Freezer 'Total suction line superheat'

[young gun]

Hey guys,
Have had a freezer unit with a Copeland scroll compressor tripping on thermal overload. The freezer is a built in 2 door freezer with an external condensing unit running on R404A. When I first got out to the unit I found the compressor out on thermal overload, I let it reset, found
High side temp = 36-38c
Lowside temp = -25 to -30c the closer the freezer got to setpoint of -22 then suction temp would reach -30c.
suction line temperature approx 30cm from compressor = 8-10c.
so total suction line superheat approx 38-40k at setpoint.

I slightly adjusted the tx valve open a few times lowering the superheat. I left it at approx 14-20k total line superheat


I was just wondering your guys thoughts on what is the lowest you believe you can go with your 'total suction line superheat' without chance of liquid floodback.

would really appreciate feedback from some of the refrigeration/freezer guru's out there. Thankyou

[Rob White]

.

It's a hard one with the low temp scrolls.

They do need good cooling and they tend to have
liquid injection to aid the cooling process.

Superheat, truly should only be set for the evap outlet,
that is true superheat and it is that superheat that the
valve is set for. If you have to adjust the valve to help
keep the compressor cool, then I would suggest you have
a slight problem?

Why would you need to adjust the valve if it was OK before?

If the valve needs adjusting, then something has changed and
by adjusting the valve you are not curing the fault, just addressing
the symptoms.

How long has the system ran, up till now?
Has it had any issues until now?

Regards

Rob

.

[young gun]

It can be difficult chasing the systems full history because of different tradesman/companies working on it. But do know the system was installed 2011, and the condensing unit has been removed and installed because of floods, and it has also had a couple of major leak repairs.

But staff at the store have said its had a past of rising in temp for a while but then coming good again, which to me sounds like its possible had a past of going out on internal overload. The temperature in the area the condensing unit is would change dramatically during the day as well, as its installed on the western side around 5-6 other condensers and other hot machinery with a colorbond fence around it on concrete. So temperature difference in the morning to afternoon could be up to 30degrees difference.

When I first got there and had only fit my gauges, I measured the very top part of compressor at 155degrees Celsius. I was happy with the high side and low side temperatures, just though the 40k superheat was excessive, and possible TX valve had not been commissioned very well, or perhaps commissioned in different conditions as it is currently middle of summer here, which is on average probabaly 20degrees hotter then winter.

It has seemed to run well since the adjustments, but has only been 48 hours, just wondering what minimal amount of superheat is required to keep the scrolls safe from floodback, but keep them running cool.

[Rob White]

It has seemed to run well since the adjustments, but has only been 48 hours, just wondering what minimal amount of superheat is required to keep the scrolls safe from floodback, but keep them running cool.

You probably have done the correct thing as it does sound like it has an interesting history :D

As for superheat????????

Can you connect to the evap outlet and set it up for 6k to 8k at that point?

Trying to set superheat to be correct at the compressor could mean you
flood the liquid out of the evap and slightly up the suction??

But that said, you have a valid point regards the discharge (comp top) temperature
and if the only way of keeping the comp cool is by adjusting the valve, then
as long as no liquid can ever get to the comp 1 deg of superheat will be sufficient.

I can't remember if you give the model number of the comp because some low temp
scrolls rely on liquid injection to alleviate this issue. Your comp doesn't have one
of them fitted?

Regards

Rob

.

[young gun]

No this compressor does not have liquid injection, but I have read about them before, they probabaly designed them to resurrect the sort of problem this unit has had lol.

I will get model number of compressor and post it up later today.

[Magoo]

From memory of the very few scrolls I have installed there is a cut off point for SST at compressor, which in turn requires liquid injection at compressor below that point. Scary, there is a port on compressor for that injection. To get away from that situation I have spec' d large suction diameters and reduce loses and maintain SST with in design. None have died to date. There is also a maximum compression ratio factor as well just compound things. Happy days.
magoo

[young gun]

copeland scroll zb38kqe-tfd-524

[Rob White]

copeland scroll zb38kqe-tfd-524


http://www.emersonclimate.com/asia/en-AP/Documents/Refrigeration/ZB_Scroll_Compressors_Manual.pdf

taz

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[young gun]

It says max superheat of 11k on page 5.....
So if you wanted to set the superheat over the evaporator to optimum 7-8k over a freezer coil (which is what ive always been taught, correct me if im wrong) ......... how are you supposed to keep the superheat at the compressor under 11k if you have long pipe run and hot warm ambient outside......????????? :s

[Rob White]

It says max superheat of 11k on page 5.....
So if you wanted to set the superheat over the evaporator to optimum 7-8k over a freezer coil (which is what ive always been taught, correct me if im wrong) ......... how are you supposed to keep the superheat at the compressor under 11k if you have long pipe run and hot warm ambient outside......????????? :s

7-8k is fine, I've always been told 6-8k so we are in the same area.

If the comp is 11k then the only way is to ensure the suction is well lagged
and the pipe runs are not too long.

Remember if you are measuring evap superheat with the gauges at the comp
you will be reading any pressure drop over the suction so the temp will apear
to be colder than it actually is.

That is why true evap superheat can only be measured at the outlet of the evap.

There is only one evap right?.............

Rob

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[Gary]

Static (non-energized) gas laws tell us that you can't have liquid where there is superheat, any superheat... however... in a running (dynamic) system, static gas laws do not apply. In a running system, it is entirely possible to have liquid droplets traveling with superheated vapor in the suction line.

The liquid droplets are entirely vaporized at roughly 5-10F superheat, depending upon flow velocity. Most compressor manufacturers recommend a minimum of 15-20F superheat near the compressor inlet.