Suction pressure issues

[jmc88]

Hoping for some help

Ive been working on a freezer system which has a very low suction pressure at the compressor (1psi) but has 20psi at the evaporator outlet. The expansion valve is working correctly as the suction temp at the evap outlet corresponds to any adjustment to the valve but there is no change to the suction pressure at the compressor. Even if i block the condenser coil and raise the head pressure from 170psi to 250psi it only pulls the suction pressure up by 1psi.

I thought there might be a blockage in one of the oil traps, i blew through with nitro down the suction but there was no change. Would an oil trap full of oil cause the suction to drop like that? Is it worth drilling into them? Its a relatively long pipe run, maybe 50 metre, condensing unit higher than evap.

Anyone any ideas what can cause a drop like that? The condensing units were changed out a few years ago by another company, could there be an oversize of some components?

The system still manages to achieve -20 though but due to the lack of suction gas to cool the compressor it gets very hot

Here is the system details and the readings ive taking:

Condensing unit

Kool tech
Model- SA-3-18DY
Volts- 230 FLA-17.4 LRA- 58.24
400 FLA-10.84 LRA- 34.14
Gas- R404a

Compressor

Frascold
Type- D3 18.1 Y 1000
Nr- 0E000205 & 0E000207
50hz 17.93m3/h 1450rpm
60hz 21.52m3/h 1450rpm
220-240 380-420 MRA 17.3 10.0 LRA 75.9 43.7
265-290 440-480 MRA 17.3 10.0 LRA 75.9 43.7

Evaporator
Searle
KS70-6
857916
3 X fans 220v 50hz 2.22amp

- Suction- 0 psi (At compressor)

- Suction- 20 psi Evaporating temp = -26 C ( At evap outlet)
- Discharge- 170 psi Condensing temp= 35 C
- Superheat- 5K
- Subcool- 13K
- Condenser Air on/off- 21/24
- Evaporator Air on/off- -18/-20
- Discharge pipe temp- 74C


Any help appreciated

Cheers

[chemi-cool]

Is there a pressure regulator between the evaporator and the compressor?
Suction line accumulator?

If you have any of these, the problem can be there as the have an internal mash filter which can be blocked or partialy blocked.

[frank]

Or the suction line can be undersized

[chemi-cool]

Frank, I take it that the system was working fine some time ago so I figure that something went wrong somewhere between the evaporator and the compressor.
There should be oil return problems in that situation.

[frank]

Agreed Chemi...but I say it because I was called to look at 3x condensing units that were coupled to an AHU providing heating and cooling to the supply air into an enclosed part of a factory.

The customer complained that it never worked right from the start.

It turned out that the liquid line was undersized leading to a severe pressure drop.

This system had been working, although on a reduced level, from the install date. This could be the same situation as the OP is now experiencing.

I've learnt to never rule out the unexpected.

Best wishes my friend

[jmc88]

There is no regulators or suction accumulator

To my knowledge the suction line is existing from installation (20 years old). The condensing units were changed out a few years ago

From looking at the evap name plate the evap is designed for R22 so im guessing when the condensing unit was changed out the system was retrofitted from R22 to R404a. Could this be an issue? If the original pipe sizes were designed for an R22 would they need to be changed if retrofitting to R404a?

Thanks for the replies

[padraic]

I changed out a suction filter today causing 1bar difference between evap and unit

some compressors have built in filter on suction valve

[Brian_UK]

Might it be worth while checking the suction pipework to see if it has suffered any damage over the years causing crushing of the pipe?

[jmc88]

Might it be worth while checking the suction pipework to see if it has suffered any damage over the years causing crushing of the pipe?

I think i might have to braze in some 1/4 shraeders along sections of the suction and pin point where it's dropping the pressure. Maybe strip all the insulation off at the bends and see if there is any kinks or damage

[TRASH101]

could be that the comp service port is knackered :-)

[Coorsman777]

The compressor would be hotter than H*** if you were starving it for cool suction gas like that.

[Gary]

The system still manages to achieve -20 though but due to the lack of suction gas to cool the compressor it gets very hot

How hot is "very hot"? The discharge line temp (74C presumably near the compressor discharge) is not excessive.

[jmc88]

74 C was taken from the discharge near the compressor. Im just worried about the lack of suction for cooling the compressor. Suction pipe temp taken near about 1 metre from compressor was 13 C

[Gary]

The compressor doesn't seem to be suffering.

I would be moderately concerned with loss of capacity/efficiency. The condensing temp is running 14K above the condenser air on temp (14K cond TD). Normal range would be about 11-20K and is an indication of heat load.

This indicates a relatively light load, although well within the normal range. Without the apparent restriction, the system might have increased capacity and higher efficiency.

If you shut down the compressor, do the two suction pressures equalize?... quickly?... slowly?

[chemi-cool]

If the unit was not cleaned well after retrofit to 404 and synthetic oil, you should expect hard slag in low places such as bottoms of U traps, receivers etc.

[jmc88]

If the unit was not cleaned well after retrofit to 404 and synthetic oil, you should expect hard slag in low places such as bottoms of U traps, receivers etc.

What do you meant by 'hard slag'?

[chemi-cool]

If a system is not cleaned good after changing from mineral oil to synthetic oil and the system is still for a while, remains of the mineral oil in low places will get like soft concrete and block the tubes.

[jmc88]

Ive read differing information about the oil going concrete like when mixed together. Have you found this happening first hand? Judging by what ive heard from the site about the guys that did the retrofit and the job they did of it(originally piped up incorrectly, wired wrong etc) i very much doubt the system was cleaned or flushed out. This could explain the drop in suction pressure. So maybe a suction line flush out is required?

Some other info I didn't include first time is there is no pump down on the system, thermostat just kills compressor and LLSV. Also the defrost termination stat was faulty causing very hot defrost temps then compressor overload trips on start up. Maybe the hot defrosts have forced some of the old oil out to the traps and low points and became concrete like?

Thanks for the replies

[chemi-cool]

I a system that was changed from 22 to 507 and wasn't cleaned well, txv was blocked, sv was blocked, compressor lost one piston and was vibrating like mad, sight glass was black.

It looks to me as the suction line should be replaced.

What you can check, is pump down the refrigerant, disconnect suction line and blow ofn through it.

[1mikeefc1]

What's the sight glass like and have you tried adding anymore refrigerant? If so is there any difference in your suction pressure at the comp? Also is the suct line freezing to touch before the comp?

[jmc88]

What's the sight glass like and have you tried adding anymore refrigerant? If so is there any difference in your suction pressure at the comp? Also is the suct line freezing to touch before the comp?

Yeah i added some added an extra 1.5kg and there was no change in suction pressure. The suction line isn't freezing and is at 13 C. I can get the suction freezing back to the compressor if i open up the expansion valve but there is hardly any change in the compressor suction pressure, only a change at the evap outlet

[1torr]

Hi where are you taking a suction pressure reading from?

[1mikeefc1]

I would have a go at changing the tev and double checking the orophis size. Seems a strange one though.