Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~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,
--2--
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
--3--
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
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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~
--6--
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.
