Suction Line Freezing

[John Kuchinic]

I just began working for a refrigeration company and I come accross Walk-In cooler systems where the suction line is frosted all the way to the compressor.
Fellow workers tell me that it is nothing to worry about. What I remember from school tells me that this should not happen at any time. Is this not a sign that the system is over charged?

[Brian_UK]

Not neccessarily.

What is the ambient temperature and what is the pipe temperature?

[Toosh]

Hi John Did you check the superheat

regards norm

[disturbed]

Hey johan
let me tell. Is U have Section Reciver used Return Section pipe?

[750 Valve]

It all depends what the temps of the system are. If that pipe temp at compressor is below zero - chances are it'll form snow/ice. The system may still be running with a good amount of superheat - for example when setting up our supermarket racks we aim for around 20K superheat max, this means on a freezer system that runs at -30sst (refrigerant temp in coil) you will likely see -10C back at the compressor (pipe temp) this is 20K of superheat but at -10C will freeze any moisture to the pipe and form ice - its still superheated though with no chance of smashing comps with floodback.

A medium temp system will generally not ice at the comp as it runs at between -10 and 0C sst (depending on design), add some superheat and the pipe at the comp is usually above freezing point and will sweat only depending on the ambient air conditions (dewpoint)

[Peter_1]

This is such a big common misunderstanding. Even teachers still teach that you may never see frost on the suction line.

[brian_chapin]

This is such a big common misunderstanding. Even teachers still teach that you may never see frost on the suction line.

I imagine those teachers would faint in the engine room of my -40f ammonia system. It has balls of ice 3" thick coming off the small uninsulated areas of pipe right before the compressor.

Ice on the pumped liquid pumps is at least 6" thick.

As was said previously - frost on the suction line is not a problem with lower temp systems.

Then again, what's the old saying? "All generalities are false - including this one"

[old gas bottle]

if its a cooler then somethings wrong assuming the rooms not below 0 deg C, are the evaps iced up,dirty,no air flow ? if thats o/k its start looking at TEV,s ,overcharging etc.

[750 Valve]

This is such a big common misunderstanding. Even teachers still teach that you may never see frost on the suction line.

Same in Australia - like brian_chapin says... show them a pumped liquid system and they'd fall over

I think they must all be domestic or air conditioning mechanics originally

[Pooh]

Same in Australia - like brian_chapin says... show them a pumped liquid system and they'd fall over

I think they must all be domestic or air conditioning mechanics originally

Not this teacher, been round the block several times and seen and worked on most things out there.

I have experienced engineers come into our workshop and flip due to the ice on the suction lines of our -20 deg C coldrooms when the students run them up, as the installations are only used for a couple of days we do not insulate due to the cost. These engineers utter those immortal words "there's liquid going back to the compressor" most of them do not understand the basics which freightens me.

Ian

[Grizzly]

Hi Pooh.
Didn't Glen Moore of Danfoss Tech. Support do a piece in the Service Engineer Mag recently on this very subject.
Incidently the scariest thing about an iced up liquid pump. Is when you have to remove the build up to service the pump. Sometimes only a hammer will do -scary!
cheers Grizzly.
Take heart your obvious proof that there are knowledgeable trainers out there!

[Pooh]

Grizzly
an ex colleague of mine turned up on site to do a liquid pump change to find the site fitter removing the ice ready for him with an air chisel at which point my mate left the building for breakfast at a cafe about two mile away. There are some very scary people out there.

Ian

[Lowrider]

To futher the discussion and perhaps a learning experiance, tell me if the amount of ice on this compressor is ok or if there's a problem.

Some info, it's a chiller delivering brine at -10 dgr C.

[Pooh]

Sanderh
is there any measurable superheat at the compressor, if so it is OK, if not then you have a problem. If you are supplying Glycol at -10deg C then you will be evaporating at nearer -20deg C so even with 8def K superheat the vapour coming down the suction line will be at -12deg C so you will get frost.

Without measuring the superheat we are all guessing if there is a problem.

Ian

[US Iceman]

I continue to be amazed at this topic.

Ice forms below the freezing temperature of water (32°F or 0°C).

If the evaporating temperature + the evaporator superheat is still below below the freezing temperature of water (32°F or 0°C) ice or frost will form.

Lack of superheat tells you if you have liquid, not frost or ice.

[Peter_1]

I think Sanderh knows the answer for his question, it's only to see if others can judge this right like Pooh.

[Grizzly]

[quote=Grizzly;86469]Hi Pooh.
Didn't Glen Moore of Danfoss Tech. Support do a piece in the Service Engineer Mag recently on this very subject.

Iceman.
Your point is very valid,
If I can I will get a copy of the article I refer to and post a copy.
It basically say's the same as you. In that "fully" trained Engineers can't seem to get their heads around the fact that Ice dosn't allways mean liquid floodback.
And it can just be sub-zero (0c) superheated vapour!
Don't give up on us yet!
Grizzly.

[Lowrider]

I think Sanderh knows the answer for his question, it's only to see if others can judge this right like Pooh.

Your right Peter! Problem was solved months ago, a no brainer! Just wanted to help "educate" my fellow ref engineers! (Don't get upset, I don't mean anything by it).

Due to sensor faults all oil was lost to the evaporator and thereby limiting refrigirant flow through the evap and thus causing the compressor to frost over!

Fault was easy to find, but then getting all the oil back! That took 4 hours, after replacing the sensors!

[US Iceman]

Now that we have this cleared up, let's look at a similar condition where liquid can exist.

If you have a suction line operating below freezing temperatures you can easily tell if liquid is present or not. If the suction line is frosted over a bit, scrape off the frost with your fingernail. If the frost does not reform on the suction line this simply means the line is cold.

If the frost reforms quickly, then there is a slight amount of liquid present in the line.

Cold gas forms frost very, very slowly while liquid will boil off and allow frost to form much more quickly.

I bet some of you older guys have noticed this too.

[Tony]

Hi 750 Valve,

this is 20K of superheat

This sounds an excessive amount of superheat - it makes the system less efficient.

5 - 7K is more than sufficient.

[Grizzly]

Now that we have this cleared up, let's look at a similar condition where liquid can exist.

If you have a suction line operating below freezing temperatures you can easily tell if liquid is present or not. If the suction line is frosted over a bit, scrape off the frost with your fingernail. If the frost does not reform on the suction line this simply means the line is cold.

If the frost reforms quickly, then there is a slight amount of liquid present in the line.

Cold gas forms frost very, very slowly while liquid will boil off and allow frost to form much more quickly.

I bet some of you older guys have noticed this too.

I have another, Clear the frost from an area, lick the end of your finger. Place it against the pipe and if it sticks or there is resistance as you pull your finger away.
Generally there is liquid there!!
And remember to remove your finger slowly! It Works, try it.
Grizzly

[US Iceman]

Yep, that works also Grizzly.

But, you learn quickly not to let your finger stick. Once is all it takes.

[hendry]

To futher the discussion and perhaps a learning experiance, tell me if the amount of ice on this compressor is ok or if there's a problem.

Some info, it's a chiller delivering brine at -10 dgr C.

i feel your photo display an abnormal state for a semihermetic comp.

my photo for sharing of normal operations.

[taz24]

I have another, Clear the frost from an area, lick the end of your finger. Place it against the pipe and if it sticks or there is resistance as you pull your finger away.
Generally there is liquid there!!
And remember to remove your finger slowly! It Works, try it.
Grizzly

Or as I once saw a student do.

Just lick the pipe!!

You can't buy inteligence

taz.

[750 Valve]

Hi 750 Valve,



This sounds an excessive amount of superheat - it makes the system less efficient.

5 - 7K is more than sufficient.

Mate thats MAX total system superheat (measured at compressor inlet), if I had 5k back at my racks I'd be a bit worried, a change in conditions in the cases/rooms (evap fan failure) on the rack will likely end up in floodback (not just ice, but the type your valves don't appreciate ).
The 5-7k you speak of is EVAPORATOR superheat, I am talking about TOTAL SYSTEM superheat.

pooh its good to know somebody's teaching them correctly

[Gary]

In a suction line, liquid droplets can and will travel with the superheated vapor. This is called "carryover". All of those droplets will disappear at about 5.5K/10F superheat.

But as 750 Valve points out, this is cutting it too close. Small fluctuations can allow liquid to reach the compressor valves.

Compressor manufacturers recommend a minimum of 11K/20F superheat at the compressor inlet.

This may or may not involve frost. There are many systems out there with frost on the compressor, yet they are not necessarily flooding.

You can only tell by measuring the superheat.

[Pooh]

Iceman, Grizzly
I too have used the sticking finger method but I beleive that without knowing the actual superheat you are as good as guessing, yes it is a guide but there is no substitute for actual accurate measurements. These then give you the information to do analysis accurately and fix the problem first time.

Ian

[US Iceman]

I'm in agreement with Ian. There is no substitute for good hard data.

The finger method or scrapping off frost is one of those "quick" ideas for trying to find the culprit quickly.

It goes back to some comments Peter made in an earlier post about using your five senses to find problems a little faster. These are invaluable if the person knows how to interpret the information so that they can begin to collect data from the problematic area.

[Grizzly]

Ian.
You guys at Star get all the toys don't you!
But I have to buy the batteries for my thermometer and I have 10 fingers.
Some of them are slightly de-sensatised. Due to picking up to many hot pieces of metal before I should of.
Although as Iceman points out I may not have for long.
And as Taz points out there is allways my tounge!
Are you telling me I have got to buy a new battery?
And has anyone tried these new Lithium ones yet?
Grizzly

[Pooh]

Grizzly
do'nt tie me to Star, other than teaching their apprentices and having some good freinds who work for them I never have (do not know why). We all use all of our senses to find faults but how many of us have been caught out by them, I have on more than one occassion and ended up doing more work than I had to.

Ian

[Gary]

Short cuts should only be used by the experienced and should never be presented as the right way to do things. They have no place in the schools.

First learn the best way. Short cuts come later and should be understood to be inferior to accurate measurements albeit sometimes faster.

That said, there is merit to using the senses. For example, imagine that you enter a very cold environment. You grasp a block of wood in one hand and a steel pipe in the other hand. They are the same temperature, but the steel transfers the heat from your hand much faster and thus feels "colder".

While the skin is a poor judge of temperature, it is a fair judge of rate of heat transfer.

It is possible to grasp a suction line and feel the ebb and flow of liquid as the TXV modulates, due to the difference in rate of heat transfer between the vapor and the liquid.

[Grizzly]

[quote=Pooh;87018]Grizzly
do'nt tie me to Star, other than teaching their apprentices and having some good freinds who work for them I never have (do not know why).
Pooh.
Sorry I must have misread some of your posts.
Gary, Pooh and anyone else I have misled. It was only ment as a light hearted comment not to be taken to seriously.
Surely sometimes, a little humour is as such a good teaching tool as being deadly ernest?
Cheers Grizzly.

[raju]

The line froast would be assessed based on the sction teperature!!!!!!

I just began working for a refrigeration company and I come accross Walk-In cooler systems where the suction line is frosted all the way to the compressor.
Fellow workers tell me that it is nothing to worry about. What I remember from school tells me that this should not happen at any time. Is this not a sign that the system is over charged?

[TXiceman]

Sounds like you need to do a bit more studying. An A/C application should not have frost or ice on the suction line. If you are operating a refrigeration unit on a freezer, the suction line will have frost of ice on it if it is not insulated.

Look at you evaporator temp and suction temp and see where you are.

Ken

[The MG Pony]

Worked on a sandwich cooler while back, R-12 all glass. Suction line had a wonderfull frost coat, well more like ice coat.

Why in a hot resteruant did it have the ice? Well humidity and a frozen up evap! Solution was simple, give her a defrost and then insulate the suction line to stop sweating and fix the defrost timer!

The freezer was the same old kit, Rest of it was fine but gave it a therough defrost dried off the suction line and insulated it up nicely but to be safe checked SH and it was just dandy, reason for the several foot thick ice was just it was running in a humid environment at a chilly temp, nothing to sweat over, frankly I get worried when I see a freezer with out some sort of frost!

[grump]

Keep it simple WORK ON AIR CONDITIONING.

Frosting back...

1. Compressor runs continuously?
2. Reduced air flow?
3. Oversized equipment/refrigerant?
4. Contactor stuck on replace?
5. Clean filters.
6. Check drive belts.
7. Fit hot gas bi pass.
8. Charge to full s/g(near enough its only air conditioning)
R22 do whatever it takes to maintain 200psig high 60psig low.
Don't injure yourself, I had to run a job at 400psig to stop frosting.
Do not get fixated on the txv!!!

Grump

[powell]

I just began working for a refrigeration company and I come accross Walk-In cooler systems where the suction line is frosted all the way to the compressor.
Fellow workers tell me that it is nothing to worry about. What I remember from school tells me that this should not happen at any time. Is this not a sign that the system is over charged?

This compressor has 15 degrees of superheat.

Frost tells you it's below dewpoint but......always check the superheat to verify there's no flooding.

[FreezerGeezer]

What the old guys always told me was that if the suction line is frozen - don't worry. If the compressor body is frozen - you've got a problem.

[TSK]

You guys need to watch out for the electrical terminals with those semi's, make sure there's a good earth cable attached, found too many with the insulators burnt right through yet still running!

[Magoo]

the enthalpy of saturated suction vapour will not form ice on suction at compressor if superheat of TX vav is correct and system is designed with correct balance. Basic design criteria. Back to to basics people.
Spot compressor so iced-up, fortunately it is a screw-semi, got injection oil cooling covered, but oil carry over is got be a problem.
magoo