Issues with screw [comparison with recip]

Hey RE members
I read many of the posts related to screw chillers.
I am maintaining a bitzer screw chiller and having issues with the calculation of its overall efficiency.
I want to compare it with my existing recip with the same operating conditions and capacity.
For part load i found recips serve better.
I want to change over to screw but am unable to calc reduction in power consumption for screw.
Capa: 175TOR
SST: 5 dC
SDT: 40dC
ref: NH3
Sys: vapor comp. PLC based auto sys with VFD on compressor motor for part load application & to maintain constant suction P.The slide valve mech is operated via the PLC provided.Energy mtr and flow mtr installed for capacity calc.

Also a second problem is of the oil carry forward which gets accumulated in the evap.This particularly happens at low load conditions.
Any suggestions for some revision in the system?Sys uses triple effect coalescer type seperator.

I am expecting a reply from senior folks USIceman,TXIceman,Josip..etc..
please help.

somebody listening der??Plz reply..i am in real trouble guys..

You need engineering data(capacity and power at certain operating conditions) for existing and new compressors. This information you can get from compressor manufacturers.

Hey segei thanx,
I tried getting them from Bitzer,somehow found that they do not have a chart or any reference for their compressor which can give me the COP wrt RPM.
Remember i am talkin about part loading.I read online that for part loading screws go inefficient wrt to recips.
I jus m searching a way to prove it:D

First. If manufacturer doesn't have information about their compressor, nobody else has it.
Second. Reason of part load inefficiency of screw compressors is slide valve. If slide valve unload, efficiency of the compressor will be reduced, especially at capacity below 50%. However, if you adjust capacity by VFD efficiency will not suffer.

Hi, vihang :)

Quote:

---

Originally Posted by vihang

Hey segei thanx,
I tried getting them from Bitzer,somehow found that they do not have a chart or any reference for their compressor which can give me the COP wrt RPM.
Remember i am talkin about part loading.I read online that for part loading screws go inefficient wrt to recips.
I jus m searching a way to prove it:D

---

be patient, please, here we are volunteers, in real life we are doing some other job/s for our daily bread and sometimes we are very occupied having no time to visit RE forums, hope you do understood that ;)


Back to your problem ...

it is true ... at part load COP of screw compressor is smaller then reciprocating compressor no need to prove it ... screws (especially with economizer) are better when running at 90-100% most of the running time ....

unfortunately, to get some diagrams with part load (consumption vs. capacity) that is available only with software for computation and balancing of components and plants. Software like that is available only to representative of manufacturers like Sabroe, Mycom, Frick, Howden etc.

One way for you is ... if you want, definitely, to use screws ... to use VFD (Variable Frequency Drive) with automatic Vi .... or reciprocating compressor/s with VFD ...

Another way is to use both screw/s and reciprocating compressor/s, where your screw/s will run, mostly, at 100% and capacity change will be done by reciprocating compressor/s ... assuming you have some constant load to keep screw compressor running at 100% ....

and the best in case if full load is relatively small and load changes are very often (when you have a lot of small rooms) is to use reciprocating compressors ... of course same type and same size .... then you have interchangeable parts what can be important sometimes ...

All above is my opinion, final decision must be yours, because you know your system the best.


Best regards, Josip :)

please see the screw vs reciprocating pdf file from york.

Moideen-Dubai

Quote:

---

Originally Posted by Segei

First. If manufacturer doesn't have information about their compressor, nobody else has it.
Second. Reason of part load inefficiency of screw compressors is slide valve. If slide valve unload, efficiency of the compressor will be reduced, especially at capacity below 50%. However, if you adjust capacity by VFD efficiency will not suffer.

---

Thanks for the reply..
I have read many of documents and studies from sabroe which makes both these types of compressor but they do not give a parameter or equation (from ARI) to calc how much am i losing on eff..Also this was the reason i have used VFD in close loop to control my chilld water out temperature.
Everybody is talking about IPLV at sabroe and nobody really wants to explain how to calc it.
moideen has probably attached the same sheet below which gives an intro of this IPLV.

and as i said i have used VFD but slide valve is controlled by PLC which means once i get to the lowest possible speed on screw with VFD n if the load is still reducing, i have to use the slide valve mechanism.But generally i try to run the comp at full load and take the capacity variation on VFD.

But my quest is HOW MUCH AM I LOSING OR GAINING by doing so??

Also i would request you to try to resolve my of oil carry fwd.Its a headache to drain the oil from the serge drum almost evry 3-4 days.Any suggestions for the sys changes??
Thanks

Quote:

---

Originally Posted by moideen

please see the screw vs reciprocating pdf file from york.

Moideen-Dubai

---

Thanks for the reply

This was the document which made me use a VFD on my compressor:)

If you have any idea about my second problem,please do post to resolve it.Its really tiring to drain a whole lot of about 50 ltrs of oil from the evaporator bottom even after using triple coalscer type seperator:(

Quote:

---

Originally Posted by Josip

Hi, vihang :)



be patient, please, here we are volunteers, in real life we are doing some other job/s for our daily bread and sometimes we are very occupied having no time to visit RE forums, hope you do understood that ;)


Back to your problem ...

it is true ... at part load COP of screw compressor is smaller then reciprocating compressor no need to prove it ... screws (especially with economizer) are better when running at 90-100% most of the running time ....

unfortunately, to get some diagrams with part load (consumption vs. capacity) that is available only with software for computation and balancing of components and plants. Software like that is available only to representative of manufacturers like Sabroe, Mycom, Frick, Howden etc.

One way for you is ... if you want, definitely, to use screws ... to use VFD (Variable Frequency Drive) with automatic Vi .... or reciprocating compressor/s with VFD ...

Another way is to use both screw/s and reciprocating compressor/s, where your screw/s will run, mostly, at 100% and capacity change will be done by reciprocating compressor/s ... assuming you have some constant load to keep screw compressor running at 100% ....

and the best in case if full load is relatively small and load changes are very often (when you have a lot of small rooms) is to use reciprocating compressors ... of course same type and same size .... then you have interchangeable parts what can be important sometimes ...

All above is my opinion, final decision must be yours, because you know your system the best.


Best regards, Josip :)

---

Hey Josip
was waiting for a reply from you;)

Firstly i will be obliged if you cn make me understand whats VFD with automatic Vi.
I have used the VFD in close loop with the chilld water out let Temp.(So as to control the outlet T and changing capacities.)

Also it would not be feasible to use both screw and recip on a single skid.The space requirement would be too high for transportation.Also inventory will increase for dif types of comp.

What i think from your suggestion is that it would be better to go for two screws where in one is controlled by VFD and second one for the lowest possible constant load.Hows that??:)

Josip i would be glad if you can also address the second issue of oil carry fwd.

Thanks for the reply.Would be patient from now on:D

Hi, vihang :)

Quote:

---

Originally Posted by vihang

Hey Josip
was waiting for a reply from you;)

Firstly i will be obliged if you cn make me understand whats VFD with automatic Vi.
I have used the VFD in close loop with the chilld water out let Temp.(So as to control the outlet T and changing capacities.)

Also it would not be feasible to use both screw and recip on a single skid.The space requirement would be too high for transportation.Also inventory will increase for dif types of comp.

What i think from your suggestion is that it would be better to go for two screws where in one is controlled by VFD and second one for the lowest possible constant load.Hows that??:)

Josip i would be glad if you can also address the second issue of oil carry fwd.

Thanks for the reply.Would be patient from now on:D

---

one discussion almost the same as yours is here too http://www.refrigeration-engineer.co...etric+capacity

a lot pro and contra;)


Automatic Vi (Internal Volumetric ratio) is helping a lot when we have compressor control capable to calculate Vi slide position (like UNISAB III) according to actual suction and condensing pressure thus move Vi slide in that position. Anyhow even with manual Vi it is possible to adjust Vi for different working conditions i.e. summer/winter. I do not know in which part of India you are, but at least you have dry/wet season thus some changes in outside temperature i.e. condensing pressure.

Two screws, one standard and another with VFD sounds OK, brand and type of compressors is not that important. More important is compressor controller like UNISAB III, with Frick or Mycom controllers I do not have experience and cannot suggest them.

Beside a lot of theory (leave it for further discussion/s) regarding oil carry-over ..... 50 ltrs of oil is tooooo much and primary pointing to .... coalescing filter is damaged or .... gasket- O-ring is/are damaged or not installed at all or complete filter and O-rings are not installed properly ... if you have the same problem since beginning i.e. with new oil separator then there can be some issue with internal parts of oil separator, with all my respect, please, first make a mechanical check of all above parts ... another possibility is oil recovery line is blocked (if installed)

you did not describe your unit neither mentioned any other discrepancy, if .... and here we have to guess what you have or not ... we are engineers and we are using drawings .... maybe is possible to upload schematic drawing of your plant to make our life more "sweet" :cool:




This is from here.... copy/paste...
http://www.engineeringtoolbox.com/co...ncy-d_410.html
IPLV - Integrated Part-Load Value

The term IPLV is used to signify the cooling efficiency related to a typical (hypothetical) season rather than a single rated condition. The IPLV is calculated by determining the weighted average efficiency at part-load capacities specified by an accepted standard. It is also important to note that IPLVs are typically calculated using the same condensing temperature for each part-load condition and IPLVs do not include cycling or load/unload losses. The units of IPLV are not consistent in the literature; therefore, it is important to confirm which units are implied when the term IPLV is used. ASHRAE Standard 90.1 (using ARI reference standards) uses the term IPLV to report seasonal cooling efficiencies for both seasonal COPs (unitless) and seasonal EERs (Btu/Wh), depending on the equipment capacity category; and most chillers manufacturers report seasonal efficiencies for large chillers as IPLV using units of kW/ton. Depending on how a cooling system loads and unloads (or cycles), the IPLV can be between 5 and 50% higher than the EER at the standard rated condition.

IPLV can be expressed as:
IPLV = 1 / (0.01 / A + 0.42 / B + 0.45 / C + 0.12 / D)
where
A = kW/ton at 100%
B = kW/ton at 75%
C = kW/ton at 50%

D = kW/ton at 25%

With all my respect to the people coming out with let's say exact formulas, but using hypothetical season I will never spend a time for above calculation ... it is better to spent time to study how to make a better refrigeration plant ...

Best regards, Josip :)

Josip..
i am so grateful...thank you
Superb post...hope it helps finding the fault with the oil seperator.

But i do not understand this fact...somehow if we go by the volumetric flow of the compressor, the oil carry over is not more than 1ppm(mg/lit)..here i take m3/hr of comp..convert it to lit/hr...and take 1 hr as the basis...am i doing some very wrong calculation?

Also please specify what other details you need about my unit..i gave the details in the first post..except that it has an economizer and that it is a flooded sys

Hi, vihang :)

Quote:

---

Originally Posted by vihang

Josip..
i am so grateful...thank you
Superb post...hope it helps finding the fault with the oil seperator.

But i do not understand this fact...somehow if we go by the volumetric flow of the compressor, the oil carry over is not more than 1ppm(mg/lit)..here i take m3/hr of comp..convert it to lit/hr...and take 1 hr as the basis...am i doing some very wrong calculation?

Also please specify what other details you need about my unit..i gave the details in the first post..except that it has an economizer and that it is a flooded sys

---

..... unfortunately, the only fact you have is lost of 50 ltrs of oil every 3-4 days .... let's go step by step and for the moment leave calculation aside

... your calculation is probably right, but for some other reason you are loosing oil and we are trying to find out what is that, agree?


so, my question is did you check what I suggest you in previous post ....?


also, details about your unit are fine, but schematic drawing of entire unit would tell us more ;)

Best regards, Josip :)

Mr vihang.
reading your interesting posts etc.,
as I read it you have a bitzer open drive screw/ or screws on ammonia, open flash economized and applied as a water chiller, with variable speed control. Plus you have oil carry over problems, with design +5 SST and +40 SDT.
Who was the design and application Engineer, is the compressor fully loaded under varaible speed control, what form of oil cooling is applied, is there economizer evap temp/pressure control, what sub cooling is acheived with economizer. Who wrote the PLC software control system. What is the minimum hertz control setting on VSD. What is the supply oil temp to compressor.
A system schematic would be helpful to interprut system application.
lots of questions I now, we all need a clearer picture of your problem.

regards Magoo.

Quote:

---

Originally Posted by Josip

Hi, vihang :)






..... unfortunately, the only fact you have is lost of 50 ltrs of oil every 3-4 days .... let's go step by step and for the moment leave calculation aside

... your calculation is probably right, but for some other reason you are loosing oil and we are trying to find out what is that, agree?


so, my question is did you check what I suggest you in previous post ....?


also, details about your unit are fine, but schematic drawing of entire unit would tell us more ;)

Best regards, Josip :)

---

Hey Josip...thanks for the guidance..

somehow not a single gasket found broken!!the coalscing filter aint damaged!!Oil return line all clear!!:(

I somehow feel that the coalscing element aint right....It allows more oil to pour out of it...

Btw i chkd the specs of the compressor its OSKA 8591 with 535 m3/Hr handling flow.

Acc. to calcs. with 1 ppm we get 14 ltrs/day of oil carry over..same is the case with me!!
Where is the calc going wrong??:(

i have started a new post for some suggestions on good coalescing media paper in the market.
I am planning to make an oil recovery system to get oil from the evap bottom and send it back to suction.
Hope that works.

Any suggestions?

Quote:

---

Originally Posted by Magoo

Mr vihang.
reading your interesting posts etc.,
as I read it you have a bitzer open drive screw/ or screws on ammonia, open flash economized and applied as a water chiller, with variable speed control. Plus you have oil carry over problems, with design +5 SST and +40 SDT.
Who was the design and application Engineer, is the compressor fully loaded under varaible speed control, what form of oil cooling is applied, is there economizer evap temp/pressure control, what sub cooling is acheived with economizer. Who wrote the PLC software control system. What is the minimum hertz control setting on VSD. What is the supply oil temp to compressor.
A system schematic would be helpful to interprut system application.
lots of questions I now, we all need a clearer picture of your problem.

regards Magoo.

---

Design engineer left the job:(
compressor fully loaded under varaible speed control--yes
what form of oil cooling is applied---elaborate plzz..
is there economizer evap temp/pressure control---- a small PM to maintain intermediate P
minimum RPM control setting on VSD----1500
supply oil temp to compressor----50
Oil---mobil gargoyle artic 300

I almost gave you all answers.

Hope it helps.


>>I AM SORRY GUYS THE PID OF THE CHILLER CANNOT BE GOT.WILL TRY TO DRAW IT ATTACH IN SOME DAYS FOR SURE.<<

This circuit on a Bitzer Oil Separator as well? Which Size?

Bitzer's manual for circuit design (sh-510-1) indicates ammonia circuits should have both a primary and a secondary oil separator. Have you got both? In their R-717 version it would appear that only the secondary separator contains a coalescer....and that for your size machine, parallel secondary separators are required. The primary version does not have access arrangement to remove anything of size, as I read the drawing.

RE: The Gargoyle: I have references that say 54 Cst Viscosity @ 40 deg C. and other sources that say 68 Cst @ 40. The 54 is pretty light stuff for your temperatures and of course that will make it carry over excessively....

Also advise: Where is your oil cooler located with respect to the oil sump, and how high is the nominal oil level with respect to the lower section of the rotor housing? Its possible that the oil cooler drains back into the oil separator on shut down; which will encourage carryover and in the worst cases cause the machine to retain oil on start up. Also there are certain operating conditions where an oil cooler can retain a vapor pocket of refrigerant which will disrupt oil injection when it finally starts to flow to the machine if the orientations of separator and piping are not correct....

Any evidences of water in your refrigerant? When you drain oil from the low side does it show water? How do you effect purging of non-condensables?

Bitzer also have performance data for a whole plethora of variables including per-cent speed operation on their sizing/ performance/ operating envelope program, available at the Bitzer web site. You can even download the constants and the polynomials on which the machine data is based....If you want to create an entire COP envelope.

http://www.bitzer.de/eng/productservice/software/3

Quote:

---

Originally Posted by sterl

This circuit on a Bitzer Oil Separator as well? Which Size?

Bitzer's manual for circuit design (sh-510-1) indicates ammonia circuits should have both a primary and a secondary oil separator. Have you got both? In their R-717 version it would appear that only the secondary separator contains a coalescer....and that for your size machine, parallel secondary separators are required. The primary version does not have access arrangement to remove anything of size, as I read the drawing.

RE: The Gargoyle: I have references that say 54 Cst Viscosity @ 40 deg C. and other sources that say 68 Cst @ 40. The 54 is pretty light stuff for your temperatures and of course that will make it carry over excessively....

Also advise: Where is your oil cooler located with respect to the oil sump, and how high is the nominal oil level with respect to the lower section of the rotor housing? Its possible that the oil cooler drains back into the oil separator on shut down; which will encourage carryover and in the worst cases cause the machine to retain oil on start up. Also there are certain operating conditions where an oil cooler can retain a vapor pocket of refrigerant which will disrupt oil injection when it finally starts to flow to the machine if the orientations of separator and piping are not correct....

Any evidences of water in your refrigerant? When you drain oil from the low side does it show water? How do you effect purging of non-condensables?

Bitzer also have performance data for a whole plethora of variables including per-cent speed operation on their sizing/ performance/ operating envelope program, available at the Bitzer web site. You can even download the constants and the polynomials on which the machine data is based....If you want to create an entire COP envelope.

http://www.bitzer.de/eng/productservice/software/3

---

Hey Sterl
Thanks.
It uses triple oil seperator (Horizontally aligned): velocity drop, demister pad..lastly Coalscer...and somehow note that acc to calcs the oil carry over is not more than 1 ppm.But are these calcs okay??

It uses 68 Gargoyle,no doubts.

This has a phe oil cooler whose inlet is located quite level of outlet of sump.So no probable draining of oil if m not wrong:)
The oil rec is located about 1 metre below the rotor casing, so does that level matters?

Also the oil injection in the compressor is not the problem because it has so mny interlocks to trip if oil doesnt reach the compressor.

Water in the refrigerant is evident upto some extent..but not always...also its too less in quantity..

Thanks for the guidance...but somehow i dint get the last paragraph...Mind if you can please make me undstand!?:)

Hello vihang.
Sterl made a good point about oil used in system particually with the design SDT of +40'c. The discharge temp from compressor and the oil temp would possibly be too high for Gargoyle 300, would be quite stable at 70> 75 'C discharge temp., can you check the actual temperature of discharge line from compressor to separator.
Generally 300 is used with low temp applications where there is some cooling effect from satuarted suction vapour.
May be a simple case of selecting a different grade oil for system

The pt100 shows me 90 dC at max at d discharge.

Hi, vihang :)

Quote:

---

Originally Posted by Magoo

A system schematic would be helpful to interprut system application.
lots of questions I now, we all need a clearer picture of your problem.

regards Magoo.

---

Seems we are at beginning of your problems. This is the most important for all of us and for you too .... to present all possible data with a kind of history, all alternation, replaced parts original or generic .... when problems started, developing of you problems, your counteraction/s ... etc to be able to help, otherwise never ending story;)

Quote:

---

Originally Posted by vihang

Hey Josip...thanks for the guidance..

somehow not a single gasket found broken!!the coalscing filter aint damaged!!Oil return line all clear!!:(

I somehow feel that the coalscing element aint right....It allows more oil to pour out of it...

i have started a new post for some suggestions on good coalescing media paper in the market.
I am planning to make an oil recovery system to get oil from the evap bottom and send it back to suction.
Hope that works.

Any suggestions?

---

It was not needed to start a new thread looking for good coalescing media paper .... just use the original element (if proved to be good) ... yes I know that one is more expensive (is it really?;)) and you try to save some money and install generic (unfortunately not good) coalescer element .... original or good replecement coalescer elements are not cheap at all, but good one (usually original) is worth each coin;) otherwise it is easy to got a headache with extra expenses much bigger then the cost of original element ...

Quote:

---

Originally Posted by vihang

Design engineer left the job:(
compressor fully loaded under varaible speed control--yes
what form of oil cooling is applied---elaborate plzz..
is there economizer evap temp/pressure control---- a small PM to maintain intermediate P
minimum RPM control setting on VSD----1500
supply oil temp to compressor----50
Oil---mobil gargoyle artic 300

I almost gave you all answers.

Hope it helps.

>>I AM SORRY GUYS THE PID OF THE CHILLER CANNOT BE GOT.WILL TRY TO DRAW IT ATTACH IN SOME DAYS FOR SURE.<<

---

Very traumatic period for him and he choose to retreat (normal act), leaving hot potatoes to you, ... seems, he was good to choose equipment according to available investment and required capacity, but he missed to follow some good engineering practice regarding oil recovery system and bigger condensers to achieve lower condensing pressure ..... old story about money savings, sometimes, with very expensive consequences ....

Quote:

---

Originally Posted by sterl

Bitzer's manual for circuit design (sh-510-1) indicates ammonia circuits should have both a primary and a secondary oil separator. Have you got both? In their R-717 version it would appear that only the secondary separator contains a coalescer....and that for your size machine, parallel secondary separators are required. The primary version does not have access arrangement to remove anything of size, as I read the drawing.


http://www.bitzer.de/eng/productservice/software/3

---

This is another good point given by Sterl ... the secondary oil separator ... I checked this page too and found Bitzer is using a primary and secondary oil separator for ammonia systems ... see this
http://www.bitzer.de/eng/productservice/p2/21 .... his link is for Bitzer softwre whic you can download and use as per your needs ...

Quote:

---

Originally Posted by vihang

It uses triple oil seperator (Horizontally aligned): velocity drop, demister pad..lastly Coalscer...and somehow note that acc to calcs

---

According to your statement, you have not installed an original Bitzer vertical oil separator, but some other horizontal type - some info about!?! ... can you tell us about oil carry over when you start up your plant when was new .... all the same from very beginning, or ...!?!



Magoo's last suggestion is maybe the best one for you (to use some other grade of oil), but anyhow you must solve the problem with coalescer element/s ... before any other action ... like installation of oil recovery system at evaporator .... I think it is better to install oil recovery system at the condenser outlet ... no need a pump to return the oil back to compressor suction ...

But, go step by step .... first try to provide and install original coalescer elements (if they were OK or try to find a good replacement, with all my respect don't manufacture them locally), maybe to lover condensing temp, if ....

Hope this is of some help to you. I believe, some other members will contribute with some other good suggestions, too to help you.


Best regards, Josip :)

Hi.
Josip's recommending Magoo's suggestion of changing the oil is spot on.
We a few years back had real issues with oil carry over.
Which we eventually cured by changing from PAO to AP grade oil.
Initially we did add another in-line coalesor but this was only on one comp out of three.

The other 2 improved with just a oil change.
Grizzly