Cryo Refrigeration Help

[Gary]

My thought is that the refrigerant condenser is fouled or under performing, so we aren't able to get adequate cooling to condense the additional refrigerant circulation when both machines are on. Still, I am wondering if there are any other likely problems that I might be overlooking.

That condenser would be my primary suspect as well. I would check the water in and out temps, the saturated condensing temp, and the leaving refrigerant liquid line temp.

Its probably a reach, but the one thing I am aware of that is different than normal is that the refrigerant (liquid) levels in the economizer and low pressure receiver are higher than usual (but still <50%). I don't suspect any liquid carryover to the compressors. Could too much refrigerant in the system lead to this problem?

Has refrigerant been added to the system recently?

The danger is not so much liquid carryover to the compressors as it is excess liquid backing up into the condenser, thus limiting the condenser's ability to reject heat.

[Cryo99]

Yes, due to some work we lost refrigerant levels. When we recharged the system I think we might have put too much (or at least more than we previously had).

There is a high pressure receiver between the economizer and the refrigerant condenser that does not have a liquid level, which is normal from the time I have been watching the unit. Since there this vessel isn't flooded, I don't believe that there is liquid backing into the condenser.

The reason I was curious about the levels is that I have observed high pressure (with just one compressor running) until level gets established in the LP receiver. I assume though that this is because, until there is a liquid level there, we are "short circuiting" the system and not getting the cooling from throttling down at the inlet to the LP receiver.

[Cryo99]

That condenser would be my primary suspect as well. I would check the water in and out temps, the saturated condensing temp, and the leaving refrigerant liquid line temp.

CW inlet T = 79F
CW outlet T = 87F
Refrigerant inlet T = 135F
Refrigerant inlet P = 217.5psig
Refrigerant outlet T = 94F
Refrigerant outlet P = 204psig

The outlet T/P falls on or just above the vapor pressure curve. I got results at a lower load and they were mvoed down the curve. I believe this means that it almost has to be a cooling limit rather than another problem. Is that correct?

The compressor discharge seems to have ~40F of superheat. Does that seem right for the screw compressor?

[Gary]

CW inlet T = 79F
CW outlet T = 87F
Refrigerant inlet T = 135F
Refrigerant inlet P = 217.5psig
Refrigerant outlet T = 94F
Refrigerant outlet P = 204psig

I don't have a pressure/temperature chart for propylene, which is why I requested saturation temperatures rather than pressures.

[Cryo99]

Sorry. The condenser outlet is on the curve (or just on the liquid side) depending on the accuracy of the measurements.

At the condenser outlet P, the sat T is 96F.

[Gary]

CW inlet T = 79F
CW outlet T = 87F
Refrigerant inlet T = 135F
Refrigerant inlet P = 217.5psig
Refrigerant outlet T = 94F
Refrigerant outlet P = 204psig/96F

87-79 = 8F dT. Water flow is adequate.

96-79 = 17F TD. Load not excessive... if anything light load.

96-87 = 9F approach. Condenser not fouled. Non-condensables unlikely.

96-94 = 2F SC. Not overcharged... if anything undercharged.

I find the pressure drop across the condenser (217.5-204 = 13.5psi) puzzling.

And we still have no explanation for the system kicking out on high discharge pressure. What is the control's pressure/saturation temp setpoint?

[Cryo99]

87-79 = 8F dT. Water flow is adequate.

96-79 = 17F TD. Load not excessive... if anything light load.

96-87 = 9F approach. Condenser not fouled. Non-condensables unlikely.

96-94 = 2F SC. Not overcharged... if anything undercharged.

I find the pressure drop across the condenser (217.5-204 = 13.5psi) puzzling.

And we still have no explanation for the system kicking out on high discharge pressure. What is the control's pressure/saturation temp setpoint?

All of this data was with only the high stage compressor running (and fully loaded). As I loaded the compressor up, the discharge pressure and temps came up, but everything moved up along the vapor pressure curve. (I can post the less loaded data if helpful.)

I can also try to get some data with the booster running (but it might not be steady state as I am not sure we can keep it running that long). The data with the booster might have to wait a week or so though, as I will be out of the plant for a while.