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wingman
07-04-2016, 12:24 PM
Hello all.


Normally I wouldn't call upon you guys but this time I'm at my wits-end.

I'll give a bit of background information so you can visualize everything.

A bit more then 1 year ago our customer (a tenant of a building) Asked us to intervene in a problem with 2 blue box precision coolers (top blowers). I did the first repair (leak in the solenoid valve.)

I noticed the load on the two units is too big and explained our customer they needed to either think of a new installation or adding additional coolers to the serverroom. To wich they hired a secondary install company. This is were the problems started. They have placed both units elsewhere in the room and removed the hats of the units and installed round spiral ducts around the room with a lot of openings. (connecting both units airwise together) Imagine the frown upon my face when I saw that. And tried to use a metaphor to explain them why that didn't work. Next to that the other company changed out the complete controller with a Elliwell (the one on small coldrooms). They have bridged several protections like the differential pressostat for the fan coil unit, belt protection etc. Also they removed the crankcase heater.

So now my problem , last time I had a problem with the TXV being stuck open and flooding the evaporator. So I had to replace the TXV with another brand (Danfoss TEZ5) with an orifice nr2 (Unit is about 24Kw cooling power). I am now getting nearly the exact same problem, on a regular time the evaporator freezes completely. I am reading a low subcooling and a low superheat. Instead of adding refrigerant I tried put some cardboard underneath the condensor to make the headpressure rise a little. The headpressure rises but my subcooling is still somewhere in the vicinity of 1,2C. Superheat is somewhere between 3,3 and 5C With a headpressure of 13,5BAR and 3,35BAR suction. (resp. 32,5C and -0,3C sat.)
Hotgas temperature is somewhere around 67-70C
The system runs on R407C. The delta T over the evaporator is around 15C, intake is around 24C.
The bulb of the TXV is not exposed to the intake air. (isolated.)


If there are more things you would like to know give me a shout. :)

redroge
07-04-2016, 03:24 PM
Have you tried switching one unit off and see how the other performs

The Viking
07-04-2016, 05:21 PM
OK,
So they added a lot of external static pressure to the evaporator fan(s)...
Never a good idea and something that need careful design to ensure the added restrictions of the ductwork is still within the design envelope of the fan itself.
On top of that, you are now left with two evaporator fans blowing against each other...

You know, sometimes the best solution is to just shake ones head, turn around and slowly walk away.

But if it was something I needed to sort out, I would start with measuring airflows and air pressures across the evaporators before I got anywhere near the fridge circuit.

:cool:

wingman
07-04-2016, 10:44 PM
OK,
So they added a lot of external static pressure to the evaporator fan(s)...
Never a good idea and something that need careful design to ensure the added restrictions of the ductwork is still within the design envelope of the fan itself.
On top of that, you are now left with two evaporator fans blowing against each other...

You know, sometimes the best solution is to just shake ones head, turn around and slowly walk away.

But if it was something I needed to sort out, I would start with measuring airflows and air pressures across the evaporators before I got anywhere near the fridge circuit.

:cool:

Well ... I've used a metaphor (metafore ) to explain my client what they have done to their installation.
This is exactly what I told them. "You are trying to pull a ten ton load with a Toyota prius on a 10% incline. What you have done is placing a spoiler on the Prius." It took them twenty minutes to absorb .....


Anyway I know about the the external pressure. I have tried decoupling the two systems with cardboard (as a test) but that did not affect the result. It is true as you say to better walk away and shake your head or frown. But then again, who will fix my clients problem or even worsen it ?
Anyhow I did figure it out on my own. It appeared to be a couple of problems. The one I stated with the intake of it's own air resulting in the low suction pressure. Secondly it was having a severe case of oil logging. The system runs a lot smoother now with the TXV not hunting anymore. I have a head pressure of 14BAR suction of around 4,5BAR suction temp of around 12C and a sub cooling of about 5C. I know the sub-cooling is still low but that is due to the Carel speed controller running a constant 30% of its speed whilst I need perhaps only 5%. (RS485 interface and software to sort that out, is on it's way )


Checking airflow is indeed something I usually do first, although it did not cross my mind until I was crouching down feeling a cold breeze in my plumber crack. ;)