New system tripping on low pressure??

[DTLarca]

Are you sure this unit is designed for cooling ambient air? Is it a fresh air unit? What is the design ambient temperature? I think that for your region, it could have been designed for 42°C ambient, and now it's working in ambient of 20° with a room of 20°C, so it's over cooling... and freezing over, of course.

I just noticed your post. You are probably right.

The subcool will increase as the ambient rises to design. It usually then decreases again when ambient exceeds design.

The superheat is high because the head pressure is too low but the superheat is also not so high because the evaporator air on temperature is also low. Raise the head pressure to design levels and we will see the superheat crash. This would confirm a low load condition which would be either because of low air flows or low air temperatures or both.

[Willsmithiraq]

Guys,

Thanks for all the info you've replied, I shall be investigating the airflow over the next few days and as Gary said will get back to you all with some pictures, which may help clear this problem up.

Really appreciate all your posts, as it's not fun being stuck on your own out here with nobody to help you!!

[mad fridgie]

There is nothing wrong with operation of the refrigerant plant per say, TXV is doing what it should do. Your problem does not lie here. Because you have extreme problems with both heat and cooling, this does indicate either lack of air flow (for what ever reason), or the incorrect coil selection.
So lets look athe air flow first.
Check fan moter rotation against blade/wheel rotation, fix if wrong!
Test either way.
Remove inlet and outlet duct from indoor coil, no air restrictions, all works well, then theres lies your problem " Ducting",
If still a problem (in heat and cooling)
then manufactures have F***** UP, wrong coil/fan configuration.
After what NNN said about the manufacturer, I would not surpise me if they have designed the coil based upon steam or hot water!
Remember when testing to test both heat and cool (lower suction pressure would be expexted just because you load is quite small any way)

[DTLarca]

If the system is for fresh air supply only then you might find the compressor circuit should not be running below say 28°C air on temps.

The TEV capacity will increase as the ambient increases even though when the ambient increases so does the air on to the evaporator, because they are the same thing. This is because the pressure differential across the from high to low will increase with the curvature of the saturation curve.

So my bet is that head pressure control with desuperheated hot gas injection will be required. If there is no side injection point on the TEV distributor for the hotgas then injecting hot gas into the suction will probably be the best solution.

The superheat being 8K at the current conditions indicates a higher than required air flow exists right now. So the most likely problem is simply a low ambient and low load combination.

[NoNickName]

If there is no side injection point on the TEV distributor for the hotgas then injecting hot gas into the suction will probably be the best solution.

That way would not prevent the coil from freezing, but only the compressor. I would suggest to inject hot gas between the TEV and evaporating coil.

[DTLarca]

That way would not prevent the coil from freezing, but only the compressor. I would suggest to inject hot gas between the TEV and evaporating coil.

It's not the hotness of the hotgas that prevents freezing. It's that it raises the saturated suction pressure. So injecting into either point works. After the TEV just improves oil return from there.

But including an EPR along with head pressure control and desuperheated hot gas injection would be good too.

Usually the distributor tubes into the evaporator have to be increased a size when hot gas is injected after the TEV.

[NoNickName]

So injecting into either point works.

mmmmhhh what about the equilibrium line and the bulb in respect to the injection point?

[DTLarca]

Here are two illustration Courtesy of Sporlan (Parker).

[NoNickName]

Well, so be it.
I still think the injection point would be better placed ahead of the evaporator, but it is MHO.

[DTLarca]

Well, so be it.
I still think the injection point would be better placed ahead of the evaporator, but it is MHO.

I agree. This is the preferred method if the evaporator is located near the condenser as in packaged equipment.

But if the evaporator is a distance from the condenser then there can be condensation along the the longer hot gas injection run. The effect is worse if the hot gas valve is located near the evaporator but can also be a problem when it is installed nearer the condenser. if it has to be done then nearer the condenser is preferable.

[r.bartlett]

Well, so be it.
I still think the injection point would be better placed ahead of the evaporator, but it is MHO.

This would be my prefered methos as it gives the hotgas chance to cool down. I have seen many with the HG line at the condenser (for cost) which causes high suction temps at the compressor

[DTLarca]

This would be my prefered methos as it gives the hotgas chance to cool down. I have seen many with the HG line at the condenser (for cost) which causes high suction temps at the compressor

When we're injecting hot gas into the suction the only concern regarding temperatures is superheated discharge temperature. Having a warmer suction vapour means it is more rarefied which means the mass flow is also reduced which helps keep saturated suction temperatures up. Comressor cooling is more dependenat on refrigerant mass flow than on refrigerant temperature. Many rotary type compressors actually have their motors located in the discharge stream.

When we use an EPR we are injecting hot gas to maintain compressor cooling by maintaining mass flow through the compressor.

The desuperheating valve is adjusted, superheat, to maintain safe superheated discharge temperatures. This is easily achieved with any of the possible configurations.

The other advantage with injecting hot gas after the TEV is good refrigerant distribution is kept up even at the low load conditions.

[Gary]

I never managed to get to the point of taking temps in heating mode, as the unit trips out on HP within 20 seconds or so. All I can say is that in heating I've felt the heating TXV and there is definately temperature difference across it, also there is cold air coming out of the condensing unit.

As the evap is now the condsensor in heating with the lack of capacity the unit trips that quickly it's like a complete condensor fan failure situation.

I'm thinking there might be an important clue here. For the unit to trip on HP that quickly, there would have to be zero airflow... or perhaps a restriction between the compressor discharge and the (indoor) condenser coil. Maybe at the coil connection?

Since this line is the suction line in cooling mode, a restriction at the coil connection would explain everything.

[Willsmithiraq]

Gary,

Not sure if I've previously said in this thread. But after the problems I had I cut both connections to the evap and blew nitrogen through from either direction at high pressure. This didn't appear to dislodge anything from the evap coil.

Going back to a comment you made earlier about one of the legs of the distributor being blocked. When the unit was operational, I had my head inside the unit and felt each leg and they "appeared" to be of equal temp going into the evap (something that I will get a probe on next time I get to the unit). Also the ice that was forming seemed to be forming equally across the coil.

[Gary]

Suction 70 psi going down to 50 psi which is the cutout
Discharge 285 psi

Liquid line temp at TXV 31 C
After TXV -2 C
Exit evap - 9 C

Subcooling 4 C
Superheat - None??

Air on cond - 20 C
Air off cond - 36 C

Air On Evap - 20 C
Air Off Evap - 9 C

Your use of minus signs is confusing me... beyond my usual level of confusion.

[Gary]

I'm wondering how the air temp leaving a frosted coil can possibly be 9C?

[Willsmithiraq]

Your use of minus signs is confusing me... beyond my usual level of confusion.

Originally Posted by Willsmithiraq
Suction 70 psi going down to 50 psi which is the cutout
Discharge 285 psi

Liquid line temp at TXV 31 C
After TXV - 2 C (minus 2C)
Exit evap - 9 C (minus 9C)

Subcooling 4 C
Superheat - 1K @ 70 psi - 8K @ 50 psi

Air on cond + 20 C
Air off cond + 36 C

Air On Evap + 20 C
Air Off Evap + 9 C

Sorry!!! Hope this clears it up!!

[Gary]

In any case, the fact that the coil frosted evenly is proof positive that the air as it immediately leaves the coil is below 0C, which means this is very definitely an airflow problem.

[DTLarca]

In any case, the fact that the coil frosted evenly is proof positive that the air as it immediately leaves the coil is below 0C, which means this is very definitely an airflow problem.

In the last month I have designed two systems each with an effective coil dew point temperature between 7°C and 9°C. The air flow for the first system, m³/kW, was 4 times that for the second system. The first system has a fresh air requirement for 120 people in a confined space and the second gave no fresh air. To meet the cooling loads in the first instance the air only needed to be cooled from 24°C to 21°C. The second system was designed for air on at 22°C and off at 10°C. The first system, even though the air was only being cooled from 24°C to 21°C, did anyhow have a higher sensible heat ratio - it had a higher moisture removal per kW than the second system.

Fresh air systems do not normally have the same Delta-T vs. Approach relationships as comfort cooling.

[DTLarca]

[quote=DTLarca;208986]The first system, even though the air was only being cooled from 24°C to 21°C, did anyhow have a higher sensible heat ratio - it had a higher moisture removal per kW than the second system.[quote]

Sorry - big mistake there - lower sensible heat ratio - not higher - the 24°C -> 21°C fresh air system with fresh air pre-cooling by enhalpy exchanger and pre-cooling coil had a lower sensible heat ratio at the final cooling coil than the comfort coiling system.

[Gary]

In the last month I have designed two systems each with an effective coil dew point temperature between 7°C and 9°C. The air flow for the first system, m³/kW, was 4 times that for the second system. The first system has a fresh air requirement for 120 people in a confined space and the second gave no fresh air. To meet the cooling loads in the first instance the air only needed to be cooled from 24°C to 21°C. The second system was designed for air on at 22°C and off at 10°C. The first system, even though the air was only being cooled from 24°C to 21°C, did anyhow have a higher sensible heat ratio - it had a higher moisture removal per kW than the second system.

Fresh air systems do not normally have the same Delta-T vs. Approach relationships as comfort cooling.

Sorry, but I'm just not understanding your point.

[Gary]

[QUOTE=DTLarca;208988][quote=DTLarca;208986]The first system, even though the air was only being cooled from 24°C to 21°C, did anyhow have a higher sensible heat ratio - it had a higher moisture removal per kW than the second system.

Sorry - big mistake there - lower sensible heat ratio - not higher - the 24°C -> 21°C fresh air system with fresh air pre-cooling by enhalpy exchanger and pre-cooling coil had a lower sensible heat ratio at the final cooling coil than the comfort coiling system.

And I still don't get it.

[DTLarca]

In any case, the fact that the coil frosted evenly is proof positive that the air as it immediately leaves the coil is below 0C, which means this is very definitely an airflow problem.

What I am saying is that for my high fresh air percentage system that was designed with an air on temperature of 24°C and an air off temperature of 21°C and what we call an apparatus dew point temperature of 7°C having a SHR of 0.68 I will have a frosty coil when my return air temperature drops to 14°C at which point my air off temperature will be 12.2°C. My air off temperature does not have to also be below freezing for the coil to freeze.

The bypass factor means there is also a contact factor. The contact factor is the portion of the air volume that leaves at coil temperature. The bypass factor is the portion that leaves, after having made no contact with the coil, remaining at return air temperature. These two air volumes mix leaving the coil to give you your air off temperature always somewhere above the coil temperature.

Now, if you had 100% contact then off course the air would leave at exactly the same temperature as is the coil. But of all the system types one can design fresh air system possibly have the highest by pass factors or the lowest contact factors.

I hope my point is clearer now

[Gary]

Okay7... I understand this. Similar to a face/bypass system. That would be an enormous bypass factor... and that's why I questioned whether there might be bypass around the coil earlier.

[Gary]

So... if I am understanding you correctly, possibly the coil has a large bypass factor and is working properly... doing exactly what it is designed to do... shutting down the compressor in mild weather and just bringing in fresh air.

This brings us to the heat mode problem. The large bypass factor is going to be a big problem in heat mode.

[DTLarca]

Okay7... I understand this. Similar to a face/bypass system. That would be an enormous bypass factor... and that's why I questioned whether there might be bypass around the coil earlier.

The bypass factor in this case was (21-7)/(24-7) = 0.82 or 82% of the air makes no contact with the coil.

This is typical of the school ventilation systems we do - particularly in the new and ever more popular Academies.

[lma]

Your system is designed for R410a... try increasing suction pressure to between 100-110 psi .. you head pressure should be .. in the region of 400 - 420 psig ... see if this achieves desired effect

[Gary]

The bypass factor in this case was (21-7)/(24-7) = 0.82 or 82% of the air makes no contact with the coil.

This is typical of the school ventilation systems we do - particularly in the new and ever more popular Academies.

Interesting... a built in bypass... clever

[Gary]

Hmmm... if we can confirm that the airflow is correct and as DTLarca suggests, this is a high bypass factor coil (fin spacing, number of rows?), then the only way this can be made to run in cooling mode in mild weather would be to raise high side pressure (and therefore low side pressure) by reducing airflow through the outdoor unit fan.

In order for it to work properly in heat mode, we would need to drop the low side pressure (and therefore the high side pressure) by again reducing the airflow through the outdoor unit fan.

Sure would be nice to have the design information on this system.

[DTLarca]

So... if I am understanding you correctly, possibly the coil has a large bypass factor and is working properly... doing exactly what it is designed to do... shutting down the compressor in mild weather and just bringing in fresh air.

That's it

This brings us to the heat mode problem. The large bypass factor is going to be a big problem in heat mode.

It may be designed to deliver heat to 0°C fresh air when taking heat from 0°C ambient air. In which case giving it 20°C and 20°C as is happening now would be the equivalent to giving a normal home AC heat pump system 40°C air on indoors with and outdoor temperature of 25°C. Of course the system will trip on head pressure - it has no capacity control.

The outdoor fan needs speed control for these conditions both for head pressure control in cooling and then for suction pressure control in heating.

Or simply choose to neither heat nor cool in these conditions but rather up or below at the more extreme conditions.

[Gary]

In the meantime, until proven otherwise, I'm sticking with the insufficient airflow theory.

[DTLarca]

In the meantime, until proven otherwise, I'm sticking with the insufficient airflow theory.

The odds are against you

The superheat, the evaporator approach and the subcool should all be low if it was an evaporator air flow problem

[Gary]

The odds are against you

The superheat, the evaporator approach and the subcool should all be low if it was an evaporator air flow problem

Also, the outdoor (condenser) TD indicates a normal load.

I may come around to your way of thinking, yet. Maybe knowing the fin spacing and number of rows will settle this.

[DTLarca]

Also, the outdoor TD indicates a normal load.

Good point - I forgot all about the condenser situation apart from the subcool.

But hang on - 285psi is about 33.5°C saturated condensing temperature. The air leaving is warmer at 36°C? They probably weren't taken at the same time but if I had to go on those readings I would say the condenser air flow is too low by far. The leaving air is being exposed to the superheated tube rows long enough to leave above saturated condensing temperature.

The TD can be brought down significantly with an increase is air flow. The subcool would increase slightly too as we turn more vapour to liquid with the increased air flow.

I may come around to your way of thinking, yet. Maybe knowing the fin spacing and number of rows will settle this.

All we need is the dry and wet bulb temperatures of the air on and off and we can determine the apparatus dew point temperature and then the current bypass factor.

[mikeref]

There is nothing wrong with operation of the refrigerant plant per say, TXV is doing what it should do. Your problem does not lie here. Because you have extreme problems with both heat and cooling, this does indicate either lack of air flow (for what ever reason), or the incorrect coil selection.
So lets look athe air flow first.
Check fan moter rotation against blade/wheel rotation, fix if wrong!
Test either way.
Remove inlet and outlet duct from indoor coil, no air restrictions, all works well, then theres lies your problem " Ducting",
If still a problem (in heat and cooling)
then manufactures have F***** UP, wrong coil/fan configuration.
After what NNN said about the manufacturer, I would not surpise me if they have designed the coil based upon steam or hot water!
Remember when testing to test both heat and cool (lower suction pressure would be expexted just because you load is quite small any way)

May i draw your attention back to this post. Would it not be reasonable to suggest the lack of air flow theory is due to restrictive ductwork or shortage of registers? The air volume has been given but not measured at the registers to confirm flow rate..

[Gary]

I consider delta-T to be the best indicator of airflow. In this case 36C-20C=16K dT. I would consider this (borderline) sufficient.

The cond TD would be 33.5C-20C=13.5K TD, which would indicate a normal load, given sufficient airflow.

As you point out, the SCT should be higher than leaving air. This could be due to the timing of the measurements or instrument error.

In any case, if there is insufficient heat load picked up on the low side, we should expect insufficient heat load to be rejected on the high side (low TD).

[Gary]

As you point out, the SCT should be higher than leaving air. This could be due to the timing of the measurements or instrument error.

Gauge calibration is especially difficult in moutainous regions as elevation has a profound affect. One good way to calibrate is to strap a temp sensor to the side of a refrigerant jug and calibrate the gauges to the temp indicated.

Zeroing the gauges is only accurate at sea level.

[NoNickName]

The deltaT or airflow theory is slippery IMHO. The acceptable range of both can only be demonstrated against the actual front surface and rows of the exchange coil. Without actual data sheets and coil selection sheets it would be impossible to determine if that is the case.
I see the experience would lead elsewhere, but I think we have to stick to what we know.
In the published data of the OP, the only anomaly, again IMHO, is the evaporating temperature.
For a A/C system, a negative evaporating temperature is not acceptable, everything else is disputable.
A negative evaporating temperature can only be caused by
1) insufficient exchange surface.
2) excess capacity

1) would be systematic and unrepairable. It would be a design fault.
2) can be caused by a number of reason, that have been showed in this same thread.

I would start from there for a proper diagnosys.

[nevgee]

I'll eat my hat on this one.... while you're chugging about bypass factors and turbulent flows in heat exchange processes, this poor guy is wanting a practical solution to a real problem. Pontificating and feather fluffing doesn't help him.

Keeping the theory to the class room ..... I think he needs to take full details of the evaporator, rows, tubes, fpi, face area and go back to the manufacturer to get them to double check he has a matched set of condensing unit and evaporator.
Either the cu has too high a capacity for the job or the evap is too small a duty for the cond unit.

If it turns out the evap is too small then most likel y he will need to add some form of capacity control. Hot gas by pass being one option to maintain a higher back pressure. Injection into the inlet of the evap coil being a better option but does not exclude the injection at the evap outlet. both ways work well if set up properly.

However, there are other complications if he is trying to use this system as a heat pump /reverse cycle arrangement. other considerations need to be thought thru.

He should really take up the fight with the supplier/ manufacturer who really are responsible for the design and mods that may be needed to correct the problem.

OK guys ...... go back to the theory . . battle on!

[mad fridgie]

First thing to remember is that faults occur in both Cooling and heating, all idications is that there is controlled flow of the refrigerant.
measured temperature differences of the air (indoor) maybe mis leading, if the coil temp is -9C (SST) then ice formation will occur on the fins stopping heat transfer futher, showing a higher air outlet temperature than what would be expected.
Of course capacity control maybe required if the unit is designed for high ambient conditions, but this not where the present problem lies. HEATING fails within 5 minutes.
So what dos this show incorrect airflow or incorrect indoor design (this being the case I bet the coil is only and 0ne or two row coil, as designed for high temperature working mediums)

[mad fridgie]

Nevgee, is sort of right, but prior to going to the manufacture, you need test the unit under free airflow conditions, remove any installation potential problems. DIS CONNECT DUCTING

[mikeref]

Some pics would go down well. What about positive pressure in the kitchen area?

[Gary]

Some pics would go down well. What about positive pressure in the kitchen area?

Good point. If there is no means of exhausting kitchen air then the flow will be reduced... like blowing into a bottle.

[Gary]

Call the manufacturer?... where's the fun in that?

[nevgee]

Good point. If there is no means of exhausting kitchen air then the flow will be reduced... like blowing into a bottle.

Most likely their doors don't fit too well and I recon the're will be loads of gaps and spaces where air will pass thru, assuming the ****roaches haven't occupied them all

[DTLarca]

Gauge calibration is especially difficult in moutainous regions as elevation has a profound affect. One good way to calibrate is to strap a temp sensor to the side of a refrigerant jug and calibrate the gauges to the temp indicated.

Zeroing the gauges is only accurate at sea level.

Great point. At higher altitudes the superheat readings are lower than actual and the subcool readings are higher than actual.

[mad fridgie]

Great point. At higher altitudes the superheat readings are lower than actual and the subcool readings are higher than actual.

Bull shyte, temperatures do not change with altitude, what changes is gauge calibration, which is pressure. Alltitude also effects mass flow of given from a fan, and heat transfer coiefficients on heat transfer. There is no problem in this thread with refrigerant control!!!!

[nevgee]

Call the manufacturer?... where's the fun in that?

Well perhaps you're right ... but this guy is on a commercial contract ... he has to get a system running, keep client happy and a reputation to maintain. If he fixes this problem, by any means, the client will be pleased. If on the other hand he pisses about fannying over theoretical issues of flow and bypass factors etc, the client ain't going to be very happy.

In a real world Gary, the pressure is on when some one has paid for a system and it isn't working as specified. They don't care about theories they want results, now , not whenever.

I feel for this guy, he needs some help to get him or the system out of the mire. He'll be reading all of these posts and be getting lost with the runaround, he needs to be offered sound practical advise that he can act on. If he were in an R & D shop then it would be different I'm sure.

[nevgee]

Bull shyte, temperatures do not change with altitude, what changes is gauge calibration, which is pressure. Alltitude also effects mass flow of given from a fan, and heat transfer coiefficients on heat transfer. There is no problem in this thread with refrigerant control!!!!

well said

[nevgee]

..... assuming the ****roaches haven't occupied them all

Roaches? Spliffs? Doobies? I am suprised