VENTILATION AND EXTRACT SYSTEMS

 

VENTILATION AND EXTRACT SYSTEMS

 

INTRODUCTION

The surrounding air enters the room whenever the door of the room is opened. The temperature and moisture content of this outside air have to be brought down to the room conditions. So this results in load on the system. The frequency of opening of the door/s is dependent upon the volume of the room.

 

VENTILATION

Ventilation is the process of supplying and removing air by natural or mechanical means to and from any space.

Ventilation air is part of the supplied air from outdoors plus any re-circulated air that has been cleaned or treated.

Inadequate ventilation can increase the effects of indoor air pollution.

Outdoor air in sufficient quantities must be brought inside to mix with and dilute the pollutant emissions and the indoor air must be vented to the outside to carry out pollutants.

 

RATE OF VENTILATION

The excess carbon dioxide water vapor, odors and air pollutants that accumulate in a building must be exhausted. At the same time an equal amount of fresh air must be introduced to replace the exhausted air. Where the polluted air in a room cannot be re-circulated to other areas, such as from kitchens and toilets, it is exhausted directly to the outdoor atmosphere. Such areas are 100% fresh air ventilated. Normally about 5 % to 25 % of the air circulated for cooling will be outdoor air.

Under certain circumstances much larger amounts of outdoor air are introduced. For example, the American Society for Heating Refrigeration and Air Conditioning Engineers (ASHRAE) recommends five times as much outside air for people who smoke than for nonsmokers (Table 6.2).

The number of air changes per hour (ACH) that a building will experience based on their appraisal of the building type, construction and use.    

To determine the required rate of ventilation, determine the cubic content of the area to be ventilated ( length x width x height ).

The next step is to determine the recommended air changes rate (Equation 6.1).

 

 

 

 

 

 

 

where,

        N    =    air changes rate (per hour)

        V    =    volume of required area to be ventilated (ft³)

 

For metric calculation use the equation below :

 

 

 

 

 

 

 

Table 6-1 : Required Rate of Ventilation

Application	Air change rate (minute)
Residential	1 to 2 min
Offices	2 to 5 min
Factories/warehouses	3 to 6 min

 

Solution :

 

The procedures to determine the air volume flow rate are :-

1. Calculate the cubic content (40ft by 30ft by 8ft = 9600ft³).
   
2. Refer to Table 6.1 which recommends a 1 to 2 min air change (an average of one air change every 1.5 minutes).
   
3. Divide the air change rate (1.5) into 9600ft³ = 64000cfm.
   
4. Select a fan size closest to the required cfm.
   

Table 6-2 : Minimum ventilation rates for different types of buildings

Type of building	Air changes per hour	Type of building	Air changes per hour
Schools Classrooms Assembly halls Changing rooms Dining rooms Dormitories Gymnasia Common rooms Staff rooms Laboratories	6 3 3 3 3 3 2 2 4	Operating theatres X-ray rooms Entrances Lavatories and Bathrooms	10 6 3 2
		Kitchens(varying according to volume of air required through canopy)	20-40
		Laundries Boiler houses Smoking rooms	10-20 10-15 10-15
Hospitals Wards Dormitories Day rooms Staff bedrooms Corridors	3 3 3 2 2

Note : For places of public entertainment such as cinemas, theatres, concert halls, assembly halls and dance halls the ventilation rate depends upon the Local Authorities Regulations, usually 28m³ per hour, per person.

 

VENTILATION SYSTEM FOR INTERNAL BATHROOMS AND TOILETS

Ventilation for these rooms should provide a minimum extract rate 20m³/hr from a toilet cubicle, or a bathroom without a water closet (W.C) and a minimum extract rate of 40m³/hr from a bathroom with a W.C. The ventilation system must be separate from any ventilation plant installed for any other purpose. In the common dust system the inlets from the bathrooms or W.C compartments should preferably be connected to the main vertical duct by a shunt duct at least 1m long This shunt duct will offer better sound attenuation between the dwellings and also tends to prevent the spread of smoke and fumes in the event of a fire.

The fans must be capable of extracting the total flow of air, plus an allowance on the fan static pressure to counteract wind pressures.

In order to keep the system operating in the event of failure of a single fan, it is recommended that two fans and motors are installed, with an automatic change-over damper.

To replace air extracted from the rooms, air should be drawn from the entrance lobby through a wall grill or a 19mm gap left under the door of the room.