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Wednesday, December 26, 2007

Energy saving - Installing high efficiency motor

The electric power required for the air compressor can be expressed as,







We can see that the electric power depends on the efficiency of the electric motor, thus, to install high efficiency motor we can save the energy as shown in the following equations,














where:
rated power = nominal power of the motor listed on its label (the power the motor delivers at full load)
load factor = the fraction of rate power at which the motor normally operates

The energy saving by replacing a motor by a high-efficiency motor can be calculated from,





Normally, the efficiency of a motor ranges between 70% to 96%. The loss is usually in the form of heat. Load factor also plays an important part in heat generation. Normally, high heat level is generated during part loading of a compressor.

Important considerations in the selection of a motor for a compressor
  • Operating profile of a compressor i.e. load vs time
  • Efficiency of the motor at part-load conditions
  • Efficiency of the motor at full-load conditions

One important rule to remember: "The efficiency of the motor decreases with decreasing load". See the following figure,




By the way, the efficiency of a motor at part-load conditions can be increased by installing a variable voltage controller.

Please keep in mind that oversizing is a bad practice beacuse the motor always operates at part-load conditions that gives lower efficiency of the motor.

Conclusion: "Using a small motor at full capacity is bettet than oversized motor, because the motors can handle occasional overloading well without any problems"

Energy saving - Repairing air leaks on compressed air lines

What is the results of air leaks on compressed air lines?

  • An air compressor works harder
  • An air compressor works longer

The above undesireable results make the compressor consumes more energy.

Eventhough air leaks is unavoidable, but with good house-keeping factory, the level of air leaks of 10% is acceptable. Most of the factories have air leaks far beyond that what mentioned.

Air leaks normally occur at the joints, flange connections, elbows, valves, filters, hoses, etc. as a result of thermal cycling and vibration.

How to detect air leaks?

  • Listen for hissing sound -- in some factories, even when in production process, we can still hear this sound if the leakage rate is high
  • Apply soap water at the location where the air leaks may present
  • Using an acoustic leak detector
  • Pressure drop test

Mechanical energy wasted caused by air leaks

The following equation expresses the actual mechanical energy wasted due to air leaks









where: 1 < n < 1.4 (isentropic) and 0.7 < hcomp < 0.9

From knowledge of fluid dynamics, we know that when line pressure P2 > 2 atm, the velocity of air at the leak site is equal to the local speed of sound.

Thus, the mass flow rate of air through a leak can be expressed as,








where:
k = specific heat ratio = 1.4 for air
Cdischarge = 0.65 (approx.)






From the above equations, you can calculate yourself to see how the energy is wasted through air leaks if the leak site has 3 mm diameter and the line pressure Pline is 5 bar. You will see that a lot of money is wasted!!

Read more details how to save the energy

Energy saving - How to calculate the cost savings

The electric motor is usually the prime mover of the air compressors. Therefore, to save the energy of compressed air system is to save the energy consumption of the electric motor.

Energy saving of the electric motor can be written as shown in the following formula







Thus,

Cost savings = Energy savings x Unit cost of energy

Facts about energy saving of compressed air system



"We are quick to identify energy losses from hot surfaces and to insulate those surfaces. But we are not so sensitive when it comes to saving compressed air, fixing air leask for instance, because most of the people think that the air is free-of-charge, but they don't reallize that to get the air at higher pressure than the atmospheric pressure requires considerable amount of work (energy)"

"The cost of electricity to operates the air compressor for one year can exceed the purchase price of the compressor"

In the next section we will see the cost of compressed air as well as the amount of energy wasted through the air leaks and so on.

Multi-stage compression with intercooling - Pressure ratio

We know from the previous section that the minimum air compressor work is achieved with isothermal compression. In practical way, we try to achieve that by involving some cooling during compression process that leads to Polytropic compression process.

Normally, this can be achieved by dividing air compression into 2 stages. The first stage builds up the pressure from P1 to Px then the compressed air is cooled by the intercooler and the second stage compressor builds up the pressure again from Px to the final pressure P2. See the following figures to understand how the energy can be saved by using intercooling between each stage.



























Fig.1 P-v diagram of polytropic compression process with intercooling




























Fig.2 T-s diagram of polytropic compression process with intercooling

We can see from Fig. 1 that the amount of compressor work saved is related to the pressure Px.

What is the optimal value of Px that yields maximum compressor work saved?

The total compressor work, for this case, is the summation of compressor work of each stage as follows,

w_total = w_1 + w_2

We can see that w_total will be lowest when w_1 = w_2. Thus,

P1/Px = P2/Px

or





That means the pressure ratio of each stage should be identical to get the lowest amount of work required for air compression.

Tuesday, December 25, 2007

Air compressors work - Compression process

To understand how to save the energy in compressed air system, it is useful to start with theory background of how the air compressors work and how the energy is consumed. As we know from Thermodynamic principle that the compressor work is,





where:
rev = reversible
in = input

Our objective is to minimize the air compressor work that means to approach the reversible process i.e. minimize the friction, and turbulence

Practical way to do this is to make v (specific volume) small by maintaining T (temperature) at low temperature during compression because v a T. In other words, to reduce the work input to a compressor, air should be cooled as it is compressed.

Effect of Cooling

  • Isentropic process: No cooling during compression
  • Polytropic process: Involve some cooling
  • Isothermal process: Involve maximum cooling
Which process yields the minimum required compressor work?
Let's consider the following equations of each process.

Assumptions:

  • All three processes are executed between the same pressure levels (P1 and P2)
  • Reversible process, gas behaves as an ideal gas (Pv = RT)

Isentropic process (Pvk = constant, k = Cp/Cv)







Polytropic process (Pvn = constant)








Isothermal (Pv = constant)






From the above 3 equations, we can plot them in P-v diagram as follows,





















where:
Red line represents an isentropic compression process (n=k)
Blue line represents a polytropic compression process (1<n<k)
Green line represents an isothermal compression process (n=1)
Yellow area represents the air compressor work required during compression process of an isothermal process

Because the area from each line to the left is the required air compressor work, we can see that an isothermal process requires loweer amount of energy than polytropic process and isentropic process respectively.

Now we understand that we can save the energy required for compression if we could have some cooling during compression.

Next topic is to see how to save the energy in polytropic compression process which is the case of most air compressors.

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