1. Field of the Invention
This invention pertains to filters, and more particularly to apparatus for preventing foreign materials from entering furnaces.
2. Description of the Prior Art
Modern high-efficiency furnaces are well known and are in widespread use. Their economical and reliable operation contributes to their popularity.
Intake air for a high-efficiency furnace is usually drawn directly from out-of-doors. The inlet air flows through an inlet tube that passes through a building wall and connects to the furnace. Similarly, exhaust gasses from the furnace flow through an outlet tube that terminates out-of-doors, usually close to the inlet end of the inlet tube. The outdoor ends of the inlet and outlet tubes are normally located within a few feet of ground level outside the building. The tubes vary in size, with two inch diameter tubes being satisfactory for most residences, and a three inch tube usually being required for commercial and industrial applications.
Despite their general acceptance, high-efficiency furnaces have a drawback that occasionally causes rather serious problems: the inlet and outlet tubes are susceptible to plugging. Common reasons for the tubes becoming plugged include leaves or other debris blowing into them. Birds are another a problem; birds have been known to fly into furnace tubes and getting killed upon reaching the furnace. Another source of plugging is children tossing stones or other items into the tubes.
When a tube becomes plugged, sensors on the furnace automatically shut it off. It is then necessary to service the furnace. The only practical way to unplug a tube is to cut it in order to reach the plugged section. Then the cut tube must be patched or replaced. The inconvenience and expense caused by a plugged tube are highly aggravating.
In an attempt to prevent furnace tubes from plugging, it is known to install strip-like strainers at their outdoor ends. However, the prior strainers are not entirely satisfactory. A disadvantage of the prior strainers is that they are recessed into the tube ends. The space between a tube end and a strainer can become filled with leaves and the like and thereby plug the tube. Although the leaves or other material can be removed fairly easily, the task of doing so, plus restarting the furnace, is nevertheless an annoyance. Another disadvantage of the prior strainers is that they reduce the area for the flowing gasses. Consequently, there is an increased likelihood that even a partially plugged strainer will cause the furnace to shut down.
Thus, a need exists for improvements in filters for modern furnaces.