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PROCESS EQUIPMENT DESIGN

مهدي کریمی97-98ترم پاییز

COMPRESSORS, BLOWERS, FANS AND

VACUUMPUMPS

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RULESOF THUMBS Fans are used to raise the pressure by about 3% [12 in. (30 cm) water] Blowers raise to less than 2.75 barg (40 psig) Compressors to higher pressures, although the blower range is

commonly included in the compressor range. For vacuum pumps use the following: Reciprocating

Piston Type down to 133.3 Pa (1 torr) Rotary piston type down to 0.133 Pa (0.001 torr) Two lobe rotary type down to 0.0133 Pa (0.0001 torr) Steam jet ejectors:

1 stage down to 13.3 k Pa (100 torr) 3 stage down to 133.3 Pa (1 torr) 5 stage down to 6.7 Pa (0.05 torr)

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RULESOF THUMBS Theoretical adiabatic horsepower

T1 is inlet temperature

Theoretical reversible adiabatic power

푃표푤푒푟 =푚푍 푅푇

푎푃푃

− 1

R = Gas Constant, Z1= compressibility factor, m = molar flow rate

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RULESOF THUMBS Outlet temperature for reversible adiabatic process

푇 = 푇푃푃

To compress air from 37.8 oC (100 oF), k = 1.4, compression ratio = 3, theoretical power required = 62 hp/million ft3/day, outlet temperature 152.2 oC (306 oF).

Exit temperature should not exceed 167–204 oC (350–400 oF); for diatomic gases (Cp/Cv = 1.4), this corresponds to a compression ratio of about 4.

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RULESOF THUMBS Compression ratio should be about the same in each stage of a multistage

unit, ratio = (Pn/P1)1/n, with n stages. Efficiencies of reciprocating compressors: 65% at compression ratio of 1.5,

75% at 2.0, and 80–85% at 3–6. Efficiencies of large centrifugal compressors, 2.83–472m3/s (6000–100,000

acfm) at suction, are 76–78%. Rotary compressors have efficiencies of 70%, except liquid liner type which

have 50%.

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DIFFERENCEWITH PUMP Major difference is that compressors handles the gases and pumps

handles the liquids. As gases are compressible, the compressor also reduces the volume of

gas. Liquids are relatively incompressible; while some can be compressed

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COMPRESSOR TYPES

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OPERATINGREGIONSOFDIFFERENT COMPRESSORS

PIP REEC001 Compressor Selection Guidelines

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OPERATINGREGIONSOFDIFFERENT COMPRESSORS

PIP REEC001 Compressor Selection Guidelines

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DYNAMICCOMPRESSORS The dynamic compressor is continuous flow compressor is

characterized by rotating impeller to add velocity and thus pressure to fluid.

It is widely used in chemical and petroleum refinery industry for specific services

Centrifugal Compressor Achieves compression by applying inertial forces to the gas by means of

rotating impellers. It is multiple stage; each stage consists of an impeller as the rotating

element and the stationary element, i.e. diffuser Fluid flow enters the impeller axially and discharged radially The gas next flows through a circular chamber (diffuser), where it loses

velocity and increases pressure

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DYNAMICCOMPRESSORS Axial flow compressor

Working fluid principally flows parallel to the axis of rotation.

The energy level of air or gas flowing through it is increased by

the action of the rotor blades which exert a torque on the fluid

Have the benefits of high efficiency and large mass flow rate

Require several rows of airfoils to achieve large pressure rises

making them complex and expensive

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ADVANTAGESANDDISADVANTAGESOFDYNAMICCOMPRESSORS

Advantages Disadvantages

Dynamic Compressors

Centrifugal • Wide operating range

• High reliability

• Low Maintenance

• Instability at reduced flow

• Sensitive to gas

composition changeAxial • High Capacity for given size

• High efficiency

• Heavy duty

• Low maintenance

• Low Compression ratios

• Limited turndown

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DYNAMICCOMPRESSORCURVE

Almost the same as centrifugal pump

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DYNAMICCOMPRESSORCURVE

Almost the same as centrifugal pump

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DYNAMICCOMPRESSORCURVE Surge curve

The surge is the minimum flow in the compressor, below with momentary reversal of rotor is occur (The flow is based on suction condition).

Anti-surge control curve is the surge flow + 10%

Stone wall (Stall) curve The maximum flow developed by the compressor is called. Below this point,

the compressor is aerodynamically unstable

Rated speed The speed that can increase the rated flow to the rated head (pressure)

Maximum continuous speed Is usually 105% of the rated capacity that compressor is tested

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DYNAMICCOMPRESSORCURVE Turndown

It is the percentage of change in capacity between the rated-capacity and the surge point capacity at the rated-head when the unit is operating at rated-suction temperature and gas composition.

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POSITIVEDISPLACEMENTCOMPRESSORS Positive displacement compressors causes movement by trapping a

fixed amount of gas

Then forcing (displacing) that trapped volume into the discharge pipe.

Rotary Compressor

The gas is compressed by the rotating action of a roller inside a

cylinder.

The roller rotates off-center around a shaft so that part of the roller is

always in contact with the cylinder.

Volume of the gas occupies is reduced and the gas is compressed.

High efficient as sucking and compressing gas occur simultaneously.

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POSITIVEDISPLACEMENTCOMPRESSORS Reciprocating compressor

Uses pistons driven by a crank shaft to deliver gases at high

pressure.

The intake gas enters the suction manifold, then flows into the

compression cylinder

It gets compressed by a piston driven in a reciprocating motion

via a crankshaft,

Discharged at higher pressure

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ADVANTAGESANDDISADVANTAGESOF POSITIVEDISPLACEMENTTYPE COMPRESSORAdvantages Disadvantages

Positive displacementcompressorReciprocating • Wide pressure ratios

• High efficiency• Heavy foundation required• Flow pulsation• High maintenance

Diaphragm • Very high pressure• Low flow• No moving seal

• Limited capacity range• Periodic replacement of diaphragm

Screw • Wide application• High efficiency• High pressure ratio

• Expensive• Unsuitable for corrosive or dirty gases

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WHYMULTISTAGECOMPRESSOR? High temp rise leads into limitation for the maximum achievable pressure rise.

Discharge temperature shall not exceed a maximum value depends on the gas composition

Required the gas in different pressure level.

Intercoolers are used in between each stage to removes heat and decrease the temperature of gas

The gas could be compressed to higher pressure without much rise in temperature

It is more difficult to compress the gas with higher temperature

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ADIABATIC ANALYSIS The equation of state for an ideal gas is:

푃푉 = 푚푅푇 푅 =푅

푀푤 The first law of thermodynamics for a steady-flow process is (per unit mass):

Neglecting the kinetic energy of the gas Assuming constant specific heat For a reversible adiabatic process

Substituting (P2 /P1)(k-1)/k for T2/T1 and k/(k - 1) for cp /R

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ADIABATIC ANALYSIS From ideal gas

Work:

The isentropic efficiency (ratio of isentropic work to actual work)

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POLYTROPIC PROCESS Polytropic process can be defined as: All the adiabatic equations apply if (n-1)/n is substituted for (k-1)/k where

If the inlet and discharge temperatures and pressures are known, n can be calculated from

where n is directly related to the heat transferred during the process by

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POLYTROPIC PROCESS The polytropic efficiency ηp (ratio of polytropic work to actual work) is given

by:

The polytropic efficiency and the isentropic efficiency can be related by:

Real gas

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MULTISTAGINGAND INTERCOOLING Why Multistaging and Intercooling?

Temperature rise and mechanical stresses limit the maximum pressure differential across a single stage of any compressor type

The volumetric efficiency decreases with increasing pressure ratio Multistaging is used to overcome these limitations and to save power

The minimum power is obtained when

ns : the number of stages; rs: the pressure ratio per stage; rt: the ratio across the compressorAssumption:

Perfect intercooling (Tinter =Tin) and no pressure losses between the stages

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MULTISTAGINGAND INTERCOOLING The maximum theoretical work that can be saved by perfect

intercooling is: