Emergency ventilation: design calculation

Emergency ventilation is a whole range of devices that ensure the elimination of contaminants and harmful impurities, as well as combustion products, gases and aerosol compounds that have arisen in the event of an unforeseen situation at the enterprise. As devices operating in this system, the main and auxiliary exhaust and supply fans, as well as other devices and automation that carry out air exchange in buildings, are used. As a rule, all devices included in such a system are equipped with warning sensors and their automatic activation.

Application features

This type of ventilation is used in all buildings and structures where an emergency release of pollution is possible, in concentrations exceeding the maximum permissible values. It is an indispensable element in buildings with a gas fire extinguishing system. With its help, gas is removed from buildings after the operation of the installation.

As a rule, it is designed and installed together with an anti-smoke system in residential buildings, administrative and industrial premises where there is a risk of fire. The joint operation of these systems is designed to ensure the unhindered evacuation of people from the area of ​​fire and increased smoke. A large volume of supply air to the flights of stairs, and the routes of people during evacuation, creates a certain barrier to the penetration of smoke into them.

Device Features

At the moment, there are several main types of such systems, which are installed depending on technological needs:

• General exhaust systems equipped with auxiliary fans to increase air flow in case of an emergency.
• Isolated from general exchange, with separate air ducts and equipment, in addition to the main ventilation, in case of insufficient air exchange.
• Exhaust system with additional equipment, which is launched in case of failure of the general exchange.
• Supply, with additional equipment, which supplies air to air curtains, flights of stairs and passageways to ensure unimpeded evacuation of people.

In residential and industrial buildings, as a rule, two types of devices are installed. Some remove polluted air, with a high specific gravity, from the lower part of the room, while others remove gases and vapors with a lower density from the upper part of the building.

There is also a simpler method of emergency ventilation, which many have long and successfully used. This method consists in installing an additional fan that communicates with the main one. If the main fan fails, the auxiliary fan will turn on. In the event of an emergency, both fans start at once.

Important!
This method only works if the capacity of the ducts makes it efficient to run the additional equipment.

Primary requirements

A number of serious requirements are applied to any emergency ventilation system, regulated by sanitary, building regulations, as well as SNiP 2.04.05-91. The main ones are:

• Polluted air must be removed through ducts located above the roof level.
• Supply air must be supplied to the premises not lower than 2 m from the ground level.
• The work of ventilation in a residential area should be arranged in such a way that the air flow flows from living rooms towards non-residential ones (kitchen, bathroom).

Carrying out calculations during design

An accurate calculation is an integral part of the preparation of project documentation before installing emergency ventilation. The purpose of the calculation is to determine the required emergency air exchange during which the amount of harmful substances, smoke and combustion products in the air will be reduced to the maximum permissible concentration.

Important:
If the normative documents do not contain data on the required air exchange in a particular room, then the value of the emergency ventilation rate should be taken at least 8 times per hour.

Based on the data on the concentration of contaminants in the air and the performance of general ventilation, you can use the formulas for calculating the non-stationary mode of the room, for the required air exchange, as well as the required time for evacuation and ventilation.

Lq_etcdt + Gdt – Lqdt = Vdt

where:

L — air exchange m³/h;
q_etc – concentration of harmful substances in the air mg/m²;
G is the amount of pollutants released mg/h;
t is time;
q — concentration of harmful substances in a ventilated room for a specific period of time mg / m³;
V — total volume of premises m³.

In addition, when designing emergency and smoke ventilation systems, data on the flow rate of smoke removed directly from the room, the number, cross section and installation locations of smoke and check valves, as well as the power and design of exhaust fans are calculated.

Advice:
The calculation of emergency ventilation is a rather complicated and responsible process, which is why only professionals should be involved in such work.