U.S. patent number RE33,082 [Application Number 06/775,661] was granted by the patent office on 1989-10-10 for combustion product condensing water heater.
This patent grant is currently assigned to Advanced Mechanical Technology, Inc.. Invention is credited to Joseph Gerstmann, Andrew D. Vasilakis.
United States Patent |
RE33,082 |
Gerstmann , et al. |
October 10, 1989 |
Combustion product condensing water heater
Abstract
In a water heating system, vapor in the products of combustion
gases is condensed in a secondary heat exchanger positioned in a
housing with the primary heat exchanger and combustion chamber. The
two heat exchangers are coaxial coils with the secondary coil
positioned below the primary. Gases flow radially through the
primary coil and then axially through the secondary coil at an
increased velocity. The gases are then used to pre-heat a gas/air
mixture in a third heat exchanger within the secondary heat
exchanger. The pre-heated gas/air mixture is burned in a burner
within the primary heat exchanger and the gas products are drawn
through the exchangers by a blower. A water storage tank is
designed to enhance stratification of hot water over cooler water.
The cooler water is used to condense vapor in the secondary heat
exchanger.
Inventors: |
Gerstmann; Joseph (Framingham,
MA), Vasilakis; Andrew D. (Bedford, MA) |
Assignee: |
Advanced Mechanical Technology,
Inc. (Newton, MA)
|
Family
ID: |
25105091 |
Appl.
No.: |
06/775,661 |
Filed: |
September 13, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
150289 |
May 16, 1980 |
04403572 |
Sep 13, 1983 |
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Current U.S.
Class: |
122/20B; 165/125;
122/33 |
Current CPC
Class: |
F24D
11/005 (20130101); F24H 1/43 (20130101); F24H
8/00 (20130101); F24D 11/004 (20130101); Y02B
30/102 (20130101); Y02B 30/00 (20130101) |
Current International
Class: |
F22B
33/00 (20060101); F22B 033/00 () |
Field of
Search: |
;122/2B,17,32,33
;237/8R,56,19 ;165/125,401,909 ;126/427 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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30162 |
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Jan 1885 |
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DE2 |
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1907987 |
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Sep 1970 |
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DE |
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3041265 |
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Sep 1982 |
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DE |
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Other References
Smay, V., High Efficiency Home Heating, Popular Science, Nov. 1979,
pp. 60, 62, 158, 162, 164. .
Boston Gas Products, "Heat Maker Brochure", 1980. .
Hydro Therm, "Hydro Pulse Brochure", A1-379. .
Water Supply Boiler/Heaters and Package Systems, Raypak, Catalog
No. 3000-d, pp. 2-7, 8/1/78. .
The Laars Type AF Heavy Duty Commercial Pool Heater, Laars
Engineers, Catalog 3.1.001-4, 6/1/70..
|
Primary Examiner: Bennett; Henry A.
Attorney, Agent or Firm: Hamilton, Brook, Smith &
Reynolds
Claims
We claim:
1. A method of heating water and storing the hot water in an
insulated storage vessel comprising:
permitting natural stratification of the water within the vessel
such that cooler water collects at the bottom of the vessel;
withdrawing cooler water from the bottom of the vessel and passing
that water in heat exchange relationship with combustion gases from
a burner assembly to cool those gases below their dew point
temperature so as to extract some of the latent heat of
vaporization from the gases and to heat the water;
returning the thus heated water to the storage vessel at a location
in the storage vessel above the cooler water and in a manner as to
avoid mixing with the cooler water, the volume of the cooler water
below the location at which hot water is returned to the vessel
being sufficiently large to provide for condensation of the
combustion gases through substantially the entire heating cycle of
the burner.
2. A water heating system comprising:
a water-walled combustion chamber defined by a primary heat
exchanger, the products of combustion gas flow path past the
primary heat exchanger providing for a first gas velocity through
the primary heat exchanger;
a secondary heat exchanger for receiving gases cooled by the
primary heat exchanger, the secondary heat exchanger providing for
a second gas velocity substantially greater than the first;
a water storage assembly including a tank having a baffle to
separate a volume of hot water from a smaller volume of cooler
water at the bottom of the tank in communication with the hot water
and a diffuser positioned over a cold water inlet into the bottom
of the tank;
means for circulating the cooler water through the secondary heat
exchanger in counterflow heat exchange relationship with the
combustion gases to cool the gases below their dew point
temperature and condense water vapor from the gases; and
means for directing the water from the secondary heat exchanger
through the primary heat exchanger to cool the combustion gases to
a temperature which is above the dew point temperature.
3. A water heating assembly comprising:
a heat exchanger and combustion chamber housing;
a water-walled combustion chamber within the housing surrounded by
a primary heat exchanger, the products of combustion gas flow path
past the primary heat exchanger providing a first gas velocity
through the primary heat exchanger;
a secondary heat exchanger within the housing for receiving gases
cooled by the primary heat exchanger providing for a second
combustion gas velocity substantially greater than the first;
a cool water inlet;
means for circulating cool water from the cool water inlet through
the secondary heat exchanger at a first flow rate in heat exchanger
relationship with the combustion gases to cool the gases below
their dew point temperature and condense water vapor from gases;
and
a water flow path from the output of the primary heat exchanger to
its input for recirculating water in the primary heat exchanger,
said water flow path and the primary heat exchanger forming a
primary heat exchanger loop;
means for mixing water from the secondary heat exchanger with a
portion of the water from the output of the primary heat exchanger
and a pump in said primary heat exchanger loop for pumping the
water mixture through the primary heat exchanger at a flow rate
greater than first flow rate to cool the combustion gases to a
temperature which is above their dew point temperature.
4. A water heating system comprising:
a water-walled combustion chamber surrounded by a primary heat
exchanger, the product of combustion gas flow path past the primary
heat exchanger providing for a first gas velocity through the
primary heat exchanger;
a secondary heat exchanger for receiving gases cooled the primary
heat exchanger, the secondary heat exchanger providing for a second
gas velocity substantially greater than the first;
water storage assembly including a volume of hot water and a
smaller volume of cooler water in communication with the hot
water;
means for circulating the cooler water from said smaller volume of
cooler water through the secondary heat exchanger in heat exchange
relationship with the combustion gases to cool the gases below
their dew point temperature and condense water vapor from the
gases;
means for directing the water from the secondary heat exchanger
through the primary heat exchanger to cool the combustion gases to
a temperature which is above the dew point temperature; and
means for directing heated water from the primary heat exchanger to
said volume of hot water in such a manner as to avoid mixing with
the smaller volume of cooler water.
5. In a liquid heater comprising an insulated storage vessel and a
fuel-fired heat exchanger assembly in which liquid is circulated
from the storage vessel to the heat exchanger assembly to be
heated, and then returned to the storage vessel, the liquid heater
characterized in that:
the heat exchanger assembly comprises a primary heat exchanger
consisting of cylindrical finned tubing coils surrounding a central
burner, the flow of combustion products from said burner being
essentially radially outward through the surrounding coil at a
first velocity, the partially cooled combustion products being
directed to flow in an axial direction at a second velocity
substantially greater than the first over a secondary heat
exchanger coil in which said gases are cooled below their dew point
temperature so as to extract some of the latent heat in said flue
gases;
liquid is withdrawn from the bottom of said storage vessel to be
circulated first through said secondary heat exchanger at a first
flow rate in heat exchange relationship with products of
combustion, then mixed with a portion of the liquid issuing from
the outlet of said primary heat exchanger, and then caused, by
action of a circulating pump placed in a primary heat exchanger
loop, to flow through said primary heat exchanger at a flow rate
greater than said first rate; and
the thus heated liquid is returned to an upper section of the
storage vessel in such a manner as to avoid mixing with the liquid
stored in the bottom of the storage vessel.
6. A water heating assembly as claimed in claim 3 wherein:
the primary heat exchanger is a coil of tubing surrounding the
combustion chamber and combustion gases flow radially over that
tubing; and
the secondary heat exchanger is a coil of tubing and gases from the
primary heat exchanger flow axially over the secondary coil.
7. A water heating assembly as claimed in claim 6 wherein the
primary and secondary coils are coaxial.
8. A water heating assembly as claimed in claim 7 further
comprising a tertiary heat exchanger located concentrically within
the secondary heat exchanger wherein an unburned fuel/air mixture
is pre-heated by the combustion gases from the secondary heat
exchanger.
9. A water heating assembly as claimed in claim 3 or 4 further
comprising means for pre-mixing fuel gas and an amount of air
sufficient for complete combustion of the gas.
10. A water heating assembly as claimed in claim 9 wherein the flow
of the gas/air mixture into the burner is induced by a blower
positioned downstream from the secondary heat exchanger.
11. A water heating system as claimed in claim 4 wherein:
the primary heat exchanger is a coil of tubing surrounding the
combustion chamber and combustion gases flow radially over that
tubing; and
the secondary heat exchanger is a coil of tubing and gases from the
primary heat exchanger flow axially over the secondary coil.
12. A water heating system as claimed in claim .[.1.]. .Iadd.4
.Iaddend.wherein the primary and secondary coils are coaxial.
13. A water heating system as claimed in claim 12 further
comprising a tertiary heat exchanger located concentrically within
the secondary heat exchanger wherein an unburned fuel/air mixture
is pre-heated by the combustion gases from the secondary heat
exchanger.
14. A water heating system as claimed in claim 4 or 13 further
comprising means for pre-mixing fuel gas and an amount of air
sufficient for complete combustion of the gas.
15. A water heating system as claimed in claim 14 wherein the flow
of the gas/air mixture into the burner is induced by a blower
positioned downstream from the secondary heat exchanger.
16. A water heating system as claimed in claim 4 wherein the water
storage assembly is a tank separated into hotter water and cooler
water volumes by a baffle.
17. A water heating system as claimed in claim 16 further
comprising a diffuser positioned over a cold water inlet into the
bottom of the tank.
18. A water heating system as claimed in claim 4 further comprising
a water flow path from the output of the primary heat exchanger to
its input for recirculating water in the primary heat exchanger,
said water flow path and the primary heat exchanger forming a
primary heat exchanger loop, and means, including a pump in said
primary heat exchanger loop, for recirculating hot water through
the primary heat exchanger mixed with water from the secondary heat
exchanger to raise the temperature of mixed water to a temperature
above the dew point temperature of the combustion gases.
19. A liquid heater according to claim 5 in which partially cooled
products of combustion are directed from said secondary heat
exchanger to flow axially through a tertiary heat exchanger located
concentrically within said secondary heat exchanger, product of
combustion in said tertiary heat exchanger being in heat transfer
communication with the unburned fuel/air mixture flowing towards
said fuel burner. .Iadd.
20. A method as claimed in claim 1 further comprising returning the
water to the storage tank at a flow rate and temperature controlled
by a thermostatic valve. .Iaddend. .Iadd.21. A method as claimed in
claim 1 further comprising varying the input to the burner to
maintain a steady
temperature of water returned to the storage vessel. .Iaddend.
.Iadd.22. A liquid heating assembly comprising:
a combustion chamber;
a primary heat exchanger in the gas flow path of products of
combustion from the combustion chamber;
a secondary heat exchanger for receiving gases cooled by the
primary heat exchanger;
a cool liquid inlet to the secondary heat exchanger;
means for circulating cool liquid from the cool liquid inlet
through the secondary heat exchanger at a first flow rate in heat
exchange relationship with the combustion gases;
a liquid flow path from the output of the primary heat exchanger to
its input for recirculating liquid in the primary heat exchanger,
said liquid flow path and the primary heat exchanger forming a
primary heat exchanger loop;
a pump in the primary heat exchanger loop for pumping liquid from
the secondary heat exchanger and a portion of the liquid from the
output of the primary heat exchanger through the primary heat
exchanger at a flow rate greater than said first flow rate to cool
the combustion gases to a temperature which is above their dew
point temperature. .Iaddend.
.Iadd. A liquid heat assembly as claimed in claim 22 further
comprising a thermostatic valve for controlling the flow rate
through the heat exchangers. .Iaddend. .Iadd.24. An apparatus as
claimed in claim 22 wherein the gas flow path past the primary heat
exchanger provides a first gas velocity through the primary heat
exchanger and the combustion gases then flow past the secondary
heat exchanger at a second gas velocity substantially greater than
the first. .Iaddend. .Iadd.25. An apparatus as claimed in claim 24
further comprising a thermostatic valve to control the flow rate of
liquid withdrawn from and returned to the storage vessel. .Iaddend.
.Iadd.26. An apparatus as claimed in claim 24 wherein:
the primary heat exchanger surrounds the combustion chamber and
combustion gases flow radially through that heat exchanger; and
a secondary heat exchanger in which gases from the primary heat
exchanger flow in an axial direction sequentially past multiple
liquid flow paths of the secondary heat exchanger. .Iaddend.
.Iadd.27. A liquid heating assembly as claimed in claim 22 further
comprising a liquid storage tank having a naturally stratified
upper zone of heated liquid and a lower zone of cooler liquid, the
volume of the lower zone of cooler liquid and the flow rate through
the primary and secondary heat exchangers being such that the
liquid provides for condensation of the combustion gases in the
secondary heat exchanger through substantially the entire heating
cycle of
the burner. .Iaddend. .Iadd.28. An apparatus as claimed in claim 22
wherein:
the primary heat exchanger is tubing surrounding the combustion
chamber and combustion gases flow radially over that tubing;
and
a secondary heat exchanger is a coil of tubing and gases from the
primary
heat exchanger flow axially over the secondary coil. .Iaddend.
.Iadd.29. A method of heating liquid comprising:
in a secondary heat exchanger passing liquid in heat exchange
relationship with combustion gases that have been cooled by a
primary heat exchanger to further cool those gases below their dew
point temperature so as to extract some of the latent heat of
vaporization from the gases and to heat the liquid;
passing the liquid heated in the secondary heat exchanger into a
primary heat exchanger in heat exchange relationship with gases
from the burner assembly to cool those gases to a temperature above
their dew point temperature to further heat the liquid but avoid
condensation of the flue gases in the primary heat exchanger;
and
mixing a portion of the liquid from the output of the primary heat
exchanger with the liquid from the secondary heat exchanger and
recirculating that liquid through the primary heat exchanger to
provide a flow rate in the primary heat exchanger greater than the
flow rate in the
secondary heat exchanger. .Iaddend. .Iadd.30. A method as claimed
in claim 29 further comprising returning the liquid to a storage
tank at a flow rate and temperature controlled by a thermostatic
valve. .Iaddend.
.Iadd.31. A liquid heater as claimed in claim 5 wherein the liquid
is water heated solely by the burner assembly to a temperature of
at least about 140.degree. F. .Iaddend. .Iadd.32. A liquid heating
assembly as claimed in claim 22 wherein the liquid is water heated
solely by products of combustion of the combustion chamber to a
temperature of at least about 140.degree. F. .Iaddend. .Iadd.33. A
method as claimed in claim 29 wherein the liquid is water to be
heated solely by the combustion gases to a
temperature of at least about 140.degree. F. .Iaddend. .Iadd.34. An
apparatus for heating liquid in a burner assembly and storing the
heated liquid comprising:
a burner assembly;
a heat exchanger assembly;
an insulated storage vessel having a naturally stratified upper
zone of heated liquid and a lower zone of cooler liquid separated
by a baffle;
withdrawing means for withdrawing cooler liquid from the lower zone
of cooler liquid and passing the withdrawn liquid through the heat
exchanger assembly in heat exchange relationship with combustion
gases from the burner assembly to cool those gases below their dew
point temperature so as to extract some of the latent heat of
vaporization from the gases to heat the withdrawn liquid; and
returning means for returning the thus heated liquid to the storage
vessel at a return location in the storage vessel above the lower
zone of cooler liquid and so as to avoid mixing with the cooler
liquid, the return location being such that the volume of cooler
liquid below it is large enough to provide for condensation of the
combustion gases through a substantial portion of the heating cycle
of the burner assembly. .Iaddend.
.Iadd.35. An apparatus for heating liquid in a burner assembly and
storing the heated liquid comprising:
a burner assembly;
a heat exchanger assembly;
an insulated storage vessel having a naturally stratified upper
zone of heated liquid and a lower zone of cooler liquid;
withdrawing means for withdrawing cooler liquid from the lower zone
of cooler liquid and passing the withdrawn liquid through the heat
exchanger assembly in heat exchange relationship with combustion
gases from the burner assembly to cool those gases below their dew
point temperature so as to extract some of the latent heat of
vaporization from the gases to heat the withdrawn liquid; and
returning means for returning the thus heated liquid to the storage
vessel at a return location in the storage vessel above the lower
zone of cooler liquid and so as to avoid mixing with the cooler
liquid, the return location being such that the volume of cooler
liquid below it is large enough to provide for condensation of the
combustion gases through a substantial portion of the heating cycle
of the burner assembly; and
a thermostatic valve to control the flow rate of liquid withdrawn
from and
returned to the storage vessel. .Iaddend. .Iadd.36. An apparatus
for heating liquid in a burner assembly and storing the heated
liquid comprising:
a burner assembly;
a heat exchanger assembly;
an insulated storage vessel having a naturally stratified upper
zone of heated liquid and a lower zone of cooler liquid;
withdrawing means for withdrawing cooler liquid from the lower zone
of cooler liquid and passing the withdrawn liquid through the heat
exchanger assembly in heat exchange relationship with combustion
gases from the burner assembly to cool those gases below their dew
point temperature so as to extract some of the latent heat of
vaporization from the gases to heat the withdrawn liquid; and
returning means for returning the thus heated liquid to the storage
vessel at a return location in the storage vessel above the lower
zone of cooler liquid and so as to avoid mixing with the cooler
liquid, the return location being such that the volume of cooler
liquid below it is large enough to provide for condensation of the
combustion gases through a substantial portion of the heating cycle
of the burner assembly; and
the heat exchanger assembly comprising a primary heat exchanger and
a secondary heat exchanger for receiving gases cooled by the
primary heat exchanger, the apparatus further comprising a liquid
flow path from the output of the primary heat exchanger to its
input for recirculating liquid in the primary heat exchanger, said
liquid flow path and the primary heat exchanger forming a primary
heat exchanger loop, and a pump in the primary heat exchanger loop
for pumping liquid from the secondary heat exchanger and a portion
of the liquid from the output of the primary heat exchanger through
the primary heat exchanger. .Iaddend. .Iadd.37. An apparatus as
claimed in claim 36 further comprising a thermostatic valve to
control the flow rate of liquid withdrawn from and returned to the
storage vessel. .Iaddend. .Iadd.38. An apparatus as claimed in
claim 36 wherein the flow path of combustion gases past the primary
heat exchanger provides a first gas velocity through the primary
heat exchanger and the combustion gases then flow past the
secondary heat exchanger at a second gas velocity substantially
greater than the first. .Iaddend. .Iadd.39. An apparatus as claimed
in claim 38 further comprising a thermostatic valve to control the
flow rate of liquid withdrawn from and returned to the storage
vessel.
.Iaddend. .Iadd.40. An apparatus as claimed in claim 38
wherein:
the primary heat exchanger surrounds the combustion chamber and
combustion gases flow radially over that heat exchanger; and
gases from the primary heat exchanger flow in an axial direction
sequentially past multiple liquid flow paths of the secondary heat
exchanger. .Iaddend. .Iadd.41. An apparatus for heating liquid in a
burner assembly and storing the heated liquid comprising:
a burner assembly;
a heat exchanger assembly comprising a primary heat exchanger and a
secondary heat exchanger, wherein the flow path of combustion gases
past the primary heat exchanger provides a first gas velocity
through the primary heat exchanger and the combustion gases then
flow past the secondary heat exchanger at a second gas velocity
substantially greater than the first;
an insulated storage vessel having a naturally stratified upper
zone of heat liquid and a lower zone of cooler liquid;
withdrawing means for withdrawing cooler liquid from the lower zone
of cooler liquid and passing the withdrawn liquid through the heat
exchanger assembly in heat exchange relationship with combustion
gases from the burner assembly to cool those gases below their dew
point temperature so as to extract some of the latent heat of
vaporization from the gases to heat the withdrawn liquid; and
returning means for returning the thus heated liquid to the storage
vessel at a return location in the storage vessel above the lower
zone of cooler liquid and so as to avoid mixing with the cooler
liquid, the return location being such that the volume of cooler
liquid below it is large enough to provide for condensation of the
combustion gases through a substantial portion of the heating cycle
of the burner assembly. .Iaddend. .Iadd.42. An apparatus for
heating liquid in a burner assembly and storing the heated liquid
comprising:
an insulated liquid storage assembly including a naturally
stratified volume of hot liquid in communication with a volume of
cooler liquid, the volumes of hot liquid and cooler liquid being
separated by a baffle;
a heat exchanger assembly for passing liquid, withdrawn from the
volume of cooler liquid, in heat exchange relationship with
combustion gases from the burner assembly to cool those gases below
their dew point temperature so as to extract some of the latent
heat of vaporization from the gases to heat the withdrawn liquid;
and
circulating means for circulating the liquid from the volume of
cooler liquid through the heat exchanger and returning the liquid
to the volume of hot liquid, the volume of cooler liquid and the
flow rate established by the circulating means being such that the
circulated liquid provides for condensation of the combustion gases
through a substantial portion of
the heating cycle of the burner. .Iaddend. .Iadd.43. An apparatus
for heating liquid in a burner assembly and storing the heated
liquid comprising:
a liquid storage assembly including a volume of hot liquid and a
volume of cooler liquid in communication with the volume of hot
liquid;
a heat exchanger assembly for passing liquid, withdrawn from the
volume of cooler liquid, in heat exchange relationship with
combustion gases from the burner assembly to cool those gases below
their dew point temperature so as to extract some of the latent
heat of vaporization from the gases to heat the withdrawn
liquid;
circulating means for circulating the liquid from the volume of
cooler liquid through the heat exchanger and returning the liquid
to the volume of hot liquid, the volume of cooler liquid and the
flow rate established by the circulating means being such that the
circulated liquid provides for condensation of the combustion gases
through a substantial portion of the heating cycle of the burner;
and
the heat exchanger assembly comprising a primary heat exchanger and
a secondary heat exchanger for receiving gases cooled by the
primary heat exchanger, the apparatus further comprising a liquid
flow path from the output of the primary heat exchanger to its
input for recirculating liquid in the primary heat exchanger, said
liquid flow path and the primary heat exchanger forming a primary
heat exchanger loop, and a pump in the primary heat exchanger loop
for pumping liquid from the secondary heat exchanger and a portion
of the liquid from the output of the primary heat exchanger
through the primary heat exchanger. .Iaddend. .Iadd.44. An
apparatus as claimed in claim 43 further comprising a thermostatic
valve to control the flow rate of liquid withdrawn from and
returned to the storage vessel. .Iaddend. .Iadd.45. An apparatus as
claimed in claim 43 wherein the gas flow path past the primary heat
exchanger provides a first gas velocity through the primary heat
exchanger and the combustion gases then flow past the secondary
heat exchanger at a second gas velocity substantially greater than
the first. .Iaddend. .Iadd.46. An apparatus as claimed in claim 45
further comprising a thermostatic valve to control the flow rate of
liquid withdrawn from and returned to the storage vessel.
.Iaddend.
.Iadd.47. An apparatus as claimed in claim 45 wherein:
the primary heat exchanger is tubing surrounding the combustion
chamber and combustion gases flow radially over that tubing;
and
gases from the primary heat exchanger flow in an axial direction
sequentially past multiple liquid flow paths of the secondary heat
exchanger. .Iaddend. .Iadd.48. An apparatus for heating liquid in a
burner assembly and storing the heated liquid comprising:
a liquid storage assembly including a volume of hot liquid and a
volume of cooler liquid in communication with the volume of hot
liquid;
a heat exchanger assembly for passing liquid, withdrawn from the
volume of cooler liquid, in heat exchange relationship with
combustion gases from the burner assembly to cool those gases below
their dew point temperature so as to extract some of the latent
heat of vaporization from the gases to heat the withdrawn
liquid;
a heat exchanger assembly comprising a primary heat exchanger and a
secondary heat exchanger, wherein the gas flow path past the
primary heat exchanger provides a first gas velocity through the
primary heat exchanger and the combustion gases then flow past the
secondary heat exchanger at a second gas velocity substantially
greater than the first; and
circulating means for circulating the liquid from the volume of
cooler liquid through the heat exchanger and returning the liquid
to the volume of hot liquid, the volume of cooler liquid and the
flow rate established by the circulating means being such that the
circulated liquid provides for condensation of the combustion gases
through a substantial portion of the heating cycle of the burner.
.Iaddend. .Iadd.49. In a liquid heater, a heat exchanger assembly
comprising:
a central burner;
a primary heat exchanger surrounding the central burner, the flow
of combustion products from the burner being essentially radially
outward through the surrounding heat exchanger at a first velocity;
and
a secondary heat exchanger which is coaxial with the primary heat
exchanger, the partially cooled combustion products from the
primary heat exchanger being directed to flow in an axial direction
sequentially past multiple liquid flow paths in the secondary heat
exchanger at a second velocity substantially greater than the
first; and
a tertiary heat exchanger located coaxially within the secondary
heat exchanger wherein an unburned fuel/air mixture is preheated by
the
combustion gases from the secondary heat exchanger. .Iaddend.
.Iadd.50. In a liquid heater, a heat exchanger assembly
comprising:
a central burner;
a primary heat exchanger surrounding the central burner, the flow
of combustion products from the burner being essentially radially
outward through the surrounding heat exchanger at a first velocity;
and
a secondary heat exchanger which is coaxial with the primary heat
exchanger, the partially cooled combustion products from the
primary heat exchanger being directed to flow in an axial direction
sequentially past multiple liquid flow paths in the secondary heat
exchanger at a second velocity substantially greater than the
first;
wherein the combustion products are diverted from a radial flow
path to an axial flow path in an annulus surrounding the primary
heat exchanger and are then directed into the secondary heat
exchanger which is axially displaced from the annulus. .Iaddend.
Description
FIELD OF THE INVENTION
This invention relates to water heaters. More particularly, it
relates to water heaters in which vapor in the products of
combustion is condensed to retrieve latent heat of
vaporization.
BACKGROUND
With the increasing cost of fuel, methods for increasing in
efficiency of heat exchanger assemblies for extracting heat from
the products of combustion of fuel burners has become increasingly
more cost effective. One means of increasing the efficiency of heat
recovery has been to burn the fuel in small-volume, water-walled
combustion chambers. Forced draft or pulsed combustion techniques
are utilized to achieve high rates of heat transfer and to vent the
products of combustion.
Recently, systems have been proposed to cool the products of
combustion below the dew point temperature of those products,
typically below 130.degree. F., in order to condense some of the
vapor and thereby extract the latent heat of vaporization of that
vapor. To cool the products of combustion to that extent and to
minimize the size, and thus the cost, of the heat exchanger
assemblies, efficient heat exchangers must be designed.
An object of this invention is to provide an efficient water
heating system in which heat is extracted from the products of
combustion by condensing vapor in the products, the system having
an acceptable size and cost.
The condensate from natural gas combustion products is mildly
acidic, and the acidic nature of the condensate is expected to be a
potential cause of corrosion. The acidic nature of the condensate
may result from sulfuric acid, nitric acid, and carbon acid.
A further object of this invention is to provide a water heating
system designed to withstand the corrosive effects of the
condensate at a minimal cost.
DISCLOSURE OF THE INVENTION
In a heating assembly of this invention, primary and secondary heat
exchangers and a combustion chamber are positioned within a single
housing. The combustion chamber is defined by the primary heat
exchanger. The combustion products flow through the primary heat
exchanger at a sufficiently low velocity to keep the temperature of
the heat exchanger walls at an acceptable level. The products of
combustion are then directed into a secondary heat exchanger in
which the velocity of the products of combustion is increased in
order to increase the heat transfer coefficient of that heat
exchanger. Cold water flows through the secondary heat exchanger in
counter flow relationship with the combustion gases to cool those
products to a temperature below the dew point temperature. Vapor in
the products of combustion is thereby condensed. After being heated
in the secondary heat exchanger, the water is mixed with hot water
from the output of the primary heat exchanger and the water mixture
is directed through the primary heat exchanger at a higher flow
rate than in the secondary heat exchanger. The hot mixture of water
entering the primary heat exchanger assures that no condensation of
the products of combustion occurs at this heat exchanger.
In the preferred water heating assembly, the primary heat exchanger
is a coil of tubing which defines the combustion chamber. Products
of combustion flow radially through that coil. The secondary heat
exchanger is a second coil of tubing coaxial with but lower than
the first. Products of combustion flow through that coil axially at
a higher velocity. Prior to combustion, the combustion air and fuel
may be preheated by combustion products in a tertiary heat
exchanger which receives those products from the secondary heat
exchanger.
In a system in which hot water is stored in an insulated storage
tank, cool water is taken from the bottom of the tank and
introduced into the burner and heat exchanger assembly, and the
heated water is returned to an upper section of the storage vessel
in such a manner as to avoid of the heated water with the cooler
water in the bottom of the vessel.
The preferred system utilizes a blower downstream of the burner and
heat exchanger assembly for inducing a draft to propel the products
of combustion.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features and advantages of the
invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
FIG. 1 is a block diagram of the water circuit of a preferred
system embodying this invention;
FIG. 2 is an elevational cross sectional view of a preferred
embodiment of the burner and heat exchanger assembly embodying this
invention;
FIG. 3 is an elevational cross sectional view of a possible storage
tank configuration for use with this invention;
FIG. 4 is an elevational cross sectional view of an insulated
plastic lined storage tank for use in this invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
A preferred system embodying this invention is shown schematically
in FIG. 1. A storage tank 12 is connected to an external gas-fired
water heater 14 by supply and return pipes 16 and 18. The water
heater 14 comprises a primary, fired heat exchanger assembly 20 and
a economizer 22 which is a secondary heat exchanger operating in
the condensing mode. A circulating pump 24 placed between the
economizer and the primary heat exchanger draws water from the
return line 18 and from the economizer 22 and drives that mixture
through the primary heat exchanger 20.
The storage tank is designed to maximize stratification between a
small volume 32 of relatively cool water in the bottom of the tank
and a larger volume 34 of stored hot water. In this case, the two
volumes are separated by baffles 33. Typically the cool volume 32
is about 20 percent of the total tank volume. If hot water is taken
from the outlet 38 of the storage tank while the heater 14 is in
the standby mode, cold water is introduced into the lower volume 32
through a diffuser from a cold water inlet 31 and pipe 35.
When the heater assembly 14 is turned on, water is drawn from the
lower volume 32 through pipe 16 if no water is being extracted
through outlet 38; or a mix of water from the cold water inlet 31
and the lower volume 32 is drawn through pipe 16 if water is being
extracted from the outlet 38. Water heated in the heater assembly
14 is returned through pipe 18 to the upper volume 34 in the
storage tank 12. The baffles 33 inhibit mixing of the hot water
from pipe 18 with the cooler water in the volume 32.
The cool water introduced into the economizer 22 through pipe 16
passes in counter flow heat exchange relationship with products of
combustion which have already been cooled somewhat in the primary
heat exchanger 20. The products of combustion and the water are
sufficiently cool when introduced into the economizer 22 that the
temperature of the products of combustion within the economizer is
below the dew point temperature. This results in condensation of
vapor in the products of combustion, and the latent heat of
vaporization is transferred to the water in the economizer.
Water which has been preheated in the economizer 22 is introduced
into the primary heat exchanger 20 which defines a combustion
chamber. There the water is heated to the high temperature
necessary for heating the water in the storage tank 12.
The purpose of the cold volume 32 should now be apparent. It
provides a sufficiently large reservoir of cool water to enable the
economizer to operate in the condensing mode throughout the
on-cycle even when no cold water is drawn through the inlet 31
during the heating cycle. The volume of cool water should be
minimized to reduce standby losses from and to limit the size of
the storage. To that end, the cool water is rationed to the heater
14 at a low flow rate to condense vapor in the exhaust gas with a
minimal amount of water.
The percentage of the storage tank which must be devoted to the
volume of cooler water 32 can be determined from the following
equation: ##EQU1## Where V.sub.H and V.sub.C are the respective hot
and cool volumes 34 and 32, T.sub.R is the temperature of the water
in return line 18, T.sub.cut-in is the temperature of water in the
storage tank at which the water heater is fired, and T.sub.Diff is
the differential between the thermostat cut-in and cut-off
temperatures. Typically, T.sub.R -T.sub.cut-in is in the range of
40.degree. to 50.degree. F. To minimize standby losses and total
tank volume, the volume V.sub.C should be less than 20 percent of
the total tank volume. Thus, the thermostat differential
temperature must be less than about 10.degree. F. A temperature
differential of 5.degree. to 10.degree. F. and a cool volume of 10
to 20 percent of the total tank volume are reasonable. For a given
burner input, the flow rate through the heater can be controlled by
a thermostatic valve 37 to maintain the desired return temperature.
Alternatively, the flow rate might be held constant and the burner
input .[.vaired.]. .Iadd.varied .Iaddend.to maintain the steady
return temperature.
Placement of pump 24 between primary heat exchanger 20 and the
secondary heat exchanger 22 is important for the following reason.
Hot water from the outlet 28 of the primary heat exchanger is
recirculated back to the inlet 30 to raise the water inlet
temperature of that heat exchanger above the dew point temperature
of the products of combustion. This is done to prevent condensation
in the primary heat exchanger. To minimize the cost of the system,
the primary heat exchanger is not protected against corrosion by
flue gas condensate.
A further advantage of recirculating water through the primary
exchanger is that it increases water flow rate and thus establishes
high water-side heat transfer coefficients. This minimizes liming
of the main heat exchanger coil. This is unnecessary in the
economizer due to the significantly low heat fluxes and water
temperatures in the economizer section.
The operating principle is best illustrated by the following
example: Consider a 100 gallon tank with a thermostat that operates
over a 10.degree. F. differential and is located one fifth of the
way from the bottom of the tank. Assume that the lower section 32
of the tank contains 20 gallons of water at an average temperature
of 80.degree. F, and that the average storage temperature is
140.degree. F.
In the proposed concept, the water heater would use the 20 gallons
of cooler water to heat 80 gallons of stored water from 135.degree.
F. to 145.degree. F. During this process, the cooler water would be
displaced by 135.degree. F. water. A heat balance indicates that
the total energy required is 15,700 Btu. If the heat output of the
water heater is 157,000 Btu/hr, then the burner-on time is 6
minutes. In this case, the circulating pump 24 would draw water at
the rate of 3.33 GPM from the bottom section and would return it to
the top section at a temperature of 175.degree. F. At the end of
the on-cycle, the mixed temperature of the upper section will have
reached 145.degree. F., and the bottom section will contain water
at 135.degree. F. The flow control is preferably accomplished by
thermostatic control of the return temperature to the tank by a
valve 37 .[.at.]. .Iadd.as .Iaddend.this will prevent excessive
temperatures if the bottom temperature, and thus the heater inlet
temperature, gets too high. Alternatively, the desired flow rate
may be set by a constant flow regulator or by a fixed orifice.
A variation of the concept might include the use of a separate,
smaller preheat tank instead of the integral volume 32.
A preferred design of the water heater 14 is shown in FIG. 2. The
primary heat exchanger consists of an integrally finned copper tube
coil 42 surrounding the combustion chamber 44. This arrangement
provides an efficient "water-wall" combustion chamber, which
minimizes combustion chamber heat losses and requires a minimal
amount of refractory insulation 46 and 48. Moreover, with radial
flow through the coil, the large area of the coil facing the
combustion chamber provides relatively low gas velocities. Such low
gas velocities are necessary to prevent excessive wall temperatures
due to the high temperature of the combustion products.
A mixture of natural gas and air is burned at a burner 50 within
the combustion chamber. In a pipe 56 combustion air from an
exterior inlet 52 is mixed with natural gas from a pipe 53 and
nozzle 54. The desired air flow rate is established by fixed
orifice 55. The mixture is drawn into the combustion chamber by a
blower 58 positioned in the flue gas outlet. Alternatively, the
mixture can be propelled by a blower placed upstream of the
burner.
Combustion gases are collected in an exhaust annulus 60 before
passing through the economizer coil. The gas temperature at the
annulus is in the range of 250.degree. to 400.degree. F.
The combustion products are cooled further in the economizer coil
62 which is designed for condensing operation. Because of the
corrosive properties of the condensate, the economizer is made of a
corrosion-resistant material, such as 70/30 cupronickel. The
economizer is designed for cross-counterflow of the combustion
products. With axial flow of gasses through this coil, the
combustion products flow at high velocities in order to achieve
high heat transfer coefficients. Here, high gas-side transfer
coefficients can be utilized without fear of excessive wall
temperatures because both the gas and water temperature are much
lower than in the main heat exchanger. Most of the system product
of combustion pressure drop will occur in this section.
FIG. 2 also illustrates a third heat exchanger section 64. This is
a counterflow pre-heater which uses the latent and sensible heat of
the exhaust products to preheat the incoming gas/air mixture. The
preheater 64 is a compact arrangement positioned concentrically
within the economizer coil 62. Exhaust gas which has passed through
the economizer is directed up through a first annulus 66 and then
back down through a second annular 68. The annuli are separated by
a cylinder 69 Gas in the annular 68 is in counter flow heat
exchange relationship with the incoming mixture of natural gas and
air. Any liquid which condenses from the exhaust gases in the
preheater is collected in a reservoir 70. Also, any condensed
liquid from the economizer 62 flows through holes 72 in the
cylinder 69 into that same reservoir. The collected liquid is taken
off through an anit-syphon tube 74 to the drain pipe 40. The
anti-syphon tube insures that exhaust gas can not leak into the
surrounding area but allows condensate to be drained from the
heater.
With sufficiently low incoming water temperatures, the preheater 64
is probably unnecessary, since it will add less than 1% to the
recovery efficiency. However, when incoming water temperatures are
high, as may occur in a hot water booster, the air preheater may
product worthwhile savings. The heat that can be recovered in this
type of preheater is limited by the heat capacity of the incoming
mixture. Typically, preheating the inlet mixture by 100.degree. F.
would increase efficiency by approximately 2.5%.
Losses in the system are minimized through the use of several
design features. The combustion chamber is small and does not have
a large inactive surface exposed to flame temperatures. The first
stage heat exchanger is small with little water inventory. The
bottom surface of the combustion chamber forms the top of the air
preheater, so that heat losses in this direction will reenter the
exhaust stream and increase the heat recovered in the preheater.
Insulation is shown on this surface only for the protection of the
metal surface which forms the bottom of the combustion chamber.
This insulation may be eliminated if the exhaust gas can be
utilized to cool this surface. The main "radiation" heat loss
occurs through the top of the combustion chamber and along the
outside shroud which contain the exhaust gas. Both these surfaces
are insulated with insulation 78 to minimize these losses. The
exhaust gas is relatively cool by the time it reaches the bottom of
the unit and this surface need not be insulated.
Other significant features include the use of an induced draft
blower which causes the unit to operate under negative craft
conditions; thus, sealing the heater is not as critical as it would
be if the unit were pressurized. Also since the exhaust gas is cool
at this point, a plastic blower may be used to reduce costs and
improved performance over a sheet metal blower.
The unit is shown using sealed combustion; that is, combustion air
is drawn directly from the building exterior and exhaust gas is
blown directly to the exterior. A natural convection stack is not
feasible because of the low exhaust temperature. An alternative is
the use of a conventional air intake plus exhaust through plastic
pipe installed as a roof vent or through an unused chimney.
Alternative storage tanks are illustrated in FIGS. 3 and 4. The
tank of FIG. 3 is conventional, with the exception of the
provisions for stratification described above. This tank is a glass
lined steel tank 80 surrounded by insulation 82 and a metal housing
84. The hot return pipe 18 .[.eenters.]. .Iadd.enters
.Iaddend.through the side of the tank and directs the hot water
upwardly into the upper volume 34. The baffle 33 is positioned
below the hot return pipe 18 to assist in the natural
stratification of the hot and cool water within the tank. Holes 86
in the baffle allow for displacement of water through the baffle as
necessary. A diffuser 88 is positioned over the inlet pipe 35 so
that flow between the volumes 32 and 34 is not induced by
introduction of cold water into the tank. A thermostat 36 controls
the operation of the heater.
An alternative version of the tank is illustrated in FIG. 4. This
preferred tank structure includes a plastic lining 90 surrounded by
insulation 92, such as foam insulation, and an outer steel tank 94.
All connections to the interior of the tank are through a bottom
plate 96. This arrangement includes the baffle 33 and the diffuser
88 as in the embodiment of FIG. 3. The thermostat 36 is also
connected through the bottom plate 96 to a remote sensing bulb
98.
It should be noted that the baffle 33 is not absolutely necessary.
It may be sufficient to have the hot return pipe outlet positioned
sufficiently high within the tank that a lower volume of cool water
remains. Also, flow directors may be mounted directly to the outlet
of pipe 18 to avoid the need for a baffle 33 fabricated within the
storage tank.
While this invention has been particularly shown and described
within references to preferred embodiments thereof, it will be
understood by those skilled in the art that various changes in form
and details may be made therein without departing from the spirit
and scope of the invention as defined by the appended claims.
* * * * *