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ESTRATEGIAS BIOCLIMATICAS

Ventilación

CONFORTCONFORT

Sensación confort = balance carga cuerpoSensación confort = balance carga cuerpo

implicaimplica

TTrr , , TToo , HR , , HR , vv , , cloclo , , metmet

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ESTRATEGIAS DE VENTILACION

VENTILACION

ACTIVA PASIVA

FORZADA

INDUCIDA

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Fluid Mechanics AIR Foil

PELICULAS

Bernoulli´s Principle

How wings work

Wind pattern simple structures

Lecture: Air Movement and Natural Ventilation,

http://www.arch.hku.hk/teaching/lectures/airvent/index.html

Chapter 9. Natural Ventilation, Santamouris & Asimakopoulos, “Passive Cooling of Buildings”

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FUGAS E INFILTRACIONES DE AIREFUGAS E INFILTRACIONES DE AIRE

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 buildingwill 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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Figure 1 Air f low around a building

FLOW AROUND BUILDINGSFLOW AROUND BUILDINGS

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WINGWING--WALL EXTRUSION FOR WALL EXTRUSION FOR IMPROVING AIR VENTILATIONIMPROVING AIR VENTILATION

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AIR FLOW THROUGH CRACK OPENINGSAIR FLOW THROUGH CRACK OPENINGS

nPkLQ )(∆=k - flow coefficient [ m3s-1Pa-n

L – Crack length [m]n – flow exponent (0.5 fully turbiulent , 1 laminar)

AIR FLOW THROUGH LARGE OPENINGSAIR FLOW THROUGH LARGE OPENINGS

ρPACQ d ∆= 2(

Cd - Discharge coefficientA – Opening Area[m2]∆P – Pressure difference across the opening [Pa]ρ – average density

( ) 31.00835.0 −∆= TTCd

Cd – Other values p. 226 Santamouis & Asimakopoulos

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WindWind inducedinduced ∆∆PP

2

2VCP p

w

ρ=

Pw – pressure due to wind flow [Pa]V – Wind speed [m/s]

aKzV

V1

10

1 =

WindWind SpeedSpeed VV

V –wind speed usaually 10 m above ground level [m/s]V1 – Actual Wind speed [m/s]K – coefficient from table bellow

Other approaches p. 227 Santamouis & Asimakopoulos

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Cp –values p. 228 Santamouis & Asimakopoulos

Cp – A parametrical model to calculate Cp is developed in p. 230-232 Santamouis & Asimakopoulos

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Gases perfectos

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The relative importance of the wind and stack pressures in a buildingdepends on building height, internal resistance to vertical air flow, locationand flow resistance characteristics of envelope openings, local terrain, and the immediate shielding of the building structure.

Figure 4 Combined effect of wind and thermal forces

Design for Natural Ventilation

The design of controlled natural ventilation systems requiresidentification of the prevailing wind direction, the strategicorientations 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 are required to determine the sizes of fans, openings, and air ducts. The methods that can be used to determine the ventilation rates include:

(a) Maximum allowable concentration of contaminants

(b) Heat generation

(c) Air change rates

(a) Maximum allowable concentration of contaminants

A decay equation can be used to describe the steady-state conditionsof contaminant concentrations and ventilation rate, like this:

Ci = Co + F / Q (3)

where Ci

= maximum allowable concentration of contaminants

Co

= concentration of contaminants in outdoorair

F= rate of generation of contaminantsinsidethe occupied space (l/s)

Q = ventilation rate (l/s)

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(b) Heat generation

The ventilation rate required to remove heat from an occupiedspace is given by:

(4)

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)

(5)

where Q = ventilation rate (l/s)

V= concentration of contaminantsin outdoor air

ACH = air change per hour

(c) Air change rates

Most related professional institutes and authorities have set up recommendedventilation rates, expressed in air change per hour, for various situations. Theventilation rate is related to the air change rate by the following equation:

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Space Air change rates per hour

Carparks 6

Kitchen 20 - 60

Lavatory 15

Bathrooms 6

Boiler rooms 15 - 30

Table 1 Recommended air change rates

Application Estimated maximum occupancy (persons per 100 m2 floor area)

Outdoor air requirements(l/s/person)

Offices

- office space 7 10- conference room 50 10Retail's Stores- street level 30 5- upper floors/arcades 20 5

Education

- classroom 50 8- auditorium 150 8- library 20 8Hospitals

- patient rooms 10 13- operating rooms 20 15

Table 2 Outdoor air requirements for ventilationASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality.

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Flow caused by wind

Major factors affecting ventilation wind forces include:

•average wind speed;

•prevailing wind direction;

•seasonal and daily variation in wind speed and direction;

•local obstructing objects, such as nearby buildings and trees;

•position and characteristics of openings through which air flows; and

•distribution of surface pressure coefficients for the wind.

Natural ventilation systems are often designed for wind speeds of half theaverage seasonal velocity because from climatic analysis there are very fewplaces where wind speed falls below half the average velocity for many hoursin a year.

GuidelinesGuidelinesforfor natural natural ventilationventilation

The following guidelines are important for planning and designing natural ventilation systems in buildings:

•a natural ventilation system should be effectiveregardless of wind direction and there must be adequateventilation even when the wind does not blow from theprevailing direction;

•inlet and outlet openings should not be obstructed bynearby objects;

•windows should be located in opposing pressure zonessince this usually will increase ventilation rate;

•a certain vertical distance should be kept betweenopeningsfor temperatureto produce stackeffect;

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•openings at the same level and near the ceilingshould be avoided since much of the air flow maybypass the occupied zone;

•architectural elements like wingwalls, parapets and overhangs may be used to promote air flow into thebuilding;

•topography, landscaping, and surrounding buildingsshould be used to redirect airflow and give maximumexposure to breezes;

•in hot, humid climates, air velocities should bemaximised in the occupied zones for bodily cooling;

•to admit wind air flow, the long façade of the building and

the door and window openings should be oriented with

respect to the prevailing wind direction;

•if possible, window openings should be accessible to and

operable by occupants;

•vertical shafts and open staircases may be used to increase

and generate stack effect;

•openings in the vicinity of the neutral pressure level may be

reduced since they are less effective for thermally induced

ventilation;

•if inlet and outlet openings are of nearly equal areas, a

balanced and greater ventilation can be obtained.

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(6)

where Q = air flow rate (m3/s)A = free area of inlet openings (m2)v = wind velocity (m/s)

Cv

= effectiveness of the openings(assumed to be 0.5 to 0.6 forperpendicular winds and 0.25 to 0.36 for diagonal winds)

The following equation shows the air flow rate through ventilation inlet openingforced by wind:

Flow caused by thermal forces

If the building's internal resistance is not significant, the flow caused by stackeffect may be estimated by:

where Q = air flow rate (m3/s)K = discharge coefficient for the opening (usually assumed to be 0.65)A = free area of inlet openings (m2)

h= height from lower opening (mid-point) to neutral pressure level (m)

Ti = indoor air temperature (K)To = outdoor air temperature (K)

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Un ab anbar con doble cúpula y captadores de viento en el desierto. Ciudad de Naeen, cercano a Yazd.

Captador de viento

http://es.wikipedia.org/wiki/Captador_de_viento

Ilustración del uso de captadores de viento y para refrescamiento.

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