Abb switchgear manual 12th edition pdf free download
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REV: J. PCM 2. Specific heat specific thermal capacity of a substance is the quantity of heat required to raise the temperature of 1 kg of this substance by 1 C. Mean specific heat relates to a temperature range, which must be stated. For values of c and , see Section 1. Thermal conductivity is the quantity of heat flowing per unit time through a wall 1 m2 in area and 1 m thick when the temperatures of the two surfaces differ by 1 C.
With solids, thermal conductivity generally does not vary much invariable only with pure metals ; in the case of liquids and gases, on the other hand, it is often strongly influenced by temperature.
Heat can be transferred from a place of higher temperature to a place of lower temperature by conduction heat transmission between touching particles in solid, liquid or gaseous bodies. The three forms of heat transfer usually occur together. Heat flow in the case of transfer by convection between a solid wall and a flowing medium:.
Heat flow between two flowing media of constant temperature separated by a solid wall:. Here, 1 and 2 are the heat transfer coefficients at either side of a wall consisting of n layers of thicknesses sn and thermal conductivities n. Thermal radiation For two parallel black surfaces of equal size the heat flow exchanged by radiation is. Index 1 refers to the radiating surface, Index 2 to the radiated surface.
C12 is the effective radiation transfer coefficient. It is determined by the geometry and emissivity of the surface table Oscillations below 16 Hz are termed infrasound and above Sound waves can occur not only in air but also in liquids water-borne sound and in solid bodies solid-borne sound. Solid-borne sound is partly converted into audible air-borne sound at the boundary surfaces of the oscillating body. The frequency of oscillation determines the pitch of the sound.
The sound generally propagates spherically from the sound source, as longitudinal waves in gases and liquids and as longitudinal and transverse waves in solids. A sound source is characterized by its sound power W.
The sound power is the sound energy radiated by a sound source in unit time. Its unit of measurement is the watt. Sound propagation gives rise to an alternating pressure, the root-mean-square value of which is termed the sound pressure p.
It decreases approximately in proportion to the square of the distance from the sound source. The sound intensity I is the sound energy flowing perpendicularly through a surface in unit time; it is therefore a vectorial value.
Since the sensitivity of the human ear is proportional to the logarithm of the sound pressure, a logarithmic scale is used to represent the various types of sound levels. Here: W Sound power radiated from the sound source Wo Reference power W The sound power level is determined indirectly by measuring sound pressure or sound intensity levels at an enveloping surface surrounding the source.
ISO , to or DIN standards can be applied as general stipulations for determining the sound power of machinery, and modified methods have also been developed for a series of machines. The sound intensity level LI is defined as the logarithm of the ratio of the sound intensity at the measuring point to the reference intensity Io.
Measurement is performed with a sound intensity probe in which two microphones are located at a short distance opposite each other. The sound intensity in the direction of the microphone axes is calculated using the sound pressure and the gradients of the sound pressure between the microphones. Further stipulations on determination of the sound power by the sound intensity method can be found in ISO , Parts 1 and 2.
The sound pressure level Lp is measured with a sound level meter as the logarithm of the ratio of the sound pressure to the reference pressure po. Measurement can be performed, for example, to IEC and The volume of a noise can be stated as a linear sound pressure level to DIN , Sheets 1 and 2 or as a frequency-dependent weighted sound pressure level to DIN , E.
Zwicker method. The weighted sound pressure levels LA, LB and LC,which are obtained by switching in defined weighting networks in the sound level meter, are stated in the unit dB with a suffix A , B or C. The total sound power level of several sound sources is obtained by adding their sound powers, i.
Addition of two sound sources of equal strength increases the sound level by 3 dB example: 2 sound sources of 85 dB together have 88 dB. With several sound sources with different volumes, the volume of the loudest sound source is dominant.
Example: 2 sound sources with 80 and 86 dB have a total volume of 87 dB. It follows from this that with two equally loud sound sources both of them attenuate, and with sound sources of different loudness only the louder ones cause attenuation. Every increase in the level by 10 dB causes the perceived loudness to double, and every reduction by 10 dB causes it to halve.
Industrial Commercial Composite generally residential Purely residential Therapy hospitals, etc. Many standards and regulations, including the TAL, make further allowances of between 3 dB and 6 dB deducted from the measured sound pressure level for noises containing tones or pulses. Other limits apply in many other countries. There are various methods for assessing the tone content of a noise: one example is that of DIN Disturbing noise is propagated as air-borne and solid-borne sound.
When air-borne sound waves strike a wall, some are thrown back by reflection and others are absorbed by the wall. If air-borne noise striking a wall causes it to vibrate, the walls transmit the sound into the adjacent space. Solid-borne sound is converted into audible air-borne sound by radiation from the boundary surfaces. Ducts, air-shafts, piping systems and the like can transmit sound waves to other rooms.
Special attention must therefore be paid to this when buildings are designed. Sound pressure and sound intensity decrease with increasing distance from the sound source.
In the open air, sound propagation is not only affected by distance, but also by reflection and absorption on buildings and plant components, and by the acoustic properties of the ground, by plants and by meteorological influences such as wind and temperature gradients.
In general, noise emissions must be kept as low as possible at their point of origin. This can often only be achieved by enclosing the noise sources. VDI provides a guideline for the practical implementation of noise reducing enclosures. The acoustic effect of an enclosure results from a combination of sound reflection from the walls on the inside, and sound absorption.
Without sound absorption, there would be an increase in sound pressure inside the enclosure as a result of the reflected sound, which would reduce the effectiveness of the enclosure. The most commonly used sound-absorbent materials are porous substances, plastics, cork, glass fibre and mineral wool, etc.
For noise reduction by more than around 10 dB, not only the choice of materials for the enclosure walls, but also the careful sealing of all openings, is important. Openings to channel heat and gas are to be fitted with acoustic dampers.
A further method of reducing noise immission is to erect an acoustic screen. The effect of an acoustic screen depends on its dimensions, position and distance from the sound source, distance of the receiver from the edge of the screen, and the absorption capacity of the wall see VDI Table Noise attenuation figures of various construction materials in the range from to Hz Structural component Attenuation dB 45 50 42 42 50 38 Structural Attenuation dB up to 20 30 30 40 15 Brickwork, rendered, 12 cm thick Brickwork, rendered, 25 cm thick Concrete wall, 10 cm thick Concrete wall, 20 cm thick Wood wool mat, 8 cm thick Straw mat, 5 cm thick.
Single door without extra sealing Single door with good seal Double door without seal Double door without extra sealing Single window without sealing Spaced double window with seal. The reduction in level L, obtainable in a room or enclosure by means of soundabsorbing materials or structures is:.
When a reduction in noise of approx. If necessary, the sound source is to be mounted on anti-vibration bearings. These can be simple rubber springs or dissipative steel springs, depending on the requirements.
For high demands, an intermediate foundation which is also installed on anti-vibration mountings may also be necessary see also VDI Mean spec. EAL Table continued Technical values of solids Material Density Melting or freezing point C 1 1 If a number of stresses are applied simultaneously to a component, i. In this case the following rule must be observed: Normal stresses z. E Fig. With stresses below the proportionality limit E elongation increases in direct proportion to the stress Hookes law.
E E Elongation E is an imagined stress serving as a measure of the resistance of a material to deformation due to tensile or compressive stresses; it is valid only for the elastic region. E is determined in terms of the load 0. If the stresses exceed the yield point s, materials such as steel undergo permanent elongation. The ultimate strength, or breaking stress, is denoted by B, although a bar does not break until the stress is again being reduced.
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