Compressor's Control Sequence?

[crab master]

Scenerio - AHU with 2 Compressors each with 2 electric unloaders - DX. What is the best way to stage all this on and why? Do you stage on Compressor 1 as first stage, compressor 2 as second stage, Compressor 1 unloader 1 as 3rd stage, Compressor 2 unloader 1 as 4th Stage, Compressor 1 unloader 2 as 5 stage and Compressor 2 unloader 2 as the 6th stage? This method I see as helping to prevent short cycling a compressor/longer run times on compressors vs. what I'll call Sequential staging - C1 stg 1, C1 U1 stg 2, C1 U2 stg 3, C2 stg 4, ...C2 U2 stg 6. I am also wondering if Sequential Staging wouldn't be better if you do a lead/lag setup on the compressors just to shorten the overall cycling of the compressors with a longer delay on stg 4. I would also think that with startup amps for the compressor it would be more economical to simply load up another head vs. starting a compressor. By the way this job is in the Northwest, Washington state - East side, so low humidity, and 60% plus of the time this part of the building would get by fine with one compressor at full capacity.

[Edited by crab master on 06-26-2005 at 02:35 AM]

[sysint]

Check the efficiency curves of the equipment and make your decision.

[dapper]

This may be similar to what sysint said but........ I would want to know the capacity of an unloaded compressor in relation to each loaded stage. I would be inclined to think like you that it doesnt make sense to start compressor #2 as the second stage but rather load up compressor #1 at least one step first.

[sysint]

I've had old Trane rooftops unloaded running 11.5 seer vs. 9 loaded. So, you would with those units start stage 2 rather than load up 1.

[crab master]

Okay that makes sense. They are mid 80's Carrier split systems so I will try to find documentation. Thanks!

[dapper]

sysint.. not that I doubt your expertise but.... I would have to wonder how 2 compressors at 11.5 SEER compares to one at a 9 SEER. I guess I just need a better explaination of SEER. I can see why an unloaded compressor would be higher. You have more condenser capacity for the load. After all isnt that how most manufacturers increase their SEER, by increasing the size of the condenser?

[smokies]

Also keep in mind the configuration of the refrigerant piping and the set-up of the evap coil. Other than energy savings we need to maintain proper oil return, coil condensation, and prevent air bypassing. The configuration of you particular unit may effect the sequence. Just a thought.

Good Luck

[lonboy]

Crabster,
If I remember right, the old Carrier step controllers staged the cooling as follows,
Stage 1 - Compressor1 runs unloaded
Stage 2 - Compressor1 1st unloader on
Stage 3 - Compressor1 2nd unloader on
Stage 4 - Compressor1 runs unloaded and Compressor2 runs unloaded

Stage 4 - Compressor1 1st unloader on, Compressor2 unloaded
Stage 5 - Compressor1 2nd unloader on, Compressor2 unloaded
Stage 6 - Compressor1 both unloaders on, Compressor2 1st unloader on
Stage 7 - Compressor 1 both unloaders on, Compressor2 both unloaders on

For oil return, I'd monitor compressor suction pressures (at the crankcase) and begin disabling unloaders as pressure drops below 62psi (R-22). I've also had good luck in the past of using suction pressure alone to control the unloaders as the low side pressure is the best indication of load and this ensures that you'll have the proper velocity to get the oil back (as long as the piping is correct).

Let us know what you did and if we can monitor the system operation from your iLON100 web page!

Cheers,
lb

[grgjohno]

Very true Smokies, a complete understanding of the mechanical setup is critical to getting the control system to function CORRECTLY. What may seem logical and in principal a correct control sequence, once implemented on site simply doesn't work as it should.
I've just completed a Siemens S7 program for a system we've designed with two staged Mcquay screw compressors, pulsed loading /unloading, twin electronic EXV's 4 row dual header evaps on a single AHU with VSD, with a single condenser with 6 fans all staged all safeties are done through the PLC. The program also controls the fire dampers and is intended to still operate under various fire zone pressure conditions.
I FAT'ed the system in Paris and it seems OK in principal, but the proof will be when I commission the thing in Korea next month.
So I agree with everyone elses comments, however be prepared to modify it, at the commissioning stage.

All the best

[crab master]

Thanks to all for the replies!

I am always looking for a better way to do something, but I also have to have enough evidence to "make it better." This is an old system with realitively minor refrigerant problems over the years so I am going to leave the sequence as it was.

grgjohno - good points. I've been a tech for 7 plus years - not that I know it all, but long enough to see what may work in some/most applications doesn't work in all so a thorough check out is a must! Let us know how Korea goes. BTW - what does "FAT'ed" mean?

Smokies - good points as well but I am confused on what you mean by prevent air bypassing. Are you talking static pressure control if you have a damper that relieves into the return?

LonBoy - the control sequence is basically as you stated and I will be just taking over the relays on the W7100. If I was ahead enough cost wise I'd add a low side crankcase pressure monitor as I really like the point of disabling/enabling unloaders according to crankcase pressure.
You know I did call Dave and ask about the iLon100 after it came out - was trying to get a sample/trial period but no luck. I mainly wanted to compare it to the TAC 511 and to see if I could use a iLon100 take over what I am doing with the 511. I did buy an ilon10 that I plan on setting up at my house.
That would be cool to give out the web address(es) to see how others set stuff up. Too bad their are so many hackers.
Thanks again for the training cause I am loving the work!

[smokies]

crabmaster -
When I mentioned air bypassing I was revering to large systems when multiple circuits where you sometimes must add dampers to the part of the evap coil that is not being used. In some events you can have air bypassing the coil that is actually cooling since air,water, and electicity all follow the path of least resistance. Its only on larger systems that you would see this but it can cause alot of headaches.

[crab master]

Okay Smokies - now I understand. Thanks.

[joey791]

For maximum efficiency though shouldnt one compressor start, load up fully, then start the 2nd one? This is the way the W7100 and 7900 work

[mdp]

From the mechanical side of the story, yes the first compressor should be fully loaded before bringing on the second compressor. Running the compressor unloaded to long can cause oil return problems. I prefer to see pressure sensors on the compressor to control unloading.