Datacenter cooling

When mixing two metals with an electrolyte (water), you create a cell. The more “anodic” (or “less noble”) metal will be the negative terminal (anode) of the cell thus made, and will oxidize.

To evaluate the intensity of the effect on a site, list all materials and compare their anodic index (this parameter is a measure of the electrochemical voltage that will be developed between the metal and gold).

In most datacenter, heat exchangers are in aluminum and many valves/other hydraulic components are in copper or brass (copper + zinc) (especially inside chillers and air handling unit), creating galvanic effect.

Air

Psychometric and vaporization charts

Humid air is mainly used for room temperature and humidity management, but also for outside air with adiabatic coolers. Cooling manager must know how to use the psychometric chart to detect energy losses or possible water condensing.

• A. Air heating. (Example: IT equipment, heating, dry cooler, etc.)
• B. Adiabatic humidification, using cold water. (Example: adiabatic coolers, etc.)
• C. Air cooling, without water. (Example: air handling units, etc.)
• D. Air cooling with saturated humidity. (Example: near heat exchangers inside air handling units or inside cooled doors, etc.)
• E. Air humidification, using vapor. (Example: humidifier, etc.)

It is also interesting to understand vaporization chart, which highlight how much energy is needed/evacuated to change phase.

Note: values are only given for example.

• A. Heating water from cold water to 100°c at P = 1 atm.
• B. Phase change, from liquid water to vapor. Temperature is fixed at 100°c, but enthalpy rise from 400 J/(Kg.K) to more than 2600 J/(Kg.K).
• C. Vapor heating, from 100°c to near 150°c, keeping 1 atm pressure but rising enthalpy.

Formulae

COP

Cooler performance is defined by the COP (coefficients-of-performance). COP is defined by:
COP = (Cooling power) / (Input electrical power)

Bernoulli

For liquids and low mach gas flows, it is possible to assume fluids to be incompressible, and use Bernoulli principle :

<m 18>{v^2/{2g}}+z+{p/{rho g}}=constant</m>

With :

• <m 18>v</m> the fluid flow speed at a point on a streamline,
• <m 18>g</m> the value of acceleration due to gravity,
• <m 18>z</m> the elevation of the point above a reference plane, with the positive z-direction pointing upward – so in the direction opposite to the gravitational acceleration,
• <m 18>p</m> the pressure at the chosen point, and
• <m 18>rho</m> the density of the fluid at all points in the fluid.

Between two points (1 and 2) of a flow (in a pipe for example), it is possible to assume with no pressure losses :

<m 18>2g+z_1+{p_1/{rho g}}=2g+z_2+{p_2/{rho g}}</m>

Now considering pressure losses :

<m 18>2g+z_1+{p_1/{rho g}} + H_1 =2g+z_2+{p_2/{rho g}} + H_2</m>

Pressure losses

There are two types of pressure losses :

• linear losses, friction of fluids along a pipe or a surface on a defined length
• singular losses, due to singular flow perturbation, for example a bent pipe or an exhaust.

Of course, losses are logical : curved shapes generates less losses than angular shapes. Same with pipes, the bigger the pipe, the less losses.

A good example is the following table, providing coefficients for singular losses, have a look on differences between shapes :

(from http://www.mecaflux.fr/perte%20charge%20singuliere.html)