ammonia suction piping risers

[Volnei]

Dear guys,

I've been studying about risers in wet suction line and I understood that the only objective of risers is to minimize the effect of liquid static head. So the use of risers is more useful in low temperatures because the liquid density is higher.

Anybody has another comment?

Volnei

[US Iceman]

Hi Volnei,

I agree with your comments. The suction risers should be sized to provide an annular flow up the risers. If the pipe is sized correctly, the higher velocity gas traveling up the center of the pipe will drag the liquid with it.

The liquid flows against the inside pipe wall and the gas travels up the center of the pipe. Of course this takes some pressure loss to accomplish the liquid lift, but the pressure loss will almost certainly be lower than the static head of a liquid column.

One other thing to be aware of is the minimum capacity that might be seen during operation. At part load the volume of gas is reduced so this can impact the minimum required pipe size to lift liquid.

Do you really have to lift the liquid is my first question? Can the liquid be gravity drained back to a pump receiver first? It is easier and more efficient to pump liquid and lift only gas, rather than lifting liquid and gas.

A good topic for more discussion.

[Andy]

Hi Volnei,

I agree with your comments. The suction risers should be sized to provide an annular flow up the risers. If the pipe is sized correctly, the higher velocity gas traveling up the pipe the center of the pipe will drag the liquid with it.

The liquid flows against the inside pipe wall and the gas travels up the center of the pipe. Of course this takes some pressure loss to accomplish the liquid lift, but the pressure loss will almost certainly be lower than the static head of a liquid column.

One other thing to be aware of is the minimum capacity that might be seen during operation. At part load the volume of gas is reduced so this can impact the minimum required pipe size to lift liquid.

Do you really have to lift the liquid is my first question? Can the liquid be gravity drained back to a pump receiver first? It is easier and more efficient to pump liquid and lift only gas, rather than lifting liquid and gas.

A good topic for more discussion.

On pumped circ systems the mains headers are usually in the roof void above the rooms. This means that all the suctions back up to the headers are risers.
If these pipes are not selected for 2 phase (liquid and vapour) they may not lift the liquid.
Correct me if I am wrong what actually happens is the liquid is pumped into the evaporator, where it remains util boiling action takes place, when the boiling starts the liquid expands into vapour and the vapour carrys the remaining liquid up the riser into the header.
Unless the suction line is way too oversized the boiling action creating vapour eventually carries the liquid up the riser.
If the vapour cannot create enough pressure to carry up the liquid the evaporator is run simply as a DX evaporator by default.

Kind Regards. Andy

[rob93291]

On pumped circ systems the mains headers are usually in the roof void above the rooms. This means that all the suctions back up to the headers are risers.
If these pipes are not selected for 2 phase (liquid and vapour) they may not lift the liquid.
Correct me if I am wrong what actually happens is the liquid is pumped into the evaporator, where it remains util boiling action takes place, when the boiling starts the liquid expands into vapour and the vapour carrys the remaining liquid up the riser into the header.
Unless the suction line is way too oversized the boiling action creating vapour eventually carries the liquid up the riser.
If the vapour cannot create enough pressure to carry up the liquid the evaporator is run simply as a DX evaporator by default.

Kind Regards. Andy

You might try a double suction riser; one line turning up 90 degrees, then turning another 90 degrees for a straight run, the second riser attaches to the top of the first pipe just before the upturned 90, rises above the 90 for the straight run and then double 90 over back into the top of the original pipe. These lines are oversized to accomodate two phase fluids and carry oil as well (some, not all). Application standards can be found in the Vilter Piping Manuals.

Hope that is moderately informative!

[US Iceman]

Hi rob93291,

Welcome to the RE forum. I have some comments to clarify your post.

The Vilter piping manual information you mentioned is ONLY for oil return up COPPER risers for R-12, R-22, & R-502. These are no longer applicable tables since the listed refrigerants are essentially done with.

These are not recommended for use on ammonia systems with vertical suction risers for liquid return.

[Andy]

Hi Rob

I have been doing a lot of thinking about suction risers recently and asking a lot of questions.I had a guy in recently who sold my company some sizing software, he said that basically if you get the oil moving in the riser, it is drawn up the pipe walls and does not fall out, so intermediate traps are a thing of the past. Maybe double risers are also
Increase the velocity in the risers by basically under sizing and then increase the size of the horizontial lines to reduce the gross pressure losses

Another point would be to cut down on the posibility of part load operation, by using electronic valves (pulse type) and not allowing them to modulate (the pulses will also aid oil return.
The eev's also flood more of the evaporator, increasing the oil movement in the evaporator.

Just a few thoughts

Kind Regards. Andy

[Josip]

Hello,

take a look at this riser for ammonia installed in Costa Rica couple of years ago.

Until now no complaints about

Best regards,

Josip

[Peter_1]

...Application standards can be found in the Vilter Piping Manuals.

Can this be bought somewhere or does a pdf-version exist?

[Cool2therule]

I'm asuming most of this discussssion about risers is with recirc systems. I have 36 flooded evaps in my system with 3 approx 6 meter risers. My problem is trying to eliminate the liquid completely from my risers and give my suction accum. transfer system a break. Is a main header knockout drum my only solution or does someone have another idea. I have about 15 levelmaster controls and 20 some RS type float evap setups. Thanks, ed

[Volnei]

Thanks for your replay US Iceman, I agree with you that pumping liquid it's better than lifting liquid and gas. Nowadays we are using riser for all evaporators included cold room evaporators, that usually are 1,0-1,5m below the main pipe. In my opinion it isn't necessary, and I would like to change this. In what situation do you use risers?

[US Iceman]

There has been a trend where all of the piping is being installed on the roofs. This offers some obvious benefits for faster installation, service, etc.

The one bad thing this results in is lifting liquid, and the associated penalties due to additional suction line pressure losses.

When the piping was installed below the evaporators, the liquid could drain by gravity which would be my preferred method. I do not care to use risers, if I have choice.

I would rather install the piping at an elevation so that the evaporator can drain by gravity, back to the pump receiver. Sometimes this requires the piping to be installed on the wall of the building.

A 1-1.5 meter riser is not too bad. I have heard of risers almost 10 meters in height at -40C and colder! This is a problem as you can imagine.

[Andy]

Hi Iceman
I would have to think a bit to find an installtion that has a gravity suction line. All or say most installtions over here have risers.
Danfoss Dircal is what I use to size 2 phase flow, I have found no alternative and it's free.

Kind Regards. Andy

[US Iceman]

hi Andy,

Unless the suction line is way too oversized the boiling action creating vapour eventually carries the liquid up the riser.

That is true. The suction riser size must be selected so the superficial gas velocity is sufficiently high enough to drag the liquid up the riser.

The superficial velocity is the calculated velocity of the gas ONLY, up the riser. The volume of gas is much larger than liquid volume, so we only use the gas volume divided by pipe area to find the minimum superficial gas velocity.

The trick is to determine the minimum superficial velocity for the evaporating temperature and the range of operation for the evaporator (full load to part load). And then it becomes even more complicated to calculate the two-phase pressure loss in the riser.

If the vapour cannot create enough pressure to carry up the liquid the evaporator is run simply as a DX evaporator by default.

That would be a good way to do this. If the evaporator was operated at a very low superheat, then you have no liquid to lift and only a small penalty due to loss of evaporator performance because of the superheat.

At least you are not lifting liquid!

If the evaporator load is reduced, the evaporator could fill up with liquid that would suddenly return when the load returns.

When the evaporator is full of liquid at part load, very little vapor is created, so the liquid bubbles up through the column of liquid in the suction riser. This looks like an old percolator type coffee pot.

This has the highest penalty due to the static head of the liquid above the evaporator.

In this case, the suction pressure would have to be reduced to provide a low enough evaporating pressure to maintain the coil evaporating temperature. This would of course require more compressor capacity and reduce the system efficiency.

[US Iceman]

Andy, Can you provide some examples of the Danfoss calculations for risers please? I have not seen this program, but would like to compare their data to what I have been using for comparison.

I would have to think a bit to find an installation that has a gravity suction line. All or say most installations over here have risers.

It seems everyone is doing this worldwide. Gravity draining for wet suction lines does not require all of the penalties for pressure loss if the piping is not trapped out.

We could have similar discussion on bottom feed coils vs. top feed coils also. The same problem exists there also.

[Andy]

Andy, Can you provide some examples of the Danfoss calculations for risers please? I have not seen this program, but would like to compare their data to what I have been using for comparison.



It seems everyone is doing this worldwide. Gravity draining for wet suction lines does not require all of the penalties for pressure loss if the piping is not trapped out.

We could have similar discussion on bottom feed coils vs. top feed coils also. The same problem exists there also.

Iceman I have Dircal on my work computer, not this one. Danfoss have promised me a new copy. I will add some calcs when I get my hands on this.

Kind Regards. Andy

[Volnei]

Andy and Iceman the DIRcalc is a good progrm to be used but first of all you need to know the level of velocities that will be used. For sizing risers I use the tables 1-16A and 1-16B of IIAR - Ammonia Refrigeration Piping Handbook.

[US Iceman]

Volnei,

In the Piping handbook the table only lists several temperatures I believe. How do you obtain velocities between the temperatures provided?

The velocities are non-linear I believe so this may pose a problem with interpolating the values.

[Volnei]

Iceman,

The book suggest that you interpolate to find values for intermidiat temperatures.

[SteveDixey]

I believe that Frigoscandia have been working on something around this. Their freezers now have the option of a dry suction header that pulls the vapour out of the coil with no liquid to act as a drag. As so many installations now run the pipes in roof voids, they reckon to gain the equivalent of 2 or 3 deg C and so have been able to design freezers down to -50c without having to pull too much extra suction at the LP receiver end.

They can retrofit this system to most freezers they have built since '95.

Steve

[US Iceman]

Hi Steve,

I believe that Frigoscandia have been working on something around this. Their freezers now have the option of a dry suction header that pulls the vapour out of the coil with no liquid to act as a drag

Can you provide additional details on what they are doing to achieve this? Sounds interesting.

Seems very similar to a critically charged evaporator of some sort.

Thanks

[SteveDixey]

Hi Steve,
Can you provide additional details on what they are doing to achieve this? Sounds interesting.

http://www.fmctechnologies.com/uploa...rigeration.pdf

[US Iceman]

Thank you Steve. This appears as though they are using a flooded coil instead of a liquid overfeed coil. The liquid feed must be metered into the suction line before the surge drum.

That would certainly reduce the penalties for the suction riser as only gas is lifted in the drawing. Although the oil draining would now be required closer to the freezer and personnel.

Looks like a promising "new technology".

[McFranklin]

Cool2therule,
Your description leads me to believe that you are overfeeding the evaporators. The various pilot float systems are intended to provide just enough liquid refrigerant as what is evaporated, which should result in only vapor being returned to the suction accumulator.

Quite often the throttling (hand expansion) valves are wide open, either never properly set at start up or "operators" with attitude if a little is good a lot is better. If your surge drums are small, the supply solenoid opens, in comes the liquid fills everything faster than desired, and you have liquid sloshing around the suction connection.
That's my guess, I could be totally off in left field. Since you are considering adding another vessel to your suction header, why not ask the contractor(s) you plan to bid the job to look at the problem and make recommendations?
Since you have two types of controls, has the plant been added on to? If so, was the increase in load taken into consideration? Is this a new problem or one that has been going on for sometime?

[luissandoval]

Dear guys

I've been studying about risers in wet suction line and I understood that the only objective of risers is to minimize the effect of liquid static head.


I have a problem because in my instalation the riser enters for the bottom of the pipe ( i attach diagram) what kind of problems can i have in my instalation?
someone say me that i will have problems in my ammonia pump is that tru or falso??

please help me.....

Best regards

Luis Sandoval Misari.

[luissandoval]

sorry

In my last e-mail i forgot to attach the file.

best regars

[Icelander]

If I understand your problem right, I think it would be good to drill 3/4" hole under pipe from the evaporator and place a pipe that is connected to a vessel use pump to empty the vessel when the level is high in the vessel. Each pipe from the evaporator to the vessel should have a valve that closes when the evaporator is in defrost.

[TXiceman]

One of the problems is that liquid from the more distant evaporators will run down on the evaporators closer to the drum.

You really need to re-pipe the suction header and loop the returns up and into the header from the top or at least on a 45 degree up from horizontal.

[US Iceman]

I agree with TXiceman. The risers should always return to the top of the suction header and with the direction of flow in the header.

Any time you pipe the branch lines on the bottom of the header the liquid can run backwards down into one of the other risers. Not good...

[luissandoval]

Dear US iceman:

This problem may cause some damage at the pump please let me know because The system is already operating now and until this moment there isn't any problem. I am especially worry that the ammonia pump doesn't work good.

sorry my english is not good.

Luis Sandoval Misari.

[US Iceman]

Dear Luis,

If these are indeed suction risers, then they have no effect on the refrigerant pumps nor do the refrigerant pumps have any effect on the risers (under normal operating conditions).

The system may work OK as you have said if the evaporators connected to these risers operate at a relative full capacity all of the time.

Did someone tell you the pumps will break because of the risers?

[luissandoval]

US ICEMAN:


A friend told me that because the riser enters from the botton of the pipe this will cause that the liquid ammonia quickly retunrs to the separator vertical vessel and this cause that mu pump won't work good. what do you think about that ?


Luis Sandoval Misari.

[Sandro Baptista]

I don't agree with this double risers for NH3.

Why should be bigger trap fills first (I don't think so). I feel that the system wouldn't work as it should. Any Comments?

DOUBLE RISER NH3.jpg

[Sandro Baptista]

I think that the riser that Josip have showed, im my opinion, it makes perfect sense and functionally.

[gwapa]

hi Josip
Could you describe a litlle bit your riser

• 
  Why do you introduse the ends of the small reiser inside the big one?
• Why you make a 45° in smaller riser?
• Why the cut is in the oposite side of the flow?
• What was the drag velocity in both reiser?

Thanks
GWAPA

[sterl]

Very good discussion.

2-phase risers for R-717 and R-744 are the subject of a current, funded and assigned research study funded by IIAR and ASHRAE. The prevailing math today is as US Iceman indicated from USSR studies in the 50's & 60's of coolant in nuke reactors. Institute at a Russian university named after the man; S. S. Kutateladze.

If you make the translation from the Russian algebra: It leans very heavily on work by Martinelli and Rossi.

The onset of difficulty occurs when the annular velocity on the liquid skin on the pipe wall inside goes negative, or put another way: The gas travels up and the liquid travels down. And 1-pipe steam heating systems have deliberately worked that way for 150 years.

There is an identifiable pressure difference in the riser well above that anticipated for frictional flow. Even at relative low recirculation rates and high vapor velocities, the actual density of the "aerosol" in the pipe is much higher than that of the vapor alone. By field measure, this can represent as much as 5-times what would be calculated for the same length of "dry" pipe passing vapor, and still not log up; that at minus 40 Evap.

Biggest "new" issue in this regard has been VFD on CSW-type evaps as opposed to full speed fan cycling.

And the FMC approach (the LVS) uses a fluid dynamic approach to separation and re-injection of liquid, such that height and separation limitiations are largely overcome. There are minimal controls involved but it works best in a narrow window of load and temperature condition....Move an LTS drum from a minus 50 to a minus 10 load and it basically acts like a tiny surge drum.


We do not use the Trap and Split type double risers except at changes in roof level. And we do not overloop the return main if the evap's pipe is 4-sizes smaller than the main: we side stab it cause the purpose of the up and over is honored by the geometry of the intersecting pipes.