Month: July 2014
What’s your “Dead Time”?
Imagine you are at your dryer and you have just taken a moisture sample.
The corn tests at 14.2% in your bench top, but you want 15.2% coming out of the dryer. What do you do?
Let’s say you decide to increase the metering roll speed (discharge rate). You make the change, but right away two thoughts strike you. First did you make a big enough change, or maybe too big of a change, and second, just how long will it be until you know the answer to your first question.
Just like there is a lag between when you turn on the shower and when you actually get hot water there is a lag between when you make a change to the metering roll speed and when you see the impact of that change. This is what is called “dead time”.
If your dryer has a cooling zone, then the absolute minimum dead time before you see any change in your outlet moisture is the time it takes for the grain to get from the bottom of the hot zone to where you take your moisture sample. This is because the grain that was already in the cooling zone will not see any change in moisture because of your rate change. In reality though you should be looking at up to a full dryer load before you see the full impact of your change. Only then can you know if you made the right change.
So, if your dryer has only a 60 minute residence time and a cooling zone, you are probably still looking at about 50 minutes to see if the rate change was correct. If you have a rack style dryer with say a 3 hour residence time then you could be looking at well over 2 hours to see the impact of your change.
Let’s use as an example the sample dryer at the right with a 3 hour residence time – 2 1/2 hours in the hot zone and 1/2 an hour in the cooling zone. If you make a discharge rate change the grain in the cooling zone will not be impacted by the change. The final moisture of this grain has already been set at this point. The grain in the bottom half of the hot zone is also unlikely to see the full impact of the change. Again in our example, by the end of the hot zone if the grain is over dried it can not be undried at this point. Therefore it is likely that to see an impact from the rate change the grain will have to at least be in the top half of the hot zone. That means you would have to wait at least 2 hours for it to exit the dryer before you might start to know if your rate change decision was correct. That’s why it is called “dead time” – time you have to wait not knowing if the right decision was made, and it is one of the reasons grain drying is so tough.
It is no wonder that so many dryer operators prefer not to make too many rate changes, and prefer to err on the side of caution (over drying a bit). If they make the wrong decision it can take up to a dryer load to get things back in order.
In our next post we will look at how Dryer Master handles the “dead time” problem to help users take the guesswork out of drying their grain.
This entry was posted in Control Technology, Drying, Grain Drying, Uncategorized and tagged dead time, drying control, grain drying.
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