# Calculating isentropic efficiency of a compressor

• MattH150197
In summary, the conversation discussed an experiment involving an electric motor driving a compressor, with instrumentation available to measure the compressor's torque and rotational speed, as well as pressure readings from 6 water-filled manometer tubes. The task was to calculate the isentropic efficiency using the given equation. The calculations resulted in an air mass flow rate of 3.89 kg/s, a difference in stagnation pressure of 1900.92 pa, air density of 1.21 kg/m3, shaft torque of 213.86 N.m, and rotational speed of 303.68 rad/s, with an isentropic efficiency value of 0.094. It was noted that the pressure difference seemed low and the value of the
MattH150197

## Homework Statement

We did an experiment which included having an electric motor provide power to drive the compressor and we had instrumentation available to measure the compressor shaft torque and shaft rotational speed and 6 water filled manometer tubes were used as measurements of the pressure. Using the manometer heights we was asked to calculate the isentropic efficiency.

## Homework Equations

The equation we were given to used: isentropic efficiency = (air mass flow rate * Difference in stagnation pressure across the compressor)/ (air density * compressor shaft torque * shaft rotational speed)

## The Attempt at a Solution

I calculated the air mass flow rate at 3.89 kg/s, the difference in stagnation pressure at 1900.92 pa, air density at 1.21 kg/m3, shaft torque at 213.86 N.m and rotational speed at 303.68 rad/s from those I got an answer of 0.094 I am new to this subject so not sure what kind of numbers I should be getting but that answer seems really low, is there any numbers you can see that seem unreasonable or could it be correct? Thanks

MattH150197 said:

## Homework Statement

We did an experiment which included having an electric motor provide power to drive the compressor and we had instrumentation available to measure the compressor shaft torque and shaft rotational speed and 6 water filled manometer tubes were used as measurements of the pressure. Using the manometer heights we was asked to calculate the isentropic efficiency.

## Homework Equations

The equation we were given to used: isentropic efficiency = (air mass flow rate * Difference in stagnation pressure across the compressor)/ (air density * compressor shaft torque * shaft rotational speed)

## The Attempt at a Solution

I calculated the air mass flow rate at 3.89 kg/s, the difference in stagnation pressure at 1900.92 pa, air density at 1.21 kg/m3, shaft torque at 213.86 N.m and rotational speed at 303.68 rad/s from those I got an answer of 0.094 I am new to this subject so not sure what kind of numbers I should be getting but that answer seems really low, is there any numbers you can see that seem unreasonable or could it be correct? Thanks
Did you mean stagnation pressure or static pressure? Also, is this a tiny little compressor or a great big compressor?

Stagnation pressure and the compressor used was a Budworth two stage axial flow compressor

MattH150197 said:
Stagnation pressure and the compressor used was a Budworth two stage axial flow compressor

Im sorry it's really kind of hard to show you how I did the calculation because for example to get the air mass flow rate I had to do 3 separate calulations to get that value so I'd have to write lots of different calculations, I really appreciate you helping but would you say that the value I got for the isentropic efficiency is plausible or not? Thanks

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The mass flow rate seems OK. The pressure difference seems low (to me), for whatever that's worth.

Okay thanks very much

## 1. What is isentropic efficiency?

Isentropic efficiency is a measure of the effectiveness of a compressor in converting energy from the input (usually in the form of gas or air) into mechanical work. It represents the ratio of the actual work output to the theoretical maximum work output under ideal conditions.

## 2. How is isentropic efficiency calculated?

The isentropic efficiency of a compressor is calculated by dividing the actual enthalpy increase of the gas during the compression process by the ideal enthalpy increase. This can be expressed as a percentage or decimal value.

## 3. What factors affect the isentropic efficiency of a compressor?

The isentropic efficiency of a compressor can be impacted by several factors, including the compressor design, operating conditions, and gas properties. Other variables such as inlet and outlet pressure, temperature, and flow rate can also affect the efficiency.

## 4. Why is it important to calculate the isentropic efficiency of a compressor?

Calculating the isentropic efficiency of a compressor allows for the evaluation of its performance and can help identify any potential issues or areas for improvement. It can also be used to compare the efficiency of different compressors or to monitor changes in efficiency over time.

## 5. What is a typical isentropic efficiency for a compressor?

The isentropic efficiency of a compressor can vary depending on the type, size, and operating conditions. Generally, a well-designed and maintained compressor can have an isentropic efficiency of 70-90%, while older or poorly maintained compressors may have efficiencies as low as 50-60%.

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