Carbon Dioxide Refrigeration

[Andy]

Hi,
just thought you all might be interested in an old type of refrigeration which has become popular again due to enviromental restraints.
Link is as follows:
Nestles Case Study
Sorry about advertising for Star, but it's an interesting topic none the less.

Now where's Marc when you want a good debate.

Regards. Andy

[PobodysNerfect]

Hi Andy,
Interesting consept. However, it would have been nice to have more details such as a schematic drawing with the actual pressures - both for the CO2 & NH3.
Saludos

[Dan]

Andy, this looks like good stuff. But the blather makes it hard to understand. No, let me say it differently. The blather makes me gag and I cannot read it for more than 30 seconds.

Could you explain what we have here in a more technical and practical manner?

[Andy]

Hi, Dan
Well if you want to complicate a thing ask aa sales man to explain it
Me I 'am too thick to be in sales and not near fat enough to pilot a desk yet.
Anyway here goes. The plant is basically two different plants, a coventional NH3 system at the high stage evaporating at -18 deg C and condensing at 35 deg C, not much in the way differing from the normal. Where things get interesting is on the low temp side, a pumped co2 system is fitted evaporating at -45 deg and condensing at -18 deg C, to all intense purposes this system again is very similar to a pumped NH3 system except for the pressures used and the design allowances to enable operation at these pressure, things like Stainless Steel pipework to withstand the pressures, thick vessel walls and heavier bodied control and shut off valves. The liquid pumps for the co2 are a bit special too being CAT powerwasher pumps (or a modified version of) these pumps work in the pressure range of 35 to 40 bar gauge, as you can see they have to be a little bit special!
Plant operation.
Heat is removed at -45 deg C and rejected to the cascade condenser where it is condensed by the cooling effect of the NH3 evaporation taking place in the same vessel, heat exchange is indirect, with the cascade vessel being basically an coil in shell heat exchanger (the only special bit of this is the way it is constructed so that in the event of a leak the two refrigerant cannot conbine into crystals within the system).
Co2 has a number of issues to be overcome as follows:-
[list=1][*]1/high pressures in the low temperature side which must be controlled and not allowed to rise even when the plant is off cycled.[*]dryness, CO2 is a very dry gas which doesnot lubricated compressors in the conventional manner.[*]defrost, it is not usually feasible to bring the CO2 pressure above 40 barg to defrost the low temp evaporator (40 bar is only 5 deg C)[*]recaction, CO2 will react with NH3 to form a crystilite structure that is corrosive to plant components.[/list=1]

All of these factors have to be overcome before a CO2 plant can be utilized, some are easily overcome such as the pressure limitation, by good plant contol, others like the defrost require different thinking to normal industry practice.
I have a system flow diagram I will try and post when I get it transfered to the correct format, in the meantime I hope this helps.
Regards. Andy

[Dan]

Thanks Andy. I have a better picture of the system and some of the challenges it presents. Pressure washer pumps, eh? Fascinating.

[Andy]

Hi,
please find attached a Pressure Enthalpy diagram (simple system) for co2, no pressure drops included as at present I have no idea how many degrees of drop will occur in the various areas.
Regards. Andy

[Andy]

Hi,
I see the enthalpy lines don't extend low enough on the above attachment, sorry about that
As for the system diagram I have attached a link to the Grasso site, they have report on co2 which can be downloaded.Grasso Index
If you follow the link to site then go to News and in there you will find the article on Co2.
Happy reading . Regards, Andy

[Friobernal]

Hi Andy;

It has been a long time since i post for the last time, but this time i have to reply about your comment:

Well if you want to complicate a thing ask aa sales man to explain it

I´m a sales man and what you say it´s true!!!!but it is the way it works. In two weeks i´m having a meeting where i am supossed to be an expert in CO2 world. But I don´t. So i have to study like in university. So i would like people to ask all you can imagine to prepare my meeting, and perhaps in a future i will be an expert in CO2 world

Bye

[Andy]

Hi Friobernal,
nice to hear from you.
Danfoss have a very good paper on R744 that can be found at http://danfoss.com
Regards. Andy

[750 Valve]

I know this thread's been hanging round for a while, but Andy, how did you overcome the defrost problem (high CO2 pressures)?

[Lancer]

Hey guy's
We are just piping up a NH3/CO2 cascaded system where I am
currently working. To overcome the defrost problem/critical temperature of the CO2, the CO2 liquid is cooled with an ancillery coil built into the CO2 receiver (little 404a unit no bigger than a fridge compressor runs this) If the ancillery fails the CO2 is vented to atmosphere at a design pressure.

Hope to have every thing going in about 6 weeks, interesting stuff, I think this system will be the first cascaded CO2/NH3 commission in Australia (maybe).

Andy, the major cost difference we found was the CO2/NH3 shell and tube heat exchanger, which was about 4 time the cost of an flooded NH3 system for about 500kW. But because the CO2 pressures are so high, the piping to and from the evaporator are small incomparrison to an NH3 system so this
negates some of this cost.


Lance.

[frank]

Hey guy's
We are just piping up a NH3/CO2 cascaded system where I am
currently working. To overcome the defrost problem/critical temperature of the CO2, the CO2 liquid is cooled with an ancillery coil built into the CO2 receiver (little 404a unit no bigger than a fridge compressor runs this) If the ancillery fails the CO2 is vented to atmosphere at a design pressure.

Hope to have every thing going in about 6 weeks, interesting stuff, I think this system will be the first cascaded CO2/NH3 commission in Australia (maybe).

Andy, the major cost difference we found was the CO2/NH3 shell and tube heat exchanger, which was about 4 time the cost of an flooded NH3 system for about 500kW. But because the CO2 pressures are so high, the piping to and from the evaporator are small incomparrison to an NH3 system so this
negates some of this cost.


Lance.

Sounds like an interesting install. Any chance of some pictures?

[Friobernal]

Hi Lance;

You´re talking about a S&t heat exchanger. I know about shell and plate heat exchangers used in cascade systems

http://www.vahterus.com/refrig_case5.htm

Fyi

Regards

[Lancer]

Frank, I'll take my digital in next week and grab a few snaps, it has been an excellent project and great experience. The system has a HT and LT unit, the HT (-13 degC) is the side load for AHU's , Chiller’s with the LT (-50 degC) driving the CO2 condenser for a 23 tier Spiral Freezer.

Frioberral, our unit is a little smaller (about 800kW) and the nozzles for the NH3 are top over bottom, that’s about the only difference.

I couldn't believe my luck when I stumbled upon this forum, I googled for some info about speed control on screw compressors and up popped this thread. Please forgive my limited grip of the refrigeration realm, electrics are my poison and I have been trying to track down some informed opinion upon screw compressor control and this seems a great place to ask.

I am curious about the consequences of using speed control rather than slide control, or a combination of both on a large screw. I have read that even under no load, a screw will still use 75% of full load duty. The screw in question is a Stal (S57 I think, may need to check this) with a 260 kW motor; 416 amps FLC @ 415V, 2 poles, 2950 RPM.

Previously the unit had been set up for R22 and controlled by an AB SLC 5/03 PLC. When called the PLC toggled the open slide solenoid, 5 sec on, 15sec off, until the demand was meet, or the motor current reached FLC limit, at which the slide was closed in the same manner until it reached a lower limit resetting the sequence. The slide has no linear position feedback just discrete limits for fully open and closed.

In the present situation, the variability of the load may be an issue due to the screws limited turn down ratio. Yes I can hear you say “ then it’s to big”, well you know it’s the old story, the equipment was brought at a fire sale for the right price with no thought given to lifecycle cost.

I was thinking of using a Variable Speed Drive to match the compressor speed with the load, obviously for a motor this size the drive would be costly, hence my hesitation, has anyone had similar experiences that they care to relate. Is this a viable option and what limitations should be considered? Will reducing the speed dramatically affect compressor performance or mechanical integrity?

Any comment appreciated

Lance.

[bruceboldy]

I know this thread's been hanging round for a while, but Andy, how did you overcome the defrost problem (high CO2 pressures)?

I am intersted and have a few potential projects in low temp cascade co2
I am also very interested in what has proven best practice in defrost of the low temp co2 coil..

bruceboldy

[Andy]

I am intersted and have a few potential projects in low temp cascade co2
I am also very interested in what has proven best practice in defrost of the low temp co2 coil..

bruceboldy

Hi Bruce

AndyP's company use a hot gas generator. Liquid is taken off the R744 condenser, fed to a pump which raises it's pressure to about 10 deg c saturation. The liquid is then boiled oil using waste compressor heat and the subsequent hot gas is used for defrosting the evaporators.

Hope this helps. Kind Regards Andy

[NoNickName]

Hi Bruce

AndyP's company use a hot gas generator. Liquid is taken off the R744 condenser, fed to a pump which raises it's pressure to about 10 deg c saturation. The liquid is then boiled oil using waste compressor heat and the subsequent hot gas is used for defrosting the evaporators.

Hope this helps. Kind Regards Andy

Let me revamp and resurrect this thread.
I don't understand the principle of operation of this defrost.
In a R717/R744 cascade system, in order to defros CO2 evaporator, NH3 is taken off the condenser and then what?

[Plank!]

Let me revamp and resurrect this thread.
I don't understand the principle of operation of this defrost.
In a R717/R744 cascade system, in order to defros CO2 evaporator, NH3 is taken off the condenser and then what?

You've missed a small detail in Andy's post - no mention was made about using NH3 (R717)

liquid R744 (CO2) is taken from the condenser - actually its from the HP reciever - then boosted in pressure and fed into the defrost hot gas generator, where heat from the high stage oil coolers is used to superheat the liquid into hot gas suitable for defrost.

Star have this system patented so perhaps if you need more info contact them or Andy P.

[crashbang]

Incressed the co2 pressure to 40 barg and use the glycol oil cooling system via heat exchanger to incress the temp to get around 20 to 30 deg C and we defrost the coolers.

[Andy]

Hi 750 valve
the higher hot gas pressures are obtained not by a compressor (manufacturers won't supply compressors to us at these pressures) but by a vapour generator. This is basically a liquid pump, a vessel and two heat exchangers and is a patented idea.
We take liquid from the main R744 liquid line and boost it with high pressure liquid pumps, then we add heat to this liquid until it becomes a saturated vapour at 10 deg C. We then add further heat to the vapour until it is superheated by about another 10K.
This vapour is then supplied to a valve station, with three control valves (Suction liquid and Hot gas) and defrosting is carried out much the same as in pump circ R717 systems.
Hope this helps, Regards. Andy.

[mcamacho]

I attended Mr. Pearson's presentation on IIR2003, regarding another warehouse using CO2 as refrigerant. There Star used the oil coolers of screw compressors to heat the CO2 required for defrost (nice idea!).

[Andy]

Hi Mcamacho
Nice to see you have heard of Star Refrigeration
We also us discharge gas from the NH3/High stage to heat the liquid into vapour for defrosting on some jobs.
Kind Regards. Andy

[Friobernal]

Hi Andy,

It´s been a long time... i keep on studying

I really don´t see what you explain about hot gas defrost CO2, is it possible, you attach a scheme or a drawing,

where are the heat exchangers?

About PHE (and sorry for the advertaising), have you heard about AlfaNova (Brazed plate heat exchager 100% SS), What do you think?

Bye

[Friobernal]

I am attaching a cycle for drawing on it, if you want, or anybody else

Hasta la vista amigos

[US Iceman]

Dear Lance,

I hope the information is useful.

Positive displacement machines (pumps or compressors) do not follow the affinity laws as centrifugal pumps or fans do. The power savings are based on the capability of the compressor, based on tip speed of the rotors and how the slide valve responds to changing the effective length of the rotors.

Think of unloading a screw compressor as changing the length of the stroke of a reciprocating compressor. The shorter stroke would decrease the dispacement. When the slide valve unloads the rotors, the effective length of the rotor used for gas compression is shortened.

When all of this is taken into consideration with variable speed operation, the part load performance starts to become linear. At least down to about 50% speed with the slide valve fully loaded.

To get the exact data, you would need the manufacturers performance program. The various data points need to be ran for the suction and discharge pressure. Then vary the speed. Once the data is plotted as a percentage of full load capability, the curve starts to make sense.

The oil pressure is a major issue. If you have a full-time oil pump driven externally by a separate motor this becomes less of an issue.

It sounds like you are using some heat recovery devices off of the compressor discharge. Depending on the application, hot water generation will be reduced during part-load operation of the screw. Your operators may not want to decrease the discharge pressure if they think it affects the water heating.

On the other side of the discussion though, reducing the discharge pressure will reduce the motor amp draw, which also saves energy. You, or they may have to find a good balance between operation and heat recovery methods.

If your system uses water-cooled condensers, the water supply will determine how low the discharge pressure could potentially be reduced. If you have evaporative condensers, any lower wet bulb temperature will allow the discharge pressure to reduce. Assuming all fans and pumps are operating at full speed.

If you have VFD's on the condenser fans, it may be better to run the fans at full speed and reduce the compressor power input. The best course of action is whatever uses the least amount of energy.

If I can be of help, let me know.

Best of luck down under,
US Iceman

[murduc74]

hi
i need help finding co2 refrigerant properties (mollier charts or p-h diagrams).
pls help!!

[Andy P]

hi
i need help finding co2 refrigerant properties (mollier charts or p-h diagrams).
pls help!!

You will find them in the ASHRAE handbook (along with loads of others - or if you prefer an electronic version check out "CoolPack" from the Technical University of Denmark. Google should take you straight to it

Cheers
Andy P

[guapo]

Hi every body,

Is it Possible to compressor to Install a VFD and fixed the slide valve to full load? The loading of the compressor is just base on the RPM.

[Josip]

Hi, Guapo

Hi every body,

Is it Possible to compressor to Install a VFD and fixed the slide valve to full load? The loading of the compressor is just base on the RPM.

Please visit this page:
http://www.sabroe.com/fileadmin/file...sors_06.03.pdf

Best regards, Josip

[frank]

You will find them in the ASHRAE handbook (along with loads of others - or if you prefer an electronic version check out "CoolPack" from the Technical University of Denmark. Google should take you straight to it

Cheers
Andy P

There is also a direct download from the RE Database on the top of each page

[US Iceman]

Is it Possible to compressor to Install a VFD and fixed the slide valve to full load? The loading of the compressor is just base on the RPM.

That is the preferred method. When the load is reduced to about one-half (half speed capacity), the compressor performance begins to reduce very quickly.

[Samarjit Sen]

Dear Andy,

I was browsing through the forums web, when I came across this thread. I am not much aquainted with CO2/NH3 refrigeration and am keen to know more about it.

In one of your posts you had referred the schematic drawing of the system. Unfortunately it appears that site has been removed. Will it be possible to to give some link where I can get the schematic drawing of the system.

Thank you and with best wishes,

[TXiceman]

It is simply a cascade system with ammonia on the high temp system and CO2 in the low temp system. The ammonia evaporator acts as the CO2 condenser for the low temp system.

Ken

[Samarjit Sen]

What is the lowest temperature one can achieve with CO2. Are their any literatures on these systems. What I understand that onle CO2 can be used as a refrigerant for a single stage compressor. Could you please provide me with some information on this.

[US Iceman]

CO2 for phase change is limited to about -67F (-55C) if I remembered the triple point correctly.

[Josip]

Hi, Samarjit Sen

What is the lowest temperature one can achieve with CO2. Are their any literatures on these systems. What I understand that onle CO2 can be used as a refrigerant for a single stage compressor. Could you please provide me with some information on this.

Can you explain this, please?

The Sabroe CO2/Ammonia Freeze Package (CAFP) concept combines the advantages of CO2 on the low-temperature side and ammonia on the high-temperature side.

A CO2 compressor has a capacity 8–12 times greater than a similar ammonia unit, making the low-temperature compressor much smaller, and the whole unit significantly more compact.

CO2/Ammonia Freeze Package (CAFP) equipment is available with capacities of 80–400 kW at -50°C or 140–650 kW at -40°C.

Please follow this link:

http://www.sabroe.com/fileadmin/file...CAFP_06.03.pdf
http://docnav.grasso-global.com/DocN..._gbr_10-04.pdf

Best regards, Josip

[Samarjit Sen]

Thank you Josip,

You see I have worked on ***** and R 404A refrigerant till now. With ammonia we have installed units for Potato Cold Storages. Now with alternative refrigerants coming in and the latest being CO2, I would like to know more about it. As far as I was made to understand that CO2 are used only for single stage compressors, but now I understand that with NH3 it could used for two stage so as to lower the temperature further. You have clarified lot of things and I have learnt a lot from you all.

With best wishes,

[Josip]

Hi,

As far as I was made to understand that CO2 are used only for single stage compressors, but now I understand that with NH3 it could used for two stage so as to lower the temperature further.

It is still a single stage compressor (for each system NH3 and for CO2) but it is two stage plant. With CO2 is not possible to use, for example: one single compressor having two stages as we can use for *****s or for ammonia.

We can obtain those temperatures also with ammonia but it is not economical at all. I have here in my country one system with 3 stage ammonia sistem (built before 20+ years) -60/-35/-10/+35 but they stop to use -60 compressors. You spend so much energy for very small work.

Best regards, Josip

[Andy P]

With CO2 is not possible to use, for example: one single compressor having two stages as we can use for *****s or for ammonia.

Hi Josip,

Why not - it's just a compressor? The CO2 compressors developed by J&E Hall in the 1880s which dominated the marine market for about 30 years were two stage. "The first CO2 machine made by J&E Hall was a two-stage compressor in which the first stage of compression was effected in the orthodox way by the action of a piston in a horizontal cylinder, and the second stage completed in a vertical cylinder containing a column of glycerine....A machine of this type was installed on 16 August 1889 in a frozen meat store in Smithfield and set to work. At 2am the pipe conveying the gas from the first to the second stage compressor burst because of a mechanical fault. 'The pipe was literally blown to ribbons' said Everard Hesketh [company chairman and designer of the compressor] 'and had it not been for the sides of the water tank surrounding the compressor...my head which was only a yard away would have been the target for some of the pieces.'"

Fortunately compressors have improved since then!!

Cheers
Andy P

PS Quotation is from "Halls of Dartford 1785-1985" by Harry Miller

[Andy]

Hi Josip,

Why not - it's just a compressor? The CO2 compressors developed by J&E Hall in the 1880s which dominated the marine market for about 30 years were two stage. "The first CO2 machine made by J&E Hall was a two-stage compressor in which the first stage of compression was effected in the orthodox way by the action of a piston in a horizontal cylinder, and the second stage completed in a vertical cylinder containing a column of glycerine....A machine of this type was installed on 16 August 1889 in a frozen meat store in Smithfield and set to work. At 2am the pipe conveying the gas from the first to the second stage compressor burst because of a mechanical fault. 'The pipe was literally blown to ribbons' said Everard Hesketh [company chairman and designer of the compressor] 'and had it not been for the sides of the water tank surrounding the compressor...my head which was only a yard away would have been the target for some of the pieces.'"

Fortunately compressors have improved since then!!

Cheers
Andy P

PS Quotation is from "Halls of Dartford 1785-1985" by Harry Miller

Hi AndyP

Have you tried, or to that matter has anyone built a compressor with an expander on it for transcritical operation ??

Kind Regards Andy

[Andy P]

Have you tried, or to that matter has anyone built a compressor with an expander on it for transcritical operation ??

The expander developed at Dresden University compresses at one end by expanding the gascooler outlet at the other. This device would sit "on top of" a lower stage compressor driven by a motor because the power recovered from full expansion is only enough to drive a portion of the compression path, due to losses in the expander and the second stage compressor. Its a neat device, and can give useful heat recovery too.

I've never tried it - they are a bit small for our stuff

cheers
Andy P

[Josip]

Hi, Andy P

Hi Josip,

Why not - it's just a compressor? The CO2 compressors developed by J&E Hall in the 1880s which dominated the marine market for about 30 years were two stage. "The first CO2 machine made by J&E Hall was a two-stage compressor in which the first stage of compression was effected in the orthodox way by the action of a piston in a horizontal cylinder, and the second stage completed in a vertical cylinder containing a column of glycerine....A machine of this type was installed on 16 August 1889 in a frozen meat store in Smithfield and set to work. At 2am the pipe conveying the gas from the first to the second stage compressor burst because of a mechanical fault. 'The pipe was literally blown to ribbons' said Everard Hesketh [company chairman and designer of the compressor] 'and had it not been for the sides of the water tank surrounding the compressor...my head which was only a yard away would have been the target for some of the pieces.'"

Fortunately compressors have improved since then!!

Cheers
Andy P

PS Quotation is from "Halls of Dartford 1785-1985" by Harry Miller

I think answer to your question is in your quotation. It is not impossible but then you are coming into high pressure area - what is dangerous, then big power consumption and so on...but as Andy said, why not to try


Best regards, Josip

[Samarjit Sen]

Dear US Iceman,

I was not aware of this. I thought that others may read it also. Anyway I am sorry for my mistake.

With best wishes,

[US Iceman]

We try to maintain an unwritten rule for single posts only to prevent accumulation of multiple posts in different areas.

I do thank you for posting the link to the article, it was useful and I think worthwhile for others to review.

Best Regards,
US Iceman

[nh3wizard]

I have some basic flow charts for CO2/Nh3 applications, but due to them being larger than this site will accept I can not post them, if you are interested let me know and Ill email them to you.

[Buckiesr]

I would be interested in the drawings for the cascade system. Please send them to me. I do not know how to email you directly. Can you find me?

[Samarjit Sen]

Dear nh3wizard,

Could you please mail the flow charts for CO2/NH3 applications to me at samarjitsen@rediffmail.com .

Thank you,

[Samarjit Sen]

Since I am not much aware of the CO2/NH3 system and I am yet to come accross one which has been and in operation in our country, I would like to ask a (maybe a stupid) question. But as I have said earlier, that I have no experience in these systems as I have been handling mostly halocarbons.

My question is that could the CO2 in the lower stage be used as a DX system with suitable TEV and Air Cooling Unit for maintaining the desired temperature in the rooms or we have to chill the brine solution and circulate the same through Coils?

From the Forum it appears that a lot of work in this field are being done in Europe and USA.

With best wishes.

[US Iceman]

My question is that could the CO2 in the lower stage be used as a DX system with suitable TEV and Air Cooling Unit for maintaining the desired temperature in the rooms or we have to chill the brine solution and circulate the same through Coils?

Hi, you need to be careful with the application of CO2 due to it's inherently higher pressures. I can't think of reason why you could not use CO2 for DX, flooded, or liquid overfeed. It's just another refrigerant.

However, like any other refrigerant you can only apply it within the guidelines of safety and equipment limitations.

If you tried to use CO2 at normal refrigeration or AC conditions of say, -30C to +4.4C you would expect to see very high pressures. These higher pressures have to be contained by the components and piping.

On the other hand, if you were to use CO2 as we normally use common refrigerants you would find very low temperatures until you get to the triple point of CO2, which is about -67F (-55C).

Using air or liquid depends on the cooling process. In either case, air or liquid/brine are secondary refrigerants. The primary refrigerant would be CO2.

[Samarjit Sen]

Thank you US Iceman. We propose to use CO2 in the lower stage and NH3 in the higher stage. The CO2 temperatures required are to be -45 Deg. C Te and -5 Deg. C Tc. There shall be three rooms. One to be maintained at - 40 Deg. C, and the other two at - 30 Deg. C. I would prefer to use a DX system as I am more aquainted with this . If you advice flooded we can also go for it.

Further you and Andy has been repeatedly emphasising on the maintaing the safety norms and controlling the pressure and temperatures. Could you please elaborate on this point, and as to how does one control the temperature of CO2 at the initial stage of commissioning and also at the idle stage.

With best wishes and thanks a lot for the guidance.