Altitude and temperature derating calculator

What the altitude and temperature derating calculator calculates

The altitude and temperature derating calculator works out how much useful air a compressor really delivers at your site compared with its catalogue rating. Free air delivery (FAD) is quoted at reference inlet conditions, commonly the ISO 1217 rating conditions of 1 bar a and 20 °C. At a higher site or with hotter inlet air, each cubic metre drawn in holds less air by mass, so the plant receives less than the nameplate suggests.

The tool takes the site altitude or a measured barometric pressure, the inlet air temperature, the rated FAD and its rating conditions, the working pressure and, optionally, the plant demand. It returns the site pressure, a density factor, the output restated to the rating conditions, the loss, the site pressure ratio and the rated FAD needed to cover the demand, with the margin left over.

Plant engineers use it when sizing a compressor, when a machine falls short in summer, or before moving a portable unit to a hill site. Drilling contractors use it before taking a down-the-hole (DTH) rig to a high site, because the hammer’s air demand does not fall with altitude while the compressor’s mass output does.

Why altitude and hot inlet air reduce compressor output

Altitude and hot inlet air reduce compressor output because they lower the density of the air entering the compressor. A positive-displacement compressor, screw or piston, sweeps roughly the same volume of air wherever it is installed. What changes is the mass of air inside that volume, and mass is what tools, hammers and processes consume.

Air density rises with absolute pressure and falls with absolute temperature. Barometric pressure falls as altitude rises: the standard atmosphere gives about 1.013 bar a at sea level, about 0.955 bar a at 500 m and about 0.846 bar a at 1,500 m. Hot air is thinner too: air at 45 °C is about 7.9% less dense than air at 20 °C at the same pressure, because 293.15 ÷ 318.15 = 0.921 in kelvin terms and 1 − 0.921 = 0.079.

Plant demand should therefore be stated at the same reference conditions as the rating and compared with the output restated to those conditions, not with the volume the machine draws in.

Pressure ratio, discharge temperature and motor limits at altitude

The pressure ratio at a high site is larger than at sea level, because the compressor starts from a lower absolute inlet pressure but must still deliver the same gauge pressure. At 7 bar g the ratio is (7 + 1.01325) ÷ 1.01325 = 7.91 at sea level and (7 + 0.955) ÷ 0.955 = 8.33 at a 500 m site. A higher ratio means a hotter discharge and, on screw machines, slightly more internal leakage.

The drive is the second limit. Standard electric motors are rated under IEC 60034-1 for operation up to 1,000 m and 40 °C ambient. Above either value the motor cools less effectively and its maker’s derating must be checked; for diesel units, the engine maker’s derating applies. The calculator warns whenever altitude exceeds 1,000 m or inlet temperature exceeds 40 °C.

Method and formulas

The method restates the compressor’s output by the mass of air it delivers, using one density factor.

• Site pressure. p_site = 1.01325 × (1 − 2.25577 × 10^−5 × h)^5.25588 bar a, where h is the altitude in metres. A measured barometric pressure, if entered, is used instead.
• Density factor. f = (p_site ÷ p_rating) × (T_rating ÷ T_inlet), with p_rating the rating inlet pressure in bar a (default 1.0), T_rating the rating temperature and T_inlet the inlet air temperature, both in kelvin (°C + 273.15).
• Output on the rating basis. Output = rated FAD × f. Loss = (1 − f) × 100%.
• Pressure ratio. Ratio at site = (p_g + p_site) ÷ p_site, where p_g is the working pressure in bar g. Sea-level ratio = (p_g + 1.01325) ÷ 1.01325.
• Rated FAD needed. Needed = demand ÷ f, where demand is the plant demand at rating conditions. Margin = (rated FAD − needed) ÷ needed × 100%. A negative margin means the machine falls short.

Conversions used: 1 m³/min = 35.31 cfm = 60 m³/h = 16.67 L/s; 1 bar = 14.50 psi.

Output as a percentage of rating by altitude and inlet temperature

The table gives the density factor, which is the output as a percentage of the rated FAD, for a rating basis of 1.0 bar a and 20 °C. At sea level and 20 °C the figure is 101.3%, slightly above 100%, because the standard sea-level pressure of 1.013 bar a is higher than the 1.0 bar a rating pressure.

Each 500 m step in altitude costs roughly 5 percentage points of output at 20 °C, from 101.3% at sea level down to 70.1% at 3,000 m. Moving from 20 °C to 45 °C inlet air costs about a further 8 points at sea level (101.3% to 93.4%) and about 5.5 points at 3,000 m (70.1% to 64.6%).

Worked example 1: 500 m site with 45 °C summer inlet air

Worked example 1 is the calculator’s default: a Hyderabad-type site at 500 m, 45 °C summer inlet air, a compressor rated 10 m³/min at 1.0 bar a and 20 °C, working at 7 bar g (101.5 psig), and a demand of 8.5 m³/min at rating conditions.

1. Site pressure: 1.01325 × (1 − 2.25577 × 10^−5 × 500)^5.25588 = 0.9546 bar a, shown as 0.955 bar a.
2. Temperatures in kelvin: rating 20 + 273.15 = 293.15 K; inlet 45 + 273.15 = 318.15 K.
3. Density factor: f = (0.9546 ÷ 1.0) × (293.15 ÷ 318.15) = 0.8796, that is 88.0%.
4. Output on the rating basis: 10 × 0.8796 = 8.796, shown as 8.8 m³/min, which is 8.796 × 35.31 = 311 cfm.
5. Loss: (1 − 0.8796) × 100% = 12.0%.
6. Pressure ratio: (7 + 0.9546) ÷ 0.9546 = 8.33 at site, against (7 + 1.01325) ÷ 1.01325 = 7.91 at sea level.
7. Rated FAD needed: 8.5 ÷ 0.8796 = 9.66 m³/min.
8. Margin: (10 − 9.66) ÷ 9.66 = 3.5%.
9. Warning: 45 °C is above the 40 °C ambient limit of standard motors, so the motor maker’s derating must be checked.

On a 20 °C winter morning at the same site, the factor is 0.9546 × (293.15 ÷ 293.15) = 95.5%, the output is 10 × 0.9546 = 9.55 m³/min (337 cfm), the loss is 4.5%, the rated FAD needed is 8.5 ÷ 0.9546 = 8.904, shown as 8.90 m³/min, and the margin is (10 − 8.904) ÷ 8.904 = 12.3%, with no warning. What this means: the summer case sets the size. The 10 m³/min machine covers the demand, but with only 3.5% in hand, so growth in demand or a hotter intake could push it short.

Worked example 2: DTH drilling at a 1,500 m hill site

Worked example 2 is DTH drilling at a 1,500 m hill site, 40 °C inlet air, a compressor rated 30 m³/min at 1.0 bar a and 20 °C, working at 25 bar g (362.6 psig), and a demand of 25 m³/min at rating conditions.

1. Site pressure: 1.01325 × (1 − 2.25577 × 10^−5 × 1,500)^5.25588 = 0.84556 bar a, shown as 0.846 bar a.
2. Temperatures in kelvin: rating 293.15 K; inlet 40 + 273.15 = 313.15 K.
3. Density factor: f = (0.84556 ÷ 1.0) × (293.15 ÷ 313.15) = 0.7916, that is 79.2%.
4. Output on the rating basis: 30 × 0.7916 = 23.75 m³/min, which is 23.75 × 35.31 = 839 cfm.
5. Loss: (1 − 0.7916) × 100% = 20.8%.
6. Pressure ratio: (25 + 0.84556) ÷ 0.84556 = 30.57 at site, against (25 + 1.01325) ÷ 1.01325 = 25.67 at sea level.
7. Rated FAD needed: 25 ÷ 0.7916 = 31.58 m³/min.
8. Margin: (30 − 31.58) ÷ 31.58 = −5.0%, so the calculator also warns that this compressor will not meet the demand.
9. Warning: 1,500 m is above the 1,000 m limit, so check the motor maker’s or, for a diesel unit, the engine maker’s derating.

What this means: the 30 m³/min machine falls short at this site. The hammer would see slower penetration and poorer hole cleaning. A machine of at least 31.58 m³/min rated FAD, with its drive confirmed for 1,500 m and 40 °C, is the starting point.

Indian site conditions that change the answer

Indian site conditions change the answer mainly through inlet temperature, and secondly through altitude. Hyderabad stands about 500–550 m above sea level, with barometric pressure about 0.95 bar a against 1.013 bar a at sea level.

• Summer intake air. Across Telangana and Andhra Pradesh, summer intake air reaches 40–45 °C. At 500 m and 45 °C the factor is 88.0%, so size on the summer case.
• Hot compressor rooms. A compressor in a poorly ventilated room breathes its own heat. Ducting outside air to the intake or improving ventilation recovers output directly; the compressor room ventilation worksheet sizes the airflow.
• Dusty sites. Quarries, crushers and construction sites choke intake filters quickly. A choked filter lowers inlet pressure below barometric, cutting output just as extra altitude does.
• Monsoon humidity. Water vapour displaces some dry air. This method ignores humidity, so monsoon dry-air output is slightly below the figure shown.
• Hill and plateau sites. Hill project sites can sit well above Hyderabad. Enter a measured barometric pressure when available.

How to interpret the result, and its limits

The result is a first estimate of the mass-flow loss from site pressure and inlet temperature: a sizing guide, not a guaranteed delivery. The method assumes a constant inlet volume; in practice volumetric efficiency falls slightly as the pressure ratio rises, and screw machines leak a little more internally, so real output can be marginally lower.

The standard atmosphere is an average, and actual pressure varies with weather. Humidity is ignored. The calculator does not derate the motor or engine; it only warns. A derated drive that forces reduced speed or pressure adds a further loss. Check also that discharge temperature stays within the maker’s limit.

How to check a compressor’s site capacity step by step

Check site capacity in eight steps, from measured site conditions to a final margin.

1. Find the site altitude, or better, measure the barometric pressure in bar a at the site.
2. Measure the hottest inlet air temperature at the intake itself on summer afternoons.
3. Confirm the rated FAD and its rating conditions; convert with the FAD and reference condition conversion tool if the basis differs.
4. Express the plant demand at the same rating conditions as the compressor.
5. Enter the values and read the factor, output and margin.
6. Repeat for the coolest season and size on the worst case.
7. If the warning appears, obtain the motor or engine maker’s derating for your altitude and temperature.
8. Add a margin for demand growth.

How Reddy Compressors helps

Reddy Compressors, operating since 2002 from its office at Tarnaka, Secunderabad, is an independent multi-brand compressor specialist covering sales of new and used/refurbished compressors, spare parts, service and rentals. Our rental fleet covers 50 to 2,000 cfm at 2 to 30 bar, with special high-pressure DTH and borewell units up to 35 bar for drilling work. Service covers all major brands across Telangana and Andhra Pradesh, and equipment is supplied across India.

What to send us

Send us these details so we can size a compressor for your site conditions.

• Site location and altitude, or a measured barometric pressure in bar a.
• Highest inlet air temperature at the compressor intake, and whether the machine sits indoors or outdoors.
• Required working pressure in bar g at the point of use.
• Air demand, with the conditions it is stated at, or the tools and hammers to be supplied.
• Nameplate details of any existing machine.
• Drive preference (electric or diesel) and the available power supply.

Frequently asked questions

Does altitude reduce a compressor’s cfm?

Yes, altitude reduces a compressor’s useful output, though not the volume it draws in. The air at a high site is thinner, so each cubic metre contains less mass, and output restated to rating conditions falls in proportion to barometric pressure. At 20 °C inlet, a compressor delivers about 95.5% of its 1 bar a, 20 °C rating at 500 m, 89.9% at 1,000 m and 79.5% at 2,000 m.

How much does high ambient temperature reduce compressor output?

High inlet temperature reduces output in proportion to the ratio of absolute temperatures. The density factor for temperature is T_rating ÷ T_inlet in kelvin. Against a 20 °C rating, inlet air at 45 °C gives 293.15 ÷ 318.15 = 0.921, so about 7.9% less air by mass from temperature alone. At a 500 m site the combined effect of altitude and 45 °C inlet air leaves 88.0% of the rated output, a loss of 12.0%.

How do I size an air compressor for a high-altitude site?

Size the compressor by dividing the plant demand at rating conditions by the site density factor. The factor combines site pressure and inlet temperature: f = (p_site ÷ p_rating) × (T_rating ÷ T_inlet). For example, a demand of 25 m³/min at a 1,500 m site with 40 °C inlet air, where f is 0.7916, needs 25 ÷ 0.7916 = 31.58 m³/min of rated FAD. Then confirm that the motor or engine is derated correctly for the altitude.

What are the ISO 1217 rating conditions for compressor FAD?

The ISO 1217 rating conditions used for this calculator are an inlet pressure of 1 bar a and an inlet temperature of 20 °C. FAD is quoted at these conditions so machines can be compared on one basis. Real sites rarely match them: Hyderabad sits at about 0.95 bar a and summer intake air reaches 40–45 °C. Restate output and demand to the same reference conditions before comparing them.

Why does a compressor run hotter at altitude?

A compressor runs hotter at altitude because its pressure ratio rises. Starting from a lower absolute inlet pressure, it must compress through a larger ratio to reach the same gauge pressure, and discharge temperature rises with ratio. At 7 bar g the ratio is 7.91 at sea level and 8.33 at a 500 m site. At 25 bar g it climbs from 25.67 at sea level to 30.57 at 1,500 m, which also increases internal leakage on screw machines.

Do electric motors need derating above 1,000 m or 40 °C?

Yes, standard electric motors need checking above 1,000 m altitude or 40 °C ambient. IEC 60034-1 rates standard motors for operation up to 1,000 m and 40 °C, because thinner or hotter air cools the windings less effectively. Beyond either limit the motor maker’s derating data must be applied before the motor is accepted for the duty. Diesel engines also lose power with altitude and heat, so the engine maker’s derating must be checked for portable units.

Related pages

• Compressed air knowledge and calculation centre
• FAD and reference condition conversion tool
• Airflow, FAD and reference conditions
• DTH compressor selection
• Diesel portable compressors
• Industrial air compressor selection guide
• Pressure unit and gauge-absolute conversion tool