Compressor Theory and Designs - Chapter 8

A set of 100 practice flashcards covering industrial air and gas compressor theory, designs, classification, and maintenance based on Chapter 8 of the Power Engineering text.

What is sometimes referred to as industry's fourth utility?

Compressed air

How is the Compression ratio of a compressor defined?

The ratio of the volume at the beginning of a compression stroke to the volume at the end.

In the SI system, what is the value of $1$ atmosphere ($1\,atm$)?

$$1\,atm = 1.013\,bar = 101.3\,kPa$$

What is the relationship between $1\,bar$ and $kPa$ in the SI system?

$$1\,bar = 100\,kPa = 10^5\,N/m^2$$

What does the term Free air delivery (FAD) represent?

The volumetric flow rate converted to the inlet pressure and temperature conditions, whether actual or standard conditions.

What is Compressor capacity?

The quantity of air delivered, which may be expressed as actual delivery or as delivery referenced to standard air conditions.

What is the effect of intercooling between stages in a multi-stage compressor?

It increases air density, improves efficiency, reduces power required, and causes moisture to condense.

Which theoretical compression process requires the least amount of work?

Isothermal compression

What are the conditions for Standard Air adopted by the Compressed Air and Gas Institute (ISO-1217)?

A pressure of $1\,bar$ ($14.5\,psia$), a temperature of $20^\circ C$ ($68^\circ F$), and a relative humidity of $0\%$.Header

What are the conditions for the International standard atmosphere?

A pressure of $1\,atm$ ($101.325\,kPa$), a temperature of $15^\circ C$, and a relative humidity of $0\%$.Header

What process occurs if compression of a gas happens with no external transfer of heat to or from the gas?

Adiabatic compression

What are the three main factors that affect barometric pressure?

Temperature of the air, altitude or elevation, and moisture content

What is the typical reduction in compressor capacity at elevated altitudes?

$3\%$ per $1000$ feet or $300$ metres

Compared to Vancouver, how much less air does a compressor in Calgary deliver due to lower atmospheric pressure?

About $10\%$ less air

What is the formula mass of water compared to the formula mass of air?

Water is $18\,kg/kmol$ while air is about $29\,kg/kmol$

What basic rule describes the relationship between humidity and air pressure?

Pressure decreases as humidity increases.

What are the two main principles used for the compression of air and gases?

Positive displacement compression and dynamic compression

What defines a positive displacement compressor?

The air is trapped and gradually reduced in volume.

What defines a dynamic compressor?

Sets of rotating and stationary blades accelerate air to high velocity and convert kinetic energy into pressure.

Which type of piston is also known as a trunk-type piston?

Single-acting piston

How many compression strokes occur per revolution of the crankshaft in a double-acting piston?

Two compression strokes

At what pressure range are single-stage air compressors generally used?

$480\,kPag$ to $700\,kPag$ ($70\,psig$ to $100\,psig$)

At what pressure range are two-stage air compressors typically used?

$700\,kPag$ to $1730\,kPag$ ($100\,psig$ to $250\,psig$)

How can a single-stage, two-cylinder compressor be recognized visually?

Both cylinders have intake air filters and the same diameter.

In a multi-stage compressor, which stage has the larger diameter?

The first stage

What are the four common cylinder arrangements for industrial reciprocating compressors?

Vertical (single and in-line); L, V, and W type; Tandem cylinder; Horizontal opposed

What configuration is the compressor in a tandem cylinder arrangement?

The cylinders are oriented end to end along a common axis with a single piston rod driving all pistons.

What is the purpose of the horizontal opposed compressor design?

To balance out the pressure and inertial forces produced in the machine.

What are the rotative speeds for heavy-duty reciprocating compressors?

$180\,rpm$ to $900\,rpm$

What materials are used for compressor cylinders based on pressure?

Cast iron for up to $1000\,kPa$, cast steel for up to $6900\,kPa$, and forged steel above that.

In oil-free reciprocating compressors, what materials are the piston rings made from?

Teflon or carbon

What are the advantages of using a separate crosshead in heavy-duty compressors?

Allows narrow piston/larger valve area, permits longer stroke/greater capacity, and separates crankcase from cylinder to control oil carryover.

What are the polished top and bottom surfaces of a crosshead called?

Shoes

How are compressor crankshafts designed to balance reciprocating and rotating forces?

They are made with counterweights.

What is a flapper valve?

A plate valve, also called a reed valve, consisting of a single strip of flexible steel ribbon.

Which company developed the channel valve?

Ingersoll Rand

What are channel valves sometimes called?

Beam valves

What component is used in channel valves to prevent metal-to-metal contact between channels and springs?

Strips of self-lubricating material

What are three environmental causes of compressor valve problems?

Corrosive contaminants, foreign material, and improper lubrication

What is a sign of valve failure regarding electrical consumption?

Lower electrical consumption or low current (amperage) draw

What does 'Intake depression' refer to on an actual indicator diagram?

The steady pressure differential required to overcome valve inertia and maintain flow.

How does a faulty suction valve on the first stage affect intercooler pressure?

It results in a slightly lower intercooler pressure.

How does a faulty suction valve on the second stage affect the intercooler?

It causes an increase in intercooler pressure and temperature.

What are the three areas for lubrication in reciprocating compressors?

The crankcase, cylinder, and packings

What is the difference between external and internal lubrication?

External refers to bearings and crossheads in the crankcase; internal refers to the inside of the cylinder and packings.

What is splash lubrication?

A process where bearings, pins, and rings are lubricated by oil thrown from the crankshaft.

What is the purpose of pressure packing?

To keep compressed gas from leaking out of the cylinder where the piston rod enters.

What is the purpose of oil wiper packing?

To keep oil from leaking out of the crankcase.

What term is used to describe cylinder and packing lube systems because they do not recirculate oil?

Terminating or once-through systems

What is an engine indicator?

A mechanical instrument for graphically recording pressure versus piston displacement through a cycle.

On a PV diagram, what does the shaded area enclosed by a cycle represent?

The work added in the thermodynamic cycle

In a PV diagram with clearance, what is the volume $V_1 - V_4$ called?

Effective volume

What does the shaded region above the discharge pressure line in an actual indicator diagram represent?

Valve bounce or the pressure rise required to open the discharge valve

How does discharge valve leakage affect effective volume?

It reduces the effective volume because expansion from the clearance space takes longer.

What is the definition of Clearance volume?

The volume of air that remains in a cylinder end when the piston is at the end of its stroke.

What is the formula for Clearance volume?

$$\text{Clearance length} \times \text{cylinder area}$$

How is Volumetric efficiency calculated?

$$\text{Effective volume} \div \text{Swept volume}$$

What happens to volumetric efficiency as the pressure ratio increases?

Volumetric efficiency decreases.

What is the characteristic constant ($R$) for air used in density calculations?

$$0.287\,kJ/kgK$$

Calculate the density of air at $100\,kPa$ and $20^\circ C$.

$$\rho = \frac{100\,kPa}{0.287\,kJ/kgK \times 293\,K} = 1.189\,kg/m^3$$

What is the formula for calculating the Displacement ($D$) of a single-acting compressor?

$$D = \frac{\pi D^2 LnN}{4}$$

Convert $10.6\,CFM$ to cubic metres per hour ($m^3/h$) given $1\,m^3 = 35.3\,ft^3$.

$$18.0\,m^3/h$$

How much volume is in $1\,m^3$ in terms of litres?

$$1000\,L$$

What is the definition of Swept volume?

The volume displaced by the piston during one complete stroke ($V_1 - V_3$).

What common features do all rotary compressors share?

They use rotating elements to provide energy and do not have inlet or discharge valves.

At what speeds do sliding vane compressors typically operate?

Up to $3000\,rpm$

What is another name for a rotary lobe compressor?

Roots blower

Why is casing lubrication not required in rotary lobe compressors?

The rotors do not touch each other or the casing.

Who developed the rotary screw compressor?

Alf Lysholm

What are the two common names for the two rotors in a twin-screw compressor?

Male rotor (convex lobes) and Female rotor (concave flutes)

What are the most common configurations for lobes and flutes on screw rotors?

$4$ lobes and $5$ flutes, or $4$ lobes and $6$ flutes

What are the three functions of injected oil in a screw compressor?

Lubrication, heat removal, and acting as a seal

What is the usual oil carryover level after passing through coalescent filters in a screw package?

$2\,ppm$ to $5\,ppm$

What advantage do silicone-based lubricants provide in rotary screw compressors?

Resistance to oxidative and thermal degradation, preventing varnish or sludge formation.

What three methods are used for capacity control in rotary screw compressors?

Variable displacement, variable speed control, and inlet valve modulation

How does a variable displacement control (slide valve) work?

It reduces the effective length of the rotors by allowing some air to be bypassed at the inlet.

What is the purpose of an automatic blowdown valve in a compressor package?

To relieve pressure from the separator after each shutdown.

What does a coalescing filter do?

It forces liquid droplets to join together until they are heavy enough to fall to the bottom.

Why is PVC pipe unsuitable for high-pressure compressed air systems?

It becomes brittle with time and is corrosive to compressor lubricating oil.

What are the two most common piping layouts for compressed air systems?

The loop system and the main-and-branch system

Why should air piping be taken from the top of a distribution header?

To eliminate the chance of liquid condensate contaminating the drop airstream.

What is the purpose of a Drip leg?

To remove collected condensate from a low-point in the header using gravity.

What is the main purpose of an air receiver tank?

To provide temporary storage and handle peaks in demand that exceed compressor capacity.

What type of filter uses activated carbon to remove odor and vapors?

Adsorption filter

How does a refrigerant dryer remove moisture?

By cooling the incoming air to depress the dewpoint and condense the moisture.

How does a deliquescent air dryer operate?

It uses a salt bed that absorbs moisture and partially dissolves.

What are the two common types of oil/water separators used for condensate?

Gravity separation and chemical adsorption

What does the 'FRL' unit stand for in a piping drop?

Filter, Regulator, Lubricator

What is a turbo-compressor?

Another name for a dynamic compressor, such as centrifugal or axial types.

In a centrifugal compressor, what is the role of the diffuser?

To transform kinetic energy (velocity) into static pressure.

What are the three main components of a turbocharger?

Centrifugal compressor, turbine, and a central connecting shaft

What provides the power to drive a turbocharger's turbine?

Engine exhaust gas

What is a single stage in an axial flow compressor?

One pair of moving blades and one pair of stationary (stator) blades

At what speeds can axial flow compressors operate?

Up to $20\,000\,rpm$

Why do axial compressor blades get shorter towards the discharge end?

Because the gas path volume becomes progressively smaller as the air is compressed.

How does the efficiency of an axial flow compressor compare to a centrifugal compressor?

It is about $10\%$ greater.

What is the main disadvantage of axial blades compared to centrifugal impellers?

They are more subject to corrosion and erosion.

In an axial flow compressor, in which direction does the gas travel?

In an axial direction along the length of the shaft

Which component in a reciprocating compressor connects the piston rod to the connecting rod?

The crosshead

What is intended by the term 'Standard CFM' (SCFM)?

Flow of free air measured and converted to standard reference conditions ($1\,bar$, $20^\circ C$, $0\%$ humidity).