The key difference between such compressors and pumps is that the compressor working fluid is a gas compressible and the pump working fluid is liquid incompressible. Fundamentals of fluid mechanics chapter 12 pumps and turbines. The analysis is based on the reduced impeller being geometrically similar and operated at dynamically similar conditions or equal. Velocitypressure development in a centrifugal compressor.
For this type of testing it is necessary that models are not only geometrically similar, but are also subjected to similar physical conditions. Centrifugal compressors are also similar to centrifugal pumps of the style shown in the adjacent figures. They are also used for scaleup purposes when performance parameters exceed that of existing pumps refer to figure 2. This means that 10 percent impeller reduction is 10 percent flow reduction. Affinity laws the turbo machine affinity laws are common used for pumps and fans to calculate capacity, head and power consumption when changing speed or wheels diameters the affinity laws of a centrifugal pump or fan express the effect on capacity, head and power consumption of a pump or fan due to. To explore this further, consider the basic equation for head, h.
The flow gpm varies proportionally with the change in impeller diameter. The reduction in speed results in a power consumption decrease. They are designed for long life, low maintenance and continuous service in dirty and corrosive applications such as petrochemical plants, refineries, natural gas processing, coal gasification and power stations. To approximate the performance of the pump, the affinity laws should be used as follows. The design of the diffuser generates a circular gas motion preventing. If the compressor flow, head and efficiency characteristics are known at a given speed, then merely applying the affinity laws at an alternate speed will produce a new curve representing the compressor performance at that speed. Centrifugal pump curves home fluid flow centrifugal pump curves this tool determine centrifugal pump performance for different impellor diameter and speed based on affinity laws. For gas compression of large volumetric flow, axial centrifugal compressors with inter.
Unlike displacement compressors that work at a constant flow, dynamic compressors work at a constant pressure and the performance is affected by external conditions such as changes in inlet temperatures. The affinity laws for centrifugal pumps and fans are used to express the influence on volume capacity, head pressure andor power consumption due to. The performance of centrifugal pumps is related to a parameter called. As the flow passes through the centrifugal impeller, the impeller forces the flow to spin faster as it gets further from the rotational axis. A detailed view is shown in figure 5 of the centrifugal compres sor section of figure 4 and can be used as a reference for selection of centrifugal compressor units.
Compressor and turbine performance and operation affinity laws for centrifugal impellers calculating effect of speed change calculating effect of impeller changes characteristic curves effect of change on operating and process parameters operational problems surge stonewall range of working efficiency. If less airflow is needed, the fan is able to reduce its speed. Dynamic simulation of a centrifugal compressor system. Changes in compressor performance can be determined whenever the speed fluctuates by simply utilizing the affinity laws. The pump speed, head, and flow relationships are expressed by the affinity laws. In the case where flow passes through a straight pipe to enter a centrifugal compressor the flow is straight, uniform and has no vorticity, ie swirling motion, so the swirl angle. Sorokes dresserrand selecting a centrifugal compressor inlet pressure p1 temperature t1. Moving up a compressor frame size has an efficiency advantage. This implies that speed reduction also reduces the compressors capability of generating pressure increase, or lift. This manual does not for designing centrifugal compressor and those parts. Types and performance of pumps and compressors eolss.
Dear mates, two question in regards to affinity laws, dimensionless numbers and behaviour prediction in centrifugal compressors. Q1 q2 n1n2 x d1d2 h1 h2 n1n22 x d1d22 p1 p2 n1n23 x d1d23 npsh31 npsh32 n1n22 per standard nomenclature, q is flow rate, h is head, p is power, and npsh3 is net positive suction head for a 3 percent head drop. If the speed should remain a constant, the affinity laws state. The affinity laws of centrifugal pumps or fans indicates the influence on volume capacity, head pressure andor power consumption of a. Centrifugal compressor surge, modeling and identification for. Compressor efficiency increases with impeller size both due to reynolds number effect and reduced relative surface roughness. The affinity laws are a set of forumlas that predict the impact of a change in rotational speed or impeller diameter on the head and flow produced by a pump and power demanded by a pump. Minimum flow point in a centrifugal or axial compressor. Affinity laws for two homologous states a and b, we can use variables to develop ratios similarity rules, affinity laws, scaling laws. Power input assuming identical pump efficiencies flow coefficient. Elliott has been manufacturing single stage centrifugal compressors for over 90 years. The following model laws thus apply to two geometrically similar centrifugal pumps operating under physically similar conditions. This is identical to an axial compressor with the exception that the gases can reach higher velocities and energy levels through the impellers. Centrifugal pump fundamentals pumps are broadly classified as kinetic or positive displacement.
Short discussion about prediction of centrifugal compressor performance running with freon assisted. Turbo machines affinity laws can be used to calculate volume capacity, head or power consumption in centrifugal pumps with changing speed or wheel diameters. These laws are mathematical expressions that define changes in pump brake horsepower bhp, head and capacity when a change is made to pump speed, impeller diameter, or both. Using the affinity laws, it is possible to determine approximate curves to estimate the centrifugal pumps new best efficiency point. Affinity laws when investigating flow phenomena, cost factors often favour the use of models which are geometrically similar to the original, fullsized equipment see similarity conditions. You may occasionally need to estimate the hydraulic performance of a centrifugal pump at a speed or impeller diameter not shown on the catalog performance curves. The affinity laws or fan laws, as they are sometimes referred to play an important role in determining centrifugal pump performance when operating conditions change. When the fan diameter and the fluid density at the fan suction are constant, the fan laws can be expressed as. Affinity fan laws for centrifugal compressor posted in student. Pdf the purpose of this research is to investigate the flow behavior inside a. Basic pump parameters and the affinity laws pdh online. Sep 25, 2012 for a variablespeed centrifugal compressor you can change the power drawn by changing the frequency of the motor and the affinity laws speed vs inlet volume flow rate, speeed vs polytropic head, speed vs gas horse power as applicable to centrifugal pumps can be applied but with limitations compared to centrifugal pumps. How to read a centrifugal compressor performance curve. Useful to scale from model to prototype useful to understand parameter changes, e.
Care must be taken to correctly read the performance data from pump curves. The fan laws of centrifugal pumps or fans, also known as the affinity laws, express the influence of changes in rotational speed, diameter, and density on volumetric flowrate, head or h pressure, and shaft power. As far as i know, this just could be applied when the compressibility factor is negligible. Affinity laws the affinity laws are derived from a dimensionless analysis of three important parameters that describe pump performance. A centrifugal compressor is a type of dynamic compressor, or turbocompressor, with a radial design. Klm technology group, 2011 after entering a centrifugal compressor, the gas is accelerated outwards by rotating impellers and then deaccelerated in the diffusers before finally leaving through the discharge nozzle. Centrifugal compressors follow the same affinity laws as that associated with centrifugal fans and pumps. Manual purpose to be used for selection, application into the system, power and cooling water estimation. The red colourmark represents the operating point of the. Affinity laws affinity laws are the performance of centrifugal pumps based on change in speed, power, flow, head, impeller diameter.
The impeller has a uniform blade length, b, of 2 in. The affinity laws for pumpsfans are used in hydraulics, hydronics andor hvac to express the. Development of modified affinity law for centrifugal pump to predict the effect of viscosity article pdf available in journal of energy resources technology 1409 april 2018 with 427 reads. Pump affinity laws for centrifugal and positive displacement. The result, which can be regarded as the modified pump affinity laws for. The affinity laws of a centrifugal pump or fan express the effect on capacity, head and power consumption of a pump or fan due to. Affinity laws the turbo machine affinity laws are common used for pumps and fans to calculate capacity, head and power consumption when changing speed or wheels diameters the affinity laws of a centrifugal pump or fan express the effect on capacity, head and.
Pdf development of modified affinity law for centrifugal. Now we will various curves and point so check in the centrifugal compressor performance curve. With a free discharge fan, the rule power varies with the cube of the speed holds true. Centrifugal pumps are studied and designed based on both mechanical and hydraulic considerations. The affinity laws of centrifugal pumps or fans indicates the influence on volume capacity, head pressure andor power consumption of a pump or fan due to. Affinity laws can be used to calculate volume capacity, head or power consumption when speed and wheel diameters are changed. Another set of the affinity laws deal with changes to the diameter of the impeller. Centrifugal pump affinity laws provide a method to determine a new pump operating point when a performance curve that indicates performance with different impeller diameters or operating speeds is not available.
Experience of performance modeling the singlestage pipeline centrifugal compressor. How to use affinity laws to calculate a new best efficiency point. For a variablespeed centrifugal compressor you can change the power drawn by changing the frequency of the motor and the affinity laws speed vs inlet volume flow rate, speeed vs polytropic head, speed vs gas horse power as applicable to centrifugal pumps can be applied but with limitations compared to centrifugal pumps. It is often used by designers in sizing and selecting the type of pump or compressor. The affinity laws formulas the affinity laws describe how changes in rpm n affect gpm, head, and hp in centrifugal pumps. In these rotary implements, the affinity laws apply both to centrifugal and axial flows. Pdf development of modified affinity law for centrifugal pump to. Large or small, we design our centrifugal compressors for economical and efficient operation.
Pump engineering calculations of brake horsepower, power, kilowatts, motor, motors, voltage drop, circular mills, transformers, turns ratio, ohms law, dc power. The affinity laws below are well known and used daily by pump people. Affinity fan laws for centrifugal compressor student. The affinity law relationships for changing impeller diameter usually work. Aug 14, 2014 the affinity laws below are well known and used daily by pump people. The cause for this is the higher volume ratio of the heavy gas. If flow is reduced below this point, cyclic variation and even reversal of gas flow and discharge pressure. This, combined with minimal system effects, make these fans excellent candidates for. Unfortunately, we are missing several other key pieces of information if we wish to apply the centrifugal compressor to another application.
The affinity laws are useful as they allow prediction of the head discharge characteristic of a pump or fan from a known characteristic measured at a different speed or impeller diameter. If you know the shape of a pump performance curve at a certain speed or with a certain impeller diameter, you can use the affinity laws to predict the. Why can we consider as acceptable the affinity fan laws to evaluate the centrifugal compressor behaviour. For the relatively small, highspeed centrifugal compressors in the 200 to 700 kw th capacity range, peak efficiency increases about 2% for each jump in. The instability is characterized by large limit cycle oscillations in compressor. Fan laws, the use and limits in predicting centrifugal compressor off design performance 93 is operated on a light gas and then operated on a heavy gas, the curve will be steeper for the light gas. Understand your process, as well as the advantages and disadvantages of each centrifugal compressor configuration, in order to choose the optimal centrifugal compressor for your application. This means that a centrifugal compressors operational range through speed variation is limited. D, in electrical submersible pumps manual second edition, 2018. The affinity laws are a group of laws and rules that govern your pumps. If the compressor flow, head and efficiency characteristics are known at a given speed, then merely applying the affinity laws at an alternate. Reprinted with permission from cep chemical engineering.
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