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Escuela Politcnica Nacional 12/11/2015

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ESCUELA POLITCNICA NACIONALFACULTAD DE INGENIERA MECNICA

Termodinmica III

Compresin de Gas

Jos Luis Palacios EncaladaQuito, 05 de Noviembre 2015

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Termodinmica III

Contenido

1. Compresin de gas

2. Compresin con interenfriamiento

3. Tipos de compresores

4. Criterios de seleccin

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Termodinmica III

Contenido

1. Compresin de gas




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Termodinmica III

Compresin de gas Los compresores son mquinas que reciben energa mecnica de un

motor impulsor. Transfieren parte de esa energa a un gas aumentando su presin. Otra parte de la energa se disipa por prdidas trmicas. El gas comprimido es almacenado en un recipiente de presin donde

posteriormente es conducido para su uso.

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Compresor reciprocante The reciprocating compressor consists of one or more cylinders each

with a piston or plunger that moves back and forth, displacing a positive volume with each stroke.

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Compresor reciprocante - Ciclo Termodinmico

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induced volume

swept volume


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Existen tres formas de clculos termodinmicos por compresin:1. Proceso Isentrpico (s=cte) , PV k = constant, k = isentropic factor

2. Proceso politrpico, PV n = constant, n = polytropic factor

3. Proceso Isotrmico, PV = constant


Termodinmica III


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=2

1d

P

PPVW

T W


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Termodinmica III

Contenido

1. Compresin de gas




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Compresin con interenfriamiento

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En el interenfriador el gas es enfriado en cada una de las etapas de compresin.


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Compresin con interenfriamiento

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Compresin con interenfriamiento - ventajas

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Se requiere menos trabajo para la misma cantidad de aire a la misma presin de suministro.

Se reducen los costos de compresin.

Mayor vol

El tamao de los cilindros puede ajustarse a la presin y volumen requeridos.

El balance mecnico del compresor es mejor.

Se logra una mejor lubricacin debido a una baja temperatura.


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Eficiencia Isentrpicas

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Contenido

1. Compresin de gas




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Tipos de compresores

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Dependiendo del aumento en la presin del gas, a las mquinas que comprimen aire se las divide en:

*P aumento de la presin (g) del gasBombas de vaco

P [atm]

0.15

2.00

0.00

Ventialdores

Sopladores

Compresores


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Segn la forma en que se produce la transformacin de energamecnica en energa de presin en el gas, los compresores seclasifican:

C. desplazamiento positivo (volumtricos)- un elemento del compresor se desplaza conjuntamente con el gas- porciones de gas son comprimidos y su vol. - proceso de compresin discontinua y pulsante

C. dinmicos (desplazamiento negativo)- un elemento giratorio con aletas o labes induce fuerza centrfuga algas aumentado su energa y cantidad de mov.

- proceso de continuo de compresin


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Termodinmica III

Contenido

1. Compresin de gas




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Seleccin de compresores

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Centrifugal compressors

Advantages

High approaching 2 stagesreciprocating compressor

Can reach pressure up to 1200 psi Completely package for plant or

instrument air up 500 hp Relatives first cost improves as

size increase Designed to give lubricant free air Does not require special

foundations

Disadvantages

High initial cost Complicated monitoring and control

systems Limited capacity control

modulation, requiring unloading for reduced capacities

High rotational speed require special bearings and sophisticates vibration and clearance monitoring

Specialized maintenance considerations


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Reciprocating compressors

Advantages

Simple design, easy to install Lower initial cost Large range of horsepower Special machines can reach

extremely high pressure Two stages models offer the

highest efficiency

Disadvantages

Higher maintenance cost Many moving parts Potential for vibration problems Foundation may be required

depending on size Many are not designed to run at full

capacity


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Understand the applicationWhat is the compressor supposed to do?

Find out the detailsGas, pressures, temperatures, ccapacities, etc.

Scope of supplyWho is to supply the motor, swithgear, piping, etc?

Size the compressor


Termodinmica III

Step I Understand the application

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Become familiar with the big picture before getting into the details.

Form a clear, concise statement describing the purpose of thecompressor.

Many compressors operate at more than one condition.

Determine why and how often the various conditions occur.


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Step II Find out the details

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To determine what type of compressor system will be needed. Avariety of detailed date is required:

Gas being handled

Suction and discharge pressure

Site elevation (local barometric pressure)

Suction temperature


Termodinmica III

Step II Gas being handle

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The gas to be compressed must be precisely identified. Gas name may be presented as a chemical formula. Data needed:

Molecular weight

n value

Critical Pressure (Pcr)

Critical Temperature (Tcr)


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Step II Pressure and Temp.

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Everything in absolute terms

Pressure

Temperature

Ps Suction Pressure

Pd Discharge Pressure

@ the compressor inlet (psia, kPa)

@ the compressor discharge (psia, kPa)

Ts Suction Temperature

R = F + 460 K = C + 273


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Step II Capacity

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To size the compressor, the capacty must be stated as the colume itwill ocupy @ compressor suction.

This volume is normally referred as inlet cubic feet per minute (ICFP). The actual cubic feet per minute (ACFM).


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Step III Scope of Supply

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The compressor is everything that is needed to take the gas at somepoint in the system and place it at another point @ high pressure.

Such a compressor is actually a process or system that consists of anumber or common items or systems:


Termodinmica III

Step IV Selecting the Proper Compressor

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Knowledge of the gas, required capacity, suction pressure, suctiontemperature, and discharge pressure will enable the compressor to besized.

The basics steps involved are:1. Calculate the compression ratio2. Choose between a single-stage or two-stage compressor3. Calculate the discharge temperature4. Determine the volumetric efficiency5. Determine the required piston displacement6. Select the compressor model7. Determine the minimum RPM required of the selected compressor8. Select an actual RPM9. Calculate the actual piston displacement10. Calculate the power required11. Select appropiate options


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Termodinmica III

Step IV Selecting the proper compressor

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Part 1 Calculate the compression ratio Compression ratio (R) is the ratio of discharge pressure to suction

pressure:


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Part 2 Choosing a one-stage or two-stage compressor This decision is mainly based on the compression ratio (R). Discharge temperatures and the duty cycle could also be considered. Here are some guidelines for choosing the proper number of stages:


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Part 2 Choosing a one-stage or two-stage compressor Comparison of a single-stage and two-stage compressor both

installed to do the same application (same capacity, gas andpressures):


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Part 3 Calculate the discharge temperature (Td) It directly affects the life of the piston rings and valves. Equation to calculate the discharge temperature for an air cooled

single-staged compressor:

Contininuos duty applications should be limited to about 149C (300F).


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Part 4 Determine the volumetric efficiency Which is the ration of the amount of gas compressed vs. the

physical size of the compressors cylinder volume. For estimating purposes:


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Part 5 Determine the required piston displacement (PDR) Piston displacement (PD) is a mesure of the compressors size and

is dependant on the size, number and type of cylinders andcompressor RPM.

Required piston displacement (PDR) will determine how large acompressor will be required to hadle the specified capacity.


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Part 6 Select the compressor size Once the choice of single-stage or two-stage and the calculation of

required piston displacement have been made, the compressor canbe sized:


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Part 7 Select the compressor size


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Part 8 Select an actual RPM Using catalogs of manufactures pick an RPM slightly above the

minimum RPM required.

Part 9 Calculate the compressors actual piston displacement (PD) The actual piston displacement can be calculated


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Part 10 Calculate the power required BHP (kW) For estimating purposes, the following formulas may be used:


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Part 11 Options Once the compressor model, RPM and required power have been

determined, various compressor options and accesories will need tobe considered:

Material compatibility : O-rings and piston ring materials need to bereviewed to ensure compatibility with the gas strea, being handled.

Suction valve unloaders: many compressors will need some type ofcapacity control system.

Seal (Piston Rod Packing) Configuration: this will depend on degreeof leakage control desired and the pressures invlolved.

External Oil Filter: for dirty or dusty locations. Extended crankshaft: if a direct drive is to be used.

BLACKMER, Steps to compressor selection and sizing, http://trukare.com/document_library/Blackmer/Compressors/CompSelectionandSizing.pdf, ltimo acceso

Diciembre 2014


Termodinmica III

Ejercicio

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A compressor to be used to draw N2 off of a cryogenic storage tankand boost the pressure a number of plant processes. The flowrequirement will vary throughout the 8 hour production day, but willaverage about 15 CFM.

Suction: N2 @ 5 psi Discharge: 65 psi Site: 1000 ft elevation, outdoors, Ambient of 0 to 100F Utilities: 460 V/3hp/60hz, 80F fresh water


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Step II Gas being handled

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N2 MW = 28.1 n = 1.40 Critical Pressure = 493 psia Critical Temperature = 228 R


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Step II Site elevation

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Step II Suction temperature

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Step II Capacity


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Part 1 Calculate the compression ratio

Part 2 Choosing a one-stage or two-stage compressor


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Part 3 Calculate the discharge temperature (Td)

Part 4 Determine the volumetric efficiency


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Part 5 Determine the required piston displacement (PDR)




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Part 8 Select an actual RPMNext higher 470 RPM

Part 9 Calculate the compressors actual piston displacement (PD)

Part 10 Calculate the power required BHP (kW)


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Referencias

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1. Cengel and Boles, Thermodynamics and engineering approach, Fifth Ed.

2. Agilent Technologies, Practical Temperature Measurements, Application Note 290, http://cp.literature.agilent.com/litweb/pdf/5965-

7822E.pdf, ltimo acceso Octubre 2014

3. Peak Sensors, Thermistor Information, http://www.peaksensors.co.uk/technical-information/thermistor-information/ , ltimo acceso

Octubre 2014

4. AVX, NTC/PTC Thermistors, http://www.avx.com/docs/masterpubs/ntc_ptc.pdf, ltimo acceso Octubre 2014

5. OMEGA, Introduction to Thermistors, http://www.omega.com/prodinfo/thermistor.html, ltimo acceso Octubre 2014

6. http://www.facstaff.bucknell.edu/mastascu/elessonshtml/sensors/tempr.html, ltimo acceso Octubre 2014

7. Micro-Chip Technologies, NTC Thermistors, http://www.microchiptechno.com/ntc_thermistors.php, ltimo acceso Octubre 2014

8. http://www.explainthatstuff.com/how-pyranometers-work.html, ltimo acceso Octubre 2014

9. Hukseflux, Thermal Sensors, SR12 Pyranometer, http://www.pyranometer.net/DE/sr12_pyranometer_de.html , ltimo acceso Octubre

2014

10. The Fundamentals of Thermoelectrics, http://www.nano.physik.uni-muenchen.de/education/praktika/f1_thermoelectrics.pdf, , ltimo

acceso Octubre 2014


Termodinmica III

Gracias

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