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Spikey
20-05-2017, 08:27 PM
Hi Guys

New to this forum so be gentle , I got a call out to a tripped out ahu with one coil with 2 circuits and separate condensers ( Airedale ) . They both have txv's, and are brought in in stages of demand. One had a leak which was fairly obvious (circuit 2) and has been repaired . I weighed out both circuits to get an idea of roughly correct weights as no documentation on site. One had 16kg (407c) the other had 6kg. I ran up circuit 1 today after recharging with the reclaimed 16kg of refrigerant and noticed after about 5-10 mins of operation that the coil started to freeze up , now the service valves on this unit are a bit limited to say the least , I have one on the discharge leaving compressor before condenser and one on the suction again on the side of the compressor, that's it. So on the basis of coil freezing I understand that airflow may be the problem , but also possibly undercharge , how will I determine correct subcooling / superheat when service valves are where they are ? I have checked superheat at condenser and am getting 15 k , but how to check subcooling if service valve is before condenser ?

RANGER1
20-05-2017, 10:24 PM
Spikey,
If I understand correctly you can read suction & discharge pressures!
If you can the use of a temperature touch probe can tell you the rest.
If you can not read a suction &/or discharge pressure impossible to get both readings.
If really long suction line etc you can allow for a bit of pressure drop of 1 to 2 deg C, but I don't think this will be necessary in your case.

Also understand both circuits are completely separate form one another, is that correct/
.
Need to read suction pressure anywhere in suction line, discharge pressure anywhere on discharge side.
Need temperature at TX valve bulb & temperature of liquid line exiting condenser or liquid receiver best.
Liquid line should be similar temperature from liquid receiver to TX valve inlet.

Let us know what you have as evaporator coil icing can be several things

- poor air flow caused by dirty coil or low fan speed, bypassing air flow, dirty filter.
- suction pressure to low
- undercharged
- TX valve issues
- blocked filter drier

Spikey
21-05-2017, 08:37 PM
Spikey,
If I understand correctly you can read suction & discharge pressures!
If you can the use of a temperature touch probe can tell you the rest.
If you can not read a suction &/or discharge pressure impossible to get both readings.
If really long suction line etc you can allow for a bit of pressure drop of 1 to 2 deg C, but I don't think this will be necessary in your case.

Also understand both circuits are completely separate form one another, is that correct/
.
Need to read suction pressure anywhere in suction line, discharge pressure anywhere on discharge side.
Need temperature at TX valve bulb & temperature of liquid line exiting condenser or liquid receiver best.
Liquid line should be similar temperature from liquid receiver to TX valve inlet.

Let us know what you have as evaporator coil icing can be several things

- poor air flow caused by dirty coil or low fan speed, bypassing air flow, dirty filter.
- suction pressure to low
- undercharged
- TX valve issues
- blocked filter drier

Hi Ranger

Thanks for answering, What i was worried about was taking my pressure reading before condenser (straight out of compressor) as thought there might be a difference in pressure between the entry point to condenser and exit of due to pipe sizing etc of coil, so would give me a false subcooling figure as attaching temp probe after condenser. Yes I have 2 circuits running through one coil in AHU, one had only 6kg and the other 16kg (the one that is now running and freezing up)which it was doing before apparently and i'm assuming trippping out panel on LP. These coils feed an operating theatre and are used mainly for controlling humidity rather than temperature, now i ran circuit 1 (16KG) in hand to get superheat and subcooling figures so at the moment i'm unsure whether airflow is the problem or as you stated the TX is fooked , I did take the suction reading pressure at service valve of compressor and temp reading about 6" back along suction pipe and was getting 15K superheat but coil was still freezing up after 10 mins, i did take subcooling reading but was working out to 30K of subcooling which i thought was a bit high which made me doubt if my readings were correct. so possibly txv is fooked , will recharge the other circuit tomorrow to see if the same thing is happening with the other one and report back

Kind Regards

RANGER1
21-05-2017, 09:17 PM
Hi Ranger

Thanks for answering, What i was worried about was taking my pressure reading before condenser (straight out of compressor) as thought there might be a difference in pressure between the entry point to condenser and exit of due to pipe sizing etc of coil, so would give me a false subcooling figure as attaching temp probe after condenser. Yes I have 2 circuits running through one coil in AHU, one had only 6kg and the other 16kg (the one that is now running and freezing up)which it was doing before apparently and i'm assuming trippping out panel on LP. These coils feed an operating theatre and are used mainly for controlling humidity rather than temperature, now i ran circuit 1 (16KG) in hand to get superheat and subcooling figures so at the moment i'm unsure whether airflow is the problem or as you stated the TX is fooked , I did take the suction reading pressure at service valve of compressor and temp reading about 6" back along suction pipe and was getting 15K superheat but coil was still freezing up after 10 mins, i did take subcooling reading but was working out to 30K of subcooling which i thought was a bit high which made me doubt if my readings were correct. so possibly txv is fooked , will recharge the other circuit tomorrow to see if the same thing is happening with the other one and report back

Kind Regards

From what you describe, liquid is banking up in condenser giving you high subcooling & a restriction in liquid line, or most likely faulty TX valve.
Starved evaporator, high superheat, low suction pressure. icing at begining if evaporator circuit.
Good luck, net test will tell.

frank
22-05-2017, 10:07 AM
I would also look at the size of the liquid line if your pipe lengths are more than 15m. If it is undersized then you would get increased pressure drop at the TEV and icing at the coil as you describe

Spikey
22-05-2017, 10:41 PM
I would also look at the size of the liquid line if your pipe lengths are more than 15m. If it is undersized then you would get increased pressure drop at the TEV and icing at the coil as you describe

Hi guys , so just an update . Went to site today to recharge circuit 2 and circuit 1 was running very happily , no frosting on coil but I noticed air flow had been increase to 45% as now on a work day the BMS had come to life and was controlling everything as it should . Sub cooling was around 10k and superheat around 6k , much better suction pressure than the other day. Circuit 2 is for second stage cooling if circuit 1 is unable to cope with demand , all good thanks for the replies