Fan Logic RSLogix

ebolbol · Nov 11, 2014

If you had four 2-speed fans on Evap type coolers cooling a process (in this case natural gas), which method of control would you go with?

Would you take the four fans and put them in a 8 step matrix to step up and down as the process temp increases and decreases with dead bands? (i.e. Step one being 1st fan in slow and step eight being all 4 fans in fast?)

Or would you consider keeping one or two fans on (in slow) at all times to prevent temperature from rising too fast to have better control with other fans?

Currently we have the former method and the bottom line is that it's not working. Temp rises quickly, fans jump into action and bring the temp waaaay down and they all have to shut off and the cycle just repeats itself.

Or would you consider somehow implementing some kind of PID control? Which I don't know how you would with non VFD fans.

Tom Jenkins · Nov 11, 2014

I wouldn't use PID - your thermostat type control should work fine. You can easily set a minimum number/speed of fans to maintain operation, provided you don't have issues with too much cooling.

You probably need to change the set points for more fan, and perhaps have a bigger dead band between steps. You could also consider adding a time delay between steps, to permit the system to react to the additional air flow before calling for the next step. It appears to me you have a control responding more rapidly than the system response.

shawn_75 · Nov 11, 2014

Do you have anything showing fan performance? For example, are two slow fans equal to one fast fan? Without this information, you're just guessing.

ebolbol · Nov 11, 2014

Tom Jenkins said:
I wouldn't use PID - your thermostat type control should work fine. You can easily set a minimum number/speed of fans to maintain operation, provided you don't have issues with too much cooling.

You probably need to change the set points for more fan, and perhaps have a bigger dead band between steps. You could also consider adding a time delay between steps, to permit the system to react to the additional air flow before calling for the next step. It appears to me you have a control responding more rapidly than the system response.

Tom, Your assumption is correct. The fans respond very quickly. However, even with on/off delays, can't seem to grab a hold of it.

I have played with db's and set points as well as on/off delays to no avail.
The target gas temp is 95 deg.

I have a normal 30 sec on delay timer (if within 3 deg db), fast 20 sec on delay if target has been (exceeded by 5 deg db), and also off delay timer if negative 6 db of target. No enchilada.

ebolbol · Nov 11, 2014

shawn_75 said:
Do you have anything showing fan performance? For example, are two slow fans equal to one fast fan? Without this information, you're just guessing.

No sir. Unfortunately I don't have fan flow rates or efficiencies at the two speeds or any thermal load information. I would say two fans in slow would be slightly more than having only one in fast as a rough guess.

Tom Jenkins · Nov 11, 2014

ebolbol said:
Tom, Your assumption is correct. The fans respond very quickly. However, even with on/off delays, can't seem to grab a hold of it.

I have played with db's and set points as well as on/off delays to no avail.
The target gas temp is 95 deg.

I have a normal 30 sec on delay timer (if within 3 deg db), fast 20 sec on delay if target has been (exceeded by 5 deg db), and also off delay timer if negative 6 db of target. No enchilada.

Not knowing your system I'm only guessing, but these seem like very short time delays and very tight dead bands. Try doing some testing. Inhibit some of the fans and see how the temperature varies with time when the fans are operating steady state - that might give you a handle on the response times needed.

ebolbol · Nov 11, 2014

Tom Jenkins said:
Not knowing your system I'm only guessing, but these seem like very short time delays and very tight dead bands. Try doing some testing. Inhibit some of the fans and see how the temperature varies with time when the fans are operating steady state - that might give you a handle on the response times needed.

It's not a bad idea. Thanks for the thoughts. I was looking
for that (AHA!!!

) idea incase I was missing something. I think playing around with it is what it's going to take to come up with some control scheme.

shawn_75 · Nov 11, 2014

Like Tom said, look at steady state conditions. I'd plot how much heat is removed from the gas for as many fan/speed combos as you can. You'll get a set of performance curves that you can use to get your process under control.

ebolbol · Nov 12, 2014

shawn_75 said:
Like Tom said, look at steady state conditions. I'd plot how much heat is removed from the gas for as many fan/speed combos as you can. You'll get a set of performance curves that you can use to get your process under control.

Good idea. I think I will do that. Good approach. Thanks again.

dginbuffalo · Nov 12, 2014

Not knowing the exact layout, I would think you'd want to try to have extra fans turn on in slow vs. speeding up a single running fan. I think that would help you keep a good constant flow across the whole square footage of the coil. I would definitely do some trending on # of fans/speed/temperature drop to come up with a good game plan. Keep in mind things may change with delta T based on outside air temp. Hopefully it's in California unlike here where it may be 90F or 10F!

ebolbol · Nov 12, 2014

dginbuffalo said:
Not knowing the exact layout, I would think you'd want to try to have extra fans turn on in slow vs. speeding up a single running fan. I think that would help you keep a good constant flow across the whole square footage of the coil. I would definitely do some trending on # of fans/speed/temperature drop to come up with a good game plan. Keep in mind things may change with delta T based on outside air temp. Hopefully it's in California unlike here where it may be 90F or 10F!

dginbuffalo,

Yeah, temperature fluctuations play a huge role. Like right now we're in the 60's but summertime, we can see temps as high as 115. Evap circulating pumps also play a role but they don't come on til 85 deg amb.

I'm sure there is a crafty magic formula to keep that gas temp @95 deg's +/- 5.... I just haven't found it.

The other day, ambient was 83 deg's, I watched the gas temp rise to 115, I saw the fans jump into action and knock that temp down to 76 deg's before shutting off again and the cycle just repeats itself.

I just have to control the fans to control the PV.

cjd1965 · Nov 12, 2014

Hi
I think you need to do some tests like others have said already. I would want to see the effect of 1 fan on slow...does it even cool? I think you need to work out cooling rate over time and then decide what fans are needed to achieve it

If 1 fan is on slow for 1 minute what effect is that? Does it cool at all? Try 2 on slow etc

Find the point where cooling is beginning to work and make that a baseline (for example 2 on slow does not cool but 3 on slow does... make 2 on slow a normal state.

You may wish to consider a duty/standby in this situation so that the 2 running all the time don't get worn out much faster than the rest

What tolerance are you hoping to get?

Tom Jenkins · Nov 12, 2014

One thing to bear in mind is that the heat transfer may not be linear. You may want to consider different time delays at different numbers of fans operating. Also, taking fans off line won't have the same impact as adding them - you could consider different tuning going up and down. The same applies with the pumps on or off - different tuning may be needed for each.

GTUnit · Nov 12, 2014

First thing I would do is check to see if the Fan Motors will handle a VFD reliably.
If so, put VFDs on the fans. Dont try to force program around a design limitation.

Sometimes this is like trying to balance yourself standing in the middle of a See-Saw while drinking a cup of coffee.

Programmers have a habit of looking at the program only.
Look at the system as a whole.
That being said I would keep one fan on all the time and 7 step the others.
Slow speeds first and then the 4 fast speeds.
You have many variables in play and like Jenkins mentioned I think the chances of this being linear are slim to none.

ebolbol · Nov 13, 2014

cjd1965 said:
Hi
I think you need to do some tests like others have said already. I would want to see the effect of 1 fan on slow...does it even cool? I think you need to work out cooling rate over time and then decide what fans are needed to achieve it

If 1 fan is on slow for 1 minute what effect is that? Does it cool at all? Try 2 on slow etc

Find the point where cooling is beginning to work and make that a baseline (for example 2 on slow does not cool but 3 on slow does... make 2 on slow a normal state.

You may wish to consider a duty/standby in this situation so that the 2 running all the time don't get worn out much faster than the rest

What tolerance are you hoping to get?

That is a great point and a good place to start. I will have to place the fans in manual mode. Wait till the gas temp rises to a 115 deg's and put on one fan in slow and wait to see what affect it has on the temp and so on like you said... and agreed on the ring logic to rotate duty... If I can maintain temp within 2 to 3 deg's of setpoint, I'm home. Never to exceed 99 degs.

Fan Logic RSLogix

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