Design & Implementation of Ventilation Systems
Evacuation of undesirable air from an environment and injecting the clean and desirable air into the same environment, using the nature of the geometry or by special equipment, is called “ventilation”. The major task of the ventilation in normal conditions is to maintain the air quality of environment, to prevent accumulation of gases produced from engine combustion in road tunnels and released gases in mines, to reduce the ambient temperature that is heated because of several phenomena such as vehicle acceleration and braking or vehicle air conditioning system. The purpose of emergency ventilation is to control the flows of the soot and hot gases and fire products, in order to establish a safe route to evacuate the passengers (means of egress) toward a point of safety, also to establish an appropriate route for emergency personnel. The tunnel ventilation in both normal and emergency conditions, is necessary.
Tunnel ventilation is generally devided into two major parts, the road tunnel ventilation and subway ventilation systems. The subway ventilation systems are studied and designed for following conditions:
|- Emergency ventilation
|- Ventilation in normal conditions
|- Ventilation in congestion condition
The ventilation systems are generally designed based on the worst operational conditions, i.e. the fire accidents. According to designed scenarios for several cases of fire, and taking into account the required standards, design constraints and client’s comments, the ventilation system design is done by selecting the appropriate equipment, determining means of egress, determining fire fighting instructions and etc.
Required Inputs and Tools
Ventilation system design process includes collecting the input data and information from client, doing several calculations and simulations for different kinds of fire cases and at last, comparing the results of calculations and simulations with standards and codes or values set by the client and proposing corrective actions when needed.
The most important inputs and tools are as follows:
|1- Client’s inputs
|2- Requirements, regulations and standards
|3- Computational tools and simulation software programs
Client’s input includes:
|- Codes and regulations proposed by client.
|- Geometrical information such as tunnel as built plans, station plans and drawings and etc.
|- Ventilation related information such as location, numbers and capacity of fans and jet-fans, fire HRR and etc.
|- Traffic information of the line such as passenger numbers, train headway, train average and maximum speed and so on.
|- Climatic information such as geographical specifications, outdoor temperature, meteorological information such as annual max. temp.
Requirements, regulations and standard codes include the formulations, criteria and assumptions which have been internationally developed to make the ventilation system design related decisions based on them. The NFPA codes, especially the NFPA 130 code, ASHRAE handbook and standards, also the Subway Environmental Design Handbook are the most important resources.
Computational tools include formulations and programs used for equipment selection and performance evaluation of them. For the issues concerning tunnel and metro ventilation design, two of the most important programs are as follows:
|- 1D SES program, for simulation of whole line and tunnels
|- 3D FDS Software, for simulation of the fire behavior at more limited spaces such as station and platform
After doing the calculations and simulations, there will be some outputs by which we can judge the designed ventilation system quality and performance accuracy, by comparing them with standard requirements. Some of the outputs are as follows:
|- Gas density
|- Gas velocity
|- Critical velocity
|- Gas temperature
|- Airflow's estimations
|- System temperature
|- Gas components density
|- Heat Release Rate per unit volume
|- Soot density and visibility estimation
|- Average, maximum and minimum values of the airflow, temperature and humidity in tunnels, shafts and stations under several conditions and arbitrary time intervals.
Using these outputs, it’s possible to decide about selecting or modifying the equipment, determining the operational configuration of the fans and jet-fans while fire accidents, determining the time of tenability for stations and tunnels, determining the means of egress, modifying the emergency exits, determining the necessary instructions while emergency conditions and etc.