Thursday, April 29, 2010

Historic Buildings Were Ventilated Naturally


Almost all historic buildings were ventilated naturally, although many of these have been compromised by the addition of partition walls and mechanical systems. With an increased awareness of the cost and environmental impacts of energy use, natural ventilation has become an increasingly attractive method for reducing energy use and cost and for providing acceptable indoor environmental quality and maintaining a healthy, comfortable, and productive indoor climate rather than the more prevailing approach of using mechanical ventilation. In favorable climates and buildings types, natural ventilation can be used as an alternative to air-conditioning plants, saving 10%-30% of total energy consumption.
Natural ventilation systems rely on pressure differences to move fresh air through buildings. Pressure differences can be caused by wind or the buoyancy effect created by temperature differences or differences in humidity. In either case, the amount of ventilation will depend critically on the size and placement of openings in the building. It is useful to think of a natural ventilation system as a circuit, with equal consideration given to supply and exhaust. Openings between rooms such as transom windows, louvers, grills, or open plans are techniques to complete the airflow circuit through a building. Code requirements regarding smoke and fire transfer present challenges to the designer of a natural ventilation system. For example, historic buildings used the stairway as the exhaust stack, a technique now prevented by code requirements in many cases.


Description

Natural ventilation, unlike fan-forced ventilation, uses the natural forces of wind and buoyancy to deliver fresh air into buildings. Fresh air is required in buildings to alleviate odors, to provide oxygen for respiration, and to increase thermal comfort. At interior air velocities of 160 feet per minute (fpm), the perceived interior temperature can be reduced by as much as 5°F. However, unlike true air-conditioning, natural ventilation is ineffective at reducing the humidity of incoming air. This places a limit on the application of natural ventilation in humid climates.

A. Types of Natural Ventilation Effects

Wind can blow air through openings in the wall on the windward side of the building, and suck air out of openings on the leeward side and the roof. Temperature differences between warm air inside and cool air outside can cause the air in the room to rise and exit at the ceiling or ridge, and enter via lower openings in the wall. Similarly, buoyancy caused by differences in humidity can allow a pressurized column of dense, evaporatively cooled air to supply a space, and lighter, warmer, humid air to exhaust near the top. These three types of natural ventilation effects are further described below.

 

 

Wind

Wind causes a positive pressure on the windward side and a negative pressure on the leeward side of buildings. To equalize pressure, fresh air will enter any windward opening and be exhausted from any leeward opening. In summer, wind is used to supply as much fresh air as possible while in winter, ventilation is normally reduced to levels sufficient to remove excess moisture and pollutants. An expression for the volume of airflow induced by wind is:
Qwind = K x A x V, where

Qwind = volume of airflow (m³/h)
A = area of smaller opening (m²)
V = outdoor wind speed (m/h)
K = coefficient of effectiveness
The coefficient of effectiveness depends on the angle of the wind and the relative size of entry and exit openings. It ranges from about 0.4 for wind hitting an opening at a 45° angle of incidence to 0.8 for wind hitting directly at a 90° angle.
Sometimes wind flow prevails parallel to a building wall rather than perpendicular to it. In this case it is still possible to induce wind ventilation by architectural features or by the way a casement window opens. For example, if the wind blows from east to west along a north-facing wall, the first window (which opens out) would have hinges on the left-hand side to act as a scoop and direct wind into the room. The second window would hinge on the right-hand side so the opening is down-wind from the open glass pane and the negative pressure draws air out of the room.
It is important to avoid obstructions between the windward inlets and leeward exhaust openings. Avoid partitions in a room oriented perpendicular to the airflow. On the other hand, accepted design avoids inlet and outlet windows directly across from each other (you shouldn't be able to see through the building, in one window and out the other), in order to promote more mixing and improve the effectiveness of the ventilation.

 

Buoyancy

Buoyancy ventilation may be temperature-induced (stack ventilation) or humidity induced (cool tower). The two can be combined by having a cool tower deliver evaporatively cooled air low in a space, and then rely on the increased buoyancy of the humid air as it warms to exhaust air from the space through a stack. The cool air supply to the space is pressurized by weight of the column of cool air above it. Although both cool towers and stacks have been used separately, the author feels that cool towers should only be used in conjunction with stack ventilation of the space in order to ensure stability of the flow. Buoyancy results from the difference in air density. The density of air depends on temperature and humidity (cool air is heavier than warm air at the same humidity and dry air is heavier than humid air at the same temperature). Within the cool tower itself the effect of temperature and humidity are pulling in opposite directions (temperature down, humidity up). Within the room, heat and humidity given off by occupants and other internal sources both tend to make air rise. The stale, heated air escapes from openings in the ceiling or roof and permits fresh air to enter lower openings to replace it. Stack effect ventilation is an especially effective strategy in winter, when indoor/outdoor temperature difference is at a maximum. Stack effect ventilation will not work in summer (wind or humidity drivers would be preferred) because it requires that the indoors be warmer than outdoors, an undesirable situation in summer. A chimney heated by solar energy can be used to drive the stack effect without increasing room temperature, and solar chimneys are very widely used to ventilate composting toilets in parks.
An expression for the airflow induced by the stack effect is:
Qstack = Cd*A*[2gh(Ti-To)/Ti]^1/2, where

Qstack = volume of ventilation rate (m³/s)
Cd = 0.65, a discharge coefficient.
A = free area of inlet opening (m²), which equals area of outlet opening.
g =9.8 (m/s²). the acceleration due to gravity
h = vertical distance between inlet and outlet midpoints (m)
Ti = average temperature of indoor air (K), note that 27°C = 300 K.
To = average temperature of outdoor air (K)
Cool tower ventilation is only effective where outdoor humidity is very low. The following expression for the airflow induced by the column of cold air pressurizing an air supply is based on a form developed by Thompson (1995), with the coefficient from data measured at Zion National Park Visitor Center. This tower is 7.4 m tall, 2.4 m square cross section, and has a 3.1 m² opening.
Qcool tower =0.49 * A* [2gh (Tdb-Twb)/Tdb]1/2, where

Qcool tower = volume of ventilation rate (m³/s)
0.49 is an empirical coefficient calculated with data from Zion Visitor Center, UT, which includes humidity density correction, friction effects, and evaporative pad effectiveness.
A = free area of inlet opening (m²), which equals area of outlet opening.
g =9.8 (m/s²). the acceleration due to gravity
h = vertical distance between inlet and outlet midpoints (m)
Tdb = dry bulb temperature of outdoor air (K), note that 27°C = 300 K.
Twb = wet bulb temperature of outdoor air (K)
The total airflow due to natural ventilation results from the combined pressure effects of wind, buoyancy caused by temperature and humidity, plus any other effects from sources such as fans. The airflow from each source can be combined in a root-square fashion as discussed in ASHRAE (2001). The presence of mechanical devices that use room air for combustion, leaky duct systems, or other external influences can significantly affect the performance of natural ventilation systems.

 

B. Design Recommendations

The specific approach and design of natural ventilation systems will vary based on building type and local climate. However, the amount of ventilation depends critically on the careful design of internal spaces, and the size and placement of openings in the building.
  • Maximize wind-induced ventilation by siting the ridge of a building perpendicular to the summer winds.
    • Approximate wind directions are summarized in seasonal "wind rose" diagrams available from the National Oceanographic and Atmospheric Administration (NOAA). However, these roses are usually based on data taken at airports; actual values at a remote building site can differ dramatically.
    • Buildings should be sited where summer wind obstructions are minimal. A windbreak of evergreen trees may also be useful to mitigate cold winter winds that tend to come predominantly from the north.
  • Naturally ventilated buildings should be narrow.
    • It is difficult to distribute fresh air to all portions of a very wide building using natural ventilation. The maximum width that one could expect to ventilate naturally is estimated at 45 ft. Consequently, buildings that rely on natural ventilation often have an articulated floor plan.
  • Each room should have two separate supply and exhaust openings. Locate exhaust high above inlet to maximize stack effect. Orient windows across the room and offset from each other to maximize mixing within the room while minimizing the obstructions to airflow within the room.
  • Window openings should be operable by the occupants.
  • Provide ridge vents.
    • A ridge vent is an opening at the highest point in the roof that offers a good outlet for both buoyancy and wind-induced ventilation. The ridge opening should be free of obstructions to allow air to freely flow out of the building.
  • Allow for adequate internal airflow.
    • In addition to the primary consideration of airflow in and out of the building, airflow between the rooms of the building is important. When possible, interior doors should be designed to be open to encourage whole-building ventilation. If privacy is required, ventilation can be provided through high louvers or transoms.
  • Consider the use of clerestories or vented skylights.
    • A clerestory or a vented skylight will provide an opening for stale air to escape in a buoyancy ventilation strategy. The light well of the skylight could also act as a solar chimney to augment the flow. Openings lower in the structure, such as basement windows, must be provided to complete the ventilation system.
  • Provide attic ventilation.
    • In buildings with attics, ventilating the attic space greatly reduces heat transfer to conditioned rooms below. Ventilated attics are about 30°F cooler than unventilated attics.
  • Consider the use of fan-assisted cooling strategies.
    • Ceiling and whole-building fans can provide up to 9°F effective temperature drop at one tenth the electrical energy consumption of mechanical air-conditioning systems.
  • Determine if the building will benefit from an open- or closed-building ventilation approach.
    • A closed-building approach works well in hot, dry climates where there is a large variation in temperature from day to night. A massive building is ventilated at night, then, closed in the morning to keep out the hot daytime air. Occupants are then cooled by radiant exchange with the massive walls and floor.
    • An open-building approach works well in warm and humid areas, where the temperature does not change much from day to night. In this case, daytime cross-ventilation is encouraged to maintain indoor temperatures close to outdoor temperatures.
  • Use mechanical cooling in hot, humid climates.
  • Try to allow natural ventilation to cool the mass of the building at night in hot climates.
  • Open staircases provide stack effect ventilation, but observe all fire and smoke precautions for enclosed stairways.
Natural ventilation in most climates will not move interior conditions into the comfort zone 100% of the time. Make sure the building occupants understand that 3% to 5% of the time thermal comfort may not be achieved. This makes natural ventilation most appropriate for buildings where space conditioning is not expected. As a designer it is important to understand the challenge of simultaneously designing for natural ventilation and mechanical cooling—it can be difficult to design structures that are intended to rely on both natural ventilation and artificial cooling. A naturally ventilated structure often includes an articulated plan and large window and door openings, while an artificially conditioned building is sometimes best served by a compact plan with sealed windows. Moreover, interpret wind data carefully. Local topography, vegetation, and surrounding buildings have an effect on the speed of wind hitting a building. Wind data collected at airports may not tell you very much about local microclimate conditions that can be heavily influenced by natural and man-made obstructions. Hints about what type of natural ventilation strategies might be most effective can often be found in a region's historic and vernacular construction practices.

C. Materials and Methods of Construction

Some of the materials and methods used to design proper natural ventilation systems in buildings are solar chimneys, wind towers, and summer ventilation control methods. A solar chimney may be an effective solution where prevailing breezes are not dependable enough to rely on wind-induced ventilation and where keeping indoor temperature sufficiently above outdoor temperature to drive buoyant flow would be unacceptably warm. The chimney is isolated from the occupied space and can be heated as much as possible by the sun or other means. Air is simply exhausted out the top of the chimney creating suction at the bottom which is used to extract stale air.
Wind towers, often topped with fabric sails that direct wind into the building, are a common feature in historic Arabic architecture, and are known as "malqafs." The incoming air is often routed past a fountain to achieve evaporative cooling as well as ventilation. At night, the process is reversed and the wind tower acts as a chimney to vent room air. A modern variation called a "Cool Tower" puts evaporative cooling elements at the top of the tower to pressurize the supply air with cool, dense air.
In the summer, when the outside temperature is below the desired inside temperature, windows should be opened to maximize fresh air intake. Lots of airflow is needed to maintain the inside temperature at no more than 3-5 °F above the outside temperature. During hot, calm days, air exchange rates will be very low and the tendency will be for inside temperatures to rise above the outside temperature. The use of fan-forced ventilation or thermal mass for radiant cooling may be important in controlling these maximum temperatures.

D. Analysis and Design Tools

Handbook methods such as those presented in ASHRAE's Handbook of Fundamentals or Bansal and Minke's Passive Building Design: A Handbook of Natural Climatic Control (ISBN: 044481745X) are very useful in calculating airflow from natural sources for very simple building geometries.
Computational Fluid Dynamics (CFD): In order to predict the details of natural airflow, numerical computational fluid mechanics models can be used. These computer simulations are detailed and labor intensive, but are justified where accurate understanding of airflow is important. They have been used to analyze new buildings including the atrium of a courthouse in Phoenix and the hangar of an air and space museum in the Washington, DC area.
An extensive list of journals, books, and other reference material regarding natural ventilation and other passive technologies is included in the Solstice Archive. For example:
DOE Building Energy Codes Program
EERE Fact Sheet: Cooling Your Home Naturally (PDF 110 KB, 8 pgs)
Software packages for natural ventilation analysis include:
AIRPAK: provides calculation of airflow modeling, contaminant transport, room air distribution, temperature and humidity distribution, and thermal comfort by computational fluid dynamics.
FLOVENT: calculates airflow, heat transfer, and contamination distribution for built environments using computational fluid dynamics.
FLUENT: A computational fluid dynamics program useful in modeling natural ventilation in buildings. It models airflow under specified conditions, so additional analysis is required to estimate annual energy savings.
STAR-CD: STAR-CD uses computational fluid dynamics to help civil engineers, architects and project managers who need better and more detailed understanding of issues involved in heating and ventilation, smoke and pollutant dispersal and fire hazard analysis, and clean room design.
Building models incorporate very limited features for deliberate natural ventilation, but they do include the calculation of natural air infiltration as a function of temperature difference, wind speed, and effective leakage area, or schedules and user-defined functions for infiltration rates.
URBAWIND: UrbaWind models the wind in urban area and calculates automatically the natural air flow rate in the buildings, according to the surrounding buildings effects and the local climatology.
Designing Low Energy Buildings with Energy-10—An hour-by-hour simulation program designed to inform the earliest phases of the design process. Runs on IBM-compatible platforms. Best operated with Pentium or higher processor and 32 Megs of RAM.
DOE-2: A comprehensive hour-by-hour simulation; daylighting and glare calculations integrate with hourly energy simulation. IBM or compatible Pentium is advisable.
ENERGY PLUS: A building energy simulation program designed for modeling buildings with associated heating, cooling, lighting, ventilating, and other energy flows.

Application

Among the primary types of buildings that can benefit from the application of natural ventilation are:
  • bus stations, picnic shelters, and other structures where stringent space conditioning is not expected,
  • barracks and other single- and multi-family housing projects,
  • most small, free-standing structures in warm and temperate climates, and
  • warehouses, maintenance pools, and other high-bay facilities in warm climates.

Relevant Codes and Standards

  • Energy Policy Act of 2005 (PDF 1.9 MB, 550 pgs)
Naturally ventilated buildings should be designed to provide thermal comfort, to achieve adequate moisture and contaminant removal, and to meet or exceed Government Energy Conservation Performance Standards.
  • Standards for building thermal comfort have been defined by ASHRAE 55.
  • Standards for adequate ventilation rates and contaminant levels can be found in ASHRAE 62.
  • Additional standards effecting ventilation practice have been developed by:
    • American Conference of Governmental Industrial Hygienists (ACGIH)
      ACGIH: provides threshold limit values for chemical substances and physical agents and biological exposure indices.
    • Occupational Safety and Health Administration (OSHA)
      OSHA (1989), Air Contaminants: examines Air Contaminants-Permissible Exposure limits (Title 29, Code of Federal Regulations, Part 1910.1000).
  • Federal energy standards: The U.S. Department of Energy (DOE) has updated 10 CFR 435 to reflect the codified version of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc./Illuminating Engineering Society of North America (ASHRAE/IESNA) Standard 90.1 to be closer to the existing voluntary sector code. This new federal standard, 10 CFR 434 Energy Code for New Federal Commercial and Multi-Family High Rise Residential Buildings, is mandatory for all new federal buildings. For existing buildings, refer to ASHRAE 100 Energy Conservation in Existing Buildings. For residential buildings, the applicable standard is ASHRAE 90.2 Energy Efficient Design of Low-Rise Residential Buildings. Methodology and Procedures for Life-Cycle Cost Analysis are described in 10 CFR 436.
Observe all codes and standards regarding transport of smoke and fire when deciding on the applicability of natural ventilation and in the design of the system.

Wednesday, April 21, 2010

MECHANICAL CIPHER

ABSTRACT:
We present a brief study of the mechanical cipher systems used in the Spanish Civil War (1936-1939). This research is based on original documents obtained from various archives. We do not address the manual methods of ciphering that were more frequently used in the fighting nor the mechanisms of simple ciphering, methods like the strip cipher, the R cipher or the RF cipher [10]. We make a brief description of the cipher device used by the Nationalist army, more commonly known as the "Clave Norte" or "North Key".
KEYWORDS: Enigma Machine, Kryha Machine, Clave Norte, North Key, Spanish Civil War, Legion Condor.

INTRODUCTION

We present a short study of the ciphering systems used by the adversaries in the Spanish Civil War (1936-1939). We do not address the manual systems used. All the systems studied in this article are documented and for this reason we do not mention the Russian cipher methods. However, recently in a Russian company's web page [8], there is mention of the use in Spain of a machine known as M-100, but there are no records of the use of this machine.

BACKGROUND

On the 17 July 1936, at 5 p.m., the Spanish troops initiated a coup in Africa, that was then followed up in practically all of Spain. The military coup having failed, the Spanish Civil War began.
On the 29 September 1936 the Nationalists decide to unify, naming General Franco as the Head of Government and General of the all the forces, celebrating his investiture on the 1st of October in the city of Burgos.

Thursday, April 15, 2010

Health and Ventilation

The public as well as health workers and0educational authorities are indebted to Dr.Wood and Mrs. Hendriksen for presenting in Ventilationt and Health the results of recent English and American investigations of the factors which make for good or bad ventilation of buildings as judged from the health standpoint.The scope of the book is limited to buildings; the ventilation of mines and tunnels is not treated.
It is only within the last 20 years that we have been brought to the realization that in general the health qualities of air are physical and not chemical; that attention must be paid to temperature, humidity and air motion rather than to the relative quantities of oxygen and carbon dioxide.

Following the presentation of this modern conception, the authors discuss ventilation practices, the ventilation of schools, end ventilation laws. It is pointed out that although comparative tests conducted in New York City Schools demonstrated that children in classrooms supplied with fresh air through open windows, showed significantly less respiratory illness than did children in rooms ventilated by mechanical systems, the laws and regulations governing the ventilation of schools in half our states provide that 30 cubic feet of (fresh) air shall be supplied for each child, each minute. Because this performance cannot be guaranteed, this requirement prohibits employing the open-windowgravity- exhaust system. Thus antiquated laws, based on a discredited and generally discarded theory, prevent thousands of children from enjoying the benefits of our present knowledge.


An accurate thermometer and its exposure where it will give representative readings for the occupied space, constitute the first step toward healthful atmospheric conditions in the classrooms. The second step, equally important, is to keep a permanent record of the temperature readings taken 6 or 8 times daily during the heating season. Merely providing thermometers will not correct overheating.Record-keeping focuses the teacher's attention
on temperature and remedial measures can be taken when overheating threatens.
The chapters on factory ventilation, ventilation for general use, ventilation costs and devices to aid ventilation appear inadequate. Further, they contain definite statements on controversial points, and so many references to the accomplishments of one particular worker in the field, as to give this section the character of a professional announcement.
To suggest that the intricate problem of adequately ventilating continuous-performance theaters in this climate (New York) throughout the year can be solved by employing modified window or any other system of natural or gravity ventilation appears unjustified. On the other hand, school auditoriums and churches, which are used intermittently and usually for relatively brief periods, may, under certain conditions of floor and air space, exposure and
outdoor temperature, be adequately ventilated by this method. No general rule can be given to govern all such cases, however; each separate problem must be studied and solved individually.
In their enthusiasm to emphasize the virtues of natural ventilation, it appears that the authors have fallen into the error of condemning mechanical ventilation in general. However true may be the opening statement in the last
paragraph on page 150-" The waste in ventilation begins with the installation of a mechanical system "-in numerous specific instances, the implication that systems of mechanical ventilation in buildings are always superfluous and wasteful is contrary to the daily experience of thousands of industrial plants and greatly weakens the force of the excellent advice in the paragraph that precedes it.
Descriptions of the sling psychrometer, the vane anemometer, and the recording wet and dry bulb thermometers might have been included to advantage in the chapter on Ventilation Instruments. The chapter on Educational Methods is filled with practical suggestions that will be welcomed by the average health worker.
Ventilation and Health is one of a very few books dealing with this newer knowledge of healthful ventilation, developed in England by Leonard Hill, and his coworkers of the Medical Research Institute, and in the United States by the New York State Commission on Ventilation, and more recently by the American Societv of Heating and Ventilating Engineers in their Research Laboratory, at the U. S. Bureau of MIines, and it is because of this, and the practical suggestions that the book is valuable.
That inaccuracies have survived the editing of this work; that descriptions of apparatus and procedures are often inadequate, and that personal opinions have at times been substituted for statements of fact, are to be regretted.

Monday, April 12, 2010

Focused on Sustainable Design

The first annual sustainable design-trend watch survey jointly commissioned by the American Society Mechanical Engineers and Autodesk found that two-thirds of respondents have worked on designing sustainable products.
The survey of ASME members is the first research conducted to understand the factors and impacts of sustainable design on mechanical engineers and their manufacturing businesses in industries including automotive and transportation, industrial machinery, consumer products and energy. Sustainable engineering refers to the design and manufacture of a volume of goods and services while using Earth’s resources more efficiently and producing less waste.
A key trend highlighted by the survey is that more than half of the practicing engineers responding reported they expect to increase their use of sustainable design practices in the next year. Primary design concerns focused on using less energy, reducing emissions and complying with environmental and regulatory standards. Additionally, a separate survey of ASME student members found that half of the respondents have encountered sustainable design practices in their studies and are extremely interested in green and sustainable information and causes.
“Engineers have to understand the impact of their decisions on built and natural systems,” says ASME Executive Director Thomas G. Loughlin. “They must be skillful at collaborating closely with colleagues in an increasingly interdisciplinary work environment to meet efficiency and resources goals impacting our only Earth.”
The results of the survey confirm that designing with sustainability in mind is now a primary aim of mechanical engineers, says Robert “Buzz” Kross, senior vice president, manufacturing industry goup at Autodesk.
Mechanical Engineering Priorities Trending Toward Renewable MaterialsAlong with creating designs that use less energy, reduce emissions and comply with regulatory standards, respondents also indicated that design priorities include using renewable, recyclable and recycled materials, reducing material waste in manufacturing and improving manufacturing processes to use fewer resources.
However, cost is a major consideration when deciding to factor sustainability into developing a new product, according to the survey. One-third of the professional engineer respondents indicated that they would consider sustainable technologies for new products only if they are cost-competitive.
Survey Methodology and Demographics
The online survey of 50,000 ASME professionals and 18,000 ASME student members was conducted over a two-week period in December. The questionnaire covered 16 questions and generated nearly 3,500 respondents in the U.S. Approximately 60% of the practicing engineers responding to the survey have careers spanning more than 20 years, with more than 25% focusing on the design and development of products, systems or equipment. Nearly 20% of the respondents work in the energy and power industry, and more than 10%, respectively, work in professional services and in manufacturing fields.

Wednesday, April 7, 2010

Using Autocad For Mechanical


AutoCAD Mechanical is a comprehensive Mechanical product design & drafting software catering to various needs of mechanical engineering companies. AutoCAD comes with a complete set of powerful drafting and detailing tools for drafting professionals - delivering the most efficient solutions in mechanical product design. Following mentioned some of the important functions and features of AutoCAD Mechanical design suite.

700,000 Standard Mechanical Components:
If you are working with machinery that requires hundreds or thousands of parts, it might take weeks or even months to draw them from scratch. Here AutoCAD Mechanical software can be of help to you. It has a comprehensive set of parts and features that you can choose for your designs. The software supports several manufactured parts such as Nuts, Screws, Washers, Rivets, Pins, Plugs, Bushings, Bearings, Structural Steel Shapes, Shaft Components, Keyways, Undercuts, Thread Ends and many more.

Powerful and Quick Dimensions:
With the use of simplified tools you can generate dimensions to easily control and expand only important variables for manufacturing. With automatic dimensioning, you can generate several dimensions with less input and force overlapping dimensions to automatically place themselves apart properly and even drive and adjust design geometry to fix in certain sizes.

Incorporation for International Drafting Standards:
AutoCAD supports BSI, ANSI, DIN, CSN, GB, ISO and GOST drafting platforms. Compliance with industry standards improve internal communication and results in reliable production outputs. The software comes with specific drafting tools for generating standards-based geometric dimensions, surface texture symbols, mechanical symbols and weld symbols. You can increase your productivity manifold and help your team deliver up-to-date, standards-based design documentation.

Automatic update across all drawings:
AutoCAD automatically redraw geometry to illustrate dashes and hidden lines of parts that are blocked by other parts in mechanical design. The hidden lines feature automatically update all relevant drawings when a change occur, practically removing lengthy manual redrawing of geometry due to repeat changes. This means you save time and efforts revising your 2D designs.

Easy Data Swapping over Different CAD Systems:
AutoCAD Mechanical suite comes with in-built industry-standard STEP (Standard for the Exchange of Product Data) and IGES (Initial Graphics Exchange Specification) formats for exchanging data between different CAD systems.

So start taking benefit of AutoCAD's comprehensive software tools to make your Mechanical drafting and drawings processes more efficient.

Monday, April 5, 2010

Flexible HVAC Duct Installation

Duct8.gif8.  Provide air space on all sides of flex duct when the duct runs through unconditioned spaces such as attics and crawlspaces. Avoid fully or partially covering flex duct with insulation. This is more important in humid climates than in dryer climates. Moisture can condense on Duct82.gifflex duct that does not have adequate airflow around it. This moisture can damage surrounding materials and can contribute to fungal growth.
4.  Support flex duct horizontally at intervals of not more than 5 feet and vertically at Duct4.gifintervals of not more than 6 feet. The maximum amount of sag between supports is 1/2 inch per foot of horizontal run. Support plenums and distribution junctions independently of the flex duct. Support straps or other support material should be at least 1 1/2  inches wide. Support by lumber is usually acceptable.
Whether it's a freezing winter morning in Minneapolis or a torrid summer afternoon in Phoenix, air leaking from ducts and air handlers can bring a little welcome relief to a home inspector braving the elements in an unconditioned attic. For clients, however, the leak that brings relief to the home inspector can represent a costly defect. Poorly installed HVAC ducts cost our clients money and reduce their comfort every minute the HVAC system runs. Installation errors can also contribute to moisture problems and related fungal growth.

This article focuses on flexible HVAC duct (flex duct), the most common material used in modern residential HVAC duct systems. While other materials such as sheet metal and duct board are still used, the cost advantages of flex duct make it a popular choice for many builders. Unfortunately, the factors that give it a cost advantage--mainly less skilled and less expensive labor--also make installation errors more likely when compared to other materials.

Costs of poorly installed ducts

In a low-pressure, forced-air HVAC system, the goal is to move the air effectively and efficiently.
  • Effectiveness is measured by occupant comfort and by maintaining reasonably consistent temperature, humidity and pressure throughout the house.
  • Efficiency is measured by total operating cost, which includes costs for utilities, maintenance and replacement at the end of the equipment's design life.
Estimates of efficiency reductions caused by poorly installed ductwork range from 10 percent for good systems up to 40 percent in poor systems. Assuming a modest annual household heating and cooling bill of $1,200 ($100/month), between $100 and $480 in utility costs alone could be wasted each year. This excludes costs for increased system maintenance and reduced system useful life, and excludes the environmental costs of pollution caused by generating the power to run inefficient systems. Adding these costs together defines a problem that deserves attention.
Like any other listed construction product, flex duct should be installed according to the terms of its listing and according to manufacturer's instructions. The following guidelines apply to most flexible duct systems.
Duct1.gif 
 
1.  Avoid bending flex duct across or around framing members, pipes and other objects. Such bends can decrease the size of the duct at the bend point, restricting airflow and increasing air friction. Over time, the duct inner core can continue to collapse at the bend point further restricting airflow. This is a common installation error.
Duct2.gif

Duct11.gif2.  Avoid bending flex duct so that the radius at the centerline is less than one duct diameter. Such bends also restrict airflow and increase air friction. This is another common installation error.
 
Duct3.gif3.  Run flex duct through spaces at least as large as the diameter of the duct inner core. While this might seem a statement of the obvious, compressing flex ducts is a common installation error. The ducts are often compressed to fit into small spaces, such as chases running between floors and the area between truss webs and truss braces in floors and attics. This installation error is also frequently found when ducts are run between different areas, such as between a garage attic and the attic over the conditioned area. Minor compression of the duct is acceptable so long as the inner core is not compressed.
 5.Avoid using flex duct to support other flex duct or construction materials such as wires or coolant lines. Over time, the weight of these materials can constrict the duct inner core.
6.  Extend flex duct to its full length. Excess duct material in a run should be less than 5 percent. Excess material increases air friction.
7.  Run flex duct out of plenums, distribution junctions and boots at least 12 inches before making a bend in the duct. Tight bends near plenums also increase air friction and decrease duct size.
9.  Provide clearance between flex duct and furnace or water heater vent flues as required by the flue type. Flex duct is considered to be combustible material in terms of clearance to combustion appliances flues.
10.  Avoid running flex duct over steam pipes and similar heat sources.
11.  Install flex duct at least 4 inches above ground level and above the design flood elevation. Do not install flex duct in tile, metal pipe or within masonry or concrete.
12.  Avoid installing flex duct where it will be subjected to direct sunlight, such as under turbine vents. Sunlight can degrade the vapor barrier.
13.  Attach flex ducts at plenums and distribution junctions to sheet metal collars that are at least 2 inches long. Use metal sleeves at least 4 inches long to splice two lengths of flex duct. Make joints substantially airtight. Use at least two wraps of approved metal tape to attach the duct inner core at collars and distribution junctions. A clamp is recommended, though not always required. Use both tape and clamps to splice two lengths of flex duct.
14.  Repair tears in the vapor barrier using recommended material.
15.  Install fireblocking where flex duct penetrates floor and ceiling assemblies, and where it penetrates concealed connections between vertical and horizontal spaces such as soffits and other dropped ceilings. Unfaced batt mineral wool or fiberglass insulation is usually acceptable fireblocking material.
16.  Avoid installing flex duct that penetrates a fire-rated assembly unless approved by the authority having jurisdiction. This includes flex duct connected to boots located in a garage.
17.  Protect flex duct from damage by sharp objects such as truss gusset plates, attic furnace support straps and nails. These objects can puncture the duct.
18.  Protect flex duct from moisture during and after construction. Once it becomes wet, flex duct insulation can remain wet for years, providing a breeding ground for fungal growth.
19.  Protect duct boots, particularly floor ducts, from contamination during and after construction.

Finding and describing installation errors

How does an inspector decide what warrants being reported as an installation error in need of repair or evaluation by an expert? In the absence of a measurable guideline (such as the one-duct diameter bend guideline), each inspector must make a common sense determination based on the HVAC system(s) in the home. Here are some common installation errors, and some suggestions for when to call for expert evaluation and/or repair.

Evaluate and/or repair:
1.  Trunk supply ducts (those that serve more than one branch duct) and central return ducts that bend across an obstruction or a support at more than a 45-degree angle. These ducts are often constricted, and those that make a vertical bend over an obstruction may become more constricted over time.
2.  Trunk supply ducts and central return ducts whose inner core is constricted in size by more than about 20 percent. Somewhat more constriction may be reasonable for a duct serving a single boot because only one boot is affected.
3.  Duct bends that do not comply with the one duct diameter bend guideline.
4.  Ducts that run horizontally out of a plenum or into a boot.
5.  Ducts that are loose at a collar or do not have a metal sleeve at a splice.
6.  Ducts with a punctured inner core.
7.  Ducts in direct contact with a heat-generating appliance vent.
8.  Ducts that leak a "significant" amount of conditioned air at a plenum collar or at a splice or junction.
9.  Ducts that touch the ground and ducts buried in insulation if there is evidence of condensation on or near the duct.
10. Ducts that breach a fire-rated assembly unless the local authority having jurisdiction accepts the practice.
11. Ducts that have wet insulation from sources such as rain or plumbing leaks.
Flex your knowledge
If you're inspecting flex duct in a newer HVAC system, there's a good chance you'll find installation errors. Armed with knowledge of recommended installation techniques and your own common sense, you can save your clients money and help the environment as well.

Thursday, April 1, 2010

Relationship Between Ventilation Rate and Odour

Odour can be regarded as a 'pollutant' or as an indicator of the presence of pollutant. Sometimes it may alert the occupant to a potential health risk, although this need not always be reliable since some highly toxic pollutants, such as radon and carbon monoxide, are odourless. More generally, odour causes discomfort, especially in sedentary environments such as the office or home. A difficulty with odour analysis is that many odours cannot be measured by instrumentation. Evaluation, therefore, has to rely on subjective testing by 'panellists', thus making the interpretation of results difficult. A comprehensive study of odour and the control of odour by ventilation has been made by Fanger (1988).

Fanger (1988). Introduction of the olf and the decipol units to quantify air pollution perceived by humans indoors and outdoors. Energy in Buildings No12 1988.
Definitions from CEN Report CR 1752 Ventilation for buildings - Design criteria:
 The strength of most pollution sources indoors may be expressed as "person equivalents", i.e. the number of standard persons (olfs) required to make the air as annoying (causing as many dissatisfied) as the actual pollution source.
Perceived air quality may also be expressed in decipol (dp), where 1 dp is the air quality in a space with a pollution source strength of one olf, ventilated by 10 l/s of clean air, i.e. 1 dp = 0,1 olf/(l/s).

Air Ventilation

Ventilation Rates

There are two basic ways of measuring ventilation rate – according to the number of people in the space and the size of the spaces.

Number of people (capacity)

This is the system that has been used in the UK for some time. The idea is that each person in the space should have a certain amount of fresh air supply (measured in metres3/person/hour or litres/person/second).

To find out how much air would be needed just multiply the number of occupants with the ventilation rate – so a rate of 60m3/person/hour would require a ventilation rate of 3000m3/hour for the room with 50 occupants (or half that for 25 seated occupants). This would probably represent a room of around 50m2.


Sometimes the guidance is quoted in litres/person/second which is more difficult to calculate but for reference:

30m3/person/hour = approximately 8 litres/person/second
60m3/person/hour = approximately 16 litres/person/second

Size of space (air changes)
 
This is a very simple calculation to understand and use. The amount of ventilation needed to fill the room once in an hour is one air change/hour. The added factor here is the ceiling height.

For the 10m x 5m (50m2) room quoted above the volume of air required for one air change depends on the ceiling height:
At a height of 2.5m (50m2 x 2.5m) the room volume would be 125m3/hr
At a height of 5m (50m2 x 5m) the room volume would be 250m3/hr

In our 50m2 floor area pub:
 
a rate of 30m3/person/hour= 12 air changes at a ceiling height of 2.5m
 = 7.5 air changes at a ceiling height of 4m
a rate of 60m3/person/hour= 24 air changes at a ceiling height of 2.5m
 = 15 air changes at a ceiling height of 4m

There are various and often conflicting views on ventilation rates to allow for smoking. The two major organisations who propose standards ASHRAE in the USA and CIBSE in the UK have both recently moved away from recommending ventilation rates in this area.
Previously ASHRAE (Standard 62 2001) had recommended a rate of 15+ litres per person per second (c.30m3/hr/person) and CIBSE (Guide B) 16 litres per person per second (c.60m3/hr/person) for a mix of smoking and non smoking occupants. Test work seems to suggest that this is more than enough to ensure that staff and customers are not exposed to any contamination above the recommended workplace exposure limits. Tests on this are ongoing.




 
 
            

Ventilation rate

The amount of air inhaled in a specified time period (e.g., per minute, per hour, per day, etc.); also called breathing rate and inhalation rate.

Smowtion

Smowtion ...