Outdoor temperature compensation for silo heating

[RonJohn]

I have a pair of outdoor uninsulated water silos that need to be maintained at or above 145°F. Both are supplied by the same source, which includes a heat exchanger for pre-heating. That same heat exchanger can be used for recirculating one silo at a time. This is a new piping arrangement to coincide with the temperature requirement.

We've had some cold nights around here as winter's approaching and my silo temps haven't been able to keep up with the outdoor temperature drop. It only got down to the 20s so far and last winter we hit just south of -20°F once. Does anybody know of a good formula/correction factor to compensate for outdoor temperature in such a situation?

I'm using Peter's SMC algorithm to control the steam and it's worked great for months, so I don't think that's an issue. It seems like I simply need to be more proactive in accounting for the cooler nights.

Thanks in advance.

 

[Tom Jenkins]

I no longer have the formulas handy, but if you look at some HVAC resources for optimized stop/start it should give you some factors. If memory serves me right the function is non-linear, but for your purposes you may be able to linearize it and get adequate results.

To tell the truth, unless the steam is free I'd think insulation would be pretty cost effective!

 

[RonJohn]

Thanks Tom, I'll see what I can look up.

To tell the truth, unless the steam is free I'd think insulation would be pretty cost effective!

Good thought! Somebody else couldn't justify the cost of an insulated silo (almost double, I'm told). Not sure how it'd work to insulate a silo after the fact. Maybe we could just put a "stocking cap" on top...

 

[dginbuffalo]

I have a pair of outdoor uninsulated water silos that need to be maintained at or above 145°F. Both are supplied by the same source, which includes a heat exchanger for pre-heating. That same heat exchanger can be used for recirculating one silo at a time. This is a new piping arrangement to coincide with the temperature requirement.

We've had some cold nights around here as winter's approaching and my silo temps haven't been able to keep up with the outdoor temperature drop. It only got down to the 20s so far and last winter we hit just south of -20°F once. Does anybody know of a good formula/correction factor to compensate for outdoor temperature in such a situation?

I'm using Peter's SMC algorithm to control the steam and it's worked great for months, so I don't think that's an issue. It seems like I simply need to be more proactive in accounting for the cooler nights.

Thanks in advance.

This sounds interesting, although I just breezed through the link that you provided regarding the control. So I'm guessing your using steam with an on/off valve vs. a modulating valve?

 

[RonJohn]

That's correct, DG. We didn't have any analog outs available on this PLC, so SMC has worked well, although I had to fudge the time constant.

A cursory search turned up one webpage that hinted at a 1°F setpoint increase for every 7°F outdoor temperature decrease with no supporting math. This is more than double what I'm currently doing. I'm finding lots of other sites that talk about how modern day HVAC controls have this optimization built in, but no factor/algorithm that I can follow.

 

[kamenges]

Where are you reading your fluid temperature?

 

[Aabeck]

For insulating existing structure, check into spray-on foam insulation. It's similar to the Great Stuff cans hardware stores sell, but applied from a spray system. Had this done to a outbuilding & it made a big difference & wasn't cost prohibitive.

http://www.msgreenconstruction.com/services/insulation/spray-foam-insulation.html

 

[RonJohn]

Good question. Each silo has a temperature transmitter mounted near the bottom of the silo.

Additionally, the supply pipe comes in about 15 feet, which is roughly half way up the silo wall. Since heat rises, I do tend to see an overshoot once the temperature catches back up. We're talking 30k gallon silos so the water is hanging around in there for a while as we try to keep them at least half full.

This is not the ideal situation but if it were easy, anybody could do it - right?