Well, you're wrong again, this time on two points.
First, in the example I gave you, the metal cubes
were replaced 5 times. That is a long way from
Second, in the heat exchanger in the furnace duct,
we sure do have infinite cubes. It's called airflow
and as long as the system is running, new colder air is
constantly moving across the heat exchanger as
long as the system is running.
Just like one cold metal cube replaces the one that
has warmed 1 minute in the simple example.
Imagine little blocks of air being changed every
minute. Then every 10 secs, then every sec,
etc. There you have the furnace heat exchanger.
Now the problem here is that
again, you never answer my questions. Do
you or do you not agree that by replacing those metal
cubes that have warmed slightly with new cold
0F cubes the 500F cube is cooled faster than
just leaving one cube there for the full 5 mins?
Here's the experiment again:
Let's say you have 10 identical
masses in the form of metal cubes. One of those
cubes is heated to 500F. The others are all at 0F.
I do the following two cases:
A - I take one of the 0F cubes and place it on top
of the 500F cube and leave it there for 5 mins.
B - I take one of the 0F cubes and place it on top
of the 500F cube for 1 min. Then I replace it with
another 0F cube for 1 min. I do that for the
same 5 min period.
Which results in the 500F cube being cooled off
the most? Which results in the most heat transferred?
Do you agree or disagree that the answer is B?
A simple yes or no at this point please. This is
a question an elementary school kid could
answer based on experience. And one that a
high school science student could answer based
And if you agree the answer is B, which we all know it is,
then what is happening is exactly the same as in
the heat exchanger in the furnace duct. The only
difference is that we're using air instead of metal
cubes to cool the heat exchanger. We don't have
infinite air, nor is that required. The MORE air
we put throught the heat exchanger, the MORE
heat that is transferred. If we put enough air
through it we've extracted all the heat.
OMG, you are so totally confused here it's unbelievable.
A car radiator has air flowing through it just as the
heat exchanger in the furnace ducts does. Both are
infinite in the sense that the airflow is continous as long as
the systems are operating. Exactly like
it would be in the metal cube example if I kept replacing
those metal cubes every 1 min for the duration of
the experiment. I actually can't believe you just made
this even more bizarre claim. This from the guy that
is always slamming the US over being behind in
technology? You really are lost in the wilderness here.
I can't understand any of this?
First, a radiator from a car would work under exactly the
same principles whether in a car or a furnace duct. The
physics don't change. Second, there is no need to use
an unspecified car radiator.
CL provided a data sheet to a heat exchanger manufactured
exactly for the purpose of being a water to air exchanger
for this kind of application. We've given you the link 3
or 4 times now. The datasheet clearly shows that as
you increase airflow the BTUs of heat transferred
increase. You can see that the increase slows as a
decaying exponential, which is EXACTLY what I told
you 10 posts ago. But you won't even address that
datasheet because there is no way around it. You
just trim it from every post.
Here it is one more time:
The spec sheet is right here:
Look at the first heat exchanger. Operating with 140F
incoming water at 6 GPM, the performance is listed
Your problems with science here are many. One
big one is that you don't understand the difference
between heat and temperature. Another is that you
think the physics of heat transfer change whether
the heat tranfer is used to warm a house or to
cool a car because in the car it's "waste" heat.
And you can't grasp the concept of infinity very
Yes, it's for 6 GPM of water flowing through the heat
exchanger. And it shows water at temps of 140F, 160F,
and 180F. And HOW MUCH HEAT IS TRANSFERRED
VERSUS AIRFLOWS OF 500, 600, 700, 800, 1000 CFM
As the airflow goes up, so does the BTUs of heat transferred.
Do you agree that is what it shows or not?
And yes, if you made another chart that showed say 3GPM
per minute of water flow instead of 6GPM, the water would
be coming out cooler. You would then have LESS heat
transferred. And you'd have a similar table showing that
as airflow increases from 500 to 1000CFM, the amount
of heat transferred increases with it.
That table for 6GPM isn't some unique point where
special rules of physics apply. It shows how the
heat exchanger works and is an operating point within
the scope of this discusssion, where if the OP built
his system he could get 30K BTUS of heat out of it.
Look at the other heat exchangers that are larger
and show greater flow rates. Find me one that
does not show that as you increase airflow, the
BTUs of heat transferred go up.
Now I agree that is one missing piece of information that
should be spec'd on the data sheet. Without it we don't
know the specific air entry temp that resulted in those
BTU numbers. However, it doesn't matter. Whatever
the air entry numbers, clearly:
The spec sheet does state:
" These air to water units are ideal for heating water in residential
and public buildings. Compact and lightweight, these units are
designed to maximize heat transfer by utilizing a series of 3/8"
copper tubes with a high density of aluminum fins. These fins are
spaced in such a way that the fin density is an impressive 12 fins per
inch. This unique design allows for heating loads of 50,000 to 60,000
Btu per square foot (please see performance chart below)."
And with water entering at 140F to 180F it's obviously being
used in that chart to heat air. At that point, it does not
matter what the incoming air temp is. Let's assume for
the moment that the particular chart used 65F air. The
chart shows the BTUs of heat tranferred versus increasing
airflow. As you increase airflow, the BTUs go up.
Now, if the incoming air were cooler, say 40F, you'd transfer
more BTUs of heat because the temp differential between
the hot and cold sides of the heat exchanger is greater.
If the incoming air is 100F, you'd transfer less BTUs because
the temp differential is smaller. You would then have a
very similar chart that starts out with a given BTU of heat
transfer at 500CFM and INCREASES as you increase the
airflow to 600, 700, 1000 CFM. Capiche?
Sure does. Do you agree that this chart also clearly
shows the BTUs of heat transferred INCREASED steadily
with the airflow? That MORE heat is transferred in that
table as the airflow increases? That it is that way for
the table on every size of heat exchanger? Yes or no?
No shit Sherlock. But remarkably, the application the
heat exchanger is targeted for is exactly what the OP was
looking for. And the water flow rate and the airflow rate
are within the range that a system to heat a home via
solar would be. In other words, that chart shows
exactly what the OP could use. It's not some curve
from some bizarre or unique heat exchanger.
I can't understand why you can't understand the simple
concept that a heat exchanger, be it this one, or a car
radiator has a CONTINUOUS MASS moving through it.
We don't have a single hand, we have say 100 people
with hands at 98F, that each place their hand on the
warm stove for 30 secs. That transfers MORE heat
than one person leaving their hand there because new
hands at 98F are constantly replacing hands that have
warmed slightly. The greater the temp delta, the greater
the heat transfer. You stated that yourself.
You wind up with 100 people with
slightly warmer hands instead of one person with a hot
hand. Does that mean less heat is transferred? Of
course not because the mass of those 100 hands
more than makes up for the fact that they are only
slightly warm. Capiche?
Same thing applies to a home radiator on a hot
water or steam system. The more air you move
past it, the lower the air temp will be, but the more
heat you will extract from the radiator and the
cooler the exit water temp will be.
Same thing applies to a car radiator. And it matter
not a wit that it's waste heat being transferred to
the atmosphere, so please don't start that again.
There you go again. It's been stated to you many times
now by Mark, CL, and I that the object of the excercise was
to heat the house. Which means getting the most heat
out of the heat exchanger, not the hottest air. You are
confusing TEMPERATURE with HEAT. Try looking up
As one more example, I could go down and turn the
blower down on my furnace to 100CFM instead of
1300CFM. Assuming it would not kick off due to the
high limit safety switch, I would then be getting very
hot air out of it. Does that mean I'm getting MORE
heat and it's going to heat my house faster, etc?
No. I'd be getting LESS heat even though the small
amount of air coming out is a lot hotter. Capiche?
There is the whole aspect of diminishing returns that both Trader and
Harry are forgetting about - PERHAPS Harry more than Trader.
The difference in efficiency is real, but how great? = particularly
since we are working with a closed loop system, and the heat not
recovered stays in the system, raising the input temp the next time
'round, which will increase the efficiency. It's not like we are
wasting any heat (or at least not huge amounts) by not recovering it
on the first pass.
And passing more air through the exchanger does increase the BTUs
extraced - but by how much, particularly when lowering the "dwell
time" does reduce the efficiency somewhat.
At what point does the loss of efficiency from the one side
over-balance the loss of theoretical heat transfer from the other.
My gut feeling is the gains/losses from either side of the arguement
are close enough that there will be a fairly broad band of overlap
where the difference in heat transferred will be RELATIVELY
insignificant in the grand scheme of things. You get to the point
where "close enough IS close enough.".
On Jan 11, 10:11 pm, email@example.com wrote:
Give me a break here. I haven't forgot about diminishing
A - I can refer you back many
posts ago where I said that the more airflow you
pass through the heat exchanger the more heat
you get out and that the additional heat gained
decays exponentially with the increased airflow.
And yes, if you put enough air through it, all the
heat is extracted and the outgoing water temp
and the air temp are equal and no more heat
can be extracted. I clearly stated that many
times here too.
B - I posted the data from the heat exchanger
data sheet you provided which fits the application
nicely. It shows that additional heat is still being
extracted at over 1000CFM. Yes, the additional
gains are diminishing.
C - This started over the issue of placing dampers
in the airflow stream. I said you might as well just
use max airflow, because there is no downside
to doing that from the heat exchange standpoint and
you will have heat exchange at a maximum. I
think you agree with that, no?
It doesn't increase the efficiency. That water leaving the
heat exchanger from the furnace is now HOTTER and
will consequently pick up LESS heat going through the
solar array, which is just another heat exchanger. Put
65F water into the solar array and you get max heat out
of it. Put in 150F water, or whatever the max temp it
is capable of achieving, and you get zero heat out of it.
You sure are wasting it, per the above.
Totally wrong, per the above.
Your gut is wrong. Sending that water back to the solar
array with all the heat extracted from it in the furnace is
how you get the maximum heat out of it, which is what
the OP wants. If that isn't possible, the next best thing
is to get as much heat as possible extracted from it.
And you do that by using maximum airflow.
On Fri, 13 Jan 2012 07:36:14 -0800 (PST), " firstname.lastname@example.org"
Not totally wrong, if when you decrease the air flow you also
increase the surface area - so you get the same CFM flow but at a
lower velocity.. All goes to the TOTAL design of the system. Can't
just change one parameter and expect to totally optimize the system.
I more or less agree with you - but with a solar "loop: if the heat is
not all extracted the first time round it just means the fluid will be
hotter coming by the next time - which may increase the efficiency so
more heat is extracted next time through.. Depends a lot on how much
solar input he is getting.
And as far as the dampers - I NEVER advocated putting dampers in the
MAIN airflow. My damper suggestion was to get away from the
"restricted airflow" straw man somebody put up, putting the heat
exchanger "off-line" to avoid the restricted air flow. And even then -
yes, they are likely pretty close to superfluous.
On Jan 13, 5:16 pm, email@example.com wrote:
But we are not changing the surface area. The surface
area is fixed for whatever heat exchanger he uses. We're
just changing the volume of air passing through. Back to the
original point that harry still disagrees with. And that was
if you increase the airflow through a water to air heat
exchanger, you get more heat out of it. The increase in
heat that you get trails off exponentially until you finally
have the air exiting and the water exiting at the same
temperature. At that point you've extracted all the
heat and the heat transfer has reached the maximum.
Increasing it beyond that doesn't extract any more
heat. I think we agree on that, don't we?
You agree that the same applies to a home radiator?
The more air you put through it, the more heat you
You agree the same applies to the auto radiator?
The more air you put through it, the more heat
you will transfer? (assuming of course the heat
is there, ie the thermostat is wide open, car block
is hot, etc)
You agree that is what the data sheet for the heat
exchanger you provided shows?
No, that is not correct. You get the most heat transfer
when the temperature delta is the greatest. If you send
the water back to the solar array at say 110F instead of
90F, it will pick up less heat when it passes through
the solar array. The amount of heat picked up is not
fixed, it's directly affected by the temp of the incoming
water. The cooler the water I put in, the more heat
I can extract from the solar array.
Yes, I acknowledged that in previous posts. I originally
thought you were advocating putting them in the main airflow,
which is why I said just put all the air through the heat
exchanger. It's simple and you get the max heat extracted.
You can see from the spec sheet on the heat exchanger
the consequences of reducing the airflow. A typical
furnace could move ballpark 1000CFM in heating mode.
From the spec sheet, that gives 38K BTUs.
If the airflow is cut to 500CFM, which is still 1/2
the available airflow, you get 28K BTUs
out of the heat exchanger. i don't know what you
think about giving up 1/3 of the available heat by
restricting the airflow, but it doesnt' sound like a good
idea to me.
I still have doubts about the viability of this thing
based on how much heat you could get out of it.
The OP never gave us data on the solar array.
I'm betting that it's sized to a water heater and
not capable of anywhere near the flow rate for
the curve in the heat exchanger spec sheet.
To get the 38K BTUs that heat exchanger has
6GPM of 140F water flowing. As I said before
relative to a water heater, that's an entire tank
of hot water in 8 minutes. Meaning I doubt
the solar array has capacity anywhere near
that. It's likely sized to a water heater application
and could have 1/10th the heat output.
In which case, the heat for the furnace likely
isn't worth the trouble.
Both theories are valid - but when the rubber hits the road - or in
this case Fluid 1 hits fluid 2 - how much difference will each make???
The only way to know is by setting up a system and testing.
Not DIFFICULT to do as a test before final install with a variable
speed blower and heat exchanger, and a flow control valve to contol
both water and air flow. Don't need to measure air temperature
change/heat absorption because if the heat leaves the water, it went
to the air. You need to be able to measure inlet and outlet water
temps and water flow, as well as air flow. That's all.
I'd definitely be doing this before making any mods to the heating/air
handling system in the house.
And PERHAPS a thermosyphon circulation system would be self
optimizing???? Just a thought.
On Fri, 13 Jan 2012 07:58:10 -0800 (PST), " firstname.lastname@example.org"
No I'm not. I'm just saying there are two things that could affect the
total efficiency and heat output of the system, and they could be , to
a point, at cross purposes.
No, they do a design analasys - and then they test it to prove it -
and they DO instrument their design - at least in the primary stages.
After they have established what works and how - and what assumptions
are valid - they go on experience. Often the documented experience of
At this point we do not have documented experience of anyone having
done this - so doing a bit of experimenting is valid.
Scientific method - defined:
a method of research in which a problem is identified, relevant data
are gathered, a hypothesis is formulated from these data, and the
hypothesis is empirically tested.
On Jan 13, 5:29 pm, email@example.com wrote:
I don't need to do that to know that the more air that
flows through a water to air heat exchanger, the more
heat that is transferred. And that the cooler the water
temp is that is entering a solar array, the more heat
you get out. Both up to the total max heat tranfer that
is possible of course, which would be when the water
exiting is equal to the air or solar array temp.
Try sticking to the example, instead of changing it.
We're not talking about changing the burner rate.
We're talking about changing the AIRFLOW rate
of the furnace with a given burner rate. Whatever
the burner rate, you get more heat out of that
example of a heat exchanger the MORE air that
flows through it. Yes, that air will be at a lower
temperature, but given the increase in air mass,
it still picks up MORE heat going through the
And where do you address the whole rest of my
post? As usual, you don't. You just try to steer
the discussion somewhere else.
exhaust air thats too cool will make occupants feel cooler espically
at higer speed airflows.
a modern high efficency forced air furnace is a excellent example,
during cool down the blower remais on and can feel cold if your
standing in front of the air vent
On Fri, 13 Jan 2012 07:54:42 -0800 (PST), " firstname.lastname@example.org"
"total system design" needs to be considered - not just one parameter.
IF, for example, a dual path heat exchanger is used, so the water
flows at half speed, yet the same amount of water flows, you MIGHT get
better efficiency AND more heat output.
You get more OF the heat into the house (higher efficiency), but less
TOTAL heat into the house.
On Jan 13, 5:20 pm, email@example.com wrote:
But nobody is talking about using a dual path heat
exchanger, nor is there any reason to use one in
this application. And you can't find a water to air
heat exchanger where you don't get more heat out
of it the more air you put through it. The spec sheet
you provided clearly shows that. Nor can you
find a solar panel where you don't get more heat
out of it the cooler the incoming water is. Those
are the parameters that matter.
I can't get harry to agree that with a simple car radiator
or home radiator, that the more air you move through it,
the more heat you get out, until you hit the max. The
max is where the outgoing water temp and air temp
are equal. I thought you agreed on that point, no?
I'm not sure efficiency is the right term, but I agree that
with the valve nearly closed,
you extract all the available heat from a tiny bit of water
and you get little heat into the house. And how do you
get more of the heat from that water to air heat exchanger
into the house? By increasing the airflow, per the datasheet.
It's funny that you can be snotty about this and be completely wrong.
Yes you want the two media in intimate contact but you then wrongly
conclude that means the velocity should be low.... So you think lower
CFM through a radiator pulls more heat out of it, obviously wrong.
When does your car overheat?
The flaw in your thinking is that you don't consider that there is
always more air to replace the air that is moving away and the colder
the air in the radiator, the more heat it pulls out.
You are the one confusing heat and temperature.
The usual case for a heat exchanger is is higher CFM = more BTU
exchanged and lower temperature.
And there you go again. Just like Mark said, you
are confusing temperature and heat. Let's recap.
The OP is proposing to take a heat exchanger
similar to a car radiator and place it in the return
duct of his furnace. He will pass hot water from
solar panels through the radiator.
Is the goal here to get the air as hot as possible?
Clearly not. The objective is to get the most
heat out of the water and into the air. By
moving more cold air
through that radiator the temperature differential
across the heat exchanger is higher and MORE
heat is transferred. Yes, the air coming out or
the radiator will not be warmed as much as if
there were less air flowing, but there is enough
additional air so that it more than make up
for the lower temperature and MORE heat is
transferred. That you can't grasp this most
basic concept just totally discredits you.
Again, you don't understand the difference between
heat and temperature. Which takes more energy in
the form of heat:
A - Raising the temp of 5000 cubic feet of air 1F
B- Raising the temp of 10 cubic feet of air 10F?
Which air is hotter?
Which of course matters not a wit.
I think it's obvious to all who's confused here.
Here's another simple experiment you can try.
You have a piece of iron that been heated with
a torch. You want to cool it off. Do you get it
cooled off faster by applying a very small stream
of water to it, in which case the water running off
is very hot, or by applying a large stream of
water and flooding it as much as possible,
in which case the water is just luke warm?
I see. So for example we can take all the coils
in the AC systems out there and make them 75%
smaller and at the same time reduce the blower
speed. That should work great. And you throw
rocks at Americans for being stupid?
The last sentence is pretty much the only thing you got
right here. And it's also why you want the maximum
airflow through that radiator. If you slow the airflow to
the limit, ie no air is flowing, the air surrounding the
radiator equals the water temp in the radiator. You
now have max temp air, but 0 air moving and 0 heat
being transferred. Now
slowly increase the airflow and while the temp of
the air moving
through starts to decrease, heat transfer starts
because you have cold air coming in contact with
the radiator. As you continue to increase the airflow
you get more heat out, more air out, but the air goes
down in temp. The process continues with a
decaying exponential in terms of the additional
heat that;s available.
1) i'm not going to debate with you, especially since you are rude...
2) you are wrong about this... try and take this as an opportunity to
3)think about what i and trad are saying... re BTU and
which holds more heat in BTU, a small red hot needle at 4000 F or
50 gallons of water at 100F?
which air flow is pulling more BTU out of an exchanger, 100 CFM at
100F or 1,000CFM at 75F?
have a nice day
The only reason you claim it's neither here nor there is
because you can't explain it with your rules of physics
which don't pertain to this universe.
A car radiator is a simple and perfectly valid
example of a heat exchanger. The heat from the
coolant is transfered to the air moving through the
radiator. With the same amount of coolant flow,
the more air you move through, the more heat that
is removed. That's one reason many cars today
are equipped with an auxialliary fan for the radiator.
If it's very hot out, you're stuck in barely moving
traffic, etc, and the coolant temp rises above the
normal range, a switch turns on the fan to move
MORE air through the radiator removing MORE
heat with it. Doesn't get any simpler than that.
If it worked the way you claim, we'd instead be
restricting the air so that the air coming out was
as hot as possible. You're confusing temperature
with heat. A car radiator with a high volume of
air moving through it will transfer MORE heat to
the air. The temp of the air coming out will be
lower, but there is MORE air and the increase
in mass of air more than offsets the fact that
it's a lower temp.
Another example would be a simple hot water
radiator in a house. With no fan you could hold
your hand by it and you'd feel some hot air
rising. Now place a fan there blowing lots of air
through it. The air will not be as warm. Does
that mean that LESS heat is being transferred?
Of course not. You're getting MORE heat, but
the air is at a lower temp because more mass is
being heated. Capiche?
Says who? It depends on what you're using the air for.
If I'm using the heat for a chemical process and want
it to be 120F, then I want it to be 120F, not 200F.
If the application requires it to be at a certain minimum
temperature, eg heating water in a water heater, then
yes you would need it to be at a certain minimum temp.
In the stated application, the goal is to transfer the
heat to air moving through a furnace to heat a
home. As long as the water
moving through the heat exchanger is above the
temp of the water, you get maximum heat transfer
with the fastest airflow, all else being held equal.
I've explained this several time now. Let' say
the water from the solar collector is at 130F.Start
at the limit of zero airflow. The air around the
heat exchanger is also 130F because no air
is moving. The water temp equals the air temp,
no heat is being transferred. Start increasing
the airflow and now you will have air coming out
that is slightly below 130F and water going back to the
collector that is slightly below 130F. Some
heat is being transferred. Ramp up the airflow
and you get more heat transferred. Will the air
temp out of the heat exchanger be less? Sure.
Does that mean less heat is being transferred?
No, because a lot more air mass is being
I've answered any questions you've put forth.
Yet I get no reply to mine. I'll state my simple
question a third time:
I give you a quiz. Our friend here
builds his proposed system. He puts an auto radiator
in the return side of his furnace duct work and connects
it to a solar collector. The solar collector heats water
to 130F and it gets pumped to the radiator. The
incoming air in the return duct is 65F. He has
a variable speed blower so that he can vary the airflow
through the radiator.
Which airflow results in the water leaving the radiator
at the lowest temp and consequently the most heat
A - 1 CFM airflow
B - 2 CFM
C - 10 CFM
D 100 CFM
E - 1000 CFM
F - 2000 CFM, the max the blower capacity?
Why won't you answer this very simple question?
No, actually we are trying to extracrt as much heat from the water as
possible - which can be more air with low net temperature gain - or
less air with high net temperature gain. The more air/lower temp
actually gets more HEAT than the low air high temperature. If you are
sitting in the airstream it FEALS like you are getting more heat with
the low flow/high temp option, but in reality, you will heat the
entire room or home FASTER withthe more air/lower temp option.
In this case, the difference would likely be marginal - but it would
be there- and would be quantifiable.
And in SOME engines, even today, if you remove the restriction of the
(open) thermostat the water flow increases velocity to the point it
does not remove enough heat from the block/heads and you get localized
overheating of the engine.
On Sat, 7 Jan 2012 07:13:25 -0800 (PST), " firstname.lastname@example.org"
NOT necessarily - with a higher TEMPERATURE primary fluid, the Delta
T at the secondary heat echanger will be higher, so the efficiency of
the exchanger will be higher, and more heat will be transferred to the
No, but if the efficiency of the heat transfer can be increased by 50%
by having a higher delta T at the secondary heat exchanger, it is
POSSIBLE that slowing the flow rate through the primary to increase
the primary fluid temperature MAY provide more actual HEAT to the
house. It would depend on the efficiencies of both heat exchangers.
It also depends if we are talking a closed series loop, where the
waterflow in both exchangers would be equal, or if we are talking
separate loops, where the GPM flow through each exchanger was
optimized. I'm ASSUMING we are talking a hybrid system, with a
frost-resistant primary fluid, and THREE heat exchangers in the
equation - solar to primary fluid - primary fluid to heat storage,
(water) and heat storage to secondary fluid (air) where the echange
fluid flows can all be separately controlled.
NOT a simple question with a simple answer that is always right.
Simple question, with simple answer that is only a SMALL part of a
more complex question.
Answer F will extract the most heat from the water IF there is more
heat available than can be extracted at 100cfm.. If 100cfm will
extract enough heat to get the water exit temperature to within a few
degrees of the duct inlet temperature, 500,000 cfm will not extract
any more heat - and the return temperature of the water will not be
below the 65 degree F air inlet temperature.
If the heat echanger was 100% efficient you would get 65 BTU per
gallon (imperial) out of the heat echanger in one minute at 1GPM or 60
GPH primary flow rate.
So to answer your "simple" question, we need to know the flow rate of
the "primary fluid", as well as it's temperature, to know how much
"heat" is available. There is only 1 btu of heat per degee F of
temperature change in 10 lbs of water. 10 times the water flow gives
10 times the POTENTIAL heat.
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