Constant Air Volume

In an all-air system, the indoor temperature can be controlled either by a constant air volume (CAV) system, which varies the temperature of the air but keeps the volume constant, or by a variable air volume (VAV) system, which maintains a constant temperature and varies the volume of the air supplied to internal spaces. 

Fume hoods connected to a common exhaust manifold offer an advantage. The main exhaust system will rarely be shut down hence, positive ventilation is available to each hood on the system at all times. In a constant air volume (CAV) system (see section 8.C.6.3.1), "shutoff dampers to each hood can be installed, allowing passage of enough air to prevent fumes from leaking out of the fume hoods and into the laboratory when the sash is closed. It is prudent to allow 10 to 20% of the full volume of the hood flow to be drawn through the hood in the off position to prevent excessive corrosion. 

The first fume hoods were simply boxes that were open on one side and connected to an exhaust duct. Since they were first introduced, many variations on this basic design have been made. Six of the major variants in fume hood airflow design are listed below with their characteristics. Conventional hoods are the most common and include benchtop, distillation, and walk-in hoods of the constant air volume (CAV), variable air volume (VAV), bypass and non-bypass variety, with or without airfoils. Auxihary air hoods and ductless fume hoods are not considered "conventional" and are used less often. Laboratory workers should know what kind of hood they are using and what its advantages and limitations are. 

A constant air volume (CAV) fume hood draws a constant exhaust volume through the hood regardless of sash position. Because the volume is constant, the face velocity varies inversely with the sash position. The fume hood volume should be adjusted to achieve the proper face velocity at the desired working height of the sash, and then the hood should be operated at this height. (See section 8.C.4.)... 

In contrast, variable air volume (VAV) systems condition supply air to a constant temperature and ensure thermal comfort by varying the airflow to occupied spaces. Most early VAV systems did not allow control of the outdoor air quantity, so that a decreasing amount of outdoor air was provided as the flow of supply air was reduced. More recent designs ensure a minimum supply of outdoor air with static... 

Skaret presents a general air and contaminant mass flow model for a space where the air volume, ventilation, filtration, and contaminant emission have been divided for both the zones and the turbulent mixing (diffusion) between the zones is included. A time-dependent behavior of the concentration in the zones with constant pollutant flow rate is presented. 

In order to have effective exchange of air in important locations in a room, the age of the air in those locations should be low. The basis for comparison is the complete mixing scenario. That scenario gives the same age for any air volume selected in the room, identical to the nominal time constant for the ventilation airflow,. A steady-state scenario is assumed. See Sutcliffe for an overview of definitions related to age of air. The various air exchange efficiency indices are presented in Table 8.6. 

Variable Air Volume Fume Cupboards This type of cupboard incorporates a variable air volume (VAV) controller that regulates the amount of air exhausted from the cupboard such that the face velocity remains essentially constant irrespective of the sash position. A sensor detects either the sash position, the pressure differential l>etween the fume cupboard interior and the room, or the vekxity at some point in the cupboard. This information is used to control either the exhaust fan speed or the position of a control damper. The supply air volume flow rate into the laboratory or workspace should also be regulated. It should be remembered that with the sash in the closed position the amount of air to dilute contaminants in both the fume cupboard and the laboratory is reduced and that there could, for example, be difficulty in reducing contaminant levels below the lower exphasive level. 

The tracer is injected into the duct at a constant rate and mixed with the flowing air. The concentration of the air-tracer mixture is measured further downstream. Assuming perfect mixing and that the air entering the test section has a zero concentration, the air volume flow rate can be calculated based on the mass balance of the tracer... 

This new burner concept (see Fig. 3.26) was tested with very positive results. The new control reduces emissions and improves the ignition behavior by adjusting the gas supply to the actual air volume. At the same time, the performance of the burner system can be adapted within a wide heat load range without increasing the emission of pollutants, as the sensor keeps the gas-to-air ratio always constant. 

An ideal gas is a relatively low-density gas. The pressure p, temperature T, and specific volume v of an ideal gas are related by an equation of state, pv = RT, where i is a constant for a particular gas and is called the gas constant. Air, helium, and carbon dioxide are ideal gases. The properties of an ideal gas can be found in tables such as air tables. 

Film No. Polyimide Doped Treatment Volume Resist, (ohm cm) Volume Dielectrig Constant Air-Side a Resist. (ohm)... 

Let us first consider the synergistic elfect that water has on void stabilization. It is likely that a distribution of air voids occurs at ply interfaces because of pockets, wrinkles, ply ends, and particulate bridging. The pressure inside these voids is not sufficient to prevent their collapse upon subsequent pressurization and compaction. As water vapor diffuses into the voids or when water vapor voids are nucleated, however, there will be an equilibrium water vapor pressure (and therefore partial pressure in the air-water void) at any one temperature that, under constant total volume conditions, will cause the total pressure in the void to rise above that of a pure air void. When the void pressure equals or exceeds the surrounding resin hydrostatic pressure plus the surface tension forces, the void becomes stable and can even grow. Equation 6.5 expresses this relationship... 

The fluidizing-air volume was adjusted during each run to maintain a constant fluidization pattern the volume of air required to achieve this was recorded in each case. 

The reader should be familiar with the use of the air-water psychrometric chart (Figure 2.5). If not, the reader should take a look at some of the problems at the end of Chapters 2 and 3. By way of review, the basic chart consists of a humidity(>/)-temperature (dry-bulb) set of coordinates along with additional parameters (curves) of constant relative humidity, constant moist volume (humid volume), adiabatic cooling curves (which are the same as the wet-bulb or psychrometric lines, for water vapor only) and the 100% relative humidity curve, also called the saturated-air curve. If any two values are known, we can determine the air-moisture condition on Figure 2.5 and evaluate all other required parameters. 

For a given relative humidity (RH) and temperature, values are assumed for two of the three variables, t, D0 and Du and the third is calculated. In this way, droplets of various initial diameter can be calculated to evaporate to any assumed diameter Dt after t seconds of fall at terminal velocity. The equation corrects for the effect of fall velocity on evaporation rate and the concurrent effect of changing diameter on fall velocity. It assumes constant air temperature and pressure, a large volume of air per droplet so that the air humidity is relatively unaffected, no air turbulence, and the absence of solutes which reduce vapor pressure or form evaporation-retarding films at the surface of the droplet. It further assumes terminal fall velocity at all times. With these restrictions, the theory appears valid for droplet sizes that obey Stokes s law. 

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