Measuring and testing – Gas analysis – Gas of combustion
Reexamination Certificate
1998-10-30
2001-11-20
Noland, Thomas P. (Department: 2856)
Measuring and testing
Gas analysis
Gas of combustion
C073S031010, C073S863110, C073S863610, C432S032000
Reexamination Certificate
active
06318150
ABSTRACT:
FIELD OF INVENTION
This invention relates generally to combustion appliances and in particular to improved apparatus for sampling gas in a combustion appliance.
BACKGROUND ART
Prior art apparatus for sampling gas in operative association with a combustion appliance, such as a furnace, boiler or water heater, have typically involved the use of a mechanical device, such as a pump, or a specially configured sampling conduit to provide a pressure difference needed to sustain a flow of gas to be sampled to a gas sensor, which is operable to measure concentration of a selected gas. A device is also needed to verify that there is sufficient gas flow to the sensor for sampling purposes. Such prior art apparatus usually focus on quantitative measurement of the gas flow and may have one or more of the following disadvantages: (1) difficulty in detecting relatively low flow rates; (2) insensistivity to flow direction; (3) flow restrictions; and (4) relatively high cost.
One application of gas sampling is to detect the presence of potentially dangerous gases, such as carbon monoxide, in the products of combustion of a furnace or in the heated air supplied by the furnace to an indoor space. Products of the combustion process are typically exhausted from the furnace through a flue after passing through a heat exchanger. An air mover, such as a blower, moves the supply air across the heat exchanger, whereby heat is transferred from the products of combustion to the air. The heated air is then supplied to an indoor space. One type of fuel used in furnaces, such as furnaces used to heat residences, is natural gas. Products of combustion of natural gas usually include water vapor, carbon dioxide and sulfur dioxide, and in the event of incomplete combustion of the natural gas, carbon monoxide. Although carbon monoxide is known to be hazardous to human health, it is usually not produced in large enough concentrations to present a danger to human beings, if the furnace is operating properly. Further, whatever carbon monoxide is produced should be confined to the inside of the heat exchanger and flue, so that under normal circumstances carbon monoxide should not be present in the supply air. However, if there is a leak in the heat exchanger, products of combustion, which may include carbon monoxide, can escape into the supply air and enter the indoor space.
Prior art techniques for preventing carbon monoxide from reaching dangerous levels in an indoor space have generally involved installation of a carbon monoxide detector either in the space or in a supply air plenum or duct downstream of the heat exchanger. A carbon monoxide detector can also be used to detect carbon monoxide in the products of combustion exhausted through the flue. The detector may generate an alarm in response to the concentration of carbon monoxide being in excess of a predetermined level, which may result in the furnace being automatically deactivated. However, carbon monoxide detectors are often unreliable and susceptible to false alarms. Also, such detectors may not be sensitive enough to accurately measure relatively low concentrations of carbon monoxide because of diffusion of the carbon monoxide gas in the supply air stream and indoor space.
In lieu of using detectors to directly measure carbon monoxide concentrations, the presence, or potential presence, of carbon monoxide in the supply air stream may be inferred if a products of combustion leak is detected in the furnace. Various procedures are known in the art for detecting products of combustion leakage from furnace components, such as heat exchangers, combustion chambers and the like. One such procedure involves introducing a non-combustible tracer gas, such as a combination of methane and nitrogen, into the heat exchanger and using a gas detector to detect the presence of any tracer gas in the supply air stream. The presence of tracer gas in the supply air stream indicates a leak in the heat exchanger through which products of combustion can escape into the supply air stream. Another procedure involves introducing a fine mist of liquid fire retardant material into the return air stream upstream of the heat exchanger and determining whether there is a change in the color of the flame in the furnace combustion chamber. A change in color from blue to orange indicates a leak in the combustion chamber wall. Other techniques involve pressurizing the heat exchanger to detect leaks. The primary disadvantages of these leak detection procedures are that they require introduction of an external gas or other fluid and the presence of a service person and special equipment.
There is, therefore, a need for an improved apparatus for sampling gas in a combustion appliance. There is also a need for an improved apparatus for detecting products of combustion leakage in a combustion appliance.
SUMMARY OF THE INVENTION
In accordance with the present invention, apparatus is provided for sampling gas in operative association with a combustion appliance, such as a furnace, boiler or water heater. The combustion appliance includes a first conduit through which combustion air is supplied to the appliance and a second conduit through which gas is discharged from the appliance after being heated. The sampling apparatus includes a third conduit communicating between the first and second conduits and a gas sensor operable to determine whether a selected gas is present in the heated gas. Operation of the appliance creates a pressure difference between the first and second conduits to draw a sample of heated gas from the second conduit into the third conduit. The gas sensor senses whether the selected gas is present in the sample of heated gas in the third conduit. In accordance with a feature of the invention, a temperature sensor is provided for sensing temperature of the heated gas in the third conduit to verify gas flow in the third conduit. If the temperature sensor does not sense at least a predetermined increase in temperature during a predetermined period of appliance operation, it indicates insufficient flow of the heated gas in the third conduit for sampling purposes.
In accordance with one application of the present invention, the gas sampling apparatus is operatively associated with a combustion appliance, such as a furnace, boiler or water heater to detect products of combustion leakage in the appliance. A gas sensor is provided to sense a selected gas known to be present in the products of combustion, such as carbon dioxide, in a gas sample taken externally to products of combustion carrying components of the appliance. A controller is provided to control the gas sensor to measure the concentration of the selected gas during a predetermined period of appliance operation and to determine whether there is an increase in the concentration during the predetermined period. An increase in concentration may be attributable to a leak in a products of combustion carrying component of the appliance. A temperature sensor is provided for measuring the temperature of the gas sample during the predetermined period. If the temperature sensor does not detect at least a predetermined increase during the predetermined period, it indicates insufficient flow of the sample to the gas sensor.
In accordance with another application of the present invention, the gas sampling apparatus is used to sense the presence of a selected gas, such as carbon monoxide, in the products of combustion of a combustion appliance, such as a furnace, boiler or water heater. In this application, a sampling conduit communicates between a combustion air intake conduit through which combustion air is supplied to the appliance and a flue through which products of combustion are exhausted from the appliance. Operation of the appliance draws a sample of the products of combustion from the flue into the sampling conduit. A gas sensor is operable to sense the selected gas in the products of combustion sample and to measure the concentration thereof during a predetermined period of appliance operation. A temperature sensor is provide
Lennox Manufacturing Inc.
McCord W. Kirk
Noland Thomas P.
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