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Tunnel Heaters

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 06 December, 2011 » Tunnel Heaters « Previous Next »

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muskrat
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Username: muskrat

Post Number: 403
Registered: 06-2009


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Posted on Saturday, 26 November, 2011 - 06:06 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

First: My excuse and disclaimer.....
Due to the horrific exchange rate we are experiencing, imported products are very, very expensive.
Farmers have an enviable reputation in this country for being accomplished 'bush mechanics'.
Now on to the actual topic ....

I want to fabricate heaters to keep the greenhouse tunnels warm in winter. Remember, we are Southern Hemisphere, so I have a few months to get this right.

I envisage a pipe with a fan at the back end, and a heater element at the front.
The mechanics of the device are easy.
I need to collective minds of the CZ to assist with the controller.
Ideally, it would be a proportional controller, rather than a bang-bang type. Probably a modified light-dimmer type circuit controlled from a temperature sensor.
I can handle both a simple analogue circuit or a PIC-based solution.
I need to stabilise the night-time winter temperatures at between 18 and 22 degrees Celsius. We get enough sun in winter to keep the temperatures high enough during the day.
Any help, suggestions, or, of course, complete design solutions would be most welcome.
Thanks in advance.
Brian
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bruce
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Username: bruce

Post Number: 711
Registered: 04-2008

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Posted on Saturday, 26 November, 2011 - 07:39 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello,
Good luck with that. I doubt I'll be able to furnish you with the kind of assistance you require, but I'm sure others will.
However, I did want to query your initial assumption that the controller must be a 'light-dimmer' type and not 'bang-bang'.
Possibly I'm not understanding your requirements , in which case, apologies. But the 'bang-bang' type ( by which I assume you're referring to the zero-crossing, pulse-width type, of heater control ) while useless for lighting, is far better when you need to control heating.
Would not a simple thermistor, to sense the air temperature, be all you needed to turn the heater on when required. Unlike lighting, which needs very fast control, heating has a large 'inertia' and something as slow as a relay is good enough.

bruce
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joe
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Username: joe

Post Number: 1215
Registered: 05-2005

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Posted on Saturday, 26 November, 2011 - 07:50 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Brian,

The only reason I can see for you suggesting trying to vary the power is to prevent sudden on/off spikes and loads on the mains - and I don't know if this is an issue for you or not. Also, your mains circuits may not be able to handle several heaters all switching on a full load at the same time. If so, then I can understand why attempting to get the heaters to a certain percentage of their output may be a good thing; or at least, cranking them up slowly. I did this recently with my evaporator unit. The trick was to creep up on the required temp and as you get with a certain percentage of the required temp, start turning down the power so as not over-shoot.

However, if your mains can handle the switching loads, then I'd agree with bruce on this...

KISS applies I think !!

If you don't like the relay idea, you could use TRIACs.


Regards,
Joe
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muskrat
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Username: muskrat

Post Number: 404
Registered: 06-2009


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Posted on Saturday, 26 November, 2011 - 08:47 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I am a firm believer in the KISS principle, but I am possibly over-complicating my specs here.
The tunnels are 30m x 40m x 5m high - quite a large volume!

I was concerned that there would be little hysteresis and the bang-bang approach would lead to excessive cycling.

Mains-borne transients are a concern (Joe - you know the sort of level of home-built electronics on the farm!), but these could most likely be filtered out with a bit of care.

I would certainly incorporate some sort of cycling controller so that all heaters do not come on at the same time.

Until my prototype is built, I have no idea as to how many heaters I'll need.

I also need to determine a position in the tunnel for the sensor(s) that will give a good indication of the overall tunnel temperature.

Who said farming was easy?

Thanks Bruce and Joe.

I'll get a prototype heater made up, then I'll play around with some sort of controller.

KISS indicates that I should start with a bang-bang (sound faintly obscene!) and if it doesn't work as expected, move on to a proportional controller.
Thanks again
Brian
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dselec
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Username: dselec

Post Number: 314
Registered: 11-2010


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Posted on Saturday, 26 November, 2011 - 11:15 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

In short u need to feed the temp sensor to a pwm (build with 4011 pulse gen with potentiometer and diode to get diff.duty cycle)that will in turn feed your dimmer ("a modified light-dimmer type circuit controlled from a temperature sensor").

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