Chiller - Excessive Water Flow 1

[TECMSC]

We had an oil level switch failure on a 300 ton air cooled chiller that was apparently caused by "excessive water flow".
The excessive water flow caused the refrigerant oil to flash and migrate from the evaporator to the condenser. If chiller is fully loaded there isn't a problem. when the chiller is satisfied and the a or b circuit is shut down the water flowing through the evaporator barrel flashes the refrigerant to a vapor that migrates to the condenser. When the chiller prepares to restart the second circuit of the chiller due to increased load the oil level switch fails causing the failed circuit to go into alarm.

Can someone exlain how this happens?
Thanks

 

[TXiceman]

Is this a dual circuit chiller barrel (HX) that operated on a TXV?

What is the refrigerant and which oil is being used?

Is it screw compressors or recips?

IS the TXV control the older thermal type valve or the newer electronic expansion valve (EXV)?

In a DX system, it is imperative that you pump down a circuit (close the liquid solenoid and let it cycle off on the circuit low pressure pump down control).

During the off cycle, the compressor oil sump heaters must be energized.

The oil level failure could be caused by refrigerant migration into the oil. The compressor will settle at some pressure between normal suction and discharge pressure when shut down. The compressor crankcase ususally is operating at suction pressure. When off, the refrigerant will have a tendency to migrate to the oil if it is not warm and/or not pumped down. If you are pumping down on shut down, you could have a leaking liquid line solenoid.

In any case, with the oil saturated with refrigerant, once you start, the suction pressure drops as the compressor pulls down the suction pressure. The suddendrop in pressure will alow the refrigerant in the oil to flash and the oil basically turns to foam and can leave the compressor. When you start up, watch the oil sight glass on the compressor and see if the oil foams out.

A screw compressor has the oil sump at discharge pressure, so oil foaming ins somwhat of a lesser problem.

But first place I would check is refrigerant migrationand oil problems.

Ken

TXiceman

http://www.rae-corp.com

 

[TECMSC]

Ken,
We're in the process of finding the root cause. To answer your questions:
-Dual circuit
-134a refrigerant but not sure on the oil
-Screw compressors
-EXV

How excessive water flow can cause a problem still confuses me. My thought is the excessive water flow i.e. excessive fps, reduced the heat transfer between the refregerant and water. The refrigerant didn't heat up causing the CHWS to rise and the chiller to work harder and harder until the water in the barrel froze...the 20 deg weather didn't help.

The info was helpful and the ice has melted form the barrel with no ruptured tubes!
Thanks,
Tom

 

[Drazen]

It should be useful to know complete operation sequence, but from what you say it seems that your water is too cold when one of chillers don't work. (I presume there is some kind of bypass valve, it is not normal that water flow just go down by 50% when one chiller is off.)

That means too small heat gain for evaporator. Did you maybe notice too high vacuum in evaporator? If expansion valve tends to throttle to the maximum and head pressure is low because of lower demand - maybe too high vacuum causes drawing of oil from oil sump?!

What should be the solution? For sure, if you have possibility to regulate compressor load, that would be the best, but it seems you don't have such option (otherwise it would work!)

The solution with throttling of compressor inlet is not so attractive any more, it is not simple and it's energy inefficient.

What you can try for sure is to change superheat setting. If you reduce superheat to possible minimum, valve will open to allow more flow, and vaccum should be reduced.

 

[sterl]

An air conditioning chiller on a 365-day/year load...Could be a control topic.

If the screws have no capacity reduction arrangement (slide valve or bypass flow arrangement) they are starting with full rotor displacement, but with the Outdoors at 20-deg they are not going to have very much in terms of discharge pressure...which means off the start, you will have a whole lot of CFM capability...

Lubricant sump on most screw machines is on the discharge side of the compressor and there should be relatively little lubricant in the evaporator. Discharge Check is usually downstream of the sump, though it may not be. Compressor suction side will be at just above sump pressure unless there is some means of off cycle implemented isolation. During off cycle, lubricant (and its not mineral oil, with R-134A) will tend to draw refrigerant.

At 20-deg O/D during off cycle, the discharge side will be at very low pressure and tend to draw refrigerant out of the lubricant providing the lubricant is warm. All the refrigerant in the condenser is going to be at that temperature, and that's where most of the refrigerant is going to be. It represents a pretty big flywheel in terms of the restart.

On initial restart, a full-swept volume compressor is going to make very high CFM for some period; and that results in very high velocities through the separator...And the oil is entrained to the discharge side. Depending on orientations and head pressure control, it will either stay with the still cold refrigerant or when the condenser gets real cold, it coule either fall to the bottom of the condenser or rise to the top of the liquid in the condenser depending on its Sg and immiscibility curve.

if it has the chance to head for the bottom, it creates some real problems when the EXV, not knowing any better, deliver a lot of oil to the chiller, which fouls the chiller and makes the suction pressure remain low for a LONG time.

So:

Check what the Lube is and whether it is heavier than R-134 at Low Temp.

If your compressor(s) have a Capacity Control arrangement, and its restart process requires it to be at minimum swpet volume on restart, and the High Side has regulators and/or mixing valves for Head Pressure Control, you can probably get there by simply slowing the compressor load-up process enough. If your compressor does not have capacity control, your head pressure control arrangement almost has to hold back pressure on the discharge side of the condenser rather than the drain line side. If this is a Full Swept Volume machine from restart and you are on Fan Speed Controls for head pressure and you don't have any other storage for refrigerant on the plant's high side: Good Luck.