Liquid Recirculation/Controlled Pressure question.

[R717Kid]

Hi,

I'm new to R-E, however I've seen and used a lot of discussion detail from search results that were talked about on here. I'm working on a NH3 liquid recirculation system that was handed to me for an energy savings project. Of the many confusing details about the system and lack of information, I'm scratching my head on how they have been controlling the CPR. The condenser(s) dump through level traps and into the CPR where the pressure in the CPR is maintained (or was) at 50 psig while the system suction pressure was running at 25-30 psig. It appears through piping that was cut off that the system was formally gas pumped and now is mechanically pumped from the recirculation vessel. I could understand the pressure control if it were gas pumped, but does anyone understand why they left the CPR that way. Raising the pressure to 65 psig in the CPR reduced flash gas produced here and overall load. It would seem to me that controlling the pressure based on current system operation is pointless especially since the liquid makeup comes from here and we need a given pressure differential across the hand expansion valve on the makeup to work properly. Any ideas?

[Segei]

Hi,

I'm new to R-E, however I've seen and used a lot of discussion detail from search results that were talked about on here. I'm working on a NH3 liquid recirculation system that was handed to me for an energy savings project. Of the many confusing details about the system and lack of information, I'm scratching my head on how they have been controlling the CPR. The condenser(s) dump through level traps and into the CPR where the pressure in the CPR is maintained (or was) at 50 psig while the system suction pressure was running at 25-30 psig. It appears through piping that was cut off that the system was formally gas pumped and now is mechanically pumped from the recirculation vessel. I could understand the pressure control if it were gas pumped, but does anyone understand why they left the CPR that way. Raising the pressure to 65 psig in the CPR reduced flash gas produced here and overall load. It would seem to me that controlling the pressure based on current system operation is pointless especially since the liquid makeup comes from here and we need a given pressure differential across the hand expansion valve on the makeup to work properly. Any ideas?
Do you have low pressure receiver? Do you have ammonia liquid pumps? Where the liquid is going from CPR? How are you going to save energy?

[R717Kid]

There is not a high pressure reciever. We do use liquid pumps that are located under the recirculation vessel. I could see pressure control on the CPR if we employed gas pumping means, but we do not. The money savings has come in the way of increasing the suction pressure from 25-30 psig to 35-40 psig. A ten pound increase that has been a tedious job. Another is reduced head pressure control. The system had been fixed to have condenser pumps and fans start at 140 psig and turn off at 120 psig. With the recip compressors, a lower overall compression ratio has netted positive gains since the head is now controlled to start the fans only at 110 psig and allow to rise to 140 psig where the water spray will begin. In the upper midwest I have found that running the fans only in this system for the majority of the year is possible while maintaining a good pressure. Since the system is for "cool storage", the evap fans have been reprogrammed to run even when not cooling which provides a more even space temp. I am pushing for VFD control on all fan motors etc. Just can't fiqure out why they had run the pressure in the CPR so low when there is no obvious benefit that I can see.

[Segei]

There is not a high pressure reciever. We do use liquid pumps that are located under the recirculation vessel. I could see pressure control on the CPR if we employed gas pumping means, but we do not. The money savings has come in the way of increasing the suction pressure from 25-30 psig to 35-40 psig. A ten pound increase that has been a tedious job. Another is reduced head pressure control. The system had been fixed to have condenser pumps and fans start at 140 psig and turn off at 120 psig. With the recip compressors, a lower overall compression ratio has netted positive gains since the head is now controlled to start the fans only at 110 psig and allow to rise to 140 psig where the water spray will begin. In the upper midwest I have found that running the fans only in this system for the majority of the year is possible while maintaining a good pressure. Since the system is for "cool storage", the evap fans have been reprogrammed to run even when not cooling which provides a more even space temp. I am pushing for VFD control on all fan motors etc. Just can't fiqure out why they had run the pressure in the CPR so low when there is no obvious benefit that I can see.
It looks like you use CPR as a receiver. Liquid will collect in CPR and from CPR will go to the recirculator. It doesn't matter what is the pressure in CPR. The only should be pressure difference between condensing pressure and CPR pressure to move liquid from condensers to CPR. Should be pressure difference between CPR and recirculator to move liquid from CPR to recirculator. What is the temperature in your coolers? Usually, operating only condenser fans is wasting of energy. Do your evaporators located in penthouses?

[R717Kid]

The evaporators are located in penthouses, 2 of the 3 coolers have duct work that exits from the evaps. The temperature of the coolers is to maintain between 30-35 degrees. Due to the size, arrangement, humidity, etc. I've been able to raise the suction pressure and still maintain the desired temp in a more controlled manner than previously done.
My "choice" for raising the pressure in the CPR came from the idea of liquid falling into the CPR and flashing off some of the liquid that had already been cooled. Raising the pressure proved this and reduced the load at the compressor. The liquid that is supplied to the recirculator is fed through a hand expansion where my understanding leads me to believe that there is a needed pressure differential in order to maintain the Refrigerating Effect across the expansion device which points me to raising the pressure in the CPR even further unless I am wrong.
Due to the size of the condensers and capacity, it seemed more logical to run the fans more consistently as opposed to cycling the fans/pumps on and off on a momentary basis. Monitoring the electrical usage has substaniated this thought.

[charlie n]

Two speed motors on the condenser fans will give you a good saving.
A better condenser pressure control will save money. Try a Danfoss EKC331 or something similar. Or better still, a floating head pressure controller that controls the condensers based on approach to wet bulb temperature. If you have screw compressors use the CPR as a flash economizer. If single stage recips. the CPR pressure setting is thermodynamically meaningless.

[Segei]

The evaporators are located in penthouses, 2 of the 3 coolers have duct work that exits from the evaps. The temperature of the coolers is to maintain between 30-35 degrees. Due to the size, arrangement, humidity, etc. I've been able to raise the suction pressure and still maintain the desired temp in a more controlled manner than previously done.
My "choice" for raising the pressure in the CPR came from the idea of liquid falling into the CPR and flashing off some of the liquid that had already been cooled. Raising the pressure proved this and reduced the load at the compressor. The liquid that is supplied to the recirculator is fed through a hand expansion where my understanding leads me to believe that there is a needed pressure differential in order to maintain the Refrigerating Effect across the expansion device which points me to raising the pressure in the CPR even further unless I am wrong.
Due to the size of the condensers and capacity, it seemed more logical to run the fans more consistently as opposed to cycling the fans/pumps on and off on a momentary basis. Monitoring the electrical usage has substaniated this thought.
Do you supply liquid to recirculator from CPR?
Assume that pressure in CPR is 50 psig. This is corresponding to temperature of 34F. Liquid from condenser 120 psig(73F) will throttle to CPR. To cool this liquid from 73F to 34F part of this liquid will evaporate(flash). Temperature of incoming liquid and liquid in CPR will be 34F, if this CPR used as receiver.
I don't know what mean about Refrigerating Effect across the expansion valve, but to supply liquid from CPR to recirculator you need pressure difference(or pump). Liquid from CPR at 50 psig(34F) will be throttled to 30 psig(17F) and part of this liquid will flash. It doesn't matter you throttle liquid from 120 psig to 50 psig to 30 psig or from 120 psig to 70 psi to 30 psig.
Evaporative condensers use much less energy if you run water pumps first and then fans. This approach will prevent scale formation as well.:)

[R717Kid]

The liquid makeup in the recirculator comes from the CPR. My delima is that the liquid coming from the condenser is 73 degrees, falling into 34 degree liquid. I watch the colder liquid boil (through sight glasses) in the CPR so a pressure/temperature drop is taking place in the CPR. Given that some liquid will flash as it is fed from the CPR to the recirculator, why would I want to cool that liquid twice? Would it not be better to use the CPR as a HPR?
Refrigerating Effect. Going from a higher potential to a lower potential via a hand expansion valve, flash gas will be produced which in turn causes a cooling effect or RE, otherwise the valve is simply metering a fluid.
In my case, one of the symptoms that lead me to this and the possibility of an oversized expansion valve is that when the liquid makeup solenoid opens and feeds the refrigerant through the expansion valve, the temperature of the refrigerant to the evaporators increases and cavitation of the pump ensues.
Thanks for the help, I am listening.:)

[Segei]

The liquid makeup in the recirculator comes from the CPR. My delima is that the liquid coming from the condenser is 73 degrees, falling into 34 degree liquid. I watch the colder liquid boil (through sight glasses) in the CPR so a pressure/temperature drop is taking place in the CPR. Given that some liquid will flash as it is fed from the CPR to the recirculator, why would I want to cool that liquid twice? Would it not be better to use the CPR as a HPR?
Refrigerating Effect. Going from a higher potential to a lower potential via a hand expansion valve, flash gas will be produced which in turn causes a cooling effect or RE, otherwise the valve is simply metering a fluid.
In my case, one of the symptoms that lead me to this and the possibility of an oversized expansion valve is that when the liquid makeup solenoid opens and feeds the refrigerant through the expansion valve, the temperature of the refrigerant to the evaporators increases and cavitation of the pump ensues.
Thanks for the help, I am listening.:)
About CPR and HPR. You go to the store and spend $4 and $6 or just $10. No difference.
HEV is just metering device. Outlet pressure of HEV is lower and corresponding temperature will be lower as well. To lower temperature of the refrigerant, part this refrigerant will evaporate. Refrigeration effect will happen in the evaporator. Refrigerant will take heat from the air and it will evaporate.
Sometimes, HEV is oversized or it is open too much. Typically it should be adjusted that solenoid of this HEV will be open 50-70% of the time. 50% means that it is open and closed equal period of time(2 and 2 min or 5 and 5 min). If it is open too much, suction pressure will increase when solenoid is open. Suction pressure will drop when solenoid close. At this moment ammonia pump can start cavitate. Adjust HEV.

[R717Kid]

Thanks, I'll give it some more effort. I appreciate the input.:)

[Segei]

Energy savings in industrial refrigeration is complicated issue. It is easy to do first steps. However, every next step will be harder and harder. Good luck.:)

[Camille]

From what I am reading it looks that the system before is gas pressure drum that was converted to a forced pump pressure liquid recirculator. Since you do not have a high pressure receiver on your system and to maintain a constant feed of liquid to the recirculator you will require a high side float on the system, whatever condenses is fed to the recirculator based on a pressure differential of the condensing pressure and system suction pressure.see attached sketch.
Liquid drain pipe from the condenser can be oversized at the mechanical room to act as holding receiver

Regards
Camille Zabbal

[R717Kid]

From what I am reading it looks that the system before is gas pressure drum that was converted to a forced pump pressure liquid recirculator. Since you do not have a high pressure receiver on your system and to maintain a constant feed of liquid to the recirculator you will require a high side float on the system, whatever condenses is fed to the recirculator based on a pressure differential of the condensing pressure and system suction pressure.see attached sketch.
Liquid drain pipe from the condenser can be oversized at the mechanical room to act as holding receiver

Regards
Camille Zabbal
Camille,

Thanks. Actually, this is exactly what we have. There is a riser that extends up from the float about 5 feet, then enlarges to the size of the condenser line. The lower side of the float dumps into the controlled pressure reciever. So in that respect, the controlled pressure reciever is acting as a holding reciever. Now, as in the sketch, the riser above the float does have one sight glass, yet rarely is there anything ever seen as it drains very fast into the CPR. If it helps to visualize, the CPR is insulated and yes it was at one time a gas pumped system with the CPR on the outside of the building and now resides indoors within 12 feet of the recirculator.

[AmmoniaMike]

About CPR and HPR. You go to the store and spend $4 and $6 or just $10. No difference.
HEV is just metering device. Outlet pressure of HEV is lower and corresponding temperature will be lower as well. To lower temperature of the refrigerant, part this refrigerant will evaporate. Refrigeration effect will happen in the evaporator. Refrigerant will take heat from the air and it will evaporate.
Sometimes, HEV is oversized or it is open too much. Typically it should be adjusted that solenoid of this HEV will be open 50-70% of the time. 50% means that it is open and closed equal period of time(2 and 2 min or 5 and 5 min). If it is open too much, suction pressure will increase when solenoid is open. Suction pressure will drop when solenoid close. At this moment ammonia pump can start cavitate. Adjust HEV.
Just a quick question from me here. If some of the flash gas comes off at a higher saturation temp and less comes off at a lower saturation temp won't that be less work for the compressor than if it all comes off at the lower saturation temp? Wouldn't that be less energy consumed by the compressor? Not sure about this but that seems to be the basic idea behind economisers.

[Segei]

Just a quick question from me here. If some of the flash gas comes off at a higher saturation temp and less comes off at a lower saturation temp won't that be less work for the compressor than if it all comes off at the lower saturation temp? Wouldn't that be less energy consumed by the compressor? Not sure about this but that seems to be the basic idea behind economisers.
You are right, but if you compress this flash gas from higher saturated temperature to the condensing pressure. To do this you need screw compressor with economizer. This plant has recip. compressors and all flash gas goes to the suction pressure. Typically, plants like this (high temperature suction) don't have economizer which apply for the plants with low suction pressures.

[sterl]

Some of these CPR arrangements returned defrost condensate to the CPR; thus the lower pressure simply replaced the setting on an inlet pressure regulator....so no regulators. Still pretty effective depending on plant geometry, etc.

Others used the CPR as a means of setting up the differential pressures by which heat reclaim arrangements could be controlled; so if you needed to divert discharge gas from one comppressor to an alternate heat exhanber than the normal condensers, both condensate streams were piped via HP floats to the CPR and thus they were not operationally interdependent, one could be operating at an entirely different pressure from the ohter.

Make sure you are not using the CPR for other purposes before you simplify to an HPR arrangement.

[R717Kid]

This is a single stage system in which the condensers drain through a high side float and into the CPR. Nothing else drains into it and is essentially used as a flash tank which then has to have a high enough pressure differential to make up liquid to the recirculator/low pressure reciever. The remaining pressure is regulated through an A4A back pressure regulator. The system has undergone many changes throughout the years having started as a critically charged system and used a pumper drum (supplied by the CPR) to supply liquid to the evaps. Another cooler was added in the early 90's and the pumps were added under the low pressure reciever while the other still used the pumper drum. Another addition in 04 and the pumper drum was eliminated and pump size increased to supply all 3 coolers.