Ventilation – Having forced recirculation – For use with heating and cooling means
Reexamination Certificate
2000-12-11
2003-08-26
Joyce, Harold (Department: 3749)
Ventilation
Having forced recirculation
For use with heating and cooling means
C062S214000, C454S044000, C454S049000
Reexamination Certificate
active
06609967
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to ventilation of a closed environment, and more particularly, to methods and apparatus for controlling flows of air supplied to the environment and drawn from the environment to satisfy various ventilation requirements.
BACKGROUND OF THE INVENTION
Ventilation of a closed environment generally is considered as a process that involves drawing air from the environment and supplying air to the environment to make up for some or all of the air drawn from the environment. For some applications, ventilation may involve a dilution process in which the air supplied to a given environment includes a mixture of outside air (e.g., fresh air obtained exterior to the environment) and recycled air (e.g., air obtained from one or more rooms in the closed environment). Accordingly, in some ventilation applications, output air which is frawn from the environment may be divided into return air which is returned to the environment at some point and exhaust air which is exhausted from the environment to the outside.
In general, a flow of air that is drawn from or supplied to a closed environment by a ventilation system may be expressed as a volume of air per unit time, for example, in terms of cubic feed per minute (cfm). Ventilation standards established by the Americal Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) provide some examples of guidelines for minimum acceptable ventilation system parameters in terms of the respective flows of air drawn from and supplied to a given environment. In particular, the ASHRAE standards establish guidelines for the flow of fresh outdoor air that should be supplied to an environment in a given time period to insure the safety and comfort of one or more persons occupying the environment from time to time.
For some ventilation applications, a variety of potentially harmful substances, or “contaminants,” may be present in the environment to be ventilated. The potential presence of a variety of contaminants in an environment may in turn affect the desirability of recycling air (i.e., returning air drawn from the environment back to the environment) in the ventilation system.
One example of an environment in which a ventilation system may be employed is a laboratory. A laboratory generally is a facility that is designed to permit the safe use of various chemicals, toxic compounds and/or other potentially harmful substances for research or other purposes. The laboratory may be equipped with one or more devices or apparatus designed to exhaust air from the lab to an outside environment to protect lab users from potentially dangerous exposure to harmful substances. For example, a laboratory may include one or more exhaust devices such as laboratory fume hoods, canopy hoods, glove boxes, or biological safety cabinets, in which potentially harmful substances regularly may be handled, and/or exhaust trunks or “snorkels” which may be located exterior to hoods to exhaust air from a particular area (e.g., a bench top or analytical instrument) where potentially harmful substances occasionally may be handled. Additionally, a laboratory may include one or more exhausted storage cabinets to store potentially harmful substances and contain harmful fumes or vapors possibly emanating from such substances. In each of the foregoing laboratory exhaust apparatus (hereinafter referred to collectively as “auxiliary exhaust devices”), generally air is drawn from the laboratory environment and exhausted to the outside, and is not recirculated to the laboratory environment.
In view of the foregoing, conventional ventilation processes in a laboratory environment generally involve supplying 100% fresh outdoor air to the laboratory environment to make up for the air exhausted from the environment. In particular, in such processes, typically no air is recirculated from the laboratory environment back to the laboratory environment even though the air drawn from the lab environment often may be clean and safe. Furthermore, due to simplicity and costs, some portions of the lab environment served by the laboratory ventilation system, such as storage areas, office areas, “dry” laboratories (e.g., where generally no potentially harmful substances are handled), and the like, are also ventilated with 100% outside air, even though the possibility of contaminants being present in these areas is remote or nonexistent.
Accordingly, the demand for 100% outside air in conventional laboratory ventilation systems often results in wasted resources (i.e., fresh outdoor air) and unnecessarily excessive operating costs.
With respect to ventilation systems in general, at least two guidelines may be considered in determining an appropriate flow of air supplied to (and drawn from) a given closed environment. One such guideline generally is referred to as a “minimum ventilation requirement,” as alluded to briefly above in connection with the ASHRAE ventilation standards. The minimum ventilation requirement relates to a volume of fresh air that should be supplied to a given closed environment (or a particular portion thereof) in a given time period to establish a minimum level of dilution ventilation in the environment. Often, the minimum ventilation requirement is expressed in terms of “air changes per hour” (ACH), but may be alternatively expressed in terms of an airflow in cubic feet per minute (cfm). In particular, a minimum ventilation requirement given in units of ACH may be converted to an airflow in units of cfm by multiplying the minimum ventilation requirement by the volume of the environment and dividing this product by 60 minutes per hour. For example, for an environment having a volume of 1,000 cubic feet, a minimum ventilation requirement of 6 ACH may be given in terms of airflow in cfm by the following conversion:
(6
ACH X
1000 cubic feet)/60 minutes per hour=100
cfm.
For purposes of the present disclosure, the term “minimum ventilation requirement” is used in a manner consistent with the description above.
Another guideline that may be considered in determining an appropriate flow of air supplied to (and drawn from) an environment in a ventilation system generally is referred to as a “thermal load requirement.” In one aspect, the thermal load requirement for a given closed environment may relate to a flow of supply air (in cfm) having a particular temperature that is required to appropriately cool (or heat) the environment (or a particular portion thereof) to a desired temperature set point. In this aspect, the thermal load requirement not only depends on the temperature of the air supplied to the environment and the desired temperature set point, but typically is also a function of a “thermal load” which may be present in the environment. The term “thermal load” in this aspect generally refers to anything in the space, such as instrumentation or other apparatus (e.g., lab analysis equipment, computer equipment, etc.) which may generate heat in the environment (or a particular portion thereof). Such thermal loads generally may be collectively characterized in terms of the number of Watts per square foot that all of the loads generate in the space.
In another aspect, the thermal load requirement for a given closed environment may relate to a flow of supply air (in cfm) having a particular moisture content that is required to appropriately humidify (or dehumidify) the environment (or a particular portion thereof) to a desired humidity. Based on the foregoing, it should be appreciated that more generally, the thermal load requirement may relate to a flow of supply air having a particular moisture content and/or a particular temperature so as to condition the environment in terms of one or both of temperature and humidity. For purposes of the present disclosure, the term “thermal load requirement” is used in a manner consistent with the foregoing description.
While a detailed explanation of the derivation of the thermal load requirement for an environment may be somewhat complicated and unnecessary for purposes
Desrochers Eric M.
Sharp Gordon P.
Joyce Harold
Phoenix Controls Corporation
Wolf Greenfield & Sacks P.C.
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