Helen
Helen
Helen
TABLE OF CONTENTS
1.Introduction……………………………………………………………..……………......1
2.types of compressor...........................................................................................................1
3. Main components of an industrial compressed air system………………….....…..…...…3
4.losses and efficency calculation of air compressor for the purpose of energy audit...........7
5. Energy performance assessment of an industrial compressed air system……….……......12
6. Typical energy conservation opportunities……………………………………………..…13
7. Reference…………………………………………………………………………....……16
List of figures
Figure1. A cross-sectional view of a reciprocating compressor………………………...….2
Figure 2.Cross section of a rotary screw compressor……………………………….……...2
Figure 3.Rotary vane compressors…………………………………………...……….….…3
Figure 4.Zero air loss drain ............................................................................................…...5
Figure 5.Filter regulator lubricator ……………………………………………………...….6
Figure 6.Components of a compressed air system …………………………………………7
figure 7. input output and losses for compressed air system...................................................7
INTRODUCTION
1. Compressor is a device that increases the atmospheric pressure of air into a desired set of
pressure according to the user need by using electrical energy as an input. And this compressed
air can be used in many different applications in fact we can definitely argue that compressed air
is the most used type of energy in wide range of application in industries. We can describe
compressed air as a form of stored energy that drives different machineries and equipment that
are used in industry or elsewhere.
A compressor is a machine that is used to increase the pressure of a gas. The
compressed air applications are highly used in the industrial sector due to its easy
transportation, safety, purity, cleanliness and storage capacity. In many countries,
compressed air systems account for approximately 10% of the industry’s total electricity
consumption. However, compressed air is expensive because around 30% of the
consumed energy is used for these systems in an industrial facility. Energy used to be
lost as leaks, pressure drops, heat, and misuse, among others. Several energy efficiencies
measures help to improve energy savings in companies such as: pressure reduction,
reduce the inlet air temperature, use a well-calculated capacity tank for storage, control
heat recovery and reduce leaks, giving potential energy savings in a range between the
20%-60%, with a return of investment not higher than two years.
. Compressor is a device that increases the atmospheric pressure of air into a desired set of
pressure according to the user need by using electrical energy as an input. And this compressed
air can be used in many different applications in fact we can definitely argue that compressed air
is the most used type of energy in wide range of application in industries. We can describe
compressed air as a form of stored energy that drives different machineries and equipment that
are used in industry or elsewhere.
An average air compressor takes about approximately seven volumes of air in atmospheric
condition and by the power of electricity it converts it to one volume of air with elevated
pressure. But when we compress air we are not only increasing the pressure of air we will also
increase the temperature since temperature and pressures are directly related. This means that
after we have used the compressed air the temperature will drop, to mitigate this we can recover
the temperature and use it for other application in industrial or other scenarios.
The reason for considering to recover the heat from the compressed air is that most of the energy
(electrical energy) that is used to compress the air is wasted during the compression process that
about 10-30% of the electrical energy forms the compressed air. The other part is wasted in
overcoming friction, noise and misuse.
2. Types of compressor
Air compressors are categorized as either positive displacement or dynamic displacement, based
on their internal mechanisms. The four most common types of air compressors you will see are:
Rotary Screw Compressor
Reciprocating Air Compressor
Axial Compressor
Centrifugal Compressor
Reciprocating air compressors: uses the reciprocating motion of the piston in the cylinder to
compress the air. When the piston moves in the downward motion the air comes into the cylinder
and when the piston goes in the upward motion the movement of the piston will compress the air
and forces it out of the compressor. Reciprocating air compressors can be multiple cylinders for
high capacity and or multiple stage for high pressures. They are often used in to supply air for
building control and automation systems.
Figure 1: a cross-sectional view of a reciprocating compressor.
Rotary screw compressors: this compressor increases the pressure of the air that is coming to it
by using two rotors that are meshed together and that have different housing. When the rotors
rotate along their place air will be trapped between the gears and the housing and the pressure of
air will be increased by the virtue of the allowable space for the air will decrease. Rotary screw
compressors are quitter and produce cooler air that is easy to dry.
Centrifugal compressors: are special types of air compressors that use a high speed rotating
impellers to accelerate air. They increase the pressure of the air coming by increasing the speed
of the air by using the rotating impellers and passing the air through small areas. They are mainly
used for high gas volume applications in industries. These types of compressors usually run at
high speeds and require noise control.
Vane compressors: a rotary vane compressor uses an elliptical slotted rotor that is housed in a
cylinder. The vanes sweeps through the cylinder as they rotate and sucking in air at one side and
discharging it on the other. They are used for small scale applications.
• Regenerative desiccant type dryers use a porous type of adsorbing mechanism that traps
the moisture when it goes through it. Desiccant can be made from silica gel, activated
alumina and molecular sieves.
• Deliquescent type dryers are the other type of dryers that uses absorption rather than
adsorption.
• Heat of compression dryers use the heat generated during compression to accomplish
desiccant regeneration.
• Membrane dryers use a semi-permeable membrane to separate water vapor from the
compressed air stream.
6. Air inlet filters: protects the compressor from small airborne particles that may enter through
the inlet.
7. Heat recovery: as we have mentioned before compressing air will result in a high temperature
air. And when we compress air in an industrial scale the generated heat will become more. So,
this heat can be recovered and can be used for different applications.
8. Pressure flow controllers: just like switch for electric circuit and valve for pipe, pressure
flow controller serves to separate the supply side of the compressor from the demand side.
9. Air receivers: are used as storages for compressed air that will be used in the event of a high
compressed air demand. They are most useful in compressed air systems that have a fluctuating
compressed air demand.
10. traps and drains: Drains (also called as traps) are needed at all separators, filters, receivers
and dryers to trap the liquid condensate and remove it from the compressed air system line.
Because if this condensate doesn’t get removed they will flow back downstream and overburden
the compressor or they will go forth and foul the end use device. Deficient drain will waste a
significant amount of compressed air. There are known methods that are used for draining among
these are:
• Zero air loss traps with reservoirs: they are the most efficient design and they use
gravity for removing the condensate because of that the installation configuration is very
important to prevent air locks.
• Electrically operated solenoid valves: this valve open for specified period of time. The
drawback in this design is that the interval that the valve opens might not be sufficient
for drainage.
• Float operated mechanical drains: this traps do not waste air when they operate but they
are prone to blockage by sediments.
• Manual drains: these drains will be installed at points where we frequently experience
moisture problems. Because this are manually operated there can be a significant loss of
compressed air.
14. Fittings: Fittings are used to connect pipes with valves and/or other pipes. The fittings must
be air tight fitting to prevent air leakage and loss of air pressure.
Figure 6:
components of a compressed air system
losses and efficency calculation of air compressor for the purpose of energy audit
4.To determine the operating efficiency, we need to apply the 1st Law of Thermo and, for that we
need too know the energy- inputs, outputs and losses:
Recall the Guiding Concept: the objective of the audit is to improve the operating efficiency of
the industral air compressor.
Reference [1 and 6] indicates that these overall efficiencies include gas friction within the
compressor, the mechanical losses (bearings, seals, gear-box, etc.), and gear-box losses. The
mechanical efficiency varies with compressor size and type, but 95% is a useful planning
number. When calculating the compressor head and discharge temperature the efficiency used
will be isentropic or polytropic (isentropic efficiency is sometimes called adiabatic efficiency).
Adding 3-4 % efficiency (mechanical losses) to the overall efficiencies will generally give a good
estimate of the thermodynamic efficiency [1and 6].
To evaluate the performance of an existing compressor, the objective is to calculate the
compressor efficiency (η) and power requirement.
The isentropic efficiency is defined by
eq 1
Where:
ηIsen = Isentropic efficiency
h1 = Suction enthalpy calculated at P1, T1, and composition (zi)
h2 = Discharge enthalpy calculated 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 compressor efficiency or power involves two steps
1. Determination of the ideal or isentropic (reversible and adiabatic) enthalpy change (h2Isen-h1)
of the compression process.
2. Determination of the actual enthalpy change (h2-h1).
The step-by-step calculation based on an EOS:
a. Assume steady state, i.e.
b. Assume the feed composition remain unchanged
c. Calculate suction enthalpy h1=f(P1, T1, and zi) and entropy s1=f(P1, T1, and zi) by EOS
d. Assume isentropic process and set s2Isen = f (P2, T2Isen, zi) = s1 = f (P1, T1, zi).
e. Calculate the ideal enthalpy (h2Isen) at discharge condition for known zi, T2 (or P2) and
s2Isen.
f. Calculate the actual enthalpy (h2) at discharge condition for known zi, T2 and P2.
g. Calculate isentropic efficiency by Equation 1: µIsen = (h2Isen – h1)/(h2 – h1)
h. Calculate power by Equation 2: power
Estimating Efficiency – Shortcut Method
The isentropic path exponent (k) or ideal gas heat capacity ratio (k=CP/CV)
Where:
T = Temperature, K (°R)
Y = Gas relative density; ratio of gas molecular weight to air molecular weight
A = 0.000272 (0.000151)
The actual discharge temperature based on an isentropic path can be estimated by
Similarly, the actual discharge temperature based on a polytropic path can be estimated by
Solving the above equation for the polytropic path coefficient (n):
Similarly, the actual discharge temperature based on a polytropic path can be estimated (ηPoly)
by:
Alternatively:
Where:
Head = Compressor head, m (ft)
Power = Compressor power, kW (HP)
R = Universal gas constant, 848 kg-m/(kmol-K) or (1545 ft-lbf/(lbmol-°R))
PS = Standard condition pressure, kPa (psia)
P1 = Suction pressure, kPa (psia)
P2 = Discharge pressure, kPa (psia)
TS = Standard condition temperature, K (°R)
T1 = Suction temperature, K (°R)
T2 = Discharge temperature, K (°R)
qS = Gas volumetric rate at the standard condition, Sm3/d (scf/day)
Za = Average gas compressibility factor = (Z1+Z2)/2
Z1 = Gas compressibility factor at the suction condition
Z2 = Gas compressibility factor at the discharge condition
MW = Gas molecular weight
the power calculation should be made per stage of compression and then summed for all stages
connected to a single driver.
The step-by-step calculation for shortcut method
a. Calculate the isentropic exponent (k) by Equation 3 using the average temperature defined by
T = (T1+3T2)/4. This form of average temperature was defined to obtain better match between
the rigorous and shortcut method results.
b. Calculate the isentropic efficiency (ηIsen) by Equation 5.
c. Calculate the polytropic coefficient (n) by Equation 7.
d. Calculate the polytropic efficiency (ηPoly) by Equation 8.
e. Calculate the isentropic and polytropic heads by Equations 9 and 10, respectively.
f. Calculate the required power per stage by either Equation 11 or 12.
REFERENCES
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11 .Miriam Benedetti