PLC/ PID for hybrid water/air blower control?

[pybyr]

hello again to the wonderful "hive mind' of the boiler room

as those of you who are following my [mis]adventures know, I am deploying an Econoburn with a really big unpressurized heat storage tank that will have 8" of surplus polyiso underneath, a foot on each side, and a foot or more on top (the "BIG tank").

All that, via primary/ secondary pumping will be hooked to a ridiciulously large water to air HX coil

that "BIG coil" will sit on top of my existing ThermoPride oil warm air furnace, which has a wonderful high volume, low velocity belt-drive blower in it, and the same blower (in the ThermoPride) will push the air through the BIG coil into my preexisting ducts

the thermostat will call for heat and turn on the secondary circulator to the BIG coil, plus the appropriate primary circulator and circs from the boiler or tank, whichever is hotter.

then, when the hot water gets moving to where the outlet of the BIG coil is hot, I'll want to kick the blower on.

Originally, I was thinking of a thermal "snap switch" to do that

now, am thinking of a PLC and thermocouple put into one of the spare 1/2 npt female joints on the water return side of the BIG coil that's now occupied by a plug

that way I can vary the parameters, and maybe someday even have a variable output from the PLC control a VFD for a 3 phase motor or a stepped multi-speed motor, so that the motor follows the heat, to maximize how much heat I can pull from the tank, through the BIG coil, even as the tank drops to 120 or even 100

[slower air flow through a cooler coil might still pull useful BTUs from a cooling tank, with less overall electricity used]

anyone have any suggestions on vendors of good PLCs and fittings for the associated thermocouples to go into a 1/2 inch female NPT?

Or am I just over-thinking this?

thanks

Trevor

 

[Redox]

You are overthinking this, but I'll play along!

We use Johnson Controls A350 controllers on our drives for a good stand alone controller. It has an analog output and lets you set the deadband. It also accepts plug in relays that allow for other functions and best of all is quite inexpensive for what it does.

Many VFDs have a PID function built into them; just add a sensor and away you go. Our drives (ABB ACH550) have two independent PID loops that can control other things as well. They aren't cheap, but the neato factor is priceless!

Chris

 

[deerefanatic]

I have one of the Coldfusionx controls set up so that when the thermocouple (laying on top of the coil surface) reaches 120F, it cloeses one of the programmable alarm relays which connects the R & G terminals of my furnace, which are the same terminals that your AC unit connects. These terminals cause the furnace blower only to come on. Then, the blower runs until the thermostat says it's warm enough... at that time, the pump turns off and the blower continues to run and extract heat from the coil until it reaches 80F........ Then it shuts down.

 

[steam man]

I have a few Siemen's 353 multiloop controllers with relay expansion boards. Digital in/out, analog in/out. Takes thermocouples, RTD's. They have incredible power but are usually found fairly pricey. I did see one on ebay some time ago for about 100 bucks. I couldn't believe it. They can do anything you are thinking of.

Mike