I have 134a traulsen commercial freezer with txv. My suction pressure is 3psi and my head is 150 psi. Ambient is 78degrees and my box temp is 9degrees. Is 3psi suction too low for 9degree box temp

depends on refrigerant being used, the ambient it is working in, the desired box temp as a general rule. gotta have more info.

http://www.arkema-inc.com/pdf/fluorochem/Forane404a_pres_temp.pdf

What is the
"Superheat" on the suction line 6" from compressor ?
Did you change the drier and evacuate to at least 500 microns ?
What model number is the condensing unit ?
Does the system have a "Crankcase Pressure Regulator"
if so is the TXV ZP type ?

why yes there is. Generally speaking, the head pressure should be around the "ambient +30" rule.
That is, measure the entering air temp to the condenser, say 75 degrees. Now add 30 to that and get 105 degrees. Look at a P/T chart and see what the pressure is for that temp which is about 253 for R-404-A.
The evap on a freezer leads the load so, as a general rule, the evap temp is going to be around minus 10 degrees to get a 0 degree box. Coolers are different. So, the pressure for a minus 10 evap is around 25 or so. But the unit has to be close to operating temp. If the box is warm, naturally the pressure is higher. And what influences the pressure is whether or not it's a TXV or a cap tube system.
If a TXV and no receiver, charge by subcooling. If a receiver, fill by sight glass. If Cap tube, charge by superheat.
Hope that answers your question.

Hello
Could be low on freon,i belive this model uses 134A.

So the suction pressure should be 31 to 35 psig,for a temp of 36 to 40.f

Also check the defrost timer and the door seal. The eveap should have a drain line on it, make sure this is working and not plugged.

God day,
Some thoughts.
1. Are the evap fan motors running, and at full speed. That would give you abnormally low suction pressure.
2. Cap tube sized too small. or partial restriction in the dryer cap tube/dryer when brazing in the lines.
3. Low ambient. What's the temp the machine is running in.
4. Over sized compressor.
5. Moisture. God, 134A is touchy on that one.
6. Was original cap tube soldered to the suction line to create a heat exchanger. Needed on many apps. Definately would cause flash back on start up.
7. Superheat a little low, should be 12 to 15 degrees.

The pressures depend, somewhat, on the expansion device,i.e. a TXV or Cap Tube. Take a pressure reading, then convert it to a temperature. The evap temp should be right at 10 colder than the box temp. Remember, heat flows to cold so the evap has to "Lead the Load" in order to cool. In many cases, the condensing pressure is usually ambient temp, plus 30, then convert to a pressure. These are general accepted practices that have been used for many years.

Hope this helps

I'll try to give you the "Readers Digest" version.
First, make sure there is no ice build up on the evap. Light frost is O.K. as long as it does not effect air flow. (Very Important). All fans need to be running. Again, an air flow thing. There should be some product in the box but not too close to the evap. Again, an air flow thing.
The expansion device can effect your pressures to some degree i.e. Cap tube vs. TXV.
As a general rule of thumb, which I use almost everyday when it comes to pressures:
Low side is based on the temp difference between evap temp and condition space in the box. In freezers, it is almost always 10 degrees. If the box is 0 degrees then the evap needs to be at a minus 10 degrees. That is a pressure for R-404A of 24.5 psi. This only holds true when the temp in the box is approching set temp. I would say, about +10 degrees. 0 Degrees for the evap for R-404A is 33.5 psi.
High side should be ambient temp +30 degrees and then convert to a pressure for the refrigerant.
Example: 75 degrees in the room, +30 degrees equals 105 degrees. Now 105 degrees converted to a R-404A pressure is 253 psi. This will get you very close to the desired high side pressure.
I would look at low side first to see if it is within reason. Don't let the high side get too high. Increases the compression ratio and overworks the compressor.
Your icing problem could be a defrost issue and not a refrigerant charge issue.

Good Luck and hope this gets you started.

The refrigerant level is low. The only thing is that the unit may not have a low charge. There can be several things that cause the unit not to refrigerate properly. It could have weak valves in the compressor or even a bad TXV(thermostatic expansion valve). Have you had this box repaired yet.? I see that this is an old post.

The first place to look is to check the charge. Using guages check the low and high side pressures and their relationship to one another. I'm assuming your unit is a R-404 unit which, if fully charged and the condenser is clean, and the fan is operating, should have about 210 to 250 high side pressure. Other things to check: evaporator coil ice/frost free; defrosting properly to obtain a clean coil; all evaporator fans running. If low side pressure is above 30 PSI you have a weak compressor. Whatever, fix the Traulsen unit, its head and shoulders above anything else on the market today.