Multiple evaporator screw type water chiller question

[coldkeeper1]

This marine chiller seems to be a custom made unit. It uses a variable capacity screw-type compressor coupled to three identical shell-and-tube evaporators. The water outlet pipe of each evaporator runs independently, before being combined into a single main outlet pipe. Simply stated, the problem is that chilled water is not coming out cool enough: 49F versus the required 44F, with compressor at full capacity.
I´ve noticed that the water outlet temperature at each evaporator is not the same (44F, 46F, 52F). They all have been internally cleaned recently. Pressure drop and water flow is about the same for each evaporator, so heat load can be assumed more or less evenly distributed
Each evaporator is feeded by an independent TXV, all of them of the same tonnage, all of them connected to a main liquid line, and all of them located in an absurdly hard to reach location.
Before getting into the nightmarish job of removing the TXV´s, I would like to ask for your opinion: is it possible, in such a parallel feeding arrangement, that the majority of the liquid refrigerant might be choosing to follow the path of less resistance (let´s say, across the two txv´s feeding the 44F and 46F water outlet evaporators) while mostly ignoring the 52F circuit maybe due to a partially closed TXV?

 

[lowedogg]

i would say that is definitely a possibility. just think about how complex the piping is on DX evaporator coil. the manufacturers take great pains to balance the refrigerant flow into the evaporator.

the txv (or the liquid line feeding it) could also be partially blocked on the barrel that is running warm. do you have a pressure reading on the refrigerant side in each evaporator barrel?

if you have plenty of capacity you could just isolate the bad evaporator to get the desired temp.

 

[KiwiMace]

I assume you have whacked the TXV's? Moisture in the refrigerant may cause scale formation in the shells which may clag up the valve. Get this tested.

 

[imok2]

What is the superheat readings on each chiller.What are the pressure gauge reading for suction and discharge Do you have sight glasses at the TXV's are they full? Is the liquid line the right size to feed all the branches? Is the sub cooling correct? (enough refrigerant)Is the head pressure to low?

 

[coldkeeper1]

Good evenning fellows and thank you all for your responses.

1. There is only a main suction pressure reading port at the compressor service valve. I´ll have to pump down the refrigerant and stop the chiller in order to have some access valves weld on the suction lines for each evaporator.

2. As I said, TXVs are really hard to get to. I think I`m going to have to disconnect the water inlet and outlet pipes and raise the chiller with a forklift in order to access the TXVs from below. Maybe I could tap the TXVs with a long screwdriver from above.

3. I´ve measured suction line temperature readings at each evaporator, but there is only one suction pressure reading point at the compressor suction valve. They read between 10 ton 18 deg F of superheat, assuming the suction pressure equals that at the compressor. There is only one sight glass at the main liquid line, which seems to be mostly full, with some traces of bubbles. There is also a liquid receiver with float type level indicators. It seems to be half full of liquid at all times.