Electric resistance heating devices – Heating devices – Convection space heater
Utility Patent
1999-10-28
2001-01-02
Walberg, Teresa (Department: 3742)
Electric resistance heating devices
Heating devices
Convection space heater
C392S476000
Utility Patent
active
06169850
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electric air heating device and, more particularly, of the type wherein an electric heating element is located within a enclosure wherein air is circulated through.
BACKGROUND OF THE INVENTION
For a central air furnace or for a heat pump, or for baseboard heating, especially suited for low pressure air, wherein fresh air from the outside is circulated within a duct by the fan of the furnace or a fan located in the duct for the baseboard heating, it is known to provide a preheated fresh air intake that is heated by an electric heating coil located within the duct. The same type of electric air heater is used in air heating home systems wherein the duct unit is used as a zone heater being incorporated in the air duct network re-circulating the air from a central air furnace. In all of these applications, it is essential to provide a thermal cut-out connected to the power line feeding the electric coil in order to prevent overheating of the duct with consequent fire hazard. This overheating is usually produced by partial or complete clogging of the duct whereby airflow through the duct is partially or totally obstructed. These thermal cut-outs are of the bimetal type and may be provided with an automatic reset or a manual reset.
The temperature differential between the on and off position or state of the automatic reset thermal cut-out is very high, for instance of the order of 30° F. to 40° F. (17° C. to 22° C.) and, therefore, the electric heater may be out of commission for an excessively long time before it is reset which may be great inconvenience in the winter time at low outside temperature, for instance, when the unit is used in a fresh air intake.
In the manual reset type of thermal cut-out, an attendant must locate the defective electric air heater unit and reset heating. This also may take a long time.
In order to obviate this disadvantage, an air pressure detector is often used in such duct type electric air heaters. Such an air pressure detector detects the change of air pressure within the duct depending on the amount of air flowing therethrough. These air pressure detectors or sensors comprise a diaphragm and, in order to be sufficiently sensitive, this diaphragm must have a large diameter. Such devices are expensive, cumbersome and are also not always efficient in case a complete obstruction of the duct would occur since they would detect an air pressure inside the duct while no air is really flowing to cool down the heating coil; this could cause an overheating of the system.
OBJECTS OF THE INVENTION
Therefore, it is the general object of the present invention to provide a duct type electric air heater device which obviates the above-noted disadvantages being provided with a novel type air flow sensor that is much more precise and less cumbersome than known air pressure detectors.
Another object of the present invention is to provide a duct type electric air heater of the character described provided with simple means to gradually decrease, up to a full cut-out if required, the electric source output upon detecting even a slight obstruction of the duct.
SUMMARY OF THE INVENTION
The electric air heating device, especially suited for low pressure air and used for heating air flowing within a duct of an air temperature control network for a house or a building, comprises an enclosure with air inlet and outlet through which variable low pressure air flow is circulated, an electric heating element mounted within said enclosure inbetween said inlet and outlet and to be exposed to and to heat the air circulated therethrough, said heating element emitting radiant heat inversely to the air flow circulating through said enclosure, a power line connected to an alternating current source and to said element, an electronic switching means series connected in said power line, a first thermistor located in said enclosure adjacent and along said heating element and exposed to the radiant heat emitted by said heating element, and a first thermistor dependent control circuit means for controlling said electronic switching means to cause the latter to change between a conducting and a non-conducting state in accordance with the radiant heat emitted by said element, said radiant heat being decreased with respect to the increasing velocity of the air flowing through said enclosure and with decreasing temperature of the air entering said enclosure, said first control circuit means includes a calibrating means to adjust a minimum and a maximum temperatures of said first thermistor between which said electronic switching means gradually changes from a conducting to a non-conducting state, resulting to a permanent heating condition of said heating element and a permanent non-heating condition respectively, said first control circuit means also includes a first pulse generating means and a first pulse duration modulating means to send a first signal to modulate the number of state changes and duration of the conducting state of said electronic switching means during each cycle of said alternating current source, between both said permanent heating and non-heating conditions; thereby providing a gradual heating of said heating element controlled by said first thermistor that acts as an air flow sensor to allow for maximal heating capability of said heating element depending on said air flow circulating through said enclosure.
Preferably, the device further includes an opto-isolator means between the power line and the first control circuit means.
Preferably, the electronic switching means is a triac, the gate of which is connected to the output of the opto-isolator means.
Preferably, the device further includes a bimetallic thermal cut-out switch series connected in said power line and located within said enclosure to be exposed to the air circulating within said enclosure and switching off said power line upon sensing a high air temperature T
H
in said enclosure and wherein said high air temperature T
H
is set such that it is reached after said first thermistor reaches its own maximum temperature.
Preferably, the heating element is either an opencoil or a tubular coil which extends longitudinally within said enclosure from said air inlet to said air outlet and said first thermistor is a thermistor bead disposed in the peripheral zone of said enclosure and approximately at mid-length of said coil and sees said coil.
Preferably, the device further includes a second thermistor which is either located about the center of said air outlet and downstream from said coil or in an enclosed space wherein the air circulated through said enclosure is discharged, a second thermistor dependent control circuit means including a second pulse generating means and a second pulse duration modulating means, said second control circuit means connected with said first control circuit means at the input of said opto-isolator means, said second thermistor and second control circuit means sending a second signal to said electronic switch to modulate the number of state changes and duration of the conducting state of said electronic switching means during each cycle of said alternating current source so as to proportionally maintain a preset air temperature at said second thermistor location, downstream of said coil or in said enclosed space.
In the device of the invention, the second signal is always overridden by the first signal.
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Van Quang
Walberg Teresa
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