Pressure level

Fig. 1. Approximate relationship between sound pressure level and distance as a function of room absorption. Numbers on the curves are metric sabins.

I oise Transmission Reduction in HVACSystems. One common use of sound-absorbing treatment is to reduce noise transmission in heating, ventilating, and air-conditioning (HVAC) systems (6). The treatments ate used to reduce the transmission of fan noise and air turbulence noise through ducts into occupied spaces. Noise transmission reduction in duct systems is described in terms of insertion loss, the difference in sound power level or sound pressure level measured at a given location before and after installation of the treatment or sound attenuation, the reduction in sound power between two locations affected by a sound source. The units ate decibels. 

Noise reduction (AIR) is the difference in the average sound pressure level between the source room and the receiving room. When the receiving room is relatively reverberant and the measurements are made in the reverberant fields of the two rooms the relationship between TL and AIR is as follows, where S is the surface area of the sound barrier between the two rooms and is the amount of sound absorption in the receiving room (7). 

Units and Rating Procedures. The unit of sound pressure level is the decibel (dB), defined as follows where L is the sound pressure level, p is the measured sound pressure, andis the reference sound pressure of 20 p.Pa. TL and AiR also are expressed in decibels. 

Rotating vanes are provided in the rooms to ensure diffuse sound fields. Sound is introduced into the source room, the average sound pressure level is measured in one-third octave bands in both rooms, and the sound-transmission loss is calculated as follows, where and die average sound... 

Jet Pumps. A jet pump is a clever way of using a high pressure motive fluid and a venturi to aspire a low pressure fluid to an intermediate pressure level. 

The electric motors are often the noisiest component of the centrifuge assembly. Most standard motors in the 75—250 kW range develop noise levels of 85 dbA (weighted sound pressure level using filter A, per the ANSI standard). A quiet motor can reduce this level by 5 dbA and should be used whenever noise is of concern. 

The optimum pressure level for gaseous diffusion operation is also determined by comparison at some pressure level the decrease ia equipment size and volume to be expected from increasing the pressure and density is outweighed by the losses that occur ia the barrier efficiency. Nevertheless, because it is weU known that the cost of power constitutes a large part of the total cost of operation of gaseous diffusion plants, it can perhaps be assumed that a practical value of r does not differ gready from the above optimum. Inclusion of this value ia the preceding equations yields... 

There are two ways of presenting steam balance data, schematically or tabulady. For both presentation types, a balance is made at each pressure level. In a schematic balance, such as that shown in Figure 9, horizontal lines are drawn for each pressure. The steam-using equipment is shown between the lines, and individual flows are shown vertically. Table 3 contains the same data as shown in Figure 9. In both cases the steam balance has been simplified to show only mass flows. A separate balance should be developed that identifies energy flows, including heat losses and power extraction from the turbines. 

Disturbance or load variable Sound pressure level Manipulated variable Number of constraints Mass flow Mass of reactants Molecular weight... 

Regulatory Control For most batch processes, the discrete logic reqmrements overshadow the continuous control requirements. For many batch processes, the continuous control can be provided by simple loops for flow, pressure, level, and temperature. However, very sophisticated advanced control techniques are occasionally apphed. As temperature control is especially critical in reactors, the simple feedback approach is replaced by model-based strategies that rival if not exceed the sophistication of advanced control loops in continuous plants. 

Sound strength is therefore location-depeudent and is often quantified as a sound pressure level (Lp) in dB based on the root-mean-square (rms) sound-pressure (ps) value, where... 

For airborne sound, the reference pressure is 2 X 10" Pa (29 X psi), which is nominally the human threshold of hearing at 1000 Hz. The corresponding sound pressure level is 0 dB. Conversation is about 50 dB, ana a Jackhammer operator is subject to 100 dB. Extreme levels such as a jet engine at takeoff might produce 140 dB at a distance of 3 m, which is a pressure amplitude oi 200 Pa (29 X 10" psi). These examples demonstrate both the sensitivity and wide dynamic range of the human ear. 

There are two approaches to fluid-generated noise control—source or path treatment. Path treatment means absorbing or blocking the transmission of noise after it has been created. The pipe itself is a barrier. The sound pressure level inside a standard schedule pipe is roughly 40-60 dB higher than on the outside. Thicker walled pipe reduces levels some at more, and adding acoustical insulation on the outside of the pipe reduces ambient levels up to 10 dB per inch of thickness. Since noise propagates relatively unimpeded inside the... 

FIG. 8-79 Valve -generated sound pressure level spectnims. 

For most chemical plants, process steam is used at pressures of 1.825 MN/m" (250 psig), saturated or lower. When combined heat and power generation is economically justified, the steam may be generated at about 5.96 MN/m" (850 psig) appropriately superheated and used to drive back-pressure steam turbines passing out process steam at the required pressure level. 

The mixed refrigerant cwcle was developed to meet the need for hq-uefying large quantities of natural gas to minimize transportation costs of this fuel. This cycle resembles the classic cascade cycle in principle and may best be understood by referring to that cycle. In the latter, the natural gas stream after purification is cooled successively by vaporization of propane, ethylene, and methane. Each refrigerant may be vaporized at two or three pressure levels to increase the natural gas coohng efficiency, but at a cost of considerable increased process complexity. 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

Noise level Motors must conform to the requirements of lEC 60034-9. A safe sound pressure level for a human body to perform better, during an 8-hour... 

The nitrogen line pressure is set to a level much less than the experimental pressure. With the nitrogen cylinder open, a check valve prevents the feed gas from getting into the nitrogen cylinder. When the surge check valve cuts off the synthesis gas feed, the unit becomes depressurized. Then pressure drops below the nitrogen pressure level, nitrogen flow will start and flush out the system. 

Suitable stoneware was used to build the earlier plants, limiting the pressure levels. The development of stainless steel gave momentum to the eontinuing seareh for proeess improvements. 

Process 2 Ammonia eombustion at 3-12 bar and subsequent nitrous gas absorption at the same pressure level... 

A high pressure level results in a higher energy requirement with possibly higher utility eosts. On the other hand, today s striet environmental pollution laws with regard to NOx diseharged into the atmosphere are easier to meet at higher pressures, and oxidation of NO to NO2 is favored by inereased pressure and low temperature. 

Although the turbomaehinery is the heart of the plant, it represents only about 15% of the total installation. Therefore, from a eost viewpoint its influenee on the ehosen proeess and pressure level is rather small. 

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