56 www.parker.com/gsfe Modular Aluminium Compressed Air Dryers Compressed air will flow down a uniform pipe at a constant velocity. For a fixed compressed air flow rate, reducing the diameter of the pipe will increase the speed of the compressed air. Changing pipe diameter can therefore be used as a means to control compressed air flow rate, which is the purpose of an FCD (Flow Control Device). If the pipe diameter is reduced far enough, the air velocity will increase until it reaches its maximum, the speed of sound (hence the FCDs common name of ‘Sonic Nozzle’). Although not needed for every application, FCDs can be used to prevent significant overflow of a desiccant dryer and help maintain a constant outlet dewpoint. They are also commonly used on multi-bank installations (two or more modular aluminium dryers connected in parallel) to prevent preferential flow if piping is unbalanced. FCDs are installed in the outlet of a dryer and are sized so the maximum flow rate of the device is above the outlet flow rate of the dryer. Below 100% of the dryers outlet capacity, they have minimal impact on differential pressure (dP) or flow. Above 100% of the dryer's rated outlet capacity, an increasingly higher differential pressure will be generated by the FCD. This will allow a small dryer overflow to occur satisfying temporary system demand, however if demand were to increase, the line pressure downstream of the FCD will start to fall indicating to the user the dryer is being overflowed. All precautions should be taken from the initial sizing of the dryer to the installation and operation to ensure the dryer is not overflowed. It is recommended that FCDs are fitted to the outlet of each modular dryer when installed in a multi-bank configuration. CDAS HL/OFAS HL/FBP HL & CDAS LE/OFAS LE/FBP LE dryers utilise the same FCDs but have different outlet flow rates. FCD max flow rate is therefore equivalent to approximately 105% of the outlet flow rate for a CDAS LE/OFAS LE/FBP LE dryer and 125% of the outlet flow rate for a CDAS HL/OFAS HL/FBP HL dryer. Benefits of Fitting a Flow Control Device (FCD) • Prevents preferential or significant overflow of the dryer • Helps to maintain a constant outlet pressure dewpoint • Indicates by high pressure drop when system demand exceeds rated capacity Multi-banking Flow Control Device (FCD) Multi-bank CDAS LE 170 Installation A unique advantage of modular aluminium dryers is the ability to access the full cross sectional area of the drying columns and fill the desiccant material with a snowstorm filler, a device which provides maximum packing density of the desiccant material. Snowstorm filling the desiccant material provides a low and equal resistance to the air flow which allows multiple drying chambers to be used without preferential flow and provides a consistent outlet dewpoint with less desiccant and in a smaller, more compact and lightweight dryer. Another advantage of snowstorm filling is that it also allows multiple dryer banks to be used without preferential flow because each individual dryer will have an equal pressure drop. This is a feature unique to modular dryers and is known as multi-banking. 2000 1500 1000 500 0 25 50 75 100 125 Air Flow Rate (% of Dryer Outlet Flow) Pressure Drop (mbar) The Affect of Flow Rate on Pressure Drop Dryer Outlet Flange Flow Control Device (FCD) Dryer Outlet Assembly Smooth Inlet to Throat The divergent section has an included angle of 12°. This is to prevent separation and help keep pressure drop low. The nozzle throat has a small cross-sectional area. Choaked flow occurs here and limits the airflow through the dryer.
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