Kilojoules to BTU for Refrigerant Enthalpy Readings

Reading a Pressure-Enthalpy Chart in Two Unit Systems

Refrigerant property data is published in SI. Open a P-h chart, a saturation table or a manufacturer's compressor selection file and the enthalpy axis is in kJ/kg, pressures are in bar and mass flow is in kg/s. Then the job in front of you is rated in tons, the customer's paperwork says BTU/h and the recovery cylinder is weighed in pounds. Every refrigeration technician who works from real property data ends up crossing that boundary, and the crossing starts with the kilojoule.

Conversion factor: 1 kJ = 0.947813394 BTU. A refrigeration effect of 150 kJ read off the chart is 150 × 0.947813394 = 142.17 BTU, and per unit mass 1 kJ/kg works out at 0.4299 BTU/lb once the pound is folded in.

What the Numbers on the Chart Actually Are

Enthalpy Is Per Unit Mass

A chart value is kJ/kg, not kJ. It only becomes an energy quantity once multiplied by the refrigerant mass that passed through, which is why mass flow belongs in every capacity calculation.

Refrigeration Effect Is a Difference

Subtract the enthalpy entering the evaporator from the enthalpy leaving it and you have the useful heat each kilogram carries away. Compressor work is the same kind of subtraction across the discharge line.

Superheat and Subcooling Shift the Endpoints

The points you read are not the saturation lines. Ten kelvin of superheat moves the evaporator outlet to the right, and subcooling moves the condenser outlet to the left, both of which change the Δh you convert.

Mass Units Cross Over Too

A kJ/kg to BTU/lb conversion is not the same number as kJ to BTU, because the mass unit changes as well. One kilogram is 2.2046 pounds, which is what drags 0.9478 down to 0.4299.

Converting a Δh Figure While the Gauges Are Still On

Read the chart, do the subtraction in SI, then convert once. Converting each endpoint separately doubles the rounding for no benefit.

Take both enthalpies off the chart first

Fix the evaporator outlet from suction pressure plus measured superheat, and the expansion-valve inlet from condensing pressure minus subcooling. Subtract the two to get the refrigeration effect in kJ/kg.

Enter that difference on the kJ side

The BTU figure follows each keystroke. A decimal comma is read the same as a decimal point, which matters when the property table you are working from was printed in Europe.

Reverse it for an imperial property table

The swap arrows send BTU back into kilojoules, which is how an older BTU/lb table lines up against an SI compressor file. The searchable lists on both sides also reach J, MJ, Wh and kWh for the electrical side of the same machine.

Carry the bare figure into the capacity sheet

The copy control above each field takes the digits alone, with no unit and no spacing, so the value goes straight into a service report or a capacity spreadsheet. Ctrl+C inside a field behaves the same way.

Convert differences, not absolute enthalpies: property tables set their zero point by convention, and different publishers pick different conventions. An absolute enthalpy converted on its own carries that arbitrary offset with it. A Δh between two points on the same table is offset-free and is the only figure worth converting.

Refrigerant Latent Heat in kJ/kg and BTU/lb

Latent heat at roughly 0 °C (32 °F) saturation, which is a fair reference for comparing how much heat each kilogram of refrigerant can absorb as it boils. Values are rounded; always work from the property table that matches your actual evaporating temperature.

Refrigerant	Type	Latent heat (kJ/kg)	Latent heat (BTU/lb)
R-717 (ammonia)	Natural, industrial	1 262	542.6
R-290 (propane)	Hydrocarbon	374.5	161.0
R-32	Single-component HFC	315	135.4
R-744 (CO₂)	Natural, transcritical	231	99.3
R-410A	HFC blend	221	95.0
R-134a	Single-component HFC	198.6	85.4
R-404A	HFC blend	165	70.9

Read down the table and the design consequence is obvious: ammonia carries roughly six times the heat per kilogram that R-404A does, so an industrial ammonia plant moves the same duty on a fraction of the charge and with far smaller liquid lines. At the other end, R-404A's low latent heat is one reason its systems circulate so much refrigerant for the capacity they deliver.

What This Helps With on a Service Call

Suction and Discharge Values Side by Side

Both boxes stay live, so a chart reading and its imperial equivalent can be compared without clearing the field and starting again between the two ends of the cycle.

Figures Clean Enough to Paste into a Capacity Sheet

Copied results contain digits only, with no unit label attached, so a converted Δh lands in a service report or a spreadsheet as a number rather than as text.

The Whole Energy List for Property Tables

Searchable dropdowns on both sides cover J, kJ, MJ, Wh, kWh, BTU and more, which covers the mix of units spread across a refrigerant table and the compressor's electrical data.

Small Enthalpy Steps Without Rounding to Zero

Compressor work per kilogram can be a fraction of the refrigeration effect. Results keep up to eight decimals, so a small Δh does not collapse to a round figure before you can use it.

Questions from the Service Van About Enthalpy Units

I have Δh in kJ/kg off the chart — how do I end up with BTU/h of capacity?

Multiply the refrigeration effect by the mass flow. At 0.05 kg/s with a 180 kJ/kg effect the evaporator is absorbing 9 kW. Kilowatts convert to BTU per hour at 3 412.14, so that is about 30 709 BTU/h — a little over two and a half tons. Do the enthalpy subtraction in SI, multiply by flow, and cross to imperial once at the very end.

Why is ammonia's latent heat so much bigger than R-410A's, and what does that change?

Ammonia is a small, strongly hydrogen-bonded molecule with a very low molar mass, so a kilogram of it contains a huge number of molecules and takes an enormous amount of energy to boil — around 1 262 kJ/kg against roughly 221 for R-410A. In practice that means an ammonia plant moves the same duty on a fraction of the refrigerant charge, with smaller pipework and less inventory to leak, which is a large part of why industrial refrigeration stayed with it.

How do I use a European compressor data sheet on a US job?

The sheet will give capacity in kW at stated evaporating and condensing temperatures, with enthalpies in kJ/kg. Convert the capacity to BTU/h by multiplying by 3 412.14, and divide by 12 000 if the job talks in tons. Check the rating conditions before anything else — European selections are commonly published at different evaporating and condensing temperatures than US practice, and that difference will move the capacity more than any unit conversion does.

Why do two tables give different enthalpies for the same refrigerant at the same temperature?

Because enthalpy has no absolute zero — each publisher picks a reference state and counts from there. The IIR convention sets liquid enthalpy to 200 kJ/kg at 0 °C; the older ASHRAE convention uses 0 at −40 degrees. Two tables can therefore differ by a constant on every line and still be equally correct. Differences between two points on the same table are unaffected, which is why every real calculation uses a Δh.

My recovery scale reads pounds but the charge is specified in kilograms — how do I reconcile that?

One kilogram is 2.2046 pounds, so a 3.5 kg factory charge is 7.72 lb. The energy meaning follows from the latent heat: with R-410A at roughly 221 kJ/kg, every kilogram lost is about 221 kJ of cooling per circulation cycle that the system no longer delivers. Converting the same figure to BTU/lb — 95.0 for R-410A — lets you quote the shortfall in the units the customer's paperwork already uses.

Kilojoules to BTU