INDUSTRIAL PLANT ENGINEERING
Module 7. AIR COMPRESSORS, BLOWERS, FANS
Objectives:
1. Discuss air compressors and its various application to industry.
2. Discuss the types of fans and blowers and its proper application.
3. Discuss how to select and assess fans and blowers.
Contents:
7.1 Compressors and air systems
Compressors and air systems are essential components in various industries, providing compressed air for
numerous applications. Compressors and air systems play a critical role in various applications across
multiple industries. Understanding their types, components, benefits, and best practices for operation and
maintenance can enhance productivity and efficiency in your operations.
Air Compressors
Air compressors are mechanical devices that convert power (often from an electric motor or a diesel engine)
into potential energy stored in compressed air. They work by sucking in air and compressing it, which increases
its pressure and allows it to be stored for later use. Here’s a more detailed look at air compressors:
Classification of Air Compressors
Air compressors are classified based on their operation principle, pressure range, and application.
1. Based on Operation Principle
A. Positive Displacement Compressors
These compressors work by trapping air and reducing its volume to increase pressure.
1. Reciprocating Compressors (Piston Compressors)
• Use a piston driven by a crankshaft to compress air within a cylinder.
• Available in single-stage (low pressure) and multi-stage (high pressure) designs.
• Applications: Small workshops, automotive, refrigeration, industrial tools.
• Example: Piston-type air compressor in garages.
2. Rotary Compressors
• Use rotating elements to compress air. Utilize two interlocking rotors to compress air continuously.
They are ideal for larger operations requiring a constant flow of compressed air.
• More efficient for continuous-duty applications.
(a) Rotary Screw Compressors
• Uses two interlocking helical screws.
• Efficient, low maintenance, and suitable for continuous operation.
• Applications: Manufacturing, HVAC, food processing, industrial plants.
(b) Rotary Vane Compressors
• Uses a rotor with sliding vanes.
• Moderate efficiency, used in medium-duty applications.
• Applications: Automotive, printing, vacuum systems.
(c) Lobe & Scroll Compressors
• Lobe: Used in low-pressure applications.
• Scroll: Quiet, oil-free compression (used in medical and clean applications).
B. Dynamic Compressors (Non-Positive Displacement)
These compressors increase air pressure by imparting velocity energy and then converting it into pressure.
1. Centrifugal Compressors
• Uses high-speed impellers to increase air velocity. Employ rotating impellers to convert speed into
pressure. These compressors are often used in large-scale operations, such as in power plants and
chemical processing.
• Converts kinetic energy into pressure using a diffuser.
• Applications: Power plants, large-scale refrigeration, petrochemical plants.
2. Axial Compressors
• Air flows parallel to the axis through multiple rotor-stator stages.
• Extremely high efficiency and pressure ratio.
• Applications: Jet engines, gas turbines, high-power industrial applications.
2. Based on Pressure Range
Type
Pressure Range Example Applications
Low-Pressure Compressors
0-150 psi
Medium-Pressure Compressors 150-1000 psi
High-Pressure Compressors
HVAC, small tools, airbrushing
Industrial manufacturing, shipyards
Above 1000 psi Breathing air (scuba, fire services), gas pipelines
3. Based on Lubrication
• Oil-Lubricated Compressors – Higher durability, used in heavy-duty applications.
• Oil-Free Compressors – For clean air applications (medical, food industry).
Final Selection Guide
• For small tools, garages → Reciprocating (Piston) Compressors
• For continuous industrial use → Rotary Screw Compressors
• For large-scale plants → Centrifugal Compressors
• For precision air supply → Oil-Free or Scroll Compressors
How Air Compressors Work
• Intake: The compressor draws in ambient air from the surrounding environment.
• Compression: The air is compressed to a higher pressure, often through mechanical means (as
described above).
• Storage: Compressed air is stored in a tank or delivered directly to tools and machinery for immediate
use.
• Discharge: When needed, the compressed air is released at a controlled pressure for various
applications.
Applications of Air Compressors
• Construction and Manufacturing: Powering pneumatic tools such as nail guns, jackhammers, and
spray guns.
• HVAC Systems: Used for air conditioning and refrigeration systems.
• Automotive: Used for airbrushing, tire inflation, and powering air tools in garages.
• Food and Beverage: Compressed air is essential in packaging, cleaning, and conveying processes.
• Medical: Provides power for dental and medical devices.
Benefits of Using Air Compressors
• Efficiency: Compressed air is a versatile energy source that can power numerous tools and equipment.
• Safety: Non-flammable and non-toxic, making it a safer choice compared to other energy forms.
• Compact Size: Air compressors are available in various sizes, making them suitable for both industrial
and DIY applications.
Maintenance Considerations
To ensure efficient operation and longevity, regular maintenance of air compressors is essential. This includes:
• Checking and changing filters
• Monitoring oil levels for lubricated compressors
• Inspecting for leaks in hoses and fittings
• Ensuring the air receiver tank is free of moisture and debris
7.2 Types of Fans and Blowers
Fans are used in various industrial and commercial applications for ventilation, cooling, air circulation, and
exhaust. They operate at relatively low pressure and high volume compared to blowers.
Types of Fans
1. Axial Fans (Move air parallel to the axis)
• Propeller Fans – Used in general ventilation, exhaust systems.
• Tube Axial Fans – More efficient than propeller fans, used in ducted systems.
• Vane Axial Fans – Higher efficiency, used in HVAC and industrial cooling.
2. Centrifugal Fans (Move air perpendicular to the axis)
• Forward-Curved Fans – High volume, low pressure (HVAC, air handling units).
• Backward-Curved Fans – More efficient, used in industrial ventilation.
• Radial Fans – High-pressure applications (dust collection, pneumatic conveying).
3. Mixed Flow Fans (Combination of axial and centrifugal)
• Used in high-performance applications like tunnel ventilation and cooling systems.
Selection Criteria
• Airflow (CFM or m³/hr) – Required volume of air.
• Static Pressure (inches of water column) – Resistance in the system.
• Efficiency & Power Consumption – Important for energy savings.
• Noise Levels – A factor in comfort and workplace safety.
• Material & Construction – Corrosion resistance for harsh environments.
Blowers are used in industrial and commercial applications where air or gas needs to be moved at a moderate
pressure. They operate between the vacuum created by fans and the high-pressure capability of compressors.
Types of Blowers
1. Centrifugal Blowers
o Use a rotating impeller to increase the velocity of air.
o Common types:
▪ Forward-curved (low pressure, high volume)
▪ Backward-curved (higher efficiency, medium pressure)
▪ Radial/blade (high pressure, lower volume)
2. Positive Displacement Blowers
o Trap air and force it into a discharge system.
o Common types:
▪ Rotary Lobe Blowers (used in wastewater treatment, pneumatic conveying)
▪ Rotary Screw Blowers (more efficient for continuous duty)
Selection Factors
• Airflow (CFM or m³/hr) – Volume of air required.
•
•
•
•
Pressure (inches of water column or PSI) – Needed for the application.
Efficiency & Power Consumption – Affects operational costs.
Material & Build Quality – Consider corrosion resistance for harsh environments.
Noise & Maintenance Requirements – Especially important for large installations.
Exhauster:
An exhauster is a mechanical device used to remove air, gas, or fumes from a space, typically creating a
vacuum or negative pressure. It is commonly used for ventilation, dust collection, industrial exhaust, and
vacuum applications.
Types of Exhausters
Exhausters can be classified based on their working principle:
A. Centrifugal Exhausters
• Use a centrifugal fan or blower to move air outward through an impeller.
• Efficient for high-volume, low-to-medium pressure applications.
• Applications: Industrial ventilation, dust extraction, fume removal.
B. Axial Exhausters
• Use axial fans to move air in a straight direction along the axis.
• Best for applications requiring high airflow with low resistance.
• Applications: HVAC exhaust systems, cooling towers, tunnel ventilation.
C. Vacuum Exhausters (Positive Displacement)
• Create a higher vacuum by using rotary or reciprocating mechanisms.
• Types:
o Rotary Lobe Exhausters – Used in pneumatic conveying, wastewater treatment.
o Roots-Type Exhausters – Common in high-vacuum and industrial air handling.
o Liquid Ring Exhausters – Used in chemical processing and vapor recovery.
• Applications: Vacuum systems, material handling, chemical industries.
Applications of Exhausters
Industrial Ventilation – Removing fumes, heat, and smoke from factories.
Dust Collection – Used in cement plants, woodworking, and steel industries.
Pneumatic Conveying – Moving powders, grains, or other materials using air.
Combustion Exhaust – Removing combustion gases from boilers and furnaces.
Vacuum Applications – Used in processes requiring negative pressure.
Factors to Consider When Selecting an Exhauster
• Airflow Capacity (CFM or m³/hr) – Required volume of air movement.
• Static Pressure (inches of water column or Pascal) – Resistance in the system.
• Efficiency & Power Consumption – Determines operating cost.
• Material & Construction – Consider corrosion resistance for harsh environments.
• Noise Levels & Maintenance Requirements – Especially important for large industrial setups.
Difference Between Fans and Blowers
Feature
Fans
Blowers
Air Movement Moves large volumes of air at low pressure Moves air at moderate pressure
Pressure Ratio Pressure ratio < 1.11
Pressure ratio between 1.11 and 1.20
Airflow
Direction
Generally parallel to the fan’s axis (axial
flow)
Perpendicular or mixed (centrifugal flow)
Types
Axial (propeller, tube axial, vane axial),
Centrifugal (forward, backward, radial)
Centrifugal (forward, backward, radial) and
Positive Displacement (rotary lobe, screw)
Applications
Ventilation, cooling, air circulation, exhaust
Pneumatic conveying, aeration, combustion air
supply
Efficiency
Higher efficiency at low pressure
More efficient for moderate-pressure
applications
Common
Uses
HVAC, cooling towers, industrial ventilation
Wastewater treatment, material handling,
furnace combustion air
Differences Between Fans, Blowers, and Compressors
Feature
Fans
Blowers
Compressors
Air Movement
Moves large volumes of air Moves air at moderate
at low pressure
pressure
Compresses air to high
pressure
Pressure Ratio
< 1.11
1.11 – 1.20
> 1.20
Airflow Direction Usually axial or centrifugal
Mostly centrifugal or positive
displacement
Positive displacement or
dynamic
Energy
Consumption
Lowest
Moderate
Highest
Applications
HVAC, cooling, ventilation
Pneumatic conveying,
aeration, combustion air
Industrial air tools,
refrigeration, gas storage
7.3 Assessment of Fans and Blowers
Assessing fans and blowers involves evaluating their performance, efficiency, suitability for specific
applications, and operational costs. Below are key criteria for assessment:
Performance Assessment
Fans:
• Airflow (CFM or m³/hr): Measures the volume of air moved.
• Static Pressure (inches of water column or Pascal): Resistance in the system.
• Fan Total Efficiency (%): Ratio of power output to power input.
• Fan Laws: Used to predict performance under different speeds and conditions.
Blowers:
• Pressure Range: Typically between 1.11 to 1.20 pressure ratio.
• Airflow Rate: Measured in CFM or m³/hr.
• Efficiency (%): Lower than fans but higher than compressors.
Energy Efficiency Assessment
• Power Consumption (kW or HP): Directly impacts operating costs.
• Motor Efficiency: Higher efficiency motors (IE3, IE4) reduce energy use.
• Variable Frequency Drive (VFD): Controls speed to optimize efficiency.
•
System Losses: Duct leakage, improper sizing, or excessive bends reduce efficiency.
Reliability and Maintenance
• Bearings & Lubrication: Proper maintenance prevents failures.
• Blade Wear & Corrosion: Especially important in harsh environments.
• Noise & Vibration Levels: Can indicate misalignment or system inefficiencies.
• Operating Life Expectancy: Affected by duty cycle and environmental factors.
Suitability for Application
Application
Fans
Blowers
HVAC & Ventilation
Best suited
Not ideal
Cooling Systems
Commonly used
Limited use
Pneumatic Conveying
Not effective
Preferred
Industrial Exhaust
Used in general cases
Used in high resistance cases
Combustion Air Supply
Not effective
Common use
Cost Analysis
• Initial Investment: Fans are generally cheaper than blowers.
• Operating Costs: Blowers consume more power due to higher pressure needs.
• Maintenance Costs: Regular cleaning and lubrication reduce long-term costs.
Note:
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•
•
Fans are suitable for applications requiring high airflow and low pressure.
Blowers are best for moderate pressure applications, such as pneumatic conveying or aeration.
Proper selection, sizing, and maintenance maximize efficiency and lifespan.
7.4 Selection of fans and blowers
Choosing the right fan or blower depends on airflow requirements, pressure needs, efficiency, and
environmental factors.
1. Define the Application
Identify where the fan or blower will be used:
• Ventilation & HVAC (low pressure, high volume) → Fans
• Cooling systems (heat exchangers, machinery cooling) → Fans
• Pneumatic conveying (moving materials with air) → Blowers
• Combustion air supply (furnace, boiler) → Blowers
• Dust collection & industrial exhaust → Fans or Blowers (depending on pressure needs)
2. Determine Airflow and Pressure Requirements
• Airflow (CFM or m³/hr) – The volume of air needed per unit time.
• Static Pressure (inches of water column or Pascal) – Resistance in the system.
• Use Fan Laws to calculate airflow and pressure changes when adjusting size or speed.
Selection Basis
Fans
Blowers
Low pressure (<1.11 pressure ratio)
Suitable
Not ideal
Moderate pressure (1.11 – 1.20)
Not effective
Best choice
High airflow (CFM)
Best choice
Less common
Ducted applications
Used
Used
3. Choose the Type of Fan or Blower
Fans (Low Pressure, High Volume)
• Axial Fans – Best for general ventilation, cooling towers, and HVAC.
• Centrifugal Fans – Used for higher static pressure, industrial exhaust, and dust collection.
Blowers (Moderate Pressure Applications)
• Centrifugal Blowers – Used for material handling, combustion air, and aeration.
• Positive Displacement Blowers – Suitable for pneumatic conveying and wastewater aeration.
4. Consider Efficiency and Energy Consumption
• Select high-efficiency models to reduce energy costs.
• Use Variable Frequency Drives (VFDs) to control speed and optimize power usage.
• Ensure proper system design (ducting, minimal bends) to reduce losses.
5. Evaluate Environmental & Maintenance Factors
• Temperature & Humidity – Corrosion-resistant materials for harsh conditions.
• Noise & Vibration – Lower speed and better mounting can reduce noise.
• Maintenance Requirements – Bearings, lubrication, and blade cleaning affect longevity.
6. Compare Costs (Initial vs. Operating Costs)
• Fans: Lower initial cost, lower operating cost.
• Blowers: Higher cost but necessary for moderate pressure applications.
Final Selection Decision
• If you need high airflow at low pressure → Choose a fan
• If you need moderate pressure and controlled airflow → Choose a blower
Formula for Compressor:
Compression Process (1 → 2) in a Reciprocating Air Compressor
The compression process in a reciprocating air compressor follows a polytropic process, meaning the
relationship between pressure (P) and volume (V) is given by:
Equation for Compression Process
Since the process follows a polytropic law, we use:
Piston Displacement for single stage:
Volumetric efficiency
Volumetric efficiency measures how effectively a compressor draws in air compared to its piston
displacement. It is defined as the ratio of the actual volume of air intake at atmospheric conditions to the
theoretical piston displacement.
Work Done in Polytropic Compression
A real compressor follows polytropic compression, where heat is exchanged during compression. The work
done is:
Work Done in Isothermal Compression
For perfect isothermal compression (constant temperature), the work done per cycle is:
or
Work Done in Adiabatic Compression
For adiabatic compression (no heat transfer), the work done is:
or
Brake Power of a Reciprocating Compressor
Brake power is the actual power required to drive the compressor, considering mechanical losses due to
friction, bearings, and other inefficiencies. It is measured at the shaft of the compressor.
Alternatively, if torque and speed are known, brake power can be calculated using:
Adiabatic Efficiency of a Compressor
Adiabatic efficiency (also called isentropic efficiency) of a reciprocating air compressor measures how
efficiently the compressor converts input power into useful compressed air under ideal (isentropic)
conditions.
Piston Speed of a Reciprocating Compressor
Piston speed is the average linear speed of the piston inside the cylinder. It is an important parameter for
designing compressors because it affects efficiency, wear, and heat dissipation.
Indicated Power of a Reciprocating Compressor
The indicated power is the theoretical power required to compress the air inside the cylinder, assuming no
mechanical losses. It is calculated based on the work done per cycle and the number of cycles per second.
Piston displacement (double acting single stage compressor)
a. Piston rod neglected
b. Piston rod considered
Formula for Fans and Blowers:
Capacity of Fan
Static head
Static head (or static pressure head) is the pressure exerted by a fluid (usually air) due to its potential energy,
independent of its motion. It is measured in inches of water column (in. WC) or Pascals (Pa).
Velocity head
Velocity head represents the kinetic energy of moving air per unit weight. It quantifies the height a column of air
would need to fall (due to gravity) to reach a given velocity. It is a crucial factor in fan and blower calculations.
Total head
Total head represents the total energy per unit weight of the air moving through a fan or blower. It is the sum of:
• Static head – pressure-related potential energy
• Velocity head – kinetic energy due to air movement
Power Output of Fan
Power Output of Fan or Air power is the useful power transferred to the air by the fan or blower. It represents the
energy required to move air at a given flow rate and pressure.
Power Input or Brake power
Brake power is the actual power required to drive the fan or blower shaft before accounting for motor losses. It
represents the mechanical power needed to overcome air resistance, system losses, and fan inefficiencies.
Motor power:
Basic Fan Laws
Fan laws (or affinity laws) describe how fan performance changes with variations in speed, diameter, or airflow
conditions. They are essential for predicting fan behavior when modifying operating parameters.
Effects of Fan Speed Variation (Using Fan Laws)
1. Flow Rate (Air Volume) Changes Proportionally with Speed:
2. Pressure Changes with the Square of Speed:
3. Power Consumption Changes with the Cube of Speed
Effects of Fan Size Variation (Using Fan Laws)
1. Flow Rate (Capacity) Varies with the Cube of Diameter:
2. Pressure Varies with the Square of Diameter:
3. Power Consumption Varies with the Fifth Power of Diameter:
Impact of Gas Density on Fan Performance
1. Flow Rate – No Change
Q2 = Q1
2. Pressure – Proportional to Density
3. Power Consumption – Proportional to Density
Sample problem: (Compressor)
1.
2.
3. The initial condition of air in an air compressor is 98 Kpa and 27°C and discharges air at 400
Kpa. The bore and stroke are 355 mm and 381 mm, respectively with percent clearance of 5%
running at 300 rpm. Find the volume of air at suction.
Practice Problem:
1.
2.
3. A two stage compressor with first stage piston displacement of 94390 cm³/sec is driven
by a motor. Motor output is 35 Hp, suction temperature 22°C, volumetric efficiency is 85%.
Mechanical efficiency is 95%, the intercooler pressure is 30 psi gage. Air temperature in
and out of the intercooler are 1O5°C and 44°C. Final discharge pressure is 100 psi gage,
suction estimated 14.5 psi. Find the compression efficiency.
Sample problem: (Fans and Blowers)
1. What hp is supplied to air moving at 20 fpm through a 2 x 3 ft duct under a pressure of 3 in
water gage?
2. A fan whose static efficiency is 40% has a capacity of 60,000 ft³/hr at 60°F and barometer of 30
in Hg and gives a static pressure of 2 inch of water density column on full delivery. What size of
electric motor should be used to drive this fan?
3. Air enters a fan through a duct at a velocity of 6.3 m/s and an inlet static pressure of 2.5 cm of
water less than atmospheric pressure. The air leaves the fan through a duct at a velocity of 11.25
m/s and a discharge static pressure of 7.62 cm of water above the atmospheric pressure. If the
specific weight of the air is 1.20 kg/m³ and the fan delivers 9.45 m³/s, what is the fan efficiency
when the power input to the fan is 13.75 kw at the coupling?
4. A fan delivers 4.7 m³/s at a static pressure of 5.08 cm of water when operating at a speed of 400
rpm. The power input required is 2.963 kw. If 7.05 m³/s are desired in the same fan and
installation, find the pressure in cm of water.
5. A fan described in a manufacturer's table is rated to deliver 500 m³/min at a static pressure gage
of 254 cm of water when running at 250 rpm and requiring 3.6 kw. If the fan speed is changed to
305 rpm and the air handled were at 65°C instead of standard 21"C, find the power in kw.
Practice Problem:
1. A fan draws 1.42 m³ per second of air at a static pressure of 2.54 cm of water through a duct 300
mm diameter and discharges it through a duct of 275 mm diameter. Determine the static fan
efficiency if total fan mechanical is 70% and air is measured at 25°C and 760 mm Hg.
2. Find the air horsepower of an industrial fan that delivers 25 m³/s of air through a 900 mm by 1200
mm outlet. Static pressure is 127 mm of water gage and air density is 1.18 kg/m³.
3. A boiler requires 90,000 ml/hr of standard air. The mechanical efficiency of fan to be installed is
65%. Determine the size of driving motor assuming fan can deliver a total pressure of 150 mm of water
gage.