Theory behind datacenter infrastructure

This page regroups basic theory for datacenter cooling management. No need to go too far in equations.

Thermal exchanges

Liquid-Liquid

Liquid-Gas

Hydraulic

Liquids

Table

Name	Formulea	Boiling point (1 atm)	Freezing point (1 atm)	viscosity (20°c)	density (20°c)
Water (liquid)	H2O	100°C	0°C	8.90 × 10−4 Pa·s (25°C)	999.97 kg/m³
Ethylene Glycol	C2H6O2	197.3 °C	-12.9°C		1.11 g/cm³
Diethylene Glycol		244.8 °C	-10.5°C		1.1184
Triethylene Glycol		287.4 °C	-7.2°C		1.1254
Propylene glycol	C3H8O2	188.2°C	-59°C		1.04 g/cm³

Glycols

All glycols produce acids in the presence of air (oxidants). The acids can reduce pH and cause corrosion. When the system pH drops below 7, rust will form on any ferrous metal, and non-ferrous metals start to corrode. Even a small amount of glycols in a circuit open to air can start corrosion ! Beware when connecting new equipments.

http://www.veoliawaterstna.com/crownsolutions/ressources/documents/2/21823,Glycol.pdf

Freezing Point of Propylene Glycol based Water Solutions

(%) by mass	0	10	20	30	40	50	60
Temperature °C	0	-3	-8	-14	-22	-34	-48

Gas

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 :

Now considering pressure losses :

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)

Wet air

Computer Room

Floor tiles

Tiles come curved, and when in place gradually become flatter. This provides stability to the floor. This is why you should be careful when removing tiles.

Mass is applying as following :

If you want to remove another tile, you should not remove a tile in the same column or in the same line, as it may cause tiles movements and permanent disharmony :

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