PDA

View Full Version : Liquid Pump Low Dif



Krohnie84
06-10-2010, 01:04 AM
Hello,
I am having some small troubles with a liquid pump that will cut out on a low dif alarm. It will only occur when the vsl lvl gets above 40%. The pressure of the vsl and the pump are constaint but it will cut out at random only when above 40% on the vsl lvl.

Any ideas would be welcome. Thank you.

NH3LVR
06-10-2010, 02:09 AM
Welcome to the forum Krohnie84!
We need a few more details. Refrigerant, type of pump, etc. Top or bottom feed coils?
Be aware that a differential pressure sensor can be tricky. I have seen cases where a compressor starting after a defrost can cause cavitation which could trip the differential sensor.

Krohnie84
06-10-2010, 07:14 PM
Thank you for your response, the Refrigerant type is R-717 and the liquid pump is an oil less Teikoku pump. I only have the Teikoku on this one vsl all the others are Baldor motors/pumps. The normal running pressure for the Teikoku is 70-75 psi. The dif may depend on the suction pressure at the time but should be between 30-40psi. I will attach two pictures, one of the pump itself and the other of a trending graph showing the pump's pressure and the vessel's level. The yellow line is the level and the green is the pump pressure.

3746

3747

charlie n
06-10-2010, 07:48 PM
The spike in the liquid level is not normal & possibly not actually happening. The situation could be ocuring when make up liquid is introduced to the vessel. (Liquid solenoid opens). If the liquid injection is not entering the vessel properly it may be causing a lot of turbulence causing a false reading at the level transmitter and also causing the pump to loose it's prime & cavitate. Check to see where the liquid makeup comes in. It should not be below the liquid level in the receiver. The best place is into the wet suction return near the receiver inlet.

Krohnie84
06-10-2010, 08:45 PM
Thanks Charlie, I checked the vessel and the liquid make up from the High pressure receiver to the med temp vessel feeds at/around the 50% mark.

Theses pumps are relatively new to the system (installed two yrs ago) but only since January of this year has this problem really become a nuisance. Before the pumps were installed the med temp only fed to four hanging evaps. All of them have TXVs on them with no liquid pumps. Two years ago we added another five units (pent houses) with hand expansions.

RANGER1
06-10-2010, 09:21 PM
It could also be when liquid makeup closes causing sudden pressure drop & pump cavitation.
Check liquid reciever level has good working level & set liquid makeup to 60-70 % on at high load condition .
Compressors should not load to quickly

Krohnie84
06-10-2010, 11:53 PM
@ ranger1-
When the receiver reaches 50% or so it starts to feed the med temp vessel. The liquid passes through a hand expansion that is set and we have checked to make sure that the flow rate has remained the same.

As for the compressors loading up too fast... the controls are set up to load very slowly; as in half hour or more.

NH3LVR
07-10-2010, 12:41 AM
@ ranger1-
When the receiver reaches 50% or so it starts to feed the med temp vessel.
I am not quite following. When the receiver reaches 50%, it feeds what?
I think we need a better description of your system.

Krohnie84
07-10-2010, 12:58 AM
I am not quite following. When the receiver reaches 50%, it feeds what?
I think we need a better description of your system.

This system has a charge of about 24,000 lbs give or take 500lbs.

I have six compressors.

Two are dedicated to med temp (Dock/cooler areas)

One dedicated to low temp

and three are able to swing between low and med temp.

The four that can run on the low side each have economizers.

I have four condensing units, one high pressure receiver, a vertical med temp vessel (where the issue is located) and a vertical low temp vessel.

As I understand it, the High pressure receiver will only feed into the med temp vessel when the liquid level in the receiver is at 50%. The feeding line has a hand expansion valve to regulate the flow of ammonia going to the med temp vessel. I have never had to mess with it besides a solenoid valve going to our purger.

NH3LVR
07-10-2010, 02:38 AM
As I understand it, the High pressure receiver will only feed into the med temp vessel when the liquid level in the receiver is at 50%.

I am a bit more clear now. You have two Low Pressure Receivers, One at low temp and another at Medium Temp. Pumps going out to the coils and wet suctions returning to the LPR's. Standard setup.
What concerns me is what you said about the Med Temp Receiver only filling when the HPR is at 50%. This sounds like a control problem. Usually this is done with a float or electronic level control on the LPR. Check the control system carefully.
Cavitation is most likely the cause of your loss of differential pressure. Some posted a good bit of information on this recently. It may have been Ranger1. Pumps with sufficient head can be very prone to this. Another point to consider is the recirculation rate. If the pumps are allowed to overfeed the coils beyond the available NPSH (Net Positive Suction Head) they will cavitate.
But look at the control system for the LPR first.
That being said sometimes unconventional control systems are used. Sometimes they even work.

Magoo
07-10-2010, 03:52 AM
I agree with NH3LVR,
cavitation is the major problem, with sudden pressure changes etc., do the vessels have anti-cavitation plates, and the pumps have the anti-cavitation infeed adaptor, and the nominal 2 + metres NPSH.

RANGER1
07-10-2010, 09:17 AM
Krohnie84,
When pump cavitates what happens control wise ie stop after time delay etc .
Does the pump when it is restarted pump straight away?

Usually when level in pump vessel increases it should help pump except if pressure drops suddenly , causing boiling of liquid causing gas bubbles & cavitation .

Another cause of cavitation is if pump has low flow through it , but this is usually prevented with a minimum flow line tee'd off discharge of pump back into wet return line or upper part of vessel .
Basically pump should be able to continue running & pumping even if all room solonoids are closed with rooms down to temp .
The bypass line must be open enough to cope with this situation as well .

How is level guaranteed in med temp vessel , does it then supply low temp vessel?
System sounds critically charged type of set up!

Krohnie84
07-10-2010, 03:55 PM
What concerns me is what you said about the Med Temp Receiver only filling when the HPR is at 50%. This sounds like a control problem. Usually this is done with a float or electronic level control on the LPR. Check the control system carefully.

I will look into the controls further, but if I understood my co-worker.. there is a float control in the HPR and it won't allow flow until it gets to 50%. The more I think about this the more fishy it sounds to me as well.


I agree with NH3LVR,
cavitation is the major problem, with sudden pressure changes etc., do the vessels have anti-cavitation plates, and the pumps have the anti-cavitation infeed adaptor, and the nominal 2 + metres NPSH.

This system was built in 1995 and the med temp vessel was originally a pressure fed system. Not until two years ago is when pumps were finnaly added to the med temp side.


Krohnie84,
When pump cavitates what happens control wise ie stop after time delay etc .
Does the pump when it is restarted pump straight away?

Usually when level in pump vessel increases it should help pump except if pressure drops suddenly , causing boiling of liquid causing gas bubbles & cavitation .

Another cause of cavitation is if pump has low flow through it , but this is usually prevented with a minimum flow line tee'd off discharge of pump back into wet return line or upper part of vessel .
Basically pump should be able to continue running & pumping even if all room solonoids are closed with rooms down to temp .
The bypass line must be open enough to cope with this situation as well .

How is level guaranteed in med temp vessel , does it then supply low temp vessel?
System sounds critically charged type of set up!

When the pumps are restarted the pressures go right back to where they belong. There has only been a few instances where there was a stall before she took off. I could say that the computer controls are restarting the pump about 50-60% of the time and we aren’t seeing an alarm.

There is a bypass line and we have "tweaked" it a little, but I think our information might be backwards. I am going to look into it to see if we have been going the right direction with this.

The Med temp vessel feeds the low temp. This system is 25% med temp 75% low. At this time we believe we might be overcharged but we aren’t really worried about that at this time.

Segei
07-10-2010, 05:35 PM
I looked at your graph. Why does this graph show pump pressure(medium pressure vessel) and level in HPR? Probably this is pressure and level in medium pressure vessel. This plant can't be critically charge, because it has liquid pumps. It means that medium pressure vessel should have level control system. In your case is should be control system+solenoid+hand expansion valve. Probably, the reason of mentioned event that expansion valve is open too much. When solenoid is open, liquid goes to medium pressure vessel and a lot of flash gas will be created. When level reach 50%, solenoid will close, but it takes time for compressors to unload. Pressure in medium vessel will go down, level up(see graph) and pump will loose prime. Operating time of this solenoid should be 50-70%. Level differential should be 5-10%.

Krohnie84
07-10-2010, 06:43 PM
I looked at your graph. Why does this graph show pump pressure(medium pressure vessel) and level in HPR? Probably this is pressure and level in medium pressure vessel. This plant can't be critically charge, because it has liquid pumps. It means that medium pressure vessel should have level control system. In your case is should be control system+solenoid+hand expansion valve. Probably, the reason of mentioned event that expansion valve is open too much. When solenoid is open, liquid goes to medium pressure vessel and a lot of flash gas will be created. When level reach 50%, solenoid will close, but it takes time for compressors to unload. Pressure in medium vessel will go down, level up(see graph) and pump will loose prime. Operating time of this solenoid should be 50-70%. Level differential should be 5-10%.


The graph I attached shows three things: The yellow line is the Med Temp Vessel liquid level, the Green line is the Liquid pump differential pressure and the light blue line below them shows the suction pressure.

That is the case, there are solenoids, hand expansions and (not verified yet) mechanical floats. The main controller for both my liquid vessels (med and low temp) are Hanson controllers plus some redundant computer controls.

Segei
07-10-2010, 07:02 PM
The graph I attached shows three things: The yellow line is the Med Temp Vessel liquid level, the Green line is the Liquid pump differential pressure and the light blue line below them shows the suction pressure.

That is the case, there are solenoids, hand expansions and (not verified yet) mechanical floats. The main controller for both my liquid vessels (med and low temp) are Hanson controllers plus some redundant computer controls.
Where is the level 50%(HPR or MPR)? For level control, you can have or solenoid+HEV or float, but not both of them.

Krohnie84
07-10-2010, 08:00 PM
Alright, I would first like to apologize if this was getting confusing. I work with two refrig systems (In the same building). The one with the pump issue has a float control going from the High Pressure Receiver to the med temp vessel. The second system (smaller one) has a solenoid and hand expansion. I got them confused/mixed up.

Another thing I have been going over is the piping diagrams and I found that the vent for each pump is wide open. I also located the return/bleed/recirculation line. I turned it open another half a turn. < I was told that this valve was open all of the way but that was not correct.

RANGER1
07-10-2010, 09:10 PM
Krohnie ,
It seems to me plant is critically charged & any excess refrigerant ends up in low temp vessel .
I'd assume liquid reciever is quite small or has thermosyphon oil coolers ?

As Sergei mentioned with unloading , compressors should unload a bit faster than they load with deadband of say 2 psi
Establish where all valves are set & see what happens .
Do you if pump does actually cavitate , or is something else just stopping it .
Is there a particular time it happens everyday ? defrost time or something?

Krohnie84
07-10-2010, 10:55 PM
Krohnie ,
It seems to me plant is critically charged & any excess refrigerant ends up in low temp vessel .
I'd assume liquid reciever is quite small or has thermosyphon oil coolers ?

As Sergei mentioned with unloading , compressors should unload a bit faster than they load with deadband of say 2 psi
Establish where all valves are set & see what happens .
Do you if pump does actually cavitate , or is something else just stopping it .
Is there a particular time it happens everyday ? defrost time or something?

Your good, the receiver is quite small. Small enough that it is suspended from the ceiling and we also have thermosyphon cooling for each of the six compressors.

At this time there is only one Med temp compressor running, I will take a look at it's settings and compare them to the other med temp comps that we have.

See, what I keep hearing is that when a pump cavitates that it sounds like there are rocks going through it? I have been right next to the pump with it alarming and I can't tell the difference. I have discussed this with my fellow tech and he and I are thinking that the pump might be stalling? The pressure on the pump is still around 40-50 psi but the dif is too low and the computer alarms.

As for the time of day, I have been trying to figure out what the other causes might be but the way our program was set up is that the defrost times with vary depending on the amount of cooling. I could complain about it more but it would seem only sensible that we should be able to set the times for defrosts every day.

Segei
08-10-2010, 05:53 AM
Alright, I would first like to apologize if this was getting confusing. I work with two refrig systems (In the same building). The one with the pump issue has a float control going from the High Pressure Receiver to the med temp vessel. The second system (smaller one) has a solenoid and hand expansion. I got them confused/mixed up.

Another thing I have been going over is the piping diagrams and I found that the vent for each pump is wide open. I also located the return/bleed/recirculation line. I turned it open another half a turn. < I was told that this valve was open all of the way but that was not correct.
I'm still confused about your system. It would be better if you give us simplified drawing of your plant.
Assume that you have float to control level in HPR and nothing to control level in MPR. This is very poor design. Every vessel with the pumps should have level control.
If you have critically charged system(float for HPR), level in MPR will be low when level in HPR is 50%. This means that your system is undercharged. When level in HPR is 40%, more liquid will be in MPR and pump will run fine. What is the level in MPR when this problem happen?

RANGER1
08-10-2010, 10:02 AM
Sergei ,
I believe liquid reciever dumps liquid into med temp vessel when above 50%
Med temp vessel dumps into low temp vessel when level above 40%.
System is charged by low temp vessel level.
This style of system can be used with Witt or Hansen float valves.

Segei
08-10-2010, 02:17 PM
Sergei ,
I believe liquid reciever dumps liquid into med temp vessel when above 50%
Med temp vessel dumps into low temp vessel when level above 40%.
System is charged by low temp vessel level.
This style of system can be used with Witt or Hansen float valves.
In this case LPR should be large vessel to accumulate liquid when load fluctuate. System should not be undercharged. I don't like this design.

RANGER1
08-10-2010, 09:18 PM
Have seen it in abbatoirs with plate freezers .
Plant has approx 30 tonne charge , so not viable to have a liquid reciever for any pump down or storage , so low side accumulator is very large .
Seems to do the job ok .
Liquid reciever is only really purpose is for thermosypon oil coolers .

Quality
08-10-2010, 09:31 PM
Sounds like there is some pump slip from your first posts not cavitation

Thats my theory

Krohnie84
09-10-2010, 10:39 PM
I'm still confused about your system. It would be better if you give us simplified drawing of your plant.
Assume that you have float to control level in HPR and nothing to control level in MPR. This is very poor design. Every vessel with the pumps should have level control.
If you have critically charged system(float for HPR), level in MPR will be low when level in HPR is 50%. This means that your system is undercharged. When level in HPR is 40%, more liquid will be in MPR and pump will run fine. What is the level in MPR when this problem happen?

This system was first made to have the med temp vessel push the liquid out via med temp pressure. When we expanned the building and added five med temp units we added pumps. The first four units were dock units and now run a cooler. At this time there are no issues with getting liquid into the med temp vessel.

The reason behind having a liquid level of 50% or higher in the high temp receiver makes sure that the thermosyphon oil coolers for the compressors have enough liquid to cool the oil. Having enough liquid to cool compressors is more important than enough for med temp vessel.

I made a few adjustments to the pumps, and I had an alarm last night. A little more on a valve or two should work pretty well.

RANGER1
09-10-2010, 10:49 PM
Sounds like there is some pump slip from your first posts not cavitation

Thats my theory


Quality can you explain what pump slip might be !

RANGER1
09-10-2010, 10:50 PM
This system was first made to have the med temp vessel push the liquid out via med temp pressure. When we expanned the building and added five med temp units we added pumps. The first four units were dock units and now run a cooler. At this time there are no issues with getting liquid into the med temp vessel.

The reason behind having a liquid level of 50% or higher in the high temp receiver makes sure that the thermosyphon oil coolers for the compressors have enough liquid to cool the oil. Having enough liquid to cool compressors is more important than enough for med temp vessel.

I made a few adjustments to the pumps, and I had an alarm last night. A little more on a valve or two should work pretty well.


Please let us know what you did for interest when you get it sorted .

Krohnie84
09-10-2010, 11:47 PM
I will do so, just so everyone will know at this time....
Each pump has an inlet, outlet and a vent. The pump is oil less and uses the ammonia to cool pump/bearings. Everything is submerged in the ammonia(take a look at my original attachment). My co-worker was told when this problem was first seen that the vent line needed to be opened more. The "vent" line has been slowly been opened all the way over the past 1yr. I have started to close these valves and have been working with another that is located off of the liquid trunk. If you would like a drawing I should be able to get something together to share, but I will still keep you guys updated with anything that may arise.
P.S. I have been reading the other threads and I have to say that this is a really good forum. All of you are very knowledgeable and helpful and I just want to thank you for helping me with this issue.

RANGER1
10-10-2010, 12:57 AM
The vent line dosn't have to be open to much as its only for cooling & lubrication of bearings .
Still surprised that it would cause cavitation though if opened to much .

Liquid trunk , is that the main discharge line off liquid pumps back into wet return of vessel ?

Krohnie84
10-10-2010, 03:42 AM
The vent line dosn't have to be open to much as its only for cooling & lubrication of bearings .
Still surprised that it would cause cavitation though if opened to much .

Liquid trunk , is that the main discharge line off liquid pumps back into wet return of vessel ?

My Co-worker was told that if he opened the vent more then it would stop any "cavitation".

It is the discharge off of the pumps going to the main liquid line, there is a T with an expansion valve that goes back to the vessel.

Quality
10-10-2010, 06:47 AM
Quality can you explain what pump slip might be !

Pump slip is either when the differential pressure is to big for the pump to overcome or when the gears are worn liquid passes by the mating faces ,hence the liquid slips by the gears

RANGER1
10-10-2010, 08:22 AM
Qualityfair enough , but in this case its a centrifugal pump .
Although if pump carbon bearings are worn the impellar can rub on housing causing possible excessive clearances .

Krohnie have you tried also to run stadby pump & notice any difference ?
Just a thought to eliminate pump itself .

Quality
10-10-2010, 08:25 AM
Qualityfair enough , but in this case its a centrifugal pump .
Although if pump carbon bearings are worn the impellar can rub on housing causing possible excessive clearances .

Krohnie have you tried also to run stadby pump & notice any difference ?
Just a thought to eliminate pump itself .

Must of misread whoops:)

Krohnie84
11-10-2010, 03:34 PM
Qualityfair enough , but in this case its a centrifugal pump .
Although if pump carbon bearings are worn the impellar can rub on housing causing possible excessive clearances .

Krohnie have you tried also to run stadby pump & notice any difference ?
Just a thought to eliminate pump itself .


We have switched the pumps after the alarm occurs, but it would still fault out hours/days after that.

Segei
11-10-2010, 06:41 PM
It isn't easy to help you if we don't have correct information about your plant, but we'll try.
Right now you have 9 med temp. units. 4 for a dock and 5 in penthouses. How many penthouses do you have? This 5 units cool 1 or 2 separate rooms.
Look at the graph. This is good information that you gave us. You have spike of the level and spike is the reason of cavitation. Reason of the spike is sudden suction pressure drop. It can happen when load suddenly drop. For example. You run 1000 cfm compressor capacity. If temperature in your cooler is satisfied your load will drop to 500 cfm, but it takes time for compressors to unload. Compressor over capacity will pull pressure in the vessel down, level will spike and pump will cavitate. Give us more information about these 9 units. Do you supply liquid to the dock units from HPR or from MPR ? What is the medium suction pressure? What are the temperatures on the dock and in the coolers?

Krohnie84
11-10-2010, 07:03 PM
It isn't easy to help you if we don't have correct information about your plant, but we'll try.
Right now you have 9 med temp. units. 4 for a dock and 5 in penthouses. How many penthouses do you have? This 5 units cool 1 or 2 separate rooms.
Look at the graph. This is good information that you gave us. You have spike of the level and spike is the reason of cavitation. Reason of the spike is sudden suction pressure drop. It can happen when load suddenly drop. For example. You run 1000 cfm compressor capacity. If temperature in your cooler is satisfied your load will drop to 500 cfm, but it takes time for compressors to unload. Compressor over capacity will pull pressure in the vessel down, level will spike and pump will cavitate. Give us more information about these 9 units. Do you supply liquid to the dock units from HPR or from MPR ? What is the medium suction pressure? What are the temperatures on the dock and in the coolers?

To make sure this comes out clearly, I have to explain this in two ways. This building was built in 1995, the med temp side at that time consisted of four hanging dock units (with txv's). Liquid was fed to them from the Med Temp Vessel via pressure (32psi) within the vessel.

Two years ago the building was expanded, that dock was "converted" into a cooler. Then five penthouse units (hand expansion valves) were installed for the new dock area. Since the length for the liquid line had now doubled if not tripled, liquid pumps were added to the vessel. The temperature of both areas maintains 33-35 degrees year round.

Segei
11-10-2010, 07:52 PM
To make sure this comes out clearly, I have to explain this in two ways. This building was built in 1995, the med temp side at that time consisted of four hanging dock units (with txv's). Liquid was fed to them from the Med Temp Vessel via pressure (32psi) within the vessel.

Two years ago the building was expanded, that dock was "converted" into a cooler. Then five penthouse units (hand expansion valves) were installed for the new dock area. Since the length for the liquid line had now doubled if not tripled, liquid pumps were added to the vessel. The temperature of both areas maintains 33-35 degrees year round.

You can't supply liquid at 32 psig and return suction at the same pressure. Probably, liquid supplied to txv units from HPR and dry suction returns to MPR(4 old units). New 5 units are supplied from MPR by the pump.
As I mention above, sudden change in ref. load will lead to this event. When temperature on the dock is satisfied, 5 units will turn off and this will lead to spike of the level and pump cavitation.
Solutions.
1. Run fans nonstop and cycle liquid solenoids. This isn't energy efficient way, but it will help to avoid pump cavitation.
2. Start up and shut off units gradually(time delay). For example. When temperature on the dock is satisfied one unit off, in 30 sec second unit off and... The same at start up.
3. During cooler weather run less than 5 units(4,3). Increase suction pressure to 35 psig.

Krohnie84
11-10-2010, 08:57 PM
You can't supply liquid at 32 psig and return suction at the same pressure. Probably, liquid supplied to txv units from HPR and dry suction returns to MPR(4 old units). New 5 units are supplied from MPR by the pump.
As I mention above, sudden change in ref. load will lead to this event. When temperature on the dock is satisfied, 5 units will turn off and this will lead to spike of the level and pump cavitation.
Solutions.
1. Run fans nonstop and cycle liquid solenoids. This isn't energy efficient way, but it will help to avoid pump cavitation.
2. Start up and shut off units gradually(time delay). For example. When temperature on the dock is satisfied one unit off, in 30 sec second unit off and... The same at start up.
3. During cooler weather run less than 5 units(4,3). Increase suction pressure to 35 psig.

You are correct, I was mistaken. When we added the pumps they attached the old liquid line with the new trunk that the installed for the penthouses. For some reason I was thinking it was that was to begin with (It sounded funny in my head when I wrote that).

Each unit has a seperate temperature probe. They will only run when the temperature in that area calls for it. The "old" dock area had about 23 or so overhead doors. The new area has over forty and some extra storage space. I think it might be 200 yards or so long.

The penthouse units have VFDs controlling them, when the units are not cooling they ramp the fans down and then speed up again when in cooling mode. The old hanging units have the option of turning off the fans when they aren't cooling but with the temperature sensitve product that is now stored there we would rather not turn them off.

Segei
11-10-2010, 09:21 PM
You are correct, I was mistaken. When we added the pumps they attached the old liquid line with the new trunk that the installed for the penthouses. For some reason I was thinking it was that was to begin with (It sounded funny in my head when I wrote that).

Each unit has a seperate temperature probe. They will only run when the temperature in that area calls for it. The "old" dock area had about 23 or so overhead doors. The new area has over forty and some extra storage space. I think it might be 200 yards or so long.

The penthouse units have VFDs controlling them, when the units are not cooling they ramp the fans down and then speed up again when in cooling mode. The old hanging units have the option of turning off the fans when they aren't cooling but with the temperature sensitve product that is now stored there we would rather not turn them off.
Step by step you give us more and more information about your plant. Try to increase(gradually) suction pressure of MPR. This will be better for the plant operation and for the plant efficiency. I think that you can go up to 40 psig. Actually, during winter operation many docks require heating but not cooling.
For sensitive product you can install ceiling fans. They are cheap and use little energy.
Make sure that your probe measure temp. and control temp. Sometimes these are separate probes.

Krohnie84
11-10-2010, 09:56 PM
Step by step you give us more and more information about your plant. Try to increase(gradually) suction pressure of MPR. This will be better for the plant operation and for the plant efficiency. I think that you can go up to 40 psig. Actually, during winter operation many docks require heating but not cooling.
For sensitive product you can install ceiling fans. They are cheap and use little energy.
Make sure that your probe measure temp. and control temp. Sometimes these are separate probes.


How would increasing the pressure to 40 make the system more efficient? That would bring the suction temperature within 7-8 degrees of my cooler temp?
Just wondering if I am missing something? The units almost run all day long as it is.

When I saw this I laughed a bit, not to insult you; I am going to take a picture of the cooler and the space above the product and the space around the cooling units. It is really kind of funny.

Segei
11-10-2010, 10:45 PM
How would increasing the pressure to 40 make the system more efficient? That would bring the suction temperature within 7-8 degrees of my cooler temp?
Just wondering if I am missing something? The units almost run all day long as it is.

When I saw this I laughed a bit, not to insult you; I am going to take a picture of the cooler and the space above the product and the space around the cooling units. It is really kind of funny.

Yes temp. difference will be 7-8 degF but you don't need 15 degF when it is cool outside. At higher suction pressure compressor and usually plant efficiency is better. How many penthouses do you have. Usually, each penthouse has 2-4 units.
About fan ceiling. This may be looks laughable when you don't have enough information. I saw docks where people store some product and still use this room as a dock. If your dock(cooler) full of racks it is different story. Anyway you don't need to run the fans nonstop. Usually, units located under the ceiling or in penthouse. Temp. probe located at air inlet. When temp. is satisfied and unit shut off, due to gravity warm air will go up and cool air will go down. Within reasonable time and temp. difference warm air will turn the unit on and product won't suffer.

Krohnie84
11-10-2010, 10:57 PM
Yes temp. difference will be 7-8 degF but you don't need 15 degF when it is cool outside. At higher suction pressure compressor and usually plant efficiency is better. How many penthouses do you have. Usually, each penthouse has 2-4 units.
About fan ceiling. This may be looks laughable when you don't have enough information. I saw docks where people store some product and still use this room as a dock. If your dock(cooler) full of racks it is different story. Anyway you don't need to run the fans nonstop. Usually, units located under the ceiling or in penthouse. Temp. probe located at air inlet. When temp. is satisfied and unit shut off, due to gravity warm air will go up and cool air will go down. Within reasonable time and temp. difference warm air will turn the unit on and product won't suffer.

How many units? Do you mean fans?

3753
This is the "old" dock area. The space between the
bar joists and the product is only a foot and a half at best.

3752
This is one of the units, it is seldom that there is empty space around the units themselves.

3754
New dock North side.

3755
New dock South side.

Segei
12-10-2010, 02:00 AM
How many units? Do you mean fans?

3753
This is the "old" dock area. The space between the
bar joists and the product is only a foot and a half at best.

3752
This is one of the units, it is seldom that there is empty space around the units themselves.

3754
New dock North side.

3755
New dock South side.
Cooling units or evaporators or coils. How many penthouses do you have? 1,2...5

Krohnie84
12-10-2010, 06:16 AM
In this system there are five med temp penthouses, four med temp hanging evaps, five hanging freezer evaps (two are blast freezers), five freezer penthouses and two convertible penthouses (med or low temp).

Segei
12-10-2010, 03:27 PM
In this system there are five med temp penthouses, four med temp hanging evaps, five hanging freezer evaps (two are blast freezers), five freezer penthouses and two convertible penthouses (med or low temp).
As I mentioned, try to increase medium pressure. How do you heat your docks during winter time?

Krohnie84
12-10-2010, 03:52 PM
We heat via hot gas, the penthouse units only run fans at 50% or so and the hot gas comes in at about 105psi.

Segei
12-10-2010, 05:32 PM
We heat via hot gas, the penthouse units only run fans at 50% or so and the hot gas comes in at about 105psi.
It is good idea to use hot gas for the heating. Do you have pressure regulator for hot gas line? Do you use 105 psig to defrost your freezers?

Krohnie84
13-10-2010, 12:54 AM
It is good idea to use hot gas for the heating. Do you have pressure regulator for hot gas line? Do you use 105 psig to defrost your freezers?

Yes, head pressure is regulated down the 105 for the penthouses on the docks and the new freezer units. The original units are regulated to 70-75 psi. There is no electric or air over coil defrosts.

Segei
13-10-2010, 04:25 AM
Yes, head pressure is regulated down the 105 for the penthouses on the docks and the new freezer units. The original units are regulated to 70-75 psi. There is no electric or air over coil defrosts.


What do you mean by original units are regulated to 70-75 psi?

Krohnie84
13-10-2010, 03:23 PM
The building was built in 1995, the original units were designed to defrost/heat with a regulated hot gas pressure of 70-75 psig. The expansion that took place in 2008 added units that are designed for 105 psig.

Segei
13-10-2010, 03:55 PM
The building was built in 1995, the original units were designed to defrost/heat with a regulated hot gas pressure of 70-75 psig. The expansion that took place in 2008 added units that are designed for 105 psig.
Usually, plan has outlet pressure regulator to keep certain constant(all year around) pressure in hot gas main. This is hot gas main. Evaporator has inlet pressure(or back pressure regulator) regulator to keep certain constant pressure inside the coil during defrosting. What do you have for new and old evaporators? What is the difference?

Krohnie84
13-10-2010, 06:42 PM
Usually, plan has outlet pressure regulator to keep certain constant(all year around) pressure in hot gas main. This is hot gas main. Evaporator has inlet pressure(or back pressure regulator) regulator to keep certain constant pressure inside the coil during defrosting. What do you have for new and old evaporators? What is the difference?


Each unit has a regulator on the pipestand before the evap.

Graph key:

Yellow = Vessel Level

Green = pump dif

Red = compressor slide valve %

Pink/purple = suction pressure

3758

After adjusting suction pressure, unload and load times on the compressor and the liquid line flow valves I got another alarm around 4:30 this morning.

Segei
13-10-2010, 08:18 PM
Each unit has a regulator on the pipestand before the evap.

Graph key:

Yellow = Vessel Level

Green = pump dif

Red = compressor slide valve %

Pink/purple = suction pressure

3758

After adjusting suction pressure, unload and load times on the compressor and the liquid line flow valves I got another alarm around 4:30 this morning.
It looks like I described before. Sudden change in refrigeration load and compressor try to handle this issue. Increase suction pressure again. Try to determine what happen with evaporators in this moment. During the night load will be lower and probably is higher that this issue will happen during the low refrigeration load.

RANGER1
13-10-2010, 09:33 PM
Krohnie ,
For liquid level to suddenly spike there is a sudden load change for this to happen as Sergei suggests .

- Can you determine pump controls that protect it like timers for pump differential & pressure setting

- pressure setting that activates cavitation timer
- time before pump cuts out
- does pump cut back in automatically , if how long before it does .
Normally pump cavitation timer would be set for 30-60 seconds so it may have time to recover hickup .
Auto restart after 2 minutes etc for 6 cycles of this before alarm goes off in a certain time period of say 30 minutes to avoid unnecassary call outs .

-Have you observed & adjusted liquid makeup hand regulating valve as previously suggested in earlier posts .

- Can you also monitor liquid makeup on/off as well ?

Krohnie84
14-10-2010, 10:48 PM
Krohnie ,
For liquid level to suddenly spike there is a sudden load change for this to happen as Sergei suggests .

- Can you determine pump controls that protect it like timers for pump differential & pressure setting

- pressure setting that activates cavitation timer
- time before pump cuts out
- does pump cut back in automatically , if how long before it does .
Normally pump cavitation timer would be set for 30-60 seconds so it may have time to recover hickup .
Auto restart after 2 minutes etc for 6 cycles of this before alarm goes off in a certain time period of say 30 minutes to avoid unnecassary call outs .

-Have you observed & adjusted liquid makeup hand regulating valve as previously suggested in earlier posts .

- Can you also monitor liquid makeup on/off as well ?

At this time there are controls and I can change the settings within the equipment's parameters. As for protection timers.... the computer system does have a timed reset and delay in place but I am unable to change those settings. < The delays and resets do work, the pumps have turned themselves back on even if the system alarms. I have seen a large number of resets that never alarmed and some that have alarmed but reset before anyone could take a look.

I have been "playing" with this valve, but at this time there has been no change. < I won't change it until I have had another dif alarm to make sure there is no other issue.

Liquid on and off to what? From the HPR to the MTV or from the MTV to the LTV? I cannot control the liquid flow from the high pressure receiver to the med temp vessel (float control on receiver< discussed in prev posts) but I am able to control the flow from the Med temp to low temp vessel. At this time I have found no connection between the two but I have tinkered with it to see if it would make a difference. As for the flow, if I decrease it I don't have enough make up for the vessel (not even a half a turn less) so I have found no reason to increase it due to the fact that it makes up the level just fine.

RANGER1
15-10-2010, 11:26 AM
I was talkig about liquid makeup from HPV to MTV .
What type of valves & controls do you have on this line ?

How is liquid makeup achieved from MTV to LTV ?
Is it off MTV liquid pump ?

Krohnie84
18-10-2010, 05:14 PM
I was talkig about liquid makeup from HPV to MTV .
What type of valves & controls do you have on this line ?

How is liquid makeup achieved from MTV to LTV ?
Is it off MTV liquid pump ?


From HPR to MTV is a mechanical float. The HPR also feeds the thermosyphons for the compressors.

There is a solenoid valve and a hand expansion right behind that. The solenoid is controlled via hanson controller.

RANGER1
18-10-2010, 08:58 PM
I was talkig about liquid makeup from HPV to MTV .
What type of valves & controls do you have on this line ?

How is liquid makeup achieved from MTV to LTV ?
Is it off MTV liquid pump ?


What about MTV to LTV , what is the setup .

Is the pump cavitation same , better or worse ?

Krohnie84
18-10-2010, 09:09 PM
What about MTV to LTV , what is the setup .

Is the pump cavitation same , better or worse ?


From HPR to MTV is a mechanical float. The HPR also feeds the thermosyphons for the compressors.

There is a solenoid valve (AKA "King Valve") and a hand expansion right behind that. The solenoid is controlled via hanson controller.

Low temp pumps have had little to no problems with cavitation (knock on wood). Med temp I am still having problems with the pump pressure falling out. I have increased the suction pressure more than a few times and messed with the expansions on the liquid return (off of the header) and the vent lines for the pumps.

sterl
29-10-2010, 01:07 AM
The TS-Pilot Receiver: otherwise HPR...most likely has an internal weir at +/- 50% and once it clears the weir, a float valve at the "high" vessel delivers liquid to the medium temp vessel: That about right?

Any chance this little condition you got follow a compressor shutting itself off?

When a compressor shuts down on a T-Sypon arrangement: The liquid in the pilot receiver (reservoir) drops quickly...That because on standby the liquid in the Thermosyphon Return Line rises all the way to the weir inside that vessel...Whereas while there was oil flow that vertical would be near-dry.

So a compressor shutting down causes you float to close...Your suction pressure was low already or the machine would have stayed on: if it shut down on low load...SO

Float valve closed. No liquid to MT vessel. MT vessel pressure drops further. Liquid in level column froths and Hansen probe sees it as quick rise in liquid level. With no flash gas to vessel compressor simply pull vessel down to lower pressure. Liquid in vessel starts to cool and that generates "submerged vapor" like what happens when you snap the cap off a bottle of warm beer.

Pump pumps froth. No differential pressure. No flow through cooling line or bypass...Pump stops on Diff Pr Control.

5-seconds later: Stability resumes and pump will start and hold....

This can also happen when a condenser pump kicks in while the outdoors is cool...the thermosyphon vessel looses its continuity of liquid cause this new condenser sucks back on the liquid delivery and the float closes...And the whole thing follows through as described. I expect your intermediate vessel sits high and the inlet piping to the pumps is not quite clean; but can't tell that from here.

Segei
30-10-2010, 12:04 AM
The TS-Pilot Receiver: otherwise HPR...most likely has an internal weir at +/- 50% and once it clears the weir, a float valve at the "high" vessel delivers liquid to the medium temp vessel: That about right?

Any chance this little condition you got follow a compressor shutting itself off?

When a compressor shuts down on a T-Sypon arrangement: The liquid in the pilot receiver (reservoir) drops quickly...That because on standby the liquid in the Thermosyphon Return Line rises all the way to the weir inside that vessel...Whereas while there was oil flow that vertical would be near-dry.

So a compressor shutting down causes you float to close...Your suction pressure was low already or the machine would have stayed on: if it shut down on low load...SO

Float valve closed. No liquid to MT vessel. MT vessel pressure drops further. Liquid in level column froths and Hansen probe sees it as quick rise in liquid level. With no flash gas to vessel compressor simply pull vessel down to lower pressure. Liquid in vessel starts to cool and that generates "submerged vapor" like what happens when you snap the cap off a bottle of warm beer.

Pump pumps froth. No differential pressure. No flow through cooling line or bypass...Pump stops on Diff Pr Control.

5-seconds later: Stability resumes and pump will start and hold....

This can also happen when a condenser pump kicks in while the outdoors is cool...the thermosyphon vessel looses its continuity of liquid cause this new condenser sucks back on the liquid delivery and the float closes...And the whole thing follows through as described. I expect your intermediate vessel sits high and the inlet piping to the pumps is not quite clean; but can't tell that from here.
As far as I remember nobody mention that this issue happen when compressor turn off. Even when it off, it takes time to cool oil in the oil cooler. So it will take time for liquid to fill up the return line.
I believe in sudden change of the load. It can happen if several evaporators off or make up solenoid close.

Krohnie84
01-11-2010, 05:54 PM
The pressure from the pump drops out almost at random. The compressors don't shut down and I have checked the discharge pressure and evap run times to see if there is a group shutting down all at once and so far nothing is coinciding. If you like I could provide another graph with some more details.

Segei
01-11-2010, 10:28 PM
The pressure from the pump drops out almost at random. The compressors don't shut down and I have checked the discharge pressure and evap run times to see if there is a group shutting down all at once and so far nothing is coinciding. If you like I could provide another graph with some more details.
What is your mid. temp. suction pressure now?

RANGER1
02-11-2010, 11:07 AM
Is there a spare pump next to this one you can turn on ?
Has pump direction been checked ?

GCAP CoolCast
10-11-2010, 04:03 PM
The vent line dosn't have to be open to much as its only for cooling & lubrication of bearings .
Still surprised that it would cause cavitation though if opened to much .

Liquid trunk , is that the main discharge line off liquid pumps back into wet return of vessel ?

the vent return into the reciever may be causing the cavitation, depending on how it is piped inside the reciever. I have seen some really poor designs lately regarding that vent return causing a "whirlpool" action inside the vessel.

That's too cold
10-11-2010, 10:23 PM
There is alot of talk for a pump cavataion problem. By now, has your issue been resloved? I would like to ask what type of compressor oil or oils are you using?

Krohnie84
11-11-2010, 01:07 AM
The problem still is occurring, at this time we have switched to a smaller compressor with economizers on the low compressors.

That's too cold
11-11-2010, 07:17 PM
I presume this is a low side recirulating vessel. If your having a cavitation problems with the sound of rocks in the pump, it is frozen oil or other frozen liquid solids. This will partily block flow through the pump and cavitate. When the pump is off for awhile, then frozen solids will melt and back to normal operation. These unwanted liquids or oils may be the pipe fitters using thread cutting oils and other residue coated pipe oils never being clean during original installation. I dealt with this problem for awhile. My final solution to get rid of it was to switch to another compressor oil. We had Frick #3 oil, this was a very bad oil choice for low temp -30F. After switching to a good low temp synthetic oil and flushing the system with oil changes that problem went completely away. We are using "Petro/Canada Reflo68". Based on the design of the vessel internal piping the pump should not cavitate. Liquid ammonia should fill the pump suction inlet and prime itself.
Let me know if this looks like a solution to the problem, if not post another reply problem...

Krohnie84
11-11-2010, 07:36 PM
The problem is on the Med temp pumps, the suction pressure is 36 psig. We do use Frick # 3 oil, but even our low temp only runs at 3psig.

That's too cold
11-11-2010, 09:28 PM
After reading posts: #3,5,7,9,10.


From the graph in post #3 the pump shuts down (1st) from dif loss, the vessel fills (2nd), and suction drops alittle?
Once the dif is satisfied, the pump restarts and they go into cavitation?
What is the pump dif set point?

Segei
12-11-2010, 12:52 AM
The problem is on the Med temp pumps, the suction pressure is 36 psig. We do use Frick # 3 oil, but even our low temp only runs at 3psig.
As I mentioned you can increase pressure to 40 psig. You can switch some evaporators off. If you have 5 evap. run #1,3,5. 2,4 off. If concerns about air circulation, run fans for 2,4 on hand mode. Problem is that capacity of your evaporators significantly greater than refrigeration load.

Krohnie84
12-11-2010, 05:39 AM
After reading posts: #3,5,7,9,10.


From the graph in post #3 the pump shuts down (1st) from dif loss, the vessel fills (2nd), and suction drops alittle?
Once the dif is satisfied, the pump restarts and they go into cavitation?
What is the pump dif set point?


Your right on, I also check the liquid valves to see if anything turned off or on close to that time and I have yet to find anything coinciding.

The pump seldom shuts down. It stays running and will only cut out if the computer times out at twenty minutes.

The pump can't see a dif of twenty psi or lower.

That's too cold
12-11-2010, 09:36 PM
The fun of ammonia nuisance problems...


The liquid line valves and evap coils will not have anything to do with this problem.
Will the software allow you to input 5 to 8 psi dif? 20 psi is too high.
Question: what starts the software "time out" at twenty minutes? When the dif switch is open/close? Upon new or restart-up of pump? Can the pump start/stop be controlled by suction pressure?

RANGER1
12-11-2010, 09:48 PM
The fun of ammonia nuisance problems...


The liquid line valves and evap coils will not have anything to do with this problem.
Will the software allow you to input 5 to 8 psi dif? 20 psi is too high.
Question: what starts the software "time out" at twenty minutes? When the dif switch is open/close? Upon new or restart-up of pump? Can the pump start/stop be controlled by suction pressure?

Disagree with 20 psi to high as even with pump off the liquid head on dicharge of pump could be higher than 5-8 psi .
As long as pump has some sort of delay before shuts down & several auto restarts before alarm it should look after itself unless something is really wrong like low level etc as well as the hundred other scenarios we have offered .

Krohnie could also try restricting pump discharge for awhile & see what happens to keep pump in pump curve . If it works go from there .
There seems to be alot of advise given & info asked for but not carried out .

That's too cold
12-11-2010, 10:43 PM
Disagree with 20 psi to high as even with pump off the liquid head on dicharge of pump could be higher than 5-8 psi

The low dif of 5 to 8 will for sure be off the low side of pump curve and in cavitation.

Yes, there will need to be "time delays" added or input changes into the software to avoid nuisance shut downs.

We can only do one task at a time until it is fixed.

Krohnie84
13-11-2010, 06:57 AM
As long as pump has some sort of delay before shuts down & several auto restarts before alarm it should look after itself unless something is really wrong like low level etc as well as the hundred other scenarios we have offered .

Krohnie could also try restricting pump discharge for awhile & see what happens to keep pump in pump curve . If it works go from there .
There seems to be alot of advise given & info asked for but not carried out .

I have tried to adjust the pump dif alarm setting but the system will not let me change it, nor will it let me change the twenty minute delay on the alarm. I have been in the room and at the computer when this happens and the pump continues to run with the low dif until it runs under 20psi for a period of 20 minutes.
I have yet to try restricting the discharge, I will start shutting it off in small increments starting Sunday (Beginning of my work week).
Don't assume that the info I have been getting from all of you hasn't gone into this problem. I have been bringing these ideas up and we have gone through and either eliminated it as the possible problem or we have been slowly changing things and watching (over periods of days) if there is any effect. Truthfully the only idea we have yet to execute is bringing the suction pressure all the way up to 40psi.

RANGER1
13-11-2010, 09:42 AM
krohnie84 ,
Can you also tell exactly whats on the pump nameplate & what the nett pump pressure is ie pump pressure minus suction pressure .

What is measurement from centre line of suction of pump to working level in HT vessel ?

Also do you have much oil in system , as in adding oil & draining oil ?

Krohnie84
13-11-2010, 10:02 PM
I will get that info tomorrow when i go in.

As for oil, we seldom have to add and it might go as long as month or two before the oil pod has to be drained.

That's too cold
15-11-2010, 08:08 PM
The pump ideal dif press is 30psi minimum??


When the pump operates normal for a period of time and then falls to 20 psi for longer than twenty minutes, the shut down occurs.
Then I would point my finger at oil slugging on the suction side of the pump. ???

Krohnie84
22-12-2010, 12:29 AM
First I would like to apologize for the time between my posts. Lots of projects going on around here; including the installation of a ammonia air handler for the production area. J

First off I was asked to get info on the pump/s that are having this issue. It is as follows:

TEIKOKU
Type: R42-21704DM
Head 124ft
Capa 90 gal per min

And the pump runs at 80 psi with a 38psi suction pressure. So a diff of 42psi.

Since the last time I talked with you guys we have changed the sequence of the Compressors and I have been noting some things.
First, the sequence change was on the med temp side (where the issue is located). We turned off the 300 hp compressor and switched to the 125 hp. Each of our low temp compressors have economizers on them so there is no worries about load issues. Since it is so nice and cold here in Minnesota we heat our dock areas from Nov to Mar.

Since I changed the compressors over at first we were still having issues with the dif pressure. I started to question the use of the economizers because they are always on when the low temp compressors are above 80%. I decided to test out just having the one 125hp comp run without any “aid” from the low side. What I found is that even though the suction pressure goes up for a longer period of time (When there is load on the Med Temp) the smaller compressor can make due just fine. Since the compressor is much smaller than the one I had running before it really isn’t slamming on to where it draws a ton of ammonia back to the vessel all at once. I had not had an alarm for two weeks (sweet!) until we switched low temp compressors and my co-worker forgot about the economizer and it is the lead compressor being at 100% all the time. The suction pressure was below 30psi until I found what he had done/forgotten to do.

So right now I am not worried about the pump, but come summer when I am running the 300hp comp (due to load) I will be having the same issues again?

Segei
24-12-2010, 06:17 PM
First I would like to apologize for the time between my posts. Lots of projects going on around here; including the installation of a ammonia air handler for the production area. J

First off I was asked to get info on the pump/s that are having this issue. It is as follows:

TEIKOKU
Type: R42-21704DM
Head 124ft
Capa 90 gal per min

And the pump runs at 80 psi with a 38psi suction pressure. So a diff of 42psi.

Since the last time I talked with you guys we have changed the sequence of the Compressors and I have been noting some things.
First, the sequence change was on the med temp side (where the issue is located). We turned off the 300 hp compressor and switched to the 125 hp. Each of our low temp compressors have economizers on them so there is no worries about load issues. Since it is so nice and cold here in Minnesota we heat our dock areas from Nov to Mar.

Since I changed the compressors over at first we were still having issues with the dif pressure. I started to question the use of the economizers because they are always on when the low temp compressors are above 80%. I decided to test out just having the one 125hp comp run without any “aid” from the low side. What I found is that even though the suction pressure goes up for a longer period of time (When there is load on the Med Temp) the smaller compressor can make due just fine. Since the compressor is much smaller than the one I had running before it really isn’t slamming on to where it draws a ton of ammonia back to the vessel all at once. I had not had an alarm for two weeks (sweet!) until we switched low temp compressors and my co-worker forgot about the economizer and it is the lead compressor being at 100% all the time. The suction pressure was below 30psi until I found what he had done/forgotten to do.

So right now I am not worried about the pump, but come summer when I am running the 300hp comp (due to load) I will be having the same issues again?
Your economizers should have back pressure regulators(BPR) to keep certain pressure in med. pressure receiver. Probably, they don't adjusted properly.
Most likely, you don't have med. tem. refrigeration load at all, because it is cold outside. However, you have load of flash gas that comes from HPR. This load can fluctuate due to modulating of the HPR float.
Assume that total capacity of all economizers is 30TR. This is constant capacity. Med. tem. load of flash gas will fluctuate from 0TR to 60TR. When load is above 30TR, mid. tem. suction pressure will gradually increases. This won't harm plant operation, because this flash gas load(not cooler load). When load is lower than 30TR, mid. tem. suction pressure will be pulled down and pulling will affect the pump operation and prime will be lost.
I suggest you the following sequence of operation. Right now, run 125HP compressors(suction 38 psig)and adjust economizers operate in the range of 40-45 psig(open 45 psig, close 40 psig). they will help to this compressor to handle fluctuation load of flash gas. During spring time you can switch from 125HP compressor to 300HP compressor. During summer operation(significant mid. temp. load) you can lower set points for economizers and they will be able to run constantly and compressor(s) will load and unload to handle the load fluctuation.

Krohnie84
28-02-2011, 09:49 PM
Well, it has been a while but I would like to give an update to the situation a lot of you have been helping me with.


Last I had told you was that we were running a 125hp compressor with no economizers. There were very few issues, but the pump still would cut out once or twice a week. We found that our lead low temp compressor was not turning it's economizer off. Once we discovered it we manually turned it off and trouble shot the controls for that particular valve.

After that it seemed like smooth sailing except for our med temp cooler couldn't maintain it's set point with us running 38 suction pressure (keep in mind that this "cooler" was once a dock and the units were only cooling about 1/3 the load they are now).

Another issue was once we had a cold spell (below 20 degrees F outside temp) the dock wanted to heat. If you recall in a previous post I informed everyone that the dock units heat and defrost with 105psi of hot gas. Once any one unit goes into defrost or heat mode the 125 hp compressor can't really keep up. The suction pressure would rise to 43ish and it will take some time before the compressor can bring it back down.

At this time we are now running our 300hp compressor again (seal was leaking a bit, decided to run it to get her to close up). With this compressor running the suction pressure no longer climbs like before but the issue is that this compressor loads up so fast that it causes the vsl to bounce around and then the pump kicks out. At this time I am working at the control set points to slow the compressor down but no luck as of yet.

Any comments or ideas would be welcome.

Thank you

Krohnie84
29-07-2011, 03:12 AM
There was two issues that were "plaguing" us.

1. The computer controls were taking too long to load the compressor up and by the time it would the controls would load the compressor past what was necessary to recover the suction pressure. If you had looked at the graph the pressure might go three maybe four psi over the set point, the graph only show system pressure, the pressure at the compressor was much higher than the set point. At times I found it above by almost 10psi. The controls via the computer won't let me change the set points, but the compressor controls will. Right now we have the med temp compressor's slide valve set to auto. This makes it follow the compressors program. The company that programmed the computer is trying to bend us over to change the littlest thing in their programs so we make due.

2. Our newest penthouse units, and their running a 105psi hot gas heat/defrost. I couldn't trend the times where the units would go into heat/defrost (Nothing tells me specifically when). Our other system (with much older controls and from another company) will let us set up the defrost times, this new system goes by hours of cooling. If it is set for defrost every 8 hours, it means that it needs to cool for 8 hours before. Not continues hours, just cooling time. Hell, it could take 8 days of only cooling for 1 hr a day before a defrost(exaggeration), but it doesn't seem the right way to go.
Well, we started to force the units into defrost/heat modes and we were able to create the issue. What kind of sucks is that the system is designed to where the low temp units will dump their hot gas into the med temp suction lines. So, we have adjusted the dock units to a lower hot gas with out any issues. We cant' do that to the low temp coolers, they do cool for long periods of time and the defrost time ends up being too long if the gas pressure is too low.

Another change is that we are running our 450hp compressor for the med temp. It is running at less amps then the 150 (Which might have blown out a gear, another story) and rarly gets above 20% capacity.

Fun times, fun times.

Any questions, love sharing crap like this.

Segei
29-07-2011, 05:59 PM
You need to adjust hot gas defrost. Three parts of defrost process are important. Hot gas supply, ammonia condensation and ammonia condensate draining. If they balanced, you will have good defrost with minimum blow-by gas.