STEAM TUBE DRYERS
Steam tube dryers have been built for a variety of customers over a period of years. Among the most sophisticated have been the TA and PTA dryers installed at ICI Petrochemicals Division's Terephthalic Acids Plant at Wilton, Cleveland. A typical outline specification for one dryer is given below.
Steam Tube Terephthalic Acid Dryer
The shell is fabricated from stainless steel plate in one section, with longitudinal and circumferential butt welded joints. The shell beneath the girth gear ring and tyres is increased in thickness with tapered edges to match the nominal shell thickness. Steel pads are welded to the thickened plates which are machined for the attachment of shims and tyre support saddles. A steel flange is welded to the thickened plate at girth gear ring location and is machined to suit the gear ring fixing.
Stainless steel heating tubes are arranged equally around the circumference of the shell, these tubes being welded into a stainless steel tubesheet at the product discharge end, and fixed by means of a stainless steel seal mounting plate at the product inlet end. Longitudinal flights can be welded to the inside of the shell on the same radial centre line as the outer row of steam tubes. The mounting plate has a tube gland sealing arrangement which allows for the differential expansion of the drum and tubes. The tubes are supported along their length by stainless steel hangers welded to the shell. The discharge end is fitted with a rotary steam union through which steam enters and condensate is removed and the tubes project through the gland housings at the feed end and are fitted with bleed plugs to bleed the air from the system and ensure the presence of steam. Steam condenses inside the tubes and the condensate is returned to the steam header by virtue of the slope of the dryer, from where it is removed through the steam union by means of a syphon tube.
The product is introduced into the dryer through a screw conveyor and stainless steel feed hood, fitted with HW Associates patented seal arrangement, and is discharged through peripheral holes in the shell into a stainless steel discharge hood and chute, which is likewise fitted with our patented seal arrangement.
The tyres are cast or forged steel machined solid section in one piece and are located on support and thrust rollers which in turn are carried on heavy steel section bedplates fitted with heavy steel adjusting screws for both support and thrust rollers. Each support and thrust roller is fitted with a pair of spherical roller bearings, together with seals to prevent the ingress of dirt, mounted on a fixed turned steel spindle.
Lubrication of all rollers is by grouped grease nipples piped and mounted at a convenient point on the bedplate.
The drive is transmitted from the motor through a vee-rope drive to the high speed shaft of a totally enclosed helical gear unit. The low speed shaft of the gear unit is coupled through a resilient grid spring coupling to the pinion shaft, which carried the pinion, meshing with the girth gear ring. The girth gear is manufactured from cast steel in halves with machined joints secured together with turned and fitted bolts.
Automatic lubrication is provided for the girth gear/pinion and the tyre/roller faces. The system for the girth gear is a fully automated air/oil spray system whilst that for the tyres is a motor driven oil drip feed system.
Diaphragm Type Patented Seal Arrangement
The purpose of the seal is to provide an efficient sealing arrangement between the rotating shell and stationary feed and discharge hoods. In order to provide for the eccentricity of running together with radial expansion, the seal design is basically a plain face seal normal to the axis of the shell. Axial expansion or contraction of the dryer shell is accommodated by a flexible diaphragm. The seals key into a one-piece seal holder ring and form a seal with a rubbing ring on the shell. The system maintains a constant interface pressure and can be readily adjusted to establish the best operative interface pressure. It is self-compensating for seal wear, and movement in the axial direction due to expansion or contraction is accounted for, maintaining a constant load on the shell. Much thought has been given to the ease of maintenance of the sealing system. The design of seal allows for ease of installation and replacement, whilst the open design of the seal loading mechanism enables the system to be readily inspected for wear and cleaning as necessary.