Maintenence

[marc5180]

Yesterday i was doing a routine maintenance visit to one of the sites that we look after and i decided to use my new digital gauges and measure superheat and subcooling to see if the system was running ok.
There are 6 daikin condensors that each feed a cooling coil on seperate Air Haundling units, i switched the controllers onto manual cooling which brought on the condensors.
On one of the condensors that i connected my gauges to i got the following readings
suction pressure- 2.52 bar -10.2degC
Superheat was 18K

Liquid pressure- 10.5bar 28degC
subcooling was 7K.

Why was the suction pressur so low was it because there wasn't much if any load?
and also the subcooling seems ok but the superheat is high, would the lack of load alter the SH/SC?

If not then it looks like there is either a restriction or the TXV isn't open enough, am i right?

[nike123]

Why was the suction pressur so low was it because there wasn't much if any load?

Could be low load, and/or insufficient air flow (dirty air filter, low fan speed).

and also the subcooling seems ok but the superheat is high, would the lack of load alter the SH/SC?

That is not evaporator superheat, that is total superheat!
Depending on pressure drop thru lines and heat gain it could be OK as well as not OK.

[marc5180]

This is where i get confused, what shoud i be measuring because i thought i should be looking at total superheat, to check if this system is performing ok.
The filters were clean, they had just been replaced and the coils were clean so air flow shouldn't be a problem.

Iv been told to work off SC of around 5-10K and the same with SH 5-10K but if this is the case then my SH is high meaning a restriction?

Depending on pressure drop thru lines and heat gain it could be OK as well as not OK.

This is where it still gets confusing for me. What is the point in taking SH and SC readings if once you have the readings they still dont help you diagnose the system. Is there something that i am missing?

[paul_h]

I don't know.
While it's good you are learning (measuring superheat and sub cooling), and sharing with us, you never mentioned ambient temp, or evap air on or off, or condenser air on or off temps on this case.
To diagnose a system, you need all the data like this, and sometimes more.

[marc5180]

I don't know.
While it's good you are learning (measuring superheat and sub cooling), and sharing with us, you never mentioned ambient temp, or evap air on or off, or condenser air on or off temps on this case.
To diagnose a system, you need all the data like this, and sometimes more.

Sorry, ambient temp was 16degc
Evap air on was 14 degC
Evap air off was 9 degC
Condensor air on was 16 degC
Condensor air off i could not get because it had a plenum bolted to the condensor which took the air outside (the condensors are in plant room)

Now by looking at these i imagine that you are going to say that everything is working fine but i know this. I just wanted to know why my guage readings were so low and my superheat high if everything is ok?

[nike123]

This is where i get confused, what shoud i be measuring because i thought i should be looking at total superheat, to check if this system is performing ok.
The filters were clean, they had just been replaced and the coils were clean so air flow shouldn't be a problem.

You still could have problem with total evaporators capacity (if there is more then one on one condenser) and air flow (lower speed then max).
You need to be sure that you have 100% evaporator capacity (nominal) for that condensing unit, fan speed of evaporators at maximum, compressor at 100% speed and condenser fans at regulated speed for optimal condensation. Then you could make your measurements.

Iv been told to work off SC of around 5-10K and the same with SH 5-10K but if this is the case then my SH is high meaning a restriction?

Your subcooling is good.
Your superheat in VRV systems (if I correctly understand) is regulated by EEV and it is affected by control logic of system. So you need to force indoor units to operate at 100% capacity to get superheat of about 5K at evaporator (where it is controlled). At compressor, superheat is affected with all superheat of indoor units connected at that outdoor unit and with speed of compressor.

[nike123]

Sorry, ambient temp was 16degc
Evap air on was 14 degC
Evap air off was 9 degC
Condensor air on was 16 degC
Condensor air off i could not get because it had a plenum bolted to the condensor which took the air outside (the condensors are in plant room)

Now by looking at these i imagine that you are going to say that everything is working fine but i know this. I just wanted to know why my guage readings were so low and my superheat high if everything is ok?

These conditions are not good for taking measurements, because they are outside of operational range of unit.
Evaporator air on is too low for air conditioning.

[nike123]

Sorry, ambient temp was 16degc
Evap air on was 14 degC
Evap air off was 9 degC
Condensor air on was 16 degC
Condensor air off i could not get because it had a plenum bolted to the condensor which took the air outside (the condensors are in plant room)

Now by looking at these i imagine that you are going to say that everything is working fine but i know this. I just wanted to know why my guage readings were so low and my superheat high if everything is ok?

Your superheat is high because EEV controls capacity by raising superheat and, by that, lowering evaporator efficiency and capacity.

[marc5180]

Y
Your superheat in VRV systems (if I correctly understand)

It isn't a VRV system just 1 condensor feeding one cooling coil

[marc5180]

These conditions are not good for taking measurements, because they are outside of operational range of unit.
Evaporator air on is too low for air conditioning.

So i should have blocked part of the condensor to raise liquid/discharge pressure before i took the readings?

[nike123]

It isn't a VRV system just 1 condensor feeding one cooling coil

It could save me lot of translating and typing if I have read that post carefully.

[nike123]

So i should have blocked part of the condensor to raise liquid/discharge pressure before i took the readings?

Yes, keep condensation at around 40°C!
Also, indoor ambient temperature should be at least 18°C.
Check TEV functioning by heating bulb with hand.

[marc5180]

So before taking superheat and subcooling readings i need to be sure
1) SCT is around 40DegC
2) Indoor ambient temperature is above 18DegC
Is there anything else?

[marc5180]

Also i could only take the compressor inlet superheat not the evaporator superheat because there were no schrader ports near the txv only near the compressor, this is mostly the case with systems that i work on.
Should i be looking at something else before i start looking at compressor superheat?

[nike123]

So before taking superheat and subcooling readings i need to be sure
1) SCT is around 40DegC
2) Indoor ambient temperature is above 18DegC
Is there anything else?

Air quantity across evaporator, but you say that you already checked that.

[nike123]

Also i could only take the compressor inlet superheat not the evaporator superheat because there were no schrader ports near the txv only near the compressor, this is mostly the case with systems that i work on.
Should i be looking at something else before i start looking at compressor superheat?

You should calculate pressure drop (and heat gain) at pipes to be sure that you have enough subcooling at TEV when conditions are "high load high ambient" and to be able to calculate "real" superheat at evaporator.
You have pipe pressure drop and heat gain calculator in CoolPack software.
This is example of suction pipe calculation of pressure drop and change in superheat thru pipe:



You see in this example how you could get 13,5K superheat if you reading pressure at compressor inlet and that, in fact, could be 5K superheat at evaporator which is OK. Also, with this tool you could evaluate if you have any restriction in pipes.

[gloucesterfridg]

Air on at 14 deg's makes this all pretty pointless. There should be some form of 'free cooling' stat to prevent the condenser running below 18.c

Forget it and come back in June...

[marc5180]

Also, with this tool you could evaluate if you have any restriction in pipes.

What parameters did you add into this for it to give you the expected superheat? Iv just been trying it but couldn't get the same as your example?

[nike123]

What parameters did you add into this for it to give you the expected superheat? Iv just been trying it but couldn't get the same as your example?

Green numbers are inputs. Don't forget to hit "calculate"
Also, check that drop down menus are right and check help for fittings.

[marc5180]

I still cant get it to what you have in the picture. What inputs did you enter, all i have is the SC of 7K and the refrigerant R22?

[nike123]

I still cant get it to what you have in the picture. What inputs did you enter, all i have is the SC of 7K and the refrigerant R22?

As I already said, I entered all green numbers on that pictures. Click on each box and when turns in blue, inter value you see on picture. Also, on drop down menus select as at picture.

[Thermatech]

Do the condensers have any low ambient control ?

If no outdoor fan speed control in low ambient 16 deg C then outdoor unit overcondensing which drags down evaporating temp at indoor coil.
Low air on coil temp makes even lower evaporating temp + some liquid flood back to outdoor unit.
For this type of aplication the outdoor unit should not be running untill indoor return air temp is much higher & outdoor temp is much higher.
If you want the units to work in this condition best make sure outdoor units have suction accumulator.

[Brian_UK]

Just a small addition to what has already been said about the airflow.

Although the coils may be clean and the filters clean you don't truly know that the airflow is correct.

You haven't been provided with the design airflow rate and I doubt if you have got the equipment, or the company's time, to measure the actual airflow through the coils.

Other things that can confuse the issue of airflow is that over time people may have been closing duct dampers and terminal dampers thereby reducing the airflow below design; these things you cannot control.

Think positive Marc, if it looks right then hopefully it is right

[marc5180]

Thanks for all of the replies guys. As i said in my first post these units feed seperate AHU's and i had to change the controller and put them on manual to get them to come on otherwise because of the low ambient temperatures these would not be on.
I took the readings without realising that i would have to instigate full load conditions but now i know i can check it again on next time i go back hopefully when its a bit warmer and see what readings i get then.
So thanks for all of the replies.