The present invention is generally directed to a method and apparatus for determining the flow rate of makeup fluid being utilized to circulate a mixed fluid of the makeup fluid and a return fluid within a volume. The present invention is more particularly directed to a method and apparatus for ventilation measurement via carbon dioxide concentration balance.
Indoor air quality is a topic which has recently gained increasing attention, for several reasons. In general, people today are much more aware of health issues. Media attention to issues such as asbestos health risks in older facilities has increased public awareness. In many facilities, budgetary controls have forced reductions in maintenance of air circulation and filtration systems.
Physical changes in occupied spaces have affected indoor air quality. Office and industrial spaces have been remodeled more frequently than in previous years, increasing the amount of airborne contaminants from construction materials, furniture and carpeting. Cubicle furniture systems have become more common. These systems can increase occupant density and can interfere with air distribution patterns within a space.
Buildings built today have less porous envelopes than older buildings. Thus, the ventilation which naturally occurred due to infiltration through the envelope has been significantly reduced.
Variable air volume (VAV) distribution systems also have become more popular because of lower operating costs. While VAV systems provide energy savings, at the same time they lower the room ventilation effectiveness. Room ventilation effectiveness relates to how well the supply air is distributed within the occupied space. Ventilation air which can not reach a portion of a space can not efficiently displace the contaminants from the space. The net effect of the lower ventilation rates and room ventilation effectiveness is an increase in space contaminant levels.
In extreme cases, Sick Building Syndrome and Building Related Illness have occurred. Sick Building Syndrome symptoms include headaches, dizziness, drowsiness, fatigue, nausea, and eye irritation. Most people experience relief shortly after exiting the "sick building." Building Related Illness is an even more serious problem. It refers to disease or infirmity resulting from exposure to indoor contaminants. It is characterized by clinical signs such as fevers or infections and prolonged recovery times. The aforementioned environmental and design changes and associated disorders have heightened attention to the problem of indoor air quality.
Indoor air quality is a function of many variables. These variables include the quality of outdoor air, the configuration of enclosed spaces, the design and maintenance of the ventilation system and the presence and strength of contaminant sources. A properly designed ventilation system must take into account all of these variables to maintain an acceptable level of indoor air quality.
There are many sources of space contaminants. Humans and their activities release a wide assortment of organic and inorganic chemicals. Personal care products, photocopy machines and other office equipment release chemicals. Growth of fungal material can release spores into the air. Standing water can allow harmful bacteria to multiply. Office furniture, partitions, paint, floor coverings and cleaning materials outgas chemicals into the air. Cracks in below-grade walls and floors can allow radon gas to enter the building. Lastly, the outdoor air itself can be a source of unwanted contaminants. This is particularly true for buildings located in large, congested metropolitan areas, or in cases where the outdoor air intake location is near loading docks, garages or near the building exhaust.
Building contaminant concentrations are a function of the amount of contaminant both entering and being generated in the space. If the contaminant concentrations in the outdoor air are negligible, then the contaminant levels in the space are inversely related to the ventilation rate and how well the air mixes within the space. Filtration can also be utilized to remove particulate contaminants from the air.
Most air source contaminants can be classified as one of the following: particulate matter, inorganic compounds, volatile organic compounds or microbes and their by-products. Particulate matter is generated by people, tobacco smoke, dust, cold water humidifiers or processes occurring inside the space. Airborne particle sizes can range from 0.01 micrometers to 100 micrometers. Particles smaller than 3.5 micrometers can penetrate into and possibly damage the lung. Depending on the size of the particle, concentrations may be reduced with either fibrous media or electrostatic attraction filtration.
Inorganic air contaminants include gases such as carbon dioxide, carbon monoxide, nitrogen dioxide, ozone or radon. Materials such as asbestos or fiberglass are inorganic particles.
Carbon dioxide is generated by the human respiratory process. The concentration level of carbon dioxide within a space can be used as an indication of the activity level and number of occupants within the space. The amount of carbon dioxide produced by an individual depends on the individual's diet and activity level. If carbon dioxide concentrations are below 1000 parts per million (ppm), the amount of ventilation air is generally considered adequate with regard to odor comfort.
Carbon monoxide is a by-product of combustion. It can enter a building if the outdoor intake is located near a garage or near a busy street. Unvented boilers, furnaces, clothes dryers or water heaters generate large levels of carbon monoxide. Combustion gases from these devices should always be vented to the outdoors. High levels of carbon monoxide can be lethal.
Nitrogen dioxide is produced by high temperature sources such as flames. Nitrogen dioxide is suspected to be a carcinogen. Local exhaust at the source is the most effective method for lowering nitrogen dioxide concentrations in a space.
Ozone is generated primarily by combination of solar radiation effecting the composition of combustion products (hydrocarbons) and by high voltage electrical equipment. In many metropolitan areas, large ozone concentrations may be present in the outdoor air. Photocopy machines and other equipment located inside a building also produce ozone. Ozone causes irritation, allergic reactions, and in some cases may make breathing difficult.
Radon is a naturally occurring radioactive gas contained in soil and ground water in some areas of the country. Lung cancer has been linked to radon exposure.
Asbestos is also an inorganic compound. Asbestos was a common insulating and building material until recently. Exposure to asbestos fibers has been linked to lung cancer. Fiberglass, another inorganic compound, can cause skin irritation, but at this time it has not been linked to any long term negative health effects.
Volatile organic compounds are generated by cleaning materials, personal care products, tobacco smoke, furniture, carpet, adhesives, paint and people. Exposure to volatile organic compounds can cause many symptoms such as headaches, dizziness, fatigue, nausea or eye irritation. If contaminant concentrations are high enough, long term negative health effects can occur. Reduction of volatile organic compound concentrations can be accomplished by dilution with outdoor air if it is of acceptable quality. Otherwise, activated carbon filters are generally used to remove volatile organic compounds from the air.
Providing acceptable indoor air quality requires a multi-disciplinary approach involving design professionals, contractors and maintenance and operating personnel. At the design level, architects and consulting engineers must provide a building layout and mechanical system design that can adequately control contaminants and provide acceptable temperature and humidity control at all load conditions. The design must adhere to applicable standards.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) has issued ASHRAE Standard 62-1989 regarding air quality for indoor spaces occupied by humans. ASHRAE Standard 62-1989 specifies minimum ventilation rates and indoor air quality that will be acceptable to human occupants and avoid adverse health affects. The standard applies to air quality in all indoor spaces that people may occupy.
Part of the design process for a ventilation system must include conformance with ASHRAE Standard 62-1989. This standard requires the measurement of the outdoor air flow rate to verify compliance with the ventilation rates specified within the standard.
One obvious solution for complying with the new ventilation rate requirements is to directly measure the outdoor air flow with a flow meter. Unfortunately, outdoor air flow rate is difficult to measure directly with accuracy.
Pitot tube type air flow stations are the most common type of air flow station. These stations sense the velocity pressure of the air as it passes through the station. The corresponding air velocity is related to the velocity pressure in a known manner. For measuring outdoor air flow, these flow stations would typically be installed near the outdoor air intake where the maximum air velocity is generally less than 500 ft/min. Outdoor air intakes are sized for such low velocities to prevent entrainment of rain, snow or dirt into the fresh air intakes. Unfortunately, manufacturers of pitot tube air flow stations generally specify low velocity sensing limits of 600-800 ft/min. so that the measurement error remains insignificant relative to the measurement signal. Thus, pitot tube air flow stations are unable to provide the required sensing accuracy when installed in a typical outdoor air intake configuration.
A less common type of air flow station utilizes heated thermistors to measure air flow. Such an air flow station uses a microprocessor to determine the air velocity as a function of the air stream temperature and the amount of power required to maintain the surface of the thermistor at a predetermined temperature. These stations are capable of accurately measuring air velocity below 500 ft/min. However, because of the high sensitivity at low air flow rates of these devices, low levels of turbulence can adversely affect the accuracy of the air flow measurement. Unfortunately, outdoor air intakes are typically very turbulent environments. Thus, thermal air flow stations are not a very good solution to the problem of directly measuring the outdoor air flow rate accurately.
Air flow rates can be measured by means of indirect, model-based techniques. These techniques utilize indirect measurements such as temperature to determine information about other variables. One possibility is a model based on energy balance and used to determine the outdoor air flow rate (CFMoa) based on the temperature of the outdoor air (Toa), temperature of return air (Tra), temperature of mixed air (Tma) as well as the mixed air flow rate (CFMma). This model is described by Equation 1. ##EQU1##
FIG. 1 shows a typical prior art fan system layout indicating where transmitters or sensors which measure the quantities in Equation 1 would be located. The ability of this model to provide an accurate measurement of the outdoor air flow rate depends on how accurately the temperatures and supply air flow rate can be measured. The most significant source of error in this model-based outdoor air flow calculator occurs when the difference between Tra and Toa becomes small. In that case, even a small error in either temperature reading can cause very large errors in the calculated outdoor air flow rate. Temperature sensing errors of .+-.1 degree Fahrenheit are the industry norm. Consequently, unacceptable errors will be calculated whenever the difference between the return and outdoor air temperatures is less than 10 degrees Fahrenheit. Most environments have significant time periods when the difference between the outdoor and return air temperatures are less than 10 degrees. Thus, a temperature based thermal energy balance is not an acceptable strategy for calculation of outdoor air flow rate.
The present invention provides a means for verifying compliance with the new ventilation standards which overcomes these limitations in direct measurement of outdoor air flow rate. The present invention provides an accurate, indirect method and apparatus for measuring the flow rate of outdoor, ventilation air as well as a method for calibrating the apparatus. The present invention finds particular application in a ventilation system for a structure which includes a supply duct for supplying mixed air to the structure, a return duct for extracting return air from the system, a recirculation duct for recirculating return air to the supply duct, and intake and exhaust ducts for admitting or venting air between the external environment and the ventilation system.