| Compressors
The type of compressor most likely to be used for an industrial compressed air system depends largely on size, cost, and reliability requirements: - Rotary screw compressors in sizes up to 500-600 hp are very popular because of their high reliability and low maintenance requirements.
- Centrifugal compressors are often used in sizes ranging from about 150 hp up to over 10,000 hp. The larger size models are relatively low in cost and small in physical size compared to reciprocating compressors.
- Reciprocating compressors are one of the oldest air compressor technologies, but are commonly used today only in sizes up to 25 hp or so. These compressors are often used for light-duty applications or in startup industrial enterprises because they are reliable and low cost.
- Rotary vane compressors are not commonly used as they tend to be energy inefficient and require higher maintenance than other compressor designs.
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| Annular Flow: A multiphase-flow regime in which the lighter fluid flows in the center of the pipe, and the heavier fluid is contained in a thin film on the pipe wall. The lighter fluid may be a mist or an emulsion. Annular flow occurs at high velocities of the lighter fluid, and is observed in both vertical and horizontal wells. As the velocity increases, the film may disappear, leading to mist or emulsion flow. When the interface between the fluids is irregular, the term wavy annular flow may be use | |
| The pressure within a system caused by fluid friction or an induced resistance to flow through the system. Most process facilities require a minimum system pressure to operate efficiently. The necessary back-pressure is often created and controlled by a valve that is set to operate under the desired range of conditions. | |
CFM = Cubic Foot per Minute. A standard measurement of airflow that indicates how many cubic feet of air pass by a stationary point in one minute. The higher the number, the more air is being forced through the system. The volumetric flow rate of a liquid or gas in cubic feet per minute. 1 CFM equals approximately 2 liters per second.
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Compressed Air System Components  Figure 1: Compressed Air System Diagram (Reprinted with permission from Compressor Engineering Data Book. Copyright 1974, 1996 by Scales Air Compressor Corp. All rights reserved)
A typical compressed air system consists of compression, cooling, storage, and distribution equipment, as shown in Figure 1. - Intake Filtering (1): Incoming air must be filtered to remove dust and other contaminants.
- Compression (2): The filtered air is compressed (typically to 80 to 110 psi) with motor-driven screw, centrifugal, or reciprocating compressors.
- Cooling (3): Compressing air raises its temperature dramatically, so cooling is required. Much of the energy "lost" in making compressed air is in the form of removed heat. Cooling is also important in the process of drying air. Much of the water vapor condenses as the air is cooled, making it easy to drain away.
- Air Storage (4): A tank called an air receiver typically is placed downstream of the cooler to provide surge capacity for the system. Some systems provide additional receiver tanks in the process area to accommodate widely variable demand.
- Drying (5): Cooled, pressurized air still carries a significant amount of moisture and lubricants from the compression process, virtually all of which must be removed before the air can be used. Drying compressed air can be very energy intensive.
- Distribution (6): A system of distribution pipes and regulators convey compressed air from the central compressor plant to process areas. This system includes various isolation valves, fluid traps, intermediate storage vessels, and even heat trace on pipes to prevent condensation or freezing in lines exposed to the outdoors. Pressure losses in distribution typically are compensated for by higher pressure at the compressor discharge.
- Point of Use (7): At the intended point of use, a feeder pipe with a final isolation valve, filter, and regulator carries the compressed air to hoses that supply processes or pneumatic tools.
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com·pres·sion Pronunciation (km-prshn)
n.
1.
a. The act or process of compressing.
b. The state of being compressed.
2.
a. The process by which the working substance in a heat engine, such as the vapor mixture in the cylinder of an internal-combustion engine, is compressed.
b. The engine cycle during which this process occurs.
3. Computer Science The process by which data is compressed into a form that minimizes the space required to store or transmit it.
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con·den·sa·tion
Pronunciation: "kän-"den-'sA-sh&n, -d&n-
Function: noun
1 : the act or process of condensing : as a : a chemical reaction involving union between molecules often with elimination of a simple molecule (as water) to form a new more complex compound of often greater molecular weight b : the conversion of a substance (as water) from the vapor state to a denser liquid or solid state usually initiated by a reduction in temperature of the vapor c : compression of a written or spoken work into more concise form
2 : the quality or state of being condensed
3 : a product of condensing
- con·den·sa·tion·al /-shn&l, -sh&-n | |
Main Entry: dis·per·sion
Pronunciation: di-'sp&r-zh&n, -sh&n
Function: noun
1 capitalized : DIASPORA 1a
2 : the act or process of dispersing : the state of being dispersed
3 : the scattering of the values of a frequency distribution from an average
4 : the separation of light into colors by refraction or diffraction with formation of a spectrum; also : the separation of radiation into components in accordance with some varying characteristic (as energy)
5 a : a dispersed substance b : a system consisting of a dispersed substance and the medium in which it is dispersed | |
dis·si·pa·tion
Pronunciation: "di-s&-'pA-sh&n
Function: noun
1 : the action or process of dissipating : the state of being dissipated : a : DISPERSION, DIFFUSION b archaic : DISSOLUTION, DISINTEGRATION c : wasteful expenditure d : intemperate living; especially : excessive drinking
2 : an act of self-indulgence; especially : one that is not harmful : AMUSEMENT | |
dis·ten·sion
Variant(s): or dis·ten·tion /di-'sten(t)-sh&n/
Function: noun
Etymology: Latin distention-, distentio, from distendere
: the act of distending or the state of being distended especially unduly or abnormally | |
| Energy loss is the action of transferring otherwise usable energy into a form that is no longer useable, either by losing control over the method the energy is transferred and converted, losing energy to waste as a function of that conversion or transfer or by the waste in conduction between mediums. | |
Energy Waste is unnecessary energy loss. Energy Waste can be prevented or corrected.
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Flow in Pipe: The directional movement of a substance through a medium, such as a pipe or chute. This movement can be measured in CFM.
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Large Leaks are those over 10mm in size. Some large leaks can be much larger than 10mm, though a 10mm leak is considered large enough to rack up high energy loss costs quickly.
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Loose Fittings are fittings that have worked loose or worn out over time. As lines grow older, fittings are subjected to high pressures and environmental damage that causes fittings to come loose or wear out.
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Pinhole leaks: Leaks under 1 mm in size.
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| Pressure Loss vs. Flow Rate | The objective is to determine the relationship between pressure loss and flow rate in a pipe of a given size conveying a liquid with known properties.
Convert all input parameters to SI units for calculation purposes and convert the results back to local units.
The Equations
The pressure loss in the pipe is due to friction within the fluid. Friction losses can only be quantified if the flow regime is known. The flow regime may be determined from the Reynolds number, calculated from the following formula:
rho x V x d
Re = ----------------
mu
where
Re is the Reynolds number (dimensionless)
rho is the density in kg per cubic meter
V is the flow velocity in m/s
d is the inside diameter of the tube in meters
mu is the absolute viscosity expressed in Pa.s
If the Reynolds number is below 2,000 the flow regime may be assumed to be laminar. If Re is above 4,000 it can be assumed to be turbulent. Between these figures the flow is in some intermediate regime that at one end may be laminar and at the other end may be fully turbulent.
Absolute viscosity (also known as dynamic viscosity) is related to kinematic viscosity by the following relation:
mu = rho x neta
where neta is the kinematic viscosity expressed in square meters per second.
For completeness here is the relation between flow (Q in cubic meters per second) and velocity in a circular conduit:
4 x Q
V = --------------
pi x d x d
where pi = 3.14159... (extend to taste)
In a laminar flow regime the Hagen-Poiseuille equation should be used:
128 x mu x L x Q
deltaP = ----------------------
pi x d x d x d x d
where deltaP is the differential pressure in pascal, i.e. the difference between the pressure at the input end of the pipe and the discharge end of the pipe, and L is the length of tube in meters.
For turbulent flow the more general Darcy-Weisbach equation for flow must be used:
f x rho x L x V x V
deltaP = -------------------------------
2 x d
where f is a dimensionless quantity known as the Darcy-Weisbach skin friction factor.
A number of equations exist to determine the correct friction factor for a given flow. For laminar flow:
64
f = -------
Re
For turbulent flow, the Colebrook-White equation may be used:
1 epsilon 2.51
------- = -2 x log10(-------------- + -------------------)
sqrt(f) 3.7 x d Re x sqrt(f)
where epsilon is the absolute roughness of the inside diameter of the tube and is expressed in meters.
Since the friction factor appears on both sides of the equation a numerical solution must be obtained.
Where a Reynolds number between 2,000 and 4,500 is obtained the friction factor is more difficult to predict. Interpolation routines may be developed based on the Moody diagram but these are generally specific to a limited range of flow conditions. A more general relation has been recently derived by Chue that covers all flow regimes including the intermediate region:
1
------- = -2 x log10(G)
sqrt(f)
2.512 epsilon
where G = ((1 – gamma) x antilog10(-sqrt(Re)/16) + (gamma x (----------------- + --------------))
Re x sqrt(f) 3.7 x d
1
and where gamma = -------------------------------------------------
Re - 3057.2516
1 + exp(----------------------)
227.52765
Chue’s equation must be solved numerically in order to obtain the friction factor. Note also that at low flow rates the friction factor given by Chue may be severely inaccurate.
Using the Equations
In using the friction loss equations alone, so called “minor” losses are neglected. Minor losses are energy losses that occur in fittings, bends, couplings and valves. In short lengths and in high flow rates minor losses can become significant and will need to be included. In simple systems using the pipe friction loss equations alone will provide a reasonably accurate indication of the performance of the system.
How the pipe pressure loss equations are to be used will depend on whether the starting point is a required flow rate or if the flow capacity of a line needs to be determined from the available pressure loss.
Specified Flow Rate
Where the required flow rate is known the process of determining the associated pressure loss for a given tube diameter is fairly straightforward. A possible routine for determining the pressure loss is given below:
1. Calculate Reynolds number.
2. If Reynolds number is less than 2,000 go to step 3 else go to step 4.
3. Calculate deltaP from Hagen-Poiseuille.
4. If Reynolds number is greater than 4,000 go to step 5 else go to step 6.
5. Use Colebrook-White to determine friction factor, go to step 7.
6. Use Chue to determine friction factor.
7. Use Darcy-Weisbach to determine pressure loss.
Specified Pressure Loss
Knowing the difference between the pressure at the inlet to the pipe and the discharge from the pipe, the flow rate can be obtained, but this approach requires more iteration. The starting point is to take a guess at the friction factor and iterate until it's value doesn’t change significantly from one iteration to the next. One routine might look like this:
1. Assume a friction factor.
2. Calculate flow rate from Darcy-Weisbach.
3. Calculate Reynolds number from the flow rate.
4. Put the new Reynolds number in to Chue to obtain a revised friction factor.
5. If the revised friction factor has not changed much from that used in step 2 then go to step 7.
6. Go to step 2 with the new friction factor.
7. The friction factor is stable from one iteration to the next so the flow rate is now correct for the pressure loss.
If the flow regime is expected to be turbulent then the Colebrook-White equation may be substituted for Chue in the above routine.
References
1. Fluid Mechanics, 1st SI Metric Edition, Streeter & Wylie, McGraw-Hill
2. Proceedings of the Institution of Civil Engineers, Part 2 Research and Theory, March 1984, Volume 77, pages 43-48, Technical Note 399, A pipe skin friction factor of universal applicability, S H Chue BE BSc(Spec) PhD.
Unit Conversions
To calculate psi from bar multiply by 14.5, although for a more accurate result divide by 0.06894757.
To calculate bar from pascals divide by 100,000.
To calculate centipoise from Pa.s multiply by 1,000. Note that 1 cP is equal to 1 mPa.s.
To calculate centistokes from square meters per second multiply by 1,000,000. Note that 1 cS is equal to 1 square millimeter per second.
1 US gallon is equal to 3.78 liters, there are 1,000 liters in 1 cubic meter.
1 foot is 0.3048 meters.
To obtain pounds per cubic foot from kg per cubic meter, divide by 16.01846. | |
PSI stands for "Pounds per Square Inch". It's a rating used to gauge pressure levels. These pressure levels are measured in force per unit area.
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Small leaks are leaks between 1mm and 10mm in size.
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| Steam Pressure Fluctuation | | Steam pressure fluctuation most likely happens when there is a malfunction in the pressure-regulating valve. This should be attended to immediately. | |
Stress Cracks: Over time, pipes are worn by both the elements and the contents they carry. Many times, these contents are under pressure and can cause stress on the pipes they travel through. These pipes eventually crack, leaving "stress cracks".
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| Ultrasonic Leak Detection | | As fluid moves from the high pressure side of a valve through the seat to the low pressure side, it produces turbulence. this turbulence generates ultrasound which is detected by an ultrasonic leak detector and translated, via heterodyning, down into the audible range. The translated ultrasounds are heard through headphones and seen as intensity increments on a meter. High frequency tuning allows users to adjust for differences in fluid viscosity (i.e. water vs. steam) and reduce any interference from stray pipe noises. | |
The pressure exerted by a vapor escaping from a liquid. It quantifies the tendency of molecules to enter the gaseous phase. The vapor pressure of water increases as temperature increases and reaches one atmosphere pressure (760 mm Hg or 14.7 psia) at the boiling point (100°C or 212°F). The activity of an aqueous solution is the ratio of vapor pressures: aw = p/po, where p = vapor pressure of a solution and po is vapor pressure of pure water. Since this is a ratio of vapor pressures, activity is not a strong function of temperature.
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| When condensate is not removed effectively from steam pipes, water hammer can result. It usually causes banging noises in the pipes. The most common type of water hammer is a traveling slug of water that impacts a fitting in the pipes, water in the steam line, or steam pockets in a return line. | |
| Steam is considered "wet" when a small test valve is opened and excessive condensate is seen-especially when an object is placed in front of the valve. Most providers typically deliver steam at a quality of 98% dryness, meaning that there can only be 2% moisture content. | |
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