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Thread: system recharge
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27-10-2008, 11:45 AM #1
system recharge
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)
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27-10-2008, 02:21 PM #2
Re: system recharge
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.Last edited by Gary; 27-10-2008 at 06:55 PM.
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27-10-2008, 06:34 PM #3
Re: system recharge
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
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28-10-2008, 04:05 PM #4
Re: system recharge
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.
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28-10-2008, 08:01 PM #5
Re: system recharge
I believe the short answer is NO.
Read Garys' post above.Brian - Newton Abbot, Devon, UK
Retired March 2015
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29-10-2008, 07:50 AM #6
Re: system recharge
Thank you very much for your help.
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29-10-2008, 03:27 PM #7
Re: system recharge
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"
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29-10-2008, 03:31 PM #8
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29-10-2008, 04:05 PM #9
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Re: system recharge
Yes, we all love simple answers to our questions!
Check my signature!
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29-10-2008, 05:46 PM #10
Re: system recharge
Last edited by Gary; 29-10-2008 at 05:49 PM.
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30-10-2008, 07:40 AM #11
Re: system recharge
[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
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30-10-2008, 05:55 PM #12
Re: system recharge
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!
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30-10-2008, 05:57 PM #13
Re: system recharge
http://www.refrigerationbasics.com/1024x768/rb1.htm
Read through that site!
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30-10-2008, 06:17 PM #14
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Re: system recharge
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!Last edited by nike123; 30-10-2008 at 06:26 PM.
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30-10-2008, 06:24 PM #15
Re: system recharge
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?
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30-10-2008, 06:29 PM #16
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30-10-2008, 06:31 PM #17
Re: system recharge
*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!
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 <_<Last edited by The MG Pony; 30-10-2008 at 06:34 PM.
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30-10-2008, 06:47 PM #18
Re: system recharge
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.
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30-10-2008, 06:50 PM #19
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30-10-2008, 06:55 PM #20
Re: system recharge
Read the site that I linked, it will explain every thing about the dead basics!
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30-10-2008, 07:07 PM #21
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Re: system recharge
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 <_<
As you pointed above for teacher, check these terms in conjunction with IR temperature measurements:
1. Emissivity
2. Distance to spot ratio
Last edited by nike123; 30-10-2008 at 07:09 PM.
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30-10-2008, 08:02 PM #22
Re: system recharge
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.
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30-10-2008, 08:03 PM #23
Re: system recharge
Oh and thanx again for all the help was very helpful
thank you all. your great!!!
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30-10-2008, 08:07 PM #24
Re: system recharge
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30-10-2008, 08:24 PM #25
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30-10-2008, 09:20 PM #26
Re: system recharge
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
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30-10-2008, 09:25 PM #27
Re: system recharge
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??
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30-10-2008, 09:53 PM #28
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30-10-2008, 10:07 PM #29
Re: system recharge
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30-10-2008, 11:06 PM #30
Re: system recharge
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30-10-2008, 11:24 PM #31
Re: system recharge
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31-10-2008, 07:35 AM #32
Re: system recharge
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)
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31-10-2008, 07:53 AM #33
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31-10-2008, 07:55 AM #34
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31-10-2008, 03:23 PM #35
Re: system recharge
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.
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11-11-2008, 07:19 PM #36
Re: system recharge
[quote=Gary;124470]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.
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.
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11-11-2008, 07:42 PM #37
Re: system recharge
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.Last edited by Gary; 11-11-2008 at 07:48 PM.
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12-11-2008, 04:13 AM #38
Re: system recharge
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.
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12-11-2008, 06:10 AM #39
Re: system recharge
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.Last edited by Gary; 12-11-2008 at 06:23 AM.
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12-11-2008, 01:26 PM #40
Re: system recharge
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.
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12-11-2008, 02:30 PM #41
Re: system recharge
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.Last edited by Gary; 12-11-2008 at 02:58 PM.
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14-11-2008, 06:06 PM #42
Re: system recharge
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.
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14-11-2008, 06:38 PM #43
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01-12-2008, 12:19 PM #44
Re: system recharge
ok, I think I got it, though if my charging procedure is wrong im gunna blame you guys thanks for all your help.
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01-12-2008, 03:49 PM #45
Re: system recharge
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.]Last edited by Gary; 01-12-2008 at 04:21 PM.
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01-12-2008, 04:18 PM #46
Re: system recharge
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.Last edited by Gary; 01-12-2008 at 04:23 PM.
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03-12-2008, 10:30 AM #47
Re: system recharge
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.
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04-12-2008, 06:42 PM #48
Re: system recharge
hey Gary,
give the guy the price for a good book,and ad a Q&A form whit it, LOL
Ice
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04-12-2008, 07:51 PM #49
Re: system recharge
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.
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06-12-2008, 06:40 PM #50
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