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Efficiency of a process

How to calculate the efficiency of a process or an equipment

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Section summary
1. Definition of process efficiency
2. Example of efficiency calculation : electrical motor
3. Example of efficiency calculation : chemical process

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1. Definition of process efficiency

The efficiency of a process (or an equipment in some cases) is the ratio in between the input to the process (typically an energy, but it can be also a material flow for instance) and the output. The higher is the ratio, the higher is the efficiency. The efficiency is often expressed in percentages.

Eff (%) = W_o / W_i *100

With :

Eff = efficiency of the process (%)
W_o = output from the process (energy, material flow, work...)
W_i = input to the process (energy, material flow, work...)

2. Example of efficiency calculation : electrical motor

Considering an electric motor which is drawing a power of 1.5 kW but is actually having a power on the shaft of 1.35 kW, the efficiency of the motor is then :

Eff (%) = Wo / Wi *100 = 1.35/1.5*100 = 90%

The efficiency of the motor is then 90%.

3. Example of efficiency calculation : chemical process

The efficiency of a process allows to calculate how optimal is the process. It is typically calculated from a mass balance : the material output (material of interest, typically saleable) to the material input. It may be necessary to account for some un-avoidable losses, for example if the chemical process is releasing a compound like water that has to be eliminated.

Eff = m_o / m_i * 100

Considering a process operating with 1500 kg/h of inlet material, and yielding 1350 kg/h of saleable material while generating 150 kg of byproduct, the efficiency is :

Eff = 1350/1500 * 100 = 90%

Note that on this process, other kinds of efficiencies, for example from an energy balance, could be calculated.

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