refrigerant physical properties versus screwcompr. efficiency

[antonio nunes]

Hej everybody
..I´m still dealing with my research on how a retrofit/refit from R22 over to R407A or (perhaps R134a) going to improve (or not) a Stal S71 screwcompressor efficiency.

In that context I would appreciate a qualified opinion concerning how refrigerant termal molecular weight , heat capacity , thermal conductivity and density will affect compressor work.

Since the compressor runs 2/3 of time in part load, I assume that a "heavier" refrigerant would force the compressor perform better, in terms of volumetric efficiency for instance..
In advance thanks a lot.

[nike123]

Are you sure in that R407A choice. Is it maybe R407C?

[antonio nunes]

..you´re right,Nike 123..it´s R407C..I had right in my notes but somehow I ended up by writting an A instead.
My assumption is, in rought terms, that a less effective refrigerant would force the compressor to internal by-pass less gas and as such lift up its efficiency-------meaning: more gas to remove the same amount of heat = higher compressor performance.
That explains my interest in the physical properties of the above mentioned refrigerants..whatever my assumption is right or not, I´m due to undrestand and explain my analysis.
Best regards AA

[Josip]

Hi, antonio nunes :)


..you´re right,Nike 123..it´s R407C..I had right in my notes but somehow I ended up by writting an A instead.
My assumption is, in rought terms, that a less effective refrigerant would force the compressor to internal by-pass less gas and as such lift up its efficiency-------meaning: more gas to remove the same amount of heat = higher compressor performance.
That explains my interest in the physical properties of the above mentioned refrigerants..whatever my assumption is right or not, I´m due to undrestand and explain my analysis.
Best regards AA

... more gas to remove the same amount of heat = higher compressor expenses ..

why .... within screw compressor's lobes we have a fixed volume for gas transportation (in partial load this volume is even smaller, but the masses we have to rotate are still the same) ... thus if you need to transport more gas to remove the same amount of heat your compressor must run longer .... spending more energy thus having only higher energy bills ....

Best regards, Josip :)

[antonio nunes]

..thank u for feedback,Josip---since the machine is running intermitently on-off (approx. every 15 min) and the Stalectronic control display shows that is performing 30% of its capacity, I suspect that it is undercompressing..that 70% of the sucked in gas is by-passing via internal slide port---my aim is to calculate/document this by means of volumetrik efficiency and "wrong" compression ratio......the way it´s running is pulling down lots of electric power anyway, wich made me think that a bigger load will give less el-consumption...
------it turns out that it´s difficult to find expertice in the matter of older screwcompressors, so any input is precious..
..best regards --------AA

[Josip]

Hi, antonio nunes :)


..thank u for feedback,Josip---since the machine is running intermitently on-off (approx. every 15 min) and the Stalectronic control display shows that is performing 30% of its capacity, I suspect that it is undercompressing..that 70% of the sucked in gas is by-passing via internal slide port---my aim is to calculate/document this by means of volumetrik efficiency and "wrong" compression ratio......the way it´s running is pulling down lots of electric power anyway, wich made me think that a bigger load will give less el-consumption...
------it turns out that it´s difficult to find expertice in the matter of older screwcompressors, so any input is precious..
..best regards --------AA

I'm working with Stal screws since 1982 and I am sure those machines still perform their job in proper way, but if they are installed in a "wrong" plant then we have discrepancies as you have .... very often goes on/off, running most of the time with part load and all of this cause a high power consumption, unfortunately ....

it is true those compressors are with fixed Vi of ... 2.6, 3.4 and 4.2 as I remember, but if you have constant suction and condensing pressure this is not important that much

You have one or more S71 screws installed in your system, what means someone made some calculation and those compressors must be compatible with the rest os the system, but maybe capacity control is not setup properly ...

I assume you have Stalectronic 300. That system I know and maybe I can help you, but I need to know much more about your system like ....

type of plant ... some description of your cooling process .... maybe different for summer/winter time
type of compressor S71-26A, -34A or -42A m (assume 26 or 34)
suction and condensing pressure .... constant or changeable
input signal for capacity control ... temp or pressure or external signal ... auto or manual

settings of switches S5, S6, S7 and S8 at SR unit

... all above are guessing and maybe I am wrong;)

Best regards, Josip :)

[antonio nunes]

Hi once again Josip..and thank you for taking interest on this issues of mine.
In a manner of fact the ship this chilling (AC) unit I´m analysing was built in Split,Croatia 16 years ago- my opinion is it has from the very start been oversized, as you may observe from the following specifications:

unit designation VSM 71 EC (M stands for marine and EC for economiser)*as far as I able to perceive indications from Stalectronic 600 control display--the economiser is only active above 75% capacity.
I spent 3 months on bord (agost,sept,oct 2010) doing my internship and during this periode the eco-circuit never opened=> no flow through economiser port.
Screwcompressor type S71 EC (2.6 volumeratio) direct driven by el-motor(P output 406 kW) 2690 rpm
*I thought of regulating frequency via VLT but I don´t think it´s realistic, since the oil pumpe is mechanical

The plant is designed for indirect cooling-R22 and water as heat transfer medium---it consists of 2 waterchillers with a total refrigerating capacity of 3500 kW, sized for chilling fresh water 525 m3/h from +11 g Celsius to +6 g Celsius. * Here I found a major "conflict" because I calculated/added up all the heat load on board and got over 4000 kW
This refrigerating capacity is stated at 32 g Celsius cooling water temperature *I assume we are speaking of sea water temperature for condensator cooling..and therefore deducte that the system is oversized by design, since the sea waters between Copenhagen and Oslo never reaches that temperature (max 18 g Celsius)

Suction and condensation pressure is varyable...between 2 and 5 bar for evap and 8 and 14 for condensing.

Actually what I intend to do is on the basis of two drift scenarios concerning load,air and sea water conditions, to calculate and compare compressor work/el-consumption when using R134a and R407C--------for those 2 scenarios I took notes of drift parameters on control display..I tried as well to measure temperatures around the unit, using a laser termometer but i´m afraid that´s too inaccurate
The intention was to determine subcooling in heat exchanger in order to plotte it in Coolpack---no sucess in that purpose.
The superheat value statet in the Stalectronic 600 display seems too high (on my humble amateur opinion) 16.6 g Celsius.
I don´t know if you can make sense of my informations at all, Josip..but if you think it helps and you mind looking at it, I can send you the pictures i took in loco from the display and a schematic view of the system where i plotted my temperature readings by Fluke laser termometer
Moreover think someone has been fiddling with the control settings since the current limitation value is stated to 300 A..when the el-motor pulls around 720 A nominal current

best regards...Antonio Afonso

[Josip]

Hi, antonio nunes :)

let's split your post to be able to follow part by part ...



Hi once again Josip..and thank you for taking interest on this issues of mine.
In a manner of fact the ship this chilling (AC) unit I´m analysing was built in Split,Croatia 16 years ago- my opinion is it has from the very start been oversized, as you may observe from the following specifications:

I do not think it has been oversized ... someone made some calculation, ship owner will not buy something what is too much oversized ... to cool down that amount of water (525 m3) you need around 3050 kW ... your compressors with economizers are designed to give around 3500 kW .... maybe you have installed utilities for 4000 kW but not in the same time .... some other load must be lower to cover that difference ...


unit designation VSM 71 EC (M stands for marine and EC for economiser)*as far as I able to perceive indications from Stalectronic 600 control display--the economiser is only active above 75% capacity.
I spent 3 months on bord (agost,sept,oct 2010) doing my internship and during this periode the eco-circuit never opened=> no flow through economiser port.
Screwcompressor type S71 EC (2.6 volumeratio) direct driven by el-motor(P output 406 kW) 2690 rpm
*I thought of regulating frequency via VLT but I don´t think it´s realistic, since the oil pumpe is mechanical

The plant is designed for indirect cooling-R22 and water as heat transfer medium---it consists of 2 waterchillers with a total refrigerating capacity of 3500 kW, sized for chilling fresh water 525 m3/h from +11 g Celsius to +6 g Celsius. * Here I found a major "conflict" because I calculated/added up all the heat load on board and got over 4000 kW
This refrigerating capacity is stated at 32 g Celsius cooling water temperature *I assume we are speaking of sea water temperature for condensator cooling..and therefore deducte that the system is oversized by design, since the sea waters between Copenhagen and Oslo never reaches that temperature (max 18 g Celsius)

I assume design calculation has been done as standard for chillers 0/40C .... thus your ship is capable to cruise in some warmer waters too, that does not mean system is oversized becuse in some places can be undersized;)

You must realize that standard difference for HE calculation is around 7-10K depending on construction of HE .... thus if your water is 18C then your condensing temperature can be as low as 28*C, so if you have a cooling water with lower temperature it is better for you, COP is higher ....


Suction and condensation pressure is varyable...between 2 and 5 bar for evap and 8 and 14 for condensing.

This is something strange ... assume those pressures are gauge pressures ...but anyhow your suction is going toooo low for chiller at 2 bargauge = 3 bar-absolute) evaporation temperature is around -15*C (for direct cooling of water is dangerous - water can be frozen within HE -maybe you have a brine system...:confused:)

... condensing pressure/temperature match design pressure of 14 barg = 15 bara = temperature of 40*C for R22...


Actually what I intend to do is on the basis of two drift scenarios concerning load,air and sea water conditions, to calculate and compare compressor work/el-consumption when using R134a and R407C--------for those 2 scenarios I took notes of drift parameters on control display..I tried as well to measure temperatures around the unit, using a laser termometer but i´m afraid that´s too inaccurate
The intention was to determine subcooling in heat exchanger in order to plotte it in Coolpack---no sucess in that purpose.

You can compare R22, R134a and R407C with CoolPack tool A.8 ...


The superheat value statet in the Stalectronic 600 display seems too high (on my humble amateur opinion) 16.6 g Celsius.
I don´t know if you can make sense of my informations at all, Josip..but if you think it helps and you mind looking at it, I can send you the pictures i took in loco from the display and a schematic view of the system where i plotted my temperature readings by Fluke laser termometer

Unfortunately I am not familiar with setup of Stalectronic 600 ... but something is wrong here too... superheat for R22 must be higher then for ammonia, but not that much and must be show in K (Kelvin) ... maybe 8-10K .... maybe that one is suction temperature shown in *C and we need to measure pressure almost in the same place (if you have gauge or pressure transmitter available there) to calculate suction superheat.. but if it is that high maybe you are low on refrigerant within HE ...


Moreover think someone has been fiddling with the control settings since the current limitation value is stated to 300 A..when the el-motor pulls around 720 A nominal current

best regards...Antonio Afonso

This is also very strange .... one question first ... Do you have possibility to give 720A to run that compressor?

Why I'm asking this .... according to your earlier explanation that your compressor is cycling ON/OFF every 15 minutes .... that is normal .... in logic of Stalectronic we have a start delay usually of 15 minutes (to cool down windings of main motor after start and to be able to restart compressor immediately after start delay went off) ... what means if you start compressor and there is no emergency-alarm stop, it will continue to run next 15 minutes and then will stop ... simultaneously you still have cooling request and compressor will start again and run for next 15 minutes ...

another thing is current limitation ... you have cooling request and compressor is going up with capacity, but very soon coming to 300A (now your rated/nominal current) and then Stalectronic issue a signal to reduce capacity until capacity reach 97% of rated/nominal current.

From one side you have request for cooling and from another side you have limitation and compressor is somewhere-nowhere...

I think your problem is probably current limitation of 300A ... maybe winter setup, but now you need full capacity and available current of 720A or whatever is rated on plate of main motor ... first try to solve that problem ...

it will be nice if you can send me a scheme of your plant ... all possible drawings and data ...

then if you have a copy of Stalectronic 600, please send a copy to me, because I do not have... maybe I can help you with setup ...

Hope this will help a little...

Best regards, Josip :)

[antonio nunes]

Hi Josip..I´ve gathered some material that might shine some light on your acessment of the system. Picture quality is not at the top, but enought to illustrate the situation.
-The current limitation issue is puzzling me as well..it "happened" somewhere in september while I was on leave..when I got back and asked who and why nobody was able to give a full explanatory answer...I allways thought that current limitation set by the unit was ment to be as close to nominal el-motor current as possible (approx 700 A) as an extra insurance towards overload.
-As far as I´ve been informed, there´s no significant difference between Stalectronic 300, 600 and even 700..the most of it lies on the software charts for different refrigerants..and offcourse the protocols are not compatible with modbus communication system for instance, but that´s another issue i´m not getting into.
-So, in your opionion the on/off periodical rate is due the unit manual settings and not the lack of heat load?
-Would you mind specifie HE? (heat exchanger?)

Hereby a compilation of display images + system overview + few info on Stalectronic + coolpack "wonderabouts"


Gratefully & with best regards AA

[Josip]

Hi, antonio nunes :)


Hi Josip..I´ve gathered some material that might shine some light on your acessment of the system. Picture quality is not at the top, but enought to illustrate the situation.
-The current limitation issue is puzzling me as well..it "happened" somewhere in september while I was on leave..when I got back and asked who and why nobody was able to give a full explanatory answer...I allways thought that current limitation set by the unit was ment to be as close to nominal el-motor current as possible (approx 700 A) as an extra insurance towards overload.
-As far as I´ve been informed, there´s no significant difference between Stalectronic 300, 600 and even 700..the most of it lies on the software charts for different refrigerants..and offcourse the protocols are not compatible with modbus communication system for instance, but that´s another issue i´m not getting into.
-So, in your opionion the on/off periodical rate is due the unit manual settings and not the lack of heat load?
-Would you mind specifie HE? (heat exchanger?)

Hereby a compilation of display images + system overview + few info on Stalectronic + coolpack "wonderabouts"


Gratefully & with best regards AA

I cannot be sure about your load .... you must see that by yourself .... logically if you need almost 3500 kW via heat exchangers (assuming) in summer time ... one compressor must run almost all the time with some capacity even in winter time ... otherwise you have no load at all or you have some big issue with your HE ... clogged or again very low with refrigerant ... this is something you must find out ..

... for main motor current you are right, value must be close to nominal motor current ... check the name plate on your motor and setup current limiter to that value ....

Regarding "-Would you mind specifie HE? (heat exchanger?)" I'm not sure what do you mean with this ... explain, please.

I know there is not a big difference between SE 300, 600 or 700, but some specific points can be there (SE 300 I know very good) ... I can help with logical setup then you can play with, but again, you must check that complete unit (plant) is without any restriction - existing cooling request, current available, heat exchange possible, enough refrigerant, water side of HE OK, etc ... all must be OK, then we can setup capacity control, otherwise no way;)

Best regards, Josip :)

[antonio nunes]

Hi Josip..well, still refering to HE (as you call it) my question was: is it short for "heat exchanger"? or?
...cuz there´s only one heat-exchanger pr. unit..the cooling medium sent over to all areas of the vessel it´s fresh water(it use to be brine once, I know cuz I took a sample- no salt no glycol)..offcourse there are plenty (15 AC ventilations units) but I´m not into that area-----my research is limited to the cooling process within compressor, condenser, evaporator and the relantionship input/output in terms of kW............
I´ll calculate the cooling request based on the a couple of scenarios/readings I made...it´s mathemtically possible to find massflow of refrigerant based on drawn el current display info.
Best regards AA

[Josip]

Hi, antonio nunes :)


Hi Josip..well, still refering to HE (as you call it) my question was: is it short for "heat exchanger"? or?
...cuz there´s only one heat-exchanger pr. unit..the cooling medium sent over to all areas of the vessel it´s fresh water(it use to be brine once, I know cuz I took a sample- no salt no glycol)..offcourse there are plenty (15 AC ventilations units) but I´m not into that area-----my research is limited to the cooling process within compressor, condenser, evaporator and the relantionship input/output in terms of kW............
I´ll calculate the cooling request based on the a couple of scenarios/readings I made...it´s mathemtically possible to find massflow of refrigerant based on drawn el current display info.
Best regards AA

I receive your documentation-plant scheme .... and now I know what is your plant looking like, unfortunately it is not complete ... water pumps and AC units are not visible ....

still we miss working points (designed suction and condensing temperature) to be able to determine compressor capacity ....

1. your main heat exchanger is evaporator where you give a service, making cold water for 15 AC unit around the ship...

2. your economizer is there to gain 10-15% of capacity, but to use

3. HE on liquid and suction line is to sub-cool liquid for evaporator and to increase suction superheat...



Hi Josip..well, still refering to HE (as you call it) my question was: is it short for "heat exchanger"? or?

what do you mean with above question Antonio? I was thinking about missing refrigerant-i.e. not enough into complete system or not enough entering into HE and after into economizer and finally into evaporator .... here we are not speaking about physical sizes ... assuming all that is correct or were correct for almost 15 years or maybe I misunderstood something ...



...cuz there´s only one heat-exchanger pr. unit..the cooling medium sent over to all areas of the vessel it´s fresh water(it use to be brine once, I know cuz I took a sample- no salt no glycol).....

it can be water only, but then your suction temperature cannot fall below 0*C otherwise you can freeze your evaporator and make a damage .... this is the reason why i'm asking for designed values....


..offcourse there are plenty (15 AC ventilations units) but I´m not into that area-----my research is limited to the cooling process within compressor, condenser, evaporator and the relantionship input/output in terms of kW.....
I´ll calculate the cooling request based on the a couple of scenarios/readings I made...it´s mathemtically possible to find massflow of refrigerant based on drawn el current display info.


what sizes are AC units? ... I think about rated cooling capacity ... you can check name plate son each unit ....

your load depend on above 15 AC units, giving you cooling request ... if there is not request for cooling (they are switched off) then you do not have any load and your compressor will not run, because there is no request for cooling ...

I'm not sure is that way good to calculate mass flow - maybe yes but how accurate will be, maybe you can describe that process) .... mass flow will change each time when there is change in suction or discharge pressure i.e. pressure ratio .... thus changing volumetric efficiency ....

usually we choose compressor by calculated and needed mass flow and swept volume to transfer that mass and not vice versa :confused:

Best regards, Josip :)

[antonio nunes]

..this is real great,Josip.. now I think we finally found a common platform to evaluate the situation/situation.
Yes, profissionals(hope I´ll get to that stage at some point) analyse/calculate the amount of needed cooling capacity and then dimension the system....in my case that´s established allready---my point of depart is:
-on the basis of two specific drift scenarios, to determine cooling capacity and compressor performance.

By the way, now I´m positive we are talking of the same component (HE) heat exchanger, the one in the system schematic draw.


My aim is to determine cooling capacity (kW) "freezing" this given situation while running on R22, since I know (from gauges and Stalectronic) the temp/pressure parameters..
This cooling output will be use to compare how other refrigerants would behave under the same circunstances..to determine if it will have a positive influence in the compressors volumetric and isentropic efficiency.

I can think of 3 ways of calculating mass flow on the grounds of the readings I have..I think(for now) the most accurate would be to make a energy balance in the condensator as following:
Q condensator = Q cooling seawater
mass flow R22 * (delta enthalpi condensator) = mass flow cooling water * specific heat capacity sea water * (delta temp seawater)
Isolating mass flow R22 I´ll have a result in kg/seconds.
If I use this result in the equation Q evaporator = mas flow R22 * ( delta enthalpi evap) I´ll determine the amount of energy the refrigerant "absorved" by the refrigerant.
...and since the cooling request from cabins,public spaces,working and service ares and so forth, equals the amount of energy/heat processed in the evaporator(not taking into account piping heat losses etc), this will be my "dimensioning" parameter to compare refrigerants and compressor behavior.
If this sounds totally sidetracked to you, let me know Josip..(what would you do instead?)
The designed values for the plant are included in one of those files I´ve posted..I´m a newbee in refrigeration technology but i´m convinced that the whole system is faulty and quite energy uneffective..
-------and closing down..let me see if I understood you rigth: even though there´s a need for cooling from above(cabins etc) will the Stalectronic program settings shut down el-motor/compressor every 15 min?

Best regards..b well...........AA

[Josip]

Hi, antonio nunes :)


..this is real great,Josip.. now I think we finally found a common platform to evaluate the situation/situation.
Yes, profissionals(hope I´ll get to that stage at some point) analyse/calculate the amount of needed cooling capacity and then dimension the system....in my case that´s established allready---my point of depart is:
-on the basis of two specific drift scenarios, to determine cooling capacity and compressor performance.

Here I'm a little bit:confused:, but I'll try to give some answer....



By the way, now I´m positive we are talking of the same component (HE) heat exchanger, the one in the system schematic draw.

6587

I attach scheme of your plant and there are HE numbered from 1 to 5 ... which one do you mean ...

HE no 1- HEAT EXCHANGER we can call that one liquid cooler or suction gas heat exchanger ... maybe in English terminology exist another name too - on both sides we have primary refrigerant - this one increase cooling load and temperature of suction gas

HE no 2 - ECONOMIZER - we can call it also liquid cooler at intermediate pressure to increase cooling capacity ... on both sides we have primary refrigerant - this one increase cooling capacity

HE no 3 - EVAPORATOR - liquid cooler in this case primary refrigerant (R22) cooling secondary refrigerant-water for 15 AC units around the ship - this one is giving main cooling load picked up by 15 AC units

HE no 4 - OIL COOLER - thermo siphon type using condensed refrigerant from condenser for oil cooling - this one increase condenser load

HE no 5 - CONDENSER - this one exchange complete load with cooling water-in your case sea water - thus transfer complete heat to mother nature



My aim is to determine cooling capacity (kW) "freezing" this given situation while running on R22, since I know (from gauges and Stalectronic) the temp/pressure parameters..
This cooling output will be use to compare how other refrigerants would behave under the same circunstances..to determine if it will have a positive influence in the compressors volumetric and isentropic efficiency.

I can think of 3 ways of calculating mass flow on the grounds of the readings I have..I think(for now) the most accurate would be to make a energy balance in the condensator as following:
Q condensator = Q cooling seawater
mass flow R22 * (delta enthalpi condensator) = mass flow cooling water * specific heat capacity sea water * (delta temp seawater)
Isolating mass flow R22 I´ll have a result in kg/seconds.
If I use this result in the equation Q evaporator = mas flow R22 * ( delta enthalpi evap) I´ll determine the amount of energy the refrigerant "absorved" by the refrigerant.
...and since the cooling request from cabins,public spaces,working and service ares and so forth, equals the amount of energy/heat processed in the evaporator(not taking into account piping heat losses etc), this will be my "dimensioning" parameter to compare refrigerants and compressor behavior.
If this sounds totally sidetracked to you, let me know Josip..(what would you do instead?)

You have a screw compressor 16 years old, with capacity of 1 500 000 kCal/h (500TR) at suction of 0*C and condensing temp of 25*C - from STAL capacity table for R22 at 50Hz thus 3600 rpm and today volumetric efficiency is at least 10-15% lower than it was at new compressor (but what about we are speaking if compressor is running at 30% of capacity:eek:)


You can see something only regarding R22 - it is in the system and you can follow pressures and temperature ... your liquid from condenser can be sub cooled etc.... etc..how to measure mass flow of sea water ....

what about load from HE no.1 and HE no.4 oil cooler -how you will determine that load and main motor power you have to add to condenser too i.e. deduce at the end .... too much inaccurate calculations ....

With all my respect to you, honestly, for me this is reverse engineering and I am sure can't be accurate without proper equipment ... but if you like you can do it by yourself ...

Otherwise, give me some time I will try (busy in the moment) to calculate the same or similar compressor for 3 different refrigerants R22,R134a and R407c, but again I need designed values of your plant, for example 0*C/35*C (evaporation temp/condensing temp) not a floating values from/to ... without right values/points I can only make it as standard design and can be different from your system...




The designed values for the plant are included in one of those files I´ve posted..I´m a newbee in refrigeration technology but i´m convinced that the whole system is faulty and quite energy uneffective..

I will not say that system is faulty ... it is designed by STAL having extraordinary designer engineers, only manufactured in Split/Croatia ... I had a chance to work with STAL engineers and learned a lot ...

.... about how much is effective is another story .... without proper setup each system is a mess .... I know that ... your plant was intended to be highly effective ....

how I know that? ... simply .... it is a ship plant with so many HE to use every single watt/calorie to reduce costs and increase cooling capacity .... thus to have a good COP .... from my point of view, deterioration of compressor within long working time is normal, but we have to accept it or to install a new one ...

Dear Antonio, I believe it was working fine for many years, otherwise owner will complain from day one ... my personal feeling is - improper setup of control devices ... of course I can be wrong ....



-------and closing down..let me see if I understood you rigth: even though there´s a need for cooling from above(cabins etc) will the Stalectronic program settings shut down el-motor/compressor every 15 min?

Best regards..b well...........AA

YES, you are right, when start delay passed, even if you have a cooling request compressor will shut down by some priority-protection signal, this time (I believe) by current limiter (did you change it to nominal motor current and check if compressor is running in different way?) or low suction temperature or something else, but I do not know what is implemented in your system, should be oil press, oil temp, discharge press or temperature, outer signal for start and stop .... it must be well protected, but you are a witness that someone change set-point of current limiter ... read my sig;)

in alarm situation it will shut down immediately, but you, possible, have only a warning signal and timer is keeping compressor running to cool down motor windings ... maybe for some reason you have problem with your PLC ...


Best regards, Josip :)

[antonio nunes]

Hej Josip
Hope all is well around there.
I´ve been busy gathering data on circulating pumpes for "brine" and seawater(condensator cooling) in order to attempt calculation of R22 massflow via heat balance--> Q condensator = Q cooling seawater
and via heat balance--> Q evaporator = Q brine
..and I got some "weird"(i think) results: massflow throughout condensator = 6.4 kg/s
massflow throughout evaporator = 2,9 kg/s
I think it´s too much since the outdoors temperature that day was 8 Celsius and the number of passengers was 900.
Meaning that the heat load removed in the evaporator corresponds to 559 kW..how about that?..we are talking about the same situation 09/10/2010(the one i´ve sended you)
Does this looks realistic to you?

Allow me to ask you the following:
Wich would be the best way to compare refrigerants then?
My original idea has been on the basis of 3 different scenarios- low part load
part load
full load
to calculate/compare refrigerants(R22-R134a-R407C) in therms of COP, efficiency grades and kWh costs.
I´m still not sure that my metode will do the trick..(you call it "up side down enginieering ;-)........
..that´s why i´ve been working on calculating heat load..the real one that day...then I would pick another day to illustrate part load and so on--------this is the skeleton of my thesis----------because as soon as I know about the massflow I´ll be able to calculate efficiencies as well.............

Hope you read this message at some point and eventually access other tools in order to solve this problem.
Best regards. AA