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BESC5240
16-03-2005, 02:57 PM
Could anyone give me an idea on the minimum velocity with which 'normal' evporators are designed?
I am confronted with the same problem on at least 10 identical installations.
The evaporators consists of a distribotor with several circuits of copper tube (5/8") in a bath of etheleen glycol. This bath kept at approx. 0°C) is a kind of buffer for a second copper coil in which a fluidum runs that has to be cooled down to +2°C.
This works. Their is no problem with the final result, but in several cases I had compressors broken down with no oil left in them. My guess is the oil is in the evaporator 'coil'. Until now I wasn't able to physicaly check the pressence of the oil in the evaporator (the units are a few thousand miles from here). But I would like to check theoretically if their is something wrong with the design of the evaporators. To me they seem too big and the internal velocities are too low. That is why I am looking for general rules on the design of an evaporator (minimum velocity for oil return, maximum slope, ...).

Brian_UK
16-03-2005, 11:07 PM
Welcome BESC5240,

Most refrigerant manufacturer's will give you details of their velocity requirements, do a Google for Refrigerant Velocity...

Also have a look at http://www.boykoeng.com/products.htm , they have a software package (expensive) but you can try it for 30days so could be useful.

Good luck,

Peter_1
17-03-2005, 06:12 AM
Hello BESC5240,

Welcome in this forum.What company you're working for if I may ask?
Didn't found anything in your public profile.
How did you found this forum?
Because I'm making some publicity for this forum at some wholesalers (Heynen, ECR,...)

What type of compressors is used, size and length of the copperlines, gas, air-cooled condenser,...?

Danfoss has a free software program (Danven) for calculating the lines.

If you're sure they are broken without any oil left in the sumps, then you don't have to guess any further where the oil is, ... it's surely in the coil.
But this can be checked easily on site if you remove the coil.

Is the coil a full length of copper that is submersed in the fluid?
Try to sketch how it looks like or post a picture. A picture says more than 1000 words.

BESC5240
17-03-2005, 08:11 AM
Hi Peter and Brian,
I have got a few line sizing programs, for normal lines. And I have quiet some experience in system design, but I wonder if there are any guidelines on velocity in evaporators. Should one take the fase change in account? Can I make my calculations only based on the velocity of the superheated gas at the end of the coil?
The compressors are Maneurop compressors, and I got them back, so I could clearly see their was no oil left. The damage (upper main bearing seized) shows clearly that their is a lubrification problem. They brake down after 6 to 12 months in service. Since the installations are overseas, i did not have the opportunity to open the coil. Until now the problem is 'resolved' by mounting an other compressor.
I will get a complete unit back (in 2 months), but this is quiet expensive. But still I would like to have some (calculated) basis to support my theory that the oil can not get out of the evaporator unless the velocity becomes high enough.

Peter,
I got to this site while searching google for some extra information on this matter. I was really surprised to see your picture here, because we know each other for some time (we even worked together). Ik heb mezelf niet geidentificeerd omdat ik niet te veel ruchtbaarheid hieraan wil geven, maar eerder informatie vergaren. Als je wil zal ik een foto van de warmtewisselaar doormailen. Maar voor de rest, hoe gaat het met jullie en de kinderen? Bruno Y.

Peter_1
27-03-2005, 01:07 PM
Bruno,

Verdoeme toch,... dat wij elkaar zo terug moeten ontmoeten.
Ik ben er bijna in gegeneerd.
Vooral omdat er een pakje klaarlag voor uwen kleinsten spruit. Wij zouden afkomen Kristien en ik, dan uitgesteld, nog eens uitgesteld , dan op een bepaald moment nagenoeg geen tijd meer omdat Kristien plots grootse plannen had (zie www.claeverbilck.be ) en nu eigenlijk voor niets nog tijd.
U zult begrijpen waarom.

Hoeveel keer ik al niet aan U heb gedacht, al dan niet omdat er een aanleiding was: wanneer ik het logo van het bedrijf zie waar je werkt, of beter noch...je paginagrote foto in C&C (heb je gemerkt dat ik er reeds een paar keer een artikeltje heb ingeschreven?), bij een verhitte discussie met een studiebureau waarbij ik de PWM ventielen van jullie niet wilde plaatsen, als ik een bestelwagen van F.. of CC tegenkom, Franck van Clima nog eens zie, bij het maken van een selctie van ventielen, jullie nieuwsbrief, ....... Echt, honderden keren.

We deden het - denk ik toch - vrij goed tezamen en het is dan ook spijtig dat contacten zo vlug verwateren.

Juist na de verhuis naar jullie nieuwe kantoren wilde ik nog eens binnenspringen bij U, tezamen met onzen Tim die toevallig met mij mee was. Ik stond natuurlijk wel bij jullie oude kantoren. Ik ben toen echter zo mijn weg kwijtgeraakt in Brussel waardoor ik in tijdsnood kwam.

Deze week heb ik echt weinig tijd gehad om op de computer te zitten en mij met dit forum bezig te houden, ook al wenst de stichter van dit forum dat wij moderators dit forum wat aanwakkeren. Kristiens moeder is met een trombose in spoed opgenomen.
Vind je het forum trouwens een goed initiatief?
Er zitten een aantal cracks tussen en ik denk dat Marc O'Brien wel diegene is die je het best kan helpen. Je moet eens gaan klijken op zijn website en naar de software die die man heeft gemaakt. Fenomenaal.
Ik heb wel gevraagd dat ze U ten volle willen helpen. Ik hoop dat er wat beweging komt bij uw vraag.

Nico B. van ECR heeft ook reeds een aantal keer kunnen helpen.

Zie ook http://www.refrigeration-engineer.com/forums/showthread.php?p=22373#post22373

Ik heb alleszins redelijk wat literatuur. Misschien kom ik eens tot bij U af. Neen,.. niet misschien...ik kom ZEKER af.
Ik bel u zeker eerstdaags eens op.
De groeten aan de familie Marissa, Chantal en met schaamrood moet ik toegeven dat ik de naam van de kleinste niet meer weet.

Peter

Peter_1
28-03-2005, 12:42 PM
Marc,

Thanks already for your reply. As you probably saw in my dutch text - I think you could understand some of it - to BESC5240, I mentioned your name.
I made some publicity for your website and your marvelous software section.
I will call BESC5240 and ask for more info.

Peter

Peter_1
28-03-2005, 04:35 PM
They're perhaps small applications but I know that before we see the final working version it took many hours of programming before it's finshed.
Honestly, ..I thnik they're VERY good.

arunchaudhari31
30-03-2005, 05:23 PM
I have a similar problem. If any one can help me.

I have installed 42 Refrigeration units with LH 124/4TC-12.2 Bitzer Condensing units with Guntner Evaporators Model GSF045.1E/27EW. Three units in each room. for Freezer application (-) 20 Deg C.

The problem is the unit gets tripped and we find that all the liquid is in Evaporator and the receiver is empty. Once we start the machine again it starts but liquid is coming in the system. I have also found that there is no oil in the system.

We have tried with adjusting the expansion valve by closing it. rotateed clockwise.

The distance between solenoid valve and the expansion valve is 15 meters. Total piping is 25 meters for each unit.

Any body can help what could be the reason ?

chemi-cool
30-03-2005, 07:01 PM
Hi arunchaudhari31,

I will check later these condensing units to see what you are talking about.

SV should be as close as possible to the TEV.

Are there oil separators installed?
Is the condensing unit above the evaporator?

What model and size TEV are you using?
Which defrost method do you employ?

Chemi :)

arunchaudhari31
31-03-2005, 05:11 AM
Thanks for your reply.

We have done the mistake in installing the solenoid valve at a distance of 15 m from expansion valve.

The Expansion valve model is TEX 5-3 Orifice No.1.

The level of the evporator and condensing unit is more or less same. though the piping in between is 25 m

There is no oil separator installed.

The solonoid valve is installed in a verticle line. Is it okay if the solenoid valve is installed in the verticle line ?

Should we close the expansion and increase the super heat ?

Lc_shi
31-03-2005, 09:54 AM
why not to use coolpack to make some caculation for the velocity of refrigerant in the pipe. if the velocity is too slow ,the oil return should be with problem. As regards to the solenoid valve installatin,it depends on the type,some must be installed in horizontal line,some unnecessary.

chemi-cool
31-03-2005, 02:41 PM
Hi arunchaudhari31,

I have looked at the condnsing unit, You have to install oil separetor, move the SV closer to the TEV, check the temp and pressure on the evaporators inlet and outlet and then you can see what need to be done.

Chemi :)

Gary
03-04-2005, 04:14 PM
Thanks for your reply.

We have done the mistake in installing the solenoid valve at a distance of 15 m from expansion valve.

The Expansion valve model is TEX 5-3 Orifice No.1.

The level of the evporator and condensing unit is more or less same. though the piping in between is 25 m

There is no oil separator installed.

The solonoid valve is installed in a verticle line. Is it okay if the solenoid valve is installed in the verticle line ?

Should we close the expansion and increase the super heat ?

During the off cycle, all available refrigerant (refrigerant not stopped by the solenoid) will migrate to the coldest point in the low side. In other words, liquid will gather in the cold evaporator.

If the evaporator is level with or above the condensing unit, gravity will transfer that liquid to the suction line and flood back to the compressor on startup, flushing out the oil.

It is a good idea to loop the suction line above the evaporator before going down to the condensing unit. This traps the liquid refrigerant in the evaporator where it will do no harm.

Adjusting the TXV is definitely NOT the answer.

jamcool
03-04-2005, 04:20 PM
have a totally different question, what is the simplest was to calculate Horsepower for compressors? here in Ja its a bit hard to call manufactors with serial numbers to find out Horsepower

Gary
03-04-2005, 04:54 PM
Why would you need to know the horsepower?

jamcool
04-04-2005, 04:22 AM
well in this end of the world the Caribbean if the comp. on a unit burns for eg. it is a copeland the wholesalers stock comps. any brand so a fridge guy just buys a comp. it is not uncommon to see a trane unit with 2 comps one a copeland the other a toshiba thats the way of the world in the tropics :rolleyes: so no matter how u give serial numbers all they want to know is what is the horsepower

BESC5240
04-04-2005, 08:00 AM
Hi Jamcool,
Horsepower is not a correct unit to size a compressor. However to some extend it is possible to compare different compressors based on their HP size.
Some examples on how some manufacturers indicate the 'HP size' in their type description:
Copeland ZR90K : (capacity in ARI conditions 60Hz= 90.000 Btu/hr) : 90.000 / 12.000 = 7,5 HP
Danfoss Maneurop MTZ72 : 72.000 / 12.000 = 6 HP
Tecumseh AH5524 : 24.000 / 12.000 = 2 HP
...

arunchaudhari31
05-04-2005, 03:24 PM
Hello everybody ! If you remember I had asked for the help for the Liquid flood Back.... I am using Bitzer condensing unit and fairly big GUNTNER evaporator ?

We had a solenoid valve installed nearly 15 min away from Expansion valve.

No we have closed the expansion valve 3 turns ( Increased superheat ) we found that now machines are not tripping on OIL Pressure switch but it is now tripping on High Discharge Pressure.

Why is it so ?

Can any body help me ?

arunchaudhari31
05-04-2005, 03:27 PM
I have read somewhere that to increase the velocity of Refrigerant in the system, we can put slight amount of R-12 in the system of R-22 is it true ?

arunchaudhari31
06-04-2005, 05:23 AM
Marc, thanks for your reply. But we have a receiver installed for this air cooled condenser. The liquid level in the receiver is now 60%. Earlier it was 30% now it is 60%.

Is it okay if we charge the R-12 in the system ?

Please confirm

Best Regards,


Arun

Peter_1
06-04-2005, 06:44 AM
... liquid level in the receiver is now 60%. Earlier it was 30% now it is 60%. Is it okay if we charge the R-12 in the system ?

You need to solve the problem itselves, not trying to search for solutions for the bad results of the problem.
You must pinpoint the problem and solve it.
Adding R12 is not the way it should be done, at least not the way a good serviceman works.

BTW, how do you know the difference from 30 to 60% in the receiver.

PS: It should have been better if you started a new post for this because this is a new thread.
We stray from the original subject.

BESC5240
07-04-2005, 10:03 AM
Comming back to my original question. In attachment you will find a picture of the evaporator.
It consists of 2 coils. One coil for the refrigerant (R134a) and the second one for the fluidum to be cooled.
The refrigerant goes through the distributor, through the distibutor lines (which to me are not optimally positioned), and than the refrigerant does in to a 5/8" (smooth) horzontal tube. The length is 6 m. In the middle of the coil (at the end of the tube) the tube goes up to a next level, and in to a 2nd layer of a 6 m 5/8" tube. This 2nd level tube also stays horizontal and goes from the inside to the outside of the coil.
Comming outside it goes into a header.
So you have 18 layers of a 6 m tube, and 9 circuits. In the middle of each circuit you have a difference in height of 16 mm (5/8").
This evaporator is put into a glycol solution and the compressor is controlled by a thermostat in the glycol.
I have calculated the velocity of the gas comming out of each circuit : 0,65 to 0,8 m/s depending on the conditions and the type of compressor used on the evaporator.
I compare this to the values that i can calculate on 'standard' heat exchangers (like shell and tube heatexchangers from Heatcraft) : I get values between 2,8 and 6 m/s. Can any one confirm these values?

PobodysNerfect
08-04-2005, 02:31 AM
This link
http://www.cibse.org/pdfs/twophase%20flow%20in%20horiz.pdf
was once posted here in another thread.

It shows the different flow patterns which exist in an evaporator coil as the refrigerant is boiling of and the velocity increases.

It can also be seen how the oil behaves and some comments about velocity is given.

Saludos,

Jan

arunchaudhari31
03-06-2005, 11:13 AM
He I wanted to send the readings of these refrigeration units. How do I attach it ?

maddymoo2
28-06-2005, 09:17 AM
I understand velocity is important but shouldn't an oil separator sort out this problem???

maddymoo2
28-06-2005, 09:20 AM
I know velocity is important but wouldnt an oil separator make this problem go away ???? regards maddymoo

BESC5240
28-06-2005, 09:38 AM
Oil separators do not solve (all) problems. They help you to overcome certain periods of low oil return to the compressors. But if the velocity in the evaporator (and the suction line) is continuously too low and the design of the system does not allow a good oil return, at the end the oil will stay in the evaporator (and/or suction line), wether you have an oil separator or not.
An oil separator will only slow down this phenomena while a smaller amount of oil is getting into the system.

maddymoo2
29-06-2005, 10:58 AM
'periods of low oil return' occur all the time in the real world and hence the need for an oil separator to stop the oil from leaving the proper area.gas velocities change constantly and if they are 'so low' all the time then I would think the presures would be low and the system would not cool effectivelyand it would be blatantly obvious that design change was required not just noticable when a compressor seized after a few months or year or whatever.also an oil separator gives the system a buffer against those low velocities so that while 'yes' some oil will make it past the separator most of it doesn't and when there is heavy load or on startup or after cycle off time when the velocities are at their highest this smaller amount will get back to the compressor and the cycle starts over.

Peter_1
29-06-2005, 07:54 PM
gas velocities change constantly .... also an oil separator gives the system a buffer against those low velocities ....

Gas velocities in a one compressor/one evaporator doesn't change at all or very slightly.
If the system is well designed and the lines were correct sized and installed, you'll never encounter problems with oil return.
I'm more than 20 years in the business and I never installed an oil separator unless on a pack.
A pack and some very special applications is the only setup where an oil separator is recommended.

BESC5240
30-06-2005, 08:10 AM
I'm sure in this case an oil separator won't help, because it is a very stable system (Tevap= -3°C and Tcond is regulated by fan speed controller; the unit works day and night in this conditions!). There is no problem with the refrigeration performance of the unit, (however, it could be optimised).
This is why I was looking for some clear figures on gas velocity is the evaporators. With coil manufacturers software I can calculate they normally work with velocities from 3 to 9 m/s. I am looking for a confirmation on these figures.

I agree with Peter: I would never use an oil separator on a one compressor/one evatorator system unless there are high variations in load and conditions to be expected (longer periods of low oil return followed by periods of normal oil return).
In this case I have a continously low speed in the evaporator. The speed only increases (a little bit) by the fact that the tubes in the evaporator get filled with oil.

benijoseph
30-06-2005, 07:27 PM
Could anyone give me an idea on the minimum velocity with which 'normal' evporators are designed?
I am confronted with the same problem on at least 10 identical installations.
The evaporators consists of a distribotor with several circuits of copper tube (5/8") in a bath of etheleen glycol. This bath kept at approx. 0°C) is a kind of buffer for a second copper coil in which a fluidum runs that has to be cooled down to +2°C.
This works. Their is no problem with the final result, but in several cases I had compressors broken down with no oil left in them. My guess is the oil is in the evaporator 'coil'. Until now I wasn't able to physicaly check the pressence of the oil in the evaporator (the units are a few thousand miles from here). But I would like to check theoretically if their is something wrong with the design of the evaporators. To me they seem too big and the internal velocities are too low. That is why I am looking for general rules on the design of an evaporator (minimum velocity for oil return, maximum slope, ...).



Mr,B.....40,

When a person makes a unit he will not make a mistake in it design because it as to perform then only
he can make some money that is one thing clear.
In your case ten unit have got similar problem , it means
there is a basic problem. I have gone through mostly all of your post ,what I dont understand is none of your post says clearly that which gas have you been using R12
or 134a If you are using R12 you have to use MINERAL OIL and for 134a synthic oil , If you use wrong grade of oil misibility of the gas and oil will not take place. it means oil
will go with the compressor pressure in fume form but boild gas will not get it back to the compressor, oil and gas have to be mixed perfectly so that the oil is back to the compressor , other wise compressor will get short of oil level and you know what will happen?
check with your compressor manufacturer regarding the oil grade, all the best.

beni joseph,

Peter_1
30-06-2005, 08:49 PM
Beni, I know Besc5240 very well and he knows very well what he's talking about.
There is indeed a basic problem here: low gas velocities.
R12??? Hey, wasn't that a gas from the middle-ages? We're not allowed to use R12 anymore.
You may use whatever oil you like Beni and mix it with the gas you like, if gas speed is too low or lines are wrong sized or wrong installed, nothing will help.
Have you checked with you compressor manufacturer BEESC5240 for the right oil :D :D :D
If Beni shoud know what I know :p ;)

BESC5240
01-07-2005, 08:58 AM
Mr,B.....40,

When a person makes a unit he will not make a mistake in it design because it as to perform then only
he can make some money that is one thing clear.
,

Dear Beni Joseph,
The systems are running on R134a and POE oil (delivered by the compressor manufactorer).
You must be lucky never having encountered a mistake in system design.
But my impression is that not all people working in the refrigeration business know what they are doing.
I can give you another example : I know a manufacterer of units ( a few thousand per year, distributed worldwide) who mounts the bulb of the expansion valve (after the hotgasbypaas to the suction line) between the suction accumulator and the compressor suction valve, in stead of where it should be: on horizontal piece of suction line, as close as possible to the evaporator outlet.
The only reason to put the bulb there is : 'it's easy to put it there and it works'.
But this is not a good system design.

benijoseph
01-07-2005, 07:23 PM
Dear Beni Joseph,
The systems are running on R134a and POE oil (delivered by the compressor manufactorer).
You must be lucky never having encountered a mistake in system design.
But my impression is that not all people working in the refrigeration business know what they are doing.
I can give you another example : I know a manufacterer of units ( a few thousand per year, distributed worldwide) who mounts the bulb of the expansion valve (after the compressorhotgasbypaas to the suction line) between the suction accumulator and the compressor suction valve, in stead of where it should be: on horizontal piece of suction line, as close as possible to the evaporator outlet.
The only reason to put the bulb there is : 'it's easy to put it there and it works'.
But this is not a good system design.

Dear B---40,

I am in this business last 28 years , I have come accross similar problem many a time but all problem are generaly connected with compressor, reason for the compr-
essor is , If you unit performance is fine .it means gas is boiling properaly in right pressure . then why oil is not coming back to the compressor? 1, compressor is pumping
out more oil then required .( I have experiance it with one
new tecumsha 2 ton compressor recently)
2,If compressor rpm is more then what it should be, (generaly it happens with pully
driven compressor )
In my opinion you are using a wrong compressor for the application, better consult compressor manufacturer! oil pumping is the defect of the compressor
not your design problem , Design problem will give you
poor performance of the unit still oil should not come out!
If oil is traping in suction side performance of the unit will be very bad.
you have mentioned about poor system design, In my opinion if the unit works with out any problem it is not a defective design!

with kind regards
beni joseph

benijoseph
01-07-2005, 07:36 PM
Beni, I know Besc5240 very well and he knows very well what he's talking about.
There is indeed a basic problem here: low gas velocities.
R12??? Hey, wasn't that a gas from the middle-ages? We're not allowed to use R12 anymore.
You may use whatever oil you like Beni and mix it with the gas you like, if gas speed is too low or lines are wrong sized or wrong installed, nothing will help.
Have you checked with you compressor manufacturer BEESC5240 for the right oil :D :D :D
If Beni shoud know what I know :p ;)

Mr, peeeteer,

If wright brother's have not done stupidity , we
wont have seen Aeroplane? STUPIDITY IS THE GREAT GRAND MOTHER OF INVENTION!
wish U all the best,
:D beni joseph.

Peter_1
01-07-2005, 08:03 PM
Compressor pumping more oil then required...
What quantity is required? None.
Every compressor pumps some oil out of the sump into the system and there isn't a need at all for pumping oil in the system.

If this oil isn't returning towards the compressor but remain in the evaporator, then you'll have a problem.
But why is it not leaving the evaporator? Can have many reasons. But that's the main question here.

Gas in the system can boil on the right pressures and you still can encouter oil return problems for more than one reason: badly sized lines, sloped wrong... Having the right pressures doesn't guarantee a proper oil return at all. Tehre has to be more then this.

Belt driven? We're talking about hermetic compressors so speed isn't an issue here.

If it should be a bad compressor, why they brake then on all these different locations worldwide?

It's definitely a system design, compressors are OK.
And let's face it in the hermetic machines: a compressor are only some pistons, driven by an electrical motor and with some valves in it. That's all. It isn't more complex than this.

And stupidity CAN be the grand mother of an invention but most of them are based on scientific backgrounds, made after many calculations.

benijoseph
03-07-2005, 07:42 PM
Dear friends,
To check Refrigernt velocity in evoporater :

velocity of refrigerant means speed at which gas is moving, It requires two pressure difference one high and one low, high side is being inlet of expansion valve and low side will be suction of the compressor, If unit is working normaly ,It means pressure in both side is with in permisable limit ,so velocity of the gas will be difference of both the side and perfect flow!
Now why oil is coming out?
1, wrong application of the compressor,
a, compressor of high temperature application,
b, Higher BTU used for low temperature,
c, higher HP rating with low BTU,
2, Compressor fault,
compressor is the heart of the unit , it is not simple to say it nothing but a motor , piston and valve plate! NO , It got its own performance like,How much gap
should be between bore and piston, More the gap,More BTU
less HP, Less stroke of the piston , less pressure 240 to 300 psi, but should not used for low temperature application , due to low suction pressure oil will be sucked through piston.
Now minimum gap between bore & piston, Low BTU, High HP,High piston stroke ,piston rings so no seepage of oil through piston, high pressure, minimum 500psi and above,THIS compressor should not be used high temperature application ,
dear friends, if proper compressor is used it will not matter , slope ,size or length ,(only efficency and performance of the unit will be less) oil will never be a problem.
Now mostly split AC cooling unit will be some times 20 feet below the compressor unit there is no problem, In automobile freezer unit are always down then compressor unit which works at -20 C,
So it means? CHECK B----40!
WITH KIND REGARDS,
BENI JOSEPH,

Peter_1
03-07-2005, 08:54 PM
So what you're saying: I may connect a suction line of 3 5/8 pipe to a 1/4 hp, inclinded away from the compressor and I will have no oil problems?????????
Size doesn,t matter, only pressure needs to be correct????????

And your theory about the dead space above the piston was also realy new for me. You see, you're never to old to learn something.

I have to go back to school or all the books and my past practical experience are useless. And I thought I understood Refrigeration. This is a real pan in the a.... I'm very depressed now.
:rolleyes:

botrous
03-07-2005, 10:08 PM
Dear friends,
To check Refrigernt velocity in evoporater :

velocity of refrigerant means speed at which gas is moving, It requires two pressure difference one high and one low, high side is being inlet of expansion valve and low side will be suction of the compressor, If unit is working normaly ,It means pressure in both side is with in permisable limit ,so velocity of the gas will be difference of both the side and perfect flow!

Beni , I as i understood from what i've quoted from your post is that the velocity of the refrigerant in the evaporator is related to the pressure (that's what i know well) but saying that the velocity equal the difference of both sides seems weird to me .

You've heard of fluid mechanics no ?
You may be able to determine an average velocity of refrigerant in the evaporator .... there are elbows and direct tubes that makes that at each point the velocity is going to change due to pressure loss (sorry i'll use french terms : perte de charge régulière et perte de charge singulière ) which affects directly the velocity of the refrigerant. And do not forget what else will affect the pressure of the refrigerant , the amount of energy it's absorbing from the room . . . (tempreture and pressure are directly related ) , that on addition to the compressor performance (french again : surface balayée par le piston du compresseur et le taux de compression) and how much the refrigerant will be cooled in the condenser etc . So calculating the velocity of the refrigerant in an evaporator is a little more complicated than you think.


dear friends, if proper compressor is used it will not matter , slope ,size or length ,(only efficency and performance of the unit will be less) oil will never be a problem.
Now mostly split AC cooling unit will be some times 20 feet below the compressor unit there is no problem

Friend, the efficency and performance will be less , because oil will be trapped in the evaporator what causes difficulty of heat exchange between the refrigerant and the space to cool, and because with time the compressor will loose OIL and have difficulties running so that makes the efficency and performance less ....
As for mostly split AC cooling unit will be at 20 feet below the compressor , have you ever heard of traps to help oil return to the compressor . If no trap is fitted the unit may work but not for a long time before it looses about 70% of the performance then the compressor burns.
If you need an evidence for that just try to disassamble an evaporator from a unit (those of 20 feet you are talking about), I advise that you where rough clothes ....

Peter_1
03-07-2005, 10:26 PM
Botrous, now I'm following again.
Thought I was missing something :p ;)

botrous
03-07-2005, 10:32 PM
Hey Peter , thanks God i've read your post , the one replying to Beni , or I would have though that I really spent good time at the university and at work but just for the fun of spending time

Peter_1
03-07-2005, 10:45 PM
Hey Peter , thanks God i've read your post , the one replying to Beni , or I would have though that I really spent good time at the university and at work but just for the fun of spending time

Was thinking almost the same Botrous.

Fact is also that I knew BESC5240 very well - we worked together some years - and he really knows what he's talking about in many different domains of ***** refrigeration.
I know that if he's asking here a question, then the answer will be not that simple.

He also knows a lot about DWM Copeland, UH and Maneurop compressors and can say without looking in catalogues what
every figure in a complete type of a unit exactly means.
Isn't it BESCY?
Say BESC, where did that 5240 comes from? Your monthly wage in Euro's :D

Peter

chillin out
03-07-2005, 10:46 PM
As for mostly split AC cooling unit will be at 20 feet below the compressor , have you ever heard of traps to avoid oil going down to the evaporator
I always thought that traps were to help the oil get back to comp rather than stop oil going to evap. :confused:

chillin out
03-07-2005, 10:53 PM
Hi peter,
If BESC5240 is as good as you say he is, then I as many have admiration for him it`s just a shame he doesn`t post more often as we could all learn from the old masters. :) :) :)

ps hi BESC5240 :)

Peter_1
04-07-2005, 06:41 AM
I always thought that traps were to help the oil get back to comp rather than stop oil going to evap. :confused:

Good one Chillin Out, hadn't even seen this one, now it's becoming even more confusing.

Well why not more posting you said.. he has the same problem as I have...time, time, time.

The OLD masters you said. We're only in our beginnig of the 40 years. LOL :p ;)

But I remember when I was a kid and my father was 40 and I found him already an old man.

botrous
04-07-2005, 08:47 AM
I always thought that traps were to help the oil get back to comp rather than stop oil going to evap. :confused:

Sorry , guys . . . . don't be confused , you're right .... it seems that i was so confused after reading Benis post that i got a malfunction in my brain . . . . I though that all I studied was fake :o

TRAP - A depression or dip in refrigerant piping in which oil will collect. A trap may be placed at the base of a suction or hot gas riser to improve oil return up the riser.

BESC5240
04-07-2005, 10:14 AM
Dear Beni,
I can assure you: there is nothing wrong with the compressor and it's application.
I would like to have your idea on a velocity I earlier mentioned: 0,6 to 0,8 m/s (calculated mean gas velocity comming out of each circuit).
Is this OK for good oil return?

Hi Peter,
No 5240 is not my monthly wages, but I'll make a suggestion to my boss. It's just my 'number'. For all the applications I need to log in for my work I try to use the same login (and password). So I used my 'number' as a login for this forum.
It's like that 80's song (from New Music?) : 'Living by number' : '...I don't want your name, just your number...
And, I'll try to find more time for this forum.

maddymoo2
04-07-2005, 12:16 PM
okay I think you guys are all abit above me in experience I'm only a refrig mechanic .anyway a couple of thoughts ,(don't know if they are good or not)if velocity is low and youre cycling head pressure by fan control could you increase control point to increase head pressure/suction pressure??also maybe the system needs a larger oil charge at installation/commisssioning .also what about installing some extra site glasses in one system as a test case at various points .you may be able to pinpopint something?

benijoseph
04-07-2005, 07:48 PM
[QUOTE=BESC5240]Dear Beni,
I can assure you: there is nothing wrong with the compressor and it's application.
I would like to have your idea on a velocity I earlier mentioned: 0,6 to 0,8 m/s (calculated mean gas velocity comming out of each circuit).
Is this OK for good oil return?

Dear friend,
If nothing is wrong , then there should not be any problem, still U say for oil pumping , connect one oil seperator and forget it or if U having academic interest visit the place even if it is in moon! sitting one thou---nd KM
away and calculating it in your calculator no point!
sorry I have no idea about mean gas velocity at present , I left university 28 years ago , basicly Iam a
electronic engineer , There was a paper in applied mechanic
and flow of fluid which I have totaly forgetten, Iam in refrigeration , its by a chance , MY father was in army and looking after refrigeration and air conditioning and I took it as
a business after my education,
one thing I will tell U , there is lot of difference in theory and when U do it practicaly,
sorry for all the post which has hurt any ones
qualification of the subject,
thanking you all,
beni joseph,

Peter_1
04-07-2005, 09:06 PM
Besc, blijkbaar weet die de klepel niet hangen. Het dringt niet tot hem door wat je eigenlijk wil vragen en waar het probleem zit. Vooral ivm die olietrap sloeg wel alles. En dan zijn uitleg ivm drukken die een oileretour garanderen.
Zo ken ik er nog wel een handvol hoor, die bricoleurs.
Ik heb je dat vergeten te zeggen: wist je dat Peter Roose is verongelukt bij het monteren van een verdamper?

Just some sidenews for BESC.

patrickj
06-07-2005, 08:45 PM
Hi Peter,
There is adifferent angle to the oil problem. For proper oil seperation there should be sufficient discharge super heat.Ex in case of R22 more than 25 deg F.Say your compressor runs with liquid flood back or sucking wet vapour you normally end up with oil problems :eek:

Superheatman
06-07-2005, 09:38 PM
Hi Peter,
There is adifferent angle to the oil problem. For proper oil seperation there should be sufficient discharge super heat.Ex in case of R22 more than 25 deg F.Say your compressor runs with liquid flood back or sucking wet vapour you normally end up with oil problems :eek:


Hi...I think this thread is about a reciprocating unit where it would be the suction superheat that is important to stop refrigerant flooding to the compressor......the discharge superheat as applicable to oil seperation is normally when there are mesh seperators on the discharge side of screw compressors.Best regards etc

Gary
09-07-2005, 12:37 PM
Comming back to my original question. In attachment you will find a picture of the evaporator.
It consists of 2 coils. One coil for the refrigerant (R134a) and the second one for the fluidum to be cooled.
The refrigerant goes through the distributor, through the distibutor lines (which to me are not optimally positioned), and than the refrigerant does in to a 5/8" (smooth) horzontal tube. The length is 6 m. In the middle of the coil (at the end of the tube) the tube goes up to a next level, and in to a 2nd layer of a 6 m 5/8" tube. This 2nd level tube also stays horizontal and goes from the inside to the outside of the coil.
Comming outside it goes into a header.
So you have 18 layers of a 6 m tube, and 9 circuits. In the middle of each circuit you have a difference in height of 16 mm (5/8").
This evaporator is put into a glycol solution and the compressor is controlled by a thermostat in the glycol.
I have calculated the velocity of the gas comming out of each circuit : 0,65 to 0,8 m/s depending on the conditions and the type of compressor used on the evaporator.
I compare this to the values that i can calculate on 'standard' heat exchangers (like shell and tube heatexchangers from Heatcraft) : I get values between 2,8 and 6 m/s. Can any one confirm these values?

Is the evaporator in the picture upside down?

BESC5240
11-07-2005, 08:12 AM
Is the evaporator in the picture upside down?
Dear Gary,
No it isn't.
The condensing unit is mounted under the the evaporator.

Gary
11-07-2005, 12:11 PM
Seems to me that it would work much better if the flow were reversed. Each path would be downhill all the way, instead of uphill. Velocity is only important where refrigerant vapor is trying to push oil uphill.