Here's a tricky one!

[R. skiffington]

Hi all,

I have a problem on a basic close control computer room a/c system that has me stumped,
(407c)
It's a Liebert Hiross downflow unit, twin compressor, two seperate evaps running to one common condensor, two seperate cond coils just mounted on one body, condensor is triple fan, HP is controled by one johnson hp controller with two transducer's, it works by giving priority to the system with the greatest cooling demand, this sounds simple doesn't it!
The problem I have is, when both systems are operating sys 1 loses presure, LP 35psi - HP +/- 170psi, eventually icing up, at this time sys 2 will be ok LP 65-70psi - HP 225psi,

sys is approx 4yo, no problem untill no, although i heard there was probs on commissioning but i knnow no details.

Sys 1 has a brand new TX valve and refrig charge was wieghed in to the labelled charge.
Now I know, bearing in mind it's 407c, sys 2 HP looks low I adjusted this to 275psi, but sys 1 still the same, I then thought that sys 1 simply looked short, I added 4.5kg, untill I had max sub cooling without any flooding back, this improved the situation, sys 1 LP 45-50 psi - hp 200 psi,

Now when i run each sys individually they're perfect, only difference is sys 1 LP runs at about 63psi when sys 2 will be at around 68-70psi,
I know what is causing the problem with sys 1, when sys 2 is running it is requiring more duty over the cond causing the hp of sys 1 to fall (eventually icing up) but i'm buggered if i know why, HP control is working fine, comp efficiency good, evap airflow good, tx valve operation ok, can't find any abnormal presure drops!
HELP!!

[Brian_UK]

What about the evaporator section?

Do you have good airflow over the coils? Is there any ductwork that may be restricted now that wasn't when first installed?

How did you "adjust the pressure from 225psi to 275psi?", is the stated charge weight correct?

[R. skiffington]

Hi Brian

Thanks for your reply,
There is no ductwork on this system, the raised computer room floor is used as the supply plenum, this has not changed long before this problem has arose.
To raise the HP I used the set point adjustment on the HP control transducer,
As for the stated charge, no i am not sure this is correct but, I added 4kg's (sys charge approx 30kg) this improved the situation but i believe the systm is now fully charged, i have sweat at the compressor and the evap seems almost flooded, meaning the entire coil is refrigerated and appears to be working efficiently. It just seems this side of the unit is asking for very little duty from the condensor.

I have looked at all logical explanations. It just doesn't seem to add up!!
Cheers

[hiitsme]

Sorry if I misunderstood you but your post was a bit "dense" for a non-native-English-speaker.... :-)

Can't this be the case (?):
one system has high HP problem and if seaparately used, the fans can handle it somehow but if you use both side at the same time it simply makes too much cool on the common-cooled condenser therefore the other part has low HP?

[R. skiffington]

Hi hiitsme,

Yes it would appear this is possible, but this is one of three systems on the site, all exactly the same, I have compared the operation of the the system that operates at a higher HP to the other systems, and found the output being required by the Fan speed control is much the same as the other systems, only asking 100v on average from the fan motors, this is certainly not excessive.
Thank you for your input

[phil17]

Hi , sure you've already checked but are the filters and evaps nice and clean? I've had a few instances where evaps are blocked due to no filters. I

[phil17]

Hi , sure you've already checked but are the filters and evaps nice and clean? I've had a few instances where evaps are blocked due to no filters. I assume there are no pressure regulators to keep the evap temp above zero in winter time? What i have done in the past on big air chillers was to put a Grasslin 1 hour timer on. This allowed me to have a 5 minute off cycle defrost in every hour to keep the evaps clear during production- may get you out of a muddle

[NoNickName]

It looks like that when condensing fan speed regulation is done on higher pressure, the lower pressure circuit is too much subcooled and as a result low pressure happens.
Try to measure SC when systems are individually working and when both are working together and compare.

[R. skiffington]

Hi Guys
Once again thank you for all the input.
Phill17 - Yes, i've checked the filters and coils, all clear, in saying that the filters serve both evaps so if there was a problem i would expect to say the same on both coils.
NoNickName - Yes, you're correct, that is exactly what's happening, the problem i have is i can't understand what's causing it and why it''s only just started happening?

One further thought I had was, a possible blockage in one of the distributor lines feeding the evap, but in my mind this shouldn't have the effect it's having on the HP, should it?
I could create a short term remedy to this prroblem by blocking part of the condensor serving the problem system, just to enough to increase the hp to a safe level, but I really need to get to the bottom of the problem, it's very frustrating!!

[NoNickName]

Why don't you just stop one fan until next summer. Or interlock it with a HP cut in, so that the fan speed regulator will act just on two fans, and the third is operated with a hp switch connected to any one circuit of your choice.

[inverter]

over condensing?

[Andy]

Hi all,

I have a problem on a basic close control computer room a/c system that has me stumped,
(407c)
It's a Liebert Hiross downflow unit, twin compressor, two seperate evaps running to one common condensor, two seperate cond coils just mounted on one body, condensor is triple fan, HP is controled by one johnson hp controller with two transducer's, it works by giving priority to the system with the greatest cooling demand, this sounds simple doesn't it!
The problem I have is, when both systems are operating sys 1 loses presure, LP 35psi - HP +/- 170psi, eventually icing up, at this time sys 2 will be ok LP 65-70psi - HP 225psi,

sys is approx 4yo, no problem untill no, although i heard there was probs on commissioning but i knnow no details.

Sys 1 has a brand new TX valve and refrig charge was wieghed in to the labelled charge.
Now I know, bearing in mind it's 407c, sys 2 HP looks low I adjusted this to 275psi, but sys 1 still the same, I then thought that sys 1 simply looked short, I added 4.5kg, untill I had max sub cooling without any flooding back, this improved the situation, sys 1 LP 45-50 psi - hp 200 psi,

Now when i run each sys individually they're perfect, only difference is sys 1 LP runs at about 63psi when sys 2 will be at around 68-70psi,
I know what is causing the problem with sys 1, when sys 2 is running it is requiring more duty over the cond causing the hp of sys 1 to fall (eventually icing up) but i'm buggered if i know why, HP control is working fine, comp efficiency good, evap airflow good, tx valve operation ok, can't find any abnormal presure drops!
HELP!!

Hi:)

If I had to work at this I would, decant both charges and recharge to the same weight on both exactly with virgin refrigerant, with the TEV's set at the same number of turns open, assuming they are both the same TEV's.

Could be contaminated or fractionated gas. Or simply a poor pumping compressor.

Hope this helps:)

Kind Regards Andy:)

[Vee-Are-Vee]

Pretty much sounds like the charge or S/H settings are out across both systems(sys 2 overcharged), I've had experience of a close control system with a similar setup to this and the gas charge was short on one circuit, the circuit with the full charge was demanding a higher fan speed therefore making the other circuit over condense and freeze up. One of the TXV phials was partially exposed as well which didnt help!

I presume all three fans are connected to the one fans speed controller?

After checking both circuit charges and S/H settings the system worked ok with the addition of some fine tuning to the pressure transducers on the condensers. Have a pair of gauges on each circuit while doing this to achieve a good balance.

Finally tested C1 on it's own, C2 on its own and then both circuits to ensure consistent operating pressures.

In theory, if both systems are set up equally the demand on the condenser fans should be more or less equal.

Good luck

[R. skiffington]

Hi guys.
Sorry the replys taken soo long, thanks everyone for there input,
I've sorted the problem, after mucking about taking all the readings i could think of, SH, SC, blablabla!
All of which were close but not perfect. After all this I still couldn't get it quite right so did exactly what andy said and started from scratch, reclaimed both charges gave each system a triple vac over 24hours, changed the driers centred both valves, I recommissioned each system individually, charging to basically a full sight glass, then set the superheats at 8. This was about a week ago and i was back there today and still running strong.
System 1 still runs a lower suction pressure at about 55psi, but this is acceptable, I found some old maintenance sheets from a few years ago not long after the units were installed and the presures were much the same.
I think there were minor isues on both systems causing the problem. I suspect maybe a small amount of non-condensables in circiut 2 and circuit 1 was slightly overcharged and the tev needed setting. But in any case it's bloody working and i'm happy! Should have done this to start with, but guess you naturally try and avoid it as it took 2.5 days to finish.
Thanks everyone for you're input!

[Josip]

Hi, R. Skiffington :)


Hi guys.
Sorry the replys taken soo long, thanks everyone for there input,
I've sorted the problem, after mucking about taking all the readings i could think of, SH, SC, blablabla!
All of which were close but not perfect. After all this I still couldn't get it quite right so did exactly what andy said and started from scratch, reclaimed both charges gave each system a triple vac over 24hours, .......
.....This was about a week ago and i was back there today and still running strong.....

....But in any case it's bloody working and i'm happy! Should have done this to start with, but guess you naturally try and avoid it as it took 2.5 days to finish.
Thanks everyone for you're input!

So, the problem is solved, all happy and the happiest guy here is Andy, because he has to send a bill to you:eek:

Don't worry be happy;)

Best regards, Josip :)

[Andy]

Hi, R. Skiffington :)



So, the problem is solved, all happy and the happiest guy here is Andy, because he has to send a bill to you:eek:

Don't worry be happy;)

Best regards, Josip :)

Hi:)
some times there is no short cut;) I have been there and I tried all the short cuts (or most of them).
Probably would have worked on R22:)

We had a VRF recently with a leak, we though we had it so the engineer took a short cut and just topped up the charge. Thing ran like a dog. It then broke a bend on the condenser and lost all the charge. Best thing really:) We were able to re calculate and weigh in the charge (and mark up the KG on the unit).

Unit runs fine now:)

Kind Regards Andy:)

[licencetochill]

hey bro, i think uv sorted this now..but..im presuming this is @ TVP?..lucky you..
just a thought, the system that is icing up-is that the sysytem with th new tx valve? who installed th valve..what size orifice has it got..have u tried opening the valve up a turn.u say ur superheat looks good tho eh? obviously if you are running a lower suction than normal,your hp will be lower as well..this is looking like thecurseofjohnlund to me.
justin

[Tiesse]

Sorry to come in so late on this thread, but where a twin circuit condenser is used, you really do have to have both circuiits set up identically, otherwise charging a circuit at a time without the other circuit running is a recipe for a major headache on these units (I work for Emerson & quite often get called about this exact set of circumstances as a tech call).