Low Pressure problem - please help urgently

[mike fisher]

Hi,
We have a Fridge Container on board, giving problems. Compressor, Drier & T.E.V. all replaced. Now it gets down to -15 degs, but suction pressure is zero and discharge pressure is about 9 bar. There is icing up at suction side of Compressor and only a couple of smally bubbles in sight glass.
We have 2.5 tons of meat ashore, waiting to come on board, so I'm not the most popular of persons around here right now. Any help would be much appreciated - thanks.

[Sturt]

Can you give us the refrigerant and confirm the suction/discharge pressure/temperatures?

[mike fisher]

Hi, thanks for replying. forgot to mention refrigerant is R134a. Suction at Compressor is just less than zero and discharge pressure at Compressor is about 9 bar. Pressure in liquid line after the Drier is about 7 bar.

[Sturt]

Hi Mike

You should have a constant pressure on the high side of the system and you state a 2 bar drop.

Suction pressure if 0 bar gauge gives an evaporating temp of -25oC on 134a. What is the suction temp so we can calculate the superheat?

Did you fully evacuate the system to <3mbar after changing the components you mention?

Discharge and ambient temperature would be useful?

Sturt.

[mike fisher]

Hi Sturt,
Apologies for not getting back to you sooner, but lots of problems here...
Ambient is 21 degs, suction temp is -11 degs, liquid temp after Drier is 38 degs, discharge gas temp is 64 degs. We vacuumed the system for about ten hours yesterday, and installed new Drier before recharging the system.
What I cannot understand is why the suction is icing up. Evap coils look good. Could it be oil in the Evap? Several people had a play with this Fridge before I got dragged in.

Mike.

[mike fisher]

Hi Sturt - also forgot to mention that I re-checked Comp'r discharge pressure and liquid pressure after the Drier using a more accurate gauge, and got the same pressure both times - 9 bar.

Mike.

[Sturt]

Hi Mike

So we have a low suction gauge and high suction superheat. This could then be:

Undercharged
Moisture in system
Choked liquid or suction strainer
Faulty BPV - not on this type of system
Faulty expansion valve (closed)
Liquid line obstructed

Thats my best guess, anyone else have thoughts on this?

Sturt.

[SteinarN]

Suction at -11C is well below frezing, thus ice on the suction pipe. This is normal as long as the compressor itself is warm. I supose you dont have any suction gas/liquid line heat exchanger......

Exactly what is the suction pressure?
Can you confirm whether the system has a suction gas/liquid line heat exchanger?
What is the make and type of the expansion valve? What adjustment have you done to the valve?
Can you confirm the evaporator fan(s) is working properly and that nothing is restricting the airflow like dirty fins?

[mike fisher]

Hi SteinarN,
We don't have a Heat Exchanger on gas / liquid lines. The Expansion valve has been replaced (ALCO TLE 2 1/2 MW 65 3C). I cannot see any adjustment on this Valve. Evap Fans are working well and there appears to be no blocking up of Evap cooling fins. Cooling coils seem to ice up nicely. If T.E.V. is closed, would we not get high pressure at Comp'r? If I add more gas, the Comp'r will ice up even more. The Fridge Container is on a ship, out at sea, so getting a Service Engineer out here is a logistics nightmare. I did speak to a guy this morning ashore, but having told him the symptoms, he was scratching his head more than me....

Thanks,
Mike.

[Sturt]

Sorry Mike I was going to finish off with saying open the expansion valve and monitor the results.

If you are happy with the system as a whole and the system is a packaged unit and or the correct charge has gone in and the expansion valve/orifice and the phial position are all correct it probably may just need the superheat setting up.

Sturt.

[SteinarN]

Based on the information so far the only thing I can say is that it seems as the superheat is a thad to high. If the suction pressure is -0,2 bar, that is -30C, then the evaporator Dt is 15C. (-15C box-(-30C) evaporating is 15C difference) This in combination with your measured suction temp indicates high evaporator super heat. However if it is not any adjustment on the valve....

Was it not any adjustment on the old valve either? Is the new valve exactly the same as the old?

[SteinarN]

If T.E.V. is closed, would we not get high pressure at Comp'r? If I add more gas, the Comp'r will ice up even more.

Thanks,
Mike.

Both suction and discharge will be lower with closed/closing tev.

What happens to the pressure if you add more gas? Does the compressor itself ice up then? Where on the compressor? What type of compressor is it?

[SteinarN]

We vacuumed the system for about ten hours yesterday, and installed new Drier before recharging the system.

Mike.

Did you change the drier before or after the vacuuming?

[mike fisher]

Hi SteinarN,
Thanks for the help with this one.
The new T.E.V. is exactly the same as the old one. spring loaded valve and power assembly were renewed. The only thing that worries me now is running the unit with negative suction pressure. By the way, this Reefer was modified about six months ago, due to problems with the Controller. The system was then modified to a basic Fridge system, and operated fine until recently. Then we started to get the Comp'r short-cycling on HP. I know the system is gas tight, as it held a vacuum for four hours (having had a vacuum pump on it for about 8 hours), so I'm just concerned about drawing in air, if a leak should develop.

Mike.

[mike fisher]

Hi SteinarN - after vacuuming.

[mike fisher]

Hi SteinarN - it's a standard six cylinder Compressor. Adding more gas does seem to cause suction temp to decrease, as we then get ice all the way to the cylinder head covers at suction valve area.

[SteinarN]

Hi SteinarN - after vacuuming.

Are you sure you didnt get any air into the system when you changed the drier? Usually I change the drier first, and then vacuum the whole system.

[SteinarN]

Hi SteinarN - it's a standard six cylinder Compressor. Adding more gas does seem to cause suction temp to decrease, as we then get ice all the way to the cylinder head covers at suction valve area.

Ice all the way to the sylinder head is a clear sign of severe liquid flodback. I supose the suction is at the motor end, opposite of the sylinder end.

It seems to be something wrong here. Adding refrigerant shouldnt result in any severe flodback if the tev is function corectly. I supose the tev is labeled for R134A.

If the system has a receiver, then adding refrigerant shouldnt do any difference until the receiver gets completely filled up with liquid refrigerant. After that the discharge pressure should start to increase and the condenser outlet temperature should start to decrease. But severe liquid floodback should still not occur.

[SteinarN]

The Expansion valve has been replaced (ALCO TLE 2 1/2 MW 65 3C)


I cant find the valve you mention. The valves for R134A is TCLE........
That valve is adjustable.

[icecube51]

did you placed the bulb on the tubes correctly ?? is the orifice correctly ??

did you take measurements at the TEV ? before and after the valve.
what is youre sub-cooling ??

Ice

[nike123]

We vacuumed the system for about ten hours yesterday, and installed new Drier before recharging the system.
_Cut_
Several people had a play with this Fridge before I got dragged in.




As SteinarN pointed before, you changed filter-dryer after evacuation. How did you done that without introducing air in system? Is that filter with replaceable core and with isolating valves? Also, how did you established that your vacuum level is OK except with evacuating time of 10 hours?

TLE 2 1/2 MW (http://www.emersonclimate.com/flowcontrols/pdf/txv/catalog/TLE.pdf) is two and half ton bi-flow TEV for R134A and with MOP and fixed superheat designated for OEM and made by their specification.

That container is probably for air conditioning purpose or for keeping temperature at, say, 14°C with cooling or heating (what ever is needed) and control has been changed to cooling meat at 2°C .

[Below 0]

Hi
More information about the container model might be more helpful.Some reefers have suction solenoid valves and quench valves, so your problem may not be the tx vlve at all. If it doesnt have these then you could always pump the system down and manually adjust spring tension on tx orifice to prevent the floodback.The compressor wont last long under the present conditions, hope this helps.

[Will]

Ditto what 'Below 0' said, a container model # and brand name would be really helpful.

[Gary]

For a proper evaluation, we need the following information:

Low side:

Evap air in temp
Evap air out temp
Saturated suction temp (SST) converted from pressure
Suction line temp at evap outlet

High side:

Cond air in temp
Cond air out temp
Saturated condensing temp (SCT) converted from pressure
Liquid line temp at receiver outlet

[jasperreefers]

Your problem may be just the current limiter setting on the controller and has caught me out a few times. factory setting is about 21amps, it works by unloading the compressor via the modulation valve giving you the low/vacume pressure reading. worth checking

[jasperreefers]

Another thought Carrier ML2i evap fan motors CAN run backwards, esp if they have had new bearings after a seize up, dark spots on the start winding will give you an indication that they may run backwards

i know this is an old post and you may have sorted it by now but stick the back of your mind

[Sledge]

I guess this is an old post, but have a question...

How does unloading the compressor cause a low suction pressure?

If the compressor is unloaded when it is not necessary to be unloaded, the suction pressure would rise.

If this was operating correctly for a number of months, but has recently failed, I am thinking components are correct.

Conditions stated of reasonable head pressure (have no idea what 9 Bar means in PSI) and starved evaporator sounds like a restriction in the liquid line. I would be thinking along the lines of a filter-drier that has blown apart.

[D.D.KORANNE]

Any time suction going to vac means , system could be picking up air/moisture through a low side leak in system . And enters moisture which may be freezing up and ends up in low suction pressure.

[jasperreefers]

Ok, Carrier have been using a current limiter on all 69NT40's It works by enrgizing the modulation valve to reduce the flow of refrigerant back to the compressor, hence the low suction/vacuum reading and a need for a quench valve for compressor cooling. Ships often use this current limiter to control thr load on the ships generators