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.

[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?

[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

[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

[US Iceman]

Thanks Andy. I look forward to reviewing that.

The material I have been using is based on thermal hydraulics in nuclear reactors. There they have to be concerned about water circulation over the control rods.

I don't understand all of that, but the methods seems to produce reliable results that agree with some of the rules-of-thumb the old timers used.

[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.

[nh3wizard]

In all our new constructions we have been using penthouses, which inturn lets gravity do the trick

[Andy]

Hi nh3wizard

I was thinking about that. Not much used in Ireland or GB, but worth considering, especially in distribution centers.

It would be used a bit in AHU installs.

Any pictures to share with us.

Kind Regards. Andy

[US Iceman]

I think the idea of using penthouses does greatly reduce the riser penalties for pressure losses.

The main problem I see with this is the first cost for the penthouses and roof curbs. These can be expensive with a first cost premium.

A secondary consideration is the use of duct work adding static pressure to fans and increasing the fan input power.

This could be minimized by careful selection of fan types and the air distribution system though.

These also have some extra benefits such as having the piping and valve stations on the roof for easier service and installation.

What has been your experience with these NH3wizard?

Any complaints or helpful suggestions?

[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".

[SteveDixey]

Although the oil draining would now be required closer to the freezer and personnel.

AFAIK, Frigo have always put oil drains in the freezer coils inside their Gyro type freezers. A Frigo Advantec freezer recently installed also had an oil drain inside the cabinet, but for some reason, the installing contractor saw fit to run the oil drain up to the roof void above the freezer. Very nice place with loads of obstructions and trip points if the oil drain suddenly "sneezed"....

Steve

[US Iceman]

...the installing contractor saw fit to run the oil drain up to the roof void above the freezer

Oh Boy... that would be convenient to work on.

Solved one problem and created another bigger one I'd say.

[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

[US Iceman]

...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...

That is true Andy as long as the gas velocity is sufficient to drag the liquid up the riser. This is the annular flow regime for two phase. If the velocity begins to decrease due to a reduction in load, the liquid will begin to shear away from the wall and collapse back down into the riser causing it to flood. This is BAD.

The liquid flooding the riser causes the evaporator pressure to increase due to static head and you loose temperature control. The boiling in the evaporator can actually stop when the static pressure increases.

..., so intermediate traps are a thing of the past. Maybe double risers are also

Double risers ARE required if the evaporator load changes. And they will certainly do this. At a partial capacity point of operation the volume of vapor being generated in the evaporator can be much less.

This then decreases the riser superficial velocity. If a single riser is used and it is too big, the riser floods.

If a single riser is used and it is sized for the part load operation, the pressure drop at full load will be excessive and produce a negative impact on the required suction pressure. The suction pressure will have to be much lower to provide the pressure differential at full load. So, you also loose compressor capacity very quickly.

The double riser is intended for evaporators where the capacity is variable. At full load both risers are carrying liquid up into the main horizontal pipe.

At part load the bottom trap in the larger riser (as shown in Josips excellent drawing) will fill with liquid. This is supposed to seal the larger riser. The smaller volume of gas then flows up the smaller riser carrying the liquid with it.

Each riser of a dual riser piping arrangement must be designed for it's respective gas velocity, which depends on the capacity reduction of the evaporator.

On interesting point I just thought of. When the liquid fills the bottom trap in the larger riser...

The trap depth should be deep enough to provide a static pressure (head) greater than the pressure loss through the smaller riser at part load. If not, the liquid can blow out of the trap seal and render the small riser ineffective.

This is the same principle for water drains on high static pressure air handling units.

[Josip]

Hello guys,

I was waiting but nobody ask about pipe size, to give some more light on this please see attachment again

Best regards,

Josip

[US Iceman]

Hi Josip,

I have a question that goes back to the unit conversion topic. What does the DN indicate and are the numbers in millimeters? I'm not familar with this.

Are the pipe wall thicknesses based on a standard or is this used as a nominal line description size?

Thanks for your help.

US Iceman

[Andy]

Hello guys,

I was waiting but nobody ask about pipe size, to give some more light on this please see attachment again

Best regards,

Josip

Hi Josip

I was holding back a bit as Iceman put me in my box when I posted above

Iceman the pipe sizes are 10" 8" and 4"

My point is with careful design double risers are not necessary, indeed lifting liquid up risers should be avoided as the suction losses arevery high.

Would it not be better to fit the surge vessel at a lower point in the system and rise using a dry suction up to the compressor plant.

Or consider flooded evaporators for the lower evaporators.

My take on the issue is if a double riser is needed I have designed a potential problem into the system and a look at the overall design is needed.

Iceman Josip has posted a link to the DirCalc software if you want to try it, you can even set it up to work in Imperial units

Kind Regards. Andy

[Josip]

Hi Josip,

I have a question that goes back to the unit conversion topic. What does the DN indicate and are the numbers in millimeters? I'm not familar with this.

Are the pipe wall thicknesses based on a standard or is this used as a nominal line description size?

Thanks for your help.

US Iceman

Hello US Iceman,

Please see attached file, everything will be clear. Yes all dimensions are in milimetres but in conversation we use expression: "bring me a piece of 2 inch pipe, please" - today, unfortunately, more often without "please" .

Here we use seamless pipes as standard for refrigeration. Attached file is from DIRCalc 1.14

Best regards,

Josip

[US Iceman]

Hi guys,

Andy, I apologize if it seemed I put you in a box. That was certainly not my intention. I get carried away typing my thoughts.

You did touch on an area I always suggest as a first course of design layout:

Would it not be better to fit the surge vessel at a lower point in the system and rise using a dry suction up to the compressor plant.

If gravity can be used to drain the liquid this is the best method, I agree. It does decrease the cost of operation to lift vapor only.

Josip, thank you for posting the information. Now I understand. The drawing is indeed for some large risers.

What is the evaporating temperature for that project?

...you can even set it up to work in Imperial units

Duly noted. I need all the help I can find.

[Josip]

Hi US Iceman,

It was ice cream factory and one riser was for surge drum working at -45C another one for surge drum working at -35C.

Plant was at the 1st floor (surge drums also) but we have to run our pipes down to the ground floor (due to hygienic demand) then again up to the 1st floor to surge drums and then again from surge drums at the 1st floor down to compressors at the ground floor

See attachment

BTW, final report (posted for ice builder) for energy conservation storage is quite interesting

Best regards,

Josip

[US Iceman]

Last year (2005) during the IIR conference in Macedonia one person had a presentation for suction risers that may be of interest to you guys.

This is from the following link:
http://www.mf.ukim.edu.mk/web_ohrid2005/abstracts.html

The proceedings listed on the above link can be ordered from IIR for about 30 € plus shipping.

ON THE DESIGN OF VERTICAL RISERS FROM AMMONIA
RECIRCULATION TYPE EVAPORATORS
H. T. HAUKAS
Consulting engineer
Strandebarm, Norway

Design criteria for vertical risers carrying two-phase ammonia flow include minimized pressure drop at stable flow conditions and prevention of flow and pressure oscillations, particularly at re-duced load. Estimates for optimum vapour velocities with respect to pressure drop in vertical up-ward two-phase flow are available.

A method to calculate the magnitude of the vertical pressure gradient is proposed. While these velocities may serve as design criterion for high riser systems (e.g. 4-5 m) at moderate evaporation temperatures, lower velocities should be applied in low tem-perature systems with less riser heights, due to greater relative significance of the other pressure drop components in the riser system.

At low and moderate vapour velocities, the liquid transport in vertical risers will be intermittent and, under certain circumstances, cause flow and pressure pulsations. This may become a major problem when pulsations in riser and evaporator interact and exacerbate instability, possibly bringing the entire sub-system into heavy oscillations. Annular flow in the riser will minimize the risk of oscillations and the requisite conditions to achieve annular flow are discussed.

At optimum vapour velocities, the flow will be well within the annular flow regime in most cases, with low risk of flow oscillations. For evaporators working over a wide capacity range, double ris-ers should be applied to keep vapour velocity above the lower limit for annular flow.

I have ordered a copy as this looks like a good investment. The abstract above also closely matches some of the research I have done so I feel comfortable spending the money.

PS. The rest of the topics look very interesting also.

[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

[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?

[dgarciaMex]

Hello guys,

I was waiting but nobody ask about pipe size, to give some more light on this please see attachment again

Best regards,

Josip

Hello Josip,

How do you know the diameter of the pipes in the riser? or how can i calculate a riser, if a have 4" suction line (ammonia) and need to rise 20ft ? Suction Temperature is 18°F.

Regards,

[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

[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.

[US Iceman]

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.

Luis,

The pump only circulates refrigerant to the hand expansion valve on the evaporator(s). As the liquid enters the evaporator a small fraction of the liquid mass will evaporate as it picks up heat from the evaporator. The remaining liquid then flows from the evaporator. The gas leaving the evaporator pulls the liquid up the riser and then enters the wet suction line. All of this liquid and vapor flow back to the vessel where the liquid separates from the vapor. The liquid falls back into the vessel (above the pump) and the vapor is pulled back by the compressor.

The liquid returning back to the vessel has no effect on the pumps, unless the liquid falls directly into the pump suction line. This might cause some cavitation, but you would not normally see this if the vessel is properly designed.

[gwapa]

dear Josip
It is very interesting the riser disign you alreary post here:I have some question

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

Thanks

[dglaab]

Hi Guys:

I am new here, but this thread applies to a current project I have going.
It seems most of this discussion centers around systems that are not overfeed systems.
I have a large overfeed system (1000 ton) that operates at -60, with large capacity swings in operation. The layout requires the main suction header (14") to be below the grade level, requiring me to lift to the LPR about 18 feet. I really don't want a knockout drum and pumps in a pit, not only for obvious safety and maintenance considerations, but area is subject to coastal flooding as well.

I am looking at a triple suction riser, and using for a sizing reference ashrae and a paper by D.K. Miller on this subject (journal of chemical engineering 1979, sizing dual suction risers.....). I am still awaiting a hard copy of the paper, as its not scanned into a database.

My query is, since the vapor pressure is equal on all traps, wouldn't the larger traps need to be sucessively lower in elevation to accomodate primary flow through the smaller risers?

Any ideas

Thanks

Dan

[Peter_1]

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

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

[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.