R404a System Troubleshooting

[MechEng1]

Hi Everyone,

I'm cooling a bulk CO2 tank with an R404a refrigeration system. My suction pressure is too high (30 psig instead of 24 psig max) and my discharge pressure is too low (200 psig instead of ~235 psig). What could the problem be? I've heard it could be a bad compressor or bad valves, but this is a brand new unit. Could it be anything else?

Thanks

[Lowrider]

What's the suction temperature, Discharge temperature, Liquid temperature and setpoint of the unit?

You say it's a CO2 cooler, so what temperature do you need to cool the CO2 to?

[MechEng1]

I'd like to cool the CO2 to 2F (300 psig), but it's currently up near 340 psig, which off the top of my head is around 9F.

Here are all the numbers I have:

Ambient Temp - 69F
Suction Pressure - 28PSIG
Evap. Temp – 5.4F
Discharge Pressure – 200PSIG
Discharge Temp – 86F
Liquid PSI – 200PSIG (taken at reservoir)
Liquid Temp – 86F

[Lowrider]

with the numbers you gave, in SI
Ambient 20,55 dgr C, Suction pressure 1.93 bar, suction temperature -20.77 dgr C, discharge pressure 13.78 bar, discharge temp 30 dgr C, Liquid 30 dgr C.

Evaporation temperature is than -20,77 dgr C, condensing temperature is 31 dgr C.

That calculates to 0 K superheat and 1 K subcooling.

If the setpoint is -16.66 dgr C and the suction temp is -20, that's an approach of less than 4K, that's pretty good.

But that low subcooling and superheat lead me to believe undercharged!

Unless you didn't do the measurements but gave me the numbers from your refridgerant ruler?

[nike123]

Hi Everyone,

I'm cooling a bulk CO2 tank with an R404a refrigeration system. My suction pressure is too high (30 psig instead of 24 psig max) and my discharge pressure is too low (200 psig instead of ~235 psig). What could the problem be? I've heard it could be a bad compressor or bad valves, but this is a brand new unit. Could it be anything else?

Thanks

Bad (inefficient) compressor would have higher than normal suction pressure and lower than normal condensation pressure and lower current draw.
What kind of evaporator is that?
What kind of condenser?
Any liquid reciver?
Any heat exchanger in suction line?


Here is a list of the temps/pressures needed to troubleshoot a system:

Evap air in temp
Evap air out temp
Low side pressure or saturation temp
Suction line temp at evap outlet

Cond air in temp
Cond air out temp
High side pressure or saturation temp
Liquid line temp at receiver or condenser outlet

The more information provided the more accurate the diagnosis.

[guapo]

I think you have a bad compressor or leaking valve plate. To mentain the pressure in the bulk tank your suction should be 1bar(-30C).

[MechEng1]

Bad (inefficient) compressor would have higher than normal suction pressure and lower than normal condensation pressure and lower current draw.
What kind of evaporator is that?
What kind of condenser?
Any liquid reciver?
Any heat exchanger in suction line?

Evaporator: distributed copper tube
Condenser: 1.5hp (8450 btu/hr at -10F evap) Copeland semi-hermetic
Liquid Receiver: yes, with 17.2lb capacity
Heat exchanger in suction: yes


To add to what I said previously, the system was only charged with about 6lbs of R404a. Considering the capacity of the receiver, it looks like it probably is undercharged. I'm going to have the technician add a couple pounds, take measurements, and do that process a few times to see which direction the numbers are going.

[chemi-cool]

Broken discharge valve or a pinhole in it from liquid in the compressor.

You said "the system was only charged with about 6lbs of R404a"
was there a leak?

for that much amount of 404 missing, you should evacuate and recharge, not add.

[Kp3]

is orifice size correct? need to flood the coil with being static. are u getting frost back on suction? is the tank empty of co2?

[guapo]

What is the capacity of co2 tank (weight)?

[abet_meneses]

Check liquid passing over sight glass,bubbles indicate undercharge system.Also check comp amp draw.What is comp hp?

[mad fridgie]

No suction super heat, low discharge temp, I would say wet suction, faulty TEV, (if new check they have installed the orifice? have seen it done before!)

[mad fridgie]

Also check that the TEV bulb is strapped to the pipe

[old gas bottle]

i have worked on one over here,sounds like a simlar set up,the object is to stop the C02 blowing off is it not ?,this one had a 3/4 hp R134A on a quite a large tank,approx 5mtr x 2mtr round and only operated on a LP switch and still does which leads me to belive its not a exact science on theese things,however this ones about 20 years old.what i,am getting round to has it had a unit change and not a expansion valve to match,may still have a old R12/R134A valve burried in the foam pad inside the coil casing.:)

[MechEng1]

Ok - I finally have a little update:

I had the R404a evacuated and it's now recharged with approximately 16lbs. The pressure dropped from the tank's relief pressure of 357 psig down to 320 psig where it's holding steady. Ideally, it would drop down to 290 and the condensing unit would shut off until pressure reached 310 psig again, so there's still some kind of problem. The following are the current system measurements:

Ambient Temp: 73F
Suction Pressure: 18 psig (-18F equivalent temp)
Suction Temperature: 0F

Discharge Pressure: 200 psig (88F equivalent temp)
Discharge Temp: 90F

Liquid Line Pressure at Receiver Outlet: 205 psig (90F equivalent temp)
Liquid Line Temp: 85F


The following are answers to some of the other questions you all have posed:

Is it a bad compressor?: I'm having a refrigeration technician check on the current draw and voltage today. I'll report back when I get this info.

Tank Capacity?: 20,000 lbs liquid CO2

Compressor horsepower?: 1.5hp

Sight Glass?: No bubbles and indicates no moisture

Expansion Valve?: I'm using a Copeland 1 ton nominal capacity valve (HFE 1 RZ specifically). Orifice is installed.

TXV Bulb?: Secured to and insulated to evaporator outlet

Unit Change?: No, this is a completely new tank and refrigeration unit.

CO2 Fill?: 80% Full

Frost on Suction?: Yes, but given a measured 18F superheat, I don't believe that there's any liquid in the line.

Kp3: What do you mean about flooding the coil? How do you do it, and what does it accomplish?

[nike123]

Discharge Pressure: 200 psig
Liquid Line Pressure at Receiver Outlet: 205 psig What is height difference between condenser liquid line highest point and receiver outlet (at port where you measured pressure)? Is it around 10' and receiver is below condenser.

Is a condenser air or water cooled?


Cond air in temp
Cond air out temp


I had the R404a evacuated and it's now recharged with approximately 16lbs.
Did you recharged new fresh refrigerant in liquid phase?

[goshen]

hi mecheng1 do a simple test try to pump down the system this will give u a indecation of the compressores condition and valves. if this test goes ok then go on checking all of the ideas mentioned above.
this usualy saves a whole lot of time diagnosing the system.

[Gary]

Liquid Line Pressure at Receiver Outlet: 205 psig (90F equivalent temp)
Liquid Line Temp: 85F


Is the liquid line where you are measuring the temp located in a hot area, such as the hot discharge air from the condenser?

If it is, measure the liquid line away from the hot area, where it is in the ambient air flow.

90F - 85F = 5F Subcooling

If the liquid line temp is true (measured in a non-heated area) then the subcooling is low and more refrigerant should be added until the subcooling is 15F.

Once the subcooling is at 15F, if the superheat is still high then not enough refrigerant is getting through the TXV. If this is the case, warm the TXV bulb. If warming the bulb brings down the superheat, then the valve needs to be adjusted. If warming the bulb does not bring down the superheat, then the orifice is undersized or there is a liquid line restriction.

I see no indication of a compressor problem.

I would strongly suggest installing a sightglass at the TXV inlet to see if it is getting solid liquid. If it is, then its down to the TXV.

The bottom line is: There is not enough refrigerant feeding through the evaporator. Either not enough refrigerant is getting to the TXV or the TXV isn't feeding it through.

[MechEng1]

Nike123 -

The difference in height between the condenser liquid line and receiver outlet is minimal - probably about 1 foot.

The condenser is air cooled. The inlet temp of the condenser air would just be ambient, correct? I'm not sure what the outlet temp is. I'll have the technician take some readings today when he gets on site.

We did recharge with new liquid refrigerant.

Goshen -

I'll have the technician pump the system down today.

Gary -

I'm actually not sure where they're measuring the liquid line temp. I'll have to ask today when I get a chance (I'm not on site).

As far as adding refrigerant goes, there's about 17 lbs in the system, which should be plenty excess for the system. The receiver capacity is only 17.2 lbs.

I haven't tried the method of warming the txv bulb, but I'll give it a try today. The txv has a capacity of about 12000 btu/hr though and the condensing unit is rated at about 8450 btu/hr at this evaporation temp so it shouldn't be a problem, but I'm willing to try everything at this point.

There is a sight glass installed, and we are getting solid liquid.

You say there's not enough refrigerant getting into the evaporator, but the suction pressure is about right. Is it possible that there just isn't enough subcooling?

[Gary]

You say there's not enough refrigerant getting into the evaporator, but the suction pressure is about right. Is it possible that there just isn't enough subcooling?

What makes you think the suction pressure is "about right"?

What is the rated TD of your evaporator?

[Gary]

Condenser: 1.5hp (8450 btu/hr at [-10F evap)

How many btu/hr do you suppose you are getting at -18F evap?... a lot less than 8450.

Suction pressure "about right"?... I don't think so.

[nike123]

Nike123 -

The difference in height between the condenser liquid line and receiver outlet is minimal - probably about 1 foot.


If that is the case, than pressure measured at compressor discharge could not be lower than pressure measured at receiver outlet.


Discharge Pressure: 200 psig
Liquid Line Pressure at Receiver Outlet: 205 psig Either your measurements are wrong or you using two different uncalibrated instruments, or you made mistake about what is what pressure.

Pressure is highest at compressor discharge valve and always drops to the compressor suction pressure as restrictions in circuit are followed. Only way that pressure in some point could be higher than previous point/points is if there is liquid column or liquid pump before that point in circuit.

Check your measurements again!


The condenser is air cooled. The inlet temp of the condenser air would just be ambient, correct?
Correct (if there is no recirculation of condenser discharged air)!



I'm not sure what the outlet temp is. I'll have the technician take some readings today when he gets on site. Every time you make measurements in different time period you should take full set of measurements again.

[MechEng1]

How many btu/hr do you suppose you are getting at -18F evap?... a lot less than 8450.

Suction pressure "about right"?... I don't think so.

Well, the condensing unit is rated at 8450 btu/hr at 90F ambient and -10F evap, which is what I was shooting for (suction pressure of 24.6 psig. It was set to 24 psig yesterday when the technician was there)

I assume "TD" is temperature difference between the inside tank temperature and evap temperature? Normally, the CO2 should be 2F and 300 psig, which would give a TD of 12F with a -10 evap temp. Right now, though, the tank is at about 6F, giving a TD of 16F.

Is it possible that everything is working correctly, but the evaporator is simply too small?

Regarding the weird pressure and temperature measurements, I don't know what say. I'm just posting what the technician is giving me, but I agree they don't make sense.

[Gary]

Well, the condensing unit is rated at 8450 btu/hr at 90F ambient and -10F evap, which is what I was shooting for (suction pressure of 24.6 psig. It was set to 24 psig yesterday when the technician was there)

I assume "TD" is temperature difference between the inside tank temperature and evap temperature? Normally, the CO2 should be 2F and 300 psig, which would give a TD of 12F with a -10 evap temp. Right now, though, the tank is at about 6F, giving a TD of 16F.

Is it possible that everything is working correctly, but the evaporator is simply too small?

If that were the case, then the evaporator would be overloaded. It is currently underloaded.

[Gary]

It was set to 24 psig yesterday when the technician was there)


What does that mean? How did he "set" the pressure?

[MechEng1]

What does that mean? How did he "set" the pressure?


He adjusted the expansion valve until the suction pressure reached 24 psig.

[Gary]

I assume "TD" is temperature difference between the inside tank temperature and evap temperature? Normally, the CO2 should be 2F and 300 psig, which would give a TD of 12F with a -10 evap temp. Right now, though, the tank is at about 6F, giving a TD of 16F.


Imagine how difficult it would be for you to calculate that TD if you were given the CO2 temperature one day and given the saturation temperature the next day under different conditions. It would be meaningless, right?

You are giving us piecemeal measurements taken at widely different time periods under different conditions and expecting us to be able to tell what's wrong with the system.

Is it so difficult to measure all of the temps/pressures at roughly the same time?

[Gary]

He adjusted the expansion valve until the suction pressure reached 24 psig.

Expansion valves are adjusted by superheat, not suction pressure. What is the superheat?

[mad fridgie]

You may have some non condensables in your CO2, this can effect the balance on your evap, do you have a vapour purge of the top of the CO2 tank and or the top of your evap

[Kp3]

i mean u need to flood the evap with liquid from ur tev. I suspect the orifice is in correct i would try a bigger size. what kw valve are u looking for on the cond unit? I do alot of these tanks that go all over the uk.

[MechEng1]

Problem Solved!!!

The TXV was connected to the evaporator outlet and the compressor was connected to the evaporator inlet. After the switch, the tank pressure dropped almost immediately. Regardless, thanks for all your help guys!