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  #1  
Old 01-11-2012, 09:13 PM
OBXandos OBXandos is offline
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Default CRAC freeze up detection

My data center recently experienced an evaporator coil freeze up on one of our CRAC units. The only indication we had was a rise in temps across the room. We checked all the usual problems but found nothing wrong. We finally had to call someone out to look into the problem and that is when we found out the real problem.

So I have been trying to come up with a possible detection method. I cannot trust a temp reading since they were coming out fine. The problem was little airflow. So I thought I could use an anemometer to measure airspeed/CFMs. The only ones I have been able to find are handheld or big ones for weather stations. Has anyone here heard of an anemometer that could connect to some monitoring software like Solar Wind'ss Orion or APC's Netbotz systems.

I would also ask if you guys could think of a different/better option.

Any help would be greatly appreciated.
JohnG
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  #2  
Old 01-12-2012, 03:48 AM
thecommis thecommis is offline
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These kind of incidents are very rare to happen in data center as we plan to have redundant CRAC and also plan in such a way that CRAC works on around 60% of capacity this will provide you buffer cooling in such incidents.

There are not too many anemometers designed for datacenters but looks like this is kind of one you are looking for.

http://www.remotemonitoringsystems.c...er_project.php
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  #3  
Old 01-12-2012, 10:15 AM
sweetgreen sweetgreen is offline
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Hi OBXandos,
Welcome to the forum.
the problem u mentioned is indeed very rare. i came across a patented device that helps monitor the airflow level and i thought it would be helpful to you. I hope it helps.
http://www.strutpatent.com/patent/07...tion-apparatus
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  #4  
Old 01-12-2012, 05:06 PM
vincent_byrne vincent_byrne is offline
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Hello OBXandos
I am please to hear that you have solved the problem of ice on the evaporator coil.

I would like to talk a little more about this problem for the benefit of the readers as we have come across this problem in a data centre for one of our clients. And I hope that you might share findings into the cause of the problems and maybe provide some details of the other possabilities that were ruled out in arriving at your conclusion.

In our case the CRAC unit in question was located in a corner of the data centre at a location which did not have to deal with a significant load. In order that we could fully assess the problem we took temperature readings at the floor grills and at two different heights through the data centre on a 4ft x 4ft grid (1200mmx1200mm). These were inputted into a newly created set of visio drawings and colour coded to give a visual view of the temperatures throughout. The temperatures were also inputted into a CFD model of the data centre which gave us a very full picture of what was happening.

1) CRAC Unit oversized for the load, with fixed speed compressors the evaporation temperature is being pulled down too low causing any moisture in the return air to freeze on the coil.

Action : Check the temperature of the return air. Check if the air from the CRAC unit in question is being correctly delivered into the cold aisle without recirculation or bypass. Check if the air is being feed into an area of low heat load.


2) Set-Point and/or differential set down too low (i.e. point at which room temp. is satisfied and compressor is turned off).

Action : Check is set point for room is not too low. It should be set at between 21 to 24 degrees without containment in place.


3) Filters are blocked, thereby restricting airflow.... compressors are running and freezing the crap out of the coil, as there is no or limited warm return air passing over the evaporator.

Action :Check for blockage in filters.

4) Incorrect superheat setting on TXV

Action : Check superheating setting on TXV, reset where necessary.

5) TXV stuck open.

Action : Check the operational status of the TXV to ensure it is not stuck in the open position.

6) Airflow switch has failed, fan motor has burned out - so no airflow or reduced airflow if multi-fan machine. Same as Item 3.

Action : Check the operational status of the airflow switch.

I hope these options will provide some food for thought for those who find themselves faced with this rather uncommon problem.

Vincent Byrne
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Old 01-12-2012, 07:17 PM
OBXandos OBXandos is offline
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Thanks for the input guys. The device that thecommis linked looks a little big for what I need. I will look into it though. The device that sweetgreen linked looks alot better but I have no idea where to get one. I'll have to do some research there too.

Vincent has some good ideas and I am working on creating a visio diagram to track some changes I would like to make.

A little background on my problem:
Around 9pm one night I noticed that temp were rising around our datacenter, about 10 degrees. We have temp sensors on a few of our racks. I went and checked the temps coming from our floor tiles and they were a few degrees warm but nothing major.

Our customer in that room of the datacenter recently launched a worldwide MMO game so they were constantly changing stuff in the environment. We did not really have a baseline because of all the changes. I just assumed that since I could not detect a problem that it was something our customer was doing.

I left work around midnight and asked the night ######## to keep an eye on it. I went to bed around 6:30am and temps kept rising. We were now around 74 degrees F. Around 9am the day shift called me and told me we were at 85 degrees. I asked them to call our service people because there was obviously a problem. They did not. They called around 1:30pm.

I got a call around 2pm from our service people that they were on their way to the datacenter. When I got to work at 4pm I was informed that our evaporator coil had froze up. The root cause was low freon pressure.

Since then I have been trying to come up with a way to detect this problem. I cannot visually check the freon levels, I could visually check the coil but that would involve taking panels off of the Liebert and I don't want anyone doing that but me for safety reasons. So I thought if we could measure a decrease in airflow/airspeed that this would be an indicator that we could follow up on.

Our datacenter can function if we lose a CRAC unit, but temps go up to what i would consider max if we do lose one. We have 4 rooms in our datacenter that are separated by walls. The raised floor and drop ceiling however are all connected. Our 4 Lieberts are pulling air from the drop ceiling and pushing air into the raised floor. When 1 goes out the other 3 have to compensate. It works ok but not great.

I am going to look into the visio doc and the CFD stuff once I get a sensor to do this with. Ill keep you guys updated with my findings.

Thanks again,
OBXandos
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  #6  
Old 01-12-2012, 09:28 PM
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KenB KenB is offline
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AKCP makes an airflow module for their network-connected SensorProbe. Might be what you're looking for. I've been satisfied using the SensorProbe for monitoring various conditions around the data center for years, but I've never used the airflow module.
http://www.akcp.com/products/intelli...ow-Sensor.html

A company called avtech also shows two airflow sensors on their site (avtech.com/products/), but I don't have any first-hand experience with that company.


Ken
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Last edited by KenB; 01-13-2012 at 01:32 AM.
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