Subprocesses are examined in order to provide proper working conditions lo­cally or to reduce emissions to environment (see Fig. 3.9).

Before considering ventilation, technical measures applied to process methods must be considered.

The last method in protection of the worker is respiratory protective equipment.

In case use of a local protection system has an effect on exposure of the process, load calculations shall be revised.


Process Methods


• Is it essential to use hazardous chemicals in this process?




No }

‘ • Could some media be substituted with another to reduce exposure?




No ]

‘ • Are the operating parameters identified and carefully controlled?

• E. g., thermal insulation


Yes, modifications

Ventilation Methods (See separate tree of local ventilation)


Return to Calculation of Local Loads

подпись: return to calculation of local loadsTotal enclosure and exhaust ventilation

(b) Partial enclosure and exhaust ventilation

(c) Local ventilation, e. g., • Push and pull

• Local exhaust ventilation

I l

Has the local protection

Return to

System an effect on the


Emission of the process?


Respiratory Protective Equipment


■B FIGURE 3.9 Local protection.

Explanations of Fig. 3.9

Step I: Loads to Local Zone

Clarify loads that influence the local zone.

Step 2: Conditions in Local Zone

Calculate the conditions in the local zone.

Step 3: Comparison to Target Levels of Local Zone

Compare calculated conditions to the target levels of the zone.

Step 4: Process Methods

Consider whether there are any process methods to reduce loads.

Step 5; Ventilation Methods

Select the optimal ventilation method to reduce emissions and/or to pro ­tect workers. (For details, see the subtree Fig. 3.10.)

Step 6: Effect of Local Protection on the Emission of the Process

Consider whether the local protection influences the emission of the pro­cess. In that case, return to Calculation of Local Loads.

Step 7: Respiratory Protective Equipment

If no other method provides protection to the worker, design respiratory protective equipment. In the design phase, the minimum is to identify the need for personal protection.

The tools for this task include:

• Balances (heat, contaminants, etc.)

• Computational fluid dynamics (CFD)

• Empirical methods

• Process methods

• Ventilation methods (see the subtree in Fig. 3.10.)

Explanations of Fig. 3.10

Step I: Source Characteristics

Characteristics are defined in Source Description.

Step 2: Alternative Prevention Methods

Select methods that could be used in this case.

Step 3: Calculation of Prevention Methods

Calculate performances of different methods and make them comparable. Step 4; Selection of the Hood

• Select the method that is optimal in this case (efficiency, economy, etc.).

• After selection, return to Local Protection (design of general ventilation system).

Step 5; Design of Conveying System

Design conveying system to convey captured contaminants away (sepa­rate tree).


FIGURE 3.10 Local ventilation.

Step 6: Design of Cleaning System

Design cleaning system for contaminants, if needed; e. g., it may be neces­sary to clean extract air or to recover some valuable substance (separate tree).