Unable to decipher high pressure fault - help!!

[Contactor]

Hello

This system is a walk in freezer room approx 2.5m x 2.5m x2.5m R404a

HP 18.5 bar (43c) 10k subcooling
LP 2.4 bar (-15c) superheat not known
Ambient 17c
Rooom should be at -18c but only holding at -8c.

I've tested for non-condensibles and there are none.

Condenser clear and fan ok, presumed over-charged so removed 0.5 kilo R404

This resulted in the following:

HP 17 bar (38c) 6k subcooling
LP 2.1 bar (-18c) superheat not known
Ambient 17c

System now undercharged with bubbles in sightglass, but room temp now dropped and holding at -13c.

(Removed 300g to start with, waited a few minutes but no significant changes so removed a further 200g until bubbles started showing)

I've had a good look at the condenser and checked for non condensibles, all fine.

Any ideas anyone?

Thanks

[chemi-cool]

I had in the past something similar with compressed air refrigerated drier, couldn't find anything wrong.

Recovered all the gas, new drier, vacuum and recharged with virgin 404.

the machine worked fine so i didn't bother to make more checks.

Give it a try and see how it goes.

[moondawn]

hi there can you check across the liquid line to see if you have a temperature drop along the line?

[Contactor]

thanks

I did wonder about the gas as the unit has had some work in the past.

Are you saying the liquid line / drier are blocked, if so why the high LP?

Thanks

[tonyelian]

h ti
if ur compressor matches the fan coil look after the txv look like too big orifice

[Quality]

I had in the past something similar with compressed air refrigerated drier, couldn't find anything wrong.

Recovered all the gas, new drier, vacuum and recharged with virgin 404.

the machine worked fine so i didn't bother to make more checks.

Give it a try and see how it goes.

I will second this as I`ve had this before on a small freezer

New charge (virgin) every time in my opinion

[Gary]

What are the condenser air on/air off temps?

[Peter_1]

Are you sure another tech filled other gas in it, partially R404 and some other gas?
Otherwise, I apply the same technique as Chemi. Recover, new dryer, vacuum and restart with new gas. This unit is too small to spend too much time on it.

[Contactor]

I willtry replacing gas and changing the drier, but does anybody think the TEV could be causing this ?

[Peter_1]

No, the TEV isn't the cause for a high HP.
Then your LP should be also far beyond normal working conditions.

Possible causes for a too high HP:
non-condensables in teh system
too small condenser surface,
clogged,
fan defective,
overcharged
Bended discharge lines

That's all...I think

[NoNickName]

Bubbles in the liquid sight glass is no evidence of an insufficient charge. Flashing can be caused by a number of reasons, least of which undercharge.
If subcooling is 6k then the amount of refrigerant is still much, despite bubbles in the sight glass.

[Contactor]

hello

changed sight glass and refrigerant and system working well, so thanks to everyone.

[razor007]

remember r 404a is a blend if you lose a certain amount of vapour it will ruin the blend. reclaim the charge of 404a and put in r 507 and see how you go

[BLACKnGOLD4LIFE]

Bubbles in the liquid sight glass is no evidence of an insufficient charge. Flashing can be caused by a number of reasons, least of which undercharge.
If subcooling is 6k then the amount of refrigerant is still much, despite bubbles in the sight glass.

great point ..... most guys i know automatically ASSume vapor in the sight glass means low charge .. some ppl have even brainwashed the customer into thinking this ... this has led to quite a few calls ive encountered where a unit ended up bein grossly over charged by a diy'r trying to save a buck by clearing the glass.

[Contactor]

If a system is known to have worked well in the past, there is no noticeable temperature drop across components in the liquid line (by touch) and there is good sub cooling as in this case, would you say it is safe to rule out pre-expansion as a possible problem.

Or are there other causes i'm not aware of?

If a liquid line is suitable for a virgin blend and some of it has leaked, then this presumably could sometimes result in pre expansion?

Thanks

[Grizzly]

If a system is known to have worked well in the past, there is no noticeable temperature drop across components in the liquid line (by touch) and there is good sub cooling as in this case, would you say it is safe to rule out pre-expansion as a possible problem.

Or are there other causes i'm not aware of?

If a liquid line is suitable for a virgin blend and some of it has leaked, then this presumably could sometimes result in pre expansion?

Thanks

Hi Contactor.
I read the below info and saved it, you may find it interesting.

Why do bubbles appear in the sight glass when I use a blend? Does this mean I don't have enough refrigerant?
There are several reasons for bubbles in the sight glass. If one of the traditional refrigerants showed vapor in the sight glass it often meant there wasn't enough liquid refrigerant being fed to the valve, and more refrigerant was added to the system.
Blends could show flashing for the same reason, however, they can also flash when there is plenty of liquid in the receiver. Ironically, this liquid in the receiver could be causing the problem, particularly when the equipment is in a hot environment. Blends will come out of the condenser slightly subcooled - at a temperature below the saturated temperature of the blend at the existing high side pressure.
Yet when the blend sits in the receiver, it can "locally fractionate," or change composition slightly by shifting one of the components into the vapor space of the receiver. This will effectively produce a saturated liquid in the receiver, at the same pressure you had before, which flashes when it hits the expanded volume of the sight glass. In most cases these bubbles will collapse when the blend gets back into the tubing which feeds the valve, and the system will operate just fine.
Check other system parameters such as pressures, superheat and amperage to confirm whether you have the right charge. Don't rely solely on the sight glass.

Grizzly

[Contactor]

I had a feeling there would be more to it... thanks

Bearing that in mind, whats the best method of gassing up a new job running a blend, I usually charge to a full sight glass, periodically check the SH and SC and leave it at that. Most of my installs are designed by others - i'm never given a charge by weight to use and even if I did then it often wouldn't be much use because there is usually a difference between whats on the final drawing and as installed....

[Peter_1]

I had a feeling there would be more to it... thanks

Bearing that in mind, whats the best method of gassing up a new job running a blend, I usually charge to a full sight glass, periodically check the SH and SC and leave it at that. Most of my installs are designed by others - i'm never given a charge by weight to use and even if I did then it often wouldn't be much use because there is usually a difference between whats on the final drawing and as installed....

Start with a rough estimate of 1 kg gas/kW for a cooling application, and 0,75 kg/kW for a freezer and 0,5 to 0.3 kg/kW for an airco application.
You never can calculate it before.
If you have to add double on a small hermetic, then there's something wrong. 80% of the receiver is also a rough guideline for standard pre-assembled groups.

As soon you've reached design conditions and the SH is 5 or 6 K (measured on the evaporator and corrected for measuring fault) or as designed, then your evaporator is fully charged, even with a not full sightglass and you have theoretically enough gas in the system.

I demonstrate several times that bubbles in a sight glass disappears many times on small hermetics when you cover the condenser with a paper and they come back when you remove it again. Most can't explain why this happens. Explaining this will lead us to the upper left corner on a cycle on a log p/g chart and the associated thermal inertia of the components which provokes this.
Same phenomena when fans on a condenser are cycled on and off and bubbles appears when switching off the fans and they disappear some time (speaking in seconds) after switching off the fans again.

[muju]

HI try using pressure temp charts while charging.
got a few tips posted earlier at this forum.

Normal Suction Pressure Example
Refrigerant type R-404A

Return air temperature (box) 20 F

Suction pressure from gauge manifold 40 psig

Box temperature - 15 F = estimated evaporator temperature 20 - 15 = 5 F

Use P/T chart to convert evaporator temperature to pressure 38 psig

Compare estimated pressure to gauge pressure 40/38 close

Normal Discharge Pressure Example
Refrigerant type R-404A

Ambient air temperature entering condenser coil 90 F

Discharge pressure from gauge manifold 305 psig

Ambient temperature + 30 F = estimated condenser temperature 90 + 30 = 120 F

Use P/T chart to convert condenser temperature to pressure 310 psig

Compare estimated pressure to gauge pressure 305/310 Same

Condenser coil temperature runs about 30 F hotter than ambient air or ambient air runs 30 F cooler than the condenser coil to promote rapid heat exchange

[Aircondmech]

Bubbles in the liquid sight glass is no evidence of an insufficient charge. Flashing can be caused by a number of reasons, least of which undercharge.
If subcooling is 6k then the amount of refrigerant is still much, despite bubbles in the sight glass.

I was thinking this. Sight glass is for moisture indication ONLY.

Never charge off sight glass.

[marc5180]

Bearing that in mind, whats the best method of gassing up a new job running a blend, I usually charge to a full sight glass, periodically check the SH and SC and leave it at that. Most of my installs are designed by others - i'm never given a charge by weight to use and even if I did then it often wouldn't be much use because there is usually a difference between whats on the final drawing and as installed....

I came across one the other day that should have been a simple compressor change.

It was a packaged heat pump where the compressor needed replacing.

There was no indication on refrigerant charge even on the commissioning report there was no details.

I recovered the refrigerant and pressure tested the system for leaks which were fine and then recharged with virgin R407c with the same amount that i took out...4.3kg.

I ran the system up and DP was low at 13b and so i blocked part of the condesor coil to stimulate design conditions.

Discharge rose to 15 bar (40C SCT) and i proceeded to take the followig figures.

evap air on 22
evap air off 14/15
Subcooling 6k
superheat 18k
Suction pressure 4bar/0C
Discharge 15bar/40C

The sight glass was flashing and there were no restrictions in the liquid line.

All filters and coils were clean as well.

I added refrigerant 250 kg at a time until i had added nearly 2kg all the figures stayed the same apart from the superheat which dropped to between 12-14k.

However when i held the bulb of the TXV the coil flooded with liquid and the SH dropped dramatically.

The bulb was placed correctly and was tightly wrapped on the pipe.

Am i right in thinking this could well be the TXV causing the restriction? Possibly undersized orifice which was a TEZ 2 no5

[Gary]

However when i held the bulb of the TXV the coil flooded with liquid and the SH dropped dramatically.

This proves that

a. There is enough liquid at the TXV inlet to flood the coil and

b. Sufficient refrigerant is able to flow through the orifice to flood the coil.

Therefore, adjusting the stem should drop the SH.

[Peter_1]

Couldn't agree more with Gary. TEV can overflood, so you have enough liquid before it.

[marc5180]

Before I left I tried adjusting the stem 1 full turn and waited 20 mins but the superheat didn't alter so I turned it back to ts original position.
Is there a general rule of thumb as to how many degrees you increase/decrease superheat by one turn of the screw?

[Peter_1]

What brand is the TEV? Some need to turn CW and others CCW to open the valve more.

[marc5180]

It's a danfoss tez2.

[Peter_1]

turn it open completely (CCW) and then back (CW) 3.5 tot 4 turns CW must give you a SH of +/- 7 to 8K.
3 turns CCW will not harm the compressor.
If it then doesn't come OK, I should add a little bit refrigerant (200 gr) at once and see if SH and/or evaporating temperature increases.

Another point: where are you measuring LP and SH because your run on R407c which is a terrible gas to make errors on the readings.

[marc5180]

Thanks for the help Peter.

I'm measuring the superheat on the suction port which is about 12" from the compressor inlet and taking the temperature from the same place.

From the evap outlet to the compressor inlet is about 3foot.

[Lodiev]

Thanks for the help Peter.

I'm measuring the superheat on the suction port which is about 12" from the compressor inlet and taking the temperature from the same place.

From the evap outlet to the compressor inlet is about 3foot.

Always the best to take your suction pressure at the evaporator just before the bulp. Most evaporators have a Schroeder valve there these days.Then do the SH conversion to the chart and u are sure u have the right measurement.

[marc5180]

I did look to see if there was a schraeder port just after the evap but there wasn't one.

I normally do measure the temp close to the compressor inlet which I know is total superheat so I suppose I thought the 3ft of pipe from the evap to the compressor wouldn't increase the superheat that much.

[Lodiev]

3ft is not much your right, but having problems with the system it is always best to eliminate all the small add ups as far as possible.
Have u ever worked with Danfoss Adap cool electronic expansion valve systems witch uses the probe or transducer to calculate the SH on the coil?

[Lodiev]

3ft is not much your right, but having problems with the system it is always best to eliminate all the small add ups as far as possible.
Have u ever worked with Danfoss Adap cool electronic expansion valve systems witch uses the probe or transducer to calculate the SH on the coil?

[marc5180]

Have u ever worked with Danfoss Adap cool electronic expansion valve systems witch uses the probe or transducer to calculate the SH on the coil?

No i've never come across these.