i need this badly thanks in advance I.Discussion (5 pts each) through a centrifu
ID: 2074899 • Letter: I
Question
i need this badly
thanks in advance
Explanation / Answer
1 A centrifugal compressor is a radial flow rotodynamic fluid machine that uses mostly air as the working fluid and utilizes the mechanical energy imparted to the machine from outside to increase the total internal energy of the fluid mainly in the form of increased static pressure head. Air is sucked into the impeller eye and whirled outwards at high speed by the impeller disk. At any point in the flow of air through the impeller the centripetal acceleration is obtained by a pressure head so that the static pressure of the air increases from the eye to the tip of the impeller. The remainder of the static pressure rise is obtained in the diffuser, where the very high velocity of air leaving the impeller tip is reduced to almost the velocity with which the air enters the impeller eye.
(summary : The idealized compressive dynamic turbo-machine achieves a pressure rise by adding kinetic energy/velocity to a continuous flow of fluid through the rotor or impeller. This kinetic energy is then converted to an increase in potential energy/static pressure by slowing the flow through a diffuser. The pressure rise in impeller is in most cases almost equal to the rise in the diffuser section )
*The pressure rise per stage is high and the volume flow rate tends to be low. The pressure rise per stage is generally limited to 4:1 for smooth operations.
the centrifugal compressors are designed for axial entry at the inlet..High Mach numbers, in the neighbourhood of 1.0, at the inlet to the impeller of centrifugal compressors may cause shock waves at the inlet. This is called "compressibility effect". To avoid it by reducing the Mach number at the inlet, the flow at the inlet is made to enter with some angle to the axial direction, causing reduction in the inlet Mach number. Such a condition is called " pre-whirl". The transonic compressors make use of such a design at the inlet of the impeller. Higher Mach numbers, especially in case of aero-engines, make the impeller more compact, but with the above said modifications
2. The impellers are assembled or mounted on a steel shaft and this assembly is known as compressor rotor (mostly in multi stage compressors). The rotor provide velocity to the gas with blades that are attached to a rotating disc. These blades can be forward-leaning , radial or backward-leaning depending upon the desired output . Most of the multistage compressors use backward-leaning blades as they provide the widest range of efficiency. The impeller extracts the gas with great velocity into a diffuser passage. The diffuser usually compromise two walls which form a radial channel. Because of these arrangements the velocity of the gas decreases and dynamic pressure is converted into static pressure. The diffuser passages are small space between adjacent diaphragms which generally turns the gas flow 180° in order to direct it towards the next impeller.
The basic purpose of a compressor is to deliver air at high pressure required for burning fuel in a combustion chamber so that the burnt products of combustion at high pressure and temperature are used in turbines or propelling nozzles (in case of an aircraft engine) to develop mechanical power. The problem of designing an efficient combustion chamber is eased if velocity of the air entering the combustion chamber is as low as possible. It is necessary, therefore to design the diffuser so that only a small part of the stagnation temperature at the compressor outlet corresponds to kinetic energy.
It is much more difficult to arrange for an efficient deceleration of flow than it is to obtain efficient acceleration. There is a natural tendency in a diffusing process for the air to break away from the walls of the diverging passage and reverse its direction.
In order to control the flow of air effectively and carry-out the diffusion process in a length as short as possible, the air leaving the impeller is divided into a number of separate streams by fixed diffuser vanes. Usually the passages formed by the vanes are of constant depth, the width diverging in accordance with the shape of the vanes. The angle of the diffuser vanes at the leading edge must be designed to suit the direction of the absolute velocity of the air at the radius of the leading edges, so that the air will flow smoothly over vanes. As there is a radial gap between the impeller tip and the leading edge of the vanes , this direction will not be that with which the air leaves the impeller tip.
3 i. Compressed gas or air: Centrifugal compressor is one of the most simple and efficient way to obtain or produce compressed air. They are best suited when the demand of air or gas is constant and excessive.
ii. Food Industry: Food processing industry depends highly on this type of compressor as it can provide oil free compressed air which are necessary for some sensitive petitions.
iii. Gas turbines: Gas turbines use either or both axial and centrifugal compressor to provide the necessary compression.
Centrifugal compressors are mostly used in gas turbine such as :-
a.) Turboshaft
b.) Turboprop
c.) Micro turbines and
d.) Auxiliary power units
iv.) Oil refiners , petrochemical and chemical plants: The centrifugal compressors used for the above purposes generally have a horizontally split casing and most of them are multistage compressors. These type of compressors are generally operated by over-sized steam engines and gas turbines.
v. Refrigeration and air control: Centrifugal Compressors support a wide variety of refrigerants and thermodynamics and are also able to supply compression in water chiller cycles due to which it has a high demand for usage in refrigerators and air conditioners.
4. Advantages over axial flow compressor :
Higher stage pressure ratio
Simple in construction
Strong in construction
More resistant to foreign materials
Less chances of blockade
Less cost of construction
Less weight
Less starting power requirement
Disadvantages over axial flow compressor:
Lesser efficiency and less flow rate
Less Flow rate is up to 200000 cfm
Efficiency 70 to 85 %
More stresses and life is less
These supply relatively less continuous flow of air.
For given mass flow rate, it requires a larger frontal area
Not suitable for multi staging
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