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Compressors , Fans & Blowers
INTRODUCTION
The main use of the fans, compressors and blowers is the transportation of gases The main item of most processes is the compressor selection There are wide variety of compressors so it is crucial to define the operating conditions before selection
Major Factors During Selection
Head or Pressure Rise Flow Rate Temperature Limitations Consumption of Power Cost
The pressure rise which is the main difference between fans, compressors and blowers can be stated as follows: ΔP(psig) Fans Blowers Compressors
2 2-10 >10
Compressors & Gas Compression • • • • • • • • •
Categories and Types Compression Process Compressor Characteristics Key Design Parameters Calculation Methods Specification Data Sheet Selection Guidelines Control Systems Typical operating Problems
Compressor Application and Classification •
Compressors are used in a variety of applications
• Example: In natural gas plants, compressors are used to establish feed gas process pressures. Compressors also provide clean, dry air for instruments and control devices • In a refinery or chemical plant, compressors are used to compress gases such as light hydrocarbons, nitrogen, hydrogen, carbon dioxide, and chlorine • These gases are sent to headers, from which they
Classification of Compressors • There are three basic designs for compressors : i) Dynamic ii) Positive displacement iii) Thermal. • Dynamic compressors include centrifugal (radial flow) and axial (straight-line) flow compressors. • Dynamic compressors accelerate airflow by drawing air in axially and spinning it outward (centrifugal compressors) or in a straight line (axial flow compressors).
• Positive displacement compressors include rotary and reciprocating compressors. • Positive displacement compressors compress gas into a smaller volume and discharge it at higher pressures. • Thermal compressors use ejectors to direct highvelocity gas or steam into the process stream, entraining the gas, then converting the velocity into pressure in a diff
Compressors Family Tree
Compressors & Gas Compression Categories and Types
The principles of compression are:
• Gases and vapors are compressible. • Compression decreases volume. • Compression moves gas molecules close together. • Compressed gases will resume their original shape when released. • Compressed gases produce heat because of molecular friction. • The smaller the volume, the higher the pressure. • Force ÷ area = Pressure. • Gas volume varies with temperature and pressure. • Liquids and solids are not compressible
Dynamic Compressors @ Centrifugal Compressor
• Gas enters a centrifugal compressor at the suction inlet and is accelerated radially by moving impellers. • Centrifugal compressors have one moving element, the drive shaft and impeller. • The impeller discharges into a circular, narrow chamber called the diff • More sensitive to density and fluid characteristics • Designed to operate at speeds in excess of 3000 rpm • Can be single stage or multistage • Single stage – designed for high gas flow rates and low discharge pressure • Multi stage – designed for high gas flow rates and high discharge pressure
The advantages of centrifugal compressors can be classified as;
-
They are more efficient than reciprocating ones They provide high flowrates They are compact, less site area They need lower maintainance requirements They are tolerant to liquid carry
Centrifugal Compressor
Compressors & Gas Compression Centrifugal Compressor
Single-stage Centrifugal Compressor Multi-stage Centrifugal Compressor
Dynamic Compressors @ Axial Flow Compressor
• Normally used for jobs where highest flow and pressure required • Request twice as many stages as centrifugal perform (8% to 10%) • Primary application of axial compressors involves transfer of clean gas such as air • Internal component are sensitive to corrosion, pitting and deposits • More lighter, more efficient and smaller than centrifugal pumps • Main purpose is in gas turbine applications
The advantages of axial compressors -
They have higher efficiency They have higher capacity (flow rate) They are in smaller size
The disadvantages of axial compressors -
They limited operating range They are more subjected to corrosion They are subjected to deposits They have higher capital costs They have lower heads
Axial Flow Compressor
Compressors & Gas Compression Axial Compressor
Combined Axial and Radial Compressor Applications
They have flow rates ranging from 50,000 to 690,000m3/hr They have pressure ratio ranging from 5.8 to 12.5
POSITIVE DISPLACEMENT COMPRESSORS
Positive Displacement Compressors
Reciprocating Compressors
Rotary Compressors
RECIPROCATING COMPRESSORS
They are the oldest type of compressors They have higher maintainance costs and lower capacity than dynamic compressors They are widely used in industry They have cylinders which are equipped with suction and delivery valves Compression cycle is composed of 3 cycles which are intake, compression and discharge
They intake gas by the help of cylinders The piston’s motion is reversed and the gas which taken in is compressed The gas is expelled during the delivery stroke
In multistage reciprocating compressors;
-
The gas is compressed to an intermediate pressure The other cylinders raise the pressure to the end pressure There also exist intercoolers
-
Positive Displacement Compressors @ Reciprocating piston Compressor
• Work by tapping and compressing specific amount of gas between a piston and cylinder wall • The back and forth motion incorporated by a reciprocating compressor pull gas on the suction and discharge on the other • Spring loaded suction and discharge valves work automatically as piston moves up and down • Have a flexible pressure range and overall capacity, low power cost high efficiency rating
Reciprocating Piston Compressor
ROTARY COMPRESSORS
The two rotating components confine a volume of gas The volume of the pocket decreases in rotation so pressure increases
The rotary compressors have high range of capacity and compression ratio The rotary compressors are classified as; Lobed, Helical – Screw, Sliding Vane
Positive Displacement Compressors @ Rotary Compressor (Sliding Vane)
• Use off center rotor with sliding vane to compress gases • body (cast iron or steel), rotor and shaft ( high strength alloy steel), sliding vanes (asbestosphenolic resin, metal) • Does not use suction or discharge valve because it is
Positive Displacement Compressors @ Rotary Compressor (Lobe) • Characterized by the two kidney bean shape impellers • Used to trap and transfer gases • Two impellers move on opposite direction during operation • Designed to have constant volume discharge pressure and constant speed drives
Positive Displacement Compressors
@ Rotary Compressor (Liquid Ring) • Unusual compressor design (combines centrifugal action, with positive displacement and rotary action) • May be found in the following application : •Hazardous gases •Toxic gases •Hot gases and vapor
Screw Compressor
HELICAL – SCREW COMPRESSORS
There are mainly two screws which are called male and female The gas is compressed between the lobes of the screw and move along the axis to an outlet port
These units can be; oil – flooded and dry The contamination of oil is prevented by dry compressors Oil flooded units are used in refrigeration systems and plant air service
FANS & BLOWERS
Blower and Fans • Simple devices typically classified as compressors • two basic design (axial flow and centrifugal flow) • mostly are single stage devices. - centrifugal blower (low pressure air systems, refrigeration unit or laboratory hoods) - fan (direct airflow into or out of ind. equipment such as cooling tower, boilers or HVAC system) • centrifugal fan – to move gases over a wide range of conditions
FANS
They are the air displacement systems moving air continuously to moderate pressures Due to little change in pressure of air in fans, air is considered to be incompressible They can have pressure rise up to 2 psig.
The characteristics of fans can be classified as;
The volumetric flowrate of the gas displaced by the fan is directly proportional with the fan speed The static pressure varies with the square of the fan speed The power consumed varies with the cube of the fan speed
FANS
Axial
Tube
Centrifugal
Vane
Radial Forward Backward Air Foil Blade Curved Curved
AXIAL FANS Gas moves parallel to the axis of rotation
-
There are two types of axial fans; Tube axial fans Vane axial fans
Tube – axial fans are used for wide range of volumes at medium pressure
In vane – axial fans there is air guide vane on the discharge side and the air flow pattern is a straight line hence improvement in efficiency and reducement in turbulance is observed
CENTRIFUGAL FANS
Gas stream moves perpendicular to the axis of rotation They are classified as; radial blade, forward curved, backward curved and air foil
wheel
RADIAL BLADE CENTRIFUGAL FANS
They are used for pneumatic transportation and exhausting process gas in high resistance systems With relatively low capacity, they can achive high static pressure They can develop high pressures with high speeds Blades clean themselves They are not used for ventilating purposes
FORWARD CURVED CENTRIFUGAL FANS
They discharge higher volume of air at slower fan speeds
They operate with a moderate amount of noise
They require little space
They are used for clean gases
BACKWARD CURVED CENTRIFUGAL FANS
They develop much of their energy directly as pressure
They develop less velocity heads by operating at medium speeds
Small variations in system volume result in small variations in air pressure
AIR FOIL CENTRIFUGAL FANS
They are backward curved centrifugal fans with an air foil cross section
They can operate more silently since air forms no turbulance while flowing through the wheels
BLOWERS
Blowers are used for supplying low pressure air up to between 2-10 psig. They consist of two parallel shaft rotors They may have 2 – 4 lobes The rotating shaft in the constitution of the blower traps some gas The compression of the gas in the blower is negligible
Blowers
They are used for; -
Pneumatic transportation of particulate material
-
Water and waste treatment
-
Providing moderate vacuum
Compressors & Gas Compression Ranges of Application
Compressors & Gas Compression Compression Process •
Gas compression is a thermodynamic process where change takes place in the physical state of the gas
•
Compression adds energy to the gas resulting in pressurevolume changes defined by ideal gas laws
•
Compression take place under conditions defined: • Adiabatic: no heat added or removed from systems • Isothermal: constant temperature in system • Polytropic: heat added or removed from system
•
Compression of ‘real’ gases in ‘actual’ compressors deviate from conformance with ‘ideality’, usually significantly, affecting compressor design.
Compressors & Gas Compression Compressor Characteristics
• Capacity/Head • Performance • Terminology
Compressors & Gas Compression Reciprocating Compressor
• Performance Diagram • Terminology • • • • •
Piston Displacement Clearance Volume Volumetric Efficiency Pressure Ratio Rod Loading
Compressors & Gas Compression Reciprocating Compressor
Compressors & Gas Compression Reciprocating Compressor
Compressors & Gas Compression Centrifugal Compressor
• Performance Curves • Terminology • • • • •
Operating Point Surge Point Stonewall Stability Turndown
Compressors & Gas Compression Centrifugal Compressor
Compressors & Gas Compression Centrifugal Compressor
Compressors & Gas Compression Centrifugal Compressor Performance
Compressors & Gas Compression Centrifugal Compressor Key Design Parameters • • • • • •
Capacity Gas Properties Pressure Head Power Efficiency Multi-Stages
Compressors & Gas Compression Centrifugal Compressor Key Design Parameters
Capacity • Flow Rates
• Normal • Maximum • Minimum
• Design Capacity
Compressors & Gas Compression Centrifugal Compressor Key Design Parameters
Gas Properties • • • • •
Composition Contaminants Molecular Weight – MW Specific Heat Ratio – /Cv Compressibility
Compressors & Gas Compression Centrifugal Compressor
10ºC 38ºC 66ºC 93ºC 121ºC
Compressors & Gas Compression Centrifugal Compressor
Compressors & Gas Compression Centrifugal Compressor
Compressors & Gas Compression Centrifugal Compressor
100ºF = 560ºR: 560/549 = 1.02 100ºF = 311K, 549ºR = 305K: 311/305 = 1.02
PV = ZmRT/MW P=100psia = 6.89 bar a T=100ºF = 37.8ºC = 310.9K = m/V = P(MW)/(ZRT) = 6.89E5x34.27/(0.946x8314x310.9) = 9.7kg/m3 = 0.61lb/ft3
Compressors & Gas Compression Centrifugal Compressor
0.077
1.02 0.973
Compressors & Gas Compression Centrifugal Compressor
0.88
Compressors & Gas Compression Centrifugal Compressor Key Design Parameters
Head • Available vs. Required Head • Available Head is Compressor Related • H(Available) = CV2/g • C = Pressure Coefficient (0.55)
• Required head is System-Related Z 1545 .T1 P2 H(Required) ( MW )( M ) P1
M
K 1 K
M is Compressio n Path Parameter K Specific Heat Ratio C P / CV Eh is Hydraulic Efficiency
1 1 Eh
Compressors & Gas Compression Centrifugal Compressor
Horsepower Calculation For centrifugal compressors the following method is normally used: • First, the required head is calculated. Either the polytropic or adiabatic efficiency is used with the companion head. H poly H AD
( N 1) / N P2 ZRT1 1 ( N 1) /( N ) P1 ( K 1) / K ZRT1 P2 1 ( K 1) / K P1
Compressors & Gas Compression Centrifugal Compressor
Horsepower Calculation H poly
ZRT1 ( N 1) /( N )
H AD
ZRT1 ( K 1) / K
P2 P 1
P 2 P1
( N 1) / N
( K 1) / K
1 1
Where: Z = Average compressibility factor: using 1 will yield conservative results R = 1544/(mol weight) T1 = Suction Temperature, ºR P1, P2 = Suction, discharge pressures, psia K = Adiabatic exponent, (N-1)/N = (K-1)/(KEp) Ep = Polytropic Efficiency EA = Adiabatic Efficiency
Compressors & Gas Compression Centrifugal Compressor
Horsepower Calculation The polytropic and adiabatic efficiencies are related as follows: P ( K 1) / K P ( K 1) / K 2 2 1 1 P P E A 1 ( N 1) / N 1 ( K 1) / KE P2 P 1 2 1 P1 P1 p
From Polytropic Head:
From Adiabatic Head:
HP = WHpoly/(Ep 33000)
HP = WHAD/(EA 33000)
BHP = HP/Em Where: HP = Gas Horse Power BHP = Brake Horsepower W = Flow, Lb/min
Compressors & Gas Compression Efficiency
• Hydraulic Efficiency • Adiabatic • Polytropic • Volumetric Efficiency • Reciprocating • Mechanical Efficiency • Drivers
Compressors & Gas Compression Centrifugal Compressor Approximate polytropic efficiencies for centrifugal and axial compressors
Compressors & Gas Compression Temperature Rise Temperature ratio across a compression stage is: T2/T1 = (P2/P1)(K-1)/K
Adiabatic
T2/T1 = (P2/P1)(N-1)/N
Polytropic
Where: K = Adiabatic exponent, /Cv N= Polytropic exponent, (N-1)/N = (K-1)/KEp P1, P2 = Suction, discharge pressures, psia T1, T2 = Suction, discharge temperatures, ºR Ep = Polytropic efficiency, fraction
Compressors & Gas Compression Temperature Rise The usual centrifugal compressor is uncooled internally and follows a polytropic path. Temperature must often be limited to: • Protect against polymerization as in olefin or butadiene plants • At T > 230-260ºC, the approximate mechanical limit, problems of sealing and casing growth start to occur. High temperature requires a special and more costly machine. Most multistage applications are designed to stay below 250300ºC
Compressors & Gas Compression Temperature Rise Intercooling can be used to hold desired temperatures for high overall compression ratio applications. This can be done between stages in a single compressor frame or between series frames. Sometimes economics rather than a temperature limit dictate intercooling. Sometimes for high compression ratios, the job cannot be done in one frame. Usually a frame will not contain more than 8 stages (wheels). For many applications the compression ratio across a frame is about 2.5 – 4.0 The maximum head that one stage can handle depends on gas properties and inlet temperature. Usually this is about 2000 to 3400m for a single stage.
Compressors & Gas Compression Surge Controls A centrifugal compressor surges at certain conditions of low flow. Surge control help the machine to avoid surge by increasing flow. • For an air compressor, a simple spill to atmosphere is sufficient. • For a hydrocarbon compressor, recirculation from discharge to suction is used.
Compressors & Gas Compression Surge Controls There are many types of surge controls. Avoid the low-budget systems with a narrow effective range, especially for large compressors. Good systems include the flow/ΔP type. The correct flow to use is the compressor suction. However, a flow element in the suction can rob excessive horsepower. Therefore, sometimes the discharge flow is measured and the suction flow calculated within the controller by using pressure measurements. The compressor intake nozzle is also sometimes calibrated and used as a flow element.
Compressors & Gas Compression Compressor Calculation Method
• Define gas properties: MW, /Cv, Z 1 • Define inlet conditions: Temp & Press. • Calculate gas flow rate: Normal and Design 1 • Establish total discharge pressure. • Calculate compression ratio and number of stages • Define selection & polytropic efficiency 1. At inlet conditions
Compressors & Gas Compression Compressor Calculation Method
cont’d
• Calculate heat capacity factor ‘M’ • Calculate ‘required’ polytropic head • Calculate hydraulic gas horsepower • Calculate discharge temperature • Calculate total brake horsepower • Estimate inter-stage cooling requirement
Compressors & Gas Compression Compressor Calculation Example 1: Calculate compressor required to handle a process gas at the following operating conditions: Inlet press and temp at 20 psia and 40ºF. Discharge pressure of 100 psia. Gas rate 2378 lb.mol/hr of the following composition and calculated properties:
Ethane
Mol% Mol/h Mol. Wt 2 48 30.1
∑
∑
Tc
∑
Pc
∑
0.60
11.96
0.24
550
11
708
14
15.70 666
633
617
587
23
551
17
Propane 95
2259
44.1
41.9
16.55
Butane
3
71
58.1
1.74
22.50 0.68
Total
100
2378
44.24
16.62
766
667
618
Compressors & Gas Compression Compressor Calculation Example 1:
cont’d
Calculation: • Inlet flow: Weight flow = 2378 x 44.24/60 = 1753 lb/min Pr = 20/618 = 0.0324, Tr = (40+460)/667 = 0.75 Compressibility factor Z = 0.97 (from generalized Z chart) Density = (MW x P1)/(10.73 x T1 x Z) = (44.24 x 20)/(10.73 x (40 + 460) x 0.97) = 0.17 lb/cu.ft Inlet volume = 1753/0.17 = 10 310 cu.ft/min
Compressors & Gas Compression Compressor Calculation Example 1:
cont’d
Calculation: • Heat Capacity Factor k = /Cv = /( – 1.99) = 16.62/(16.62 – 1.99) = 1.137 M = (k-1)/(kEp) Assume Ep = 77%: M = (1.137 – 1)/(1.137 x 0.77) = 0.156
Compressors & Gas Compression Compressor Calculation Example 1:
cont’d
Calculation: • Polytropic Head, Hp
H poly
ZRT1 P2 M P1
M
1
= 0.97 x (1545/44.24) x (40 + 460)/0.156 x [(100/20)0.156 -1] = 30 988 ft
Compressors & Gas Compression Compressor Calculation Example 1:
cont’d
Calculation: • Discharge Temperature, T2 T2 = T1(P2/P1)M = 500(5)0.156 = 643ºR = 183ºF • Gas Horsepower (GHP) & Brake Horespower (BHP) GHP = W . Hpoly/(33000Ep) = 1753 x 30988/(33000 x 0.77) = 2140 BHP = 2140/0.98 = 2180 (Assume Mechanical Eff. = 98%)
Compressors & Gas Compression Example
Calculate the Brake Horsepower for the following Compressor:
Compressors & Gas Compression Example
Calculate the Brake Horsepower for the following Compressor:
Calculate Gas Mixture Properties Composition:
Composition Hydrogen Nitrogen Total Gas Mix
H2 = 65.6/(65.6+21.4) = 75.4 vol% N2 = 100 – 75.4 = 24.6 vol% Mole% 75.4 24.6 100.0
Mole Wt 2 28
∑MW 1.51 6.89 8.40
mass% 18.0 82
Use Z = 1 for conservative results
14.3 1.04 11.04
∑MW 2.57 0.85 3.42
Compressors & Gas Compression Example
Calculate the Brake Horsepower for Compressor: Cont’d
Let’s look at the first stage: First calculate Polytropic Head: H poly
ZRT1 ( N 1) /( N )
P2 P 1
( N 1) / N
T2/T1
= (P2/P1)(N-1)/N
ln(T2/T1)
= (N-1)/N ln(P2/P1)
(N-1)/N
= ln(T2/T1)/ln(P2/P1)
1
T1 = 22ºC = 295K T2 = 99ºC = 372K P1 = 2418 kPag = 2518 kPa a P2 = 4300 kPag = 4400 kPa a
= ln(372/295)/ln(4400/2518) = 0.416 Hpoly
= 1 x (8.314/8.4) x 295 x ((4400/2518)0.416 -1) 0.416 = 183.4 kJ/kg
Compressors & Gas Compression Example
Calculate the Brake Horsepower for Compressor: Cont’d
First stage: /Cv
= /(-R) = 3.42/(3.42-8.314/8.4) = 1.4
(N-1)/N
= (K-1)/(KEp)
Ep
= (1.4 -1)/(1.4 x 0.416) = 0.69
W = (107 000/22.414) x 8.4 = 40100kg/h = 11.14 kg/s
Compressors & Gas Compression Example
Calculate the Brake Horsepower for Compressor: Cont’d
First stage: Gas Horsepower
= W . Hpoly/Ep = (11.14 x 183.4)/0.69 = 2960 kJ/s = 3.0 MW
Similar for stage 2, 3 and Recycle: GHP(stage 2) = 2.9MW GHP(stage 3) = 3.3 MW GHP(recycle stage) = 1.0 MW Total GHP = 3.0 + 2.9 + 3.3 + 1.0 = 10.2 MW A good assumption for Mechanical Efficiency = 95% BHP = 10.2/0.95 = 10.6 MW
Compressors & Gas Compression
Compressors & Gas Compression
ing Equipment in a Compressor System
ing Equipment in a Compressor System
Intercooler and after cooler heat exchanger - compression of gases create heat in compressor - control high temperature - intercooler lower the temperature as gas is discharge out of first stage of compressor - as the gas is compressed (create more heat), discharge into after cooler before go to receiver Safety valve - used to relieve excess pressure that could damage operating equipment - sized to handle specific flow rates
ing Equipment in a Compressor System (cont….)
Silencers - most compressor exceed OSHA standards noise pollution - muffle some of the damaging noise produced by compressor - should be mounted on the inlet and outlet of a compressor Demister - designed to remove liquid droplets from gas - function as a cyclone - heavier component fall to the bottom of the demister and removed - clean gas escapes out the discharge line on the top of the demister Dryer - for dry air service, discharge of a compressor is run through a dryer - filled with moisture adsorbing chemicals called desiccant dryer (alumina, mol sieves and silica gel) - operation uses parallel or series dryer
Compressor system
Start up and shutdown a dynamic compressor
Start up and shutdown a positive displacement compressor
Troubleshooting a centrifugal compressor
Troubleshooting a reciprocating compressor
Compressor Symbols
CONCLUSIONS
The fans have wide range of flowrate The material selection is important during manufacturing fans The blowers have low power and pressure applications The blower is less efficient method of compression
CONCLUSIONS
The centrifugals are tolerant to liquid carry The liquid with the gas can cause erosion and severe damage in centrifugals In axial equipment, high compression efficiency is observed The axial equipment is applied for high flow andlow discharge pressures
CONCLUSIONS
The reciprocating systems are applicable for low flow rate of high pressure ratio The oil contamination is important in reciprocating systems The reciprocating systems have higher maintainance cost The reciprocating systems are not suited to dirty gasses The process gases that are taken in should be clean and dry in axial equipments The reciprocating systems are not tolerate liquid droplets in the suction flow
CONCLUSIONS
The screw compressor have higher initial cost than reciprocating compressors for the same duty The sliding vane compressors have low pressure applications The sliding vane compressors operate at low speeds The noise level of the sliding vane compressors is low
INTRODUCTION
The main use of the fans, compressors and blowers is the transportation of gases The main item of most processes is the compressor selection There are wide variety of compressors so it is crucial to define the operating conditions before selection
Major Factors During Selection
Head or Pressure Rise Flow Rate Temperature Limitations Consumption of Power Cost
The pressure rise which is the main difference between fans, compressors and blowers can be stated as follows: ΔP(psig) Fans Blowers Compressors
2 2-10 >10
Compressors & Gas Compression • • • • • • • • •
Categories and Types Compression Process Compressor Characteristics Key Design Parameters Calculation Methods Specification Data Sheet Selection Guidelines Control Systems Typical operating Problems
Compressor Application and Classification •
Compressors are used in a variety of applications
• Example: In natural gas plants, compressors are used to establish feed gas process pressures. Compressors also provide clean, dry air for instruments and control devices • In a refinery or chemical plant, compressors are used to compress gases such as light hydrocarbons, nitrogen, hydrogen, carbon dioxide, and chlorine • These gases are sent to headers, from which they
Classification of Compressors • There are three basic designs for compressors : i) Dynamic ii) Positive displacement iii) Thermal. • Dynamic compressors include centrifugal (radial flow) and axial (straight-line) flow compressors. • Dynamic compressors accelerate airflow by drawing air in axially and spinning it outward (centrifugal compressors) or in a straight line (axial flow compressors).
• Positive displacement compressors include rotary and reciprocating compressors. • Positive displacement compressors compress gas into a smaller volume and discharge it at higher pressures. • Thermal compressors use ejectors to direct highvelocity gas or steam into the process stream, entraining the gas, then converting the velocity into pressure in a diff
Compressors Family Tree
Compressors & Gas Compression Categories and Types
The principles of compression are:
• Gases and vapors are compressible. • Compression decreases volume. • Compression moves gas molecules close together. • Compressed gases will resume their original shape when released. • Compressed gases produce heat because of molecular friction. • The smaller the volume, the higher the pressure. • Force ÷ area = Pressure. • Gas volume varies with temperature and pressure. • Liquids and solids are not compressible
Dynamic Compressors @ Centrifugal Compressor
• Gas enters a centrifugal compressor at the suction inlet and is accelerated radially by moving impellers. • Centrifugal compressors have one moving element, the drive shaft and impeller. • The impeller discharges into a circular, narrow chamber called the diff • More sensitive to density and fluid characteristics • Designed to operate at speeds in excess of 3000 rpm • Can be single stage or multistage • Single stage – designed for high gas flow rates and low discharge pressure • Multi stage – designed for high gas flow rates and high discharge pressure
The advantages of centrifugal compressors can be classified as;
-
They are more efficient than reciprocating ones They provide high flowrates They are compact, less site area They need lower maintainance requirements They are tolerant to liquid carry
Centrifugal Compressor
Compressors & Gas Compression Centrifugal Compressor
Single-stage Centrifugal Compressor Multi-stage Centrifugal Compressor
Dynamic Compressors @ Axial Flow Compressor
• Normally used for jobs where highest flow and pressure required • Request twice as many stages as centrifugal perform (8% to 10%) • Primary application of axial compressors involves transfer of clean gas such as air • Internal component are sensitive to corrosion, pitting and deposits • More lighter, more efficient and smaller than centrifugal pumps • Main purpose is in gas turbine applications
The advantages of axial compressors -
They have higher efficiency They have higher capacity (flow rate) They are in smaller size
The disadvantages of axial compressors -
They limited operating range They are more subjected to corrosion They are subjected to deposits They have higher capital costs They have lower heads
Axial Flow Compressor
Compressors & Gas Compression Axial Compressor
Combined Axial and Radial Compressor Applications
They have flow rates ranging from 50,000 to 690,000m3/hr They have pressure ratio ranging from 5.8 to 12.5
POSITIVE DISPLACEMENT COMPRESSORS
Positive Displacement Compressors
Reciprocating Compressors
Rotary Compressors
RECIPROCATING COMPRESSORS
They are the oldest type of compressors They have higher maintainance costs and lower capacity than dynamic compressors They are widely used in industry They have cylinders which are equipped with suction and delivery valves Compression cycle is composed of 3 cycles which are intake, compression and discharge
They intake gas by the help of cylinders The piston’s motion is reversed and the gas which taken in is compressed The gas is expelled during the delivery stroke
In multistage reciprocating compressors;
-
The gas is compressed to an intermediate pressure The other cylinders raise the pressure to the end pressure There also exist intercoolers
-
Positive Displacement Compressors @ Reciprocating piston Compressor
• Work by tapping and compressing specific amount of gas between a piston and cylinder wall • The back and forth motion incorporated by a reciprocating compressor pull gas on the suction and discharge on the other • Spring loaded suction and discharge valves work automatically as piston moves up and down • Have a flexible pressure range and overall capacity, low power cost high efficiency rating
Reciprocating Piston Compressor
ROTARY COMPRESSORS
The two rotating components confine a volume of gas The volume of the pocket decreases in rotation so pressure increases
The rotary compressors have high range of capacity and compression ratio The rotary compressors are classified as; Lobed, Helical – Screw, Sliding Vane
Positive Displacement Compressors @ Rotary Compressor (Sliding Vane)
• Use off center rotor with sliding vane to compress gases • body (cast iron or steel), rotor and shaft ( high strength alloy steel), sliding vanes (asbestosphenolic resin, metal) • Does not use suction or discharge valve because it is
Positive Displacement Compressors @ Rotary Compressor (Lobe) • Characterized by the two kidney bean shape impellers • Used to trap and transfer gases • Two impellers move on opposite direction during operation • Designed to have constant volume discharge pressure and constant speed drives
Positive Displacement Compressors
@ Rotary Compressor (Liquid Ring) • Unusual compressor design (combines centrifugal action, with positive displacement and rotary action) • May be found in the following application : •Hazardous gases •Toxic gases •Hot gases and vapor
Screw Compressor
HELICAL – SCREW COMPRESSORS
There are mainly two screws which are called male and female The gas is compressed between the lobes of the screw and move along the axis to an outlet port
These units can be; oil – flooded and dry The contamination of oil is prevented by dry compressors Oil flooded units are used in refrigeration systems and plant air service
FANS & BLOWERS
Blower and Fans • Simple devices typically classified as compressors • two basic design (axial flow and centrifugal flow) • mostly are single stage devices. - centrifugal blower (low pressure air systems, refrigeration unit or laboratory hoods) - fan (direct airflow into or out of ind. equipment such as cooling tower, boilers or HVAC system) • centrifugal fan – to move gases over a wide range of conditions
FANS
They are the air displacement systems moving air continuously to moderate pressures Due to little change in pressure of air in fans, air is considered to be incompressible They can have pressure rise up to 2 psig.
The characteristics of fans can be classified as;
The volumetric flowrate of the gas displaced by the fan is directly proportional with the fan speed The static pressure varies with the square of the fan speed The power consumed varies with the cube of the fan speed
FANS
Axial
Tube
Centrifugal
Vane
Radial Forward Backward Air Foil Blade Curved Curved
AXIAL FANS Gas moves parallel to the axis of rotation
-
There are two types of axial fans; Tube axial fans Vane axial fans
Tube – axial fans are used for wide range of volumes at medium pressure
In vane – axial fans there is air guide vane on the discharge side and the air flow pattern is a straight line hence improvement in efficiency and reducement in turbulance is observed
CENTRIFUGAL FANS
Gas stream moves perpendicular to the axis of rotation They are classified as; radial blade, forward curved, backward curved and air foil
wheel
RADIAL BLADE CENTRIFUGAL FANS
They are used for pneumatic transportation and exhausting process gas in high resistance systems With relatively low capacity, they can achive high static pressure They can develop high pressures with high speeds Blades clean themselves They are not used for ventilating purposes
FORWARD CURVED CENTRIFUGAL FANS
They discharge higher volume of air at slower fan speeds
They operate with a moderate amount of noise
They require little space
They are used for clean gases
BACKWARD CURVED CENTRIFUGAL FANS
They develop much of their energy directly as pressure
They develop less velocity heads by operating at medium speeds
Small variations in system volume result in small variations in air pressure
AIR FOIL CENTRIFUGAL FANS
They are backward curved centrifugal fans with an air foil cross section
They can operate more silently since air forms no turbulance while flowing through the wheels
BLOWERS
Blowers are used for supplying low pressure air up to between 2-10 psig. They consist of two parallel shaft rotors They may have 2 – 4 lobes The rotating shaft in the constitution of the blower traps some gas The compression of the gas in the blower is negligible
Blowers
They are used for; -
Pneumatic transportation of particulate material
-
Water and waste treatment
-
Providing moderate vacuum
Compressors & Gas Compression Ranges of Application
Compressors & Gas Compression Compression Process •
Gas compression is a thermodynamic process where change takes place in the physical state of the gas
•
Compression adds energy to the gas resulting in pressurevolume changes defined by ideal gas laws
•
Compression take place under conditions defined: • Adiabatic: no heat added or removed from systems • Isothermal: constant temperature in system • Polytropic: heat added or removed from system
•
Compression of ‘real’ gases in ‘actual’ compressors deviate from conformance with ‘ideality’, usually significantly, affecting compressor design.
Compressors & Gas Compression Compressor Characteristics
• Capacity/Head • Performance • Terminology
Compressors & Gas Compression Reciprocating Compressor
• Performance Diagram • Terminology • • • • •
Piston Displacement Clearance Volume Volumetric Efficiency Pressure Ratio Rod Loading
Compressors & Gas Compression Reciprocating Compressor
Compressors & Gas Compression Reciprocating Compressor
Compressors & Gas Compression Centrifugal Compressor
• Performance Curves • Terminology • • • • •
Operating Point Surge Point Stonewall Stability Turndown
Compressors & Gas Compression Centrifugal Compressor
Compressors & Gas Compression Centrifugal Compressor
Compressors & Gas Compression Centrifugal Compressor Performance
Compressors & Gas Compression Centrifugal Compressor Key Design Parameters • • • • • •
Capacity Gas Properties Pressure Head Power Efficiency Multi-Stages
Compressors & Gas Compression Centrifugal Compressor Key Design Parameters
Capacity • Flow Rates
• Normal • Maximum • Minimum
• Design Capacity
Compressors & Gas Compression Centrifugal Compressor Key Design Parameters
Gas Properties • • • • •
Composition Contaminants Molecular Weight – MW Specific Heat Ratio – /Cv Compressibility
Compressors & Gas Compression Centrifugal Compressor
10ºC 38ºC 66ºC 93ºC 121ºC
Compressors & Gas Compression Centrifugal Compressor
Compressors & Gas Compression Centrifugal Compressor
Compressors & Gas Compression Centrifugal Compressor
100ºF = 560ºR: 560/549 = 1.02 100ºF = 311K, 549ºR = 305K: 311/305 = 1.02
PV = ZmRT/MW P=100psia = 6.89 bar a T=100ºF = 37.8ºC = 310.9K = m/V = P(MW)/(ZRT) = 6.89E5x34.27/(0.946x8314x310.9) = 9.7kg/m3 = 0.61lb/ft3
Compressors & Gas Compression Centrifugal Compressor
0.077
1.02 0.973
Compressors & Gas Compression Centrifugal Compressor
0.88
Compressors & Gas Compression Centrifugal Compressor Key Design Parameters
Head • Available vs. Required Head • Available Head is Compressor Related • H(Available) = CV2/g • C = Pressure Coefficient (0.55)
• Required head is System-Related Z 1545 .T1 P2 H(Required) ( MW )( M ) P1
M
K 1 K
M is Compressio n Path Parameter K Specific Heat Ratio C P / CV Eh is Hydraulic Efficiency
1 1 Eh
Compressors & Gas Compression Centrifugal Compressor
Horsepower Calculation For centrifugal compressors the following method is normally used: • First, the required head is calculated. Either the polytropic or adiabatic efficiency is used with the companion head. H poly H AD
( N 1) / N P2 ZRT1 1 ( N 1) /( N ) P1 ( K 1) / K ZRT1 P2 1 ( K 1) / K P1
Compressors & Gas Compression Centrifugal Compressor
Horsepower Calculation H poly
ZRT1 ( N 1) /( N )
H AD
ZRT1 ( K 1) / K
P2 P 1
P 2 P1
( N 1) / N
( K 1) / K
1 1
Where: Z = Average compressibility factor: using 1 will yield conservative results R = 1544/(mol weight) T1 = Suction Temperature, ºR P1, P2 = Suction, discharge pressures, psia K = Adiabatic exponent, (N-1)/N = (K-1)/(KEp) Ep = Polytropic Efficiency EA = Adiabatic Efficiency
Compressors & Gas Compression Centrifugal Compressor
Horsepower Calculation The polytropic and adiabatic efficiencies are related as follows: P ( K 1) / K P ( K 1) / K 2 2 1 1 P P E A 1 ( N 1) / N 1 ( K 1) / KE P2 P 1 2 1 P1 P1 p
From Polytropic Head:
From Adiabatic Head:
HP = WHpoly/(Ep 33000)
HP = WHAD/(EA 33000)
BHP = HP/Em Where: HP = Gas Horse Power BHP = Brake Horsepower W = Flow, Lb/min
Compressors & Gas Compression Efficiency
• Hydraulic Efficiency • Adiabatic • Polytropic • Volumetric Efficiency • Reciprocating • Mechanical Efficiency • Drivers
Compressors & Gas Compression Centrifugal Compressor Approximate polytropic efficiencies for centrifugal and axial compressors
Compressors & Gas Compression Temperature Rise Temperature ratio across a compression stage is: T2/T1 = (P2/P1)(K-1)/K
Adiabatic
T2/T1 = (P2/P1)(N-1)/N
Polytropic
Where: K = Adiabatic exponent, /Cv N= Polytropic exponent, (N-1)/N = (K-1)/KEp P1, P2 = Suction, discharge pressures, psia T1, T2 = Suction, discharge temperatures, ºR Ep = Polytropic efficiency, fraction
Compressors & Gas Compression Temperature Rise The usual centrifugal compressor is uncooled internally and follows a polytropic path. Temperature must often be limited to: • Protect against polymerization as in olefin or butadiene plants • At T > 230-260ºC, the approximate mechanical limit, problems of sealing and casing growth start to occur. High temperature requires a special and more costly machine. Most multistage applications are designed to stay below 250300ºC
Compressors & Gas Compression Temperature Rise Intercooling can be used to hold desired temperatures for high overall compression ratio applications. This can be done between stages in a single compressor frame or between series frames. Sometimes economics rather than a temperature limit dictate intercooling. Sometimes for high compression ratios, the job cannot be done in one frame. Usually a frame will not contain more than 8 stages (wheels). For many applications the compression ratio across a frame is about 2.5 – 4.0 The maximum head that one stage can handle depends on gas properties and inlet temperature. Usually this is about 2000 to 3400m for a single stage.
Compressors & Gas Compression Surge Controls A centrifugal compressor surges at certain conditions of low flow. Surge control help the machine to avoid surge by increasing flow. • For an air compressor, a simple spill to atmosphere is sufficient. • For a hydrocarbon compressor, recirculation from discharge to suction is used.
Compressors & Gas Compression Surge Controls There are many types of surge controls. Avoid the low-budget systems with a narrow effective range, especially for large compressors. Good systems include the flow/ΔP type. The correct flow to use is the compressor suction. However, a flow element in the suction can rob excessive horsepower. Therefore, sometimes the discharge flow is measured and the suction flow calculated within the controller by using pressure measurements. The compressor intake nozzle is also sometimes calibrated and used as a flow element.
Compressors & Gas Compression Compressor Calculation Method
• Define gas properties: MW, /Cv, Z 1 • Define inlet conditions: Temp & Press. • Calculate gas flow rate: Normal and Design 1 • Establish total discharge pressure. • Calculate compression ratio and number of stages • Define selection & polytropic efficiency 1. At inlet conditions
Compressors & Gas Compression Compressor Calculation Method
cont’d
• Calculate heat capacity factor ‘M’ • Calculate ‘required’ polytropic head • Calculate hydraulic gas horsepower • Calculate discharge temperature • Calculate total brake horsepower • Estimate inter-stage cooling requirement
Compressors & Gas Compression Compressor Calculation Example 1: Calculate compressor required to handle a process gas at the following operating conditions: Inlet press and temp at 20 psia and 40ºF. Discharge pressure of 100 psia. Gas rate 2378 lb.mol/hr of the following composition and calculated properties:
Ethane
Mol% Mol/h Mol. Wt 2 48 30.1
∑
∑
Tc
∑
Pc
∑
0.60
11.96
0.24
550
11
708
14
15.70 666
633
617
587
23
551
17
Propane 95
2259
44.1
41.9
16.55
Butane
3
71
58.1
1.74
22.50 0.68
Total
100
2378
44.24
16.62
766
667
618
Compressors & Gas Compression Compressor Calculation Example 1:
cont’d
Calculation: • Inlet flow: Weight flow = 2378 x 44.24/60 = 1753 lb/min Pr = 20/618 = 0.0324, Tr = (40+460)/667 = 0.75 Compressibility factor Z = 0.97 (from generalized Z chart) Density = (MW x P1)/(10.73 x T1 x Z) = (44.24 x 20)/(10.73 x (40 + 460) x 0.97) = 0.17 lb/cu.ft Inlet volume = 1753/0.17 = 10 310 cu.ft/min
Compressors & Gas Compression Compressor Calculation Example 1:
cont’d
Calculation: • Heat Capacity Factor k = /Cv = /( – 1.99) = 16.62/(16.62 – 1.99) = 1.137 M = (k-1)/(kEp) Assume Ep = 77%: M = (1.137 – 1)/(1.137 x 0.77) = 0.156
Compressors & Gas Compression Compressor Calculation Example 1:
cont’d
Calculation: • Polytropic Head, Hp
H poly
ZRT1 P2 M P1
M
1
= 0.97 x (1545/44.24) x (40 + 460)/0.156 x [(100/20)0.156 -1] = 30 988 ft
Compressors & Gas Compression Compressor Calculation Example 1:
cont’d
Calculation: • Discharge Temperature, T2 T2 = T1(P2/P1)M = 500(5)0.156 = 643ºR = 183ºF • Gas Horsepower (GHP) & Brake Horespower (BHP) GHP = W . Hpoly/(33000Ep) = 1753 x 30988/(33000 x 0.77) = 2140 BHP = 2140/0.98 = 2180 (Assume Mechanical Eff. = 98%)
Compressors & Gas Compression Example
Calculate the Brake Horsepower for the following Compressor:
Compressors & Gas Compression Example
Calculate the Brake Horsepower for the following Compressor:
Calculate Gas Mixture Properties Composition:
Composition Hydrogen Nitrogen Total Gas Mix
H2 = 65.6/(65.6+21.4) = 75.4 vol% N2 = 100 – 75.4 = 24.6 vol% Mole% 75.4 24.6 100.0
Mole Wt 2 28
∑MW 1.51 6.89 8.40
mass% 18.0 82
Use Z = 1 for conservative results
14.3 1.04 11.04
∑MW 2.57 0.85 3.42
Compressors & Gas Compression Example
Calculate the Brake Horsepower for Compressor: Cont’d
Let’s look at the first stage: First calculate Polytropic Head: H poly
ZRT1 ( N 1) /( N )
P2 P 1
( N 1) / N
T2/T1
= (P2/P1)(N-1)/N
ln(T2/T1)
= (N-1)/N ln(P2/P1)
(N-1)/N
= ln(T2/T1)/ln(P2/P1)
1
T1 = 22ºC = 295K T2 = 99ºC = 372K P1 = 2418 kPag = 2518 kPa a P2 = 4300 kPag = 4400 kPa a
= ln(372/295)/ln(4400/2518) = 0.416 Hpoly
= 1 x (8.314/8.4) x 295 x ((4400/2518)0.416 -1) 0.416 = 183.4 kJ/kg
Compressors & Gas Compression Example
Calculate the Brake Horsepower for Compressor: Cont’d
First stage: /Cv
= /(-R) = 3.42/(3.42-8.314/8.4) = 1.4
(N-1)/N
= (K-1)/(KEp)
Ep
= (1.4 -1)/(1.4 x 0.416) = 0.69
W = (107 000/22.414) x 8.4 = 40100kg/h = 11.14 kg/s
Compressors & Gas Compression Example
Calculate the Brake Horsepower for Compressor: Cont’d
First stage: Gas Horsepower
= W . Hpoly/Ep = (11.14 x 183.4)/0.69 = 2960 kJ/s = 3.0 MW
Similar for stage 2, 3 and Recycle: GHP(stage 2) = 2.9MW GHP(stage 3) = 3.3 MW GHP(recycle stage) = 1.0 MW Total GHP = 3.0 + 2.9 + 3.3 + 1.0 = 10.2 MW A good assumption for Mechanical Efficiency = 95% BHP = 10.2/0.95 = 10.6 MW
Compressors & Gas Compression
Compressors & Gas Compression
ing Equipment in a Compressor System
ing Equipment in a Compressor System
Intercooler and after cooler heat exchanger - compression of gases create heat in compressor - control high temperature - intercooler lower the temperature as gas is discharge out of first stage of compressor - as the gas is compressed (create more heat), discharge into after cooler before go to receiver Safety valve - used to relieve excess pressure that could damage operating equipment - sized to handle specific flow rates
ing Equipment in a Compressor System (cont….)
Silencers - most compressor exceed OSHA standards noise pollution - muffle some of the damaging noise produced by compressor - should be mounted on the inlet and outlet of a compressor Demister - designed to remove liquid droplets from gas - function as a cyclone - heavier component fall to the bottom of the demister and removed - clean gas escapes out the discharge line on the top of the demister Dryer - for dry air service, discharge of a compressor is run through a dryer - filled with moisture adsorbing chemicals called desiccant dryer (alumina, mol sieves and silica gel) - operation uses parallel or series dryer
Compressor system
Start up and shutdown a dynamic compressor
Start up and shutdown a positive displacement compressor
Troubleshooting a centrifugal compressor
Troubleshooting a reciprocating compressor
Compressor Symbols
CONCLUSIONS
The fans have wide range of flowrate The material selection is important during manufacturing fans The blowers have low power and pressure applications The blower is less efficient method of compression
CONCLUSIONS
The centrifugals are tolerant to liquid carry The liquid with the gas can cause erosion and severe damage in centrifugals In axial equipment, high compression efficiency is observed The axial equipment is applied for high flow andlow discharge pressures
CONCLUSIONS
The reciprocating systems are applicable for low flow rate of high pressure ratio The oil contamination is important in reciprocating systems The reciprocating systems have higher maintainance cost The reciprocating systems are not suited to dirty gasses The process gases that are taken in should be clean and dry in axial equipments The reciprocating systems are not tolerate liquid droplets in the suction flow
CONCLUSIONS
The screw compressor have higher initial cost than reciprocating compressors for the same duty The sliding vane compressors have low pressure applications The sliding vane compressors operate at low speeds The noise level of the sliding vane compressors is low
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