SCR heating, DX cooling...

[davem]

For a research lab laser room, I'm asked to control an oversized DX cooling unit and an SCR-controlled duct heater. The room is only 320 square feet, but the required air changes per hour, and the expected heat load of the equipment, requires a 4-ton cooling unit (unfortunately single-stage), with a 12KW duct heater modulated with a 0-10V signal. Given that I'm supposed to control the room temperature +/- 1F, I can tell I'll have to use the heater to temper the air when the room is calling for cooling (45F blasts of cold air into the room will make maintaining +/- 1F a tough chore...)
However, since most controllers (I'm thinking of using a Johnson product, given the building's existing controls network...) make a point of separating the cooling and heating controls, and never shall the two meet, I need some help configuring a control to include some sort of tempering using the heater, even in cooling mode. I have flexibility in using whatever sensors I need (duct sensor before the heater? after the heater? whatever I need...), but I can't seem to find an HVACpro configuration to fit this particular sequence. I'd use a DX9100, if I had a clear idea of how to sequence the heater in heating and cooling modes.
The mechanical company looking for my ideas on this is not real crazy about the notion of keeping the compressor permanently energized while I simply modulate the heater to maintain room temperature, so if there's a single-stage cooling with modulating heat sequence that has worked for someone, I'd love to hear it.

[controldude]

We have done this many times with reznor mua's with dx and modulating gas ( maxitrol valves w/ a-200 amplifiers one per gas section ) but we would use sporlan electronic hgbp board with electronic evap pressure regulator valve, w/short loop hot gas and desuperheating valve
since you have scr's this type of control should work even better. any time oa temp > 68deg adj. start cooling, ddc loop to maintain dat of 68deg adj. the heating sections we would ramp too 100% then reset to 0% before enableing next section and ramp both sections together. hope this gives you some ideas

[sonc]

I've had good luck with these little guys, and pretty cheap too. The anolog output is 4-20ma, but you can throw in a 500 ohm resistor for the heater input.

[sonc]

I've had good luck with these little guys, and pretty cheap too. The anolog output is 4-20ma, but you can throw in a 500 ohm resistor for the heater input.

http://www.process-controls.com/Alph...ontroller.html

[sonc]

I've had good luck with these little guys, and pretty cheap too. The anolog output is 4-20ma, but you can throw in a 500 ohm resistor for the heater input.

http://www.process-controls.com/Alph...ontroller.html

[sonc]

Can someone tell me how the fock to delete a post? I give up

[drrtu]

You may want to consider a de-humidification loop. You may end up with a room that runs at 60% RH. I have done a few QC labs for the injection molding industry. We would size the unit based upon air changes to get the room down to class 100,000 clean. At the large flowrate 2 to 8 CFM/ft the coil is way over sized and the unit would short cycle. You can use a Johnson AHU controller (it has a de-humd algorithm) but your swings will be greater than +1 /-1. If you stick with a CV terminal re-heat design, you can nail the temperature and also control RH.

[davem]

Guys, thanks for the replies. The dehum routine looks to be just the ticket - easy to implement with a UNT, fairly simple to tune (given that the humidity is not a critical loop anyway), and allows the compressor and heater to be enabled simultaneously. Many thanks, drrtu.

[crab master]

Sonc - I could not figure out how to delete a post either - but remember you can go back in and edited your original post if you screwed up/forgot to mention something. I would be interested in hearing if anyone else was able to delete a post because I screwed up and hit submit two or three times thinking something locked up only to see two/three posts all saying the same thing.

"The mechanical company looking for my ideas on this is not real crazy about the notion of keeping the compressor permanently energized while I simply modulate the heater to maintain room temperature" - ask the mechanical company if they would rather have the compressor running continously or short cycling like mad? Explain to them the compressor does not care what the supply temperature is only that the return temp is not too low and that you are only further tempering the air so it does not overshoot due to such tight tolerances.