How to Estimate Compressor Efficiency
Manufacturers of compressors usually design, develop, and produce these products in order to meet their customers’ specific energy efficiency requirements. However, it is essential to determine the compressor efficiency at the early stages of development.
Compressor efficiency is often evaluated via calculating the coefficient of performance, which is defined as the ratio of the cooling capacity to the power input. Compressor efficiency comprises the following:
- Compression efficiency
- Motor efficiency
- Volumetric efficiency
- Mechanical efficiency
This article primarily covers how to calculate the efficiency of a compressor, or compressor efficiency from composition, measured flow rate, discharge and suction pressures, and temperatures.
There will be a rigidly accurate calculation based primarily on an equation of state, including a shortcut process that will also be considered.
Calculating Compressor Efficiency
Compressor efficiencies generally vary with throughput, compressor size, and type. Therefore, experts can only determine compressor efficiencies via compressor tests, though it is not uncommon for compressor manufacturers to provide excellent estimates.
However, 95 percent can be a beneficial planning number for planning purposes. When estimating the compressor head as well as discharge temperatures, the efficiency employed will be polytropic or isentropic. Isentropic efficiency is, in specific cases, referred to as adiabatic efficiency.
Adding 3 to 4 percent efficiency – which is mechanical losses – to the overall efficiencies in Table One generally gives an excellent estimate of the thermodynamic efficiency.
Table One: Overall Compressor Efficiencies
|Centrifugal||0.70 – 0.85|
|Rotary screw||0.65 – 0.75|
|Low speed reciprocating||0.75 – 0.90|
|High speed reciprocating||0.72 – 0.85|
In order to efficiently evaluate the performance of a compressor, the primary objective is to calculate compressor efficiency as well as power requirement.
The following are the measured and known properties:
- Gas composition (zi)
- Discharge pressure (P2) and temperature (T2)
- Gas mass rate or standard condition gas volume flow rate (qs)
- Suction pressure (P1) and temperature (T1)
Here’s the rigorous method of estimating efficiency:
The heart of every commercial flow simulation software can be traced to the equation of state. However, a cubic EOS – such as Peng-Robinson or Soave Redlich-Kwong, is employed because of their relative accuracy and simplicity.
Using proper binary interaction coefficients, this process simulation results of the 2 calculations are almost identical. This is why the Soave Redlich-Kwong was utilized in this particular work.
This equation defines isentropic efficiency:
ηIsen = Isentropic Enthalpy change/Actual Enthalpy change = (h2isen – h1/h2 – h1)
The compressor power is calculated this way:
Power = (m)(h2 – h1) = (m)(h2Isen – h1/ηIsen)
ηIsen = Isentropic efficiency
h1 = Suction enthalpy estimated at P1, T1, and composition (zi)
h2 = Discharge enthalpy estimated at P2, T2, and composition (zi)
h2Isen = Isentropic discharge enthalpy at P2 (or T2)
S2Isen = S1, and composition (zi)
M = mass flow rate.
The computation of compressor efficiency generally involves 2 steps:
- The first step consists in determining the isentropic or ideal (adiabatic and reversible) enthalpy change (h2Isen – h1) of the compression process
- The second step involves determining the real enthalpy change (h2 – h1)
Here’s the step-by-step calculation of compressor efficiency, usually based on EOS:
- Assume steady state
- Assume that the feed composition remains untouched or unchanged
- Calculate suction enthalpy h1 = f(P1, T1, and zi) as well as entropy s1 = f(P1, T1, and zi) by EOS.
- Assume isentropic process and set i.e. s2Isen = f (P2, T2Isen, zi) = s1 = f (P1, T1, zi).
- Estimate the ideal enthalpy at discharge condition for known T2 (or P2), zi, and s2Isen.
- Compute the actual enthalpy at discharge condition for known P2 and T2, zi.
- Compute isenthropic efficiency by Equation 1: µIsen = (h2Isen – h1)/(h2 – h1)
- Calculate power by equation 2: Power = (m)(h2 – h1)
Factors That Affect Compressor Efficiency
Several factors are responsible for making a compressor grow less efficient over time and without adequate maintenance. Here they are in no particular order:
- Hot air
Hot air has a far lower density than cold air. But cold air is naturally more ‘compressed’ than hot air. This implies that pumping cold air into a compressor is far more efficient than doing the same for hot air.
Pumping cold air into your compressor helps in maximizing its efficiency, resulting in significant productivity gains over time.
Water is known to have high specific heat, meaning it requires a lot of energy to change its temperature. In addition, water collects within the compressor as condensation and triggers corrosion or rust.
This is why it is highly essential to ensure incoming air is as dry and cool as possible. This is one of the most efficient and best ways of protecting your costly investment and boosting the efficiency of your compressor.
- Impure air
Impure air usually contains dust, dirt, or other contaminants or impurities. These can cause the accumulation of gunk inside the compressor. The build-up wears heavily on compressor components and also reduces the internal volume.
This can considerably increase costs due to repeated compressor maintenance procedures or sessions.
Controls That Assist in Boosting Compressor Efficiency
The following are a few controls that assist in boosting compressor efficiency:
- Start and stop controls
These controls inform the compressor’s system to switch off and on, depending primarily on the pressure desired.
- Modulating controls
Compressors are designed to track their own needs – e.g., flow, etc. – and modulating controls allows you to tune this crucial parameter. In addition, multi-step controls offer the unique ability to operate without full pressure load.
- Variable displacement
This critical parameter allows the compressor to function in multiple conditions of partial load.
- Load/unload controls
These controls work in tandem with the start and stop controls, informing the compressor to release pressure.
- Auto dual and dual controls
These controls allow you to select between the load and unload or start and stop switches.
Many industries rely heavily on compressed air as a vital means of powering their most crucial tools or appliance. Therefore, maintaining compressor efficiency is essential as it helps gauge compressors’ operability over time.
Therefore, estimating compressor efficiency is essential as it helps you determine the capacity of your unit over time and watch out for the factors that could negatively impact its efficiency and productivity.
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