Chiller Troubles

[Stiff Nipples]

I just started up a Medium Temp Glycol Rack. I am having trouble getting my Glycol supply temp down to the 20 F spec'd. The rack has 2 plate type chiller barrels with 2 SEH 100 EEV valves (1 per barrel and sized correctly). Both EEV's are controlled by the Sporlan Chiller Controller. Glycol Supply Pressure is 50 psi and Glycol Return Pressure is 15 psi My Suction is running about 44 psi/ 8 F (R-507) Supply glycol temp is 24 F and return Glycol Temp is 27 - 28 F. I've had some issues getting the EEV's to feed properly with out flooding out the compressors. Superheat measured by both the Sporlan controller (Suction Transducer and SH sensor mounted in a well in the suction) and by standard means (gauges and Temp probe) seems to be fine not dropping below 8 degrees, if set for 10 on the controller. Yet for some reason the EEV's seem to still allow a bit of liquid by. This can be corrected by bumping my SH set point on the controller so I'm not sure if this is the only reason I can't get my supply temp down. I'm wondering if there is something else I can look at, maybe my system Gpm and if so what exactly should I be looking for? Or is it a system design issue. I've heard of other glycol racks that have 2 TXV's per barrel one 60 ton and one 40 ton with only one or the other feeding at any given time. Any Ideas would be appreciated.

[750 Valve]

When you say flooding the comps out, is it diluting the oil? What sort of comps - recips? Are you using the return vapour to subcool liquid lines or anything of the sort? Seems like it may be an issue with setup of superheat controls, maybe programming - like PID loops or if not are you sure your probe is mounted in the best spot possible?

[Stiff Nipples]

They're Recips, 9 in total. We put in 10 gallons of oil initial charge and 4 more after start up, for a total oil charge of 14. Maybe Flooding was a bad choice of words but basically it's been difficult keeping a steady level of oil in the comps. I added 2 gallon to the resevoir and the next day it was gone? Nothing has be tripping on oil failure since relocation of the Superheat sensor which is placed at the 4:00 position, in a well, in the suction. The oil doesn't have far to travel so I'm a little hesitant in adding to much more oil with out finding out where the rest of it went. There is no subcooling loop or heat exchange of any kind on the suction header. Though the idea has been entertained. So the PID of the controller is set up like this. The "P" factory setting for the Large valve (100 tons +) is 45 steps open/closed per degree superheat over/under superheat set point. I changed a couple days ago to 30. At the factory setting you can actually see the valve open (50 psi) and the valve close (30 psi) all within a 20 sec time period. At 30 it seems to be more stable 48 - 44 pressure swing. The "I" factory setting of 10 sec for every time the valve references it's position, superheat, superheat set point. I changed that to 1 sec given how close the chillers are to the comp. The "D" I'm not sure what the factory setting is as I haven't changed it.

[Stiff Nipples]

What about heat trace around the accumulators? If so what kind of wattage would I be looking to put out?

[wambat]

sounds to me like the glycal flowrate is too hi can you slow it down slowly and see if you can get the temp down?

[Stiff Nipples]

There are 2 Triple Duty Valves on the outlets of either pump. They're supposed to be an isolation valve, check valve and a flow regulating valve. A port on the inlet and outlet for measuring inlet and outlet pressure and marker rings on the shaft(supposedly for gpm) Is there any way to calculate my gpm by supply return pressure without using those ports? According to my sys spec sheet total system gpm is 450. Could it be possible that I have to much glycol in the system running supply 50 psi and return 15 psi?? Could incorrect setting of case balance valves could affect my total system gpm( ie. too high a setting)? The impeller on the pump say it is rated for 460 gpm is that the max that pump will move?

[Gary]

Sounds to me like a fouled evaporator (normal to low superheat, high approach temp). You may need to acid clean the glycol side. Rare but possible is oil logging on the refrigerant side. Either way we are looking at an insulative coating between the refrigerant and the glycol.

If the system is new and/or has never worked right, then we need to look at the flow. Is it set up counterflow, possible air locking, right GPM, suction line sizing for oil return, oil traps, etc.?

[BUSHY]

hi fellas instead of having your superheat sensor mounted in a well try and strap it straight to the suction line and insulate it if you still find that you are getting liquid back to the compressor it may be that the sensor can not pick up the temp in the well straight away also it would be worth checking the sensor against a digitemp or some other calibrated thermometre, if it still happens you will need to look at the pid setup of the controller to get it to react quicker

[Gary]

The supply glycol temp is 24F, while the refrigerant temp is 8F. That's a HUGE spread. With good heat transfer these temps would be much closer to each other.

The superheat is low, because the load is low, because there is very little heat being transferred from the glycol to the refrigerant... and the oil isn't returning for the same reason. At low load there is insufficient refrigerant flow to bring it back.

Forget about the superheat. The problem is in the chiller barrel.

Is this a new machine?

Has it ever worked right?

[Stiff Nipples]

So heres an update as of the end of work on Thursday. When I originally started the rack up and had issues with oil return I figured I drive my head up from 150 to 180 and increase the velocity in my lines as more compressors cycled on.....well as it turns out I am my own worst enemy! :) Turns out that by doing so I increased the pressure drop across my EEV and in turn increased the tonnage of the valve. By looking at the system specs and the valve specs. I require just over 50 tons per barrel and with a 150 psi pressure drop across the valve I was making the valve operate as a 75 ton valve. By dropping my head to 150 and bringing my suction up to 46 I was able to get the valve to act more like a 60 ton. And was able to maintain my 24 degrees and start dropping my superheat with out building frost on the end bells of my compressors. Of course I had to figure that out at the end of the day and work week! I still don't think that it's the whole problem but at least it's moving forward.