Did you know:
40percent-bypass

Or that: 

30percent-hot-air

In-fact:

Screen Shot 2015-03-27 at 15.13.08 (2)

Here’s why:

Too often I hear or read about a certain Data Hall temperature, i.e. “the data hall temperature was 25°C”.

I am normally confused as I cannot grasp what is going on. In a data hall there are generally four different groups of temperatures.

These are:

Return air to CRAHs: set point 22°C

Supply air from CRAHs: 12 to 20 °C

Supply air to IT equipment: 15 to 40°C

Exhaust air from IT equipment: 18 to 45°C

 

At Operational Intelligence we use these 4 temperatures to determine how efficient data centres are. It is an currently R&D tool available for public use. Try it here.

Clearly, there is no single temperature representing the data hall.

The following diagram illustrates typical legacy data centre air flow rates by the width of the arrows and temperatures in a colour scale.

Screen Shot 2015-03-26 at 15.57.47

 

Note:

CRAH: Computer Room Air Handling Equipment

NP: Negative Pressure

BP: By-Pass

IT: IT Equipment

R: Recirculation

 

 

The Breakdown

With regards to the example above (representative of a typical data centre), the first point to note is that while we may have a good control of the air temperature returning to CRAH units (as they are controlled on this set point), there is a very high range of temperatures entering the IT equipment. ASHRAE recommend these be 18 to 27°C, and allow 15 to 32.2°C.

Clearly not only are we frequently outside both the recommended and allowed ranges, we appear to have no control over this.

A small amount of air is induced from the hall to the floor plenum due to high under-floor velocities (Venturi effect). The velocity pressure is higher than the total pressure, creating a negative static pressure (note NP in drawing for negative pressure) that induces the air down. This is a bad place to locate a rack as cold air does not come out of the floor grille.

Of all the air that is supplied to the data hall, on average around 50% of this air returns to the CRAH units without reaching the IT equipment. This is noted as BP (bypass). Typically, this is from floor grilles not located in the proximity of IT equipment air inlets and open cable cut-outs at the back of the racks.

Normally, as a consequence of insufficient air at the inlet to IT equipment and given that IT equipment have their own fans, air is recirculated (noted as R in diagram) from the exhaust back into the inlet. This recirculated air mixes in different proportions to the cooled air supplied, and hence results in a wide range of values. Note arrows representing cooler and warmer temperatures.

The exhaust air temperature from the IT equipment will now vary in terms of the inlet temperature and the IT equipment load. This air then mixes with the bypass air resulting in a cooler temperature that returns to the cooling units (CRAH).

The air temperature supplied by the CRAHs will normally vary with the load of each unit if controlled on return air temperature. I would suggest that we specify the following temperatures when referring to a data centre. It will help us to understand what is going on.

Remember & Apply

– Cooling units: air in (return air to CRAHs)

– Cooling units: air out (supply air from CRAHs)

– IT equipment: air in (supply air to IT eq.)

– IT equipment: air out (exhaust air from IT eq.)

Do you know what they are? A better understanding of this will help us improve both reliability of IT equipment and energy efficiency.