problems with coldroom icing up

[goody]

a few months ago i had a problem with a cold room icing up. It was an integral coldroom on r404a with hot gas defrost. An engineer was called before me and replaced a condensor fan motor and cleaned the condensor and defrosted the coil and put the problem down to these faults. I attended site and found that when it went onto defrost the heat in the discharge line would dissapear and the suction pressure would only go up to 4.5 bar. I degassed the system and found it had a full charge of refrigerant so i vacced it and recharged it. I rang my senior engineer for advise and was advised to condemn the compressor which was then changed(along with the drier) and ran back up. the coldroom would run down to temperature but still would have no heat in the discharge line when it went on to defrost. The compressor was still running in defrost so i am unsure why it would not have heat in the discharge line.

[Brian_UK]

Ho Goody and welcome to the forum.

Do we assume that you have a solenoid valve operating your hot gas line?

By 'discharge line' do you mean the liquid line from the receiver, the condenser or the compressor discharge pipe?

[Jus1]

When you say coldroom, do you mean +temperatures? i.e +2/+5?
If so, why not get rid of the hot gas defrost and have the evap fan operating constantly on defrost ( off cycle ) This is ok in plus temps, and also if compressor is operating correctly in refrigeration cycle mode.

[Grizzly]

Like Brian I am slightly confused as to which is what
within your explanation.

Why would you expect the suction pressure to rise above 4.5 bar?

What pressure does your Condenser fans cycle in at?

Again as Brian asks what is happening with your Hot Gas Defrost line?

It sounds like based on the limited info you supply
that the evap coil is not going into defrost mode.

Or rather insufficient heat is being supplied or generated.

Is this system critical charge or more likely does it have a receiver.

What basis are you going by when you say it was charged correctly?

Please give us more so we can help.
Grizzly

[smpsmp45]

Though the data is insufficinet as stated by Brian, the possibility in the Hot gas defrost line can not be ruled out

[goody]

It's a coldroom at +2 degrees c. it is a critically charged system with no reciever, the weight of refrigerant required is stamped on the manufacturers plate. the condensor fans only run when in refrigeration cycle. By discharge line i mean the discharge off of the compressor and is controlled by a solenoid valve for the hot gas defrost and when the system goes onto defrost the solenoid valve opens and the compressor still runs but then there is no heat in the compressor discharge line so it doesnt warm the evap coil

[nike123]

It's a coldroom at +2 degrees c. it is a critically charged system with no reciever, the weight of refrigerant required is stamped on the manufacturers plate. the condensor fans only run when in refrigeration cycle. By discharge line i mean the discharge off of the compressor and is controlled by a solenoid valve for the hot gas defrost and when the system goes onto defrost the solenoid valve opens and the compressor still runs but then there is no heat in the compressor discharge line so it doesnt warm the evap coil

What is high side pressure when solenoid valve opens?

[Kh1971]

Hi,

Please check your circuit again or let somebody check it for you, there is something wrong!!!!!!

[BUSHY]

i too have had a simillar problem except mine was with a freezer room. first problem found was that there was no check valve on suction line, next was it was getting a gut full of liquid back because the unit would not pump the evaporator out before it went on defrost causing low discharge temps and lastly it was a single compressor running one evap with no auxillary evaporator to get its heat for deicing primary evap,the unit was not capable of producing enough heat to deice system

[nike123]

the condensor fans only run when in refrigeration cycle.

You mean evaporator fans?
Try to be more precise, this is not poetry.

[philfridge]

Sounds like unit is either short of gas or solenoid valve faulty as reverse cycle pressure is very low. What is the running pressure ??

[Samarjit Sen]

It is not a solution, but the best system for defrosting I feel is electrical. Earlier I used to opt for hot gas, but found they are not very effective for low temperature. Now we are installing electric defrost system and there is no problem

[Gary]

a few months ago i had a problem with a cold room icing up. It was an integral coldroom on r404a with hot gas defrost. An engineer was called before me and replaced a condensor fan motor and cleaned the condensor and defrosted the coil and put the problem down to these faults. I attended site and found that when it went onto defrost the heat in the discharge line would dissapear and the suction pressure would only go up to 4.5 bar.

Get out your P/T chart and translate this to temperature. The suction pressure is telling you the temperature of the coil. It has nothing to do with anything else. That pressure is not going to rise above a pressure corresponding to freezing temperature until the ice is melted.

[Grizzly]

Hi Goody!
So if I am correct in this system when it goes on defrost.
The discharge gasses are diverted down the hot gas line to defrost the Evap.

Therefore the surplus liquid that would normally sit in the evap has to go somewhere.

IE the Condenser!
I assume if this is how it works?
There is a non return valve on the discharge line after the hot gas Solenoid Valve.

To the prevent surplus liquid in the condenser sub cooling the Discharge Gasses.

When the defrost cycle is initiated does the liquid line solenoid valve shut. Pumping the evap down before the H.G. S.V. Opens.

Sorry to keep asking questions but Their is a simple explanation here I just can't see it.

When in normal running IE freeze cycle does the compressor provide sufficient lift. (high enough discharge pressure)

If it does then the issue is as to what in the defrost
cycle is preventing enough heat to be generated.

Hence my question about a non return valve on the discharge line.

Could it be something stupid like a passing L.L.S.Valve.

Keep us informed as to how you get on please
cheers Grizzly.

[Grizzly]

It is not a solution, but the best system for defrosting I feel is electrical. Earlier I used to opt for hot gas, but found they are not very effective for low temperature. Now we are installing electric defrost system and there is no problem

Samarjit.

I have to disagree.
Hot gas defrosting is Very very effective at low temp applications!

Over here Electric is very expensive.
whereas the hot discharge gasses are allready being produced.

If sized and designed correctly it is an extremely effective cheap option.

Admittedly it is usually only installed on larger systems.
Grizzly

[nike123]

Samarjit.

I have to disagree.
Hot gas defrosting is Very very effective at low temp applications!

Over here Electric is very expensive.
whereas the hot discharge gasses are allready being produced.

Only if you have two or more evaporators on one condensing device! If it is only one, then you have not much savings in electricity consumption.

Admittedly it is usually only installed on larger systems.

I think that above mentioned is partially reason for that.

[Gary]

By discharge line i mean the discharge off of the compressor and is controlled by a solenoid valve for the hot gas defrost and when the system goes onto defrost the solenoid valve opens and the compressor still runs but then there is no heat in the compressor discharge line so it doesnt warm the evap coil

There is heat is anything that is above absolute zero, so this statement makes no sense.

Since this system has only one evaporator and that evaporator is currently condensing rather than evaporating, there is no source of heat other than what little is gained in the suction line and the heat of compression. I would expect the discharge to be relatively cool.

It will eventually defrost the coil, but this system is not going to defrost quickly.

What terminates the defrost cycle?

[Grizzly]

Guys
URM! this system is at +2 .

Why are we having to defrost in the first place
Grizzly ( I only just noticed the obvious +2c)

[dear.mr.davidov]

check your back pressure valve regulation

[Gary]

Guys
URM! this system is at +2 .

Why are we having to defrost in the first place
Grizzly ( I only just noticed the obvious +2c)

Because the system is undersized and therefore not off long enough to defrost itself?

[Gary]

check your back pressure valve regulation

I read through all of the posts and have seen no mention of a back pressure regulator/EPR on this system. Have I missed something?

[nike123]

This is nice manual with some details about hot gas defrost:
http://tinyurl.com/yp49zm

[Jus1]

like i said in my previous post. +2'c only needs off cycle defrost. remove the hot gas. Alter the wiring, enough said. Then atleast the compressor has a rest over then just when off on stat.

[Gary]

like i said in my previous post. +2'c only needs off cycle defrost. remove the hot gas. Alter the wiring, enough said. Then atleast the compressor has a rest over then just when off on stat.

If the off cycle was enough to defrost the coil, then he wouldn't have a frozen coil and we wouldn't be having this discussion.

[taz24]

Because the system is undersized and therefore not off long enough to defrost itself?

Why would you assume it is undersized?



We can only go on the information we have been given. The thing is running now the same as it was before changing the gas and the compressor.
Assuming the system ran ok in the past then what has changed for it not to run ok now?

There are more questions than answers.

Goody needs to post more information.
As for the defrost gas not being hot, comparing hot to absolute zero is not much help.

The hot gas pipe will be hot to touch for a few seconds only then it will cool down as the pressure drops. The hot gas will be injected straight to the evap and then back through the evap and suction to the comp. There is no pressure drop so the discharge and suction pressures will almost equalise out.
This cannot be alowed to last to long other wise the comp overheats and will fail.

The termination of the defrost needs to be checked.
Does it terminate on time, temperature or both?

More questions than answers

taz

[Gary]

Why would you assume it is undersized?

I don't... I simply offered a possible answer to the question posed by Grizzly.

We can only go on the information we have been given. The thing is running now the same as it was before changing the gas and the compressor.
Assuming the system ran ok in the past then what has changed for it not to run ok now?

There are more questions than answers.

Goody needs to post more information.
As for the defrost gas not being hot, comparing hot to absolute zero is not much help.

The hot gas pipe will be hot to touch for a few seconds only then it will cool down as the pressure drops. The hot gas will be injected straight to the evap and then back through the evap and suction to the comp. There is no pressure drop so the discharge and suction pressures will almost equalise out.
This cannot be alowed to last to long other wise the comp overheats and will fail.

The termination of the defrost needs to be checked.
Does it terminate on time, temperature or both?

More questions than answers

Yep... we can't see it from here... we need a much better description of the system and a lot more information.

[Gary]

As for the defrost gas not being hot, comparing hot to absolute zero is not much help.

Is it less helpful than saying the discharge line has no heat?

[Chunk]

It could be that the defrost is not set up correctly ie:not going on for long enough.As said in above posts we need more info.

I think that we need unit manufacturers name and this should be easier to solve.

[Gary]

It could be that the defrost is not set up correctly ie:not going on for long enough.

Based on very limited information, I would agree with this. We know that it goes into defrost. The suction pressure tells us that the coil is iced. Given long enough, it will defrost itself.

[Grizzly]

In the words of the song "There are more questions than answers!".
Another thought as we are all guessing!

Could a partially blocked evap coil create this scenario?
Grizzly

[Chunk]

I had a similar problem with a freezer yesterday that was only installed a week ago(i know this wont help with your problem but)someone had installed an epr and thus the evaporator wouldnt reach a high enough temperature to clear any ice.

I have also had a situation where the hot gas was being sucked straight back up the suction before it got to the evap giving you a higher suction pressure but not defrosting anything.So if you have a suction solenoid valve make sure it shuts on defrost.

Also whilst i am making wild guesstimations check if you have an evaporator temperature probe which is not shutting the hot gas prematurely.

Thats it.Have fun going back to work tomorrow because i`m on holiday.

[Gary]

In the words of the song "There are more questions than answers!".
Another thought as we are all guessing!

Could a partially blocked evap coil create this scenario?
Grizzly

Something (time/temperature/pressure) ends the defrost cycle.

The defrost cycle is apparently ending before all of the ice is melted.

A partially blocked evap (or anything else that reduces the airflow) could cause excess ice build-up. If the defrost is terminated by time duration, then this could cause the defrost cycle to end before all of the ice is melted.

If the defrost is terminated by temperature or pressure, then the coil should completely defrost itself, regardless of build-up.

[Chunk]

How about cycling the condenser fans with a hp switch if you are not getting required heat.This unit may be oversized for application and losing all heat before it has any effect.

I like guessing as it gets my posts up and makes me look brainier than i am.

[Gary]

I had a similar problem with a freezer yesterday that was only installed a week ago(i know this wont help with your problem but)someone had installed an epr and thus the evaporator wouldnt reach a high enough temperature to clear any ice.

I have also had a situation where the hot gas was being sucked straight back up the suction before it got to the evap giving you a higher suction pressure but not defrosting anything.So if you have a suction solenoid valve make sure it shuts on defrost.

Also whilst i am making wild guesstimations check if you have an evaporator temperature probe which is not shutting the hot gas prematurely.

Thats it.Have fun going back to work tomorrow because i`m on holiday.

How about cycling the condenser fans with a hp switch if you are not getting required heat.This unit may be oversized for application and losing all heat before it has any effect.

I like guessing as it gets my posts up and makes me look brainier than i am.

I'm going to take a wild guess and say you work on multiple evaporator systems, probably racks.

[Chunk]

Hi Gary.

Yes i do.

Am i not making any sense then?

[Jus1]

If the off cycle was enough to defrost the coil, then he wouldn't have a frozen coil and we wouldn't be having this discussion.

It is iced up probably because the hot gas defrost is not working and also the evap fan will probably not be operating in defrost mode either. so i do beleive +2'c heating on defrost is not really needed. every 6 hrs/30mins/12'c defrost term is ideal OFF CYCLE!

[Gary]

Hi Gary.

Yes i do.

Am i not making any sense then?

You are making perfect sense... if the system has multiple evaporators.

The non-defrosting evap(s) provide a heat source for defrosting and to keep up the head pressure.

On a single evap system, hot gas is in fact cool, the head pressure remains low, regardless of fan setting and an EPR would have little to no affect on defrosting.

[Grizzly]

Something (time/temperature/pressure) ends the defrost cycle.

The defrost cycle is apparently ending before all of the ice is melted.

A partially blocked evap (or anything else that reduces the airflow) could cause excess ice build-up. If the defrost is terminated by time duration, then this could cause the defrost cycle to end before all of the ice is melted.

If the defrost is terminated by temperature or pressure, then the coil should completely defrost itself, regardless of build-up.

Thanks Gary
I was trying to say if this system used to "work".
Then what is differant.... a partially blocked coil was one option.

Your point about the way the defrost cycle is totally relevant I agree.

I might even suggest that the reason the chamber no longer defrosts.
Could be because the product being stored has changed!!!
A higher moisture content needs a longer defrost , does it not.

I am going to let this 1 go now as I have speculated enough!

Hears hoping that Goody comes back to us with some more info or an update!!"

Cheers Grizzly

[Chunk]

With regards to the epr on the system i had,the suction pressure was being held off so the evaporator would fill with cool gas as it was supposed to but was taking longer to defrost.

When i removed epr from system it defrosted a lot quicker as there was no flow restriction.As it was only a small single evap system i saw no need for epr(or should i say that all the other units that i have worked on with the same set up,had no epr fitted).

[Gary]

With regards to the epr on the system i had,the suction pressure was being held off so the evaporator would fill with cool gas as it was supposed to but was taking longer to defrost.

When i removed epr from system it defrosted a lot quicker as there was no flow restriction.As it was only a small single evap system i saw no need for epr(or should i say that all the other units that i have worked on with the same set up,had no epr fitted).

I take it the EPR was between the hot gas and the evap?

Usually on a single evap system the hot gas would dump into the coil just downstream from the metering device.

On rack systems the flow direction is reversed during defrost, but still the EPR/SORIT would not come between the hot gas and the coil.

[andywill]

Hi
Had same problem on a zanotti integral freezer cold room. What I found was the discharge line was nearly blocked with braze. Replaced defrost line weighed gas in and all was ok

Andy