Minimum normal suction pressure for R134a chiller

What is the lowest normal operating suction pressure that is acceptable on R134a chiller running water with no glycol or other low temp additives?
We are operating a 280 ton R134a chiller using a 45 deg F evaporator leaving temperature setpoint. ~630 gpm chilled water flow with an evaporator inlet temperature of 46-55 degrees depending on the load.

We has issues in the past with the mechanical TXVs that were too slow to respond and resulted in liquid flood back during rapid unloading. We are currently installing EXVs and revising the PLC logic to control the EXVs and maintain superheat control best that we can. Bitzer recommends that we run as much superheat as we can on these CSH9571 compact screws, but I am running into issues with running reduced suction pressures in order to maintain 15+ deg of suction superheat.

During the entire load range suction temperature varies between ~45deg at low load and ~48 deg at high load. During lower load operation suction pressures will operate steady state at ~26psig (30 deg sat temp). Evaporator inlet pressures will be no lower than 32 psig at minimum load.

I have always been told to NEVER operate steady state below 28psig (32deg sat temp) to prevent freezing. Due to the water being at least 46 deg at the evaporator inlet and the suction gas being superheated 15 deg (measured suction temp of >45 deg) do I still have to worry about freezing when we operate at 26 psig suction pressure?

The only way I can maintain 15 deg or more suction superheat, but not operate with suction pressure below 28psig is to increase the water temperature which is currently fixed at 45 deg evap outlet due to design requirements and calculations.

What kind of evaporator do you have? brazed heat exchanger, liquid flooded, tube?
whats the model of chiller?

Shell and tube evaporator with dual refrigerant circuits. It is a custom chiller unit using Bitzer compressors.

So the refrigerant is in the shell and water in the tube?
After exv the refrigerant goes into the bottom of the shell, and suction pipe is connected to the top of the shell?
What you have then is a liquid flooded evaporator, so it should be easy to maintain a lower difference between water outlet and evaporating temperature.

If you have 45 degree water outlet and 30 degree evap temp. the superheat will never be higher than the lowest temperature which in this case is 45 degree. (45-30=15)
Try to set the superheat to 8-12k to bring up the evap temp.

I would not have run the system over time with 30 degree evap temperature, since the water is going through the copper pipes 3/4" and the shell is filled with liquid boiling off.

sorry, water is on the shell side and refrigerant on the tube side.

Hi Rallas
That is correct ,you never let the suction pressure go below the freezing point of water ie (R134A 32DegF =28 PSIG ) as in the chiller barrel you can get areas of poor water movement which can end up with localised freezing around the tubes and then crushing the tubes, causing failure. Ideally you should have a Low pressure switch set at 28 PSI to stop the machine in that situation.
Your problem seems due to the fact your trying to run the chiller with much too high superheat. This has 2 effects 1) you run with a low suction gauge 2) you lose system capacity.
It is good your fitting Electronic expansion valves as with them you can run with lower superheats and with increased capacity
You can easily run with superheats down to 8 or 10F or possibly lower providing you use a complete dedicated EEV superheat controller. A custom programmed PLC may end up giving a poorer and slower control than the original TEVs

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Originally Posted by rallas

What is the lowest normal operating suction pressure that is acceptable on R134a chiller running water with no glycol or other low temp additives?
We are operating a 280 ton R134a chiller using a 45 deg F evaporator leaving temperature setpoint. ~630 gpm chilled water flow with an evaporator inlet temperature of 46-55 degrees depending on the load.

We has issues in the past with the mechanical TXVs that were too slow to respond and resulted in liquid flood back during rapid unloading. We are currently installing EXVs and revising the PLC logic to control the EXVs and maintain superheat control best that we can. Bitzer recommends that we run as much superheat as we can on these CSH9571 compact screws, but I am running into issues with running reduced suction pressures in order to maintain 15+ deg of suction superheat.

During the entire load range suction temperature varies between ~45deg at low load and ~48 deg at high load. During lower load operation suction pressures will operate steady state at ~26psig (30 deg sat temp). Evaporator inlet pressures will be no lower than 32 psig at minimum load.

I have always been told to NEVER operate steady state below 28psig (32deg sat temp) to prevent freezing. Due to the water being at least 46 deg at the evaporator inlet and the suction gas being superheated 15 deg (measured suction temp of >45 deg) do I still have to worry about freezing when we operate at 26 psig suction pressure?

The only way I can maintain 15 deg or more suction superheat, but not operate with suction pressure below 28psig is to increase the water temperature which is currently fixed at 45 deg evap outlet due to design requirements and calculations.

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To operate at suction temperature below 32F you need water flow protection. If water pump motor trip or pump broken(no flow), the water can be frozen inside the chiller.

I would advise (if it is possible) to install suction line heat exchanger in order to increase subcooling and total superheat. That way, you can use evaporator superheat as low as possible, and total superheat as required by Bitzer. Also, you gain in capacity with higher subcooling.
With such decrease in superheat, you are running with higher evaporation temperatures and moving further away from water freezing point.

http://www.funke.de/files/funke_shell_tube_he_e.pdf

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Originally Posted by Glenn Moore

Hi Rallas
That is correct ,you never let the suction pressure go below the freezing point of water ie (R134A 32DegF =28 PSIG ) as in the chiller barrel you can get areas of poor water movement which can end up with localised freezing around the tubes and then crushing the tubes, causing failure. Ideally you should have a Low pressure switch set at 28 PSI to stop the machine in that situation.
Your problem seems due to the fact your trying to run the chiller with much too high superheat. This has 2 effects 1) you run with a low suction gauge 2) you lose system capacity.
It is good your fitting Electronic expansion valves as with them you can run with lower superheats and with increased capacity
You can easily run with superheats down to 8 or 10F or possibly lower providing you use a complete dedicated EEV superheat controller. A custom programmed PLC may end up giving a poorer and slower control than the original TEVs

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We initially ran with 8 deg superheat and the system worked, but we tried all the PID tuning in the book and could not get superheat to swing less than +-4.0 deg. The 4.0 deg minimums superheat we observed was just too low. Per Bitzer's recommendations we increased the superheat setpoint as high as possible. Tried 20 deg initially, but suction pressures were in the very low 20's. Moved to 15 deg of superheat and suction pressures would stay in the 26 psig range and operation was rock solid with superheat staying within 0.5deg of setpoint during steady state operation and even during transients we wouldn't see swings more than +-4 deg.

We are going to try to reduce the setpoint to 12 deg during our next round of testing. Based on the minimum suction temp we usually see of 45 deg, we should not have any refrigerant in the evaporator below 33 deg or 28.5 psig. We did limited testing using 12 deg superheat while at full load and it was stable, but have not tested operation with 12 deg at lower loads.

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Originally Posted by nike123

I would advise (if it is possible) to install suction line heat exchanger in order to increase subcooling and total superheat. That way, you can use evaporator superheat as low as possible, and total superheat as required by Bitzer. Also, you gain in capacity with higher subcooling.
With such decrease in superheat, you are running with higher evaporation temperatures and moving further away from water freezing point.

http://www.funke.de/files/funke_shell_tube_he_e.pdf

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Unfortunitely we have no room to add any heatexchangers like this. The footprint is fixed and the chiller is located in a small room.

Hi Rallas
Have you looked at the Danfoss EKD 316 controller for the ETS stepper expansion valves In these superheat controllers you can choose between 2 control modes.
1) The controller comes at default with the control mode on MSS control.
MSS control is (Minimum Stable Superheat) where the controller constantly tries to run with the lowest possible stable superheat between parameter set points ie Min S/heat and Max S/heat. This works well with steady loads, and can give enhanced duties.
2) By changing from MSS control to what we call ( UPLOAD ) control, you can set the controller parameters Min S/heat and Max S/heat depending on the opening degree of the valve and the chiller duty. On chiller's where you have capacity control the superheat setpoint will change within the MIN/MAX s/heat set points.
So if for instance we set the MIN S/heat parameter at 5 F and the MAX S/heat at 9F then when the opening degree of the EEV is between 0-25% the control will try to run the s/heat at around the 5 F mark, as the duty increases and the load and the opening degree of the valve increases the S/heat set point will increase also, ie at full load duty /opening of valve above 75% the S/heat setpoint will be at the 9 F.
As you know the liquid refrigerant inside the chiller at minimum load is like water simmering in a pan bubbling away but fairly stable , as the load increases the boiling effect starts to make the water start to bubble and spit and becomes not so stable. As the load increases further the boiling effect becomes more and more erratic and unstable which is the same as the refrigerant inside the chiller. At full load the liquid and vapour bubbles at the chiller exit make it difficult for the valve and controller to keep the superheat stable, as like the water boiling there are spits of water trying to get out with the steam.
So the EKD controller changes the superheat set point depending on the load of the chiller. Ie the S/heat at Min load is low and S/heat at MAX load is High .When I set the controller up I run the chiller in manual and the EKD in manual with a fixed S/heat setpoint and then run the chiller at MAX load and find the best stable S/heat and then run it at MIN load and find the best stable S/heat . These settings are then transferred to the controller and then put into AUTOMATIC control. This allows the chiller to run and give good control right through the capacity range but also gives enhanced part load efficiency.
As with any expansion valve,thermostatic or electronic the liquid injection should always give STABLE CONTROL to the system under any load.

To improve dynamic of your chiller try to to slow down loading and unloading of your compressors.

The point is what water temperature you like to maintain, then make sure don’t allow Evap temp fall below 32F to protect from freeze up.