The aesthetic value of open fireplaces is such that their inefficient heating abilities are endured even to the point of reducing overall fuel efficiency for the buildings in which they are employed. The reasons for the latter are well known. The fireplace, itself, is an inefficient heat source because most of the heat of combustion escapes up the chimney and the strong draft thereby created exhausts warm air from the building thus lowering overall building temperature outside the immediate fireplace area. This, in turn, calls upon the central heating system to stabilize the heat loss.
Convective heating systems have long been employed in conjunction with conventional fireplace structures as a means of recovering a portion of that heat normally lost to chimney draft and replacing, with recuperatively heated air, at least a portion of the withdrawn room air. Convective heating systems conventionally employ a fuel burning stove or firebox positioned within a fireplace enclosure in spaced relation to the back, sides, and/or bottom walls of the enclosure. The firebox is vented to a chimney or stack and sealed with respect to the space between the firebox and fireplace enclosure. As fuel is burned in the firebox limited room air is withdrawn, to support combustion, through a firebox inlet grate and the products of combustion are exhausted to the chimney or stack. When the firebox walls become heated a convective air flow is established in the space between the firebox and fireplace enclosure withdrawing relatively cool room air from adjacent the floor which is heated as it passes inwardly and upwardly within the fireplace enclosure prior to its reintroduction into the room from the upper portion of the enclosure. In addition to providing room heat by radiation, the firebox is the heat source to establish and heat a convective flow of room air. Firebox improvements since the early "Latrobe" system (U.S. Pat. No. 4,744) have included an elongate, glass fronted construction whose generally trapezoidal shape in horizontal section approximates that of a fireplace enclosure for purposes of improving convective flow and retaining the aesthetic appearance of a conventional fireplace; improved combustion air controls; and specially configured outer wall constructions for improved heat exchange with the convective flow path as exemplified by U.S. Pat. Nos. 4,026,264; 4,026,263 and 4,015,581, respectively. The use of glass doors on the front wall of the firebox constituted a major design improvement which is now the accepted mode of construction in that such doors ameliorate draft induction of room air while retaining the aesthetic value of an "open fire".
Aside from such basic firebox improvements the general trend in convective heating systems has been in the direction of improving recuperative efficiency with respect to a given heat source. Exemplary are improved heat exchange techniques in the form of fins and/or flow path directors and methods for increasing convective mass flow such as by the use of blowers and the like.
The problem has been attacked from the wrong end. The limitations inherent in the heat source have been accepted as more or less given parameters to be tolerated or ignored. Stated differently, for a given quantity of the same fuel and external factors being equal, the available BTU's for convective heat exchange does not vary significantly among the various systems that have been in use for years. The key to dramatic increases in overall unit efficiency lies with the heat source (firebox) itself which, historically, has been one of the most inefficient heating units ever designed.
In the ensuing discussion explanatory of the foregoing it must be borne in mind that the concern herein is for elongate fireboxes retaining the visual aesthetics of an open fireplace since many of their inherent limitations derive from this general configuration.
In considering, for purposes of discussion, a conventional elongate wood burning firebox having a generally centralized flue and supplied by drafted combustion air below a glass fronted wall; the hottest portion of the fire is centrally of the firebox. Indeed, in many instances the outer ends of the fuel logs either do not burn at all and must later be stoked to the center or only become consumed after a substantial bed of glowing embers is established. In such conventional firebox there is a central zone which maintains ignition temperatures while areas transverse of the central zone remain below ignition temperature. The reasons are twofold. The net mass flow of combustion products is upward to a central flue creating a centrally flowing draft (i.e. away from the outer ends of the fuel logs) which, in turn, creates a centrally directed flow of incoming combustion air to the center of the firebox even though combustion air inlets may extend completely across the front of a closed firebox. Once the overall central flow of combustion products and incoming combustion air is visualized then the inherent creation of a central ignition zone is readily understandable on the basis of general thermal theory that upon attainment of flame supported ignition temperature (i.e. that temperature at which the local rate of heat generation is sufficient to propogate the flame throughout the combustible) the same will maintain until fuel exhaustion or quenching occurs. Since quenching, or localized quenching as applied to the present discussion, occurs because of:
(1) a rate of heat loss such as to cause local chilling below ignition temperature; or PA1 (2) insufficient oxygen to support combustion,
it will be seen how the central flow of combustion products and incoming combustion air contribute individually, and collectively, to localized chilling and decreased oxygen partial pressures transversely of the central flow zone which quenching effect increases directly as a function of net mass flow velocity. The result, following initial ignition by highly combustible materials, is transverse quenching dilimiting the central ignition zone. If, as is the usual case, initial ignition is effected centrally of the firebox, the remote ends of the box tend to remain well below ignition temperature.
Expressed differently, a conventional firebox whose central ignition zone is bounded on either side by sub-ignition temperature zones exhibits large temperature gradients transversely of the firebox which peak centrally and drop off rapidly, below ignition temperature, toward both ends of the firebox. The effect is readily visible from the greater amount of smoke emanating from the ends of the logs and the greater soot and resin depositions adjacent the ends of the firebox.
The value of vertical temperature gradients vary greatly depending upon their position within the firebox as would be expected from the above discussion of central draft to flue. Considering a central portion of the firebox, the temperature drops somewhat from the point where oxidation of the combustible gases take place to the flue entrance but this central, vertical gradient becomes quite small (lying wholly within the ignition temperature range) as the firebox interior is further heated by radiation. Similarly, vertical gradients adjacent remote ends of the firebox are quite small (lying within the sub-ignition temperature range). Looking, however, to those diagonal temperature gradients extending from outside the central area of the firebox, upwardly toward the flue entrance (the direction of induced draft); the value of such diagonal gradients are quite large (extending from sub-ignition to ignition temperature ranges). Similarly, those vertical temperature gradients intermediate the central and remote portions of the firebox, i.e. lying just outside the axis of flue exhaust, exhibit a large value as they extend vertically from an ignition temperature range adjacent the burning fuel source upwardly to a sub-ignition temperature range transverse of central flue exhaust. The minimal value of the central vertical gradient as contrasted with the larger vertical gradient transversely thereof is, of course, an indication of the large amount of heat being lost up the flue.
Since the fuel source is positioned rearwardly of the firebox to avoid overheating the glass front it will be seen that the aforedescribed temperature gradients define a generally frustoconically shaped combustion zone of relatively high (ignition) temperature as contrasted with the lower temperature zones bounding either side and the front thereof. Accordingly it is the central portions of the top and backwalls of the firebox which provide the most effective heat exchange for the convective flow path with the remainder of the firebox walls available for heat exchange being at a substantially lesser temperature.
Although the aforedescribed central drafting effect of a central flue can be somewhat ameliorated and the generally conical combustion zone somewhat elongated at the truncated end thereof by the use of an elongated flue of the type shown in U.S. Pat. No. 4,026,264; the small advantage is more than offset by the fact that down drafts from such a flue whirl the flames transversely and forwardly overheating and sooting the doors. Additionally, thermal expansion and contraction of such an elongated flue inevitably breaks its seal to the connected flue or chimney, thus allowing loss of convected room air up the chimney.
The primary purpose of the invention is to substantially reduce both the horizontal and vertical temperature gradients within the firebox to the extent that the aforedescribed combustion zone, fueled with a like charge, is expanded to encompass a generally rectangular volume approximating that of the firebox. This is effected by precluding the direct escape of hot rising flue gases and momentarily trapping the same to lie, in effect, as a hot air blanket in heat exchange relation over the entire lower surface of the upper firebox wall prior to continuing displacement of the same to flue by subsequently rising, hotter flue gases. The substantial elimination of direct flue escape produces a concomitant decrease in the centralizing components of the combustion air draft permitting combustion air to be introduced equally to the fuel across the length of the firebox. The latter, taken with that radiant heat downwardly directed from the overlying hot air blanket, maintains ignition temperatures at extreme ends of the firebox the rising flue gases from which join and supplement the hot air blanket. The result is a generally rectangular combustion zone maintained at ignition temperatures throughout substantially the entire firebox except immediately adjacent the glass doors. The effect is augmented and efficiency is further increased by preheating the combustion air prior to its entry into the firebox via a preheat manifold construction which not only provides a measure of air shielding for the glass doors but limits the forwardmost extent of fuel placement to prevent overheating of the glass.
The increase in both radiant and convective heating efficiency is dramatic. The most obvious advantage is that substantially the entire surface area of each of the back, top, bottom and side walls is now maintained at a much higher temperature than was previously possible thereby greatly increasing convective heat exchange efficiency without the expense of heat exchange assistants such as fins, convoluted flow paths and the like. An ancillary advantage supplementing the foregoing and desirous in and of itself is the virtually complete combustion effected within the firebox as a consequence of the greatly increased path length along which the combustion products must traverse the combustion zone prior to exiting the flue. This is evidenced by the virtual elimination of both soot within the firebox and resinous buildup in the chimney. Immediate visual recognition, during burning, is had by virture of the fact that fire logs burn evenly from end to end in a virtually smoke free environment immediately following full ignition.
Although, as previously indicated, the use of glass doors on units of the type herein proposed has become fairly standard in the industry the problem of glass breakage due to uneven heating is still prevalent. Major contributing factors are continuing localized cooling adjacent the lower edge of the glass by incoming combustion air and momentary, intense localized heating due to flash fires. An additional advantage in preheating the combustion air prior to firebox entry is that it reduces localized glass cooling. A combination of air shielding and baffles alleviate flash fire effects on the doors.
The firebox construction herein described is adapted for use with convective heating systems employing a free standing, or fabricated, fireplace unit as well as conventional firebrick enclosure. When used with a free standing unit over a combustible floor surface, the unusually intense heat radiated from the firebox necessitates special safety precautions exceeding those required for previous units and takes the form of air cooling to supplement the usual metal and insulative shielding.
Another purpose of the invention as applied to free standing units is to utilize convected room air to effect such cooling and then utilize the air thus heated for separate space heating or for reintroduction into the room heated by the conventionally convected air flow.
Secondary heat recovery is frequently effected by directing the convective flow in heat exchange relation with the flue pipe to extract further heat destined for loss to atmosphere. It is a further object of the invention to enhance the efficiency of this exchange by increasing both the sensible heat available for exchange and the surface area for effecting the same. This is accomplished by creating an upper heated air trap, within the flue, analogous to the aforedescribed entrapment of heated air within the firebox.