Earth’s atmosphere is composed of air. Air is a mixture of gases of 78% nitrogen and 21% oxygen with traces of water vapor, carbon dioxide, argon, and various other components. Air is a uniform gas with properties that are averaged from all the individual components. Air at sea level static conditions for a standard day depends on the pressure and temperature of the location on the earth and season of the year. Gas is composed of a large number of molecules which are in constant and random motion.
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Air pressure and temperature changes from day to day, hour to hour, and sometimes even minute to minute during severe weather. Standard value of air shown in the diagram are just average values used by engineer in assist to design and calculate machines. Gravity is the key important factor because it holds the atmosphere to the surface. As altitude changes, the state-of-the gas factors will change, which is why the typical values given are at static conditions – sea level. As altitude increases, air density, pressure, and temperature decrease.
Wind Direction and Speed
Understanding Wind.
Wind can be defined as a simple of air movement across the earth’s surface and can be in any direction. which is cause by the differences in air density, thus causing in horizontal differences in air pressure greatly than it causes the vertical pressure. These pressure systems are essentially the cause and result of spatial differences in atmospheric pressure/circulation.
There are general characteristics to describe wind, wind Speed and wind Direction, which create different types of wind. Examples of wind include breeze, which is a long duration of low speed wind; gusts, a short burst of high speed wind; strong immediate winds like squalls; and lastly strong intense winds like hurricane or typhoon. Wind speed is the velocity obtained by a mass of air travelling horizontally through the atmosphere. The common measurements for wind speed are kilometres per hour(kmph), miles per hour (mph), knots and meters per second by using a anemometer. The direction of wind is measured by an instrument called a wind vane.
There are two main that effect wind direction and speed
Pressure-gradient force
Coriolis force and friction.
*and lastly friction.
These factors work coherently to change the wind in different directions and at different speeds.
Pressure-Gradient Force
Pressure gradient force is the primary force influencing the formation of wind. Wind always blows from high pressure area to low pressure area on a horizontal gradient. Vertically, wind flow from low pressure area to high pressure area. This pressure gradient force that causes the air in motion and causing the air to move in motion with increasing speed down the gradient. Uneven heating on the earth’s surfaces causes the continual generation of these pressure differences. The greater the pressure difference over a certain horizontal distance, the greater the force and therefore, the stronger the wind.
On weather map surfaces, the variations of air pressure over the earth’s surface is indicated by drawing isolines of pressure, called isobars.
The spacing of the isobars indicates the amount of pressure change over a given distance. The closely space in the isobar show steep pressure gradient indicate strong winds, relatively, widely spaced isobars indicate a weak pressure gradient and light winds.
The Coriolis force
The rotation of the Earth creates another force, known as the Coriolis force which effects the direction of the wind and other object objects in motion in very predictable ways. Newton’s first law of motion – The law of Inertia, state that forces are balanced. Air will remain moving in a straight line unless it is altered by an unbalancing force. Instead of wind blowing directly from high pressure area to low pressure area, Coriolis force opposes the pressure gradient acceleration and changes the moving air direction. Wind is deflected to the right of the gradient in the Northern Hemisphere, while in the Southern Hemisphere wind is deflected to the left.
Key note*
Coriolis force only effect the wind direction and not the wind speed.
There is no deflection of winds on the equator of the earth, but maximum deflection at the poles
Friction layer Wind
Friction is the last force that influenced both speed and direction winds. Friction is only operative only close to the Earth’s at about 2,000 feet above earth’s surface. Friction greatly reduces speed of surface air and reduces the Coriolis force. As a result, the reduced Coriolis force alter the pressure
Gradient force, to move the air at right angles across the isobars toward the area of lower pressure. Surface winds on a weather map does not blow parallel to the isobars in geostropic and gradient wind, instead surface wind cross the isobars vary at an angle from 10 to 45 degrees. Over the ocean where frictional drag is less, and reduced the angle to as little as 10 degrees.
Hospital and Air
General Principles of infection control
Isolation precaution is an important strategy in the practice of infection control. The spread of some infections can be impeded if infected patients are segregated from those who are not infected yet. Although there is no single study showing the effectiveness of isolation.
The concept of isolation can be traced back to biblical times when lepers were segregated from the rest of the populace. Towards the end of 19th century, there were recommendations for patients with infectious desease to be placed in separate facilities, which ultimately became known as infectious diseases hospitals. However, in the early 1950s, many of these infectious disease hospitals closed and the patients were moved to general hospitals. The need for proper isolations of infections in the context of general hospitals thus become an important issue.
Spatial separation is critically important when using isolation precautions because many infectious airborne contaminations are spread mainly through direct contact when patients are near to one another. Special ventilation controls are required for diseases that can be transmitted over long distances by droplet nuclei (x). However, most diseases are not of this category. Proper isolation is critically important for infectious diseases that can be transmitted through long distance which can result in large clusters of infection in a short period.
Infection Control and Isolation Practices
Three level of controls must be considered when using isolation precautions. When setting up levels of control for isolation system in hospital, attentive attention must be given for the system to work effectively. Failure in doing so will result all three levels not working and supporting each other.
First level of control
Administrative control is the first level of control measure that needs to be taken to ensure that the entire system proceed effectively.
Implementing proper procedures for triage of patients
Detecting infections early
Separating infectious patients from others
Transporting the patients
Educating the patients and staff
Designating responsibilities clearly and correctly
Communicating with all relevant partners
Second level of control
“environmental and engineering controls” is the second level so isolation.
Cleaning of the environment
Spatial separation
Ventilation of spaces
Third level of control
The third level of control is to further decrease the risk of transmission of infectious disease
Personal protection
Provide personal protective equipment
Sanitor provided in hospital
Uses of Air Pressure Differences in Hospital
In a hospital setting, certain populations are more vulnerable to airborne infections including immune-compromised patients, new-borns and elderly people. This also include hospital staff and visitors can also be exposed to airborne infections as well.
Negative Room Pressure to Prevent Cross – Contamination
A negative pressure room in a hospital is used to contain airborne contaminants within the room. In the hospital is surrounded by harmful airborne pathogens include bacteria, viruses, fungi, yeasts, moulds, pollens, gases, volatile organic compounds, small particles and chemicals are part of a larger list of airborne pathogens.
Negative pressure is created by balancing the room’s ventilation system so that more air is exhaust out from the room than it is supply. A negative pressurize room is architecturally design so that air flows from the corridor, or any adjacent area into the negative pressure room. This is to ensure and prevent airborne contaminants from drifting to other areas of the hospitals and contaminating patients, staff and sterile equipment.
Rooms to be Pressurize Negatively
According to the 2014 FGI Guidelines and Standard 170-2013, there are a list of rooms in healthcare architecture that needs to be negatively pressurized.
ER waiting rooms
Radiology waiting rooms
Triage
Restrooms
Airborne infection isolation rooms
Darkrooms
Cytology, glass washing, histology, microbiology, pathology, sterilizing laboratories and nuclear medicine
Soiled workrooms
Soiled or decontamination room for central medical and surgical supply
Soiled linen and trash chute rooms
Holding rooms
Autopsy rooms
Janitors’ closets
Architecture Design for Negative Pressure Room
In a well-designed negative pressure room, there should only be one source of air input to the room. Air is pulled through a gap under the door, other than the small opening, the room should be air tight as possible to prevent air from entering. Room must be regularly maintained to prevent any crack or opening in the room.
There are certain criteria and guidelines that a negative pressure room should fulfilled
A negative pressure differential of ‰ 2.5 Pa
Isolation room with ‰12 air changes per hour (ACH) for new building, ‰6 ACH in existing old buildings
An airflow differential >123-cfm (56 l/s) exhaust
Airflows from clean to dirty
Sealing of room, allowing approximately 0.5 square feet (0.046 m2) leakage
An exhaust to the outside
With recent approval from World Health Organization guidelines, natural ventilation can be used for airborne precaution rooms.
Positive Pressure in Healthcare Design
Healthcare centre are surrounded by pollutions, germs and airborne infection, and these can severely be hazardous to patients, healthcare employees and visitors when exposed. Visitors in healthcare centre are usually patients suffering from allergies, asthma, cardiopulmonary diseases, hyper sensitive to chemicals or having a weaker immune system and are seriously threatened by airborne micro-biological contamination could worsen their condition.
Room adjacent to a negative pressure room are positive pressure. Positive pressure in rooms is to ensure that airborne pathogens do not contaminate the patient or supplies in that room. Operation room are example use of positive pressure, which is use to protect the occupant and sterile medical and surgical supplies. The design intention of a positive pressure room is to optimize the condition for clean, invasive procedure, thus reducing infectious risks to patient. These rooms are often considered the cleanest room in a healthcare facilities.
Examples of positive pressure procedure rooms
Cardiac catheterization or interventional radiology in a radiology suite
Trauma or emergency surgical procedure rooms
Other invasive procedures such as the insertion of pacemakers or electrophysiology procedures carried out in other locations of inpatient and outpatient facilities
Criteria for a positively pressurise operating room
‰15 air changes per hour (ACH) airflow out of the room
Examples of Drawing Layout for Negative Isolation Room
Positive Pressure vs Negative Pressure
When total cubic feet per minute from supply air is more than return air, the room is under positive pressure and the air will flow out of the room. (Supply air > Return air)
When return air is more than supply air, the room is under negative pressure and the air will flow into the room. (Return air > Supply Air)
CHAPTER 3 – ARCHITECTURE PROPERTIES OF CONTROLLING AIR
Architecture
Natural Ventilation of Health Care Facilities
Ventilation
Contemporary healthcare centre relies heavily on mechanical ventilation to keep indoor spaces ventilated and pressurise. The uses of mechanical ventilation require high amount energy and often do not work as expected. Equipment failure, poor maintenance, utility service and other management failure may interrupt a normal mechanical operation in healthcare centre. Instead of being an important system for controlling disease and infection, failure in mechanical ventilation systems may result in uncontrollable spread of disease through health-care facilities which could cause huge problem, outbreak of diseases. To ensure performance of mechanical system is not compromised, high cost of money is needed for installation and maintenance cost for the operation. Backing up all mechanical ventilation equipment is expensive and unsustainable is required for continuous operation if the system services a critical facility.
Conditional recommendation when designing naturally ventilated healthcare facilities, overall airflow should bring the air from the agent sources to areas where there is sufficient dilution.
Ventilation
“Ventilation” the common term use in contemporary architecture, and is an important factor in building design. Ventilation provide healthy air for breathing by moving outdoor air into a building or a room, and channels the air within the building or each respective room. There are three basic elements in building ventilation to be considered:
Ventilation Rate – ventilation flow rate can be referred to as the absolute amount of inflow air per unit time and the air-change rate as the relative amount of inflow air per unit time. (Annex X.)
Airflow Direction – the overall airflow direction into a building.
Air distribution or airflow pattern – each part of the space should be distributed by the external air in an efficient manner. Air flown pattern effects the way airborne pollutants is removed in an efficient manner because pollutants is generated in each part of the space.
Natural Ventilation
One of the fundamental aspects of architecture is to provide comfort to the inhabitant. This is done by wall insulating, heating, protecting from the sun and managing fresh air intake. Natural ventilation enhances air quality by dissolution of pollutants and refreshing thermal comfort in building. Ventilation based on natural forces should always be preferred to mechanical ventilation especially in European climates, as it can efficiently complete comfort and energy objectives without mechanical energy consumption.
Driving Forces of Natural Ventilation
From our understanding from chapter 2 (Architecture and Air) that wind is a natural phenomenon causes by pressure-gradient force and coriolis forces therefore is the most influential factor for natural ventilation. Wind creates air flow insides building by creating high and low pressure on different building facades. These movement is strongly dependent on wind pressure gradients. Wind flow and wind pressure distribution. The second natural forces affecting natural ventilation Differential of indoor and outdoor air density causing thermal buoyancy force, stack pressure. Natural ventilation drives outdoor natural air into building envelope openings and other architectural purpose-built openings include windows, doors, solar chimneys, wind towers and trickle ventilators. Wind pressure and stack pressure are two of the natural forces that drives natural ventilation and is important
Wind Pressure
When wind flows around a building, it can produce a very high suction pressures. Pressure is induced on the building when wind strikes a building. Positive pressure on the windward face which is the direction of upwind from the building; negative pressure on the leeward face, pulling rather than pushing on the building. This drives the air to flow through windward openings into the building to the low-pressure openings at the leeward face. Windward pressure differs along the height of the building, while the leeward pressure is constant. These pressures occur mainly on the leading edges of the roof, and the cladding on these areas has to be firmly fixed to the structure and the roof has to be firmly held down.
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The wind pressure generated on a building surface is expressed as the pressure difference between the total pressure on the point and the atmospheric static pressure. Wind pressure data can usually be obtained in wind tunnels by using scale models of buildings. If the shape of building, its surrounding condition and wind direction are the same, the wind pressure is proportional to the square of outdoor wind speed. Thus, the wind pressure is usually standardized by being divided by the dynamic pressure of the outdoor wind speed.
The standardized wind pressure is called the wind pressure coefficient and symbolized as (Cp). The outdoor wind speed is usually measured at the height of the eave of the building in the wind tunnel. Calculation for wind pressure acting on the building surfaces can be found in Annex X.
Natural Architectural Ventilation System
Windows and Openings
Cross flow
Trickle Ventilators
Wind Screen
Stack Pressure
Stack pressure or thermal buoyancy force is generated from the air temperature or humidity difference (sometimes defined as density difference) between indoor and outdoor air. This difference generates an imbalance in pressure gradients of the interior and exterior air columns, causing a vertical pressure difference. Air buoyancy allows movement of air into and out of buildings, chimneys, flue gas stacks or other containers. The effectiveness of stack ventilation is influenced by the effective area of openings, the height of the stack, the temperature difference between the bottom and the top of the stack and pressure differences outside the building.
There are two effective uses of stack ventilation which occurs in a room and stack effect in a high-rise building. Examples two different uses are given as below.
When the room air is warmer than the outside air, the room air is less dense and rises. Air enters the building through lower openings and escapes from upper openings; on the other hand, when the air is colder than the outside air, the room air is denser than the outside air, the direction of air flow is reverse to an insignificant degree. Air is then entering the building through the upper openings and escapes through the lower openings. Stack driven flows in a building are driven by indoor and outdoor temperatures. The ventilation rate through stack is the result of pressure differential between two openings of the stack.
“When air heat up, it becomes less dense thus more buoyant, causing air to rise up.” Understanding the properties of air in chapter 2, we are able to use this effect to naturally ventilate buildings. Cooler air from outside of the building is drawn into the building at the lower level and is heat up by user, equipment, heating or solar heat gain within the building. Hot air that rises up in the building is vent out at a high level. The tendency of warm air to rise results in pressure differences at various levels of the building. Pressure on the lower levels and basements of a building falls below the atmospheric pressure. On the upper levels of the building, pressure of air will be higher than atmospheric pressure. In between the point of high pressure and low pressure zones lies the neutral pressure plane where the pressure will be neutral. Internal air pressure above the neutral plane will be positive pressure, forcing air to be drawn out the building; wheres, below the neutral plane, the internal air pressure will be negative and drawing air into the building.
The neutral pressure plane is often located at the vertical mid-point of the building. A building with similar leakage rates at all levels will have neutral plane at the mid-point. However, when the basement is leaky and sealed top floor of the building, the building will have a lower neutral pressure plane. Similarly, when the building has a leakier top floor and sealed basement the neutral pressure plane will be higher than the mid-point.
Natural Architectural Ventilation System
Solar Chimney and Atrium
Trombe Wall
Bernouli’s Principle
Identical to stack ventilation using air pressure for passive ventilation, except the difference between bernouli’s principle and stack ventilation is where the pressure difference comes from. Unlike stack ventilation which utilizes temperature difference to move air, bernouli’s principle uses wind speed difference to move air. In general principle of fluid dynamics, faster moving air has lower pressure. This lower pressure can help suck fresh air through the building. From an architectural point of view, outdoor air further from the ground is less obstructed, causing it to move faster than air at lower altitude, thus resulting in lower pressure. Site surrounding is an important factor to be accounted for, with less obstruction for wind to travel.
Natural Architectural Ventilation System
Example use of Bernouli’s principle are wind cowl’s and wind tower which utilizes the faster winds above roof tops for passive ventilation.
Wind Cowl
Fast roof top wind is scooped into the building through the intake valve and at the larger outlet valve creates lower pressure which naturally suck the air out. Stack effect will also help to pull air out through the same exhaust vent.
Architectural Design taking Advantage of Stack Ventilation and Bernouli’s Principle
Designing for stack ventilation and Bernoulli’s principle are similar, and a structure built for one will generally have both phenomena at work. In both strategies, cool air is sucked in through low inlet openings and hotter exhaust air escapes through high outlet openings. The ventilation rate is proportional to the area of the openings. Placing openings at the bottom and top of an open space will encourage natural ventilation through stack effect. The warm air will exhaust through the top openings, resulting in cooler air being pulled into the building from the outside through the openings at the bottom. Openings at the top and bottom should be roughly the same size to encourage even air flow through the vertical space.
To design for these effects, the most important consideration is to have a large difference in height between air inlets and outlets. The bigger the difference, the better.
Towers and chimneys can be useful to carry air up and out, or skylights or clerestories in more modest buildings. For these strategies to work, air must be able to flow between levels. Multi-story buildings should have vertical atria or shafts connecting the airflows of different floors.
Reference
http://www.mintie.com/assets/img/resources/ASHRAE_Article-on-VentilationChanges.pdf
http://www.tsi.com/uploadedFiles/_Site_Root/Products/Literature/Brochures/Room-Pressure-Solutions-for-Healthcare-Facilities_2980067_US.pdf
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