underfloor air distribution

[tikitime]

I have a long rectangular computer server room (20' x 70')that I am designing a underfloor hvac system for. The underfloor system will provide air directly to the servers through cut-out floor panels. The problem is that the client wants the hvac units in the far corner of the room. With the long rectangular nature of the room, I am afraid that the supply air underneath the floor will never make it to the other side of the room. Does anyone have any suggestions? I see that there are underfloor fan units available but I am not familiar with how well these work.

 

[lilliput1]

You can have access floor grilles located to match your design air flow distribution provided the grilles have balancing dampers. Have the balancing contractor set, mark & lock the dampers accordingly. Check pressure drop through the grilles & access floor plenum (reduce effective cross section to consider obstructions) & verify CRAC(s) (Computer Room AC Units) have enough capacity. Optimize also location of individual CRAC units for approximately even air distribution to match distributed loads.

 

[EagleCove1]

Be very careful by insisting that all penetrations and wall seams of the under floor construction are sealed air tight. Cable Trays, electrical conduits, communications lines are all sources of leakage. Unless this is done, you run the risk of having a significant pressure loss at the end of the room and airflow that is insufficient. Location of the floor grilles should be carefully located to be near the sources of heat in the space.

 

[McCormick93]

Air discharge at 70' is not too far away as long as raised floor penetrations are sealed, as EagleCove says. If you put the CRAC too close to the load, the high air velocity under the floor will actually create a venturi effect and suck air down into the plenum.

In addition to sealing the holes, make sure there aren't too many cable trays under the floor which will block air from reaching the target area.

Also, be sure your client doesn't make the mistake of placing several rows of servers all facing the same direction. Most server racks draw air into the front and exhaust out the back. You don't want hot exhaust from one row blowing into the intake of the next row. This will diminish the capacity of the AC system. Server racks should be lined up face to face and back to back, creating "hot aisles" and "cold aisles". There is good research on this published on

http://www.upsite.com

 

[chicopee]

FIRST YOU HAVE TO KNOW HOW MUCH CFM AND AIR TEMPERATURE YOU NEED TO PROVIDE THE SERVER. SECONDLY,YOU NEED TO KNOW THE BLOWER PERFORMANCE WHICH YOU CAN GET FROM THE MANUFACTURER IN THE FORM OF PERFORMANCE CURVES. THESE CURVES BASED ON SEVERAL RPM VALUES ARE FOR AN AIR TEMPERATURE OF 70 DEG.F. THIRDLY, FROM ASSUMED CFM'S FLOWING THRU THE DUCT, CALCULATE CORRESPONDING FRICTION LOSSES. PLOT THE FRICTION LOSSES VS. AIR FLOW ON THE SAME GRAPH USED FOR THE FAN OR BLOWER PERFORMANCE CURVES. THE POINT OF INTERSECTION BETWEEN THE FRICTION LOSS CURVE AND THE FAN PERFORMANCE CURVES ARE THE APPROXIMATE CFM's THAT YOU WILL FOR THE SERVER AFTER TEMPERATURE CORRECTION. ALSO USE THE APPROPRIATE UNITS FOR FRICTION LOSS IN INCHES OF WATER, AIR FLOW IN CFM, FAN DELIVERY IN CFM AND STATIC PRESSURE IN INCHES OF WATER.