What will cause eva pressure fluctuate?

[wilsoncheung]

I have a question to discuss and hope you will be interested.

Its a air-cooled cooling air unit, thermal expansion valve with distributor. Air come out of evap is about 10 degree. Now the problem is the evaporating pressure fluctuates about 50 kPa, I wanna know what cause the fluctuation.

Any possible answer provided is welcome!

[coolments]

Lots of things could cause that. Stab in the dark but how does the unit control head pressure, if the condenser fan cuts in and out on a pressure switch that could cause it or maybe the thermostaic expansion valve is hunting.

need lots more info to help, how frequent is the fluctuation, air on air off running conditions gas type and running pressures, delta T etc.

[TXiceman]

Probably caused by fan cycling on the econdensre and not letting the TXV to stabilize. Try opeing up the band on the fan cycling controls.

First, check to see if the suction pressure is sort of laging the head pressure.

Ken

[wilsoncheung]

Lots of things could cause that. Stab in the dark but how does the unit control head pressure, if the condenser fan cuts in and out on a pressure switch that could cause it or maybe the thermostaic expansion valve is hunting.

need lots more info to help, how frequent is the fluctuation, air on air off running conditions gas type and running pressures, delta T etc.

We have changed the TXV for several times, still the evap pressure fluctuate.

[wilsoncheung]

Probably caused by fan cycling on the econdensre and not letting the TXV to stabilize. Try opeing up the band on the fan cycling controls.

First, check to see if the suction pressure is sort of laging the head pressure.

Ken

The condensing pressure is quite stable, so it may not be the fan problem of the condenser.

[Gary]

Air come out of evap is about 10 degree.

What temperature going into the evap?

Insufficient airflow through the evaporator reduces the load, possibly to the point where the orifice is too large for the load... which causes the TXV to hunt.

[Gary]

Now the problem is the evaporating pressure fluctuates about 50 kPa, I wanna know what cause the fluctuation.

The pressure doesn't tell us anything if we don't know what refrigerant is in the system.

[wilsoncheung]

What temperature going into the evap?

Insufficient airflow through the evaporator reduces the load, possibly to the point where the orifice is too large for the load... which causes the TXV to hunt.

Thanks, gary!

The temperature enters the evap at about 15 degree. The air flow is sufficient but not uniform, I guess that is one of the cause. But we have 2 same units, the other one is so far so good.

[wilsoncheung]

The pressure doesn't tell us anything if we don't know what refrigerant is in the system.

Sorry to tell the refrigerant, it's R22.

[Peter_1]

A too big orifice

[wilsoncheung]

I have changed the TXV for several times, all new.
But there seems to be no effect, it's still hunting.

You guys have talked about insufficient air flow, or the condenser fan problem causing the condensing pressure hunting. But I have other system with the same evap air flow and the same condensing pressure. The cond air flow is from the same condensing fan .

[nike123]

What is compressor model#, designed evaporation and condensation temperature, TXV model#, orifice#, actual evaporator air in temperature, condenser air in and out temperature?

[coolments]

How about giving us some readings of the fluctiation, air on, air off, system suction & discharge pressure, super heat, at the highest and lowest peak, whats the humidity level in the room.

Also does the TEV have an external equaliser if so have you made sure its not blocked or is it a power assembly.

Whats the room set point your trying to acheive, is it fresh air or does it recycle. how big is the room, whats the duty of the system.
We realy are just stabbing at a possible solution to your problem here, you need to give us more date so we can help.

[wilsoncheung]

Hi,coolments!
Here is the thing:
This unit has four compressors,each is a circuit. The four system s have the same cond temperature(50 degree), but different evap pressure. The indoor air pass the four evaporators sequentially, which makes the four evaporators evaporate at 13, 3, -3, -6 degree(from one to four system)

1,2 and 4 system run quite well and only the 3rd system's evap pressure fluctuates.

[Gary]

Hi,coolments!
Here is the thing:
This unit has four compressors,each is a circuit. The four system s have the same cond temperature(50 degree), but different evap pressure. The indoor air pass the four evaporators sequentially, which makes the four evaporators evaporate at 13, 3, -3, -6 degree(from one to four system)

1,2 and 4 system run quite well and only the 3rd system's evap pressure fluctuates.

As Peter1 stated earlier, the TXV orifice is too large.

[wilsoncheung]

The 3rd system has one evaporator, two TXVs and one refrigerant distributors, which means the R22 was expanded by two TXVs and then enter the same distributor.

As for the other three circuit, they all have just one bigger TXV, would that be a cause of the fluctuation?

[nike123]

The 3rd system has one evaporator, two TXVs and one refrigerant distributors, which means the R22 was expanded by two TXVs and then enter the same distributor.

As for the other three circuit, they all have just one bigger TXV, would that be a cause of the fluctuation?

I think that it is obvious!