Help with r404a Freezer

Hey guys ive been working on a site which has an old freezer unit that has been reinstalled. It runs on R404A. Anyway ive noticed the installation company has under sized the liquid line. The original size was 1/2 inc h and they have installed 3/8. The problem is this freezer is running at low pressures when the temp inside the freezer is -6 my back pressure is 40 psi. This does not match with my p.t chart for r404a. The compressor is getting nice sweat back untill about -10 degrees C then the suction line starts to ice up. The freezer takes a really long time to get down to temp and stops at about -16 when i need it at -20 . Upon inspecting the condensing unit ive found that the compressor is a scroll compressor that has a liquid injection line running back to the compressor, This line has a solenoid valave on it. I did a bit of research and found these lines are for cooling the compressor.There are no sensors to set this valve up as i understand it there should be a temp sensor on the head of trhe compressors that opens and closes this valave. I intend to leave this disconnected. I am 100% sure that the unit is not short on refrigerant and there are no nnon condenseables in the system as ive personally vacuumed and charged system tot a full sight glass. i cant really check superheats as the system is not down to desired temp. I guess my question is will this undersized liquidline be causing my low suction pressures due to a pressure drop in the liquid line as its smaller.

i think that there is a current relay in the terminal box in older scroll models to control the liquid injection into the compressor .what is your discharge pressure ,you should check that both your coils are clean and fans blowing the correct direction as 40psi at -6 room temp sounds about right

hi head pressure is sitting between 250 psi & 200 psi the condenser fan is cycled by a hp controller. my gauges indicate that at - 6 my back pressure should be at 58 psi. Can someone explain symptoms from havving an undersized liquid line?

Undersized liquid line will cause flash gas and chill the liquid line, flashing in sight glass The 3/8"is good for around 7.0 Kw at 40'C Condensing -18'C evaporating at 10 metre equivelant length, 1/2"double that.

Pressures seem OK,
Undersize means preesure drop, pressure drop means possible flashing, flashing means reduced mass flow flow through TEV. Reduced mass flow means, straved evap. Starved evap, low suction pressure and high superheat.

ok can anybody see any problems in the dissconnection of the liquid injection? surely these scrolls can run without it. Its the first time ive seen one on a unit. anyone know why i would be getting ice back on the suction line at the compressor when it gets below - 10 and before that im getting a really nice sweat back. Would it just be the suction line temp causing this. Id assume if liquid was reaching the compressor id get ice build up the whole time and not just when it gets below a certain temp.

You get line frozen when line is at temperature below 0°C, or sweating when line is below dew point of surrounding air.
If you have any superheat at compressor entrance, than compressor doesn't getting any liquid regardless if suction line is sweating or freezing.
Compressors manufacturers prefer 20-30K superheat at compressor entrance or else installation of suction accumulator.

Measure evaporator and total superheat (10" from compressor inlet) and we will see if that is OK or not.

Liquid injection will be required when (and if) superheat is increased and evaporation temperature is low, and therefore, mass flow is lowered.
You now have high evaporation temperature and, by description of your line, low total superheat.
That mean that liquid injection for compressor cooling is, maybe, not necessary now, but when evaporation temperature drop to -30°C or -25°C, and if total superheat is high enough, you may need liquid injection to keep compressor cold enough.

You have 40Psi when freezer is at -6°C.
40 PSI is -14°C saturation which gives 8K TD.
TD sound good.

You could measure superheat even when temperature is not near required if you have TXV as expansion device.
TXV keep superheat at set value. That is its job, and if it has liquid refrigerant at inlet, superheat should be maintained fairly stable.

Cover portion of condenser to get stable 200 PSI and check subcooling at TXV inlet (20 cm from TXV)

Ah ok will measure superheat tomorrow if temps in the room are low enough. Other than the system being old are there any reasons for this thing to take so long to get down to temp (-18) takes about 3 - 4 hours to hit -17. Thanks for all the help btw

Undersized for actual heat load:

• To many air changes
• Bad insulation.
• To many fresh goods.
• Not designed for freezing of goods, only for temperature maintenance

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Originally Posted by jbomba

Ah ok will measure superheat tomorrow if temps in the room are low enough.

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Give us full set of measurments:

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Here is a list of the temps/pressures needed to troubleshoot a system:

Evap air/water in temp
Evap air/water out temp
Low side pressure or saturation temp
Suction line temp at evap outlet
Suction line temp at compressor inlet

Cond air/water in temp
Cond air/water out temp
High side pressure or saturation temp
Liquid line temp at receiver or condenser outlet
Liquid line temp at TXV inlet
The more information provided the more accurate the diagnosis.

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Nike, beware of normal superheat settings for LT applications, in many cases it is limited to 10K without other methods of head cooling (injection, force air etc)

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Originally Posted by mad fridgie

Nike, beware of normal superheat settings for LT applications, in many cases it is limited to 10K without other methods of head cooling (injection, force air etc)

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You are right.
Here it is operation envelope WITH liquid injection of Maneurop LFZ scrolls for low temperature:

http://i56.tinypic.com/1zlrqsj.jpg

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Originally Posted by jbomba

... my gauges indicate that at - 6 my back pressure should be at 58 psi.

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Wrong. Your P/T chart (or gauge temp scale) tells you the saturation temperature of the refrigerant, not the temperature of the freezer. The saturation temp must be lower than the freezer temp.