GB2117893A - Hot air heating system - Google Patents

Abstract

A hot air heating system utilizes the otherwise wasted heat and moisture in the gases of combustion to pre-heat and humidify the room air. The hot flue gases discharged from the furnace are mechanically induced by a blower to pass through an auxiliary heat exchanger 5 in countercurrent flow to the cold air being returned from the rooms along duct 3 to the furnace, thereby pre- heating the return air and cooling the exhaust combustion gases. The products of combustion include a considerable quantity of water vapor. Cooling these gases results in condensation of this water, which is then collected in a sump 14 and utilized to humidify the room air. Condensation of the vapors releases their latent heat of vaporization adding considerably to the heat recovery of the system. Because the water does not contain any minerals the humidifier does not have the usual problems of mineral deposit build-up. <IMAGE>

Description

SPECIFICATION Hot air heating system Background of the Invention Most heating systems depend on convection of hot exhaust gases to draw combustion air into the furnace and to discharge the exhaust gases to the atmosphere. In recent years, primarily since the concern over energy shortages and the increased cost of energy, some domestic heating systems have been equipped with a blower to induce the circulation of air through the combustion chamber and to exhaust the products of combustion to the atmosphere. This system gains efficiency because the optimum quantity of combustion air, technically termed the stoichiometric air/fuel ratio, can be more readily controlled and also because it avoids the heat loss that occurs with a natural convection system which tends to draw warm air out of the furnace room even when the system is not operating.

More advanced versions of the force draft system reclaim some of the heat in the exhaust gases through an auxiliary heat exchanger of some type, as shown in United States Patents 3,813,039, 3,934,798 and 4,241,874. The exhaust gases generally leave the combustion chamber at temperatures in the range of approximately 4500 F. This is reduced in the auxiliary heat exchanger and the heat derived from this heat exchanger is then used in one manner or another.

Summary of the Invention The invention relates to an accessory to a hot air heating system which utilizes the gases of combustion to pre-heat the return air, and more particularly which utilizes the condensate generated through cooling of the combustion gases to humidify the circulating room air. As a separate unit it can be retrofit to an existing furnace or installed as an auxiliary unit with a new furnace.

With the invention, the hot gases of combustion from the furnace are drawn through a heat exchanger, which is located in the return room air duct, by a blower so the combustion gases pass in countercurrent relation the return air, thereby pre-heating the return air before it enters the furnace. As the cooling of the combustion gases results in condensation of considerable quantities of water, the water is collected in a sump outside the heat exchanger.

The room air being circulated by the furnace passes across the body of condensate in the sump so that the return air will be humidified. In an alternate method, the condensate may be conducted to a sump in the warm air duct where it will be picked up by the heated circulating air.

The humidifying system has a further advantage over conventional humidification systems which rely on tap water for humidification, in that the condensate, which is used for humidification, is free of dissolved salts or minerals, so there will be no precipitation of salts or minerals as the water is evaporated by the circulation of air.

The water is removed from the heat exchanger through a water trap so that the exhaust gases do not contaminate the circulating room air. The water trap is transparent plastic or glass so it can be observed if the correct pressure (positive or negative) is being maintained within the heat exchanger. A negative pressure is desirable in order to avoid the possible contamination of the room air by the products of combustion.

The heating system of the invention provides substantial energy savings in that the waste gases of combustion, including t,he latent heat of vaporization of the water vapor container therein, are utilized to pre-heat the return air. In addition, the condensate resulting from cooling of the combustion gases is employed to humidify the return air being returned to the furnace.

With the system of the invention, the gases of combustion are forced, or preferably drawn, through the heat exchanger by a blower. In the preferred version the induced draft results in negative pressure in the heat exchanger, which would prevent leakage of exhaust gases into the surrounding area in the event a crack or other defect would appear in the heat exchanger. The induced draft system also eliminates the need of a chimney as the waste gases of combustion can be directly discharged to the atmosphere by operation of the blower, because the convection otherwise created by a chimney is produced by the blower.

The use of the transparent water trap along with the blower makes it possible to adjust the air/fuel ratio for optimum combustion efficiency and also to observe if that adjustment is maintained.

Other objects and advantages will appear in the course of the foilowing description.

Description of the Drawings The drawings illustrate the best mode presently contemplated of carrying out the invention.

In the drawings: Fig. 1 is a side elevational view of a hot air heating system incorporating the invention; Fig. 2 is a vertical section showing the heat exchanger in the return air duct; and Fig. 3 is a section taken along line 3-3 of Fig. 2.

Description of the Illustrated Embodiment Fig. 1 illustrates a typical hot air heating system including a furnace 1 having a fuel burning unit which acts to heat air flowing through the heating plenum in the furnace. A blower, not shown, discharges air from the furnace through a supply duct 2 to the rooms or other area to be heated and the cool air is returned to the furnace through a return duct 3.

In accordance with the invention, the hot waste gases of combustion pass from the furnace through a conduit 4, then flow through a heat exchanger 5 that is mounted within the return duct 3 and are discharged through a pipe 6 to the atmosphere. Blower 7 is mounted in the pipe 6 and provides a forced draft to draw air into the combustion chamber and the waste gases out through the heat exchanger 5 and outlet pipe 6.

The heat exchanger 5 includes an inlet header 8 and an outlet header 9, which are connected by a plurality of tubes 1 0. The headers 8 and 9 have a generally tubular configuration and, as shown in Fig. 2, the inlet header 8 is preferably located at a higher level than the outlet header 9.

The waste gases of combustion enter the inlet header 8 through the conduit 4 and then pass through the tubes 10 and are discharged from the header 9 through conduit 11 to outlet pipe 6. The cool return air flowing through return duct 3 passes in heat transfer relation across the headers 8 and 9 and tubes 10 and heat is thereby transferred from the combustion gases, which are normally at a temperature in the range of 4500F to 4750F, to the return air, which is generally at a temperature in the range of 600F to 650F.

The heat transfer to the return air and the resulting cooling of the combustion gases results in the condensation of considerable quantities of water and the condensate flows downwardly through tubes 10 to the header 9.

To collect the condensate, the lower end of header 9 is provided with a drain 1 2, which is connected to a U-shaped transparent water trap 1 3 that extends downwardly from header 9. One leg of trap 13 is connected to drain 12, while the other leg communicates with a sump 14, so that condensate will drain through trap 13 and into sump 14 without permitting the exhaust gases to escape at this location. The difference in condensate level in the two legs of trap 1 3 is an indication of the pressure differential between the return duct 3 and heat exchanger 5.

The sump 14 is preferably formed of transparent material and the lower surface 1 5 is sloped so that an increasing volume of water will present an increasing surface area to the circulating room air. The proper design of the sloped surface 15, or a combination of slopes will provide an approximate balance between the need for humidification and the availability of the water to provide the humidification.

if closer control of humidification is desired, the level of water in the sump 14 can be controlled through use of an adjustable outlet or weir 16 which is mounted for sliding movement with respect to an overflow opening in the side wall of the sump. The outlet 1 6 can be controlled manually or by a humidistat to adjust the water level in the sump, thereby adjusting the surface area of water available to be picked up by the circulating room air. The overflow, if any, from the humidifier can be connected through a suitable pipe or hose, not shown, to a drain.

A vacuum breaker can also be incorporated in the return air system to insure that a negative pressure differential is maintained between the pressure in the return duct and the exhaust gas pressure in the heat exchanger. The vacuum breaker may take the form of a flap or damper 17 connected to rod 1 8 which is mounted for pivoting movement in the side walls of the sump. Flap 17 is spring loaded, or biased by gravity, to a closed position, and a positive pressure differential between the atmosphere and the pressure in return duct 3 will cause flap 17 to open to maintain substantial atmospheric in the return duct. This insures that the pressure in duct 3 will be greater than the pressure in the heat exchanger, thereby preventing the possibility of any contamination of room air with the products of combustion.This also insures adequate supply of circulating air to the room air blower, even if the cold air returns are inadequate, or are blocked by furniture, carpeting, or other items.

A damper 1 9 is also provided in outlet pipe 6 and can be adjusted in position to provide the optimum air/fuel ratio. The transparent water trap 13 and transparent sump 14 enables the operator to visually determine the differential in liquid height in the two legs of the trap which corresponds to the pressure differential between duct 3 and heat exchanger 5. By adjustment of damper 1 9 the pressure differential. as seen in trap 13, can be controlled to obtain the desired air/fuel ratio for optimum efficiency of the system.

The invention provides a substantial energy saving by using the heated combustion gases to pre-heat the return room air. In addition, the condensate from cooling the combustion gases is utilized to humidify the room air, which also saves energy by making a lower thermostat setting more comfortable to the occupants. The humidification systems utilizes moisture condensed from the products of combustion to humidify the circulating room air with water free of minerals that would tend to cause problems in conventional humidifiers.

As the combustion gases are drawn through the heat exchanger by blower 7, which is located on the downstream side of the heat exchanger 5, a negative pressure results in the heat exchanger which prevents leakage of the combustion gases into the return air stream in the event of a defect or fracture of the heat exchanger tubes. The vacuum breaker 1 7 in the return air systems insures that a negative pressure differential is maintained between the air pressure in the duct and the exhaust gas pressure inside the heat exchanger.

The construction of the invention is simple and economical to manufacture and maintain, and is readily adapted to existing heating systems, as well as new installations, because it is incorporated in the return air duct rather than into the furnace itself.

Claims (17)

1. In a hot air heating system, a furnace to burn a fuel and heat air passing through the furnace, a duct system connected to the furnace for distributing heated air from the furnace to a zone to be heated and returning cool air from said zone to the furnace, exhaust gas conduit means connected to the furnace for discharging the exhaust gases of combustion to the atmosphere, heat exchange means interconnecting the duct system with said conduit means and disposed to transfer heat from said exhaust gases to the return air in said duct system to thereby heat the return air and cool the combustion gases, condensate collecting means associated with the heat exchanger for collecting condensed water from the cooled combustion gases, said collecting means having an outlet, humidifying means in the duct system and communicating with said outlet, and means associated with the outlet for permitting the flow of condensate through the outlet to the humidifying means but preventing flow of said combustion gases through said outlet to said humidifying means, the air flowing through said duct system passing across said humidifying means to thereby humidify the air.

2. The heating system of claim 1, wherein the duct system includes a return duct for returning cool air from said zone, said heat exchange means is disposed within the return duct.

3. The heating system of claim 2, wherein said heat exchange means includes an inlet header connected to said conduit means, and outlet header, and a plurality of tubes connecting said inlet header and said outlet header, a first of said headers being at a higher level than the second of said headers, whereby said condensate will flow to said second header, said collecting means being connected to said second header.

4. The heating system of claim 1, and including a blower in said conduit means and located downstream of said heat exchange means, said blower acting to create a negative pressure in the heat exchanger and draw the exhaust gases through said heat exchange means.

5. The heating system of claim 2, wherein said humidifying means comprises a reservoir disposed in the return duct, the condensate being discharged through said outlet and being collected in said reservoir.

6. A hot air heating system, comprising a furnace to burn a fuel and heat air passing through the furnace, a supply duct connected to the furnace for supplying heated air to a zone to be heated, a return duct for returning cool air from said zone to the furnace, a flue gas conduit connected to the furnace for discharging hot flue gases, a heat exchanger disposed within the return duct and having an inlet connected to said flue gas conduit, said heat exchanger having an outlet, a discharge conduit connecting the outlet of the heat exchanger to the atmosphere, a blower located in the discharge conduit for drawing the flue gases through the heat exchanger and discharging the flue gases to the atmosphere, heat from the flue gases being transferred to the return air in said return duct to thereby pre-heat the return air and cool said flue gases, a sump located below the heat exchanger for collecting water condensed from the cooled flue gases, said heat exchanger having an outlet in the lower end thereof, said sump disclosed in the return duct beneath said outlet, and flow control means for permitting the flow of condensate through said outlet to said sump and for preventing the flue gases from passing through said outlet to said sump, said return air passing over the water in said sump to thereby humidify the return air.

7. The heat system of claim 6, wherein said heat exchanger has a high end and a low end, and said sump is located at the low end of the heat exchanger.

8. The heating system of claim 6, wherein said sump is provided with a sloping bottom wall whereby a variation in the depth of water in said reservoir will result in a corresponding change in surface area of said water.

9. The heating system of claim 8, wherein said sump includes a side wall having a discharge overflow opening, and said system includes an adjustable member mounted for movement with respect to said overflow opening to regulate the level of condensed water collected in said reservoir.

10. The heating system of claim 6, wherein said heat exchanger includes an inlet header connected to said flue gas conduit and an outlet header connected to said outlet conduit, and a plurality of tubes connecting said inlet header and said outlet header, a first of said headers being at a higher level than the second of said headers, whereby said condensate will flow to said second header, said sump communicating with said second header.

1 The heating system of claim 10, wherein the flue gases in said heat exchanger flow in counter-current relation to the flow of return air in said return duct.

12. The system of claim 6, wherein said flow control means comprises a generally U-shaped tubular member having a pair of vertically extending legs, one of said legs connected to said outlet and the other of said legs communicating with said sump.

13. The system of claim 12, wherein said tubular member is transparent whereby the differential in level of condensate in said legs can be observed.

14. The system of claim 6, and including damper means disposed in said discharge conduit for controlling the pressure in said heat exchanger.

1 5. The system of claim 2, wherein said return duct includes an aperture, a movable closure to close and open said aperture, and biasing means for biasing said closure to a closed position, said biasing means being arranged to be overcome by a pressure differential between the atmosphere and the return duct to thereby open said closure and maintain atmospheric pressure in said return duct.

1 6. A heating system, comprising a heating unit to burn a fuel, a flue gas conduit connected to the heating unit for discharging hot flue gases from said heating unit, an air supply duct connected to said heating unit for supplying air to said heating unit, a heat exchanger unit removably connected in said air supply duct and having an inlet connected to said flue gas conduit, said heat exchanger having an outlet, a discharge conduit connecting the outlet of the heat exchanger to the atmosphere, a blower located in the discharge conduit for drawing the flue gases through the heat exchanger unit and discharging the flue gases to the atmosphere, heat from the flue gases being transferred to air flowing in said air supply duct to said heating unit to thereby pre-heat said air, said air supply duct having an opening in the lower surface thereof, said heat exchange unit being secured within said opening and including a plurality of heat exchange tubes through which said flue gases flow and located at least partially within said duct, said heat exchange unit also including a sump to collect condensate and located beneath the level of said duct.

17. The system of claim 16, wherein said heat exchange unit also includes an aperture disposed beneath the level of said duct and above said sump, a movable closure to close and open said aperture, and biasing means for biasing said closure to a closed position, said biasing means being arranged to be overcome by a pressure differential between the atmosphere and said duct to thereby open said closure and maintain atmospheric pressure in said duct.