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Equation 8 · The Physical Plant: Power, Cooling, and Networks in an AI Datacenter

What does this equation mean?

ρcp≈4180 kJ/(m3⋅K)\rho c_p \approx 4180\ \mathrm{kJ/(m^3{\cdot}K)}

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This equation gives an approximation: it relates the quantities while allowing an approximation. Read the equation part by part below; each part has a contextual explanation and a link to its mathematical background.

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ρ\rho

Symbol ρ

ρ is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

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cpc_p

Symbol c_p

the specific heat capacity.

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≈

≈

Approximately equal to; the equality is not exact.

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multiplication

multiplication

Multiply the quantities on either side.

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subscript

subscript

The lower label selects a particular version, component, or indexed member of the quantity. For example, x₀ and xₜ can be values at different positions.

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superscript

superscript

A raised number can be a power. When it is a label or bound, it selects a case or the upper limit of a sum; the formula’s structure distinguishes these uses.

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Its accuracy depends on the assumptions and range of use described in the article.

What the article says around this equation

The following is my own worked calculation, using standard property values rather than any cited source. At around 27 °C and atmospheric pressure, air has ρ\rho cpc_p ≈\approx 1.17\ kJ/(m3⋅K)\mathrm{kJ/(m^3{\cdot}K)} ; liquid water has ρ\rho cpc_p ≈\approx 4180\ kJ/(m3⋅K)\mathrm{kJ/(m^3{\cdot}K)} . The ratio is roughly 3,600 to one. Rejecting 100 kW at a 15 K rise therefore requires about 5.7\ m3/s\mathrm{m^3/s} of air — near 12,000 cubic feet per minute, through a single rack aperture — or about 1.6\ L/s\mathrm{L/s} of water, which is a garden hose. There is no engineering cleverness that closes a factor of 3,600; the only levers are a larger Δ\Delta T or a larger duct, and both run out.

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