PDA

View Full Version : What should superheat be on...



Rango
31-10-2003, 04:10 AM
Have a small countertop cooler that is used for salads. etc. Kept at 36 to 40 F. Small hermetic featuring 134a. Few weeks ago, ceased to cool One of our guys added schraders on both sides and charged, but nobody can find the leak. My question: factory plate states 6 oz., but starting from situation where system is not evacuated, howdoes one adjust to the optimum charge. What should the superheat be on this sytem. By the way, it's cap tube.Obviously, I'ma boobie, so go easy on me, ha ha. Thanx in advance>

FreezerGeezer
31-10-2003, 09:44 AM
Hi Rango, and welcome to the forum.

I used to do these quite a bit a few years back. The technique I was taught, and that seemed to work well, was this:

Having repaired the leak, pressure test the system. (to find the leak, the easiest way is to pressurise the system with oxygen free Nitrogen (OFN) to about 10 Bar. DON'T USE ANY OTHER GAS FOR THIS - YOU COULD BLOW YOURSELF UP IF YOU DO!)
I generally pressure test to the same as the leak test, i.e. 10 Bar.
Then fit your vacuum pump, and vac the system out to no higher that 2 Torr. Isolate the pump from the system, and leave it for 15 mins to an hour. If you can, it's preferable to leave it for 24 hours, but I reckon that no-one does in the real world! ;-) If the pressure does not rise significantly, ie. no more that 1/4 Bar over the half hour, then you're okay to charge. All this time, you'll have had your bottle of R134a by the unit you're working on, so it should be at roughly the local ambient temperature. Connect the liquid side of the bottle to the system (which is still at 2 Torr or better, take care not to let any air get in!) open the bottle to the system, and leave it to 'bomb charge' once the pressure in the bottle & the system have equalised, isolate the bottle from the system. Now run the cabinet. I would expect about 6 C superheat, a suction pressure of around 1.6 Bar or 23 PSIg (for 40 F), and the suction pipe would show sweating, perhaps light frosting, up to an inch or so of the compressor inlet. To be honest, I never bothered with the superheat. If the rest is correct, then the superheat is in the ballpark. 9 times out of 10, bomb charging like this will get you very close to the correct charge. If you need to top up the system to achieve the correct pressures etc, then charge vapour into the suction side, just a little at a time.

Sorry if all that has taught my gran to suck eggs, but you did say you're a boobie! :-)

Rango
31-10-2003, 11:14 PM
Small brain fart, FreezerGeezer, think that was noobie.

Rango
31-10-2003, 11:16 PM
Small brain fart, FreezerGeezer, think that was noobie. And thanks for the info. What about toppping off an existing charge?

mrchatts
31-10-2003, 11:20 PM
Rango,

I think every one has their own technique with this one.

If it was me, I would recover refrigerant and pressure test to about 150psig using oxygen free nitrogen, locate leak with well-maintained leak tester or the old famous soap solution. Once the leak is found, repair and change drier, pressure test again then carry out triple vac.

When your happy with vacuum connect to liquid connection on refrigerant bottle, if you haven got liquid connection connect to vapour and turn bottle upside down, open bottle and purge line. If you have scales and refrigerant charge stamped on name plate (job sorted) weigh correct amount in. If not then add small amount then switch on, keep adding refrigerant to keep compressor out of vacuum, remember SLOWLY your adding liquid refrigerant, regulate the flow of refrigerant using your gauges (always liquid charge R134a)

Now! as your adding refrigerant watch your frost over the coil, if fan assisted coil it would be good to switch fans off. Take your time its easy to overcharge the system. Keep watching the frost as it eventually covers the evaporator, the frost should stop when it enters the suction accumulator (if fitted) or just before leaving evaporator section, you don’t want the frost getting too close to the compressor.

That’s done, box up. Now that’s how you teach your granny how to suck eggs J

Next!!

Peter_1
01-11-2003, 12:30 PM
Rango,
We in Belgium follow the Kyoto protocol as much as possible.
Sorry for us all (and especially for our kids and our grandchildren) that the US won't follow this recommendations. We care for the environment. (I'm not saying you don't) Therefore we have to find the leak before adding any refrigerant. This is can be found in the Belgium 'Code of Good Practice' If you have to top off, then there is defenitly somewhere a leak.

Gary
02-11-2003, 06:05 PM
Unfortunately, if there is no wiegh-in option available, then there is only one or two days in the year when the systems charge can be accurately adjusted by using the superheat method.


Yet one can get very close to ideal charge with superheat, if done properly, perhaps closer than weigh-in since "identical" components in fact vary, and there is no better method for charging the system.

FreezerGeezer
02-11-2003, 11:03 PM
Marc & Daliti, you're right, of course. I should have made that clear - DO NOT PUT ANY MORE GAS IN IF YOU CAN'T FIND THE LEAK!!!
As to summer / winter conditions, I didn't take that into account, as I was working in air 'conditioned' supermarkets, which generally didn't have a lot of temperature variance over the year. To be perfectly honest, we were so damned busy that we never really had time to get things spot on. Close enough had to be good enough, generally.
Having said that, we were getting things a lot better when the client decided not to renew the contract - all that hard work for nowt! Thankfully, I had left by that time.

Rango
02-11-2003, 11:41 PM
I agree, no adding without leak check. However, this unit was leakchecked and recharged by one of our other guys and I just believe that thngs aren't quite right. Hence the question.

rango

angryk
03-11-2003, 03:46 AM
I have a strong feeling this is a restricted cap tube.

rbartlett
03-11-2003, 08:56 AM
get it frosting back when it's near down to temperature,-or better still when it's cut out a few times... then let out some gas.

watch the ice melt off the suction line back as far as you can see..button up the unit and go on to the next job..

cheers

richard


cheers

richard

mrchatts
03-11-2003, 10:30 PM
Don't know penguinkiller
Thats wot it says on the bottle "Liquid Charge" I must be wrong :(

mrchatts
03-11-2003, 10:37 PM
Originally posted by rbartlett
get it frosting back when it's near down to temperature,-or better still when it's cut out a few times... then let out some gas.

watch the ice melt off the suction line back as far as you can see..button up the unit and go on to the next job.


Cant believe you said that "then let out some gas."
:o

Gary
03-11-2003, 11:49 PM
get it frosting back when it's near down to temperature,-or better still when it's cut out a few times... then let out some gas.


I can't believe you would charge a system this way. Especially a medium temp system. What is so difficult about checking subcooling and superheat?

rbartlett
04-11-2003, 08:34 AM
Originally posted by mrchatts
Cant believe you said that "then let out some gas."
:o


i hope you are not assuming that i would recommend letting gas out by any other method than the safe refrigerant handling guidelines in accordance with modern good practice...??

cheers

richard

mrchatts
04-11-2003, 09:23 PM
Well richard it did seem to come across that way........Ok my mistake

Gary
12-11-2003, 06:13 AM
I always use both subcooling AND superheat on everything, Marc. :D

Peter_1
12-11-2003, 08:45 AM
[QUOTE]Originally posted by rbartlett
[B]get it frosting back when it's near down to temperature,-or better still when it's cut out a few times... then let out some gas.

watch the ice melt off the suction line back as far as you can see..button up the unit and go on to the next job..

Doing it just the same way for capillary systems. We never repair capillary systems on site because you lose to mutch time between the different steps and the cooling time is that slow and the labor cost to expensive.
Mostly we charge the gas in the morning. We don't look at it until we come back around noon to eat something. Check the frosting, release a little bit of gas or ad a litlle bit more and leave it again till the evening. Doing it this way, the labor cost for the client remains fair.
We release the gas always via 2 schraedervalves on the HP and LP side.
A friend of mine is a Refr. teacher. He teaches his students to do it also this way. He then evacuate the charged gas again from the system and measure the weight to prove that the weight of the added refrigerant is almost exact the weight prescribed on the nameplate. He also demonstrated already 15 years ago on an R12 household freezer to evacuate the R12 and recharged it with propane.

Gary
12-11-2003, 07:50 PM
A friend of mine is a Refr. teacher. He teaches his students to do it also this way. He then evacuate the charged gas again from the system and measure the weight to prove that the weight of the added refrigerant is almost exact the weight prescribed on the nameplate.

This only works if everything else is working properly. That's a very big if.

The customer didn't call you to come out and charge his system. He called you to make sure everything on the system is working properly. Charging is just one small part of it.


However, they don't make superheat charts for countertops, I doubt.

Superheat charts are handy, but not entirely necessary. At design load, we want to flood the coil, but not flood the compressor. Superheat (at design load) tells the story.


And overcharging a captube system to establish subcool when it should not have any can lead to early compressor failure through repeated flooded starting.


When we compare subcooling and superheat, we are watching the transfer of refrigerant from the high side to the low side, seeing how they balance, whether the high side is holding too much refrigerant (restriction), low side holding too much (overfeed), both sides too much (overcharge), or both sides too little (undercharge).

By monitoring both subcooling and superheat, we are observing all of the potential refrigerant side problems.

Our job isn't simply to charge the system. Our job is to make sure the entire system is working properly before we walk away. Contrary to popular opinion, it doesn't take any longer to do it right. It is not difficult.

I find it hard to believe that the schools are still teaching these hack methods, but I look around and I see that it is so.

P.S. I know that Marc knows all of this. I'm thinking he is yanking my chain, and doing a fine job of it... LOL

Gary
12-11-2003, 08:43 PM
And overcharging a captube system to establish subcool when it should not have any can lead to early compressor failure through repeated flooded starting.


To answer this one more specifically, if a cap tube system has proper superheat at design load with little or no subcooling, the cap tube is in fact feeding vapor instead of liquid (not restrictive enough). This could be an error in design or it could be intentional. (On an A/C system it probably means someone installed the wrong piston).

Making a cap tube less restrictive has its advantages in that it increases the refrigerant flow rate, making for a faster pulldown. If fed solid liquid, the pulldown could be much faster, but this would flood the compressor at design load, therefore an accumulator would be needed to catch the overflow.

In any case, the superheat entering the compressor must be sufficient to prevent floodback at design load.

Peter_1
13-11-2003, 07:57 AM
[QUOTE]Originally posted by Gary

The customer didn't call you to come out and charge his system. He called you to make sure everything on the system is working properly. Charging is just one small part of it.


If there was a leak, or he pinched a hole in an evaporator or i see oil around a nut (i just give you some examples), then correct charging is not the smallest part but the biggest (after repairing the leak).
I'm sitting also every year in a jury for graduating bachelor Refrigerating students. Every year I'm surprised, sometimes positive but many times negative. Sometimes I'm realy thinking they graduate rather as a carpenter.
There is sometimes Gary a big difference what can be read in books and what has to be done on site.

When we compare subcooling and superheat, we are watching the transfer of refrigerant from the high side to the low side, seeing how they balance, whether the high side is holding too much refrigerant (restriction), low side holding too much (overfeed), both sides too much (overcharge), or both sides too little (undercharge).
By monitoring both subcooling and superheat, we are observing all of the potential refrigerant side problems.


You can monitore this, agree, but if you don't know the design subcooling and superheat temperatures? I think that was the explanation of Marc. And sometimes temperatures (subcooling and superheat) on these little things are far away from what you should normally expect.
Till the time you found this out, it will have cost more then a complete unit. The customer won't appreciate this.
Again, if you are in the field, especially with those little systems like household or countertop stuff, you must act efficient, otherwise you will ose clients.
In fact, whe do this job 1st because we like, but 2nd, because we have to earn money.


Our job isn't simply to charge the system. Our job is to make sure the entire system is working properly before we walk away. Contrary to popular opinion, it doesn't take any longer to do it right. It is not difficult.

Mostly, it will take mutch longer to do certain jobs right. Ever had a freezer with melting products where you can't evacuate long enough, otherwise all the frozen goods will be garbage?
You evacuate as long as possibel (not needed) and replace drier and come back the other day to evacuate again when goods are already frozen.
Or you solder a leack where it should be better to replace the whole tube net?
Time, time and time and... money, money, money.


I find it hard to believe that the schools are still teaching these hack methods, but I look around and I see that it is so.

This isn't a hack method of all. I see that other posters do it the same way. It is only a personal opinion of you. You should read everything or i wasn't complet with my explanation (english is not my native language) He is doing it this way to prove something.
And after that, all his students can chareg a household correct

Don't misunderstand me, i'm not saying you're wrong.
I'm only saying that
there are different solutions for a problem
is more then one good method to solve this problem,
there are even diffrenet approaches to determine a problem
the 'field methods' are sometimes different (and better) from the 'school methods'
field practice is very important,
a client must be served correct (a good job for a reasonable price) which won't say you did the job like you wanted it to do.The customer didn't call you to come out and charge his system. He called you to make sure everything on the system is working properly. Charging is just one small part of it.

Gary
13-11-2003, 07:01 PM
You seem to be assuming that I am suggesting methods which are theoretical and impractical in the field. I have been a hands on service tech for over 35 years, working on everything from the smallest fractional HP systems, cascade freezers, A/C systems, all the way up to 3600 HP cetrifugals. None of the procedures I advocate are theoretical, nor difficult, nor time consuming. They are the minimum necessary to do the job right.

Yes, there are many ways to do things. I have invested a great deal of research in finding the best ways. I apologize if my remarks about schools teaching hack methods offends you, but they are indeed hack methods. The schools teach inappropriate short-cuts, which are not short-cuts at all, as the right way to do things. Hack, hack, hack.

And yes, there are times when short-cuts are necessary and appropriate, but these are few and far between and should not be taught as "the right way". They are not the right way, they are short-cuts.

Gary
13-11-2003, 07:22 PM
Just a quick one to say I didn't want to comment on your replies yet before I added this quick reminder that I was talking about flooded starts, not flood backs

Flooded starts are an entirely different issue. To be sure, the more refrigerant is in the system, the more refrigerant will be in the compressor on startup, but the amount of refrigerant is not the cause of it being there.

If there is liquid in the compressor on startup, it is either migrating there during the off cycle or migrating to the evaporator and then being transported to the compressor.

The first would indicate that the compressor is cold and needs a crankcase heater.

The second would be an untrapped evaporator which is above the compressor. Because it is untrapped, the migrated liquid is transported by gravity down the suction line. It can usually be trapped by simply running the suction line above the evaporator and then down to the compressor. Gravity then holds the liquid in the evaporator. In any case undercharging the system is not the answer.

Gary
13-11-2003, 11:20 PM
Still sounds a little theoretical to me, Gary, if I can borrow the mood from your second last post.


Not so much theoretical as oversimplified and incomplete. To cover these subjects fully, for all types of systems, under all conditions, takes a lot more than just a few paragraphs.


Any system charged by superheat and subcool in low ambients without the aid of an imperical guide will be overcharged.


Of course, and those which are charged by frostline under such conditions are doomed. As I have been trying to point out in this thread, everything must be taken into consideration in order to do it right.

Maybe I should add a line to my sig saying something like, "However much I tell you, there is always more to it". :D

RogGoetsch
14-11-2003, 07:44 AM
Guess I'll put my two cents in. I need some abuse.

Remember we're talking about a 6 or 7-ounce charge. When the customer uncrates the thing and plugs it in, almost the entire refrigerant charge is in the compressor. (Refrigerant migration to lower vapor pressure in oil mixture, for the newbies.) This is the worst flooded start it will ever see, but no problem because the mass of refrigerant is too small to foam out all the oil.

During the off cycle, most small cap units need to equalize for low torque start. Liquid will be in the evap, usually with a small accumulator at the evap outlet. Start-up will flood briefly but again, not significantly.

The mass of refrigerant is kept small both to facilitate equalization and to protect the compressor.

There should be no subcooled liquid in the condenser and at full load, not in the drier either. A capillary tube cannot pass gas as easy as liquid, which is how capillary feed is actually controlled. It would be impossible to mass-produce a system whose liquid flow rate required such precision as perfect captube diameter, length, and identical captube entrance effects on every unit.

It's just like Gary said: even the nameplate charge is not going to be the optimal charge for every unit, only the "average" unit.

Rather, the capillary tube is selected that allows somewhat more than enough flow and the final restriction is provided by gas when all the available liquid has been fed in. Bubbles only restrict at the beginning of the captube because as more liquid flows in behind them and the temp of the captube rapidly drops to ambient and then suction line temp, the bubbles condense.

If you add excess refrigerant to a captube system, it will overfeed if the system is properly designed. If it backs up into the condenser, the captube or drier is too restrictive.

Subcooled liquid in the condenser is a very bad idea because it robs condensing surface with no purpose since the captube is the best subcooler made. Also the excess liquid slows equalization and will go live in the compressor when the unit is shut down for any length of time.

But we were originally talking about a leaker.

In such a case, no success in finding the leak, I would add ultraviolet fluorescing dye (gasp, shudder!) and add refrigerant until close to the correct suction pressure with a clean condenser. (Ambient is usually a hot kitchen). With a warm box, this would be a low charge, so I would add a bit more as I watched my superheat and the box came to temp. I wouldn't get too concerned if I had 1/2 ounce too much on a 6-ounce leaker.

Remember, you are allowed a leak rate of up to 1/3 of the charge per year. On my hypothetical unit, I don't even know what the leak rate is at this point. This way I get the customer cooling again, get set up for a quick and thorough leak detection on the next service, have begun a record to determine leak rate, and am on to the next problem.

In about 9 months I get a call from the customer, play the UV light on the unit, and sell a new evaporator (usually) or tell him to buy a new box if there is corrosion everywhere.

My invoices have a check-off box for "job not complete" and a line to fill in something like: "leak-check with UV, next service". I always point this out to the customer and let him know what his options will be when I have to come back.

I know many people get apoplectic when the subject of ultra-violet dye comes up. No, it doesn’t void the compressor warranty, destroy the compressor, or ruin hoses and sight glasses. It may stunt your growth or discolor your teeth, though, if you drink too much, so be careful.

Rog

Gary
14-11-2003, 10:12 AM
In such a case, no success in finding the leak, I would add ultraviolet fluorescing dye (gasp, shudder!) and add refrigerant until close to the correct suction pressure with a clean condenser. (Ambient is usually a hot kitchen). With a warm box, this would be a low charge, so I would add a bit more as I watched my superheat and the box came to temp. I wouldn't get too concerned if I had 1/2 ounce too much on a 6-ounce leaker.


As you have pointed out, subcooled liquid backing up into the condenser is not a good thing. Given the tendency of R134A/POE systems in particular to plug driers and/or cap tubes, how do you know you are not overcharging the system due to a partial restriction without having some indication of subcooling?

Gary
14-11-2003, 12:42 PM
Somehow, this discussion wants to expand beyond the tiny self-contained indoor units we were originally discussing. Fine, let's take it all the way.

High temp, medium temp, low temp, TXV, EEV, cap tube, piston, whatever type of refrigerant, any type of system, you name it.

With the caveat of "there is always more to it", under (real or simulated) design conditions, if the superheat is not too high and not too low, there is enough refrigerant to do the job.

If the subcooling is high, there is liquid backing up into the condenser.

Within these two limitations, on a variable orifice system more refrigerant can/should be added, but on a fixed orifice system more cannot/should not be added.

There we have it. Charging in a nutshell, using both subcooling and superheat.

But as always, the devil is in the details. :D

Gary
15-11-2003, 02:12 AM
Low load with warm ambient will give you maximum refrigerant flow and minimum superheat, and high load with cold ambient will give you minimum refrigerant flow with maximum superheat. But does this translate to low subcooling for the former and high subcooling for the latter? It ain't necessarily so.

Low load tends to reduce the difference between SCT and ambient while warm ambient tends to make the condenser less efficient, increasing that difference.

By the same token, high load tends to increase the SCT over ambient, while cold ambient makes the condenser more efficient, decreasing the difference.

In addition there is the relative condenser sizing and airflow to consider. An oversized condenser and/or fan will reduce the SCT over ambient.

Subcooling is always equal to or less than the SCT over ambient. For example, if the SCT is 10F over ambient, the subcooling cannot possibly be more than 10F, because the liquid at the outlet of the condenser cannot drop below ambient temperature.

In order to accurately judge subcooling, the SCT must be at least 20-25F over ambient. This may require blocking off the condenser airflow.

Peter_1
15-11-2003, 09:16 AM
What is the meaning of the abbreviation SCT?

Gary
15-11-2003, 02:08 PM
SCT = Saturated condensing temperature = high side pressure converted to temperature on a pressure/temperature chart.

RogGoetsch
18-11-2003, 08:16 AM
In a cap tube system, as Gary says, subcooling in the condenser indicates excess restriction, either from blockage or bad design. (I say bad design, meaning that if you plot the performance on a pressure-enthalpy diagram and calculate performance values for the same system both with and without condenser subcooling, your efficiency will be less with condenser subcooling.)

And for a cap tube system, optimal charge would depend on what conditions of ambient and load you expected to see. Minimum superheat at these conditions would give the best performance. At other conditions, the charge would not be optimal. This is a fundamental limitation of cap tube systems.

(But I would never recommend opening a sealed system that was performing adequately, but not optimally. That way lies madness!)

Mark C
12-03-2004, 12:06 AM
Originally posted by Peter_1
Rango,
We in Belgium follow the Kyoto protocol as much as possible.
Sorry for us all (and especially for our kids and our grandchildren) that the US won't follow this recommendations. We care for the environment.

Pseudo Science. But for your information... We in the USA are not allowed to vent gas, either. It is against the law.