Rotary Joints And Condensate Evacuation Systems

Removal of moisture from the paper by drying is an expensive process. The drying section of a paper machine accounts for a large portion of the overall running cost. With rising fuel costs and increased demand for higher dryer capacity, producers need to focus on maximising efficiency and reducing costs. Low efficiency of a paper drying system can be attributed to too much blow-through steam and may result in higher steam consumption as well as higher differential pressure. Rotary joint and syphons are the heart of any paper drying system.

The right selection and implementation ensure maximum capacity realisation, operational flexibility and hence the economy of operation.

Condensate Behaviour Inside a Dryer

Condensate behaviour inside a dryer is a very significant factor in choosing the right syphon design. Condensate behaviour can be divided into 3 stages, puddling, cascading and rimming.

Puddling

At low speeds, the condensate accumulates at the dryer bottom. Differential pressure is needed to push the condensate up to the dryer centre and then out.

Cascading

During puddling and cascading, the best condensate evacuation happens when the syphon tip is submerged in the condensate. This is known as a flooded syphon. Condensate in the syphon is a solid column of water and can be discharged through a trap.

Rimming

At higher speeds, the condensate travels along the dryer surface, forming a rim. This requires a higher differential pressure to overcome additional centrifugal forces. During rimming, the syphon shoe is positioned just above the condensate layer and some quantity of steam is allowed into the syphon along with the condensate. This drastically reduces the density of steam and water column in the syphon, thereby reducing the differential pressure requirement. This is known as a blow through.

Forbes Marshall Rotary Joints and Syphons

Forbes Marshall rotary joints and syphons, suitable for every type of machine and speed, improve drying rates and can reduce steam consumption by up to 5%.

In a 175 TPD kraft machine in Egypt, the dryer surface temperatures varied between 60 and 80 deg C.

After replacing existing syphons with Forbes Marshall FBS rotary joints and syphons, the temperatures improved to between 100 to 120, indicating an average 35 deg C improvement.

At a 140 TPD kraft unit in Vapi, Forbes Marshall FBS rotary joints and syphons increased the average dryer surface temperatures by 25 deg C.

Now the machine is operating with a differential temperature of 27 deg C between steam and dryer surface.

By replacing rotary syphons at a mill in central India with Forbes Marshall high-speed stationary (HSS) rotary joints and syphons along with Forbes Marshall Turbumax™,

the dryer surface temperatures improved from an average of 80 deg C to 95 deg C.

Here, machine production improved from 175 to 190 TPD. Also, the writing/printing 500 mpm machine is now running with a drying rate of 22 in the pre-size press and 24 in the post-size press section.

Dryer surface temperatures improved from 55 to 65 deg C by changing from traditional scoop type syphons to Forbes Marshall high-speed stationary (HSS) rotary joints and syphons.