Heat recovery system with combustion gas accelerator

A system is disclosed for recovering heat from high temperature combustion gases escaping from a heater such as for examples a domestic warm air furnace, boiler or the like. The system includes a first conduit leading to a chimney and a second conduit leading to a heat recovery unit which in turn is connected to a vent. A "T" connection interconnects the first and second conduits to each other and to the heater. The heat recovery unit artificially induces a draft in the second conduit which overcomes the natural draft induced in the first conduit by virtue of its connection to the chimney. An accelerator in the T connection increases the velocity of combustion gases flowing past the first conduit into the second conduit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to systems for recovering heat from high temperature 
combustion gases. 
2. Description of the Prior Art 
A system for recovering heat from high temperature combustion gases is 
disclosed in U.S. patent application Ser. No. 380,606 filed May 21, 1982 
now U.S. Pat. No. 4,497,439. In this system, combustion gases may flow 
through either a first conduit leading to a conventional chimney, or a 
second conduit leading to a heat recovery unit which in turn is connected 
to a vent. The first and second conduits are interconnected to each other 
and to the source of combustion gases by a common "T" connection, and a 
draft is artificially induced in the second conduit to overcome the 
natural chimney draft and thereby divert the flow of combustion gases 
through the heat recovery unit and out through the vent. Should the means 
for artificially inducing the draft in the second conduit become 
inoperative, the combustion gases automatically divert to the first 
conduit leading to the chimney as a result of the natural draft present 
therein. However, under extreme conditions, where for example the chimney 
is subjected to usually high winds, the natural chimney draft may become 
strong enough to overcome the artificially induced draft, with the result 
that the high temperature gases will bypass the second conduit leading to 
the heat recovery unit and continue along the first conduit to the 
chimney. This results in a loss of valuable energy. 
SUMMARY OF THE PRESENT INVENTION 
The basic objective of the present invention is to avoid the 
above-described bypass problem by providing a gas accelerator in the T 
connection between the first and second conduits. The accelerator 
increases the velocity of the combustion gases flowing past the first 
conduit into the second conduit. This increase in velocity, when combined 
with the artificially induced draft in the second conduit, is sufficient 
to overcome the natural chimney draft, even in extreme and unusual 
situations. However, if the means for inducing the artificial draft in the 
second conduit becomes inoperative, the aforesaid increase in velocity 
will be insufficient by itself to overcome the natural chimney draft, with 
the result that the gases will still divert to and be exhausted safely 
through the first conduit and the chimney. 
Preferably, the accelerator comprises a tapered third conduit located 
within the T connection, with an inlet at one end through which the 
combustion gases enter from the heater, and with a smaller outlet at the 
opposite end through which the combustion gases exit into the second 
conduit. 
Ideally, the first and second conduits have circular cross sections, and 
the third conduit consists of a hollow truncated cone. 
Preferably, the first conduit extends laterally with respect to the third 
conduit, and the outlet of the third conduit is located approximately at 
the center line of the first conduit.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENT 
Referring initially to FIG. 1, a conventional heater, such as for example a 
domestic warm air furnace, is shown at 10. The furnace may be fired by any 
conventional means, including for example an oil or gas burner 12. A first 
combustion gas conduit 14 connects to furnace to a chimney 16. A second 
combustion gas conduit 18 leads to a heat recovery unit generally 
indicated at 20. The first and second conduits 14, 18 are interconnected 
to each other and to the furnace 10 by means of a common "T" connection 
22. 
The heat recovery unit 20 includes a sump 24 containing water. The second 
conduit 18 leads to a hood 26 overlying and in communication with the 
sump. A vent 28 extends from the sump 24 to a remote outlet (not shown). 
The furnace has an outlet air duct 30 which carries heated air to the rooms 
to be heated, and a return air duct 32 through which room air is brough 
back for reheating. 
The first combustion gas conduit 14 has natural draft induced therein by 
virtue of its connection to the chimney 16. The heating recovery unit 20 
has means associated therewith for inducing a draft in the second conduit 
18. The said means includes a water pump 34 connected to the sump 24 by 
suction line 36. The pump discharge line branches into lines 38, 40. Line 
38 leads to a first nozzle assembly 42 in conduit 18, and line 40 leads to 
a heat exchanger 44 in the return air duct 32. The heat exchanger 44 is in 
turn connected in a line 46 to a second nozzle assembly 48 in the hood 26. 
The nozzle assemblies 42, 48 operate to artificially induce a draft in 
conduit 18, thereby diverting combustion gases from conduit 14 through the 
conduit 18, the hood 26 and sump 24, and finally out through the vent 28. 
The water sprayed by nozzle assemblies 42, 48 is heated by the combustion 
gases before being collected in the sump 24. As that heated water is 
thereafter circulated through the heat exchanger 44, it gives up heat to 
the air being returned through duct 32 to the furnace. By preheating the 
return air in this manner, important cost savings are realized. If the 
pump 34 becomes inoperative, the artificially induced draft in conduit 18 
will cease to exist, in which event the natural chimney draft will take 
over, causing combustion gases to be exhausted via conduit 14 to the 
chimney. 
As mentioned previously, under unusual conditions, the natural chimney 
draft may become strong enough to overcome the artificially induced draft 
in conduct 18, thereby causing the combustion gases to bypass the heat 
recovery unit 20. In order to prevent this from happening, a gas 
accelerator 50 is arranged in the T connection 22. As can be best seen in 
FIG. 2, the accelerator comprises a tapered third conduit 52, preferably 
consisting of a hollow truncated cone. The third conduit has its enlarged 
inlet end 54 arranged to receive the combustion gases from the furnace 10, 
and its smaller or restricted outlet end 56 arranged to discharge 
combustion gases into an elbow 58 forming part of the second conduit 18. 
As the combustion gases pass through the accelerator, they experience an 
increase in velocity which when combined with the artificially induced 
draft in conduit 18, is sufficient to overcome the chimney draft, even in 
the most extreme and severe conditions. However, in the event of an 
interruption of the artificially induced draft, the natural chimney draft 
will be more than enough to overcome the effects of velocity increase in 
accelerator 50, thus insuring that the combustion gases automatically 
divert to and continue to be exhausted safely through conduit 14 and 
chimney 16. 
Preferably, the outlet end 56 of the third conduit 52 is located 
approximately on the center line 60 of the first conduit 14.