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Bachuss
27-10-2008, 11:45 AM
Hallo folks, I'v just started a course in ref engineering about 4 weeks ago. I was just wondering if you can figure out how much refrigerant you need to recharge a system (any system) by the pressure, temperature relation?

I only ask because some of the system I'v been working on dont say and have only done two weeks of my course so far and although they touched on this area I cant find it in my notes :( (must keep better notes)

Gary
27-10-2008, 02:21 PM
The pressure/temperature relationship does not tell you IF you need refrigerant, much less how much.

It simply tells you the temperature at which the refrigerant is changing state from liquid to vapor or vice versa, under current conditions.

icecube51
27-10-2008, 06:34 PM
on most systems there is a detailed sign where it marks the current,cooling or heating capacity,kind of refrigerant its working on,and sometimes the amount of refrigerant in the outside unit. the amount stands for a line distance of 4 or 5 or 6 mtr. the rest you have to add by calculation.in small units its around 30 or 50 gram a meter of tube.
if you are not sure of the remaining amount, you can empty the complete setup and put it on a scale.find the exact amount needed via brochures or i..net or local store and add the difference.

its a good start.

Ice;)

The MG Pony
28-10-2008, 04:05 PM
You need a P&T chart and a set of guages and depending on TXV or Cap tube you go by either sub cooling or super heat once the system is optimumly charged and running record how much gas it took.

Brian_UK
28-10-2008, 08:01 PM
I believe the short answer is NO.

Read Garys' post above.

Bachuss
29-10-2008, 07:50 AM
Thank you very much for your help.

Bachuss
29-10-2008, 03:27 PM
I believe the short answer is NO.

Read Garys' post above.

Thank you Gary for your oh so enlightening and in-depth answer to my question, I especially like the fact you pointed me in the direction of other material which may be of interest to me in regards to the subject at hand. On a more personal note I believe your caption should read something more like "also total tool"

Bachuss
29-10-2008, 03:31 PM
You need a P&T chart and a set of guages and depending on TXV or Cap tube you go by either sub cooling or super heat once the system is optimumly charged and running record how much gas it took.

Definatly most helpful answer so far, though a little more depth would have made you my god. to everyone else thankyou (except Brian_the grump_uk)

nike123
29-10-2008, 04:05 PM
Yes, we all love simple answers to our questions!:rolleyes:
Check my signature!

Gary
29-10-2008, 05:46 PM
Thank you Gary for your oh so enlightening and in-depth answer to my question, I especially like the fact you pointed me in the direction of other material which may be of interest to me in regards to the subject at hand. On a more personal note I believe your caption should read something more like "also total tool"

I told you what you needed to know... and helped you more than you realize.

It's not that you don't know. It's that you don't know that you don't know.

You are working on systems before you are ready... and others are willing to teach you enough to be dangerous.

Bachuss
30-10-2008, 07:40 AM
[quote=Gary;124289]I told you what you needed to know... and helped you more than you realize.

I am so sorry gary, you did. that reply was intended for Brian_UK not you, these assessments have my head in a total mess.

again I am so sorry

The MG Pony
30-10-2008, 05:55 PM
First put in a base charge and record it, now run the unit till it is within the designed temp range, Now if cap tube we go by superheat, ideally we want 20k.

So attach the gauge to the suction line and take the pressure, refer to the P&T chart to see the SST, take the actual pipe measure of temp and see what the SH is, add or take gas till you get a nice 20K SH and then record the final charge!

Read up more on it becuase there are clearer posts then mine!

The MG Pony
30-10-2008, 05:57 PM
http://www.refrigerationbasics.com/1024x768/rb1.htm

Read through that site!

nike123
30-10-2008, 06:17 PM
First put in a base charge and record it, now run the unit till it is within the designed temp range, Now if cap tube we go by superheat, ideally we want 20k.



To correctly charge a capillary or metering piston system, you must put in enough refrigerant to cool the
compressor, but never so much that it allows floodback under any operating conditions. So to charge using the superheat method, you must consider:
- Outdoor temperature
- Evaporator temperature: which reflects airflow, system conditions, indoor humidity and temperature.
- Suction line temperature: which reflects superheat.
- Condenser efficiency

For that purpose some manufacturers has developed charging charts.
If they are not available for your case than you should simulate borderline conditions and adjust charge accordingly.
Sticking to fixed superheat value could result with broken compressor depending on conditions when charged.
Best method for capillary/orifice is to charge by weight.


Check reasoning in this article (http://www.mediafire.com/?wzgtwmyhzty)!

The MG Pony
30-10-2008, 06:24 PM
To correctly charge a capillary or metering piston system, you must put in enough refrigerant to cool the
compressor, but never so much that it allows floodback under any operating conditions. So to charge using the superheat method, you must consider:
- Outdoor temperature
- Evaporator temperature: which reflects airflow, system conditions, indoor humidity and temperature.
- Suction line temperature: which reflects superheat.
- Condenser efficiency

For that purpose some manufacturers has developed charging charts.
If they are not available for your case than you should simulate borderline conditions and adjust charge accordingly.
Sticking to fixed superheat value could result with broken compressor depending on conditions when charged.
Best method for capillary/orifice is to charge by weight.

well now that he has given you every thing <_< you chose the peak conditions of your area, for example most comercial kitchen units are charged assuming 45c condensing and -10 evaperating (In my local) So since the average temp is 20 to 15c I need to simulate 45c condensing to properly charge, can you gues how I can do so?

nike123
30-10-2008, 06:29 PM
well now that he has given you every thing <_< you chose the peak conditions of your area, for example most comercial kitchen units are charged assuming 45c condensing and -10 evaperating (In my local) So since the average temp is 20 to 15c I need to simulate 45c condensing to properly charge, can you gues how I can do so?

Cover part of condenser with blanket and measure temperature at the end of condenser with IR thermometer and adjust covered portion acordingly!

The MG Pony
30-10-2008, 06:31 PM
Cover part of condenser with blanket and measure temperature at the end of condenser with IR thermometer!

*Places face in palm of hands and sighs*

We must leave some thing for him self to learn! You are not helping by giving every thing to him:confused:!

I knew this very well! It was for him to learn then answer!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

A good teacher will guid the student with out telling him the answer, but giving him enough guestions to seek it out!

FYI I use a K type thermocuple and card board, I wouldn't trust any thing to an IR therm <_<

Gary
30-10-2008, 06:47 PM
A good teacher would take away his gauges and teach him everything there is to know about delta-T's and TD's. First things first.

The most important tool in our toolbox is the thermometer, not the gauge.

Bachuss
30-10-2008, 06:50 PM
A good teacher would take away his gauges and teach him everything there is to know about delta-T's and TD's. First things first.

The most important tool in our toolbox is the thermometer, not the gauge.

:eek:Whats a delta-T?:rolleyes:

The MG Pony
30-10-2008, 06:55 PM
Read the site that I linked, it will explain every thing about the dead basics!

nike123
30-10-2008, 07:07 PM
We must leave some thing for him self to learn! You are not helping by giving every thing to him:confused:!

I knew this very well! It was for him to learn then answer!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

A good teacher will guid the student with out telling him the answer, but giving him enough guestions to seek it out!

:confused:

I have had impression that you are asking that question.


FYI I use a K type thermocouple and card board, I wouldn't trust any thing to an IR therm <_<I use both, and sometimes PT1000 probes and Elliwell to read temperatures and every now and then I check my IR readings and I am very satisfied with precision (+-1K).
As you pointed above for teacher, check these terms in conjunction with IR temperature measurements:
1. Emissivity
2. Distance to spot ratio
;)

Bachuss
30-10-2008, 08:02 PM
what the hell have I started. I just wanted to know if there was a quick mathmatical way of going, ok, this system runs on such an such gas which functions at this pressure. the temp is *blech* so "BAM" I need this much refrigerant (in pressure) to top it up
but no
you guys have to turn my thread in to a debait on the fine art of teaching
well I quit. :(

Bachuss
30-10-2008, 08:03 PM
Oh and thanx again for all the help was very helpful
thank you all. :D your great!!!

Gary
30-10-2008, 08:07 PM
:eek:Whats a delta-T?:rolleyes:

A delta-T is a change in temperature of a single substance or flow of substance. In this case it is the difference between air temperature entering a coil and air temperature leaving the coil.

An increase in delta-T (or dT) is caused by a decrease in air flow.

nike123
30-10-2008, 08:24 PM
what the hell have I started. I just wanted to know if there was a quick mathmatical way of going, ok, this system runs on such an such gas which functions at this pressure. the temp is *blech* so "BAM" I need this much refrigerant (in pressure) to top it up
but no
you guys have to turn my thread in to a debait on the fine art of teaching
well I quit. :(
It apears that there is not such a thing as "quick mathmatical way of going" with this subject.
If you quit now, how much of time will pass till your next quick quiting? And again till next? And then again till next?..........:eek:

Bachuss
30-10-2008, 09:20 PM
A delta-T is a change in temperature of a single substance or flow of substance. In this case it is the difference between air temperature entering a coil and air temperature leaving the coil.

An increase in delta-T (or dT) is caused by a decrease in air flow.

ow I already knew about these, didn't know they had a fancy name (should be about 15 deg C difference yes?). thanks for the info but I dont see what they have to do with my specific question, I know about fault finding or that is to say i am finding out about fault finding on a daily bases, as usual thanx for you help, much appreciated

Bachuss
30-10-2008, 09:25 PM
It apears that there is not such a thing as &quot;quick mathmatical way of going&quot; with this subject.
If you quit now, how much of time will pass till your next quick quiting? And again till next? And then again till next?..........:eek:

Damn you and your common sense, ok I wanted to know if I could use the pressure and temperature along with my comparitor to tell me how much or maybe when to stop add refrigerant, I think I have it.I can use the superheat to tell me when optimal running pressure has been used??? Yes/No??

nike123
30-10-2008, 09:53 PM
Damn you and your common sense, ok I wanted to know if I could use the pressure and temperature along with my comparitor to tell me how much or maybe when to stop add refrigerant, I think I have it.I can use the superheat to tell me when optimal running pressure has been used??? Yes/No??
If you go on link in post#14 in this thread, and read that article, than you could find your answer there.

The MG Pony
30-10-2008, 10:07 PM
:confused:

I have had impression that you are asking that question.

I use both, and sometimes PT1000 probes and Elliwell to read temperatures and every now and then I check my IR readings and I am very satisfied with precision (+-1K).
As you pointed above for teacher, check these terms in conjunction with IR temperature measurements:
1. Emissivity
2. Distance to spot ratio
;)

Know both terms ;) I don't like having to make little black dots on every thing to get the correct filters! a nice clamp and bead prob makes for fast messure with out need for dots! Too many veriables for IR therms, so I chose the one with least veriables!

Gary
30-10-2008, 11:06 PM
ow I already knew about these, didn't know they had a fancy name...

Most people confuse delta-T (dT) with temperature difference (TD).

TD is a comparison of temperatures of two different substances or flows of substances.

For example, the evap TD is the difference between the entering air temp and the saturation temp of the refrigerant.

Gary
30-10-2008, 11:24 PM
ow I already knew about these, didn't know they had a fancy name (should be about 15 deg C difference yes?).

Not necessarily.

Evaporator dT varies with application and humidity level.

Condenser dT varies with ambient temperature, humidity, system efficiency and load.

Bachuss
31-10-2008, 07:35 AM
Most people confuse delta-T (dT) with temperature difference (TD).

TD is a comparison of temperatures of two different substances or flows of substances.

For example, the evap TD is the difference between the entering air temp and the saturation temp of the refrigerant.

So TD is the difference between the air and the refrigerant

and dT is the difference between the air entering and the air exiting the coils

so why call them delta-Ts what is the meaning and thank you again for the help (never knew there were different terms for different temp diffs)

Bachuss
31-10-2008, 07:53 AM
If you go on link in post#14 in this thread, and read that article, than you could find your answer there.

Although there is alot of helpful info here I dont have the money to fund canadas road network so the sight is pritty limited for me, I couldn't find what I was looking for but thanx anyway.

Bachuss
31-10-2008, 07:55 AM
Although there is alot of helpful info here I dont have the money to fund canadas road network so the sight is pritty limited for me, I couldn't find what I was looking for but thanx anyway.

infact I have found, tell me I'm right if the system has a cap type controler for txv to use the head pressure

Gary
31-10-2008, 03:23 PM
So TD is the difference between the air and the refrigerant

and dT is the difference between the air entering and the air exiting the coils

so why call them delta-Ts what is the meaning and thank you again for the help (never knew there were different terms for different temp diffs)

Here is another term to learn: "Approach"

As the air moves over the coil and refrigerant moves through the coil, the temperatures of each move closer to each other. IOW, the temps "approach" each other.

The difference between the leaving air temp and the saturation temp is the approach temperature.

dT tells you about airflow.

TD tells you about load.

Approach tells you about heat transfer.

These should be checked on both evaporator and condenser.

Bachuss
11-11-2008, 07:19 PM
[quote=Gary;124470]Here is another term to learn: &quot;Approach&quot;

As the air moves over the coil and refrigerant moves through the coil, the temperatures of each move closer to each other. IOW, the temps &quot;approach&quot; each other.
You dont say, just some how I dont think that is a technical term, at least only so far as "tighten" or "exchange" is. lmfao.

Gary
11-11-2008, 07:42 PM
I'm not making this up. "Approach" is a real term with real meaning.

It is more commonly used for water cooled condensers and liquid chiller bundles. When the approach is too high (insufficient heat transfer between the refrigerant and the leaving water), the pipes are fouled and in need of acid cleaning.

But the term also applies to air to air systems as it identifies condenser coatings (carbon coatings in heavy automotive traffic areas) and oil logging in the evaporator.

camerond
12-11-2008, 04:13 AM
Just started reading this thread, so sorry if someone answered this.
But in regards to charging or recharging via pressure, are we talking about standing pressure or operating pressure? And what size system are we talking about? The only time i've ever charged going by pressure was when charging the low stage of -80 freezers.

Gary
12-11-2008, 06:10 AM
Just started reading this thread, so sorry if someone answered this.
But in regards to charging or recharging via pressure, are we talking about standing pressure or operating pressure? And what size system are we talking about? The only time i've ever charged going by pressure was when charging the low stage of -80 freezers.

The low stage of a cascade system is the only situation where charging by pressure is valid.

Why? Because while standing at room temperature there is only vapor in the low stage of the system. Once there is both liquid and vapor (saturation), pressure corresponds to temperature.

Given sufficient refrigerant to form a liquid/vapor mixture, the pressures have little to do with quantity of refrigerant and everything to do with the temperature and volume of air moving through the coils.

Davidoroyal
12-11-2008, 01:26 PM
Er depends on the refrigerant used- you should be able to identify the correct head and suction pressure in relation to the ambient with a pressure relationship chart- you can get one of these in any Gas wholesalers.

Gary
12-11-2008, 02:30 PM
Er depends on the refrigerant used- you should be able to identify the correct head and suction pressure in relation to the ambient with a pressure relationship chart- you can get one of these in any Gas wholesalers.

IOW, in relation to the temperature of the air moving through the condenser... and assuming sufficient volume of air.

But having the correct pressures doesn't necessarily mean it has the correct charge.

Charging by pressures is not valid. Systems must be charged by subcooling and superheat... after proper airflow has been confirmed.

dudthefridge
14-11-2008, 06:06 PM
Hallo folks, I'v just started a course in ref engineering about 4 weeks ago. I was just wondering if you can figure out how much refrigerant you need to recharge a system (any system) by the pressure, temperature relation?

I only ask because some of the system I'v been working on dont say and have only done two weeks of my course so far and although they touched on this area I cant find it in my notes :( (must keep better notes)
ive always found that if you have no other means of judgeing how much refrigerant to put in a system this simple method will work ( although getting tube sizes and pipe length and talking to manufacturer should help aswell ).

put a base charge of refrigerant into the system ( no more than receiver size, if it has one )
take your ambient temp at condenser and add 16c, relate to press.
when this press/temp is achieved leave to run for a while or untill product temp is near on ( e.g room temp ).
then for more critical charge look at suction temp/pressure and complete charge by this gauge, also look at sight glass and comp run amps, manufacturer should say what ideal amps are.if there is a similar unit near by take the amps from this and use as a guide.
this is what my teacher taught me and has always proved successful.

ignore on vrv sytems though.

nike123
14-11-2008, 06:38 PM
ive always found that if you have no other means of judgeing how much refrigerant to put in a system this simple method will work ( although getting tube sizes and pipe length and talking to manufacturer should help aswell ).

put a base charge of refrigerant into the system ( no more than receiver size, if it has one )
take your ambient temp at condenser and add 16c, relate to press.
when this press/temp is achieved leave to run for a while or untill product temp is near on ( e.g room temp ).
then for more critical charge look at suction temp/pressure and complete charge by this gauge, also look at sight glass and comp run amps, manufacturer should say what ideal amps are.if there is a similar unit near by take the amps from this and use as a guide.
this is what my teacher taught me and has always proved successful.

ignore on vrv sytems though.

And it still could result in overcharging! Without superheat and/or subcooling it is still walk in the dark.

Bachuss
01-12-2008, 12:19 PM
ok, I think I got it, though if my charging procedure is wrong im gunna blame you guys ;) thanks for all your help.

Gary
01-12-2008, 03:49 PM
Assuming the airflow through both coils is right:

Take the basic question:

Is the system charge right?

and divide it into two questions:

Is the high side charge (subcooling) right?

Is the low side charge (superheat) right?

If the high side is undercharged and the low side is undercharged, then the system is undercharged.

If the high side is overcharged and the low side is overcharged, then the system is overcharged.

If the high side is overcharged and the low side is undercharged, then there is a restriction.

If the high side is undercharged and the low side is overcharged, then the system is flooding (mis-adjusted or stuck open TXV, compressor inefficient, etc.).

If the high side charge (subcooling) is right and the low side charge (superheat) is right, then the system charge is right.]

Gary
01-12-2008, 04:18 PM
ive always found that if you have no other means of judgeing how much refrigerant to put in a system this simple method will work ( although getting tube sizes and pipe length and talking to manufacturer should help aswell ).

put a base charge of refrigerant into the system ( no more than receiver size, if it has one )
take your ambient temp at condenser and add 16c, relate to press.
when this press/temp is achieved leave to run for a while or untill product temp is near on ( e.g room temp ).
then for more critical charge look at suction temp/pressure and complete charge by this gauge, also look at sight glass and comp run amps, manufacturer should say what ideal amps are.if there is a similar unit near by take the amps from this and use as a guide.
this is what my teacher taught me and has always proved successful.

ignore on vrv sytems though.

The difference between saturated condensing temp (SCT) and ambient temp is called the "Condenser TD".

If the heat load is light, then the TD will be less. If the heat load is heavy, then the TD will be more.

Also, high efficiency systems use larger condenser coils than standard efficiency systems, therefore a high efficiency system will have lower TD.

So you see... the TD may vary considerably.

Charging by TD may get you in the ballpark (or maybe just in the neighborhood of the ballpark) but it is definitely not right.

sherwin
03-12-2008, 10:30 AM
the most accurate in charging refrrigerant refer to equipment manual. it is indicated how much amount of refrigerant you will charge in terms of kilos/pound and ect. dont ever base on pressure and temperature because it will vary depent the ambient temperature.

icecube51
04-12-2008, 06:42 PM
hey Gary,
give the guy the price for a good book,and ad a Q&A form whit it, LOL

Ice

Gary
04-12-2008, 07:51 PM
the most accurate in charging refrrigerant refer to equipment manual. it is indicated how much amount of refrigerant you will charge in terms of kilos/pound and ect. dont ever base on pressure and temperature because it will vary depent the ambient temperature.

The quickest and most cost efficient way to charge is by weight as determined by the factory, but is it the most accurate? How does the factory come up with the charge weight?

They control the temperature and volume of air flowing through both coils and then they charge by subcooling and superheat. No two (same model) systems are exactly the same, so they repeat this procedure on a number of systems and come up with an average charge weight.

Thus it is possible to tailor the charge to your exact system by doing what the factory does and end up with a more accurate charge.

ahmed_helmy226
06-12-2008, 06:40 PM
:off topic:
You need a P&T chart and a set of guages and depending on TXV or Cap tube you go by either sub cooling or super heat once the system is optimumly charged and running record how much gas it took.

Gary
06-12-2008, 07:34 PM
Actually, it is on topic, but not the best way to charge a system.

Let's imagine that you are charging a TXV system. Further imagine that the service tech who worked on the system before you adjusted the TXV wide open. By the time you get the subcooling up, the compressor will be full of liquid and the system will be grossly overcharged.

Or worse yet, if the compressor valves are broken, you are not going to get the subcooling up and you are not going to clear the sight glass, no matter how much refrigerant you dump into the system.

You would have found these problems if you monitored the superheat as well as the subcooling.

On a cap tube system, if you are just monitoring the superheat, you will miss restrictions and non-condensibles, as well as inefficient compressor.

It pays to monitor both subcooling and superheat on both TXV and fixed orifice systems.

Note: It is usually much easier to monitor the superheat at the compressor inlet rather than the evaporator outlet and serves the purpose.

Gary
06-12-2008, 07:57 PM
Here is a little story that may be slightly off topic, or perhaps not:

I was inspecting a rooftop A/C system, which had 4 cap tubes.

The superheat was within specs, but the subcooling was 38F/21K.

I cut the cap tube assembly open to find that one of the cap tubes was brazed shut at its inlet.

This was a factory braze.

This unit was 18 years old and had been inspected every three months since new.

That's 72 inspections by a wide variety of service techs and nobody noticed the problem.

It pays to check subcooling as well as superheat.

Miko
30-07-2009, 09:26 AM
How do we charge an empty 2 stage cascade system?

Do we charge both High Stage and Low Stage at the same time or

Charge High Stage to reach a correct superheat/subcool or based on manufacturer specs the only proceed to Low Stage to a correct superheat/subcool or maufacturers specs?

Miko
30-07-2009, 09:28 AM
Sorry.. there is a typo error on the spelling.. so here it goes again..

How do we charge an empty 2 stage cascade system?

Do we charge both High Stage and Low Stage at the same time or

Charge High Stage to reach a correct superheat/sub cool or based on manufacturer specs then only proceed to Low Stage to a correct superheat/sub cool or manufacturers specs?