Help with Review of Ammonia Cold Storage Plant

[carpetman]

Hi all who will listen,

I have recently graduated from a degree in Mechanical Engineering and now find myself working as an industrial refrigeration engineer with little experience and big projects (problems?)

I have recently been tasked to review an Ammonia Cold storage facility which is not running as efficiently as possible. I have done the heat load calculations for the entire plant; 17 chillers at 0C, 22 freezers at -18C and 3 blast freezers at -30C. Giving a total load of 1012kW. My superiors believe the problem with the current system is the refrigeration lines diameter is too small, increasing running costs to an unacceptable rate.

My problem is I have no senior engineers or persons of more experience to depend on for guidance so I was hoping to get at least a nudge in the right direction from this website. Now to my question; Am I best to gather the information on current compressors, condensers, intercoolers, recievers, and what ever else there maybe in the system and from those produce what should be the running costs of the system and compare with current running costs?
This question is just one of many, and I am really hoping for some guidance to kick me off in the right direction for the review and then hopefully the design of a system upgrade.

I am currently awaiting further information on the current system from the plant manager, so if I haven't supplied enough info here please let me know.
This is all new to me and I will be forever greatfull to any who can help with some guidance.

Thanks in advance.

[lraja]

Your supervisor seem to have a valid point, take few min to check those suspected lines against minimum requirements (diameters) for such capacities to start with. You could use the information in a standard refrigeration hand book or ASHRAE guidelines

[carpetman]

Thanks Iraja,

I've been going through the Ashrae refrigeration handbook, which is obviously a great help, the thing is I don't have any experience with line sizing so I'm still unaware of how to go about producing a result which is guaranteed correct.

Do I size the line coming into the evaporator and then work back increasing the line size for every junction I come across? And then do the same for the wet return? And how about the line sizing between the inter-cooler, compressors, condensers and receivers?

Sorry if this is an obvious question, the system I am looking at just seems a lot more complex than anything I have dealt with through university; 7 compressors, 3 condensers, an inter-cooler, 2 receivers, many hundred meters of pipe and probably lots more components I am unaware of.

Thanks again.

[mad fridgie]

How lucky you are, to be paid for a job you know nothing about, I struggle to get paid for what I do know.
down to business.
Your compressors, get SST and SCT, power draw, check against manufactures data.
This will give you "mass flow", you can work out the density at the varing different conditions. From there you should be able to calculate the pressure drop through respective pipes. (you could then, check against bigger pipes) Nowadays I am to old for all these calcs, i just use some software. lol

[carpetman]

Hmm, taking many hours to solve a simple task.. doesn't sound lucky to me.
Thanks for your help though! As I said I'm very new to all this so forgive me if I sound like I don't know anything (I don't) but I'm trying to learn as fast as possible.

I've sized the liquid line and suction line for each individual evaporator (57 in total), as well as determined the pressure drop and required sub cooling. What I'm still unsure about is do these lines just increase in diameter as they meet and run back to the plant room? (I'm asking simple questions in the hope of getting simple answers) and where could I get guides on the diameter increase? Am I going about this correctly?

What about sizing the feed lines or the discharge lines for the 7 compressors?

Thanks.

[RANGER1]

The main headers have to be bigger to cope with pressure drop which should be kept to a minimum say 1 to 2 deg C.

Are we assuming correctly that plant is liquid recirculation ?

You also have to allow for circulation rate of 4-1 in suction returns to plant room vessels.
How many vessels do you have & what are saturated temps in plant room for each vessel .

If there are any long suction risers returning into mains on say blast freezers as you will pay a penalty from liquid head of ammonia in units if pipework is not designed correctly ( say anything higher than 3-4 metres). This means you have to run suction line at a lower pressure to achieve temps in air cooler .

Basically any pressure drops you pay a penalty , so they should be kept to a minimum .
If you have high quality accurate gauges & place them around plant you might find some problem areas .

[Sandro Baptista]

Carpetman

First at all congratulations for your arrive at the RE.

Do you have plug points where you can measure by connecting a gauge?

For instead: At the cold stores -18ºC, if you are evaporating at pressure corresponding -25ºC of saturated temperature, for example (just at outlet of the evaporator) then if it is at 100 meters of tube plus valves and connecting accessories until to the suction of each compressor then you may expect about 2K of pressure loss (the ideal would be zero but that would be infinitly huge lines and valves and no hidrostatic penalty could occur). Of course this is a criterion but if you have about 10 K this I can sure you it's not normal - you would sucking the gas at the compressor at -35ºC which would be a big penalty on the COP and with a great decrease on the capacity.

So try to use first all the instrumentation you have and you can use so you can be approach of the reality of each refrigeration plant.


Regards

[Segei]

Hi all who will listen,

I have recently graduated from a degree in Mechanical Engineering and now find myself working as an industrial refrigeration engineer with little experience and big projects (problems?)

I have recently been tasked to review an Ammonia Cold storage facility which is not running as efficiently as possible. I have done the heat load calculations for the entire plant; 17 chillers at 0C, 22 freezers at -18C and 3 blast freezers at -30C. Giving a total load of 1012kW. My superiors believe the problem with the current system is the refrigeration lines diameter is too small, increasing running costs to an unacceptable rate.

My problem is I have no senior engineers or persons of more experience to depend on for guidance so I was hoping to get at least a nudge in the right direction from this website. Now to my question; Am I best to gather the information on current compressors, condensers, intercoolers, recievers, and what ever else there maybe in the system and from those produce what should be the running costs of the system and compare with current running costs?
This question is just one of many, and I am really hoping for some guidance to kick me off in the right direction for the review and then hopefully the design of a system upgrade.

I am currently awaiting further information on the current system from the plant manager, so if I haven't supplied enough info here please let me know.
This is all new to me and I will be forever greatfull to any who can help with some guidance.

Thanks in advance.
I'm really surprised with your superiors.
1. Why did they give this project to you (without senior engineer)?
2. Why do they think that plant inefficient and lines undersized?
You can check the line diameters. Assume, that suction line is undersized. Will you recommend to replace it? I found that very often the problem is inefficient operation. Give us more information about this plant we try to help you. I assume that this plant has evaporative condensers. Is it one stage or two stages? Give us operating pressures, temperatures.

[Sandro Baptista]

The operation sequence of the compressors / management of the compressor capacities is very important as well of the condensers. Operating at part load...or at full load with glycol systems if you sufficient thermal inertial volume to avoid start-stop-start-stop...and so on

[mad fridgie]

You add the mass flow of refrigerant for each evap, as it is been added to the suction, or removed from the liquid.
So your suction should get bigger in theory after each evap (no always the case because you are limited to the actual pipes that are available) You then calculate, each pressure drop for each leg.
Liquid becomes smaller.
You also have to look at the diversity of the rooms, was it designed to have all evaps operating at the same time or not, "if not" being the case, smaller mains are installed.

Do you have saperate compressor(s) for the blast freezers and the storage freezers, are are they on a common LT system?

What is the quality of the insulation of the pipework, if this poor, then this increases infiltration, increasing pipe pressure drops (need to run lower suction pressures)

[carpetman]

Firstly, thank you all for your input!

The plant is a number of hundred kms away from my work station so I am yet to see it with my own eyes. I'm currently relying on the plant manager to relay information back to me as needed (A slow process though).

Here are the specs I know; The plant has 17 chillers at 0C, 22 freezers at -18C and 3 blast freezers at -30C. High temp heat load (chillers) is 1012kW, Low Temp heat load (freezer & blast) 997kW. There is 7 compressors (3 for LT + 2 which can be switched), an inter-cooler, 3 condensers, a high temp drum and low temp drum. The system runs 24/7, defrost using hot gas 4 times a day for 15mins for each individual room (except blast).
I am waiting for other information from the pant manager but I believe condenser temp is about 40C, low temp suction -40C and high temp suction -18C, I have based my line calculations on these values.
I'm still learning about what information is needed to carry this out successfully so I have probably missed many things.

I will post information as I receive it.

Segei, unfortunately there are no senior engineers at my company. I will be able to get some guidance in the near future but this is a friendly offer not something I can rely on for every problem I come across and so I'm trying to do things as independently as possible (I know relying on RE is not exactly independent).
The plant was upsized a number of years ago and since has not been running well. I have made no judgements as to what the problem might be, only listened to my superiors.

Thank you all. I hope this is making some sense

[mad fridgie]

Firstly, thank you all for your input!

The plant is a number of hundred kms away from my work station so I am yet to see it with my own eyes. I'm currently relying on the plant manager to relay information back to me as needed (A slow process though).

Here are the specs I know; The plant has 17 chillers at 0C, 22 freezers at -18C and 3 blast freezers at -30C. High temp heat load (chillers) is 1012kW, Low Temp heat load (freezer & blast) 997kW. There is 7 compressors (3 for LT + 2 which can be switched), an inter-cooler, 3 condensers, a high temp drum and low temp drum. The system runs 24/7, defrost using hot gas 4 times a day for 15mins for each individual room (except blast).
I am waiting for other information from the pant manager but I believe condenser temp is about 40C, low temp suction -40C and high temp suction -18C, I have based my line calculations on these values.
I'm still learning about what information is needed to carry this out successfully so I have probably missed many things.

I will post information as I receive it.

Segei, unfortunately there are no senior engineers at my company. I will be able to get some guidance in the near future but this is a friendly offer not something I can rely on for every problem I come across and so I'm trying to do things as independently as possible (I know relying on RE is not exactly independent).
The plant was upsized a number of years ago and since has not been running well. I have made no judgements as to what the problem might be, only listened to my superiors.

Thank you all. I hope this is making some sense
Good answers, what a total waste of energy.
Blast freezer, should be on its own compressor(s), lets call it this LT and should be SST -40
Your -18C rooms should be at a higher SST -30
and your 0C rooms should be at SST -10C
All give and take a bit.
Has the client dramatically increased the load in each of the rooms. If so they may have just added more refrigeration grunt, lowering all the suctions pressures, to increase the evaporator splits, hence increasing the evap duties.

[Segei]

Firstly, thank you all for your input!

The plant is a number of hundred kms away from my work station so I am yet to see it with my own eyes. I'm currently relying on the plant manager to relay information back to me as needed (A slow process though).

Here are the specs I know; The plant has 17 chillers at 0C, 22 freezers at -18C and 3 blast freezers at -30C. High temp heat load (chillers) is 1012kW, Low Temp heat load (freezer & blast) 997kW. There is 7 compressors (3 for LT + 2 which can be switched), an inter-cooler, 3 condensers, a high temp drum and low temp drum. The system runs 24/7, defrost using hot gas 4 times a day for 15mins for each individual room (except blast).
I am waiting for other information from the pant manager but I believe condenser temp is about 40C, low temp suction -40C and high temp suction -18C, I have based my line calculations on these values.
I'm still learning about what information is needed to carry this out successfully so I have probably missed many things.

I will post information as I receive it.

Segei, unfortunately there are no senior engineers at my company. I will be able to get some guidance in the near future but this is a friendly offer not something I can rely on for every problem I come across and so I'm trying to do things as independently as possible (I know relying on RE is not exactly independent).
The plant was upsized a number of years ago and since has not been running well. I have made no judgements as to what the problem might be, only listened to my superiors.

Thank you all. I hope this is making some sense
"has not been running well". What do you mean? Plant doesn't keep temp. or something else.
High suction pressure is too low. Most likely evaporators are overdefrosted. Where is this plant located? Condensing temperature 40C or 104F is too high for ammonia plant and for evaporative condensers.

[Sandro Baptista]

Good answers, what a total waste of energy.
Blast freezer, should be on its own compressor(s), lets call it this LT and should be SST -40
Your -18C rooms should be at a higher SST -30
and your 0C rooms should be at SST -10C


If the LT refrigeration kWs is at most part for cold stores -18ºC then operating at -40ºC is like mad says a lot of waste of electric energy. Separating the circuits of cold stores and blast freezers would be handy but costly.

About the intermediate pressure, -18ºC, change the set-point of the HP compressor for -10ºC, like mad says. The LP compressors would give a little less but the overall COP will be higher and you will have more compressor capacity available (safety).

Also check if there is air in the system. See if the discharge line from the condensers to the receiver is well below the condensing temperature.

[Sandro Baptista]

presence of air will force the compressor work a little harder to discharge at a higher pressure » lower COP, lower capacity, more wear, more service on the HP compressors

[carpetman]

Hello once again,

A little more info from the plant manager "Our condenser runs around 950-1050 kpa in summer it can get up to around 1200kpa depends on how much load we have.

With the pipe work i have done some checks and have found pipes are to small and the amount of load coming back into the drums is way to big for the drums
"

What I've decided to do is to simply finish the pipe calculations (this is good practice anyway), compare, discuss with superiors and then continue by tackling the potential problems which you have all been kind enough to open my eyes to.

What do you guys think? Is this a good strategy or just a waste of time? I'm trying to stick to what my superiors suggested before I go off on tangents and re-design the whole plant.

Can you guys think of any other information that I should gather? As sated many times above, I still do not know all the information that is needed and it is proving harder than originally thought to get small pieces of info which I know I require.

Thanks a million more times!

[mad fridgie]

Hello once again,

A little more info from the plant manager "Our condenser runs around 950-1050 kpa in summer it can get up to around 1200kpa depends on how much load we have.

With the pipe work i have done some checks and have found pipes are to small and the amount of load coming back into the drums is way to big for the drums
"

What I've decided to do is to simply finish the pipe calculations (this is good practice anyway), compare, discuss with superiors and then continue by tackling the potential problems which you have all been kind enough to open my eyes to.

What do you guys think? Is this a good strategy or just a waste of time? I'm trying to stick to what my superiors suggested before I go off on tangents and re-design the whole plant.

Can you guys think of any other information that I should gather? As sated many times above, I still do not know all the information that is needed and it is proving harder than originally thought to get small pieces of info which I know I require.

Thanks a million more times!
In situations like this you can not just rely on theory for all your calculations (even though is good practice)
If the system is running at mucher lower pressures than designed, then your pipework for a fixed duty will be incorrect.
You NEED to take the oppotunity to take actual measurements, and get a full understanding of the system and the process.
There is a saying
"Shyte in; Shyte out"
You may be looking at a symptom and not the cause.
You also maybe right.
But how do you know, unless you use true data.

[Sandro Baptista]

It seems you are condensing at a "good" pressure, Tc=+28ºC up +34ºC. No lack of condenser seems to exist.

About the surge drums are the compressors are sucked liquid from the surge drums. Are the high level switch from the drums cutting out the compressors?

I think we have giving a lot of suggestions so please make reads, see the pressure drops and see the set-points of the machines and so on and give us the feed back.

[Sandro Baptista]

If the system is running at mucher lower pressures than designed, then your pipework for a fixed duty will be incorrect.


It doesn't mean at 100% (even so I also thought so) that the problem is from the DN of suction pipes.

It could be the compressor are consigned to operating at a lower suction pressures than needed (so unnecessary extra compressors are working to get so low compressors).

[carpetman]

Yes, many suggestions indeed. Thanks once more. This forum is a fantastic place!
Mad fridgie, you are most certainly right! I must see with my own eyes. I have simply been trying to understand the system before I go and just see be metal objects making noise in front of me. This has given me a much better understanding, I'm sure ill print this thread out when I head up to the plant.

Ill get back to you when I have some answers of my own (hopefully)!

Thanks.

[mad fridgie]

If you want some brownie points, why do you not enquire about recieving
RECs certificates for the possible upgrade. 1credit ($40)/1Mw saving/10years.
Also if the piping and pots are incorrect then you can use the oppotunity to increase efficiency.
By seperating the different loads (you can then likly use existing pipe work and add additional. this would be cheaper that replacing the total mains and pots with the correct size

[Sandro Baptista]

By seperating the different loads (you can then likly use existing pipe work and add additional. this would be cheaper that replacing the total mains and pots with the correct size

I'm plenty agree.

[Segei]

Hello once again,

A little more info from the plant manager "Our condenser runs around 950-1050 kpa in summer it can get up to around 1200kpa depends on how much load we have.

With the pipe work i have done some checks and have found pipes are to small and the amount of load coming back into the drums is way to big for the drums
"

What I've decided to do is to simply finish the pipe calculations (this is good practice anyway), compare, discuss with superiors and then continue by tackling the potential problems which you have all been kind enough to open my eyes to.

What do you guys think? Is this a good strategy or just a waste of time? I'm trying to stick to what my superiors suggested before I go off on tangents and re-design the whole plant.

Can you guys think of any other information that I should gather? As sated many times above, I still do not know all the information that is needed and it is proving harder than originally thought to get small pieces of info which I know I require.

Thanks a million more times!
I think that you overfocused on piping. Assume that piping undersized. Estimate the benefits of changing the size to the right one. Do this estimation for real refrigeration load not design load. Design load happens only for less than 1% of operating time. Load will be lower for the rest 99%. Compare savings with investment that should be done to change piping and estimate simple payback. In North America it should be 2 years or shorter.
I guarantee you that lowering condensing pressure from 1000 kpa to 800 kpa will save more than repiping and you don't need huge investment for this action.

[carpetman]

Thanks Segei,

I'm focusing on piping for the beginning, it may end up being a waste of time but it still teaches me about piping design for future purposes (and I can do this from my desk). Once I have completed this I will go onto the other various options mentioned above. I'm not ultra concerned about time frame for this as it's the first real life project I've ever worked on and I really just want it to be a learning experience (along with a successful outcome).
The more options I investigate the more I will begin to understand.

No doubt I will be back with more questions at some stage though.

Thanks.

[Segei]

Thanks Segei,

I'm focusing on piping for the beginning, it may end up being a waste of time but it still teaches me about piping design for future purposes (and I can do this from my desk). Once I have completed this I will go onto the other various options mentioned above. I'm not ultra concerned about time frame for this as it's the first real life project I've ever worked on and I really just want it to be a learning experience (along with a successful outcome).
The more options I investigate the more I will begin to understand.

No doubt I will be back with more questions at some stage though.

Thanks.
You are welcome.
I were focused on energy efficiency in industrial refrigeration for the last ten years. Definitely, designing approach(right piping, additional condensers...) will save some energy. However, significant investments are required. Very rare you can get a good payback. Optimization of a refrigeration plant operation can give us good energy savings and a few months payback.

[mad fridgie]

I think that you overfocused on piping. Assume that piping undersized. Estimate the benefits of changing the size to the right one. Do this estimation for real refrigeration load not design load. Design load happens only for less than 1% of operating time. Load will be lower for the rest 99%. Compare savings with investment that should be done to change piping and estimate simple payback. In North America it should be 2 years or shorter.
I guarantee you that lowering condensing pressure from 1000 kpa to 800 kpa will save more than repiping and you don't need huge investment for this action.
Hi Segei, he is in Australia, and the wet bulb temp can be above 30C (depending where the plant is), so it is difficult to drop the heads pressure to what you would expect as being normal. Most of the time his winter is better than your summer. (Or I should say if your prefer the warm weather)

[RANGER1]

22 freezers with 4 defrosts a day is interesting in itself .
Typically Australian hot gas is returned to low temp vessel instead of high temp .
If its an old plant the ammonia may have significant amount of water in it as well , being in a vacuum .
Its easy to be critical of design but its normally built to a price & power was cheap until a few years ago.
Sorry I don't believe a junior guy can achieve much , I think an expert is required .

[Sandro Baptista]

Carpetman,

When you go visiting the plant can you take some photos to the plant equipaments? Also a scheme of the refrigeration plant, if possible. I'm curious about that plant.

Don't hurry I will not compete with you on anyway. I'm from Portugal, to far of Australia. :)

[Segei]

Hi Segei, he is in Australia, and the wet bulb temp can be above 30C (depending where the plant is), so it is difficult to drop the heads pressure to what you would expect as being normal. Most of the time his winter is better than your summer. (Or I should say if your prefer the warm weather)
Hi, Mad.
This is the reason I asked him about location of this plant. In our days we can go to internet and check summer, winter, current wet bulb temperatures. However, maximum head pressure 1200 kpa is not bad. I'm just curios about 22 freezers. May be they have 22 evaporators. Why do they have 22 freezers with one evaporator in each? Temperature in all freezers is -18C.

[RANGER1]

Carpetman,

When you go visiting the plant can you take some photos to the plant equipaments? Also a scheme of the refrigeration plant, if possible. I'm curious about that plant.

Don't hurry I will not compete with you on anyway. I'm from Portugal, to far of Australia. :)


No-one is competing its just common sense .

Obviously this client looks after themselves because they already know more than the rest of us .

Its called false economy .

[mad fridgie]

I think this young fellow is on steep learning curve, (especially without having a tech superviser) i find it funny he has come straight of uni, and got a job as an industrial refrigeration engineer. I could understand this if he had a masters in refrigeration.
No offense to the OP, good on him for going to uni and getting a job

[carpetman]

Thanks Mad,
It's all very exciting, although a little daunting. I look forward to being able to tackle problems like this without too much trouble.

Ranger, you are most probably right. I'm not assuming Ill be able to complete this job as well as anybody with experience but it is still a nice learning curve to be struggling up (although a steep one). Once I've reviewed all the options mentioned here Ill be taking up a friendly offer from a senior to review my work and give feedback. At least this way Ill know if what I've done is correct or not. Oh and yes the plant is old as the trees.

Ill PM you some stuff to you shortly Sandro.


Thanks

[Sandro Baptista]

okay, Carpetman I'll be waiting for :)

[750 Valve]

If I were you, given the fact the plant manager is calling undersized pipework i would start there - he represents the company paying the bills.

You have the max design heat load, go to site and do some in depth research on the piping arrangements, line sizes, vessel sizes and all running conditions. Sketch the plant and its pipework, verify the pressure drops thru the existing lines for each load and refrig mass flow rates. Working out existing pipework characteristics should be easy as you are not selecting from scratch just verfying what is there, same with vessel sizing.

Once you verify if the current pipework is adequate then move from there - if its inadequate then has it always been inadequate? have they added the blast on and are compensating by just running lower temps? Can anything be done to bring the operation of the current plant into line and reduce pressure drops in current pipework?

If the pipework is proved adequate then at least you have ruled out a major factor and can prove to the plant manager that this is not the problem and start to focus attention on other issues such as total LT load running at blast freezer SST