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SkyWalker
03-09-2009, 07:19 PM
hi guys, could someone explain to me why when a split lets say in a server room (i know this sint ideal, but hell it happens up and down the country) with alot of load, too much for the system,

why isit that if the unit never gets any thermo off time, it ice's up? i went to one today complete block, the unit is located above a server rack, ambiant was 25c indoor was seeing 32c. heat straight out of rack.

what i dont understand is all the ice? if the refrigerant charge is correct why do they still ice up over time? suction sat temp surely should never drop below 1c ragardless how long its been on?

Kirky
06-09-2009, 01:12 PM
Could be a number of things. Firstly could be that the unit is simple too small for applications and is working too hard. I had a mitsi split located in a server room with same problem. The IT guys had set the unit to low in a room with temp pushing 30degrees the unit will never get anywhere near set point. All I did was up setpoint a couple of degrees gave the filters a good clean and it ran fine. However the unit was so undersized had to install another one to meet heat load

nike123
06-09-2009, 04:42 PM
hi guys, could someone explain to me why when a split lets say in a server room (i know this sint ideal, but hell it happens up and down the country) with alot of load, too much for the system,

why isit that if the unit never gets any thermo off time, it ice's up? i went to one today complete block, the unit is located above a server rack, ambiant was 25c indoor was seeing 32c. heat straight out of rack.

what i dont understand is all the ice? if the refrigerant charge is correct why do they still ice up over time? suction sat temp surely should never drop below 1c ragardless how long its been on?

Because ordinary split system is not designed to deal with WB temperature below 18°C or lower. His designed TD is about that 18°K.
Because of lack of latent heat of wet air (when air is dry like in server room after some time and without fresh air introduction) there is not enough heat in air to rise evaporator temperature above 0°C.
For that use, evaporators must be of much bigger size than ordinary split systems to be able to absorb more heat from relatively dry air (lower TD evaporators).

Thermatech
06-09-2009, 04:48 PM
Standard split systems are designed for comfort cooling.
This type of application will have people in the room & some fresh air infitration so there will be some latent heat load due to humidity being produced in the occupied space. So typical air humidity in the room will be 50% RH.
Most mass produced split systems will have a sensible heat ratio of arround 0.75 making 75% sensible cooling & 25% latent cooling = 100% total cooling.
They are designed to work with room air on coil arround 50% RH.
This type of system has compact cooling coil & rather small air volume as this is cheaper to manufacture & is ok for standard comfort cooling application.The typical evaporating temp will be quite low & the unit will extract lots of moisture from the air as condensation.
Good quality systems will have a coil frost prevention function which will prevent frost build up on the coil.

You will find that systems designed for computer room high sensible cooling will have much much larger indoor coil & larger air volume for high sensible cooling.
The system will operate at higher evaporating temp to avoid coil frost.

Most small computer rooms have 100% sensible load.
But the split system is constantly cooling & dehumidifying.
The recirculated room air soon becomes very dry at 25 to 30 % RH.
At this condition the small amount of moisture in the air freezes on the coil. Any small amount of frost on the coil reduces the air volume even further making the coil temp even lower & the unit soon become a block of ice if it does not have coil frost prevention function or this fuction is not working correctly.

This is why some installation contractors resort to always installing a big drip tray underneath wall mounted units in computer rooms because of repeated defrosting problems.

Most manufacturers of standard split systems quote a lowest limit of 15 degC wetbulb room on coil in the performance data for the lowest acceptable humidity level in the room.
But if you take humidity measurments in most computer rooms you will find the wet bulb much lower at around 11 to 13 deg C WB.
So no contractor can accurately select a standard comfort cooling split system for a computer room application because the manufactures do not produce any data for lower than 15WB.
This data is not produced because the manufacturers know that at lower than 15WB the system will have coil frost problems & the cooling performance of the system is very poor.
In this case the contractor has to oversize the unit & install more units to offset the reduced cooling available from the standard split system working in this extreem condition for which it was not designed.

Typically you will find for computer rooms the fixed speed splits will run for a few mins & then stop for a few mins on coil frost prevention mode. So the system is only operating 50% of the time & therfore less than 50% capacity.
Inverter split systems will constantly run at slow compressor speed to avoid coil frost & therefore minimum cooling capacity.
Most split system low ambient head pressure control set up is not ideal for this type of aplication as it is set up for comfort cooling application high cooling performance & energy efficiency. So the system will be allowed to operate at rather low discharge pressure & low evaporating temp / pressure.This tends to make the coil frost problem even worse.
But this type of HP control stratagy is not ideal for the system operating in the computer room 100% sensible cooling application as you ideally need evaporating temp higher than coil frost conditions.

There are some common sense modifications which can be deployed to split systems which help to prevent frost / ice problem but the best is to install a proper high sensible cooling system which has been designed for this type of application.

back2space
06-09-2009, 09:54 PM
Fans set on max speed and try whereever possible introduce fresh moist air into the room either through supply fan or open windows/grilles?

acnerd
07-09-2009, 04:51 PM
That's a great explanation Thermatech. Any chance I can download your brain ...... well just the technical side?