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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