Tons of Refrigeration to Horsepower for Cold Storage

Sizing a Cold-Store Compressor From a Tonnage Figure

A cold-store heat load arrives from the calculation sheet in tons: fabric gain, product pull-down, respiration, infiltration through the doors, fan and lighting heat, people working in the chamber. Turning that total into a compressor selection means expressing it as power first, then asking how much shaft work the plant must spend to move it — and at freezer suction temperatures those two numbers are much closer together than anyone new to the trade expects.

Conversion factor: 1 TR = 4.716177291 hp, since 3 516.85 W of heat divided by 745.7 W per horsepower gives 4.7162. A 25 TR frozen store is therefore 117.90 hp of heat flow — and with a low-temperature COP near 1.6, the compressors need roughly 74 hp of shaft power to keep shifting it.

What the Tonnage Figure Is Telling You

The room heat load arrives in tons

Load sheets for chambers, corridors and docks are usually totalled in tons because evaporator catalogues are written that way. It is heat gained by the space over 24 hours, spread across the running hours.

4.7162 hp of heat per ton

This is the ceiling, not the answer. It is the shaft power a plant would need if it were performing no better than a resistance heater run backwards, at a COP of exactly 1.

Suction temperature sets the shaft power

Divide the heat-flow figure by COP at your design suction. A −5 °C chill duty near COP 3 needs a third of it; a −40 °C blast duty near COP 1.2 needs almost all of it.

Motor frame size against compressor size

The selected motor sits above the calculated shaft power to survive pull-down, high condensing on a summer afternoon and a starved evaporator after defrost. Never order a motor equal to the design duty.

Turning a Cold-Room Heat Load Into a Compressor Selection

Plant rooms are built out of a schedule of chambers, each with its own temperature and its own suction group. The conversion is the same for all of them; the divisor is not.

Put the chamber load in as tons

Enter the room total — 5, 25, 40, 60 TR — and the horsepower equivalent of that heat flow appears beside it. Spaces are ignored and a comma is accepted as a decimal point.

Divide by the COP at your suction

Take that horsepower and divide by the coefficient of performance for the design saturated suction temperature. The result is compressor shaft power, which is the number a selection program will confirm.

Read a compressor catalogue backwards

When a supplier answers with duties in horsepower, press the swap arrow (↔) for hp → TR and see what each machine is worth as heat flow before COP is applied. Useful for spotting a quotation that has already divided once.

Send clean numbers to the load sheet

Copy hands over the bare figure without a unit or spacing, which is what a spreadsheet column of chamber duties wants. Ctrl + C from inside a field behaves the same way.

Add the plant's own heat before you divide: evaporator fans, defrost heaters and pump work all end up inside the chamber as load. On a deep-freeze store they can add ten to fifteen per cent to the tonnage the compressors have to carry.

Cold-Room Duties, Tonnage and the Shaft Horsepower Behind Them

Chamber duties from typical distribution and processing plant. Column four is the straight conversion of the load into horsepower of heat flow; column five is the compressor shaft power once the COP for that suction temperature is applied, rounded to the nearest catalogue size.

Chamber duty	Room / evaporating temp	Load	Heat flow in hp	Compressor shaft hp
Produce chill room, small store	+2 °C room, −5 °C evap	5 TR	23.58 hp	≈ 8 hp
Dairy and meat chill chamber	0 °C room, −8 °C evap	15 TR	70.74 hp	≈ 25 hp
Distribution-centre chill hall	0 °C room, −8 °C evap	60 TR	282.97 hp	≈ 100 hp
Frozen store, holding load	−20 °C room, −30 °C evap	25 TR	117.90 hp	≈ 75 hp
Frozen store, pull-down after a full load-in	−20 °C room, −30 °C evap	45 TR	212.23 hp	≈ 135 hp
Ice cream hardening store	−25 °C room, −35 °C evap	10 TR	47.16 hp	≈ 34 hp
Plate freezer, fish processing	−40 °C evap	30 TR	141.49 hp	≈ 125 hp
Batch blast freezer	−35 °C air, −40 °C evap	40 TR	188.65 hp	≈ 160 hp

Follow the last two columns downwards and the story is plain. Up at chill duties the compressors buy their heat cheaply — a third of the heat-flow figure or less. Down at plate-freezer temperatures the shaft power has climbed to nearly nine tenths of it, so a plant that mixes chill and freeze duties on one suction group throws away most of the advantage the chill rooms should have had.

How the Converter Fits the Plant-Room Workflow

Both ends update as you walk the room schedule

Every chamber on the list can be typed straight over the last one, in either box, with the opposite figure following immediately — no clearing, no convert button.

Turn a supplier's horsepower quote back into tons

One press of the swap arrow runs the pair the other way, which is how you check whether a vendor's hp figure is heat flow or shaft power before comparing two offers.

Kilowatts and BTU/h for a mixed-standard plant room

Ammonia packages tend to be quoted in kW while the evaporators alongside them come in BTU/h; the searchable dropdowns hold every power unit on both sides of the page.

Clean digits for the plant-room load sheet

Copy strips the unit and the spacing, and results carry up to eight decimals, so a column of chamber duties stays consistent all the way through the calculation.

Cold-Store Plant Room Questions

Why does a freezer need so much more compressor power per ton than a chill room?

Two things move at once. The pressure ratio the compressor has to work across roughly doubles between a −8 °C and a −35 °C suction, and the gas returning to the machine is far thinner, so each revolution carries less mass. A chill duty near COP 3 spends about a third of the 4.7162 hp per ton; a −40 °C duty near COP 1.15 spends more than four fifths of it. The same tonnage costs three to four times the shaft power.

Does an ammonia plant need less shaft power than an HFC one for the same tonnage?

Usually a little, and the gap widens as the store gets colder. Ammonia's very large latent heat and favourable pressure-volume behaviour give it a modest COP advantage over an HFC blend at low temperature, and two-stage or economised ammonia arrangements push that further. The trade-off is the plant room itself: machinery rooms, gas detection, safety valves and trained operators. Below about 100 TR the paperwork usually wins the argument for HFC or CO₂; above it, the running cost usually wins for ammonia.

How do I allow for defrost when I convert a room load into tonnage?

Two separate allowances. Heat that a hot-gas or electric defrost leaves behind in the chamber becomes load the compressors must remove afterwards, so it goes into the tonnage. Separately, an evaporator that is defrosting is not cooling, so the running-hour base shrinks — a store on six defrosts a day may only cool for 18 or 20 hours, and the design tonnage has to rise to fit the 24-hour heat gain into that shorter window.

Should compressors be sized on pull-down or on holding load?

Size the machinery for the worst realistic daily intake, not for the empty-store holding figure and not for the once-a-decade commissioning pull-down. The table above shows a frozen store at 25 TR holding and 45 TR while a full load-in is being brought down — a plant chosen for the smaller figure simply never catches up on a busy day. Commissioning pull-down from ambient is normally handled by accepting a longer time, sometimes with a hired package, rather than by buying permanent capacity that will idle for the next twenty years.

What happens to shaft horsepower when a screw compressor unloads?

It does not fall proportionally. A slide valve reduces capacity by shortening the effective rotor length, but internal leakage, oil pumping and bearing losses carry on, so a screw at half capacity may still draw around 60 to 70 per cent of full-load power. Below roughly 50 per cent the curve gets noticeably worse. This is why plants are built with several machines and only one of them trimming, or with variable-speed drive on the swing compressor — running two screws at 50 per cent each is markedly more expensive than running one at full load.

Tons of Refrigeration to Horsepower