heat exchanger plans...

[mobile_bob]

i realize that there is interest in an exhaust gas exchanger that elminates the need for periodic
cleaning (yes Jens i feel you pain buddy)

the exchanger that i built last year about this time, has been working excellently since in testing, but
i have not been able to do a couple things that i feel is necessary before i release the plan set.

while the unit works very well at near full loading, i need to do more testing at low and part loads
to determine is efficiency and whether or not it can maintain itself (self clean)

and most importantly i need to work out a formula for sizing the unit for specific enigne's, hp output, rpm,
exhaust temps, and the output the engine is likely to be operating under.

because of its design "one size" likely will not suffice for all engine's, and likely not for all load levels.

in order to work out the formula at least get a working baseline to build from i need some input from you guys
with other enignes.

1. what engine would you want an exchanger to work on?  (listeroid, changfa, other)

2. how many cylinders?

3. displacement?

4. rpm

5. exhaust port diameter (in inches if possible)

6. max hp rating

7. intended power output range ( for instance a 6/1 listeroid running at 2.0 kwatt average, etc)

8. exhaust temp as near the port as you can measure

9. orientation you would like to use the unit in, horizontal or vertical?

the unit i have working on my 195 changfa which is run at 90-100% loading 1800rpm, 7 plus kwatt output
has an input range of 640 degree's F. and an exhaust outlet of ~230 degree's F.  it is approx 40" long
and approx 7 inches in diameter (with insulation installed)

i would expect that a listeroid 12/2 would be very close to the same physical size if run at near full load is a requirement
and a 6/1 might be half the physical size in length,, in any case the diamter will remain the same whatever we drive it with.

the last thing i want to do is release plans for something that might not perform as well as it should because of me not
accounting for the variables and sizing the internal parts inappropriate to what the engine can deliver.

btw for what its worth, my unit has been proven to remove ~75% of the waste heat from the exhaust stream, while this
is not the most efficient perhaps the fact that one does not have to clean it is the tradeoff.

also keeping the exhaust temp over 212F output keeps down condensation issues with the remainder of the exhaust system.

any input would be useful in trying to work out the final plans

thanks
bob g

[bschwartz]

Add one vote for standard 6/1 listeroid at nominal 2KW load.  If loads drop off for any period of time (say 2 hours) but then pick up, do you think this would load up?

[Apogee]

I would vote for one 6/1 in size and a second chanfa/12/2 in size.

One of each would be perfect for me!!!!

Thanks Bob!

Steve

[mobile_bob]

bswartz:

i haven't tried that in testing yet, at least with any real observation
i will put that on the list of tests to run.

my thinking is maybe i can simulate what you are asking by dropping the load on my changfa
to around a kwatt for an hour or two and then ramp back up to full load to see if there is any indication
of degradation of performance or increase in exhaust back pressure.

i don't believe it would be a problem handling that, however
it probably would not be a good idea to run the engine for long periods of time at low or no load
because of buildup of tars and oils internally?  i just don't know at this point.

for the unit to self clean it must be allowed to cycle from cold to full operating temperatures with some frequency.
especially if part load operation is the goal for the majority of the time.

interesting questions, have to give it more thought.

thanks
bob g

[mobile_bob]

thanks for the input guys

Jens:

i am thinking the sizeing of my exchanger is very close to being appropriate for your listeroid 2 cylinder

your displacement is quite large, comparatively speaking to the changfa, but the flow works out to about
a wash between the two,

if you can, measure the heat leaving the Ypipe betwen the heads, i would like to know the peak temperature
at that point if possible.

also i am thinking the operating window is sufficiently large to obtain a relatively flat efficiency curve from 5-9kwatt
output, the target of 7 kwatts would be a good place to design to, and allow some dip on either side i would think
would be acceptable.

thanks
bob g

[mobile_bob]

i think that would be fine, in my testing allowing the unit to cool to ~75 degree's seems to be good enough
to allow for the thing to do it cleanup function on restart, it might be that even a higher temperature at cold
startup would work as well.

the units need for a clean cycle seems to be more a function of running at very light or no loading for a protracted
period,  i don't think it would need a cleanup cycle if it were to run at over 5kwatts and higher most of the time.

your application i am not as concerned with, i think it will work very well with it, however
most of my concern relates to the 6/1 lopeing along at a kwatt output with an oversized cooling system
where the exhaust temps are so low and exhause flows are low as well.

that is where sizing is very important

bob g

[mobile_bob]

270C is about 500 degree F.

at what loading was the engine running at at this temp?  5kwatts?

bob g

[mobile_bob]

ok that makes sense now, for some reason i was thinking you had a 12/2
but in reality you have a 20/2?

is there anyway to measure the temp right at about 1" out from the exhaust port on each head?

what i am thinking is you have air dillusion coming in from crossover on the opposing cylinder mixing at
the Y pipe and cooling the exhaust a bit?

500 degree's F is probably pretty close though for a 20/2 at 7.5kwatt loading

your at about 75% loading on a 20/2 at 7.5kwatt, so  500 / .75 = ~660degree F (which is about what i would expect
the engine to be doing at full 10kwatt loading.

a bit more number crunching and i think we can get the sizing formula worked out to get you right in the sweet spot
for your application.

glad i asked the question, now that i think about it.  this has been something that has been nagging at me for many months

the design is interesting, but the reality is i got very lucky in sizeing the various important parts of the exchanger for my application

now the importance of sizing is more clearly defined in my mind at least, and i think i have enough info to work out the rest.

the only thing left that might shift the efficiency a bit at this point is the coolant temperature you are running at (engine coolant
not exhaust exchanger coolant), so long as you are running at or near 195-200 i don't think there will be effect.

bob g

[Dail R H]

    I have a 12/2Metro with a 7.5 st head. It's not in operation yet,so no run data available,sorry. With no hardware in place yet,orientation ie,horizontal versus vertical is not an issue. I could plan either.

[AdeV]

As you know Bob, I've been hankering after one of these for a while now So I'll definitely vote for a 6/1 sized unit.

At the moment, I can easily provide low load testing - because I don't actually have a load hooked up to my engine. I need to do a bit of work with temp sensors & the like, but now I've got my lathe & can do screw cutting, I'm well on the way there.

How about this for an idea? I'm assuming that, if this heat-x works out as well as you're anticipating, that you'd quite like to make a commerical system - either planset or finished items. If that's the case, maybe it would be an idea to get some "beta testers" together, who can build a unit up & provide a set of running data. The test results can then be correlated & a decent stab can be made at predicting some generalised performance figures. I think that to cover all of the variables involved, one - or even just 2 or 3 - testers will not be able to provide sufficient variability; but 10 or 15 may just be able to do it.

Anyway, gotta go, lunch is calling. More thoughts later...

[cognos]

Very ambitious project this, designing a waste heat recuperator... technically, a gas/liquid heat recuperator...

Some random thoughts...

I'm going to wager that the liquid side is clean enough that fouling won't be an issue. It's the exhaust gas side that's going to be problematic.

Shell and tube design for ease of cleaning? Since the pressures on the exchanger are low, durable/inexpensive/highly efficient materials can be used - like thin steel.

On-line water or solvent flush, lowering the need for shutdown and disassembly for cleaning? Or - off-line water or solvent "soak" (fill it, let it sit, drain it?) Combination of both?

Corrugated tube for turbulence? Tube side gas turbulence = higher heat transfer efficiency, but at the cost of more diffficult cleaning.

Single gas pass or double pass - or more? Single pass = long, narrow exchanger with low flow restriction, while multiple pass = shorter, larger diameter, higher restriction.

Shell side baffles? Increases the liquid side turbulence, for higher efficiency, but harder to make and increases flow restriction.

Sized properly, the outlet gas temperature should not fall below the condensation temperature of anything in the gas, nor pass the boiling point of anything on the liquid side. Tough to do if the load on either side of the exchanger varies - flow rates or temperatures. But possible.

There are some basic control schemes that can help with size versus load matching over a load curve....
One can put a recirculation bypass on the liquid side - joining the outlet back to the inlet - so if the total liquid flow (heat demand) is high enough to drop the outlet gas temperature below 230°F, some warm outlet liquid can be mixed back in with the inlet liquid to decrease the heat uptake...
And a low liquid heat load demand (flow) can be dealt with by bypassing some exhaust gas around the exchanger, so the liquid outlet temperature doesn't get too high...

There's an "el cheapo" formula for fouling, that we used to determine when an exchanger's heat transfer had become so low due to fouling that it needed cleaning...
you needed some baseline inlet and outlet temps (and differential pressures, if possible, but not mandatory) - then you monitored the fall of the outlet temp on the "cold" side (in your case, the water side) and rise of the outlet temp on the "hot" (exhaust) side...

I'll see if I can find them, if anybody wants them...

[BioDBilly]

Hi Bob

I'll cast votes for one each of a 6/1 and a design that would accomodate the Redstone engine, which it sounds like you have.  It so happens that I have a Goodway heat exchange tube cleaning tool ( http://www.goodway.com/client_ucs/products/133-psm-500.aspx ). I am hoping to find a heat exchanger design that can be maintained with it.  Sizing the tubing to their brushes would be my issue there.

Regards,
Will C

[mobile_bob]

the design has no "tubes" to have to clean, and the prototype is fully welded and sealed so that disassembly
is near impossible without destroying the thing.

i am not at all sure there would be any benefit to building it so that is could be taken apart except for maybe making
the alterations internally to allow for operation under other parameters.

basically it doesn't operate anything like conventional heat exchangers do.

bob g

[veggie]

I have both a Changfa 195 and a Listeroid 6/1.
I vote for both. !

Veggie

[cognos]

So... even more ambitious than I first thought... a non-conventional exchanger design! I'll be very interested in seeing the proposed unit.

Every gas/liquid design I've seen where the gas is the "dirty" exchange medium is a shell and tube exchanger of some sort, even if it might be 5 storeys tall and look - and act - like a boiler. Cleaning takes many forms - on-line steam cleaners that rotate, water injectors, burners in the gas stream fired intermittently to burn off accumulated crud, rattles that spall off hard stuff - there are many solutions to this problem. But they all end up requiring disassembly and power-washing, at some point. At least on the tube side...