Trane DX twin system

[barty]

Just bouncing idea's

We have a customer with 3 x 80 ton dx condensers each with 2 systems. The unit has 6 cond fans which are all on and the head is controlled by those dampers (3 on each circuit). Now this isn't too sucessfull + the temp stat which cuts of some of the fan contactors (two fans per contactor) have on all 3 condensers been wired out (assume some form of HP control trouble led to this?)

Now given that there are 6 fans and two independant circuits

What form of head pressure control would you recommend as these are for a server room it's 24/7 etc and they just love to ice when the cold lowers the hp which drops the refrigerant flow.

Cheers

Richard

[berg2666]

How about 4 fan cycling switches, the ones that run off the hiside pressure, that control the fan contactors.

Just an option that will be inexpensive. Rancos are about 60 bucks each here...

I can probably think of a few others with some time.

[vbhvac]

It sounds like in cold ambient fans with dampers should be running and in warmer fans without dampers should be added. This additional switches are they field installed? If so - first of all remove them and check what kind of head pressure unit is actually maintaining - you may have problems with this dampers.

[esdd]

How about just fixing the controlls back to the way they were ment to be, the factory fan stepping and damper arangement works pretty daggone well when set-up correctly.

[barty]

How about 4 fan cycling switches, the ones that run off the hiside pressure, that control the fan contactors.

Just an option that will be inexpensive. Rancos are about 60 bucks each here...

I can probably think of a few others with some time.

That assumes that if circuit A only is running then the 3 fans on that side will be sufficient to maintain H.P. ?

Cheers

Richard

[barty]

How about just fixing the controlls back to the way they were ment to be, the factory fan stepping and damper arangement works pretty daggone well when set-up correctly.


The units were installed in '98 we first saw them in Dec 06. So a lot of water has passed under the bridge..I simply have no idea why they were disconnected and this place is SUPER CRITICAL to the company -and millions of mobile phone users too!!

I worry that we re-instate the stat git-r-done (most of the dampers are in place still) then find that there was a good reason for removing them and I'm facing a court case for HUGE losses!!

I'm going back soon so will attempt to see if there is any history on site..


Cheers

Richard

[stanbyyourword]

commision back to factory specs or my preference would be vfd with transducers 1 per circuit

[absrbrtek]

What is the model number of the condenser? What is the pressure the damper actuaters are rated for? Are the fans set up 3 per circuit?

[jogas]

IMHO, condenser fan damper control (Robertshaw) is just a leak waiting to happen (at the damper shaft seal) and you lose refrigerant. Also, it just doesn't work at low ambients.
We had the same situation with two older single (hi side) circuit trane 75 and 100 ton condensing units,each with 3 large fans. We installed a VFD on each that feeds all 3 fan motors. Each motor has it's own fuse block. A transducer at the king valve controls head.
We used the VFD alarm contact in the Reset relay circuit so we don't have to rely on the HPC when there is a VFD problem.
This setup has worked flawlessly since installed.
jogas

[barty]

The low ambient problem is one that worries me. These are prone to icing the coils due to the head dipping down below 200 psig. I would love to get it rock solid at 250 -slightly higher than normal perhaps but I'm sure it would make all the difference.

BTW We have a new damper waiting to be fitted so I agree with the 'leak waiting...' part. Only one of the three has all six in place still working and at $600 each + fitting they are an expensive option.

The customer is wanting to nurse these units for another 1-2 years before a complete swap out. My recommendation was change one unit (keeping the old on site for spares) then see what happens but given the speed of the market in mobile technology they don't want to spend $50k if the whole system becomes unnecessary in that time.

I like the VFD idea but I'm guessing it would be beyond their price range now...

I may re-instate one of the stats whilst I'm on site next just to see what happens...

Cheers

Richard

[onetime]

There may be no fix with fans and dampers or VFD depending on how cold it gets, with no fans running and some wind it can lower HP to a problem pressure. Some form of headmaster may be the best fix.

[hvac697]

How About Hot Gas Bypass Energized Below A Certain Low Ambient Temp-this Would Reduce Compressor Capacity & Keep Your Coils From Icing Up.

[onetime]

How About Hot Gas Bypass Energized Below A Certain Low Ambient Temp-this Would Reduce Compressor Capacity & Keep Your Coils From Icing Up.


That could help but by the sounds of it you would be feeding so much hot gas and for such a long time that you could easily overheat the compressor and cause a compressor failure. Hot gas and unloaders are hard on compressors when they run for long periods of time, not enough cooling for the compressor. Maybe the best thing would be economizer operation.

[barty]

It has full fresh air bypass up to 12 deg c outdoor air . However due to the high heat load, above this temp requires DX cooling..

Cheers

Richard

[The Doctor]

would trane have anything to say that would be helpful?
on a unit of that tonnage, could you install a switch to hold a fan off below certain OA temps, say like below 20*F , and another to take out the 2nd of the three fans (??) per circuit at what ever might give you target liquid temp and head pressure?
those motor bearings should be able to handle a P66 or similar fan speed too.

[bertoh]

you say it has full fresh air to 12 C (54F) ? thats not that low, but you need proper head pressure control, the trane dampers are a joke. install the pressure controled VFD (P66 i think).if the motor is not rated for it, the relacement in a few months will be.link the transformer control for VFD1 throu AUX switch on COMP1 and visa versa on SYS2. really and truly a system like this should have a liquid receiver below the condenser with a hot gas pressure control to keep that pressure up. if it is big time mobile phone server site, right it up with all the bells and whistles and get it signed, thats your insurance if they want the band aid approach, you got the i told you so approach.
open those dampers up, VFD on first fan of each system and cycling press switch on the rest and link them thru comp aux or relay stage 1 stage2 etc.

you need some sort of unloaders (hydraulic or electric) if electric how are theycontrolled by AHU inside? that might have something to do with low press freeze up's. if they are strictly off of room temp, you needto change it to leaving air temp.