Efficiency compares the inputs used by a system to the outputs produced. It is a commonly used concept, but one which is prone to a great deal of misuse in many industries. This article provides insight into the parameter known as “efficiency,” how it’s calculated, and importantly, it’s uses and limitations in predicting blower energy consumption and comparing alternate system designs.
What is Efficiency?
In engineering, efficiency usually refers to the ratio of the work done or the energy developed by a machine, engine, etc., to the energy supplied to it. It is usually expressed as a percentage, although in most calculations it is used as a decimal. Efficiency is always less than 100%.
For any system, efficiency can be calculated simply:
This simplicity is deceptive. The discussion of blowers encompasses many different types of efficiency:
- Isentropic efficiency
- Polytropic efficiency
- Design point efficiency
- Average efficiency
- Wire-to-Air efficiency
- Bare blower efficiency
The frame of reference, the calculation method, and the operating point have a bearing on the value of efficiency. It’s easy to see that it’s important to define terms and parameters clearly in discussions or comparisons of blower efficiency.
The most common method for evaluating aeration blowers is isentropic efficiency. Isentropic compression is an ideal, reversible process. It is an adiabatic process; i.e., no heat transfer occurs. Real compression never meets these conditions, of course. However, if all calculations and comparisons for a given blower system are made based on the assumption of an isentropic process then comparisons will be accurate. Power calculations will also be correct if parameters are the same for calculating efficiency and calculating power.
The isentropic efficiency of a bare blower is the isentropic power of the discharge air divided by the input power.
%ηs = isentropic efficiency, percent
Ps = isentropic power of air stream, hp
Pin = actual power input to the blower or blower system, hp
qm = mass air flow rate, lbm/min
k = ratio of heat capacities, cp/cv, = 1.4 for air at standard conditions
Rair = specific gas constant for air/water vapor mixture, = 53.51 ft∙lbf/lbm∙°R for air at
Ti = inlet temperature, °R = °F + 460
pd,i = discharge and inlet pressure, psia = psig + barometric pressure
ρ = density, lbm/ft3
qv = volumetric flow rate, ft3 /min
Subscription users only!
Subscribers are able to view the whole article. Please register/subscribe (it's free and easy) to read all articles.