Superheat decreasing down stream of evaporator

[samjohn]

Hello,
Is it normal for the temperature of my suction line to "decrease" as it goes down stream toward the compressor?
The reading closest to the evaporator is 36f so that seams right for 507 at 70psi. However down stream past the accumulator near the suction valve it reads 33f. The 30 ton compressor seems to be cold in a lot of spots as well. More so than how my other compressors run. Last time I brought this system down to my target temp (31f RSW) a large part of the compressor was frosting. It's a semi-hermetic Carlyle.
Are these indicating an overcharge? I recently added a 25lb bottle.
Am I in danger of flooding back if the refrigerant reaching the compressor doesn't have a couple of degrees of superheat (which would be 30f at my 70psi suction pressure "507")?
Thanks for any help. It may be hard to reply (on fishing boat at sea. Spotty internet)

[RANGER1]

If you have a liquid/suction line heat exchanger with an internal leak ,that can explain why superheat is lower at compressor, as well as floodbacks to compressor.It is usually close to evaporator, or inside casing.

[samjohn]

Ranger1 so you are saying that my super heat should be increasing as it gets closer to the compressor?
The evaporator is a shell and tube design (507 in the titanium tubes). Only 3 years old! The evaporator is submerged in the fish hold.
I recently charged up the system. Could this problem be a result of an over charge. Where the accumulator is filled too high with liquid (unable to flash)?
The accumulator is frosted up pretty good but that seems normal and also happens on my other two systems which are working well

[joe-ice]

Super heat can decrease if your return piping passes through an ambient that is at a colder temperature than the gas in the pipe , The gas would give up the super heat it has acquired to the ambient with the possibility of turning back into a liquid.

[samjohn]

Joe-ice, so that wouldn't be the case in my situation. The evaporator/chiller is submerged in chilled water (currently at 35f) and plumbing goes through bulkhead into engine room (usually around 70f).
If my accumulator is completely frosted (a foot upstream from compressor) is that a good indication that it is mostly vapor and I am not in danger of flooding back?
Thanks for the help

[samjohn]

Joe-ice, so that wouldn't be the case in my situation. The evaporator/chiller is submerged in chilled water (currently at 35f) and plumbing goes through bulkhead into engine room (usually around 70f).
If my accumulator is completely frosted (a foot upstream from compressor) is that a good indication that it is mostly vapor and I am not in danger of flooding back?
Thanks for the help

[joe-ice]

What type of system is it , what is the metering device , If i converted f to celsius correctly your superheat would appear very low at around 2k . Evaporating at a temperature to chill water i would not expect to see any ice building on compressor body ,

[RANGER1]

If you have a liquid/suction line heat exchanger with an internal leak ,that can explain why superheat is lower at compressor, as well as floodbacks to compressor.It is usually close to evaporator, or inside casing.

Samjohn,
I was assuming you had an air cooler which sometimes has liquid line running through suction line heat exchanger to subcool liquid before expansion device.
You may still have one, I don't know?
If superheat decreases closer to compressor, your instruments are accurate ie gauges & thermometer, then big chance expansion valve is overfeeding.
You may have something in suction line, or line is undersize giving a pressure drop at compressor, but still sounds like liquid present, compare it to "good" machine.
Compressor should be warm to touch on sump & under heads.
Discharge temperature can be an indicator as well that you are getting some kind of flood back.

[samjohn]

Thanks joe-ice, with yours and ranger1 latest comments I think it is going to be best to almost close down my TXV and open it up little by little. Until I have a good superheat number. I was working with what I thought was a good number (8f) but I was noticing that the superheat temp that I am taking on the suction line as it comes through the bulkhead into the engine room "is" a bit of a moving target. It starts out at my target but then as time goes by and the condensation builds it just keeps going lower. Is this something that you experience? I tried to insulate the thermostat with rags but not a perfect scenario in 75f engine room. Thanks for your help

[samjohn]

Thanks ranger1. I think I will try and tighten down TXV and monitor. There has been a little over adjusting going on with TXV (read comment to joe-ice) trying to find perfect superheat.
Thanks for your help I will try and come back with an update.

[The Viking]

And we agree that superheat is the differential between the refrigerant's boiling point at the given pressure and the measured temperature of the refrigerant NOT the measured temperature of the pipe?

:cool:

[samjohn]

Joe-ice, Ranger1
Problem solved; delivered fish. Opened submerged evaporator up and found loads of fish bits clogging the circulation water inlet. cleaned it all out and plugged hole where fish got into piping. now have a well running system.
Moral to story; if heat transfer is not taking place then cold refrigerant will be making its way back to compressor.
Also learned that if there is a problem it is not a good time to start monkeying around with TXV superheat setting. It should in most cases be adjusted when system is just beginning it's cooling. Not when it is near its target temp and compressor is freezing!
Thanks again for all your help.

[samjohn]

Viking, i agree 100% but what will be the best method for getting the refrigerant temp. I am currently using a little capillary tube type device that came with my fairly new multimeter. And as I said attaching it to the out side of the copper pipe. I would like to think it is giving a reading within 1-2f degrees accuracy.
Any suggestions?