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heat exchanger capacity

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Posted by Eric Yancey on September 20, 2006, 2:59 pm
 
I've been designing a solar system to provide hot water for my shop's
radiant heat system.  In an attempt to keep material costs down, I'm trying
to decide if building my own heat exchanger is the way to go.  After a bit
of research, it seems the best design for my application is a 2 tube single
wall unit.

Let me back up a bit and describe my system.  The entire system is closed,
and I'll have 2 thermal panels in one circuit pumping glycol for freeze
protection.  This will be pumped through the heat exchanger and simply
return to the collectors.  The opposite side of the heat exchanger will be a
thermosyphon to and from a standard hot water heater which has been acting
as my heat source and will be my heat storage device.  From the water heater
the hot water is pumped through the radiant floor.

So, back to the exchanger.  The first design I looked at was a 1" outer tube
with a 3/4" inner tube.  I was going to build this to be about 4' long.
Then I started researching fitting prices for 1" tubing ($$), and realized
that I can probably build the unit describe below for the same price and
perhaps will be more efficient.

Second design: 3/4" outer tube with 1/2" inner tubde.  To provide enough
capacity with the smaller size tubing I think I'll need three of these,
running in parallel, to achieve the desired heat exchange.  I think this
design is actually more efficient than the first since there is more overall
surface area for the exchange.

My question is, if I go with the 2nd design, how long should the tubes be,
especially considering I would like to achieve a thermosyphon?  Will 3 tubes
be enough?

Would the first design be better, or should I go with a different approach
altogether?


Thanks in advance for help/suggestions!

Eric





Posted by SJC on September 20, 2006, 3:50 pm
 

I've been designing a solar system to provide hot water for my shop's
radiant heat system.  In an attempt to keep material costs down, I'm trying
to decide if building my own heat exchanger is the way to go.  After a bit
of research, it seems the best design for my application is a 2 tube single
wall unit.

Let me back up a bit and describe my system.  The entire system is closed,
and I'll have 2 thermal panels in one circuit pumping glycol for freeze
protection.  This will be pumped through the heat exchanger and simply
return to the collectors.  The opposite side of the heat exchanger will be a
thermosyphon to and from a standard hot water heater which has been acting
as my heat source and will be my heat storage device.  From the water heater
the hot water is pumped through the radiant floor.

So, back to the exchanger.  The first design I looked at was a 1" outer tube
with a 3/4" inner tube.  I was going to build this to be about 4' long.
Then I started researching fitting prices for 1" tubing ($$), and realized
that I can probably build the unit describe below for the same price and
perhaps will be more efficient.

Second design: 3/4" outer tube with 1/2" inner tubde.  To provide enough
capacity with the smaller size tubing I think I'll need three of these,
running in parallel, to achieve the desired heat exchange.  I think this
design is actually more efficient than the first since there is more overall
surface area for the exchange.

My question is, if I go with the 2nd design, how long should the tubes be,
especially considering I would like to achieve a thermosyphon?  Will 3 tubes
be enough?

Would the first design be better, or should I go with a different approach
altogether?


Thanks in advance for help/suggestions!

Eric

I assume you are refering to something like the quad rod exchanger.

http://kingsolar.com/catalog/mfg/21stcent/5ftquadrod.html

This is just for Domestic Hot Water (DHW) and not for home heating.
You would have to look at your total BTUs over time to see if any of
this would be what you want. I don't know that a couple of panels is
going to do it, but maybe you have a small shop. Also, floor radiant
heating usually requires some pretty hot fluid to get the heat transfer.





Posted by Eric Yancey on September 20, 2006, 5:08 pm
 

Thanks for your input.  Yes, that exchanger you linked to is similar to what
I'm thinking of building.  The design that I'm starting from is actually on
page 4 of this PDF:
http://www.builditsolar.com/Projects/WaterHeating/MSClosedLoop.pdf   (sorry,
I should have included this initially but I forgot this was on the web).

My shop is about 750 ft2.  The panels I'm looking at using are 4x12, so I'd
have a total of 96 ft2 of collector.  The shop is very well insulated and
the small hot water heater kept it warm enough for my taste last winter.
Basically if I keep the shop at 55-60F it's warm enough.  It's used for
woodworking so it needs to be a minimum of 50F so the finishes will cure and
glue will dry.

The floor is a 4" concrete slab and I kept the water heater on its lowest
setting last winter (the floor is perimeter insulated AND 100" insulated
underneath) which I believe is about 120F.  I kept the air thermostat at 60F
and it was very comfortable, almost too warm at times.  The material that I
read said that the fluid circulating in the slab should be between 100-140F.
If the direction I'm heading won't achieve this, please let me know where
I'm going wrong.  Better to find out now than later after I've plumbed the
whole thing :)

Thanks again!

Eric







Posted by SJC on September 20, 2006, 5:38 pm
 


Thanks for your input.  Yes, that exchanger you linked to is similar to what
I'm thinking of building.  The design that I'm starting from is actually on
page 4 of this PDF:
http://www.builditsolar.com/Projects/WaterHeating/MSClosedLoop.pdf   (sorry,
I should have included this initially but I forgot this was on the web).

My shop is about 750 ft2.  The panels I'm looking at using are 4x12, so I'd
have a total of 96 ft2 of collector.  The shop is very well insulated and
the small hot water heater kept it warm enough for my taste last winter.
Basically if I keep the shop at 55-60F it's warm enough.  It's used for
woodworking so it needs to be a minimum of 50F so the finishes will cure and
glue will dry.

The floor is a 4" concrete slab and I kept the water heater on its lowest
setting last winter (the floor is perimeter insulated AND 100" insulated
underneath) which I believe is about 120F.  I kept the air thermostat at 60F
and it was very comfortable, almost too warm at times.  The material that I
read said that the fluid circulating in the slab should be between 100-140F.
If the direction I'm heading won't achieve this, please let me know where
I'm going wrong.  Better to find out now than later after I've plumbed the
whole thing :)

Thanks again!

Eric

It depends on whether you intend to use electric or natural gas as well
on the water heater to get it to 140F. Most flat plate collector efficiency
goes down when the difference between ambient air and water temperature
increases. In other words, if it is 30F outside and you are circulating 120F
water to try and get it to 140F, the collectors will not be that efficient.
I do not know what collectors you are using, but I use the charts available
for the SunEarth collectors to gauge efficiency at the temperature difference
expected.


http://sunearthinc.com/empire_series_flat_plate.htm

If you look at the spec sheet for their best painted collector, you might
get just over 300 BTU per square foot per day on an average winter day
with a 90F difference between air and water temperature. If 30K BTU
will do what you want, then you are all set.







Posted by Eric Yancey on September 20, 2006, 6:35 pm
 

Thanks very much for the additional info.  I do not intend to use the water
heater to heat the water in addition to the collectors if I can help it.  It
is acceptable to me to have the shop cool off somewhat during the night and
since woodworking isn't my full time job and I don't mind if it isn't heated
100% of the time.

I envision that during the day most of the heat would be transferred to the
floor, since during the night the floor would have cooled considerably and
used up most (or probably all) of the heat in the storage tank.  This
should, according to my fairly limited knowledge, be the most efficient time
for the system - hot collectors + cold floor + cold storage tank.
Throughout the day the floor would heat up and eventually the air thermostat
would tell the system to stop heating the floor.  Once that happens, the
tank would receive most of the heat and the efficiency would begin to
decrease (at that point I think it would be late in the day anyway, so no
big deal).  At least this is how I envision the system working.

At any rate, I think that 30K BTU will get me where I need to be without any
problem.

Thanks for your help!



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