A paper drying system provides steam at the right pressure to every dryer in the machine as per requirement, and ensures an efficient evacuation of condensate from inside the dryers. A good paper drying system facilitates this over the varied paper grades that the machine intends to produce.
The objective of a paper drying system is the attainment of the desired dryer surface temperature with minimum steam consumption and least drive load on the machine.
Classification of paper drying systems
There are two categories into which paper drying systems can be classified –
1) BLOW THROUGH SYSTEMS
There are two configurations possible for a blow through system –
a) Thermocompressor based b) Cascade type
2) TRAP BASED SYSTEM
1) BLOW THROUGH SYSTEMS
A blow through system uses the principle of steam assisted condensate evacuation from paper dryers. This is known as the ‘blow through’ phenomenon, which is achieved by passing ‘blow through’ steam through the syphons. The ‘blow through steam drives the condensate out of the dryer as it forms along with itself. Machines with speeds greater than 250 mpm (with 1.5m dryer; for higher dryer size the speed criteria reduces) need a blow through system to facilitate condensate evacuation from dryers. Slow speed machines can also be provided with a blow through system to facilitate better condensate evacuation.
Thermocompressor based Paper DryingThermocompressor based systems
Thermocompressor based systems recirculate the flash and blow through steam back into the steam supply headers for the group. A well designed Thermcompressor is the heart of this system and therefore the nomenclature. The benefits of this system are,
- This system provides maximum pressure in all dryers with independent pressure control on each section. This allows for maximum production rates.
- While operating finer grades, this system can provide the minimum required pressures in the individual dryer sections, thereby eliminating / reducing the need for shutting of few dryers and associated problems.
- A thermocompressor system recirculates all the flash and blwo through steam and is the most efficient paper drying system. The specific steam consumption of a machine with a thermocompressor based paper drying system (all other things remaining the same) is the lowest.
- Automatic differential pressure control due to the thermocompressor.
- Allows for minimum electrical load on the machine due to complete evacuation of condensate and enhances life of bearing, journals, etc.
In a cascade system, the flash and blow through steam is cascaded into other groups at lower pressures. A well designed cascade system allows for a blow through system to be operated where high motive pressure availability is limited.
A cascade system design requires the utmost attention and a proper study of the machine, the paper grades and the operating conditions.
Cascade systems operate the machine at the lowest pressures possible thereby enhancing the steam economy and providing better specific steam consumption.
Essentials of a Good Cascade System
- Minimum dryers venting to condenser
- Providing desired differential pressure on each and every dryer
- Top bottom configuration design, if required
- Correct calculation of the quantity of flash and blow through steam from each section and determining its utilization down the cascade chain
Cascade Paper Drying System
Selection of the right control valves, dP control and level control valves, the right control system whether a DCS or PLC is crucial.
A well engineered ‘blow through’ type of paper drying system has the following sub systems, each of which is an engineered system in itself.
- Steam pressure control system complete with moisture separators, strainers
- Individual pressure control sections
- Automatic and adequate differential pressure control
- Correctly selected syphon and rotary joints
- Air venting
- Flash and blow through separator vessels
- Level controls for separators
- PLC / DCS SCADA system
- Paper break logic
Trap based system
This system provides, on each dryer, an individual ball float trap module complete with double window sight glass, air vent and a bypass arrangement. The traps are fitted with a steam lock release arrangement.
A trap based system is suitable for machine speeds upto 250mpm (with 1.5 mtr dryer).
To maximize the paper machine efficiency and paper quality, the condensate in the dryer must be evacuated constantly.
This can be ensured by a well engineered system that takes into account various factors like paper grade, basis weight, drying rates.
(Fig. Steam Trap Module)
Selection and sizing of each and every component that forms a part of the TRAP BASED system is very critical.
- Good quality pressure reducing / de-superheating stations with the requisite turndown and range ability are a key requirement.
- Pressure control valves with required turndown
- Moisture separators with baffle design are recommended for their higher efficiency
- Engineered strainers with the correct mesh sizes and suitable free flow areas are required – these need to be designed correctly
- Fixed bend stationary syphons are the correct choice along with suitable rotary joints
- Ball float steam traps with inbuilt steam lock release arrangement – the sizing of the trap is very important
- Disc check valves are to be selected, other types of non return valves are sub-optimal
- Double window sight glass with good quality borosilicate lapped glass for high viewing efficiency
- Flash vessels to recover flash steam and pressure powered pumps for returning condensate back
- Wireless trap monitoring systems are recommended for on line diagnosis of leaking or waterlogged traps
- Isolation valves and bypass valves on the system need to be piston type – other isolation valves are not suitable as it is a bi-phase flow. Piston valves are zero leak valves and thereby save fuel too.
- Automatic capsule type air vents with air pockets need to be fitted on every dryer as an integral part of the trap based system.