Maker Pro
Maker Pro

Liquid to liquid heat exchanger idea

DJ said:
I was told "...your shift is over when tank three hits 40C..." ;-).

Faster flow was NOT better in that case ;-).

A faster fluid velocity always increases the heat transfer rate, since
the boundary layer between the fluid and the pipe wall is thinner, with
more conductance. That's basic, but it may not get you home earlier :)

There's a tradeoff in solar pond and Big Fin and greywater pipe coil heat
exchangers with pressurized DHW inside and unpressurized still heat store
water outside: copper is more conductive but holds less water than plastic,
for the same money, which makes a plastic pipe with a volume comparable to
an average hot water burst more efficient than copper by day's end, even
with slower flow and less conductance.

We now have two victi... err, beta test sites in the Philadelphia area
for full-size solar pond and ersatz Big Fin solar heaters.

Clint's pond might be 5'x8'x2' deep, with a large flat elliptical 1" pipe
spiral under the 2" cover and EPDM over the cover and a 5 watt submersible
pump moving water over the cover during the day. A local greenhouse supplier
has Dynaglas polycarb roofing in standard 4'x12' pieces for about $50, so
the pond might have 2 4'x6' pieces as the upper glazing. The demo unit is
happily humming away at Clint's house now :)

On another front, 2 cheap Big Fins (2' widths of 0.018" brown-painted Al
coil stock attached to 3 1/2" copper pipes) might be in a 4'x12'x3" thick box
on the south wall of Moise's house, glued to polyiso board, with Dynaglas
over that and an unpressurized 6'x6'x2' deep box tank upstairs in the house
and a flat coil in the box. In Phila, with no overhang, the wall unit would
produce about 500 Btu/ft^2-day in January and 1500 in July. A 2.7' overhang
above a 1.3' separation would shade it completely on 6/21. How much overhang
would limit this to 1000 Btu/ft^2-day in July? Overtemp and freeze protection
could come from a $5 fountain pump in the tank that turns on when the fins
cool to 40 F and also turns on when the tank water is greater than 140 F,
with 2 $10 thermostats, like this:

on if box <40 F
---------X--------------
| E38 "Easy heat" |
| $10.99 at Lowes |
| |
| |
| on if tank >140 F | ------
-------------X------------------| |
Grainger 2E365 (SPDT) | |
| 5 W |
120 V | pump |
| |
--------------------------------| |
------

Another 2E365 might turn on a fan when the tank is warmer than 140 F
and the house needs heat. If the fan's ever running at dawn, the pump
may need replacement.

Home Depot (sku 125467) and Lowes (sku 17376) both sell Amerimax Home
Products 24"x50'x0.018" brown-painted Al coil stock for about $63. Make
a jig by screwing 2 5' 1x3s to a 2x8 5/8" apart, place flashing over
the groove with 1/2" copper pipe over that, pound for 30 seconds with
a big rubber mallet, remove the pipe, spread silicone caulk in the round
groove, place the fin over a 10' pipe, glue it down to 2" polyiso board
with more silicone with the brown side and the pipe groove up, Al-foil
tape the board edges, add polyiso perimeter spacers and closure strips
and the 4'x12' Dynaglas and screw it to the south wall of the house.

20 PI=4*ATN(1)
30 K=95'Al thermal conductivity (Btu/h-F-ft^2)
40 T=.018/12'Al coil stock thickness (feet)
50 D=2'Al coil stock width (feet)
60 OD=5/8/12'Cu pipe OD (feet)
70 L=D/6-PI*OD/4-OD/2'half-fin width (feet)
80 HB=1.5/2'one-sided airfilm conductance (Btu/h-F-ft^2)
90 NL=SQR(2*HB/(K*T))*L'fin efficiency term
100 TANHNL=(EXP(NL)-EXP(-NL))/(EXP(NL)+EXP(-NL))'tanh(nl)
110 FEFF=100*TANHNL/NL'fin efficiency (%)
120 W=72*(L+OD/2)'tip-to-tip fin width (inches)
130 PRINT W,FEFF'fin width (in) and efficiency (%)
140 AC=2*W/12*10'collector area (ft^2)
150 ACEFF=AC*FEFF/100'effective collector area (ft^2)
160 AG=48'total glazed area (ft^2)
170 SUNPOWER=.9*250'full-sun heat into box (Btu/h-ft^2)
180 FINPOWER=ACEFF*SUNPOWER'full-sun into fins (Btu/h)
190 BOXPOWER=AG*SUNPOWER'full-sun heat into box (Btu/h)
200 TA=34'average daytime outdoor temp in Phila in January (F)
210 BOXNET=BOXPOWER-FINPOWER'net global sun into box (Btu/h)
220 BOXTT=TA+BOXNET/AG'Thevenin equivalent box temp (F)
230 DHWTEMP=100'average collector water temp (F)
240 DHWPOWER=FINPOWER+(BOXTT-DHWTEMP)*1.5*ACEFF'water heating (Btu/h)
250 BOXTEMP=TA+(BOXPOWER-DHWPOWER)*1/AG'air temp in box (F)
260 SUNHOURS=4'full-sun hours in January in Phila
270 DHWENERGY=SUNHOURS*DHWPOWER'dhw energy (Btu/day)
280 COST=2*AG+2*AC'water heater materials cost ($)
290 PRINT AC,BOXTEMP,DHWENERGY,COST

fin width efficiency

21.05476" 80.81405%

fin area box air daily heat cost

35.09126ft^2 102.9664F 29958.45Btu $166.1825

Nick
 
N

News

Jan 1, 1970
0
Raising a fluid velocity ALWAYS increases heat transfer.

Nick

As long as the fluid remains the same temperature. Having a large cylinder
with stored water at 80C will transfer that heat faster if at a high
velocity and the heat exchanger can "take" and transfer the heat. Say a
boiler on a low flow can deliver 80C on the flow pipe. If you increase the
flow rate the temperature may be cooler at the flow pipe.
 
News said:
...Say a boiler on a low flow can deliver 80C on the flow pipe. If you
increase the flow rate the temperature may be cooler at the flow pipe.

Not if you increase the VELOCITY with the same volume flow rate.

Nick
 
C

Chris W

Jan 1, 1970
0
Chris said:
In order to heat my water and still use the existing hot water heater,
I was thinking about building my own liquid to liquid heat exchanger.
Since there would be antifreeze in the solar collector, I need some
way to make sure it will never contaminate the water in the hot water
tank. So I came up with this design, tell me what you think.

http://www.thewishzone.com/cdw/HeatExchanger/
Here's another idea. What if the tube the potable water was running
through were so thick as to guarantee a crack was virtually
impossible? Say a .75 ID and a 1.25 OD for a .25 wall. Obviously
thicker is going to take longer to conduct the heat but 1/4" of solid
copper is going to conduct heat a lot better than some thermal past or
oil in the middle chamber. Of course there is the problem of finding a
tube like that. I doubt you could buy one, you would probably have to
get a machine shop to gun drill a solid copper rod. Then soldering
would be fun with a copper tube that thick.

--
Chris W

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
One stop wish list for any gift,
from anywhere, for any occasion!
http://thewishzone.com
 
P

Paul

Jan 1, 1970
0
Chris W said:
Here's another idea. What if the tube the potable water was running
through were so thick as to guarantee a crack was virtually impossible?
Say a .75 ID and a 1.25 OD for a .25 wall. Obviously thicker is going to
take longer to conduct the heat but 1/4" of solid copper is going to
conduct heat a lot better than some thermal past or oil in the middle
chamber. Of course there is the problem of finding a tube like that. I
doubt you could buy one, you would probably have to get a machine shop to
gun drill a solid copper rod. Then soldering would be fun with a copper
tube that thick.

--
Chris W

Gift Giving Made Easy
Get the gifts you want & give the gifts they want
One stop wish list for any gift, from anywhere, for any occasion!
http://thewishzone.com

I think in our part of the country we have codes and safety standards.
You may be willing to rig whatever you want, but when it comes to
your family, guests and people that buy your house, codes win out.
 
N

News

Jan 1, 1970
0
Not if you increase the VELOCITY with the same volume flow rate.

......and increase the burner rate. A burner can only transfer so much heat
to the water. A boiler raises the temperature of water - the temp rise. Say
it can give a 40C rise at n water velocity. If water entering the return
pipe is 30C then 70C out the flow, but only if the velocity of the water is
n. Have n x 2 and the flow may be far lower as the water doesn't stay in
the boiler long enough to have its temperature risen. You can run your
fingers through a bunsen burner flame and no problems (we have all done it),
but keep your fingers there and see what happens. Also when the flow temp
is lower the return temperature is cooler too, so a double whammy.
 
E

Ecnerwal

Jan 1, 1970
0
Chris W <[email protected]> said:
tube like that. I doubt you could buy one, you would probably have to
get a machine shop to gun drill a solid copper rod. Then soldering
would be fun with a copper tube that thick.

And meanwhile you can actually meet code, cheaper, with off-the-shelf
double wall heat exchangers which are actually designed to do the job
correctly, and efficiently.
 
Y

YouGoFirst

Jan 1, 1970
0
...if you use a large pipe and can reduce the flow rate through the heat
Higher velocities INcrease heat transfer...

Nick

That is true, but you can reach a point where your fluid is traveling so
fast through the heat exchanger that there is little to no effect occuring.

Also, the point I intended was that if you want to achieve the greatest
temperature differential with a water heating system, the fluid that is
being heated should travel as slower so that it is able to receive
sufficient heat to obtain a desired temperature.
 
N

News

Jan 1, 1970
0
Steve Spence said:
It's bigger than you are ....

Take heat out fast enough and you get tundra. But....you can reverse the
process in summer and use the tundra to cool a building. If you a cold
winter hot summer area then that is the way to go.
 
B

Bert Menkveld

Jan 1, 1970
0
Brian Whatcott said:
When I wanted to preheat a hot water tank, I used an extra heater tank
with a heat exchanger element screwed into it. This is a commercial
item, that has a spaghetti array of pipes from a round header of the
size usually associated with an electric heater element. I used non
poisonous antifreeze as the fluid in the solar panel loop feeding the
exchanger.

This commercial heat exchanger would be of interest to many of us DIYers.
Do you have any pointers to where you might find such a heat exchanger that
fits in place of a standard electric hot water heating element?
 
P

Paul

Jan 1, 1970
0
Bert Menkveld said:
This commercial heat exchanger would be of interest to many of us DIYers.
Do you have any pointers to where you might find such a heat exchanger
that fits in place of a standard electric hot water heating element?
I generally think in terms of hot water heat exchangers like the quad rod.
http://www.kingsolar.com/catalog/mfg/21stcent/3ftquadrod.html
They go outside the tank and attach to fittings that are already on the
tank.
A heat exchanger as small as an electric element probably would not have
enough surface area. Solar tanks by Rheem and others usually use a copper
coil inside.
http://www.rheem.com/consumer/catalogRich_detail.asp?id=58
 
N

News

Jan 1, 1970
0
DJ said:
Well, remember though, I was talking a big pond, not ground. And a
spring-fed pond at that, so one with continuous energy input.

A flowing spring is the best source of heat.
 
T

Tom

Jan 1, 1970
0
Maybe IM missing something but wouldn't the design be much simpler is
you just used propylene Glycol? I believe it's food grade and won't
hurt you.
 
Top