INTRODUCTION TO RVPD and SHOVEL DRYER:
RVPD (Rotary Vacuum Paddle Dryer) as well as, Vacuum Shovel Dryer are two of the similar looking and commonly used industrial dryers (especially popular in manufacturing of Pharmaceutical, Chemical, Food and Polymer group Products), however, has a few uniqueness that differentiates them from each other. While both these types of industrial dryers consists of a horizontal agitator shaft with blades rotating insides a concentric cylindrical static drum, the type of arms (and blades at the end of each arm) attached to the shaft differentiates them both. Let's do a comparative study as it is one of the most common question I come across from many of my clients.
RVPD or RVD or ROTARY VACUUM PADDLE DRYER:
Rotary Vacuum Paddle Dryer (RVPD) is simply a stationary horizontal drum dryer, which shuffles the material (inside the drum) with the help of the concentric horizontal agitator (with its arms and paddles rotating around its longitudinal axis). The paddles are attached to the arms of the agitator (mostly bolted, however can be welded too). The paddles usually are set at an angle such that those pushes the material to be dried towards the center. So the orientation of the paddles between the center of the dryer and the driving arm is exactly opposite of those paddles which lie between center and the opposite end of the drum. This arrangement at a low RPM ensures the material gets continuously shuffled within the dryer ensuring high level of heat transfer and hence highly effective and fast drying.
Vacuum Shovel Dryer is almost similar in construction, except the arrangement of the shovels or ploughs. The shovels Or ploughs because of its unique shape not only breaks the lumps (if any), but also (and most importantly) lift the mass continuously while pushing it out towards the inner wall of the the drum). This continuous action at a high RPM initiates the mechanical fluidization of the material to be dried. Based of the factors like the size and shape of the shovel or plough, distance between the adjacent ploughs and the RPM, a centrifugal vortex motion takes place and hence the material to be dried starts getting thrown in all directions. This effect brings out a through mixing of the material (especially all the particles of different sizes and bulk densities), because of which high level of heat transfer takes place within the mass which expedites the process of drying in multiples.
Applications and Advantages:
Both RVPD and Vacuum Shovel Dryer are highly popular for Vaccum Drying of any lumpy, sticky and sensitive material where the material to be dried has inherent moisture Or solvent to be removed. Many a times the material to be dried not only needs to be effectively dried, but also the color Or texture of the material needs to be maintained even at a higher temperature. Shovel Dryers are preferred over RVPD only when the material has a tendency of hard lump formation and/or the RPM required isnhigher.
Construction and Major Components:
1. Horizontal Drum: The Horizontal Cylindrical Jacketed/Limpeted Drum Or Vessel is basically designed for high vacuum, as well as for moderate pressure at high temperature. At the drive end, a mechanical seal is usually used to ensure there are no leakages from the drive end side. The other end Or non-drive end of the horizontal drum Or vessel is either cantelever Or is supported with skirt Or Legs. The drum Or vessel is either limpeted Or jacketed with a provision of the circulation of hot water Or low pressure steam. Thermic fluid can be an option too (depending on what is the desired drying temperature and what temperature difference (delta T) we want to achieve. The other end of the vessel is mostly sealed with a concentric round end plate (either clamped with a hinged joint Or is bolted to the body flange).
2. Shaft and Blades: As discussed above, the most important differentiating factor between RVPD and Vacuum Shovel Dryer is the types of blades used on the central horizontal hollow shaft agitator. Usually, the shaft and sub-arms of the shaft are also jacketed (ie Heating & Cooling fluid can be circulated in the agitator and arms). Blades are usually bolted to the arms of agitator. This is so because blades corrodes much faster than the agitator shaft and arms.
3. Bottom discharge of the material: The material to be dried is removed from the bottom valve (either a flush bottom valve Or a knife gate valve). Most of the times the preferred valve is zero holdup valve Or Flush Bottom Valve. At the same time, Knife Gate Valve Or Butterfly Valve are also not very uncommon. Except, the hold up volume is high incase of Knife as well as Butterfly valve.
4. Material Charging Nozzle: The suitable size material feed nozzle (mostly with a removable hopper) is provided on the top of the horizontally installed drum Or vessel. Where feed is automatic (i.e. comes from say the upper floor reactor etc by gravity), the nozzle is a concealed one (i.e. the feed pipe coupled to it) by a TC or a Flanged joint.
5. Vacuum Nozzle and Vapor Filter: Just like the Material Charging Nozzle, the Vacuum Nozzle too is provided on the top of the vessel and mostly it is located in the center of the top of the horizontal drum. This nozzle is usually big in size (designed based on the vapor load expected) and a concentric vertical cylindrical container (mostly insulated and cladded) is installed which houses a single or series of filterbags (usually 5 to 10 microns). The objective of the filter housing is to arrest the powder particles flying away due to turbulent shuffling of the material inside the vessel and vacuum applied inside the vessel.
6. Concentric Vacuum Pipe, Condenser(s) and Receiver(s): The vacuumized heating of the feed material (as well as the solvent Or water content of the feed) results into vapor generation which needs to be condensed effectively. A shell and tube heat exchanger is usually preferred along with either a vertical Or a horizontal cylindrical receiver (solvent collector pot). The cooling media used is either chilled water Or cooling water. The sizing of the condenser (Heat Transfer Area calculation) is done based on the Heat Load expected from the dying operation (based on the stoichiometry of the feed material).
7. Vacuum Pump and Vacuum Connection: Whenever the batches of drying material are not very big, the vacuum pumps can be either water ring pump Or oil seal pumps. Mostly water ring pumps are preferred as they are economical (not very expensive). However, incase of high vacuum requirement, oilseal pumps Or screw pumps are preferred.
8. Drive Assembly including Gearbox and AC Motor with VFD: A 3 phase heavy-duty AC motor (selected based on the torque calculations) is used along with the variable frequency drive. A suitable gearbox is used to achieve the desired rotational speed. Incase (especially applicable in Pharma) the drive is not needed in cleanroom or production room, the agitator shaft is taken long enough and is passed through the dividing wall. In such cases, a mechanical seal or a good quality glandrope is used in the hole of the wall ensuring the drum is fully inside the production room Or clean room (if applicable), whereas, the drive assembly (motor, gearbox etc) are outside of the room wall.
9. Operating Panel and Control Panel: The operating panel can be either a PLC driven Touch Screen HMI based Panel Or would be a push-button type relay based simple Panel. The control panel is basically for housing the controls such as VFDs and circuit breakers. Control panels are mostly kept in the non-production area Or some designated area (or in utility section area).
10. Water Heating and Circulation System Vs Low pressure steam: Whenever the drying temperature is not very high, the heating media preferred is either Hot Water or Low Pressure steam. Incase of Hot Water, a separate tank is taken with water inlet and outlet connection. This cylindrical tank (usually skid mounted) has either in-tank electric heater or intank heating coil carrying low pressure steam inside. A centrifugal pump is used to circulate the water constantly to the dryer jacket Or limpet. The set point of the temperature controller in the jacket of the dryer vessel is used to throttle the flow of steam through PID or cut off the electric heater (depending on what is used as a heating media).
The operating panel can be either a PLC driven Touch Screen HMI based Panel Or would be push-button type relay based simple Panel. The control panel is basically for hosing the controls such as VFDs and circuit breakers. Control panels are mostly kept in the non-production area Or some designated area (or in utility section area).
Material of Construction:
Depending on FDA Or any other governing body's guidelines and/or Need of the Procee, all Contact Parts can be SS316 Or Higher Grade, and Non Contact Parts can be SS304 (Or Mild Steel in case of Non-GMP set up). Incase of Highly Corrosive chemicals, the Contact Parts are recommended to be Hastelloy (C22 Or C276), Or Halar Coated SS304 Or PU Coated SS304 for both RVPD and Vacuum Shovel Dryer.
Surface Finish:
As per GMP (Good Manufacturing Practice) guidelines, the product contact surface needs to be extremely smooth. This helps in preventing the surface corrosion due to electrolytic cell formation. Hence the Product Contact surface is no less than 240 grit Mirror Finish and Non Contact surface is 180 grit matt finish. Incase of a Non-GMP set up, if Mild Steel is used (for Product Non Contact in some cases), the surface is either epoxy painted Or coated with protective layer.
Let us discuss the design and other need based considerations that goes into planning and designing an RVPD and a Vacuum Shovel dryer in PART -II of this post. Meanwhile, please feel free to contact incase of any queries, questions Or suggestions on this Or any other topic at shg.techo@gmail.com
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