Variable air volume

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... 

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. 

In thermal models, the ventilation airflow rates normally arc input parameters, to be defined by the user or to be calculated by the program on the basis of a nominal air exchange or flow rate) and some control parameters (demand-controlled ventilation, variable air volume flow ventilation systems), in airflow models, on the other hand, room air temperatures must be defined in the input (see Fig. 11.49). 

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. 

Flares ideally bum waste gas completely and smokelessly. Two types of flares are normally employed. The first is called the open flare, the second is called the enclosed flare. The major components of a flare consist of the burner, stack, water seal, controls, pilot burner, and ignition system. Flares required to process variable air volumes and concentrations are equipped with automatic pilot ignition systems, temperature sensors, and air and combustion controls. 

TCV = Temperature Control Valve TE = Temperature Element VAV = Variable Air Volume XP = Positioner for Fan Volume Control, such as a Blade Pitch Positioner... 

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. 

Variable air volume (VAV) laboratories are rapidly replacing traditional CAV laboratories as the design standard. These systems are based on fume hoods with face velocity controls. As the users operate the fume hoods, the exhaust volume from the laboratory changes and the supply air volume must adapt to maintain a volume balance and room pressure control. An experienced laboratory ventilation engineer must be consulted to design these systems, because the systems and controls are complex and must be designed, sized, and matched so they operate effectively together. 

For the office environment, the more common system used today is the variable-air-volume (VAV) system. This approach was originally developed as a cooling system, but with proper apphcation of control it will serve equally well for heating. In climates where there is need for extensive heating, perimeter treatment is required to replace the skin loss of the building structure. An old but reUable method is fin-tube radiation supplied with hot water to replace the skin loss. A system that is being seen with more regularity is in the form of perimeter air supply. Care should be taken with the apphcation of perimeter air systems to ensure that wall U-values are at least to the level of ASHRAE Standard 90.1. If this is not done, interior surface temperatures will be too low and the occupants in the vicinity will feel cold. 

On variable-air-volume (VAV) systems, install variable-frequency drives (VFDs) to control supply fan and return fan speed as a function of system pressure set-point in place of inlet-vane control. This is applicable for airfoil or backward-curve fans, not forward-curve fans, which have relatively efficient unloading with inlet-vane control. 

Most of the chemical fume hoods considered here consist of a cabinet or enclosure set at waist level (above a table or storage cabinet) that is connected to a blower located above the hood or external to the hood through a duct system. The cabinet has an open side (or sides) to 2illow a user to perform work within. A movable transparent sash separates the user from the work. Most chemical fume hoods have a sill that functions as an airfoil at the work surface below the sash. The connection to the blower might be by use of a v-belt, or it may be direct drive. This allows provision of a smooth flow of air with minimal turbulence. In some installations, axially mounted blowers are used, especially if multiple hoods are ducted into a common blower. Baffles located in the rear of the cabinet provide control of the air flow patterns, and can usually be adjusted to provide the best air flow around the experiment or procedure being performed. Many chemical fume hoods are equipped with air flow indicators, low flow monitors and alarms, and differenti2d pressure sensors to allow the user to operate safely. The major types of chemical fume hoods include the standard/conventional, W2dk-in, bypass, variable air volume, auxiliary air, or ductless types. Additional types include snorkels and canopies that are portable. Each type must be understood to be operated most efficiently within specifications (see the section below on safe operation). 

A fan motor consumes 40 kilowatts of power, operates 2,080 hours per year and delivers 35,000 CFM (constant volume). The air distribution system is converted to variable air volume and the same fan supplies an annual average of 20,000 CFM at the same static pressure. The annual amount of energy saved, assuming 0.08 kW-h, is most nearly ... 

The best way to control the fan supply pressure for a variable air volume HVAC System is to ... 

A system consisting of variable air volume (VAV) air handling units. 

A properly designed pocket filter can perform efficiently from 25% up to 150% of the normal air flow, i.e. it is particularly suitable for variable air volume systems. Pocket filters are available for numerous applications in many configurations, and are capable of maintaining a low pressure drop for over 8000 hours. 

Where cost-effectiveness or initial capital cost is not a critical factor, flat panel fine filters can be combined with pocket type filters, to provide second-stage filtration, or electrostatic filters may be used for the retention of even finer particles. If the latter are of the dry type, collection of dust from the electrostatic filters must be by pocket or automatic roll filter. Dry-type electrostatic filters usually combine such dust collectors in an integral unit. Both pocket type filters and electrostatic filters are particularly suitable for variable air volume systems, where air volumes can vary from 20% to 110% of design value. Other types of filters are less effective for variable volume flow. Variable air volume systems are becoming increasingly prominent because of the energy savings that are possible. 

Apart from the effect of variable air volumes on the effectiveness of certain types of filter, other system characteristics may dictate the choice of filter type, particularly as regards flow velocities. In general, the higher the efficiency of a filter, the lower the permissible design flow rate hence in many cases it may be necessary to downgrade... 

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