Ventilation Kholid

8/2/2019 Ventilation Kholid

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Ventilation


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Definition

VENTILATION is the process by which fresh air

is introduced and ventilated air is removed

from an occupied space.

The primary aim of ventilation is to preserve

the qualities of air. Sometimes, ventilation

may also be used to lower the temperature

inside an occupied area.


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Natural ventilation

Natural ventilation is the process of supplying

and removing air by means of purpose-

provided aperture (such as openable

windows, ventilators and shafts) and the

natural forces of wind and temperature-

difference pressures.


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Natural ventilation

Controlled natural ventilation is intentional displacement ofair through specified openings such as windows, doors, andventilations by using natural forces (usually by pressuresfrom wind and/or indoor-outdoor temperature

differences). It is usually controlled to some extent by theoccupant.

Infiltration is the uncontrolled random flow of air throughunintentional openings driven by wind, temperature-difference pressures and/or appliance-induced pressuresacross the building envelope. In contrast to controllednatural ventilation, infiltration cannot be so controlled andis less desirable than other ventilation strategies, but it is amain source of ventilation in envelope-dominatedbuildings.


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Mechanical ventilation

Mechanical or forced ventilation is the process ofsupplying and removing air by means ofmechanical devices, such as fans. It may be

arranged to provide either supply, extract orbalanced ventilation for an occupied space.

There are also specialized areas in whichventilation is vital, such as ventilation for

industrial processes, mines, tunnels andunderground development. However, in thislecture we will focus only on natural ventilation.


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Purposes of ventilation

Maintaining human comfort and health are two key reasons for providingventilation in buildings. To achieve these purposes, a ventilation system should beable to meet the following criteria:

provide sufficient supply of air/oxygen for the physiological needs of human beings(a minimum of 0.2 l/s/person is required for breathing purpose) and/or livestock;

provide sufficient supply of air/oxygen for industrial, agricultural and otherprocesses (for example, provision of oxygen for burning and combustionprocesses);

remove the products of respiration and bodily odor (including those from smoking)of human and/or animal occupants;

remove contaminants or harmful chemicals generated by processes or frombuilding materials;

remove heat generated by people, lighting and equipment inside the occupiedspace;

create some degree of air movement which is essential for feelings of freshnessand comfort (usually a velocity of 0.1 to 0.3 m/s is required).


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Principles of Natural Ventilation

For air to move into and out of a building, a pressuredifference between the inside and outside of the building isrequired. The resistance to flow of air through the building

will affect the actual air flow rate. In general, controllednatural ventilation and infiltration are driven by pressuredifference across the building envelope. The pressuredifference is caused by:

wind (or wind effect);

difference in air density due to temperature differencebetween indoor and outdoor air (stack or chimney effect);or

combination of both wind and stack effects.


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Wind effect

When air flow is due to wind, air enters

through openings in the windward walls, and

leaves through openings in the leeward walls.

The pressure distribution patterns due to wind

in a number of cases are illustrated in Figure 1.


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Wind pressures are generally high/positive on the windward side of abuilding and low/negative on the leeward side. The occurrence andchange of wind pressures on building surfaces depend on:

wind speed and wind direction relative to the building;

the location and surrounding environment of the building; and

shape of the building. Mathematically, pressure on building surfaces may be expressed as:

Pw- Po =Cp p vw2

where

Pw = mean pressure on the building surface (N/m2 or Pa)

Po = static pressure in undistributed wind (N/m2 or Pa)

vw = mean wind velocity (m/s) = density of air (kg/m3)

Cp = surface pressure coefficient


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Stack effect

When air movement is due to temperature difference between theindoor and outdoor, the flow of air is in the vertical direction and isalong the path of least resistance. The temperature differencecauses density differentials, and therefore pressure differences,that drive the air to move. During the winter season the following

stack effect occurs: indoor temperature is higher than outdoor temperature;

the warmer air in building then rises up;

the upward air movement produces negative indoor pressure at thebottom;

positive indoor pressure is created on the top; warmer air flows out of the building near the top; and

the air is replaces by colder outside air that enters the building nearits base.


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When thermal force is acting alone, a neutral pressure level(NPL) exists, where the interior and exterior pressures areequal. At all other levels, the pressure difference betweenthe interior and exterior depends on the distance from the

neutral pressure level and the difference between thedensities of inside and outside air.

where Ps

= pressure difference due to stack effect (N/m2 or Pa) = density of air (kg/m3)

g = gravitational constant = 9.81 m/s2

h = height of observation (m)

hneutral = height of neutral pressure level (m)

T= absolute temperature (K) (subscripts i= inside and o = outside)


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Design for Natural Ventilation

The design of controlled natural ventilation

systems requires identification of the

prevailing wind direction, the strategic

orientations and positions of openings on the

building envelope. These openings include

windows, doors, roof ventilators, skylights,

vent shafts, and so forth.


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Ventilation rates

When designing a ventilation system, the ventilation rates arerequired to determine the sizes of fans, openings, and airducts. The methods that can be used to determine theventilation rates include:

(a) Maximum allowable concentration of contaminantsA decay equation can be used to describe the steady-state

conditions of contaminant concentrations and ventilationrate, like this:

Ci= Co + F / Q where Ci= maximum allowable concentration of contaminants

Co = concentration of contaminants in outdoor air

F= rate of generation of contaminants inside the occupied space (l/s)

Q = ventilation rate (l/s)


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(b) Heat generation

The ventilation rate required to remove heatfrom an occupied space is given by:

where H = heat generation inside the space (W)

Q = ventilation rate (l/s)

cp = specific heat capacity of air (J/kg.K) = density of air (kg/m3)

Ti= indoor air temperature (K)

To = outdoor air temperature (K)


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(c) Air change rates

Most related professional institutes and authorities haveset up recommended ventilation rates, expressed in airchange per hour, for various situations. The ventilation rate

is related to the air change rate by the following equation:

whereQ = ventilation rate (l/s)

V= concentration of contaminants in outdoor air

ACH = air change per hour

Table 1 gives some recommended air change rates for typical spaces. Table 2 provides someexamples of outdoor air requirements for ventilation.


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Space Air change rates per hour

Carparks 6

Kitchen 20 - 60

Lavatory 15

Bathrooms 6

Boiler rooms 15 - 30

Application Estimated maximum

occupancy (persons per

100 m2floor area)

Outdoor air requirements

(l/s/person)

Offices

- office space 7 10

- conference room 50 10

Retail's Stores

- street level 30 5

- upper floors/arcades 20 5

Education

- classroom 50 8

- auditorium 150 8

- library 20 8

Hospitals

- patient rooms 10 13

- operating rooms 20 15

Table 1 Recommended air change rates

Table 2 Outdoor air requirements for ventilation


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Guidelines for natural ventilation

natural ventilation system should be effective regardless of wind direction

and there must be adequate ventilation even when the wind does not

blow from the prevailing direction;

inlet and outlet openings should not be obstructed by nearby objects;

windows should be located in opposing pressure zones since this usuallywill increase ventilation rate;

a certain vertical distance should be kept between openings for

temperature to produce stack effect;

openings at the same level and near the ceiling should be avoided since

much of the air flow may bypass the occupied zone; architectural elements like wingwalls, parapets and overhangs may be

used to promote air flow into the building;


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topography, landscaping, and surrounding buildings should be used toredirect airflow and give maximum exposure to breezes;

in hot, humid climates, air velocities should be maximised in the occupiedzones for bodily cooling;

to admit wind air flow, the long faade of the building and the door and

window openings should be oriented with respect to the prevailing winddirection;

if possible, window openings should be accessible to and operable byoccupants;

vertical shafts and open staircases may be used to increase and generatestack effect;

openings in the vicinity of the neutral pressure level may be reduced sincethey are less effective for thermally induced ventilation;

if inlet and outlet openings are of nearly equal areas, a balanced andgreater ventilation can be obtained.


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Barriers to the application of natural

ventilation A successful application of natural ventilation strategies is only possible when there are no problems in many

areas at various levels from the design stage to actual operating demands placed on the building users (Allard,1998). These potential barriers include:

Barriers during building operations Safety concerns

Noise from outdoor

Dust and air pollution

Solar shading covering the openings

Draught prevention

Knowledge of the users about how to take the best advantage of natural ventilation

Barriers during building design Building and fire regulations

Need for acoustic protection

Difficult to predict pattern of use

Devices for shading, privacy & daylighting may hamper the free flow of air

Problems with automatic controls in openings

lack of suitable, reliable design tools

Other barriers

Impact on architectural & envelope design Fluctuation of the indoor conditions

Design a naturally ventilated building requires more work but could reduce mechanical system (design fee on a fixed percentageof system's cost)

Increase risk for designers

Lack of suitable standards

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Ventilation Kholid