ASHRAE Journal - September 2023 - 42

TECHNICAL FEATURE
FIGURE 5 Multiple compact AGR integration with the tubular HX of natural gas furnaces.
Multiple
Compact AGRs
technologies on cost, effi ciency,
condensate acidity and emissions
control. The overall cost penalty
for a 2 L (0.5 gallon) AGRenabled
furnace is $120, less
than the fi ne imposed by the
new California standard. The
neutral condensate, as enabled
by AGR, allows it to be released
into the sewer system or recycled
for use in indoor humidifi cation,
eliminating the need for a second
drainage system.
to recycle the trapped SOx in an offl ine regeneration
reactor. This can effectively avoid environmental issues
related to SOx slip. SOx trapping degradation does not
affect the AGR's capability to achieve neutral condensate
and ultralow NOx emissions.
Furnace manufacturers highly recommend annually
scheduled maintenance.6 Thus, it is appropriate to
combine the regular furnace maintenance with AGR
regeneration service.
The condensing furnace with the Pt AGR component
was operated for approximately 400 hours to test longterm
reliability and durability. Figure 4a shows the pH
values of the condensate collected during the 400 hour
testing period. The recorded pH values remained at ~7,
with 99.9% acidic content removal achieved during the
tested period. Meanwhile, all the furnace components,
including the fan, combustion
system and control continued to work
normally. Following the 400 hour
test, NOx and formic gas/CO/HC/CH4
were still removed effectively or
oxidized without a cost penalty.
Figure 4b compares NOx emissions
before and after the test. Additionally,
during this 400 h long-duration test no
incident of fl ue gas leakage occurred
due to the AGR component nor was
there abnormal pressure drop of the
AGR component.
AFUE
Unit Price ($)
AGR Cost ($)
Condensate
NOx Emissions
SOx Emissions
Formic Gas
CO Emissions
Discussion
Table 1 compares the AGR-enabled
condensing furnace prototype with
two popular commercial furnace
42
Condensing
Heat Exchanger
AGR technology allows simple installation and a
more simple, less expensive furnace design by using
a large, low-cost secondary HX instead of the specifi c
corrosion-resistant super-ferritic stainless steel alloy
HXs used in furnaces that are required to withstand
the high acid content resulting from cyclic burner
operation in a condensing furnace. The large, low-cost
secondary HX can yield high furnace effi ciencies at
low cost.
On the other hand, the novel furnace with AGR does
not alter the original commercial unit confi guration
in the building. All the original furnace components,
including the fan, combustion components and
control, continued to operate normally. This indicates
high compatibility with commercial furnaces
currently on the market. The proposed technology can
TABLE 1 Summary of the retrofitted AGR condensing furnace compared with two commercial furnace
technologies, including two-stage modulating condensing and noncondensing furnaces.
NONCONDENSING
FURNACEa
TWO-STAGE MODULATING
CONDENSING FURNACEb
80%
834
-
-
~40 ng/J
1 - 2 ppm
9 - 14 ppm
400 ppm (cold start)
40 ppm (steady state)
-
96%
1,448
-
Acidic (pH = 3-4)
~40 ng/J
<1 ppm
9 - 14 ppm
400 ppm (cold start)
45 ppm (steady state)
Corrosion-resistant
stainless-steel alloy
heat exchanger
ULTRACLEAN CONDENSING
FURNACE WITH 2 L AGRc
97%
1,098
~120
Neutral (pH = ~7)
0 - 1 ng/J
0 ppm
0 ppm
1 ppm (cold start)
1 ppm (steady state)
Low-cost alumina alloy
heat exchanger
ª A commercial 80% noncondensing furnace.
b A commercial 96% two-stage modulating condensing furnace.
c The furnace with AGR is retrofi tted based on a commercial 92% non-modulating condensing furnace.
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