Liebert hiross lp fault

[Benhurlow]

Hi, I'm a new member to this forum and this is my first post, so I hope I'm posting in the correct forum.
I've been called out to a liebert hiross model m66uaoo1v102000po, which over the weekend cut out on low pressure. Having checked discharge (14bar), liquid (12bar) and suction pressures (2bar), all were running well below normal leading me to believe the unit to be short of gas (r407c), but I've just completed reclaiming the charge, which is correct according to the commissioning info. I'm going to put the unit on pressure test anyway, but I'm not convinced that there is going to be a leak on the system.
Does anybody have any suggestions?
Thanks in advance.

[Gary]

Evap air on temp?
Evap air off temp?
Evap superheat?

[Gary]

Evap air on temp?
Evap air off temp?
Evap superheat?

[The Viking]

The problem with R407c is that it sometimes fractionates.

In situations like this when you reclaimed the whole charge and found that it was the correct charge you are likely to find that when you recharge the system with the same gas the system is running properly.
This does however assume that you reclaimed the refrigerant in to one (clean) cylinder and that you when you recharge the system charge the refrigerant in it's liquid phase.
(By putting it in to a single cylinder you have in effect remixed the refrigerant)

If you reclaimed in to several cylinders you might even find that they now got different pressures in them, this would confirm the fractionation theory and you would have to get a larger cylinder to mix the whole charge together in (or charge the system with virgin refrigerant)

The above do assume that no previous leaks has been repaired on the system and the system only been partially recharged at that point.

If the above doesn't work then we do need to look at running conditions like sub cooling and super heat.

:cool:

.

[RusBuka]

May be trouble is http://savepic.org/3182994m.jpg (http://savepic.org/3182994.htm)

[hyperion]

Any chance that the system has been over-condensing, forcing the discharge and liquid pressure down which will create a false suction pressure. With R407C this could cause all sorts of unusal operating conditions.

[RusBuka]

Onboard filter dryer can be jammed.

[Benhurlow]

Thanks everyone for your help
I've pressure tested the system overnight which it passed and recharged the system. Having turned the unit back on I still get 2bar suction but now I'm getting 17bar liquid. My air on is 23deg C and air off is 17deg C and my superheat is 30k and sub cooling is 12k, if I measured correctly. I've manually opened the expansion valve which increased the suction to 3bar and reduced the superheat to 14k but that is with the expansion valve now fully open. Does this mean the expansion valve needs replacing?

[Gary]

Superheat adjustment is most accurate when the room is down to design temperature. Once the room cools down you may need to close it off a little.

The question then is why did it need adjusting? Either someone closed it off or it may have a partially plugged inlet screen. Given the fact that the adjustment alters the superheat means it probably doesn't need to be replaced.

[The Viking]

if measured correctly

To measure subcooling and superheat a set of gauges needs to be connected to the system.

The different between the refrigerant's saturated temperature as read from the correct scale on the suction gauge (or comparator if your gauges haven't got an R407c scale) and the actual temperature of the suction pipe by the expansion valve's bulb is your superheat (measurments in degrees C gives you the superheat in K)

The different between the refrigerant's saturated temperature as read from the correct scale on the HP gauge (preferable attached to the liquid line) and the actual temperature of the liquid line gives you the subcooling.

The more accurate the measurements the better the result.


If your measurements is correct, it might mean the TEV needs replacing. It would be worth checking your findings against the commissioning sheet.
Some Liebert units are notoriously operating on high superheats, probably a design thing to reduce their dehumidification.

:cool:

[Gary]

If a sight glass is full (I guess it is with the 12K sub cooling) and a superheat is 14K with the TXV fully open, than "yes", the expansion valve needs to be replaced.

Unless there is a heavy heat load causing the liquid to boil off faster than it can feed through the orifice. It is not at all unusual for a txv to have high superheat during pulldown.

[RusBuka]

Check all liquid line on sudden change of temperature. Don't forget about delta T bubble\dew.
Check air flow, may be speed of the fun is lower for some reasons.