilation alone is inappropriate for highly toxic substances, carcinogens, dusts or fumes or for widely fluctuating levels of pollutants. Since hygiene standards are often revised (usually downwards), specifications of existing systems may prove inadequate. 

Dilution ventilation Design ventilation systems to control low-level toxics. Design locker rooms with good ventilation and special areas or enclosures for contaminated clothing. Design ventilation to isolate operations from rooms and offices. Design filter press rooms with directional ventilation. 

The two major control techniques are environmental controls and personal protection. Environmental control reduces exposure by reducing the concentration of toxicants in the workplace environment. This includes enclosure, local ventilation, dilution ventilation, wet methods,... 

There are two types of ventilation techniques local and dilution ventilation. 

Table 3-12 Nonideal Mixing Factor kfor Various Dilution Ventilation Conditions1...

If the contaminant cannot be placed in a hood and must be used in an open area or room, dilution ventilation is necessary. Unlike hood ventilation, where the airflow prevents worker exposure, dilution ventilation always exposes the worker but in amounts diluted by fresh air. Dilution ventilation always requires more airflow than local ventilation operating expenses can be substantial. 

For exposures to multiple sources the dilution air requirement is computed for each individual source. The total dilution requirement is the sum of the individual dilution requirements. The following restrictions should be considered before implementing dilution ventilation ... 

Xylene is used as a solvent in paint. A certain painting operation evaporates an estimated 3 gal of xylene in an 8-hr shift. The ventilation quality is rated as average. Determine the quantity of dilution ventilation air required to maintain the xylene concentration below 100 ppm, the TLV-TWA. 

The hood requires significantly less airflow than dilution ventilation and prevents worker exposure completely. 

Frequently, processes cannot be constructed outside. In this case local and dilution ventilation systems are required. These ventilation systems were discussed in detail in chapter 3, section 3-4. 

Local ventilation is the most effective method for controlling flammable gas releases. Dilution ventilation, however, is also used because the potential points of release are usually numerous and it may be mechanically or economically impossible to cover every potential release point with only local ventilation. 

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