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Author Topic: Why is it so ?  (Read 2400 times)

mycuppa

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Why is it so ?
« Topic Start: 24/03/2012, 10:56 PM »
Alas, the one roasting mystery that eludes me has become a frustrating quest in search for the answer.

It has to do with commercial roasters and why they roast slower when it's hot and faster when it's cold.
 
Common sense makes you think otherwise, but all my testing and fine tuning of the optimizer on my profiler by using an input variable for ambient temp offset just means I end up chasing my tail in trying to predict and forecast the absolute accurate deviation for temp and time for a given ambient temp and roast batch type.

Whilst that statement did not come out very clear - it's like this..........

I measure the current ambient temp when I am roasting. We have the roaster in an insulated room that is opened for natural ventilation - so it stays relatively constant once we commence roasting.

On warm days - and I consider 25 degrees to be a warm day - the roaster can actually run around 10 seconds longer on batches. This is counter to what your brain should be telling you - the higher ambient means the green beans are warmer before being injected into the roasting chamber, as is the make-up air on the gas burner (although that makes little difference to combustion).

The resultant roast is to specification (level, final temp, etc.) and considerable cuppings at various ambient temps on a batch we run almost daily indicates it makes absolutely zero discernible difference when the roaster runs a few seconds longer on warmer days

On cooler days - say sub-20 degrees, the roaster actually runs faster and can finish anywhere from 1 to 10 seconds earlier. You are literally talking about a few seconds over 15 mins, or 1.1 % deviation.

I am in the habit of making a slight compensation on the profiles because I want it perfect every time.

Where is Professor Julius Sumner-Miller when you need him ?

One theory I have been toying with is the actual rate of change is slowed when it is warmer - that is, the dry-out phase and turning point are reached a little sooner and the roaster "backs off" somewhat, thus slowing the rate rate of temp increase and therefore thermal momentum is effectively slower over set time intervals.

All things being equal it may possibly work in reverse on cold days, with the roaster charging at the roast hard and therefore building up thermal inertia.

Why I am puzzled by this phenomenon is because the roast is never running at full energy for the entire batch cycle - it has heaps of power in reserve and only runs the burner at intervals (more bursts at the early phase, less or not at all in the latter stages). So it's not like it can't cope.

Given that condition, you would think that ambient temp means very little at all because the roaster is controlling the air and bean temp throughout the batch cycle and the outside air cannot affect what is going on inside the drum.

I have thought about the Carnot Cycle and I also wondered about the effects of gas pressure at higher temps affecting adiabatic heating/cooling.

I realize for home roasters the ambient temp plays a larger part in roast batch variation as there are more exposure and less energy being applied. For commercial roasters I would have thought ambient temp would not matter.

One day I will discover the reason.
 



Brett H

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Re: Why is it so ?
« Reply #1: 24/03/2012, 11:14 PM »
Wow Jeff... That is some serious academia right there.  Just for the sake of interest how does the humidity play out in regards to your climate down there?
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mycuppa

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Re: Why is it so ?
« Reply #2: 24/03/2012, 11:17 PM »
Wow Jeff... That is some serious academia right there.  Just for the sake of interest how does the humidity play out in regards to your climate down there?

Thanks Brett. The humidity does not affect much at all.

A week ago we had a many days of Brisbane-like tropical humidity and the most painful thing was cooling the beans and of course running out of Pepsi and beer at night.

3rd Crack

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Re: Why is it so ?
« Reply #3: 24/03/2012, 11:41 PM »
I believe it is to do with the vaporization front.

puff goes the magic dragon ..  8)

Beanz

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Re: Why is it so ?
« Reply #4: 24/03/2012, 11:59 PM »
It is a long time since I thought about this but I wonder if it is the inbuilt hysteresis / on-off differential built into the temperature control.
On the hotter days as it approaches the set point the enegy input is backed off to avoid overshooting the set temperature.
On the colder days the energy input is higher and it does not back off the energy input at the same point as it approaches the set point because it sees a larger temperature difference between ambient and set point and consequently coasts in a few seconds earlier.
I am sure if you Google Hysteresis there will be plenty of better explanations and some graphs. Hysteresis is built into control circuits to stop systems constantly switching on and off as they cycle above and below the set temperature, effectively a built in lag to slow the response.
The observations you note seem to fit

Kelsey

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Re: Why is it so ?
« Reply #5: 25/03/2012, 07:35 AM »
I'm not sure the phenomenon is limited to commercial roasters.

The last few roasts I did prior to last night were in the hottest parts of summer took  noticably longer than they had previously, the one in the cool weather last night was shorter.
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RichardM

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Re: Why is it so ?
« Reply #6: 25/03/2012, 09:23 AM »
Don't discount humidity. Remember as temperature increases two relevant things happen:

1) The capacity of the air to hold moisture increases
2) The density of the air decreases.

Is it possible that the density of the incoming "cool" (ambient) air having a lower density reduces the cooling effect (though I'd think it would be *exactly* counteracted by the temperature variance).

I dunno Jeff, you're the engineer  :D
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mycuppa

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Re: Why is it so ?
« Reply #7: 25/03/2012, 10:49 AM »
Thanks Beanz - yes I had already thought about hysteresis (from my early days in the BHP steelworks).

I don't think it comes into play so much. There is a natural hysteresis exhibited by the energy control due to the time lags for burner on/off - it is necessary to allow for a air purge sweep each time before the main valve on the burner is fully opened (to prevent gas buildup and explosion).

The thermal measuring points are also enclosed away from the ambient air being in the drum and the exhaust air pipe.

RM, your thoughts on humidity might be partially relevant.......the heated air passing through the roaster you would think might dry out added moisture pretty quickly - maybe, who knows. I also noticed that the phenomenon occurs on hot dry days where humidity is quite low.

K - nice to know I'm not alone  :thumb:

Koffee Kosmo

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Re: Why is it so ?
« Reply #8: 25/03/2012, 11:14 AM »
I cant put my finger on it but?
I have a feeling its due to a combination of charged particles, air pressure (as in high or low )  and magnetic forces in the air
This is caused by weather - and pressure changes
Its funny how something that can weigh millions of tons can float - ie clouds

These things you cant see and your roaster cant measure without the correct instruments

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mycuppa

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Re: Why is it so ?
« Reply #9: 25/03/2012, 11:39 AM »
Entirely possible KK.

We are talking about around 1% deviation (or a tad +/-), so when it's getting that small it can be literally anything as simple as the temp stability before the greens are loaded and whether the burner comes on 10 or 15 seconds earlier in the roast cycle as it works against the temp drop due to the fresh greens being injected into the chamber.

As Seasonal Blends are becoming boring and passe, perhaps I need a Lunar or Quantum Blend.........yep, that's what I will do, create the Quantum Lunar Blend today.



Fresh Coffee

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Re: Why is it so ?
« Reply #10: 25/03/2012, 02:28 PM »
Different design coffee roasting plant, different behaviours  ;)

If you are recycling hot air and have the capability to change the point at which the recycle cycle is shut off (as in where you stop recycling and bring in only 100% ambient air instead) , try changing the recycle cut off point to a few different set points and see what happens & if that affects anything, significantly, marginally, or otherwise.

My equipment is also very well insulated and enclosed nevertheless I am always making adjustments dependent on the prevailing weather, and have never really gone into the "why's", just the "arh harhs" of it all! Learn something new every day, especially with the very very different type of weather we've had this summer. which resulted in my making quite some changes to compensate for what I want in the roast. All good!  This is the difference between having higher end 21st century equipment that can be "manipulated" according to the whims of the in house master of coffee, and old technology roasting plant where you cant do anything except vary inflow of gas to the burners and air flow through the flue.

I expect those that are using old technology / budget type equipment with little control will have been having a very interesting summer, make no mistake.

Brett H

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Re: Why is it so ?
« Reply #11: 25/03/2012, 03:21 PM »
I think the big positive to come out of this thread is the Quantum Lunar Blend! Sounds like it might be time to place an order  :thumb:
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