Side-by-side comparisons
One question, two or three named options, and an honest account of where the comparison stops being useful. These pages sit beside the field lessons — they answer the difference; the lesson explains the system.
What a comparison page is for
A field lesson explains how a system works. A comparison answers one question about two or three named options: what differs, what each one commits the site to, and which conditions tend to point toward each. Where the honest answer depends on site conditions, hazard analysis, adopted standards or an authority having jurisdiction, the page says so rather than declaring a winner.
These pages are educational reference material. They do not select equipment, specify a design, establish compliance or replace qualified engineering with verified site data.
CRAC vs CRAH: what is the difference?
Both sit on the data hall floor and both move room air. What actually differs is where the heat goes after it leaves the coil.
CRAC — A room-air unit with its own direct-expansion refrigerant circuit.
CRAH — A room-air unit with a chilled-water coil fed from a central plant.
Hot aisle vs cold aisle: two halves of one layout
These are not two competing designs. They are the two aisles a single layout creates — and the useful comparison is what each one is for and what fails in it.
Cold aisle — The aisle equipment air inlets face, supplied with conditioned air.
Hot aisle — The aisle equipment exhausts face, carrying heated air back to the cooling units.
Wet pipe, dry pipe and preaction sprinklers compared
All three are sprinkler systems and all three operate head by head. What differs is what sits in the pipe and what has to happen before water reaches an operating sprinkler.
Wet pipe — Pipes hold pressurised water up to the sprinklers at all times.
Dry pipe — Pipes hold pressurised gas; a valve admits water once a sprinkler opens.
Preaction — Pipes hold supervised gas; a detection event is part of admitting water.
Top-of-rack vs end-of-row switching
Where the access switch sits decides what runs through the cable trays, how many ports sit idle, and how much of the estate a single switch failure touches.
Top-of-rack — One or two access switches per cabinet, with short copper inside it.
End-of-row — Shared access switches in a cabinet at the end or middle of a row.
N+1 vs 2N: what the labels actually claim
Both describe how much more than the required capacity is installed. Neither says whether the extra capacity can be reached when something upstream fails.
N+1 — The capacity the load needs, plus one additional unit of the same kind.
2N — Two full systems, each able to carry the load on its own.