Matching Chiller and CRAH Tonnage to a Data Hall's kW of IT Load
A data centre keeps two sets of books. The white space is measured in kilowatts — that is how the design load, the UPS blocks, the metered draw and the capacity report are all written. The cooling equipment that has to remove that heat arrives from the vendor labelled in tons: a 30-ton CRAH, a 200-ton chiller, a 500-ton plant. Reconciling the two is a daily job for anyone commissioning a hall or checking whether a new deployment still fits under the plant.
Why Both Units Live on the Same Drawing
Plant catalogues still count in tons
White space is contracted in kilowatts
Room heat is more than the servers
Nominal tonnage is a label, not a promise
Reading a Cooling Schedule Against the Facility Capacity Report
The sequence below is what a facility engineer runs when a vendor quotation lands, or when an operations question — can this hall take another hundred kilowatts? — needs an answer before the change board meets.
Type the nameplate tonnage in
Put the unit's ton figure in the left field — 20, 30, 100, 500 — and the kilowatt equivalent appears as you go. Half sizes are common on room units, and a decimal comma works as well as a dot.
Take the in-room losses off the top
The kilowatt figure is total heat removal, not IT capacity. Knock off the fan, distribution and lighting share before comparing it with a committed load; on most halls a 10 % allowance is a sensible first pass.
Reverse it when the hall load is the known figure
With a 640 kW design load already fixed and the chiller order still open, press the swap button (↔) to run kW → TR. Dividing by 3.5168528 gives about 182 TR of duty before any redundancy is added on top.
Lift the clean number into the capacity sheet
The copy button hands over the plain figure with no unit attached, which is what a spreadsheet cell or a capacity-tracking field expects. Ctrl + C inside a field does the same thing.
Cooling Unit Tonnage and the IT Load It Can Carry
Nominal equipment sizes as they appear on quotations, converted to kilowatts of cooling, with the IT load each supports once about 10 % of the hall's heat is set aside for fan work, in-room distribution losses and lighting.
| Nominal size | Cooling (kW) | IT load carried | Where it turns up |
|---|---|---|---|
| 5 TR | 17.58 kW | ≈ 16 kW | Comms room or a single edge cabinet enclosure |
| 10 TR | 35.17 kW | ≈ 32 kW | Branch server room on a dedicated split system |
| 20 TR | 70.34 kW | ≈ 64 kW | In-row units serving one contained aisle segment |
| 30 TR | 105.51 kW | ≈ 96 kW | A single perimeter CRAH in a modest hall |
| 50 TR | 175.84 kW | ≈ 160 kW | Large downflow room unit or a small modular chiller |
| 100 TR | 351.69 kW | ≈ 320 kW | Air-cooled chiller module in an N+1 bank |
| 200 TR | 703.37 kW | ≈ 639 kW | Water-cooled machine on a small central plant |
| 500 TR | 1 758.43 kW | ≈ 1 599 kW | Centrifugal chiller feeding a 1.5 MW hall |
Read down the middle column and the arithmetic that trips people up becomes obvious: a "100 ton" chiller is not a 100 kW machine, it is a 352 kW machine, and three of them running in an N+1 bank protect roughly 640 kW of IT rather than 960 kW. These are also cooling kilowatts, not electrical ones — at a PUE of 1.4 a 320 kW IT load pulls about 448 kW from the meter, but only the white-space share is what the plant has to lift.
What the Converter Gives You During a Capacity Review
Tonnage and kW track each other as you type
Walk an equipment schedule from the smallest in-row unit up to the central plant without clearing the box between entries — the other side follows every keystroke.
Flip to size plant from a committed load
The swap control turns the page into kW → TR, the direction you need when the hall's design kilowatts are fixed and the chiller order is not.
Reach for megawatts when the campus total is the question
Searchable dropdowns on both sides carry every power unit, so the same page steps a plant figure up to MW or across to BTU/h without leaving the tab.
Figures that paste straight into the capacity model
Results run to eight decimals where they matter and copy as a bare number, so nothing needs cleaning up before it lands in a spreadsheet.
Data Hall Cooling Questions
Should I size against a CRAH's total capacity or its sensible capacity?
Sensible, every time. A data hall has almost no moisture to remove — it is a tightly sealed space with tiny outside-air rates and no wet load — so the latent portion of a catalogue's total capacity is capacity you will never use. Room units built for IT duty run at a sensible heat ratio close to 1.0 for exactly this reason. If a schedule quotes a comfort-cooling total figure, take the sensible column before converting to kilowatts, or the hall gets over-promised by a noticeable margin.
How much of the electricity a rack consumes actually comes back as heat?
Effectively all of it. A server does no mechanical work on the outside world; the energy leaves as a trickle of light down a fibre, a little acoustic noise from the fans, and heat. Treating IT watts as watts of heat one for one is standard practice and is accurate to well inside the tolerance of any load estimate. The extra heat you must allow for comes from everything else in the room, not from a shortfall in that assumption.
Chilled water or direct expansion for a new hall?
DX room units keep the refrigerant circuit inside each machine, which makes small and phased builds simple: you buy tonnage as the load arrives and one failure stays contained. Chilled water centralises the refrigeration in a plant room, moves heat around in pipes rather than refrigerant lines, scales far better past a few hundred tons and opens the door to economiser operation. Below roughly a hundred tons the DX route usually wins on cost and simplicity; above it, water tends to take over.
How do I count tonnage once N+1 redundancy is applied?
Only the units left standing after the worst single failure count towards capacity. Four 100 TR chillers in an N+1 arrangement give 300 TR of usable duty — 1 055 kW, not 1 407 kW — and the fourth machine exists to cover a fault or a maintenance window. The same logic applies to room units: if a hall needs six CRAHs to hold temperature, seven get installed, and the capacity report is written against six.
Do free-cooling hours change the tonnage I have to install?
No — they change what the plant costs to run, not what you install. The equipment still has to hold the full load on the hottest afternoon of the design year, so peak conditions set the tonnage. What economisers change is the energy story: in a cool climate, a raised chilled-water temperature can let them carry a large share of the year's hours with the compressors idle or unloaded. Size for the peak, budget the energy on the hours.
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