Scution pressure problem

[Daveal3D]

Hi guys,


I'm running a Maneurop MT64 compressor through a heat exchanger as a condenser and a heat exchanger as evaporator. I'm having trouble setting superheat, right now i have 22*C (i know it's much) my suction pressure is 42psi and suction temp around 15*C. Condensing pressure 218psi and subcooling 8*C. I have danfoss TEX2 TXv with a no 6. orrifice. We cooling dampenning solution for a printing press to 10*C . And i'm sure it's not the HE, because on the next same machine i got the same HE running at 66psi suction and arround 6*C superheat.

I tryed adding refrigerant and only the discharge pressure raises and no change with suction.

I figure my TXV isnt working properly?

[Peter_1]

If you have 8°C SC, then you probably have sufficient refrigerant in the circuit.
Is there a sightglass?
If you're sure about the orifice in the TEV, then it's probably broken, the bulb most partially it's gas or the mesh screen on the inlet of the TEV is partially blocked.

Nobody serviced it before you with the wrong gas in it?

Because hours are expensive, recover gas and replace it at once?

[Daveal3D]

This machine was rebuild several times before i started servicing, as i was told. Because there were leaks in several places i recovered the gas and vacuumed so i know it's pure 22 in there.

My current orifice is the largest for this TXV series. I tried replacing it with another .6 and still nothing so it's not the orifice.

The ambient is 25-28°C in that hall because of the running presses it can reach over 30°C in hot summer days.

No there's no sigh glass, apparently it was removed for some reason. I'm gonna install a new one next week.

I tried increasing the discharge pressure to 280psi today and nothing, still reads the same 42 psi suction.
Because of this i cant get superheat below 20°C without the suction line freezing over.

[Peter_1]

An orifice rarely breaks or fails: it's just a spring controlled hole.
You must be sure it's the proper one: not too big, not too small.

If you're orifice meets the capacity of the compressor and you're sure you get full liquid before the valve, then I should install a new valve.

Increasing HP doesn't change in such a way that it will function. If you have 6 bar DP (aha, that's how much psi??) then you have more than enough.

Try to use SI units between brackets. I'm not used to work with IP units and I read not thoroughly post in IP units.

[Daveal3D]

Decided to install a sight glass today and i was actually surprised. There was allot of bubbles, i mean allot i could see refrigerant running full force.
When i was installing the sigh glass i decided to take a look in to the plate heat exchanger (evaporator) and it was full of oil. :eek:
If the evaporator HE is has oil in it than no doubt the condensing HE is full of oil as well.

Could that be the cause for low suction pressure, if oil is running with refrigerant?

[Peter_1]

Oil acts as an isolator whereby the refrigerant doesn't get any load or at least not sufficient.
Oil can' t boil off and blocks the evaporator, not only a PHE but any evaporator.
Is the PHE bottom fed?
You can open the TEX fully for some minutes so that refrigerant flushes the oil out the PHE.
The Maneurops can withstand liquid slugging for some time.
Try this once.

[Daveal3D]

TXv is on the bottom, I'll try opening it.
I know that oil wont cause the suction pressure drop, but can it block the TXV?

I tried lowering the superheat today and couldn't get it lower than 18°C the suction line starts to freeze. If i turn the superheat any lower the pipe goes from 12°C to -2°C in a few seconds.

[Peter_1]

TXv is on the bottom, I'll try opening it.
I know that oil wont cause the suction pressure drop, but can it block the TXV?

It can definitely drop the suction pressure because there's no load on the evaporator due to the isolating oil


tried lowering the superheat today and couldn't get it lower than 18°C the suction line starts to freeze. If i turn the superheat any lower the pipe goes from 12°C to -2°C in a few seconds.

Don't look that the pipe frosts, let it run for several minutes with TEV full open and see if you measure improvement after you set the TEV as it was set before.
I even should say that you replace with 2 sizes bigger orifice for some minutes.
Measure if you see a difference after a flush.

[Daveal3D]

Right now I've got the largest orifice for this TXV range -.6 that's gives about 16kw previous one gave about 10kw.

I have already lowering the superheat to about 10°C for 5 minutes but i put it back because the pipe froze over pretty quick, didn't see any difference in pressure, it was the same 3bar (42Psi) suction.

I'll leave it open longer next time to see if anything changes.