Simple boiler control for off-grid?

The crux of the problem here is the burner control sensor within the boiler.
It would seem that this component is too sensitive to the boiler water
temperature, resulting in excessive actuation cycling.

If there is an inspection port or aperture on the boiler, you might be able
to try the following suggestion:

Create a thermal shield around the burner control sensor by placing a length
of ceramic tube in such a manner as to surround it. This ceramic tube would
probably have to be epoxied or glued to the wall of the boiler at the burner
sensor location. The opposite end of the tube would just be open, so that
the tube fills with water and ultimately allows the sensor to heat up and
cool down in a manner that is more 'gradual' than the original arrangement.

The length and diameter of the ceramic tube will probably be subject to
experimentation. I would start off with a small diameter ceramic tube, large
enough to cover the burner sensor. The longer you make the tube, the more
hysteresis effect you will observe. ... Maybe a piece of vinyl pipe of some
variety would serve as the ceramic tube. I would be wary of using ordinary
PVC plastic pipe, since PVC is subject to embrittlement and weakening in
thermally stressful conditions.

Regards,
Dave
Dallas, TX
USA

Nicolas Boretos wrote:

Many thermostats use a small magnet to create hysteresis. With some, the
position of the magnet -- hence the hysteresis -- is adjustable. With
others, you may be able to bend the mount. Replacing the magnet with a
stronger one (rare earth?) or mounting one where there was none, are
other possibilities.

Jerry
--
Engineering is the art of making what you want from things you can get.
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Where is the primary thermostat located at. What is it measuring,
firebox temperature or boiler water temp? Describe the boiler a bit
more.

One possibility that might work, depending on your answer to my
questions above, is to add a timer to the burner fan control so that it
stays on for a time after the thermostat stops the auger. Depending on
the mechanical configuration of the firebox, boiler drum and thermostat,
this might continue to heat the system which will delay the thermostat
from cooling. If so it will also increase overall boiler efficiency and
lower your ash disposal needs.

Olive kernel may be cheap but the amount of power consumption the auger
takes is proportional to heat needed, times efficiency. Changing the
hysteresis of the thermostat will change the length of the on/off cycle
but not the amount of on time and this is proportional to fuel moved to
the furnace. Adding solar panels may be the way to go here. Not so cheap
but you can't change the laws of physics. Look into some other method of
moving the olive kernel other than an auger. This consumes lots of
power. A hopper and conveyor would work.
--
Paul Montgomery
Progressive Gauging Inc.

A. Paul Montgomery wrote:

Thanx to all who replied. I'll try and clarify a bit here...

Both stats are in a oil filled well which is immersed in water, thus
measuring water temperature.
Describe the boiler a bit
Fairly crude, "cube ina a cube" with about 2" spacing left, back and
right between them. The internal cube is the firebox, and one of its
sides is the firebox door. There are several 2" pipes, left to right, at
the bottom of the firebox which act as a heat exchanger as well as a
"rack" to hold a log should you want to burn wood. There's also a few
more pipes towards the top, again left to right, at 45 degrees to
horizontal, as additional heat exchanger. Finally, at the top there's
several tubes again, open to the firebox this time,  front to back where
the the flue gases pass through and into the back exhaust manifold.

This seems like a good idea. I can replace a plate on the door (made to
take a normal oil burner) with some fireglass and play with the timer....

Alas, this is only so true Paul...
There is a hopper on the system, and the auger sits under the hopper. My
next move would be to replace the current auger motor with something a
bit more efficient. The 4 pole motor is 180 watts; with a 50:1
reduction, I'm looking at extreme torque overkill....(I can turn the
auger quite easily by hand even with a full hopper...

Solar panels have to wait....

I'm attaching a solution someone posted in another ng to control the
temp with two tstats and a latching relay...I think it should work...

"You set the main thermostat to 25 degrees. You use a second
thermostat,connected to the supply *after* the main stat and set to 45
degrees, to power a relay with contacts across the 25 degree stat.

Initially, both stats are closed and the 25 deg stat is shorted by the
relay. The temp will rise until the 25 stat opens. No problem, the
supply is maintained by the relay contacts.
  The temperature will rise until the 45  stat opens. This opens the
relay, the 25 stat is already open - so no power then goes to the load
or the 45deg stat or its relay.

Only when the temp has fallen to 25 deg will the main stat close,
restarting the cycle. "

regards,

nikos

Hello,

A temperature controller with adjustable hysteresis is easy to find at
Omron.
Choose the E5CN and you have a hysteresis that you can set at the value you
want,1 or 10°C or more.

Good luck.
Ronny

An alternative approach if you are so inclined:

Obtain a length of Pyrex glass tubing, seal one end and partially fill with
a temperature sensitive opaque fluid, i.e. alcolol, oil, etc. Calibrate the
tube with marks at the desired on and off temperatures. Place
LED/Phototransistor pairs at each temperature setpoint, in a manner such
that the opaque fluid intercepts each LED beam. Insert the tube assembly in
the original thermostat well. Disconnect or remove the original burner
thermostat. Depending on your choice of logic levels, a TTL AND  gate and a
TTL solid state relay will allow you to interface the new temperature sensor
assembly to your burner and fan control.

N.B.: If you are not exchanging heat between your flue gas exhaust pipe and
your return water flow, you are losing efficiency. A simple spiral of copper
tubing around the exhaust flue and some appropriate insulation could
significantly improve the system. This technique is always used in large
scale power plant boilers.

regards,
Dave

'Thou has past the double rocks and art in a foreign land'
    ....  Medea

dcorliss@ieee.org says...

Commonly referred to as an economizer. Wonder why?
--
Paul Montgomery
Progressive Gauging Inc.

David Corliss wrote:

   ...


Large-scale boiler installations use forced and induced draft, and so
can tolerate low flue temperatures.  Cooling the exhaust makes it
denser, to the chimney's efficiency as a pump declines.

   ...

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

says...

I had to read between the lines here to figure out your point which I
think is that cooling the stack gas in a simple natural drafted
firebox/stack setup by adding a heat exchanger in the stack causes the
problems of decreased natural draft and clogging the stack with things
like creosote. Olive kernels may not produce much condensates so the
second problem may not be. The first problem is seldom a problem in this
type of situation because there is almost always excess draft.

As an exercise in design possibilities this thread may be useful to us,
but to the originator who wants to decrease his electrical usage from
the auger without spending any money, it is useless. Changing the way
the controls work will not produce much savings. Since he stated that
the fuel is cheap, the only reason to increase efficiency is to lower
the amount of fuel used and hence the power requirements from the auger.
He needs instead to look at a motor that draws less current. The maker
most likely used a motor 10 to 100 times more powerful than needed to
prevent stalls. Replacing this with a smaller one and putting up with
some jamming seems appropriate to me in this case. A Gerber wheel should
be considered.

--
Paul Montgomery
Progressive Gauging Inc.

The situation here, as I understand it, uses a motorized fan which activates
at the same time as the feed screw. In  effect, this is a forced draft, and
there should be sufficient draft for the boiler operation. An improvement in
efficiency should occur if the return water flow can be heated by the
exhaust gas.

If the system here were strictly natural draft, there might be a reduced
exhaust gas velocity effect due to cooling. Whether this would be
significant or insignificant would depend upon the coupling efficiency of
the exhaust gas heat exchanger. In this application, I doubt whether it
would be of consequence.

.... good point though.

....

David Corliss wrote:

Paul Montgomery made another good point about the motor. It is surely
overpowered for normal operation, but using a motor that consumes less
power needn't make jams more likely. The motor is very likely a shaded
pole, and there's hardly a less efficient motor of commerce. Replacing
it with a PM motor and a switching power supply could save substantial
drain when the load is light.

Although fuel is cheap, feeding it is apparently not, Increasing the
efficiency of the heating system reduces the amount of fuel to be fed.

jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

says...

From what I understand this guy is not on the electrical grid and is
most likely getting power from solar, batteries and inverter. Switching
to a DC PM motor would also eliminate the loss in the inverter.
--
Paul Montgomery
Progressive Gauging Inc.

A. Paul Montgomery wrote:

A good boost-buck switcher can be about 90% efficient even at quarter
load and takes care of changing battery voltage. Running a 12V motor off
a fully charged (13.5V) lead-acid cell wastes more power than the
switcher consumes, and you still get 12V when the battery is down to 9.
It's an extra that can always be added later. The major benefit is from
the PM motor. With some sophistication, a brushless PM motor can combine
the regulation with the drive electronics. The more I think on on, the
niftier the possibilities become.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

True. I was talking about the lose from using an inverter and this does
not have the efficiency of a straight switcher because of the
transformer. It depends a lot on the quality of the inverter. I have a
little cheap one for my car that will drain the battery in two days with
no load connected.

I'm not for sure that running at 13.5 v will waste juice though. The
motor runs faster which will auger faster and thus shorten the run time.
I'll take your word for it though since I am too lazy right now to dig
out the efficiency curves for dc motors. If there is a slight gain in
power savings, I still would have to be convicted that the cost of the
switcher and the increased complexity is worth it.
--
Paul Montgomery
Progressive Gauging Inc.

A. Paul Montgomery wrote:

   ...


You may be right about the motor not staying on as long because it's
running faster. With a brush-type motor, copper- or silver-filled
brushes have slightly lower loss than pure carbon, and can probably beat
the semiconductor losses in a brushless design. Squeezing every last bit
of oomph out of a battery isn't something I'm accustomed to thinking about.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:

I would hesitate to use metal brushes in a high-current application,
though.  While I've never been able to get a straight answer from an app
engineer my understanding is that carbon brushes are more robust to
momentary overloads, which is what you'd get with simple control of a motor.

Using a switching amplifier to drive a motor would let you bring up the
motor voltage slowly, which would significantly reduce the current spike
at startup over using a relay.  If you didn't want to get a brushless
motor and controller you could also use a correctly built switching
amplifier to limit the motor current & hence be able to use metal brushes.

I'd just be crude and start with a PM brushed motor and a relay, then
improve things later.  This gets you going, will certainly show you if
you've improved efficiency and even lets you use inexpensive automotive
parts for motive power if you're feeling like a cheap skate.  If it
works you can eke out a bit more efficiency later.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Jerry Avins wrote:

Or, if the primary source of power from the batteries is 12VDC, just
using a 12VDC motor and switching it in would do.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com