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antonio nunes
16-01-2011, 10:29 PM
Hi everybody..I wonder if I could find some assistance concerning capacity down-regulating on a screw-compressor..the issue is that I assume that current limiting is shortning itīs efficiency. My problem is calculating how much..thanks in advance
Best regards Antonio( marine engineer trainee)

lana
17-01-2011, 06:49 AM
Hi Antonio,

When screw compressors unload, i.e. capacity reduction, then their power consumption is higher than recips. This means they are less efficient than recips at part load.
If you are looking for exact numbers then you have to contact the manufacturers but I doubt that they give you that graph. As far as I have looked there is no data about this in their published literature.

Cheers
LANA

antonio nunes
17-01-2011, 11:50 PM
Thanks lana..u r right..Iīve tried 2 get hold of some sort of documentation/graphs, but they r not very keen on sharing that type of information. Iīm stucked, and since Iīm writting on this issue in order to finish my studies, Iīm starting to worry.

Magoo
18-01-2011, 12:04 AM
Screws operate most efficiently at 100% /fully loaded.
Use a variable speed drive to alter compressor speed to maintain efficiency. Minimum hertz around 25

Magoo
18-01-2011, 12:18 AM
The Danfoss HVAC series VSD's are excellent for the application.
Start compressor fully unloaded at 25 Hz , then step load the compressor to 100%/ fully loaded, then step up speed to max Hz, capacity control by speed control, current protection etc., protects motor.

antonio nunes
18-01-2011, 12:22 AM
;-)..I wish I could suggest/base my thesis on that solution, sir..Itīs far too expensive. My idea is to change inlet/outlet ports and thereby decrease load capacity since I assume that the whole system is oversized..but to do that I must document my assumptions and attest that running on/off in current limiting mode is affecting its efficency..see what I mean?...best regards and thank u for relating to my confused state of mind.

Magoo
18-01-2011, 12:49 AM
Base your project thesis on cost effectiveness. As in one up capital cost for equipment and subsequent down track savings. With the system basis I described and using HEM's motor. the cost savings/ efficiency is on -going, accountants love cost savings. They bark at capital investment costs, but pay back periods will sell concept. A VSD drive is in general terms cheaper than say a Star delts / DOL / soft starter type starter system. Look at the big picture and don't be side tracked by the initial numbers/ $, take advantage of technowledgy devlopments

Segei
18-01-2011, 03:47 AM
You can get a lot of information(graphs) on Industrial Refrigeration Consortium website http://www.irc.wisc.edu/

antonio nunes
18-01-2011, 12:32 PM
..well, you both made me reconsider the project main outlines..I will adresse the company and ask about their pay-back period for tech. investments..their ferryboats are running on R22 as well, so a retrofit it will be necessary at some point. I want you to know that all the help I can get here is very valuable to me, since that area of thermodynamics is a sort of negleted within marine engineering studies around here..AC is not common in Scandinavia. Best regards

Josip
28-01-2011, 12:27 AM
Hi, Antonio Nunes :)


;-)..I wish I could suggest/base my thesis on that solution, sir..Itīs far too expensive. My idea is to change inlet/outlet ports and thereby decrease load capacity since I assume that the whole system is oversized..but to do that I must document my assumptions and attest that running on/off in current limiting mode is affecting its efficency..see what I mean?...best regards and thank u for relating to my confused state of mind.

Change inlet/outlet ports!?! no need try to close suction valve and see what will happen - then, what about screws still with the same suction volume .... you are running into vacuum...

Current limiting mode has nothing with efficiency - it is only protection for your power source - there is not enough power for more current or protection to not overcome discharge pressure to open safety discharge valve and to lose refrigerant .....

If compressor is running on/off that means it has enough capacity ... too much ;)

Each screw compressor has a low efficiency running below 90% of capacity - means bad design - it will be better for you to have two smaller units running in parallel - one in winter time and both in summer time;)

If we are speaking about VFD regulation for STAL screw S71-26E (assuming you have this unit-read about it in your's another post) I think it will be very expensive, not impossible, but almost :eek: ... why...

... maybe due to the big electrical motor, not sure, but probably 355 kW - VFD will be very expensive and must be compatible with motor ;)

... maybe due to direct coupled oil pump ... I do not know what is its minimum RPM - now is running at 2000 RPM to give a proper oil pressure, and maybe when you decrease its RPM you will lost oil pressure.. thus you must install external oil pump ... additional costs and leaks...

... maybe due to old compressor, very expensive spare parts ....and sometimes even not available...

Seems it is oversized for your application and area, but what you have you have ... leave it to cycle on/off until it will last ... for me it is much better to go on/off then to run endlessly at 10-20-30%

Still you have good and reliable compressor:) take care about that baby. A lot of them still are running happily in my country. Not that much efficient with Stalectronic 300 regulation, but doing their duty proudly for decades.

Hope this will help regarding your confusion ;)

Best regards, Josip :)

shooter
31-01-2011, 12:40 PM
a screw compressor has often a capacity slide inside, however it is not very efficient, better is 100% run and a VFD in it.
if valve is on minimum say 10% for mycom the capacity is about 25% and power is about 50%.

shooter
31-01-2011, 12:41 PM
with mycom it is 10% position
capacity is 25%
power is 50%.

sterl
10-02-2011, 09:07 PM
Look up Manske and Reindl. Manske's PHD thesis is on the web deals with this topic specifically....

Performance curves for a specific older compressor may be tough to source but this was the driving force for variable-Vi machines and those at least 15-years developed...And every manufacturer eventually went there. In doing so they inherently publiished how "bad" the fixed-Vi machines were....

There is also a question of operating conditions (as there always is) and the nominal Vi of a given machine and HOW THE SLIDE VALVE POSITION effects actual Machine Vi: This a characteristic of the machine's discharge port and rotor proportions: A long machine with small diameter rotors can be managed more effciently by a slide valve than a short machine with Big Diameter rotors...But for a generic set of curves and otherwise an effective treatment see Link below...



http://www.fjb.co.uk/Downloads/large%20screw%20compressors.pdf