arpadveress 
HAC stands for hazardous area classification and can be made on 2 ways: bubble and room methodology.
1) bubble means we consider all emission sources individually and if any change comes, we shall redo and redo HAC again. Detailed knowledge of all parameters is essential for each and every part of machinery, piping and more.
2) room means we consider the usage of all hazardous materials and define the area as one or more zones. That gives flexibility in terms of changes if any.
Commercial there are differences both on CAPEX but OPEX side as well.
From LEAN side there is a big push to reduce cost of stocking, which is more against to option 1 (here any later changes may mean immediate effect on surrounding Ex installations’ compliance) – request for investment which is not authorized there, so delay in production.
Reduction of costing prefers option 1 in case of CAPEX, but OPEX side shall prefer it less and more the option 2, because it gives cost-effective flexibility for total lifetime of Ex safe operation – see above.
In many countries we are aware of the rule: if hazardous areas (m3) go above 40% of the area, here anyway we shall do the HAC for the complete volume.
So which aspect is stronger more depends on the phase of project or already in operation.
Shall I give a good example, think about a pharmaceutical unit…
We always recommend to go ahead with the room concept, still in some cases option 1 is prefered.
So keep up good work! Pro bono!
Arpad
