airedale problem cus 40 dhi

hello can you help??
i have one of the above on 407c with a 48 kw ahu on with 1 1/8" liquid line, the unit seems to run well on comps 1 and 3 (both on differant circuits) if i run comps 2 and 4 aswell the suction pressure drops eventually freeze the evap then unit stops as lack of air flow, i have checked gas charge and think it's right 10.5kg condencer 0.58kg per m over 20 m for pipe and estimeted 5 kg for evap have full sight glass no aparent blockages/ temp differances in drier on liquid line tev appears to be responding to temp change phail is connected correctly, filters clean, fan running correctly
pressures on 1 comp per circuit 4 ish bar suction with 16/17 bar ish liquid,
pressure on 2 comp per circuit 2.5/3 bar suction and liquid around 16/17 bar

scratching my head tried loads of other things too but runing out of ideas

please help my head is spining!

Do you have the correct airflow through the air handling unit coils?

Is the unit providing the correct air temperatures with 2 compressors?

Are the four compressors staged through a controller of some sort? Is it working correctly?

You say that the exspansion valves appear to be working have you measured there superheat values to see if they are ok, Ifthis is new kit have they got the right size orifice ?

hi it's old ish kit 6 years and worked ok till last winter since then only had it working on 2 comps which seem to cope but as the customer says it's not a it should be so want's a resolve!, air flow not changed! superheat about 8 oC

the compressors are staged by a dixell xr 440 c which i think( dont know enough about them) is set correct but this doesn't stop the fact of icing on 2 comps on a circiut working?

1 1/8" Liquid line for circa 48Kw unit ??? Is this for each circuit ?

If comps 2 & 4 run with 1 & 3 off do you still get ice forming ??

Whats the temp diff across the coil with 2 & 4 comps running ??

What's the capacity of the exv's ?

Is the condensing unit twice the size it should be...

hi with 2 and 4 running i get no problem with icing,

it's a common suction and discharge on 1 and 2
same on 3 and 4

so twin circuit and twin comp per circuit. one tev per circuit suction 1 1/8 liquid each circuit i guess is about 2 3/8 ish

temp differance across coil about 8/10oC varying on time system runing

Check your condenser, exv's & AHU all match each other for capacity..

should all match it was designed by another company and has been worknig ok for 5 odd years i'm a bit unsure on the dixell step control but every thing apears ok?

Hi dubati

Just to clarify things, is there 1 or 2 coils in your AHU ?
If 2, are they interlaced or back-to-back ?
What is the duty of the, or each coil ?

If the total duty of your coil(s) is 48Kw, ring Airedale tomorrow- 01132 391 000 & check the duty of your condensing unit matches..

Under a high load do 2 comps maintain temperature ??

What model is the indoor unit? does raising head pressure help with suction pressures?

Alec

hi update coming looks like major miss sizing fromorigional install and made to work by off setting controls........

Hi again dubati

Glad to hear you're getting somewhere now.

Do you know yet if the condensing unit is too big or is the AHU too small ? Are the exv(s) matched to the condenser or coil ? Does the unit as it is have enough duty to cope when loaded ?

If the condensers too big you could set it up so no more than 2 compressors run at any one time.

Best bet is to find out what the duty is for the area that your conditioning & work back from there..

:)

yes to all of the above!! it explains why it will work only on comp 1 and 3 or 2 and 4

rough figures

evap 48 kw total of twin circuits,
tev's 30 kw per cicruit,
condencer roumoured at 90 kw,
comps upto 38kw each depending on system conditions

so getting there!....

oh and load for the area supposed to be sized and qouted paid for

60kw with 20% leaway!

Oooops :confused:

May be see if you could up-rate your AHU without exceeding the ductwork specs..

Are the fans pulley driven ?

Think i need to sit down, i can feel a hot-gas by pass coming on..

air flow is belts and pulleys but the amount is critical for product and to be fair it is coping with full load on two comps looks like it's capacity control with breakers off!! i'm looking into coils sizing and possible changes implications! costs too