Condenser tripping on high head pressure

[jakb21599]

I have a cooler that has 4 evaporators pulled by 1 compressor. they share one common suction and liquid line. The thermostat controls liquid line valve. when temperature is met valve closes and unit pumps down and cuts off on low pressure. the problem is this. the compressor is tripping on high head pressure before pumping down to shut unit off. I`m thinking the valve is not closing all the way or has trash in it because compressor suction pressure does not drop quick enough before head pressure raises and trips unit out. you can valve suction off at compressor and it will pull 20 inches hg in about 30 seconds. do the same with liquid line and get the same result. so the only thing left is the liquid line valve. the running suction pressure at setpoint temp is 60psi. head pressure is 245psi. sc 11 sh 33 on the pressure switch the cut in is set at 80psi and cut out is at 50psi Is their any thing else that would cause this other than liquid line valve ?

[Jeff Winsor]

How about if system overcharged and the reciever and condensor not large enough to absorb all the liquid?

[Grizzly]

I'm with Jeff on this one, Sounds like an overcharged system.
Or. Air in the system, choked liquid strainer or maybe partially?
Do your condenser fans continue to run on pump down?
Good Luck Grizzly.

[monkey spanners]

Sounds like the reciever is too small for the refrigerant charge.

If the liquid line solenoid is only partially closing as you suspect then i would think the cooling load would drop due to lower refrigerant flow and hence the amount of heat the condenser has to reject would drop also as would the head pressure. I wonder if the system has a partially blocked drier and has been overcharged to fill the sight glass?

Found a system that had been on pumpdown since install and had been tripping on hp every time it stopped. So much for commisioning checks!

[Magoo]

Hi Jakb.
is this a new installation, or has problem occured with an originally stable existing system.

[mikeref]

And what is the high pressure switch actually tripping at? (Kpa or Psi).

[Gary]

What are the condenser entering air temp and leaving air temp?

[jakb21599]

the system has been in place for 3 years. if it were overcharged. wouldn`t that show up in the sc and sh measurements ? i dont think the suction line filter is clogged as their is less than 1 degree temp drop across it. the customer has stated that the room will not keep temp (40F) the delta T for the condenser is around 28F with the site glass full you can still here the hissing sound at all the TXV valves on the evaporators which leads me to believe the liquid line valve maybe the problem, unless all the strainers on all the txv valves are clogged, but from what i have seen in the past is when the txv is clogged it will ice up. all these are not icing up. if the liquid line valve is partially clogged wouldnt their be a temp drop across it ?

[Gary]

you can valve suction off at compressor and it will pull 20 inches hg in about 30 seconds. do the same with liquid line and get the same result.

If the unit doesn't trip on high pressure when you manually close the liquid valve, then why would it trip on high pressure when the liquid line solenoid valve closes?

If it were overcharged or had an undersized receiver, manually closing the liquid valve would trip the high pressure control.

If your liquid line solenoid valve were leaking through, then the problem would be cycling after pumpdown. Is it cycling after pumpdown?

Are there any discharge line devices (muffler, check valve, etc.) that may be restricted?... or possibly a kink in the discharge line? Might the discharge line be undersized?

[Gary]

Since you haven't told us what refrigerant is in the system, I'll take a wild guess and say R404a.

[Gary]

the delta T for the condenser is around 28F

What are the entering and leaving air temps?

[Gary]

if it were overcharged. wouldn`t that show up in the sc

Not necessarily. It all depends on where you measure the liquid line temp. If the liquid line is in a hot area (say the condenser leaving air), then it will measure higher, leading you to believe that the subcooling is not excessive, when in fact it may be grossly overcharged. If you move down the liquid line to a spot that is in the ambient air, you get a much more accurate measurement.

[D.D.KORANNE]

Gas lock in receiver ....... Do you have any vent back to codenser from receiver ????

[jakb21599]

Not necessarily. It all depends on where you measure the liquid line temp. If the liquid line is in a hot area (say the condenser leaving air), then it will measure higher, leading you to believe that the subcooling is not excessive, when in fact it may be grossly overcharged. If you move down the liquid line to a spot that is in the ambient air, you get a much more accurate measurement.

this could be the problem then, cause i had the temp clamp on the liquid line inside the condensing unit. the entering air temp is 86f and leaving air temp is 116f. the refrigerant is 404A as you said. if the unit is over charged, why would all the txv valves be hissing like it was low on charge and the room cant get to temp.? the person that was thier before me probably added ***** and overcharged it then.
if it is over charged, a couple years ago i seen a guy recover 18lbs of refrigerant with out a recovery machine using the condensing unit. How did he do this ? My recovery machine is being replaced by manufactor and i wont get it back for another week.

[jakb21599]

If the unit doesn't trip on high pressure when you manually close the liquid valve, then why would it trip on high pressure when the liquid line solenoid valve closes?

If it were overcharged or had an undersized receiver, manually closing the liquid valve would trip the high pressure control.

If your liquid line solenoid valve were leaking through, then the problem would be cycling after pumpdown. Is it cycling after pumpdown?

Are there any discharge line devices (muffler, check valve, etc.) that may be restricted?... or possibly a kink in the discharge line? Might the discharge line be undersized?

no discharge line devices. the unit does trip on high head pressure when you close off liquid line at condenser. the suction side is very slow at pulling down to cut out setpoint while the high side constantly goes up till it trips on head pressure. I even moved the head pressure cut out to 475 psi from 360psi and still tripped out so i put it back to 360

[Tony]

if it is over charged, a couple years ago i seen a guy recover 18lbs of refrigerant with out a recovery machine using the condensing unit. How did he do this ?

Connect a hose from the liquid receiver service valve to a recovery cylinder.
Place the cylinder on scales.
Purge air out of the hose.
Mid seat the liquid receiver service valve.
Open cylinder valve and watch the scales go up.

Hope this helps.

[Gary]

In the initial post, you said the high side pressure was 245psi. I find this doubtful. I'm guessing the liquid line temp was at least 116F, being in the leaving air stream. If we add the 11sc, that makes the SCT at least 127F. I'm guessing you meant to say that the high side pressure was 345psi. This puts the TD at 42F over ambient, which is excessive. Since the dT is not excessive, that leaves overcharge.

I'm guessing overcharge.

[jakb21599]

In the initial post, you said the high side pressure was 245psi. I find this doubtful. I'm guessing the liquid line temp was at least 116F, being in the leaving air stream. If we add the 11sc, that makes the SCT at least 127F. I'm guessing you meant to say that the high side pressure was 345psi. This puts the TD at 42F over ambient, which is excessive. Since the dT is not excessive, that leaves overcharge.

I'm guessing overcharge.
The unit was overcharged. I removed 10lbs. Now it is not tripping on head pressure when in pump down. But i can not get the room temp below 42F. That is what it would keep the room at before i removed excess ***** to. The txv valves for each evaporator are still hissing. it was like this before i removed excess ***** and is still doing it now. ( is that normal ?) They want the room to be 35F It has been running all day with doors closed and no movement in or out and still wont get below 42F The cooler size is 28x23x13 with 4 Bohn WK075AC low velocity evaporators. the entering air into evaporator is 42F and exiting air is 32F. ( is this ok for the delta T for this type evaporator ?) coils are clean and still have OEM fans on them. The super heat at each evaporator is 12 degrees for all of them. AT the condenser the low side is 71 psi the high side is 318 psi. the sh is 30 and sc is 4 the outside ambient temperature is 100F the entering condenser air temp is 100F and exiting condenser temp is 129F So, with the info above. Do you see something that may be wrong as to why it will not get below 42F ? Maybe thats all it can do ? Am i missing something that i need to correct ? Any insight would be great, thanks for the help

[Gary]

Where did you measure the SC?

[jakb21599]

Where did you measure the SC?

on the outside of the case away from exiting fan air, like you had suggested to me in an earlier post.

[Gary]

The unit is now undercharged. Add refrigerant until the SC is about 15F.

Warm one of the TXV bulbs to see if the suction line temp will drop. If it does not drop, then there is a restriction. If it does drop, then the TXV needs to be adjusted.

[jakb21599]

The unit is now undercharged. Add refrigerant until the SC is about 15F.

Warm one of the TXV bulbs to see if the suction line temp will drop. If it does not drop, then there is a restriction. If it does drop, then the TXV needs to be adjusted.
I done this on all the txv valves and the temp drops from around 42F to about 28F in a few minutes on all of them. as far as the charge being low now. I`m confused. If i add any more to the system it will cut out on high head pressure during pump down, which is set at 425psi and cut out pressure is set at 48psi. I didnt have 15F SC when it was overcharged this morning before i removed excess *****. I checked it on the outside of the unit before i removed ***** and it was only 8F then. Why would this be ? the liquid line temp was around 104F outside and i checked it at all the evaporators and it was 103F it is around a150ft run from condenser to evaporators. the liquid line is 1/2 up to the evaporators and drops to 3/8 right before entering inlet of txv if this is of any info that might be needed

[Gary]

Hmmm...

318psi=122F SCT
Condenser air out is 129F

The SCT should be higher than the leaving air temp. I'm wondering if your temp tester might be out of calibration?

[mad fridgie]

Adjust TEV open valve reduce superheat setting.
You have indicated that you have removed 10lb, showing overcharge, you could have taken to much out making undercharge,

[Gary]

I done this on all the txv valves and the temp drops from around 42F to about 28F in a few minutes on all of them.

It is not possible for the suction line temp at the bulb to be higher than the temp surrounding it. IOW, if the surrounding air is 42F and the saturated suction temp (SST)is 30F, then it is not possible to measure more than 12F SH at the bulb.

Adjust the TXV's until the SH is about 8F.

[jakb21599]

Hmmm...

318psi=122F SCT
Condenser air out is 129F

The SCT should be higher than the leaving air temp. I'm wondering if your temp tester might be out of calibration?

i dont know if it matters but the condenser air out temp was taken at the top of the coil which was 129F when i moved the probe to the bottom the temp was 112F does it matter where at the temp is taken? top of coil....bottom of coil.... middle of coil ? my temp tester may be out of calibration as you said to. I will check that also

[mad fridgie]

Missed this one, 150ft liquid line, that could be quite a pressure drop (not knowing the size of the equipment), without sub cooling flashing is likely to occur, This is what i think happened, so add more refrigerant, lift head pressure, increase natural sub cooling, reduce flashing more stable superheat. Pump down no room for excessive charge. out on HP
Removed charge drop head pressure and SC, flashing occurs in liquid line, equalibrium is achieved with TEVs, reduced mass flow to evaps. (starving evaps)
Is the liquid line the right size, does it pass through warm areas, does it have many bends?

[jakb21599]

Missed this one, 150ft liquid line, that could be quite a pressure drop (not knowing the size of the equipment), without sub cooling flashing is likely to occur, This is what i think happened, so add more refrigerant, lift head pressure, increase natural sub cooling, reduce flashing more stable superheat. Pump down no room for excessive charge. out on HP
Removed charge drop head pressure and SC, flashing occurs in liquid line, equalibrium is achieved with TEVs, reduced mass flow to evaps. (starving evaps)
Is the liquid line the right size, does it pass through warm areas, does it have many bends?
I do not know if the liquid line is the right size.the 1/2 feeds all 4 evaporators yes it passes thru hot area ( up in the ceiling of the building) and has about 8 90 degree bends. the site glass at condenser is clear and then flashes small bubbles. i guess cause the txv valves are regulating

[Gary]

Missed this one, 150ft liquid line, that could be quite a pressure drop (not knowing the size of the equipment), without sub cooling flashing is likely to occur, This is what i think happened, so add more refrigerant, lift head pressure, increase natural sub cooling, reduce flashing more stable superheat. Pump down no room for excessive charge. out on HP
Removed charge drop head pressure and SC, flashing occurs in liquid line, equalibrium is achieved with TEVs, reduced mass flow to evaps. (starving evaps)
Is the liquid line the right size, does it pass through warm areas, does it have many bends?

If the coil floods when the bulb is warmed, then there is sufficient liquid at the TXV, despite the 150ft run.

[jakb21599]

If the coil floods when the bulb is warmed, then there is sufficient liquid at the TXV, despite the 150ft run.

so i need to check the calibration on temp probe. then check the sc again before i adjust the txv to 8F ? why would the txv valves still hiss when it was overcharged ?

[Gary]

If the coil floods when the bulb is warmed, then go ahead and adjust the TXV. We'll deal with the SC later.

[jakb21599]

If the coil floods when the bulb is warmed, then go ahead and adjust the TXV. We'll deal with the SC later.
ok. I will do this monday and give you the latest. Thanks for your help

[mad fridgie]

If the coil floods when the bulb is warmed, then there is sufficient liquid at the TXV, despite the 150ft run.
Incorrect there is "sufficient refrigerant mass flow"
a TEV is proportinal band control only, so even though there is highish superheat the valve is not fully open, by increasing the superheat (by placing the your hand on the bulb) you further open the valve, increasing the mass flow and the refrigeration effect. This likely to be a liquid vapour mix.(which is not ideal)
I did state that the valve was at equalibrium,

[Gary]

And I did state "sufficient" not "ideal", so perhaps we are saying the same thing.

[ks guy]

Where is your liquid line solenoid located on the system?

[texas64]

Can I recommend something? Try changing the filter drier...