AIR TRAY DRYER (ATD) OR TRAY DRYER (TD)

WHAT IS ATD OR AIR TRAY DRYER:

Air Tray Dryer is the simplest and most economic form of a Static Dryer which works on the principle of Heat Transfer by forced convection. Hot Air is circulated inside a closed cabinet, which help raise the temperature of the wet mass to an extent that the solvent Or water gradually starts getting evaporated and hence forced out of the closed cabinet.

Air Tray Dryer is widely used in all industries including Pharmaceutical (API), Chemicals and Intermediates, Food, Cosmetics etc.

TYPES OF AIR TRAY DRYERS:

Depending on the pattern of air circulation, there can be three popular types of ATDs viz.

1. O Type Flow: Air enters the dryer from the lateral side of the chamber (usually from the lower lateral side) and gets exhausted from the top damper Or exhaust nozzle. The ratio of volume of air exhausted to the volume of air in circulation is 3:10. This type of dryers are also known as a Horizontal flow Tray Dryer. This is the most economic and simplest form of the Tray Dryer and usually doesn't need an inlet AHU. Merely, a simple pre-filter in the inlet is considered to be sufficient. However, if the process demands, HEPA filters might be needed in the inlet. Exhaust AHU can be optional.

2. U Type Flow: Air enters the dryer from the top of the chamber and gets exhausted from the top damper Or exhaust nozzle, taking a U shaped path. If the air inlet point is on the top left, the exhaust would be on the top right and vice-versa. The ratio of volume of air exhausted to the volume of air in circulation is 9:10. This type of dryers are also known as a Vertical Flow Tray Dryer. To facilitate the U shaped path of the Air, the inside wall of the chambers are shaped specifically. Here, usually the inlet air Filtration and Heating are facilitated by external AHU (also known as inlet AHU), which can consist of Pre Filter, Dehumidifier (optional), Steam Coil Or Ele. Heaters, Fine Filter and HEPA Filter (Optional). Exhaust AHU can be purely optional too.

3. W Type Flow: Two stream of air enters the dryer from the two oppositely placed nozzles on the top of the chamber, and gets exhausted from the central damper Or exhaust nozzle on the top, taking a W shaped path. This is also another example of a Vertical Flow Tray Dryer. The ratio of volume of air exhausted to the volume of air in circulation is 9:10 Or 10:10. To facilitate the W shaped path of the Air, the inside wall of the chambers are shaped specifically. Here too, the inlet air Filtration and Heating are facilitated by external AHU (also known as inlet AHU), which can consist of Pre Filter, Dehumidifier (optional), Steam Coil Or Ele. Heaters, Fine Filter and HEPA Filter (Optional). Exhaust AHU can be purely optional too.

PRICIPLE OF WORKING: 

ATD Or Air Tray Dryer works on the Principle of 'Heat Transfer by Forced Convection'. Dry air, when passed over a set of Hot Finned Tubed Coils (carrying Low Pressure Steam) Or over Electric Heaters, gets heated up and further removes the moisture Or solvent from the wet mass to be dried. The most important factors to be taken into consideration here are (a) Amount of Heat Required in KCal for drying the wet mass (b) Heat Transfer Area of the Heating Coils (c) Volumetric Flow Rate of the Air required to achieve the desired temperature and (d) Ratio of Volume of Air in circulation to the Volume of Air to be Discharged.

ADVANTAGES OF ATD OR STD OVER OTHER DRYERS:

• Most Economic Operation with Minimum Energy Consumption:   Air Tray Dryer uses Heat energy provided either by Low Pressure Steam or Electric Heaters. Flow of air is induced with the help of a blower fan operated by AC frequency motor.
• Fast Drying with Minimum consumption of Heat Energy: ATDs are operated with Hot Air as a drying media. Because of the recirculation of the hot air inside the Hot Cabinet, the moisture and/or solvents from the wet mass starts evaporating quickly,  resulting into fast drying. Due to the interlocking mechanism, the heaters get cut off upon sensor reaching the desired temperature (incase of electrically heated ATD) Or  Low Pressure Steam is throttled through PID control valve (incase of the STD or Steam Tray Dryer).
• Possibility of De-Humidification: Incase of coastal areas Or area where the Air carries a lot of humidity, an inline de-humidifier can be added in the air suction line Or inlet AHU (inlet Air Handling Unit), which drags the moisture from the Air by condensing it and producing Dry Air (suitable for heating and drying purpose).
• Ease of Operation: All the important parts of the ATD viz. Inlet AHU, ATD Chamber, Trolley for Trays, Number of Trays, Ducting (if applicable), Exhaust AHU, Blower Fan etc are all accessible and hence cleanable. Some of the parts like External AHU may require ladder Or operating platform if those are placed at a height.
• Minimum Powder Loss Or Maximum Drying Yield: ATD being a static dryer, the powder loss can only be due to handling loss, hence, is controllable. With an Optional Filtration assembly in exhaust (Or AHU in exhaust), the entrained powder particles can be arrested and hence, maximum dry powder is collected by the end of the operation.
• Easy availability of Spares, Gaskets and Filters: All of the spares used in ATD are easily available in the market (and also are majorly available in almost all countries where Pharma Production industry exists).
• Possibility of fully automated operation with Optional 21 CFR: ATD operations can be made fully automated with HMI and PLC and/or SCADA as per the appropriate Zone Classification, as well as, integration of 21 CFR chapter 11.

DISADVATAGES OR LIMITATIONS OF ATD:

• Not Suitable for Air Sensitive Or Hazardous Or Inflammable Products: Certain products due to their properties, cannot withstand direct contact with air Or blow of air. Also, there can be products which are either hazardous Or may have low flash point solvents to be removed. While, there can be some other products which are simply not suitable for ATD. All these products are usually dried under Vacuum.
• Not suitable for Too Wet Or Very High density powder particles:  Hot Air usually penetrates through the wet cake due to the porous nature of the cake. However, if the cake is too wet, Or, if the bulk destiny of the product is very high, the penetration of the air stays limited, hence, either drying requires very long time, Or ATD is not recommended.
• Sheet thicknesses are very low: Cladding over the insulated material is of very thin sheets (usually 18 guage) SS304 Or lower grade Stainless Steel, making is highly vulnerable to mechanical abuse Or impact.
• Possibility of frequent Chocking of the HEPA filters: Incase of the exhaust filtration line, the series of filters usually includes Pre Filter (10 micron), Fine Filter (5 or 3 micron) and HEPA filter (0.3 micron). Many products in the later stages of drying generates highly fine particles, which easily escapes the Pre and Fine filter and get arrested by HEPA. This can lead to frequent chocking of the HEPA filter, and might need frequent replacements.
• Inlet and Exhaust AHU (Optional) needs bigger space: The space needed for suction Air Handling Unit as well as Exhaust Air Handling Unit is usually high. Also, the connecting ducting too requires a lot of space. Incase of an Inlet AHU, usually it is placed in the technical area Or non-operational area, whereas the ATD is in the clean room Or operational area. The ducting has to pass through the cleanroom wall.  Similarly, the exhaust AHU along with its blower and motor must be kelp outside of the cleanroom and is usually kept in the open space. The ducting sometimes has to travel in a really zigzag fashion, because, many a times the technical area has no space left due to HVAC ducting. One has to keep these challenges in mind when planning the ATD. 
• Proper Insulation needed for safety purpose, as well as, to avoid heat loss: ATD body, inlet ducting (optional), as well as, sometimes the exhaust ducting (optional) requires proper insulation and cladding (so as to avoid heat loss) as it carries the Hot Air. This is important in both safety point of view, as well as, for preventing the heat losses. 

MATERIAL OF CONSTRUCTION:

Since Pharmaceutical, Food and even Cosmetics follows GMP guidelines (sometimes due to statutory mandates), usually the Contact Parts are SS316 Or Higher Grade, and Non Contact Parts are SS304. In some cases, Non - Contact can be of Mild Steel (especially true in case of Non-GMP set up). Further, just like other Process Equipments, incase of Highly Corrosive chemicals, the ATD Contact Parts can be Hastelloy (C22 Or C276), Or Halar Coated SS304 Or PU/ PFA Coated SS304.

SURFACE FINISH: 

As per the GMP (Good Manufacturing Practice) guidelines, the product contact surface needs to be extremely smooth. This also helps in preventing the surface corrosion due to electrolytic cell formation. Hence the Product Contact Surface usually is minimum 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.

MAJOR COMPONENTS OF AN ATD:

The major and important components of the ATD can be listed as under:

1. ATD Chamber: The rectangular doubled walled chamber consists of inner 18 guage stainless steel walls which are insulated with LRB or Rockwool insulation and are cladded with external 16 guage steel sheet. Depending on the Type of the dryer (O, U Or W), the inlet and exhaust air points are decided.

2. ATD Door: The chamber door is of the same MOC as that of the chamber. High density Door Gasket (food grade silicon) is fixed on the door, however, it loosens up frequently.

3. Internal trolley with PU wheels: A vertical trolley (mostly SS304 Or SS316) is made up of light weight stainless steel sections. It is designed ergonomically, in such a way that it can hold maximum number of trays (usually 48 trays) and the operator can find it convenient for loading and unloading. Wheels are usually antistatic PU which can withstand high temperature.

4. Trays (for carrying the mass to be dried):  Just like the VTD, the trays are usually rectangular open boxes of size 800mm (32)" x 400mm (16)" x 25mm to 30mm (1 or 1½)" with Or without bidding.

5. Recirculation Fans: The Horizontal ATD has a recirculation fan, which pull the air from the filtration air and pushes it into the recirculation loop internally. The motor is always outside of the chamber body, and fan is inside the body.

6. Inlet Filtration and Heating: Depending upon the type of dryer, the nature of the product and process demand, the Inlet Filtration and Heating Coil can be either completely inside of the chamber body (with a lateral partition housing the heating coil and the re-circulation fans) Or can be completely external by means of an Inlet AHU and Ducting.

Components of the Inlet AHU can be further elaborated as under:

A. Inlet AHU Box: The Rectangular AHU box is built in either SS304 Or in Mild Steel. It's components can be described as follows:

B. Pre Filter Assembly: Usually PP Air Filters of mesh size of 10 to 15 microns are used to filter the inlet air and arrest the suspended Or unwanted solid particles.

C. Dehumidification Assembly (Mostly Optional): Incase of Humid Air, the Dehumidifying coils are used with the chilled water Or brine solution circulating from within the coils. This drags and condenses the water Or moisture from the air and makes it dry. The chilled water piping (supply and return line), as well as, the control valve are outside of the AHU.

D. Fine Filter Assembly (Mostly Optional): PP Air Filters of mesh size of 3 to 5 microns are used to filter the inlet air (already passed through the Inlet Filter and/or over the Dehumidifier) and arrest the suspended Or unwanted solid particles (if any).

E. Steam Heated Coil Or Electrically Heated Coil Assembly: Usually Galvanized Steel Tubes Or SS304 Tubes are used to form the coil with fins (welded over the coil) to increase the Heat Transfer Area. The coil carries the Low Pressure steam. When the clean dry air passes over the coil and fins, it gets heated. The temperature of the Hot Air is controlled through the PID operated gate valve.

F. HEPA FIlter (Mostly Optional): Depending on the process requirements, sometimes, the inlet air stream needs to pass through the HEPA filter assembly. However, HEPA filter  due to its highly fine mesh ( micron) increases the pressure drop across it and hence the higher capacity suction blower might be needed to create fluidization effect.

G. Ducting (Optional): Inlet Air ducting sometime is needed  if the inlet AHU is kept away from the FBD. Usually, Pharma uses either SS304 Or SS316 ducting with Or without insulation. 

H. Instrumentation: Instruments like DPG Or DPT (Differential Pressure Guage/ Transmitter), PG/PT (Pressure Guage/ Transmitter), Temperature Indicator/ Transmitter etc are generally mounted on the AHU and forms an important part of the control philosophy.

7. Exhaust AHU: Again, depending upon the type of the dryer, the nature of the product and process demand, the exhaust of the air can be either simply exhausted in the open air Or can be passed through the External AHU and Ducting. Incase, a HEPA filter is used in the exhaust AHU, a separate suction blower along with the differential pressure guage Or transmitter might be needed.

Safety Features and Instrumentation: 

The common safety features includes -

A. ATD door sensor and blower/Fan motor interlock

B. Rupture disk and and blower/ Fan motor interlock

C. Differential Pressure Transmitter across the HEPA and sometimes across the Fine filters.

D. Earthing element

Instruments such as PT 100 sensors, PID for steam assembly, FLP Or Non FLP earthing arrangements, Timer, Pressure Gauges/ Transmitters, Temperature Gauges/ Transmitters are used.

Further, the operating panel can be either relay based push button type panel Or  HMI (Human Machine Interface) operated PLC (Programmable Logic Control) based panel and can be always suitable to appropriate Zone classification. 

FACTORS TO BE TAKEN INTO CONSIDERATION WHILE DESIGNING AN ATD:

1. Orientation of the ATD, Available Floor Space, Availability of the Utilities and Floor Layout/ Elevation: This is one of the most overlooked Or underestimated point. An important point to be noted here is that, ATD can be installed (a) either completely inside the Clean Room Or (b) Only Door and Skirting  Frame inside and rest of the body in the Technical Area by passing it through the cleanroom wall. As a usual practice, ATD with its AHUs, Blowers, Ducting etc must be planned in sync with each other. Also, a GAD (General Arrangement Drawing) is superimposed on Autocad Layout Drawing to see the feasibility of installation during the design stage

2. Material of Construction:  The appropriate MOC selection is very important with respect to the adherence to the quality and process parameters, as well as, longevity of the ATD.

3. Level of Automation: Depending on the needs of the Production Department, as well as, budget allocation, the level of automation can be decided.

4. Statutory and Safety compliance: Sometimes, due to company policy and FDA mandate, special features are incorporated such as Touch Screen HMI (Human Machine Interface) with PLC (Programmable Logic Control). In some cases, there can be further sophistication by adopting special Data Security measures such as 21 CFR Chapter 11 compliance, which allows password protected individualistic logins and maintains record of each activity by each user. Overall, Safety, GMP guidelines along with respective FDA mandates has to be followed while designing an ATD.

5. Batch Size Calculations: A basic Process Calculations must be done including Heat Load, Drag Force and hence Volumetric Air Flow rate calculations. This helps in deciding the optimum batch size, as well as, the time cycle, hence monthly Or timely production throughput.

6. Product Specific Operational Challenges (based on the previous experience Or Studies): Since it is a capital purchase, the careful attention must be given to the specific needs of the Production Team, concerns and apprehensions of the Engineering and Maintenance, as well as, the Safety and Statutory department, which helps ensure the production goes smooth.

7. Mechanical and Process Design, as well as, Sizing of the Equipments: Stress analysis/ calculations must be carried out under varying parameters viz temperature and pressure. Similarly, the thickness calculations must be done keeping in mind the static and dynamic loads.

HOW TO SELECT THE RIGHT ATD MODEL/ ATD VENDOR (Tips and Tricks):

Here are a few tips that will help you choose/ select the most appropriate ATD brand Or Vendor who will manufacture the ATD for you. These tip and suggestions are over and above your Vendor Qualification protocols and procedures, and purely focuses on the Product/ Machine under discussion.

1. Drying Capacity Calculations: As far as possible, we must see to it that the vendor's design fulfills Or matches our calculated container volumes, thicknesses, finger bags area, heat load, blower fan dia and flowrate and other design parameters. If vendor has their own calculations available, we must see and evaluate them on our specifications and requirements.

2. Material of Construction,  Quality of Gaskets and Makes of BoughtOuts,  Adherance to GMP guidelines: Just like an FBD, ATD too is very a commonly manufactured product and hence a lot of vendors claim to have made outstanding ATDs. Therefore, apart from all other criteria, we must evaluate them on the selection of Material of Construction, Quality of Gaskets and Other BoughtOut items, Aesthetically Pleasing Quality and Surface finish etc. Pharma's requirement of Validation Documentation along with the Test Certificates too is an additional and important parameter.  

Please feel free to contact me regarding any questions, queries Or additional assistance needed on Design, Sales, Qualification or other aspects at   shg.techo@gmail.com