Refrigeration 101

[Gary]

Thanks Gary - excellent stuff.

I'd appreciate if you could touch on refrigerant charge determination, at some point in the thread.

Think of the system charge as consisting of low side charge and high side charge.

Assuming there is proper airflow through both coils (and that everything else is functioning normally):

On the low side, at design refrigerated space temp, if the coil outlet superheat is right, then the low side charge is right.

Coil outlet superheat varies with type of system. Generally speaking, a freezer should have 6-8F/3.5-4.5K superheat, a cooler should have 8-10F/4.5-5.5K superheat, and an A/C should have 12-16F/6.5-9K superheat.

On the high side the subcooling should be no more than 15F/8.5K because this is the point at which liquid will start to back up into the condenser.

On a cap tube/fixed orifice system, if the superheat is right (at design space temp) and the subcooling is not more than 15F/8.5K then the system has the right charge.

Note that minimum subcooling is not a factor on cap tube/fixed orifice systems. They are critically charged.

On a TXV system there is a minimum and maximum charge. When the superheat is right (at design space temp) and the subcooling is not more than 15F/8.5K then there is enough refrigerant in the system to do the job. This is the minimum charge.

Additional refrigerant can then be added to bring the subcooling up to 15F/8.5K. This is the maximum charge.

It is good practice to bring the charge up to maximum because vapor bubbles flowing through the TXV will wear out the needle and seat. This also provides additional refrigerant for heavy loads.

[Rich1281]

Hi Gary, it’s been a long time since we had lunch in Michigan at Bob Evans :-) Hope you have been well.
Quote:
Originally Posted by Gary

Coil outlet superheat varies with type of system. Generally speaking, a freezer should have 6-8F/3.5-4.5K superheat, a cooler should have 8-10F/4.5-5.5K superheat, and an A/C should have 12-16F/6.5-9K superheat..

We installed a Russell freezer system last week and I thought of you. They are saying Copeland now wants 30F superheat at the compressor MINIMUM. They don't seem to care about the superheat outlet at the coil. What happened to Copeland’s 12F standard?

On the high side the sub cooling should be no more than 15F/8.5K because this is the point at which liquid will start to back up into the condenser.

I have yet to see a system that can obtain a 15F sub cooling, the most I have seen ever was 10 and the newer systems seem to be around 4f. Is this due to a design issue? I always test a system before leaving to make certain it can pump down under load and not be a problem. Over 10f sub cooling I have yet to see possible. ?????

Also, I'd like to mention to newbie’s that these numbers may not be obtainable on converted systems. On a converted system you may have an overly charged system trying to obtain even small sub cooling temperatures. Also, one must be concerned with pump down capacity where LONG liquid lines are used and the solenoid valve is located inside the condenser. I have one customer with a 7/8 liquid line where the original installer put the solenoid valve at the unit and the line was over 100 actual feet. Needless to say, we moved the solenoid valve to the evaporator and the systems pump down capacity was ample since we didn't have the entire refrigerant in the liquid line pumping into the condenser.

......Additional refrigerant can then be added to bring the sub cooling up to 15F/8.5K. This is the maximum charge.

Is this a new standard? Like I stated, I've seen systems overcharged at 10 Sub Cooling.

Eagerly waiting for your guidance.

Richard

[Gary]

Hi Gary, it’s been a long time since we had lunch in Michigan at Bob Evans :-) Hope you have been well.

Hi Rich. A very long time. Nice to hear from you again.

We installed a Russell freezer system last week and I thought of you. They are saying Copeland now wants 30F superheat at the compressor MINIMUM. They don't seem to care about the superheat outlet at the coil. What happened to Copeland’s 12F standard?

As far back as I can remember, Copeland has called for a minimum of 20F superheat and a maximum of 30F superheat at the compressor inlet.

Being a compressor manufacturer, their primary concern is the superheat at the compressor inlet rather than the evaporator outlet. No surprise there.

I have yet to see a system that can obtain a 15F sub cooling, the most I have seen ever was 10 and the newer systems seem to be around 4f. Is this due to a design issue? I always test a system before leaving to make certain it can pump down under load and not be a problem. Over 10f sub cooling I have yet to see possible. ?????

Are you measuring subcooling on the liquid line near the receiver?... or on the drip leg between the condenser and the receiver?

[chillyblue]

Are you measuring subcooling on the liquid line near the receiver?... or on the drip leg between the condenser and the receiver?

Does this make a difference?? if so why??

I have yet to see a system that can obtain a 15F sub cooling, the most I have seen ever was 10 and the newer systems seem to be around 4f. Is this due to a design issue? I always test a system before leaving to make certain it can pump down under load and not be a problem. Over 10f sub cooling I have yet to see possible. ??????

I've seen this, but i am very doubtfull how the system will cope when the ambient air into the condenser reaches 32 deg C (design temp) I can see hp switches tripping.

CB

[Slother]

Hi All;
enjoy the site, I'm a sparky working on hot water heat pumps and have a couple of questions regarding fundamentals.

We were called to a fault, the outlet from the TX valve was frosting up. The fridgie explained that the TX valve was the problem.

The unit was working fine, the reported fault (rattly). Both pressures LP and HP were a little low, there was some oil but no leaks could be found. I suggested topping up refrigerant, it was explained that doing this would only put more refrigerant into the TX outlet (and frost it up more).

I have read some posts about checking superheat temp in the fault finding of a TX, not sure how this is done. If someone could give me advice on checks to carry out if TX valve outlet frosts up (including adjustment) I'd appreciate it.

[nike123]

Hi All;
enjoy the site, I'm a sparky working on hot water heat pumps and have a couple of questions regarding fundamentals.

We were called to a fault, the outlet from the TX valve was frosting up. The fridgie explained that the TX valve was the problem.

Frosting of valve outlet at air-con unit could indicate at problem, but at refrigeration unit that is normal. What is that unit used for?

The unit was working fine, the reported fault (rattly). Both pressures LP and HP were a little low, there was some oil but no leaks could be found.

If there is oil, there is leak!
How much low, we need numbers?
We also need temperatures of air in and out of evaporator and condenser to tell if pressure is low or not.
Also we need pipe temperature at evaporator outlet, compressor inlet, condenser outlet.

I suggested topping up refrigerant, it was explained that doing this would only put more refrigerant into the TX outlet (and frost it up more).

Topping up refrigerant could be done only after is established that refrigerant missing.That is done by measuring what I said above and judging from acquired measurements.
If measurement say that subcooling is low, than refrigerant is missing in condenser. It could be leak or restriction on low side. If you have oil spots and nobody before is not added refrigerant you are probably short of refrigerant.
If you have sight glass, than you will be having lot of bubbles in glass.

I have read some posts about checking superheat temp in the fault finding of a TX, not sure how this is done. If someone could give me advice on checks to carry out if TX valve outlet frosts up (including adjustment) I'd appreciate it.

Please do not touch TXV before we establish that it is cause of trouble, especially, do not adjust it.

[ewart]

Hey Gary
How would you determine the correct refrigerant charge in a water cooled system when water flow is controlled by a water regulating valve and it depends on head pressure to regulate. Say we are working with R404 and condenser water temp. of 50F?????

[nike123]

Hey Gary
How would you determine the correct refrigerant charge in a water cooled system when water flow is controlled by a water regulating valve and it depends on head pressure to regulate. Say we are working with R404 and condenser water temp. of 50F?????

If you have regulated condenser water flow than your condenser water out temperature should be at or more than cca 70°F. Why that low? Or you are speaking about water in temperature?

[ianparker0801]

Hi, I'm new to all this. have found the forum amazing usefull, having been sent on a couple of basic courses, which mainly teach you all the legislation, then left a static sight with no guidense or help. I have a slight confusion which is probably easy to resolve. After reading the guide to calculating superheat and subcooling (which made alot make sense) I use my companies service sheets to record the information needed, but instead of asking for liquid line temp. and press. they ask for discharge temp, press to be recorded. I would have assumed discharge would suggest after the compressor, are they the same thing or is this more likely because the sheets are incorrect. This probably seems like I'm being pedantic but untill I found this forum it was causing me all kinds of confusion.

[Benners]

Hi Gary ,
I havn't been a member long and am currently in an apprentaship in the trade, i have about 6 months experience and things are clicking into place for a newby if that makes sense ha. I just want to say that your postings are brilliant and are helping me alot. Looking forward to reading some more many thanks
Ben

[Rich1281]

Are you measuring subcooling on the liquid line near the receiver?... or on the drip leg between the condenser and the receiver?

On AC systems its the outlet side of the condenser, on refrigeration units its usually at the drip since I ususally can't get onto the outlet of the condenser. Wouldn't the liquid line just outside the reciever be the larger/greater subcooling difference?

BTW, someone stoll my books, where can I get replacements?
Rich

[Gary]

BTW, someone stoll my books, where can I get replacements?
Rich

Just click on my signature line.

[Deniver45]

Gary,

When you say "Generally speaking, a freezer should have 6-8F/3.5-4.5K superheat, a cooler should have 8-10F/4.5-5.5K superheat, and an A/C should have 12-16F/6.5-9K superheat."

At what abient temp are we talking here?

75 degree's?

[Gary]

Gary,

When you say "Generally speaking, a freezer should have 6-8F/3.5-4.5K superheat, a cooler should have 8-10F/4.5-5.5K superheat, and an A/C should have 12-16F/6.5-9K superheat."

At what abient temp are we talking here?

75 degree's?

Normally, the ambient temp would have relatively little effect on the superheat.

[desA]

Normally, the ambient temp would have relatively little effect on the superheat.

In my heat-pump trials, I have observed an influence on evaporator SH, due to ambient air temperature movements.

If Ta,in falls during the range of test, SH will roll down a little more than if Ta,in is constant.

If Ta,in rises during the range of test, SH can often end up being almost constant over the test duration.

The influence can be in the range of ~ 1-3K, depending on the operating SH of the machine.

[Gary]

In my heat-pump trials, I have observed an influence on evaporator SH, due to ambient air temperature movements.

If Ta,in falls during the range of test, SH will roll down a little more than if Ta,in is constant.

If Ta,in rises during the range of test, SH can often end up being almost constant over the test duration.

The influence can be in the range of ~ 1-3K, depending on the operating SH of the machine.

In your heat pumps, the ambient air would be the air entering the evaporator. As I interpreted the posters question, I believe he was referring to the air entering the condenser.

[desA]

Gary,

When you say "Generally speaking, a freezer should have 6-8F/3.5-4.5K superheat, a cooler should have 8-10F/4.5-5.5K superheat, and an A/C should have 12-16F/6.5-9K superheat."

At what abient temp are we talking here?

75 degree's?

This is the post I was referring to, Gary. There is a mix of applications & conditions here.

It could be useful to split the air-breathing evaps from the closed-space evaps, in regards to the SH question - it will be less confusing.

[Gary]

This is the post I was referring to, Gary. There is a mix of applications & conditions here.

It could be useful to split the air-breathing evaps from the closed-space evaps, in regards to the SH question - it will be less confusing.

I agree... both the question and the answer could have been more specific.

[nicehitsam]

Think of the system charge as consisting of low side charge and high side charge.

Assuming there is proper airflow through both coils (and that everything else is functioning normally):

On the low side, at design refrigerated space temp, if the coil outlet superheat is right, then the low side charge is right.

Coil outlet superheat varies with type of system. Generally speaking, a freezer should have 6-8F/3.5-4.5K superheat, a cooler should have 8-10F/4.5-5.5K superheat, and an A/C should have 12-16F/6.5-9K superheat.

On the high side the subcooling should be no more than 15F/8.5K because this is the point at which liquid will start to back up into the condenser.

On a cap tube/fixed orifice system, if the superheat is right (at design space temp) and the subcooling is not more than 15F/8.5K then the system has the right charge.

Note that minimum subcooling is not a factor on cap tube/fixed orifice systems. They are critically charged.

On a TXV system there is a minimum and maximum charge. When the superheat is right (at design space temp) and the subcooling is not more than 15F/8.5K then there is enough refrigerant in the system to do the job. This is the minimum charge.

Additional refrigerant can then be added to bring the subcooling up to 15F/8.5K. This is the maximum charge.

It is good practice to bring the charge up to maximum because vapor bubbles flowing through the TXV will wear out the needle and seat. This also provides additional refrigerant for heavy loads.

Yes sir I'm a newbie but I like this, this is really great stuff.Thanks for all u do.

[nicehitsam]

Gary this is great info I do appreciate it could you tell me what is meant by " if the superheat is right at( design space temp) and the subcooling is not more than 15F/8.5K then the system has the right charge.