SECTION 9 AIR AND GAS COMPRESSORS AND VACUUM SYSTEMS Estimating the Cost of Air Leaks in Compressed-Air Systems 9.1 Selecting an Air Motor for a Known Application 9.4 Air-Compressor Cool
Trang 1SECTION 9 AIR AND GAS COMPRESSORS AND VACUUM SYSTEMS
Estimating the Cost of Air Leaks in
Compressed-Air Systems 9.1
Selecting an Air Motor for a Known
Application 9.4
Air-Compressor Cooling-System Choice
for Maximum Coolant Economy 9.10
Economics of Air-Compressor Inlet
Sizing Rupture Disks for Gases and Liquids 9.39
Determining Airflow in Pipes, Valves, and Fittings 9.40
System Economics and Design Strategies
ESTIMATING THE COST OF AIR LEAKS IN
COMPRESSED-AIR SYSTEMS
Find the cost of compressed air leaking through a 0.125-in (0.3175-cm) diameterhole in a pipe main of a typical industrial air piping system, Fig 1, to the atmo-
are the same as given above?
Calculation Procedure:
1. Find the volume of air discharged to the atmosphere
Air flowing through an orifice or nozzle attains a critical pressure of 0.53 times theinlet or initial pressure This reduced pressure occurs at the throat or vena contracta,which is the point of minimum stream diameter on the outlet side of the air flow
If the outlet or back pressure exceeds the critical pressure then the vena contracta
Trang 2FIGURE 1 Typical compressed-air system main and branch pipes (Factory
Manage-ment and Maintenance).
or throat pressure rises to equal the backpressure Air flow through a hole in a pipe
or tank replicates the flow through an orifice or nozzle
⫽ ⬚F⫹460
2. Determine the annual cost of the air leakage
This compressed-air plant operates 7500 h / yr Since the leakage rate is 328.8
are several leaks of this size, or larger, in the system
3. Find the rate of leakage at the higher line pressure
When the backpressure is less than the critical pressure, a different flow equation
critical pressure is greater than the backpressure Air leakage through the hole is
symbols as before
Trang 4Substituting, W⫽0.5303(0.012272)(1.0)(64.7) / (530)0.5⫽0.01829 lb / s (0.0083
(29.89 kg / h)
4. Compute the annual cost of air leakage at the higher pressure
times as great This points out the fact that higher pressures in a compressed-airsystem can cause more expensive leaks
Related Calculations. Compressing air requires a power input to raise the airpressure from atmospheric to the level desired for the end use of the air Whencompressed air leaks from a pipe or storage tank, the power expended in compres-sion is wasted because the air does no useful work when it leaks into the atmo-sphere
In today’s environment-conscious world, compressed-air leaks are considered to
be especially wasteful because they increase pollution without producing any eficial results The reason for this is that the fuel burned to generate the power tocompress the wasted air pollutes the atmosphere unnecessarily because the air pro-duces only a hissing sound as it escapes through the hole in the pipe or vessel
ben-SELECTING AN AIR MOTOR FOR A KNOWN
APPLICATION
Show how to select a suitable air motor for a reversible application requiring 2 hp(1.5 kW) at 1000 rpm for an industrial crane Determine the probable weight ofthe motor, its torque output, and air consumption for this intermittent duty appli-cation An adequate supply of air at a wide pressure range is available at theinstallation
Calculation Procedure:
1. Assemble data on possible choices for the air motor
There are four basic types of air motors in use today: (1) radial-piston type; (2)axial-piston type; (3) multi-vaned type; (4) turbine type Each type of air motor hasadvantages and disadvantages for various applications Characteristics of these airmotors are as follows:
(1) Radial-piston air motors, Fig 3, have four or five cylinders mounted around
a central crankshaft similar to a radial gasoline engine Five cylinders are preferred
to supply more horsepower with evenly distributed power pulses In such a unitthere are always two cylinders having a power stroke at the same time The radial-piston motor is usually a slow-speed unit, ranging from 85 to 1500 rpm It is suitedfor heavy-duty service up to 20 hp (15 kW) where good lugging characteristics areneeded Normally they are not reversible, though reversible models are available atextra cost
(2) Axial-piston air motors, Fig 4, are more compact in design and require lessspace than a four- or five-cylinder radial-piston motor Air drives the pistons intranslation; a diaphragm-type converter changes the translation into rotation Thisarrangement supplies high horsepower per unit weight Axial-piston motors are
Trang 5AIR AND GAS COMPRESSORS AND VACUUM SYSTEMS 9.5
FIGURE 3 (a) Five-cylinder piston-type radial air motor used in sizes from about 2 hp (1.5 kW) to 22 hp (16.4 kW) and at speeds from 85 to 1500 rpm (b) How five-cylinder air motor distributes power Two cylinders are always on power stroke at any instant (Gardner-Denver
Company).
available in sizes from 0.5 to 2.75 hp (0.37 to 2.1 kW) They run equally well ineither direction To make the motor reversible, a four-way air valve is inserted inthe line
(3) Multi-vaned motors, Fig 5, are suitable for loads from fractional hp (kW)
to 10 hp (7.5 kW) They are relatively high-speed units which must be geared downfor usable speeds The major advantages of multi-vaned motors is light weight andsmall size However, if used at slow speed, the gearing may add significantly tothe weight of the motor
(4) Air-turbine motors deliver fractional horsepowers at exceptionally highspeeds, from 10,000 to 150,000 rpm, and are an economical source of power Theyare tiny impulse-reaction turbines in which air at 100 psi (689 kPa) impinges onbuckets for the driving force Force-feed automatic lubrication sprays a fine film ofoil on to bearings continuously, minimizing maintenance
Based on the load requirements, 2 hp (1.5 kW) at 1000 rpm, a reversible piston air motor, Table 1, would be a suitable choice because it delivers up to 2.8
Trang 6radial-Diaphragm converts piston translation
to rotation
Pistons
(four or five)
Output shaft
FIGURE 4 Axial-piston air motor available in various output sizes (Keller Tool
Com-pany).
FIGURE 5 Typical multi-vane type air motor, available in fractional hp sizes and up to
some 10 hp (7.5 kW) (Gast Manufacturing Company).
Trang 7AIR AND GAS COMPRESSORS AND VACUUM SYSTEMS 9.7
TABLE 1 Specifications of Typical Air Motors
Rated hp*
(kW)
Speed at rated
hp—rpm
Free speed rpm
Weight
lb (kg)
Stall torque ft—lbs
Air consumption
free air/min Radial piston motors (non-reversible)*
2.9 (2.2) 1,500 3,200 130 (59)
3.3 (2.5) 1,300 3.000 130 (59)
3.8 (2.8) 1,200 2,700 130 (59)
Radial piston motors (reversible)* 2.5 (1.7) 1,200 2,200 135 (61.3)
2.8 (2.1) 1,000 1,950 135 (61.3)
3.2 (2.4) 900 1,600 135 (61.3)
5.2 (3.9) 750 1,600 200 (90.8)
Ingersoll-Rand.
The weight of this motor, Table 1, is 135 lb (61.3 kg)
2. Compare the advantages of air motors to other types of motive power
Air motors have a number of advantages over their usual competitors—electric motors These advantages are: (1) In explosive or gaseous environments, air motors are lower in cost than larger, heavier, explosion-proof electric motors Air motors operate relatively trouble-free in moist, humid environments where the electric mo-tor may suffer from a buildup of fungus and corrosion And since the air momo-tor requires little maintenance, it can be mounted in inaccessible locations (2) With
an air motor, the output speed can be varied from zero to free-speed no-load rotation
by merely changing the volume of air supplied to the motor Controls are simple
in design and use (3) Air motors can weigh as little as one-quarter that of electri-cally-powered units; their physical dimensions are about 50 percent those of elec-trical devices Further, air motors do not spark; they cannot burn out from over-loading; the air motor is not injured by stalling Air motors start and stop positively; they have a consistent output torque which can be changed by varying the inlet air pressure
Air motors do, however, have limitations Thus: (1) Compared to electric motors, air motors are inefficient An air motor requires about 5 hp (3.7 kW) input to the air compressor to produce one horsepower (0.7 5kW) at the motor outlet (2) Air motors are rarely practical in sizes greater than 20 hp (15 kW) Their most efficient range is 1 / 20 to 20 hp (0.04 to 15 kW) (3) The initial cost of an air motor is high;
in larger sizes, above 1 hp (0.75 kW), air motors cost up to five times that of equivalent electric motors
3. Check the motor duty cycle and load against the unit’s characteristics
When selecting an air motor, the first factor to be considered is the type of duty cycle, intermittent or continuous A crane, for which this motor will be used, does have an intermittent duty cycle because it is not normally used continuously There
is a rest period while the crane load is being put on the crane and again while being off-loaded from the crane
The great majority of air-motor applications have a low-load cycle; the air mo-tors are used for only a few seconds continuously and have long off-duty periods
Trang 8Horsepower Governor controlled curves
0
Stall speed
FIGURE 6 Performance curve of a typical air motor Note how a built-in governor can change
the shape of the curve by limiting the maximum speed of the air motor (Product Engineering).
The duty cycle will usually determine the type of motor and the size of compressorthat must be used
4. Check the horsepower and speed required
Performance curves, Fig 6, show an air motor’s torque and horsepower (kW) output
at various rpm Such curves can be varied somewhat by using governors or bymodifying the air intake or exhaust ports However, the basic shape of the perform-ance curve depends on the fundamental design of the air motor It is common
practice to rate an air motor at its maximum output, i.e., at the top of the
dome-shaped performance curve The reversible radial-piston motor chosen here has equate horsepower and speed for the anticipated load
ad-5. Determine the effect of air pressure and quantity on the air motor output
Table 2 shows how the air pressure available at the motor inlet affects both thepower output and rpm of typical air motors For the motor being considered here,the output would be sufficient at the lowest air pressure listed Thus, the motorchoice is acceptable
Trang 10Related Calculations. When using tables of air-motor performance, it is portant to keep in mind that the stall torque and air consumption vary for eachmotor Hence, these values are not listed in the usual performance tables There are
im-so many variables in air-motor choice that stall torque and air consumption areunique for each application and are supplied by the motor manufacturer when themotor choice is made
As a general rule of thumb, stall torque ranges between 2 and 2.5 times thetorque developed when operating at maximum horsepower output
In small motors, up to 2.5 hp (1.9 kW), air consumption varies from 35 to 40
con-sumption rates apply to non-reversible motors Reversible air motors consume 30
to 35 percent more air
The data, tables and illustration in this procedure are from Product Engineering
magazine
AIR-COMPRESSOR COOLING-SYSTEM CHOICE
FOR MAXIMUM COOLANT ECONOMY
Select a suitable cooling system for a two-stage 5000-hp (3730-kW) engine-drivenair compressor, Fig 7, installed in a known arid hard-water area when the rated
kPa) Water conservation is an important requirement for this compressor because
of the arid nature of the area in which the unit is installed Use standard water requirements in estimating the capacity of the cooling system
cooling-Calculation Procedure:
1. Assess the types of cooling systems that might be used
Several types of cooling systems can be used for air compressors such as this.Because the air compressor is used in an arid area subject to water shortages, arecirculating system of some type is immediately indicated Since both the engineand air-compressor cooling water require temperature reduction in such an instal-lation, the two requirements are usually combined in one cooling system
The first arrangement that might be chosen, Fig 8, combines a heat exchangerfor engine power-cylinder cooling and a cooling tower for raw-water cooling forthe compressor Either a natural-draft cooling tower, such as that shown, or a me-chanical-draft cooling tower might be used The cooling-tower choice depends on
a number of factors In an arid area, however, natural-draft towers are known toperform well in dry climates Further, they require much less piping and electricwiring than mechanical-draft towers
Another possible cooling-system arrangement uses a closed coil in the coolingtower for both the power and air cylinders, Fig 9 This totally closed system doesnot allow contact between the compressor and engine cooling water with the at-mosphere This means that the compressor and engine cooling water can be treated
to reduce scale formation Raw water recirculated through the cooling tower doesnot contact the compressor coolant
Where installation costs are critical, raw water can be used to cool the compressor cylinders, Fig 10 The engine power cylinders, which usually operate
Trang 12FIGURE 8 Heat exchanger, center, for power-cylinder cooling and raw-water cooling
for the compressor (Ingersoll-Rand Co.).
FIGURE 9 Closed cooling system for power and air cylinders utilizing pipe coil in
the cooling tower (Ingersoll-Rand Co.).
Trang 13AIR AND GAS COMPRESSORS AND VACUUM SYSTEMS 9.13
FIGURE 10 Raw water cools the air cylinders; power cylinders use
closed system protected by thermostatic valve (Ingersoll-Rand Co.).
FIGURE 11 Cooling-tower recirculating system is not recommended because of the
possibility of scale and impurities buildup (Ingersoll-Rand Co.).
at a higher temperature, are cooled by a closed system protected by a thermostaticvalve
An open cooling-tower system, Fig 11, is not recommended for installationssuch as this because of the possible heavy scale buildup However, such an opencooling system might be used where the economics of the installation permit it andscale buildup is unlikely to occur
2. Determine the air-compressor cooling load
Use flow rates given in Table 6, page 9.24 to estimate the cooling water flow ratefor this air compressor Thus, with the intercooler and jacket in series, using the
Trang 14higher flow rate of 2.8 gal / min (364.3 L / s) per 100 ft3/ min (100 m3/ s), the
3. Compute the engine jacket-water cooling load
The engine jacket water cooling load is computed separately using a cooling-water
as given in the Internal-Combustion Engine section of this handbook Further, theusual jacket-water flow rate is 0.25 to 0.60 gal / (min bhp) (0.02 to 0.05 kg / kW)
3000 gal / min (189.3 L / s)
Additional cooling water may be used for the turbocharger, if fitted, and foraftercooling Steps for calculating these cooling-water flows are given in the sectioncited above Such cooling-water flows are usually additive to the jacket-water flow,depending on the cooling arrangement used
Related Calculations. Cooling systems for air and gas compressors are portant for reliable and safe operation of these units Hence, great care must beexercised in choosing the most reliable and economic cooling system
im-Today, both mechanical-draft and natural-draft cooling towers are popularchoices An economic study is needed to determine the best choice when the cool-ing effectiveness of both types of towers are about equal Data given on coolingtowers elsewhere in this handbook can be helpful to the designer in choosing thebest type of tower to use for a given installation of air or gas compressors
Straight flow-through cooling of small compressors and their drive engines isoften used where adequate water supplies are available Thus, in large cities thecooling water may be taken from the water main and discharged to the sewer afterpassage through the compressor and engine Cost of the water may be small com-pared to the investment in a cooling tower But with increased environmental con-cerns, this scheme of cooling may soon be extinct
ECONOMICS OF AIR-COMPRESSOR INLET
LOCATION
A plant designer has the option of locating an air-compressor inlet pipe either insidethe compressor building or outside the structure The prevailing average indoor
200-hp (149.1 kW) compressor drive motor operates at full load throughout the
7500 hr load year Determine which is the best location for the compressor intakebased on power savings with an electric power cost of $0.04 / kWh
Calculation Procedure:
1. Determine the power savings possible with cooler intake air
Trang 15AIR AND GAS COMPRESSORS AND VACUUM SYSTEMS 9.15
re-quired at the air inlet temperature
re-quired at the lower intake temperature) / intake volume rere-quired at the higher intake
2. Find the annual power saving with the lower intake temperature
hp)(0.746 kW / hp)(annual operating hours) Since the compressor operates at full
$3276.43 If an outside inlet were more expensive than an indoor inlet, this savingcould be used to offset the increased cost
Related Calculations. As a general rule, an outside air intake, Fig 12, is moreeconomical than an inside air intake when the air in the building is at a highertemperature than the outside air The only time an outside air intake might be lessdesirable than an indoor air intake is when the outside air is polluted with corrosive
vapors, excessive dust, abrasive sand, etc., which would be injurious to people or
machines Under these circumstances the designer might elect an indoor air intake.However, before choosing an indoor intake, review the efficacy of outdoor air filters
of various types, Fig 13
over 4 lb (1.8 kg) will be carried into the compressor during 1 week’s operation.Frequently, much of the dirt carried in the air is abrasive If this dirt is allowed toget into the compressor cylinders it will mix with the lubricating oil and cause rapidwear of piston rings, cylinder walls, valves, and other parts
Intake-air filters, Fig 13, can reduce much of the danger of abrasive particles
in the supply air Each type has its favorable features Viscous coated wire filters,
Fig 13a, are often used for small- and medium-size compressors Centrifugal flow units, Fig 13b, and traveling-curtain oil-bath filters, Fig 13c, are popular for
air-larger air compressors The final choice of an intake filter is a function of pressor capacity, intake-air quality, annual operating hours, and expected life of thecompressor installation Filter manufacturers can be most helpful to the plant de-signer in evaluating these factors
com-The general procedure given here is valid for air compressors of all types:
cen-trifugal, reciprocating, vane, rotary, etc., and the procedure can be used for air
compressors in plants of all types—chemical, petroleum, manufacturing, marine,
industrial, etc This procedure has universal application because it is based on the
properties of air, the compressor power input, the annual operating hours, and thecost of power These values can be found in any application of air compressors inindustry today
Trang 16tile tunnel for outdoor-air intake.
Compressed-Air and -Gas System
Components and Layouts
POWER INPUT REQUIRED BY CENTRIFUGAL
COMPRESSOR
Trang 17AIR AND GAS COMPRESSORS AND VACUUM SYSTEMS 9.17
(a)
FIGURE 13 (a) Viscous coated-wire intake-air filter (Air-Maze Corp.) (b) Centrifugal air-flow oil-bath intake-air filter also acts as a silencer (c) Traveling-curtain oil-bath intake- air filter cleans itself in the oil (American Air Filter Co., Inc.).
Trang 18The air is delivered from the compressor at a pressure of 70 lb / in2(abs) (482.4
20 ft (6.1 m) above the suction pipe The weight of the jacket water, which enters
is the horsepower required to drive this compressor, assuming no loss from tion?
radia-Calculation Procedure:
1. Determine the variables for the compressor horsepower equation
The equation for centrifugal compressor horsepower input is,
2 2 w (t ⫺t )⫹R
2. Compute the input horsepower for the centrifugal compressor
compres-COMPRESSOR SELECTION FOR
COMPRESSED-AIR SYSTEMS
Determine the required capacity, discharge pressure, and type of compressor for anindustrial-plant compressed-air system fitted with the tools listed in Table 3 Theplant is located at sea level and operates 16 h / day
Trang 19AIR AND GAS COMPRESSORS AND VACUUM SYSTEMS 9.19
TABLE 3 Typical Computation of Compressed-Air Requirements
Calculation Procedure:
1. Compute the required airflow rate
List all the tools and devices in the compressed-air system that will consume air,
tool Enter this value in column 1, Table 3 Next list the number of each type oftool that will be used in the system in column 2 Find the maximum probable airconsumption of each tool by taking the product, line by line, of columns 1 and 2.Enter the result in column 3, Table 3, for each tool
The air consumption values shown in column 3 represent the airflow rate quired for continuous operation of each type and number of tools listed However,few air tools operate continually To provide for this situation, a load factor isgenerally used when an air compressor is selected
re-2. Select the equipment load factor
factors for compressed-air operated devices are usually less than 1.0
Two variables are involved in the equipment load factor The first is the time
factor, or the percentage of the total time the tool or device actually uses
com-pressed air The second is the work factor, or percentage of maximum possible
work output done by the tool The load factor is the product of these two variables.Determine the load factor for a given tool or device by consulting the manufac-turer’s engineering data, or by estimating the factor value by using previous ex-perience as a guide Enter the load factor in column 4, Table 3 The values shownrepresent typical load factors encountered in industrial plants
Trang 20TABLE 4 Approximate Air Needs of Pneumatic Tools
3. Compute the actual air consumption
Take the product, line by line, of columns 3 and 4, Table 3 Enter the result, i.e.,
the probable air demand, in column 5, Table 3 Find the sum of the values in column
4. Apply allowances for leakage and future needs
Most compressed-air system designs allow for 10 percent of the required air to be
lost through leaks in the piping, tools, hoses, etc Whereas some designers claim
that allowing for leakage is a poor design procedure, observation of many lations indicates that air leakage is a fact of life and must be considered when anactual system is designed
Future requirements are best estimated by predicting what types of tools anddevices will probably be used Once this is known, prepare a tabulation similar toTable 3, listing the predicted future tools and devices and their air needs Assume
Trang 21AIR AND GAS COMPRESSORS AND VACUUM SYSTEMS 9.21
TABLE 5 Air Compressor Brake Horsepower (kW) Input*
5. Choose the compressor discharge pressure and capacity
In selecting the type of compressor to use, two factors are of key importance:discharge pressure required and capacity required
(620 kPa) at the tool inlet Hence, usual industrial compressors are rated for a
loss in the piping between the compressor and the tools Since none of the toolsused in this plant are specialty items requiring higher than the normal pressure, a
Where the future air demands are expected to occur fairly soon—within 2 to 3years—the general practice is to choose a compressor having the capacity to satisfy
com-pressor would be chosen
6. Compute the power required to compress the air
Table 5 shows the power required to compress air to various discharge pressures atdifferent altitudes above sea level Study of this table shows that at sea level a
investment because this hp will be saved for the life of the compressor The usuallife of an air compressor is 20 years Hence, by using a two-stage compressor, the
(13.1 kW)
7. Choose the type of compressor to use
Reciprocating compressors find the widest use for stationary plant air supply Theymay be single- or two-stage, air- or water-cooled Here is a general guide to thetypes of reciprocating compressors that are satisfactory for various loads and ser-vice:
(1034 kPa), for light and intermittent running up to 1 h / day
(1034 kPa), for 4 to 8 h / day running time
Trang 22Single-stage air-cooled compressor up to 15 hp (11.2 kW) for pressures to 80
compres-sor
Single-stage horizontal double-acting water-cooled compressor for pressures to
operating time
Two-stage, single-acting air-cooled compressor for 10 to 100 hp (7.5 to 75 kW),
5 to 10 h / day operation
Two-stage double-acting water-cooled compressor for 100 hp (75 kW), or more,
24 h / day, or less operating time
Using this general guide, choose a two-stage double-acting water-cooled cating compressor, because more than 100-hp (75-kW) input is required and thecompressor will operate 16 h / day
recipro-Rotary compressors are not as widely used for industrial compressed-air systems
as reciprocating compressors The reason is that usual rotary compressors discharge
Centrifugal compressors are generally used for large airflows—several thousand
quan-tities, such as steel-mill blowing, copper conversion, etc As a general rule,
compressors.
Using these facts as a guide enables the designer to choose, as before, a stage double-acting water-cooled compressor for this application Refer to the man-ufacturer’s engineering data for the compressor dimensions and weight
two-8. Select the compressor drive
Air compressors can be driven by electric motors, gasoline engines, diesel engines,gas turbines, or steam turbines The most popular drive for reciprocating air com-pressors is the electric motor—either direct-connected or belt-connected Whereeither dc or ac power supply is available, the usual choice is an electric-motor drive.However, special circumstances, such as the availability of low-cost fuel, may dic-tate another choice of drive for economic reasons Assuming that there are nospecial economic reasons for choosing another type of drive, an electric motorwould be chosen for this installation
With an ac power supply, the squirrel-cage induction motor is generally chosenfor belt-driven compressors Synchronous motors are also used, particularly whenpower-factor correction is desired Motor-driven air compressors generally operate
at constant speed and are fitted with cylinder unloaders to vary the quantity of airdelivered to the air receiver A typical power input to a large reciprocating com-
Air compressors are almost always rated in terms of free air capacity, i.e., air
at the compressor intake location Since the altitude, barometric pressure, and air
temperature may vary at any locality, the term free air does not mean air under
standard or uniform conditions The displacement of an air compressor is the
compressor, the displacement is that of the low-pressure cylinder only