CA1201950A

CA1201950A - Hot air heating system

Info

Publication number: CA1201950A
Application number: CA000425059A
Authority: CA
Inventors: Robert C. Brauer
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-04-05
Filing date: 1983-03-31
Publication date: 1986-03-18
Also published as: GB2117893B; DE3312260A1; GB2117893A; GB8309142D0; DE3312260C2

Abstract

HOT AIR HEATING SYSTEM

Abstract of the Disclosure A hot air heating sytem utilizing 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 in countercurrent flow to the cold air being returned from the rooms to the furnace, thereby pre-heating the return air and cooling the exhaust combus-tion gases. The products of combustion include a con-siderable quantity of water vapor. Cooling these gases results in condensation of this water, which is then collected in a sump 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.

Description

~IOT AIR HEATING SYSTEM
Background oE the Inven-tion Most heating systems depend on convection o~
ho-t exhaust gases -to draw coZnbustion air into the furnace S and to discharge the exhaust gases -to the atmosphere.
In recent years, primarily since -the concern over energ~
shortages and the increased cost of energy, some domestic heating systems have been equipped wi-th a blower -to induce the circulation of air through the combustion chamber and to exhaust the products oE combus-tion to the atmosphere. This system gamsefficiency because the optimum ~uantity 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 sys-tem 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 450~. This is reduced in the auxiliary heat exchanger and the heat ~5 derived from this heat exchanger is then used in one manner or another.
S~nm~ of -the Invention The invention relates to an accessor~ to a hot air heatin~ system which utilizes the gases of combustion to pre-heat the return air, and more particularl~ which util-izes the condensate generated through cooling o~ the combustion gases to humidify -the circulating room air. As a separate unit it can be retrofit to an existing ~urnace 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 re-turn room air duct, by a blower so the combustion gases pass in countercurrent relation the return air, thereby pre-heating the re-turn air before it enters the furnace. As the cooling of the combustion gases results in condensa-tion of considerable quan-tities of water, the water is collected in a sump out-side the heat exchanger. The room air being circulated by the furnace passes across the body oE 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 b~,r 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 humidifcation, is free Os 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 e~changer. ~ negative pres-sure is desirable in order to avoid the possible contam ination of the room air by the products of combustion.
The heating s~stem of the invention provides substantial energy savings in that the waste gases of combustion, including the 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 o~
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 hea-t exchanger, which would preven-t leakage oE exhaus-t gases into the surrounding area in -the even-t a crack or other defect would appear in the heat exchanger. The induced draft sys-tem 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 adjus-t -the air/fuel ra-tio for optimum combustion efficiency and also to observe if that adjustmellt is maintained.
Other ohjects and advantages will appear in the course of the following description.
Description of the Dra _ ngs 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 illustra-tes a -typical hot air heating system including a furnace 1 having a ~uel 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 Exom the Eurnace through a con-duit ~I then flow through a heat exchanger 5 that ismounted within the return duc-t 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 ou-tle-t pipe 6.
The heat exchange.r S incoudes an inlet header 8 and an outlet header 9, which are connected by a plurality of tubes 10. The headers 3 and 9 have a generally tubular configuration and, as shown in Fig. 2, the inlet header 8 is preferably located at a higher level th~n the outlet header 9.
The waste gases oE combustion enter the inlet header 8 through the condui-t 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 3 and 9 and tubes 10 and heat is there-by transferred from the combustion gases, which are norm-ally at a temperature in the range of 450F to 475~F, to the xeturn air, which is generally at a temperature in the range of 60F to 65F.
The heat transfer to the return air and the result-ing cooling of the combustion gases results in the conden-sation ofconsiderable 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 12, which is connected to a U-shaped transparent water trap 13 that extends downward-ly ~rom header 9. One leg of trap 13 is connected to drain 12, while the other leg con~unicates with a sump 14, so that condensate will drain through trap 13 and into sump 1~ without permitting the exhaust gases to escape at this locati.on. The difference in con~ensate level in the two legs of t.rap 13 is an indication of the pressure differential between the return duct 3 and heat exchanger 5.
The sump 1~ is preferably formed of transparent material and the lower surface 15 is sloped so that an increasing volume of water will present an increasing :~2~

surface area to -the circula-ting room air. The proper design oE the sloped surrface 15, or a combina-tion of slopes wil] provide an appro~ima-te balance be-tw~en the need ~or humidification and the availability of the water to pro~ide 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 wier 16 which is mounted for sliding movement with respect to an over-flow opening in the side wall of the sump. The outlet16 can be controlled manually or by a humidistat -to adjust the water level in the sump, -thereby adjusting the surface area of watér available to be picked up by the circulating room air. The overflow, if any, from the h~unidifier can be connected through a suitable pipe or hose, no-t shown, to a drain.
A vacuum breaker can also be incorporated in the return air system to insure that a negative pressure di~ferential is maintained between the pressure in the return duct and the exhaust gas pressure in the heat exchanger. The vacuum breaker ma~ take the form of a flap or damper 17 connected to rod 18 which is mounted for pivoting movement in the side walls of the sump.
~lap 17 i~ spring loaded, or biased by gravity, to a closed position, and a positive pressure di~erential between the atmosphere and the pressure in return duct 3 will cause flap 17 to open to mainkain substantial atmospheric in the return duct. This insures that the pressure in duct 3 wil] be greater than the pressure in the heat exchanger, thereby preventing the possibility o~ any contamination o~ room air with the products of combustion. This als¢ 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.

9~

A damper 19 is also provided in outle-t pipe 6 and can be adjusted in position to provide the optimum air/fuel ratio. The transparent water trap 13 and trans-parent sump 14 enables the operator to visually determine the differential in liquid height in the two legs of the trap which corresponds to t:he pressure differential between duct 3 and heat exchanqer S. By adjustrnent of damper 19 the pressure di~ferential, 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 ~he return room air. In addition, the condensate from cooling the combustion gases is utilized to humidify the roo~ air, which also saves energy by making a lower thermostat setting more com~ortable to the occupants.
The humidification systems utilizes moisture condensed from the products oE combustion to humidify the circul-ating 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 o~ 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 o a defect or fracture of the heat exchanger tubes. The vacuum breaker 17 in the return air systems insures that a negative pressure differential is maintained between the air pressure in the duct and .30 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

1. In a hot air heating system, a combustion unit to burn a fuel and heat air passing therethrough, an air duct connected to said combustion unit, exhaust gas conduit means connected to said combustion unit for discharging the exhaust gases of combustion to the atmosphere, a pre-assembled heat exchange unit remov-ably connected in said air duct, said heat exchange unit including a first header, a second header, a plur-ality of tubes interconnecting said first and second headers, said heat exchange unit also including a hous-ing to contain said headers and said tubes, said second header located at a lower level than said first header, a first conduit connecting the first header to said exhaust gas conduit means, a second conduit connecting the second header to the atmosphere, condensate collec-tion means disposed at a level beneath said second header, the exhaust gases passing through said heat exchange unit to heat the air and cool the exhaust gases and condensed water from the cooled exhaust gases being collected in said second header, and flow control means for delivering the condensed water from the second header to said collection means and for preven-ting the passage of exhaust gases from said second header to said collection means.

2. The heating system of claim 1, and including a blower disposed in said second conduit for drawing said exhaust gases through said heat exchange unit.

3. The heating system of claim 1, wherein said housing is provided with an opening disposed above said condensate collection means, said opening provid-ing communication between said housing and the atmos-phere, a movable closure to close and open said open-ing, and biasing means for biasing said closure to a closed position, said biasing means being arranged to be overcome by a preset pressure differential between the atmosphere and said air duct to thereby open said closure and restore atmospheric pressure to said air duct.

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

5. The heating system of claim 1, wherein said flow control means comprises a generally U-shaped tube having a pair of generally vertical legs, one of said legs being connected to said second header and the other of said legs communicating with said collection means.

6. The system of claim 5, wherein said tube is transparent whereby the differential in level of condendsate in said legs can be observed.

7. The system of claim 1, wherein said combustion unit is a furnace for supplying heated air to a zone to be heated, and said condensate collection means includes a humidifier positioned and arranged to humidify said air.

8. The system of claim 1, wherein said air duct is disposed to return air from said zone to said furnace, and said humidifier is associated with said air duct.